Provided by: afnix_2.1.1-3_amd64 bug

NAME

       vol-1 - afnix programmer's guide

GETTING STARTED

       AFNIX   is  a  multi-threaded functional engine with dynamic symbol bindings that supports
       the object oriented paradigm. The system features a state of the art runtime  engine  that
       runs  on both 32 and 64 bits platforms. The system specification offers a rich syntax that
       makes the functional programming  a  pleasant  activity.  When  the  interpreter  is  used
       interactively,  text is entered on the command line and executed when a complete and valid
       syntactic object has been constructed. Alternatively, the interpreter can execute a source
       file  or  operates  with  an  input  stream.  AFNIX  is a comprehensive set of application
       clients, modules and services. The original distribution  contains  the  core  interpreter
       with   additional  clients  like  the  compiler,  the  librarian  and  the  debugger.  The
       distribution contains also a rich set of  modules  that  are  dedicated  to  a  particular
       domain.  The  basic  modules  are  the  standard  i/o  module,  the  system module and the
       networking module. Above modules are services. A  service  is  another  extension  of  the
       engine that provides extra functionalities with help of several modules. This hierarchy is
       strictly enforced in the system design and provides a clear functional separation  between
       application  domain.  When  looking  for a particular feature, it is always a good idea to
       think in term of module or service functionality.  AFNIX  operates with a set of  keywords
       and  predicates.  The  engine  has  a native Unicode database. The set of standard objects
       provides support for integers, real numbers,  strings,  characters  and  boolean.  Various
       containers  like  list,  vector,  hash table, bitset, and graphs are also available in the
       core distribution. The syntax incorporates the concept of lambda expression with  explicit
       closure.  Symbol  scope  limitation  within  a lambda expression is a feature called gamma
       expression. Form like notation with an easy block declaration is  also  another  extension
       with  respect  to  other  system. The object model provides a single inheritance mechanism
       with dynamic symbol resolution. Special features include instance parenting, class binding
       instance inference and deference. Native class derivation and method override is also part
       of the object model with fixed  class  objects  and  forms.  The  engine  incorporates  an
       original  regular  expression  engine  with  group  matching,  exact  or partial match and
       substitution.  An  advanced  exception  engine  is  also  provided  with  native  run-time
       compatibility.   AFNIX   implements  a true multi-threaded engine with an automatic object
       protection mechanism against concurrent access. A read  and  write  locking  system  which
       operates  with  the  thread  engine  is  also  built in the core system. The object memory
       management is automatic inside the core interpreter. Finally, the engine is written in C++
       and  provides  runtime  compatibility  with it. Such compatibility includes the ability to
       instantiate  C++  classes,  use  virtual  methods  and  raise  or  catch   exceptions.   A
       comprehensive  programming  interface has been designed to ease the integration of foreign
       libraries.

       First contact
       The fundamental syntactic object is a form. A form is parsed and immediately  executed  by
       the  interpreter.  A  form  is  generally  constructed  with  a function name and a set of
       arguments. The process of executing a form is called  the  evaluation.  The  next  example
       illustrates  one  of  the simplest form which is supported by the engine. This form simply
       displays the message hello world.

       Hello world
       At the interpreter prompt, a form is constructed with  the  special  object  println.  The
       unique argument is a string which is placed between double quotes.

       (axi) println "Hello World"
       Hello World

       The  interpreter  can be invoked to enter one or several forms interactively. The form can
       also be placed in a text file and the interpreted called to execute it.  The  als  is  the
       referred  extension  for  a  text  file,  but  it  can be anything. A simple session which
       executes the above file -- assuming the original file is  called  hello.als  --  is  shown
       below.

       zsh> axi hello.als
       Hello World

       In  interactive  mode,  the  interpreter  waits  for  a  form. When a form is successfully
       constructed, it  is  then  immediately  executed  by  the  engine.  Upon  completion,  the
       interpreter  prompt  is  displayed  and  the  interpreter is ready to accept a new form. A
       session is terminated by typing ctrl-d. Another way to use  the  engine  is  to  call  the
       compiler  client  called  axc, and then invoke the interpreter with the compiled file. The
       interpreter assumes the .axc extension for compiled file and will automatically figure out
       which file to execute when a file name is given without an extension.

       zsh> axc hello.als
       zsh> axi hello.axc
       Hello World
       zsh> axi hello
       Hello World

       The  order  of  search  is  determined  by  a special system called the file resolver. Its
       behavior is described in a special chapter of this manual.

       Interpreter command
       The interpreter can be invoked with several options, a file to execute  and  some  program
       arguments. The h option prints the various interpreter options.

       zsh> axi -h
       usage: axi [options] [file] [arguments]
       [h]           print this help message
       [v]           print system version
       [m]           enable the start module
       [i   path]    add a resolver path
       [e   mode]    force the encoding mode
       [f assert]    enable assertion checks
       [f nopath]    do not set initial path
       [f noseed]    do not seed random engine
       [f   seed]    seed random engine

       The v option prints the interpreter version and operating system. The f option turns on or
       off some additional options like the assertion checking. The use of program  arguments  is
       illustrated  later  in this chapter. The i option adds a path to the interpreter resolver.
       Several i options can be specified. The order of search is determined by the option order.
       As  mentioned earlier, the use of the resolver combined with the librarian is described in
       a specific chapter. If the initial file name to execute contains a  directory  path,  such
       path  is  added automatically to the interpreter resolver path unless the nopath option is
       specified.

       Interactive line editing
       Line editing capabilities is provided when the interpreter is  used  interactively.  Error
       messages  are  displayed  in red if the terminal supports colors. Various accelerators are
       bound to the terminal as indicated in the table below.

       Binding     Description
       backspace   Erase the previous character
       delete      Erase at the cursor position
       insert      Toggle insert with in-place
       ctrl-a      Move to the beginning of the line
       ctrl-e      Move to the end of the line
       ctrl-u      Clear the input line
       ctrl-k      Clear from the cursor position
       ctrl-l      Refresh the line editing

       The arrow are also bound to their usual functions. Note that when  using  the  history,  a
       multi-line command editing access is provided by the interpreter.

       Binding   Description
       left      Move the cursor to the left
       right     Move the cursor to the right

       up        Move up in the history list
       down      Move down in the history list

       Command line arguments
       The interpreter command line arguments are stored in a vector called argv which is part of
       the interp object. A complete discussion about object and class is covered  in  the  class
       object  chapter.  At this time, it is just necessary to note that a method is invoked by a
       name separated from the object symbol name with a semicolon. The example below illustrates
       the use of the vector argument.

       # argv.als
       # print the argument length and the first one
       println "argument length: " (interp:argv:length)
       println "first argument : " (interp:argv:get 0)
       zsh> axi argv.als hello world
       2
       hello

       Loading a source file
       The  interpreter object provides also the load method to load a file. The argument must be
       a valid file path or an exception is raised. The load method returns nil. When the file is
       loaded, the interpreter input, output and error streams are used. The load operation reads
       one form after another and executes them sequentially.

       # load the source file demo.als
       (axi) interp:load "demo.als"
       # load the compiled file demo.axc
       (axi) interp:load "demo.axc"
       # load whatever is found
       (axi) interp:load "demo"

       The load method operates with the help of the interpreter resolver. By default the  source
       file  extension  is  als.  If  the  file  has been compiled, the axc extension can be used
       instead. This force the interpreter to load the compiled version. If you are not sure,  or
       do  not  care about which file is loaded, the extension can be omitted. Without extension,
       the compiled file is searched first. If it is not found the source file  is  searched  and
       loaded.

       The compiler
       The  client  axc  is the cross compiler. It generates a binary file that can be run across
       several platforms. The h option prints the compiler options.

       usage: axc [options] [files]
       [h]            print this help message
       [v]            print version information
       [i] path       add a path to the resolver
       [e  mode]      force the encoding mode

       One or several files can be specified on the command line. The  source  file  is  searched
       with  the  help  of  the  resolver. The resolver i option can be used to add a path to the
       resolver.

       Writing structure
       The structure of file is a succession of valid syntactic objects separated by blank  lines
       or  comments.  During  the  compilation or the execution process, each syntactic object is
       processed one after another in a single pass. Reserved keywords are an  integral  part  of
       the  writing systems. The association of symbols and literal constitutes a form. A form is
       the basic execution block in the writing system. When the form uses reserved  keyword,  it
       is customary to refer to it as a special form.

       Character set and comments
       The  writing system operates with the standard Unicode character set. Comments starts with
       the character #. All characters until the end of line are consumed. Comments can be placed
       anywhere in the source file. Comments entered during an interactive session are discarded.

       Native objects
       The  writing  system  operates  mostly  with objects. An object is created upon request or
       automatically by the engine when a native representation  is  required.  To  perform  this
       task,  several  native  objects, namely Boolean for boolean objects, Integer , Relatif for
       integer numbers, Real for floating-point number, Byte, Character and String for  character
       or  string  manipulation are built inside the engine. Most of the time, a native object is
       built implicitly from its lexical representation, but an explicit representation can  also
       be used.

       const boolean  true
       const integer  1999
       const relatif  1234567890R
       const real     2000.0
       const string   "afnix"
       const char     'a'
       trans symbol   "hello world"
       trans symbol   2000

       The  const and trans reserve keywords are used to declare a new symbol. A symbol is simply
       a binding between a name and an object. Almost any standard  characters  can  be  used  to
       declare  a  symbol.  The  const reserved keyword creates a constant symbol and returns the
       last evaluated object. As a consequence, nested const constructs are possible like trans b
       (const  a  1). The trans reserved keyword declare a new transient symbol. When a symbol is
       marked transient, the object bound to the symbol can be changed while this is not possible
       with a constant symbol. Eventually, a symbol can be destroyed with the special form unref.
       It is worth to note that it is the symbol which is destroyed and not the object associated
       with it.

       Stop and resume parsing
       The  parsing  process  is stopped in the presence of the ◀ character (Unicode U+25C0). The
       parsing operation is resumed with the ▶ character  (Unicode  U+25B6).  Such  mechanism  is
       useful  when  dealing with multi line statements. This mechanism is also a good example of
       Unicode based control characters.

       Forms
       An implicit form is a single line command. When a command is becoming complex, the use  of
       the standard form notation is more readable. The standard form uses the ( and ) characters
       to start and close a form. A form causes an evaluation. When a  form  is  evaluated,  each
       symbol  in  the  form  are  evaluated  to  their  corresponding  internal object. Then the
       interpreter treats the first object of the form as the object to execute and the  rest  is
       the  argument  list  for the calling object. The use of form inside a form is the standard
       way to perform recursive evaluation with complex expressions.

       const three (+ 1 2)

       This example defines a symbol which is initialized with the integer 3, that is the  result
       of  the  computation  (+  1  2). The example shows also that a Polish notation is used for
       arithmetic. If fact, + is a built-in operator which causes the arguments to be summed  (if
       possible).  Evaluation can be nested as well as definition and assignation. When a form is
       evaluated, the result of the evaluation is made available to  the  calling  form.  If  the
       result is obtained at the top level, the result is discarded.

       const  b (trans a (+ 1 2))
       assert a 3
       assert b 3

       This  program  illustrates  the mechanic of the evaluation process. The evaluation is done
       recursively. The (+ 1 2) form is evaluated as 3 and the result  transmitted  to  the  form
       (trans  a  3).  This form not only creates the symbol a and binds to it the integer 3, but
       returns also 3 which is the result of the previous evaluation. Finally, the form (const  b
       3)  is  evaluated,  that is, the symbol b is created and the result discarded. Internally,
       things are a little more complex, but the idea remains the same. This program  illustrates
       also the usage of the assert keyword.

       Lambda expression
       A  lambda expression is another name for a function. The term comes historically from Lisp
       to express the fact that a lambda expression is analog to the concept of expression  found
       in  the  lambda  calculus.  There are various ways to create a lambda expression. A lambda
       expression is created with the trans reserved keyword. A lambda expression takes 0 or more
       arguments  and  returns  an object. A lambda expression is also an object by itself When a
       lambda expression is called, the arguments are evaluated from left to right. The  function
       is  then called and the object result is transmitted to the calling form. The use of trans
       vs const is explain later. To illustrate the use of a lambda expression,  the  computation
       of an integer factorial is described in the next example.

       # declare the factorial function
       trans fact (n) (
         if (== n 1) 1 (* n (fact (- n 1))))
       # compute factorial 5
       println "factorial 5 = " (fact 5)

       This  example  calls  for several comments. First the trans keyword defines a new function
       object with one argument called n. The body of the function is defined with the if special
       form  and  can  be  easily  understood.  The  function is called in the next form when the
       println special form is executed. Note that here, the call to  fact  produces  an  integer
       object, which is converted automatically by the println keyword.

       Block form
       The  notation used in the fact program is the standard form notation originating from Lisp
       and the Scheme dialect. There is also another notation called the block form notation with
       the use of the { and } characters. A block form is a syntactic notation where each form in
       the block form is executed sequentially. The form can be either an implicit or  a  regular
       form. The fact procedure can be rewritten with the block notation as illustrated below.

       # declare the factorial procedure
       trans fact (n) {
         if (== n 1) 1 (* n (fact (- n 1)))
       }
       # compute factorial 5
       println "factorial 5 = " (fact 5)

       Another  way  to  create a lambda expression is via the lambda special form. Recall that a
       lambda expression is an object. So when such object is created, it can  be  bounded  to  a
       symbol. The factorial example could be rewritten with an explicit lambda call.

       # declare the factorial procedure
       const fact (lambda (n) (
           if (== n 1) 1 (* n (fact (- n 1)))))
       # compute factorial 5
       println "factorial 5 = " (fact 5)

       Note that here, the symbol fact is a constant symbol. The use of const is reserved for the
       creation of gamma expression.

       Gamma expression
       A lambda expression can somehow becomes very slow during the execution, since  the  symbol
       evaluation  is  done  within  a set of nested call to resolve the symbols. In other words,
       each recursive call to a function creates a new  symbol  set  which  is  linked  with  its
       parent. When the recursion is becoming deep, so is the path to traverse from the lower set
       to the top one. There is also another mechanism called gamma expression which  binds  only
       the  function  symbol  set  to the top level one. The rest remains the same. Using a gamma
       expression can speedup significantly the execution.

       # declare the factorial procedure
       const fact (n) (
         if (== n 1) 1 (* n (fact (- n 1))))
       # compute factorial 5
       println "factorial 5 = " (fact 5)

       We will come back later to the concept of  gamma  expression.  The  use  of  the  reserved
       keyword  const  to  declare  a  gamma  expression  makes  now  sense.  Since most function
       definitions are constant with one  level,  it  was  a  design  choice  to  implement  this
       syntactic  sugar.  Note that gamma is a reserved keyword and can be used to create a gamma
       expression object. On the other hand, note that the gamma expression  mechanism  does  not
       work for instance method. We will illustrate this point later in this book.

       Lambda generation
       A  lambda  expression  can be used to generate another lambda expression. In other word, a
       function can generate a function, an that capability is an  essential  ingredient  of  the
       functional  programming  paradigm.  The  interesting  part  with  lambda expression is the
       concept of closed variables. In the next example, looking at the lambda expression  inside
       gen,  notice  that  the  argument to the gamma is x while n is marked in a form before the
       body of the gamma. This notation indicates that the gamma should retain the value  of  the
       argument  n  when  the closure is created. In the literature, you might discover a similar
       mechanism referenced as a closure. A closure is simply a variable which is closed under  a
       certain  context.  When  a variable is reference in a context without any definition, such
       variable is called a free variable. We will see later more programs  with  closures.  Note
       that it is the object created by the lambda or the gamma call which is called a closure.

       # a gamma which creates a lambda
       const gen (n) (
         lambda (x) (n) (+ x n))
       # create a function which add 2 to its argument
       const add-2 (gen 2)
       # call add-2 with an argument and check
       println "result = " (add-2 3)

       In  short, a lambda expression is a function with or without closed variables, which works
       with nested symbol sets also called namesets. A gamma expression is  a  function  with  or
       without  closed  variable  which is bounded to the top level nameset. The reserved keyword
       trans binds a lambda expression. The reserved keyword const binds a  gamma  expression.  A
       gamma expression cannot be used as an instance method.

       Multiple arguments binding
       A lambda or gamma expression can be defined to work with extra arguments using the special
       args binding. During a lambda or gamma expression execution, the special  symbol  args  is
       defined  with the extra arguments passed at the call. For example, a gamma expression with
       0 formal argument and 2 actual arguments has args defined as a cons cell.

       const proc-nilp (args) {
         trans result 0
         for (i) (args) (result:+= i)
         eval result
       }
       assert 3 (proc-nilp 1 2)
       assert 7 (proc-nilp 1 2 4)

       The symbol args can also be defined with formal arguments. In that case, args  is  defined
       as a cons cell with the remaining actual arguments.

       # check with arguments
       const proc-args (a b args) {
         trans result (+ a b)
         for (i) (args) (result:+= i)
         eval result
       }
       assert 3 (proc-args 1 2)
       assert 7 (proc-args 1 2 4)

       It is an error to specify formal arguments after args. Multiple args formal definition are
       not allowed. The symbol args can also be defined as a constant argument.

       # check with arguments
       const proc-args (a b (const args)) {
         trans result (+ a b)
         for (i) (args) (result:+= i)
         eval result
       }
       assert 7 (proc-args 1 2 4)

       Nameset and bindings
       A nameset is a container of bindings between a name and symbolic variable. We use the term
       symbolic  variable  to  denote any binding between a name and an object. There are various
       ways to express such bindings. The most common one is called a  symbol.  Another  type  of
       binding  is  an  argument. Despite the fact they are different, they share a set of common
       properties, like being settable. Another point to note is the nature of the nameset. As  a
       matter  of  fact,  there  is  various  type of namesets. The top level nameset is called a
       global set and is designed to handle a large number of  symbols.  In  a  lambda  or  gamma
       expression,  the  nameset  is  called  a local set and is designed to be fast with a small
       number of symbols. The moral of this little story is to think always in terms of namesets,
       no  matter how it is implemented. All namesets support the concept of parent binding. When
       a nameset is created (typically during the execution of a lambda expression), this nameset
       is  linked with its parent one. This means that a symbol look-up is done by traversing all
       nameset from the bottom to the top and stopping when one is found. In  term  of  notation,
       the  current  nameset  is  referenced  with the special symbol '.'. The  parent nameset is
       referenced with the special symbol '..'. The top level  nameset  is  referenced  with  the
       symbol '...'.

       Symbol
       A  symbol is an object which defines a binding between a name and an object. When a symbol
       is evaluated, the evaluation process consists in returning the  associated  object.  There
       are  various  ways  to create or set a symbol, and the different reserved keywords account
       for the various nature of binding which has to be done depending on  the  current  nameset
       state.  One  of  the  symbol  property is to be const or not. When a symbol is marked as a
       constant, it cannot be modified. Note here that it is the symbol which  is  constant,  not
       the object. A symbol can be created with the reserved keywords const or trans.

       Creating a nameset
       A  nameset is an object which can be constructed directly by using the object construction
       notation. Once the object is created, it can be bounded to a symbol.  Here  is  a  nameset
       called example in the top level nameset.

       # create a new nameset called example
       const example (nameset .)
       # bind a symbol in this nameset
       const example:hello "hello"
       println example:hello

       Qualified name
       In  the  previous  example, a symbol is referenced in a given nameset by using a qualified
       name such like example:hello. A qualified name defines a path to access a symbol. The  use
       of  a qualified name is a powerful notation to reference an object in reference to another
       object. For example, the qualified name .:hello refers to the symbol hello in the  current
       nameset. The qualified name ...:hello refers to the symbol hello in the top level nameset.
       There are other use for qualified names, like method call with an instance.

       Symbol binding
       The trans reserved keyword has been shown in all previous example.  The  reserved  keyword
       trans  creates  or set a symbol in the current nameset. For example, the form trans a 1 is
       evaluated as follow. First, a symbol named a is searched in the current nameset.  At  this
       stage,  two situations can occur. If the symbol is found, it is set with the corresponding
       value. If the symbol is not found, it is created in the current nameset and set.  The  use
       of  qualified  name is also permitted -- and encouraged -- with trans. The exact nature of
       the symbol binding with a  qualified  name  depends  on  the  partial  evaluation  of  the
       qualified  name. For example, trans example:hello 1 will set or create a symbol binding in
       reference to the example object. If example refers to a nameset, the symbol  is  bound  in
       this  nameset. If example is a class, hello is bounded as a class symbol. In theory, there
       is no restriction to use trans on any object. If the object does not have a symbol binding
       capability, an exception is raised. For example, if n is an integer object, the form trans
       n:i 1 will fail. With 3 or 4 arguments, trans defines automatically a  lambda  expression.
       This notation is a syntactic sugar. The lambda expression is constructed from the argument
       list and bounded to the specified symbol. The rule used to set or define  the  symbol  are
       the same as described above.

       # create automatically a lambda expression
       trans min (x y) (if (< x y) x y)

       Constant binding
       The  const reserved keyword is similar to trans, except that it creates a constant symbol.
       Once the symbol is created, it cannot be changed. This constant property is  hold  by  the
       symbol  itself. When trying to set a constant symbol, an exception is raised. The reserved
       keyword const works also with qualified names. The  rules  described  previously  are  the
       same.  When  a  partial  evaluation  is  done,  the  partial object is called to perform a
       constant binding. If such capability does not exist, an exception is raised. With 3  or  4
       arguments,  const  defines  automatically  a gamma expression. Like trans the rule are the
       same except that the symbol is marked constant.

       # create automatically a gamma expression
       const max (x y) (if (> x y) x y)

       Symbol unreferencing
       The unref reserved keyword removes a symbol reference in a given context. When the context
       is  a  nameset,  the  object  associated  with  the  symbol  is  detached from the symbol,
       eventually destroyed with the symbol removed from the nameset.

       # create a symbol number
       const x 1
       # unreference it
       unref x

       Arguments
       An expression argument is similar to a symbol, except that it is used only  with  function
       argument.  The concept of binding between a name and an object is still the same, but with
       an argument, the object is not stored as part of  the  argument,  but  rather  at  another
       location  which  is  the  execution  stack. An argument can also be constant. On the other
       hand, a single argument can have multiple bindings. Such situation  is  found  during  the
       same  function  call  in  two different threads. An argument list is part of the lambda or
       gamma expression declaration. If the argument is defined as a constant argument a sub form
       notation  is  used  to defined this matter. For example, the max gamma expression is given
       below.

       # create a gamma expression with const argument
       const max (gamma ((const x) (const y)) (if (> x y) x y))

       A special symbols named args is defined during a lambda  or  gamma  expression  evaluation
       with the remaining arguments passed at the time the call is made. The symbol can be either
       nil or bound to a list of objects.

       const proc-args (a b) {
         trans result (+ a b)
         for (i) (args) (result:+= i)
         eval result
       }
       assert 3 (proc-args 1 2)
       assert 7 (proc-args 1 2 4)

       Special forms
       Special forms provides are reserved  keywords  which  are  most  of  the  time  imperative
       statement,  as  part  of  the  writing  system.  Special forms are an integral part of the
       writing system and interact directly with the interpreter. In most cases, a special  forms
       returns the last evaluated object. Most of the special forms are control flow statements.

       If special form
       The  if  reserved  keyword takes two or three arguments. The first argument is the boolean
       condition to check. If the condition evaluates to true the second argument  is  evaluated.
       The  form  return  the result of such evaluation. If the condition evaluates to false, the
       third argument is evaluated or nil is returned  if  it  does  not  exist.  An  interesting
       example  which combines the if reserved keyword and a deep recursion is the computation of
       the Fibonacci sequence.

       const fibo (gamma (n) (
           if (< n 2) n (+ (fibo (- n 1)) (fibo (- n 2))))

       While special form
       The while reserved keyword takes  2  or  3  arguments.  With  2  arguments,  the  loop  is
       constructed  with  a  condition  and  a  form.  With 3 arguments, the first argument is an
       initial condition that is executed only once. When an argument acts as a  loop  condition,
       the  condition  evaluate  to  a  boolean. The loop body is executed as long as the boolean
       condition is true. An interesting example related to integer arithmetic with a while  loop
       is the computation of the greatest common divisor or gcd.

       const gcd (u v) {
         while (!= v 0) {
           trans r (u:mod v)
           u:= v
           v:= r
         }
         eval u
       }

       Note in this previous example the use of the symbol =. The qualified name u:= is in fact a
       method call. Here, the integer u is assigned with a value. In this case, the symbol is not
       changed.  It  is  the  object  which  is  muted. In the presence of 3 arguments, the first
       argument is an initialization condition that is executed only once. In this  mode,  it  is
       important  to note that the loop introduce its own nameset. The loop condition can be used
       to initialize a local condition variable.

       while (trans valid (is:valid-p)) (valid) {
         # do something
         # adjust condition
         valid:= (and (is:valid-p) (something-else))
       }

       Do special form
       The do reserved keyword is similar to the while reserved keyword,  except  that  the  loop
       condition is evaluated after the body execution. The syntax call is opposite to the while.
       The loop can accept either 2 or 3 arguments. With 2 arguments, the first argument  is  the
       loop  body and the second argument is the exit loop condition. With 3 arguments, the first
       argument is the initial condition that is executed only once.

       # count the number of digits in a string
       const number-of-digits (s) {
         const len (s:length)
         trans index 0
         trans count 0
         do {
           trans c (s:get index)
           if (c:digit-p) (count:++)
         } (< (index:++) len)
         eval count
       }

       Loop special form
       The loop reserved keyword is another form of loop. It take four arguments.  The  first  is
       the  initialize form. The second is the exit condition. The third is the step form and the
       fourth is the form to execute at each loop step. Unlike the while and do  loop,  the  loop
       special form creates its own nameset, since the initialize condition generally creates new
       symbol for the loop only.

       # a simple loop from 0 to 10
       loop (trans i 0) (< i 10) (i:++) (println i)

       A loop can also be designed with a Counter object. In this case, a counter is created with
       an  initial  and final count values. The counter step-p method can then be used to run the
       loop

       # a counter from 0 to 10
       trans cntr (Counter 10)
       # a simple loop from 1 to 10
       loop (cntr:step-p) (println cntr)

       In this example, the counter prints from 1 to 10 since the counter is designed to  operate
       from 0 to 9, and the println function is called after the step-p predicate.

       Switch special form
       The  switch  reserved  keyword  is  a condition selector. The first argument is the switch
       selector. The second argument is a list of various value  which  can  be  matched  by  the
       switch value. A special symbol called else can be used to match any value.

       # return the primary color in a rgb
       const get-primary-color (color value) (
         switch color (
           ("red"   (return (value:substr 0 2)))
           ("green" (return (value:substr 2 4)))
           ("blue"  (return (value:substr 4 6)))
         ))

       Return special form
       The  return  reserved  keyword indicates an exceptional condition in the flow of execution
       within a lambda or gamma expression. When a return is executed, the associated argument is
       returned  and  the execution terminates. If return is used at the top level, the result is
       simply discarded.

       # initialize a vector with a value
       const vector-init (length value) {
         # treat nil vector first
         if (<= length 0) return (Vector)
         trans result (Vector)
         do (result:add value) (> (length:--) 0)
       }

       Eval and protect
       The eval reserved keyword forces the evaluation  of  the  object  argument.  The  reserved
       keyword  eval is typically used in a function body to return a particular symbol value. It
       can also be used to force the evaluation of a protected object. In  many  cases,  eval  is
       more  efficient  than  return.  The  protect reserved keyword constructs an object without
       evaluating it. Typically when used with a form, protect return the  form  itself.  It  can
       also  be  used  to prevent a symbol evaluation. When used with a symbol, the symbol object
       itself is returned.

       const add (protect (+ 1 2))
       (eval add)

       Note that in the preceding example that the evaluation will  return  a  lambda  expression
       which is evaluated immediately and which return the integer 3.

       Assert special form
       The  assert  reserved  keyword  check for equality between the two arguments and abort the
       execution in case of failure. By default, the assertion checking is turn off, and  can  be
       activated  with  the  command  option  f  assert.  Needless to say that assert is used for
       debugging purpose.

       assert true   (> 2 0)
       assert 0      (- 2 2)
       assert "true" (String true)

       Block special form
       The block reserved keyword executes a form in a new local set. The local set is  destroyed
       at  the  completion  of the execution. The block reserved keyword returns the value of the
       last evaluated form. Since a new local set is created, any  new  symbol  created  in  this
       nameset is destroyed at the completion of the execution. In other word, the block reserved
       keyword allows the creation of a local scope.

       trans a 1
       block {
         assert    a 1
         trans     a (+ 1 1)
         assert    a 2
         assert ..:a 1
       }
       assert 1 a

       Built-in objects
       Several built-in objects and built-in operators for arithmetic and logical operations  are
       also  integrated in the writing system. The Integer and Real classes are primarily used to
       manipulate numbers. The Boolean class is used to for boolean  operations.  Other  built-in
       objects include Character and String. The exact usage of these objects is described in the
       next chapter.

       Arithmetic operations
       Support for the arithmetic operations is provided with  the  standard  operator  notation.
       Normally,  these operators will tolerate various object type mixing and the returned value
       will generally be bound to an object that provides the minimum loss of  information.  Most
       of the operations are done with the +, -, * and / operators.

       (+ 1 2)
       (- 1)
       (* 3 5.0)
       (/ 4.0 2)

       Logical operations
       The  Boolean  class  is used to represent the boolean value true and false. These last two
       symbols are built-in in the interpreter as constant symbols. There are also special  forms
       like not, and and or. Their usage is self understandable.

       not true
       and true (== 1 0)
       or (< -1 0) (> 1 0)

       Predicates
       A  predicate  is  a  function  which  returns a boolean object. There is always a built-in
       predicate associated with a built-in object. By convention, a  predicate  terminates  with
       the  sequence  -p.  The  nil-p  predicate is a special predicate which returns true if the
       object is nil. The object-p predicate is the negation of the nil-p predicate.

       Predicate     Description
       nil-p         check nil object
       eval-p        check evaluation
       real-p        check real object
       regex-p       check regex object
       object-p      check for non nil object

       string-p      check string object
       number-p      check number object
       method-p      check method object
       boolean-p     check boolean object
       integer-p     check integer object
       character-p   check character object

       For example, one can write a function which returns true if the argument is a number, that
       is, an integer or a real number.

       # return true if the argument is a number
       const number-p (n) (
         or (integer-p n) (real-p n))

       Special  predicates  for  functional  and  symbolic  programming  are also built-in in the
       engine.

       Predicate     Description
       class-p       check class object
       thread-p      check thread object
       promise-p     check promise object
       lexical-p     check lexical object
       literal-p     check literal object
       closure-p     check closure object
       nameset-p     check nameset object
       instance-p    check instance object
       qualified-p   check qualified object

       Finally, for each object, a predicate is also  associated.  For  example,  cons-p  is  the
       predicate for the Cons object and vector-p is the predicate for the Vector object. Another
       issue related to evaluation, is to decide whether or not an object can be  evaluated.  The
       predicate eval-p which is a special form is designed to answer this question. Furthermore,
       the eval-p predicate is useful to decide whether or not  a  symbol  is  defined  or  if  a
       qualified name can be evaluated.

       assert true  (eval-p .)
       assert false (eval-p an-unknown-symbol)

       Class and instance
       Classes  and  instances  are  the  fundamental objects that provide support for the object
       oriented paradigm. A class is a  nameset  which  can  be  bounded  automatically  when  an
       instance  of  that class is created. The class model is sloppy. Compared to other systems,
       there is no need to declare the data members for a  particular  class.  Data  members  are
       created  during  the  instance  construction.  An instance can also be created without any
       reference to a class. Methods can be bound to the  class  or  the  instance  or  both.  An
       instance can also be muted during the execution process.

       Class and members
       A  class  is  declared  with  the reserved keyword class. The resulting object acts like a
       nameset and it is possible to bind symbol to it.

       # create a class object
       const Circle (class)
       const Circle:PI 3.1415926535
       # access by qualified name
       println Circle:PI

       In the previous example, the symbol Circle is created as a class object. With the help  of
       a  qualified  name,  the  symbol PI is created inside the class nameset. In this case, the
       symbol PI is invariant with respect to the instance object. A form can also  be  bound  to
       the  class  nameset.  In both cases, the symbol or the form is accessed with the help of a
       qualified name.

       Instances
       An instance of a class is created like any built-in object. If a method called  preset  is
       defined for that class, the method is used to initialize the instance.

       # create a class
       const Circle (class)
       trans Circle:preset (r) {
         const this:radius (r:clone)
       }
       # create a radius 1 circle
       const c (Circle 1)

       This  example  calls  for several comments. First the preset lambda expression is bound to
       the class. Since preset is a reserved name for the class object, the form is automatically
       executed  at  the instance construction. Second, note that the instance data member radius
       is created by the lambda expression and another reserved keyword called this  is  used  to
       reference the instance object as it is customary with other programming systems.

       Instance method
       When a lambda expression is bound to the class or the instance, that lambda can be invoked
       as an instance method. When an instance method is invoked, the instance nameset is set  as
       the  parent nameset for that lambda. This is the main reason why a gamma expression cannot
       be used as an instance method. Therefore, the use of the  reserved  keyword  this  is  not
       recommended  in a gamma expression, although it is perfectly acceptable to create a symbol
       with such name.

       # create a perimeter method
       trans Circle:perimeter nil (
         * (* 2.0 Circle:PI) this:radius)
       # call the method with our circle
       trans p (c:perimeter)

       It must be clear that the perimeter symbol defines a method at  the  class  level.  It  is
       perfectly  acceptable  to  define a methods at the instance level. Such method is called a
       specialized method.

       Miscellaneous features

       Iteration
       An iteration facility is provided for some objects known as iterable  objects.  The  Cons,
       List  and  Vector  are  typical iterable objects. There are two ways to iterate with these
       objects. The first method uses the  for  reserved  keyword.  The  second  method  uses  an
       explicit iterator which can be constructed by the object.

       # compute the scalar product of two vectors
       const scalar-product (u v) {
         trans result 0
         for (x y) (u v) (result:+= (* x y))
         eval result
       }

       The  for reserved keyword iterate on both object u and v. For each iteration, the symbol x
       and y are set with their respective object value. In the  example  above,  the  result  is
       obtained by summing all intermediate products.

       # test the scalar product function
       const v1 (Vector 1 2 3)
       const v2 (Vector 2 4 6)
       (scalar-product v1 v2)

       The  iteration  can  be  done  explicitly  by  creating  an  iterator for each vectors and
       advancing steps by steps.

       # scalar product with explicit iterators
       const scalar-product (u v) {
         trans result 0
         trans u-it   (u:get-iterator)
         trans v-it   (v:get-iterator)
         while (u:valid-p) {
           trans x (u:get-object)
           trans y (v:get-object)
           result:+= (* x y)
           u:next
           v:next
         }
         eval result
       }

       In the example above, two iterators  are  constructed  for  both  vectors  u  and  v.  The
       iteration is done in a while loop by invoking the valid-p predicate. The get-object method
       returns the object value at the current iterator position.

       Exception
       An exception is an unexpected change in the execution flow. The exception model  is  based
       on a mechanism which throws the exception to be caught by a handler. The mechanism is also
       designed to be compatible with the native implementation.  An exception is thrown with the
       special  form  throw.  When  an  exception  is  thrown,  the  normal  flow of execution is
       interrupted and an object used  to  carry  the  exception  information  is  created.  Such
       exception object is propagated backward in the call stack until an exception handler catch
       it. The special form try executes a form and catch an exception if one  has  been  thrown.
       With one argument, the form is executed and the result is the result of the form execution
       unless an exception is caught. If an exception is caught,  the  result  is  the  exception
       object. If the exception is a native one, the result is nil.

       try (+ 1 2)
       try (throw)
       try (throw "hello")
       try (throw "hello" "world")
       try (throw "hello" "world" "folks")

       The  exception  mechanism  is also designed to install an exception handler and eventually
       retrieve some information from the exception object. The reserved symbol what can be  used
       to retrieve some exception information.

       # protected factorial
       const fact (n) {
         if (not (integer-p n))
         (throw "number-error" "invalid argument")
         if (== n 0) 1 (* n (fact (- n 1)))
       }
       # exception handler
       const handler nil {
         errorln what:eid ',' what:reason
       }
       (try (fact 5)       handler)
       (try (fact "hello") handler)

       The  special  symbol  what stores the necessary information about the place that generated
       the exception. Most of the time, the qualified name what:reason or what:about is  used.The
       only  difference is that what:about contains the file name and line number associated with
       the reason that generated the exception.

       Regular Expressions
       A regular expression or regex is an object which is used to match certain  text  patterns.
       Regular  expressions  are  built  implicitly  by  the  parser  with the use of the [ and ]
       characters. Special class of characters are defined with the help of the $ character.  For
       example,  $d  is  the  class  of  character  digits  as defined by the Unicode consortium.
       Different regular expression can be grouped by region to be matched as  indicated  in  the
       example below.

       if (== (const re [($d$d):($d$d)]) "12:31") {
         trans hr (re:get 0)
         trans mn (re:get 1)
       }

       In  the previous example, a regular expression object is bound to the symbol re. The regex
       contains two groups. The call to the operator == returns true if  the  regex  matches  the
       argument string. The get method can be used to retrieve the group by index.

       Delayed evaluation
       The special form delay creates a special object called a promise which records the form to
       be later evaluated. The special form force causes a promise to  be  evaluated.  Subsequent
       call with force will produce the same result.

       trans   y 3
       const   l ((lambda (x) (+ x y)) 1)
       assert  4 (force l)
       trans   y 0
       assert  4 (force l)

       Threads
       The  interpreter  provides  a  powerful mechanism which allows the concurrent execution of
       forms and the synchronization of shared objects. The engine provides supports the creation
       and  the  synchronization  of  threads  with a native object locking mechanism. During the
       execution, the interpreter wait until all threads are completed. A threads is created with
       the  reserved  keyword launch. In the presence of several threads, the interpreter manages
       automatically the shared objects and protect them against concurrent access.

       # shared variable access
       const var 0
       const decr nil (while true (var:= (- var 1)))
       const incr nil (while true (var:= (+ var 1)))
       const prtv nil (while true (println "value = " var))
       # start 3 threads
       launch (prtv)
       launch (decr)
       launch (incr)

       Form synchronization
       Although, the engine provides  an  automatic  synchronization  mechanism  for  reading  or
       writing  an  object,  it  is  sometimes necessary to control the execution flow. There are
       basically two techniques to do so. First, protect a form from being  executed  by  several
       threads.  Second,  wait  for one or several threads to complete their task before going to
       the next execution step. The reserved keyword sync can be used to synchronize a form. When
       a  form,  is  synchronized,  the  engine guarantees that only one thread will execute this
       form.

       const print-message (code mesg) (
         sync {
           errorln "error  : " code
           errorln "message: " mesg
         }
       )

       The previous example create a gamma expression which make sure that both  the  error  code
       and error message are printed in one group, when several threads call it.

       Thread completion
       The  other  piece  of  synchronization  is  the  thread  completion  indicator. The thread
       descriptor contains a method called wait which suspend the calling thread until the thread
       attached  to  the  descriptor  has been completed. If the thread is already completed, the
       method returns immediately.

       # simple flag
       const flag false
       # simple shared tester
       const ftest (bval) (flag:= bval)
       # run the thread and wait
       const thr (launch (ftest true))
       thr:wait
       assert true flag

       This example is taken from the test suites. It checks that a boolean variable is set in  a
       thread.  Note  the  use  of  the  wait method to make sure the thread has completed before
       checking for the flag value. It is also worth to note that wait is one of the method which
       guarantees  that a thread result is valid. Another use of the wait method can be made with
       a vector of thread descriptors when one wants to wait until all of them have completed.

       # shared vector of threads descriptors
       const thr-group (Vector)
       # wait until all threads in the group are finished
       const wait-all nil (for (thr) (thr-group) (thr:wait))

       Condition variable
       A condition variable is another mechanism to  synchronize  several  threads.  A  condition
       variable  is  modeled  with the Condvar object. At construction, the condition variable is
       initialized to false. A thread calling the wait method  will  block  until  the  condition
       becomes  true.  The mark method can be used by a thread to change the state of a condition
       variable and eventually awake some threads which are blocked on it. The use  of  condition
       variable  is  particularly  recommended when one need to make sure a particular thread has
       been doing a particular task.

       The interpreter object
       The interpreter can also be seen as an  object.  As  such,  it  provides  several  special
       symbols and forms. For example, the symbol argv is the argument vector. The symbol library
       is an interpreter method that loads a library. A complete description of  the  interpreter
       object is made in a special chapter of this book.

LITERALS

       This  chapters  covers  in  detail  the  literals  objects  used to manipulate numbers and
       strings. First the integer, relatif and real numbers are described. There is a broad range
       of  methods  for these three objects that support numerical computation. As a second step,
       string and character objects are described. Many  examples  show  the  various  operations
       which  can  be  used  as  automatic  conversion between one type and another. Finally, the
       boolean object is described. These objects belongs to the class of literal objects,  which
       are  objects  that have a string representation. A special literal object known as regular
       expression or regex is also described at the end of this chapter.

       Integer number
       The fundamental number representation is the Integer. The integer is a 64 bits signed  2's
       complement number. Even when running with a 32 bits machine, the 64 bits representation is
       used. If a larger representation is needed, the Relatif object might be more  appropriate.
       The Integer object is a literal object that belongs to the number class.

       Integer format
       The default literal format for an integer is the decimal notation. The minus sign (without
       blank) indicates a negative number. Hexadecimal and binary notations can also be used with
       prefix 0x and 0b. The underscore character can be used to make the notation more readable.

       const a  123
       trans b -255
       const h  0xff
       const b  0b1111_1111

       Integer  number  are constructed from the literal notation or by using an explicit integer
       instance. The Integer class offers standard constructors. The default constructor  creates
       an integer object and initialize it to 0. The other constructors take either an integer, a
       real number, a character or a string.

       const a (Integer)
       const b (Integer 2000)
       const c (Integer "23")

       When the hexadecimal or binary notation is used, care should be taken to avoid a  negative
       integer.  For example, 0x_8000_0000_0000_0000 is the smallest negative number. This number
       exhibits also the strange property to be equal to its negation since with 2's  complement,
       there is no positive representation.

       Integer arithmetic
       Standard  arithmetic  operators are available as built-in operators. The usual addition +,
       multiplication * and division / operate with two arguments.  The  subtraction  -  operates
       with one or two arguments.

       + 3 4
       - 3 4
       - 3
       * 3 4
       / 4 2

       As  a  built-in  object, the Integer object offers various methods for built-in arithmetic
       which directly operates on the object. The following example illustrates these methods.

       trans i 0
       i:++
       i:--
       i:+ 4
       i:= 4
       i:- 1
       i:* 2
       i:/ 2
       i:+= 1
       i:-= 1
       i:*= 2
       i:/= 2

       As a side effect, these methods allows a const symbol to be modified.  Since  the  methods
       operates on an object, they do not modify the state of the symbol. Such methods are called
       mutable methods.

       const i 0
       i:= 1

       Integer comparison
       The comparison operators works the same. The only difference is that they always return  a
       Boolean  result.  The comparison operators are namely equal ==, not equal !=, less than <,
       less equal <=, greater > and greater equal >=. These operators take two arguments.

       == 0 1
       != 0 1

       Like the arithmetic methods, the comparison operators are  supported  as  object  methods.
       These methods return a Boolean object.

       i:=  1
       i:== 1
       i:!= 0

       Integer calculus
       Armed with all these functions, it is possible to develop a battery of functions operating
       with numbers. As another example, we revisit the Fibonacci sequence as demonstrated in the
       introduction  chapter.  Such  example  was terribly slow, because of the double recursion.
       Another method suggested by Springer and Friedman uses two functions to perform  the  same
       job.

       const fib-it (gamma (n acc1 acc2) (
           if (== n 1) acc2 (fib-it (- n 1) acc2 (+ acc1 acc2))))
       const fiboi (gamma (n) (
           if (== n 0) 0 (fib-it n 0 1)))

       This  later  example is by far much faster, since it uses only one recursion. Although, it
       is no the fastest way to write it, it is still an elegant way to write complex functions.

       Other Integer methods
       The Integer class offers other convenient methods. The odd-p and  even-p  are  predicates.
       The  mod  take  one  argument  and  returns the modulo between the calling integer and the
       argument. The abs methods returns the absolute value of the calling integer.

       i:even-p
       i:odd-p
       i:mod 2
       i:= -1
       i:abs
       i:to-string

       The Integer object is a literal object and a number object. As a literal object,  the  to-
       string  and  to-literal  methods  are  provided  to obtain a string representation for the
       integer object. Although the to-string method  returns  a  string  representation  of  the
       calling integer, the to-literal method returns a parsable string. Strictly speaking for an
       integer,  there  is  no  difference  between  a  string  representation  and   a   literal
       representation. However, this is not true for other objects.

       (axi) const i 0x123
       (axi) println (i:to-string)
       291
       (axi) println (i:to-literal)
       291

       As  a number object, the integer number can also be represented in hexadecimal format. The
       to-hexa and to-hexa-strign methods are designed to obtained such  representation.  In  the
       first form, the to-hexa method return a literal hexadecimal string representation with the
       appropriate prefix while the second one does not.

       (axi) const i 0x123
       (axi) println (i:to-hexa)
       0x123
       (axi) println (i:to-hexa-string)
       123

       Relatif number
       A relatif or big number is an integer  with  infinite  precision.  The  Relatif  class  is
       similar to the Integer class except that it works with infinitely long number. The relatif
       notation uses a r or R suffix to express a relatif  number  versus  an  integer  one.  The
       Relatif  object  is  a  literal  object  that  belongs  to the number class. The predicate
       associated with the Relatif object is relatif-p.

       const a  123R
       trans b -255R
       const c  0xffR
       const d  0b1111_1111R
       const e (Relatif)
       const f (Relatif 2000)
       const g (Relatif "23")

       Relatif operations
       Most of the Integer class operations  are  supported  by  the  Relatif  object.  The  only
       difference  is that there is no limitation on the number size. This naturally comes with a
       computational price. An amazing example is to compute  the  biggest  know  prime  Mersenne
       number. The world record exponent is 6972593. The number is therefore:

       const i 1R
       const m (- (i:shl 6972593) 1)

       This  number  has 2098960 digits. You can use the println method if you wish, but you have
       been warned...

       Real number
       The real class implements the representation  for  floating  point  number.  The  internal
       representation  is machine dependent, and generally follows the double representation with
       64 bits as specified by the IEEE 754-1985 standard for binary floating  point  arithmetic.
       All integer operations are supported for real numbers. The Real object is a literal object
       that belongs to the number class.

       Real format
       The parser supports two types  of  literal  representation  for  real  number.  The  first
       representation  is  the  dotted  decimal  notation.  The second notation is the scientific
       notation.

       const a  123.0 # a positive real
       const b -255.5 # a negative real
       const c  2.0e3 # year 2000.0

       Real number are constructed from the  literal  notation  or  by  using  an  explicit  real
       instance.  The  Real class offers standard constructors. The default constructor creates a
       real number object and initialize it to  0.0.  The  other  constructors  takes  either  an
       integer, a real number, a character or a string.

       Real arithmetic
       The  real  arithmetic  is  similar  to the integer one. When an integer is added to a real
       number, that number is automatically converted to  a  real.  Ultimately,  a  pure  integer
       operation might generate a real result.

       + 1999.0 1   # 2000.0
       + 1999.0 1.0 # 2000.0
       - 2000.0 1   # 1999.0
       - 2000.0 1.0 # 1999.0
       * 1000 2.0   # 2000.0
       * 1000.0 2.0 # 2000.0
       / 2000.0 2   # 1000.0
       / 2000.0 2.0 # 1000.0

       Like the Integer object, the Real object has arithmetic built-in methods.

       trans  r 0.0 # 0.0
       r:++       # 1.0
       r:--       # 0.0
       r:+ 4.0    # 4.0
       r:= 4.0    # 4.0
       r:- 1.0    # 3.0
       r:* 2.0    # 8.0
       r:/ 2.0    # 2.0
       r:+= 1.0   # 5.0
       r:-= 1.0   # 4.0
       r:*= 2.0   # 8.0
       r:/= 2.0   # 4.0

       Real comparison
       The comparison operators works as the integer one. As for the other operators, an implicit
       conversion between an integer to a real is done automatically.

       == 2000 2000   # true
       != 2000 1999   # true

       Comparison methods are also available for the Real object. These methods  take  either  an
       integer or a real as argument.

       r:=  1.0 # 1.0
       r:== 1.0 # true
       r:!= 0.0 # true

       A complex example
       One  of  the most interesting point with functional programming language is the ability to
       create complex function. For example let's assume we wish to compute the value at a  point
       x  of  the  Legendre  polynomial of order n. One of the solution is to encode the function
       given its order. Another solution is to compute the function and then compute the value.

       # legendre polynomial order 0 and 1
       const lp-0 (gamma (x) 1)
       const lp-1 (gamma (x) x)
       # legendre polynomial of order n
       const lp-n (gamma (n) (
           if (> n 1) {
             const lp-n-1 (lp-n (- n 1))
             const lp-n-2 (lp-n (- n 2))
             gamma (x) (n lp-n-1 lp-n-2)
             (/ (- (* (* (- (* 2 n) 1) x)
                   (lp-n-1 x))
                 (* (- n 1) (lp-n-2 x))) n)
           } (if (== n 1) lp-1 lp-0)
         ))
       # generate order 2 polynomial
       const lp-2 (lp-n 2)
       # print lp-2 (2)
       println "lp2 (2) = " (lp-2 2)

       Note that the computation can be done either with integer or real numbers. With  integers,
       you  might  get  some  strange  results anyway, but it will work. Note also how the closed
       variable mechanism is used. The recursion capture each level of the polynomial until it is
       constructed. Note also that we have here a double recursion.

       Other real methods
       The   real  numbers  are  delivered  with  a  battery  of  functions.  These  include  the
       trigonometric functions, the logarithm and couple others. Hyperbolic functions like  sinh,
       cosh,  tanh, asinh, acosh and atanh are also supported. The square root sqrt method return
       the square root of the calling real. The floor and ceiling returns respectively the  floor
       and the ceiling of the calling real.

       const r0 0.0       # 0.0
       const r1 1.0       # 1.0
       const r2 2.0       # 2.0
       const rn -2.0      # -2.0
       const rq (r2:sqrt) # 1.414213
       const pi 3.1415926 # 3.141592
       rq:floor           # 1.0
       rq:ceiling         # 2.0
       rn:abs             # 2.0
       r1:log             # 0.0
       r0:exp             # 1.0
       r0:sin             # 0.0
       r0:cos             # 1.0
       r0:tan             # 0.0
       r0:asin            # 0.0
       pi:floor           # 3.0
       pi:ceiling         # 4.0

       Accuracy and formatting
       Real  numbers  are  not  necessarily accurate, nor precise. The accuracy and precision are
       highly dependent on the hardware as well as the nature of the operation  being  performed.
       In  any  case,  never  assume  that a real value is an exact one. Most of the time, a real
       comparison will fail, even if the numbers are very close  together.  When  comparing  real
       numbers,  it  is preferable to use the ?= operator. Such operator result is bounded by the
       internal precision representation and will generally return the desired  value.  The  real
       precision  is  an  interpreter  value  which is set with the set-absolute-precision method
       while the get-absolute-precision returns the interpreter precision. There is also  a  set-
       relative-precision  and get-relative-precision methods used for the definition of relative
       precision. By default, the absolute precision is set to 0.00001 and the relative precision
       is set to 1.0E-8.

       interp:set-absolute-precision 0.0001
       const r 2.0
       const s (r:sqrt) # 1.4142135
       (s:?= 1.4142)    # true

       Real  number  formatting  is  another  story. The format method takes a precision argument
       which indicates the number of digits to print for the decimal part. Note that  the  format
       command might round the result as indicated in the example below.

       const pi 3.1415926535
       pi:format 3  # 3.142

       If  additional  formatting  is  needed,  the Stringfill-left and fill-right methods can be
       used.

       const pi  3.1415926535        # 3.1415926535
       const val (pi:format 4)       # 3.1416
       println (val:fill-left '0' 9) # 0003.1416

       Number object
       The Integer, Relatif and Real objects are all derived from the Number object  which  is  a
       Literal  object.  As such, the predicate number-p is the right mechanism to test an object
       for a number. The class also provides the basic  mechanism  to  format  the  number  as  a
       string. For integer and relatif, the hexadecimal representation can be obtained by the to-
       hexa and to-hexa-string methods. For integer and real numbers, the format  method  adjusts
       the  final  representation with the precision argument as indicated before. It is worth to
       note that a formatted integer gets automatically converted into a real representation.

       Character
       The Character object is another built-in object. A character is internally represented  by
       a  quad  by  using  a  31  bit representation as specified by the Unicode standard and ISO
       10646.

       Character format
       The standard quote notation is used to represent a character. In that  respect,  there  is
       hare  a  substantial  difference  with other functional language where the quote protect a
       form.

       const LA01 'a' # the character a
       const ND10 '0' # the digit 0

       All characters from  the  Unicode  codeset  are  supported  by  the   AFNIX   engine.  The
       characters  are  constructed  from  the literal notation or by using an explicit character
       instance. The Character  class  offers  standard  constructors.  The  default  constructor
       creates  a null character. The other constructors take either an integer, a character or a
       string. The string can be either a single quoted character or the literal  notation  based
       on the U+ notation in hexadecimal. For example, U+40 is the @ character while U+3A3 is the
       sigma capital letter.

       const nilc (Character)        # null character
       const a    (Character 'a')    # a
       const 0    (Character 48)     # 0
       const mul  (Character "*")    # *
       const div  (Character "U+40") # @

       Character arithmetic
       A character is like an integer, except that it operates in the range 0 to 0x7FFFFFFF.  The
       character  arithmetic  is simpler compared to the integer one and no overflow or underflow
       checking is done. Note that the arithmetic operations take an integer as an argument.

       + 'a' 1 # 'b'
       - '9' 1 # '8'

       Several Character object methods are also provided for  arithmetic  operations  in  a  way
       similar to the Integer class.

       trans  c 'a' # 'a'
       c:++         # 'b'
       trans  c '9' # '9'
       c:--         # '8'
       c:+ 1        # '9'
       c:- 9        # '0'

       Character comparison
       Comparison  operators  are  also working with the Character object. The standard operators
       are namely equal ==, not equal !=, less than <, less equal <=, greater > and greater equal
       >=. These operators take two arguments.

       == 'a' 'b' # false
       != '0' '1' # true

       Other character methods
       The  Character  object  comes with additional methods. These are mostly conversion methods
       and predicates. The to-string method  returns  a  string  representation  of  the  calling
       character.  The to-integer method returns an integer representation the calling character.
       The predicates are alpha-p, digit-p, blank-p, eol-p, eos-p and nil-p.

       const LA01 'a'  # 'a'
       const ND10 '0'  # '0'
       LA01:to-string  # "a"
       LA01:to-integer # 97
       LA01:alpha-p    # true
       ND10:digit-p    # true

       String
       The String object is one of the most important built-in  object  in  the   AFNIX   engine.
       Internally,  a  string  is  a vector of Unicode characters. Because a string operates with
       Unicode characters, care should be taken when using composing characters.

       String format
       The standard double quote notation is used  to  represent  literally  a  string.  Standard
       escape sequences are also accepted to construct a string.

       const hello "hello"

       Any  literal  object  can  be  used  to construct a string. This means that integer, real,
       boolean or character objects are all valid to construct strings. The  default  constructor
       creates a null string. The string constructor can also takes a string.

       const nils (String)      # ""
       const one  (String 1)    # "1"
       const a    (String 'a')  # "a"
       const b    (String true) # "true"

       String operations
       The  String  object  provides  numerous  methods  and  operators. The most common ones are
       illustrated in the  example  below.  The  length  methods  returns  the  total  number  of
       characters  in  the string object. It is worth to note that this number is not necessarily
       the number of printed characters since some characters might be combining characters used,
       for  example,  as diacritics. The non-combining-length method might be more adapted to get
       the number of printable characters.

       const h "hello"
       h:length       # 5
       h:get 0        # 'h'
       h:== "world"   # false
       h:!= "world"   # true
       h:+= " world"  # "hello world"

       The sub-left and sub-right methods return a sub-string, given the position index. For sub-
       left,  the index is the terminating index, while sub-right is the starting index, counting
       from 0.

       # example of sub-left method
       const msg "hello world"
       msg:sub-left  5 # "hello"
       msg:sub-right 6 # "world"

       The strip, strip-left and strip-right are methods used to strip blanks and tabs. The strip
       method combines both strip-left and strip-right.

       # example of strip method
       const str " hello world "
       println (str:strip) # "hello world"

       The  split method returns a vector of strings by splitting the string according to a break
       sequence. By default, the break sequence is the blank, tab  and  newline  characters.  The
       break sequence can be one or more characters passed as one single argument to the method.

       # example of split method
       const str "hello:world"
       const vec (str:split ":" # "hello" "world")
       println (vec:length) # 2

       The fill-left and fill-right methods can be used to fill a string with a character up to a
       certain length. If the string is longer than the length, nothing happens.

       # example of fill-left method
       const pi  3.1415926535  # 3.1415926535
       const val (pi:format 4) # 3.1416
       val:fill-left '0' 9     # 0003.1416

       Conversion methods
       The case conversion methods are the standard to-upper and  to-lower  methods.  The  method
       operates  with the internal Unicode database. As a result, the conversion might change the
       string length. Other conversion methods related to the  Unicode  representation  are  also
       available.  These are rather technical, but can be used to put the string in a normal form
       which might be suitable for comparison. Such conversion always uses the  Unicode  database
       normal form representation.

       # example of case conversion
       const str "hello world"
       println (str:to-upper) # HELLO WORLD

       String hash value
       The  hashid method is a method that computes the hash value of a string. The value depends
       on the target machine and will change between a 32 bits and a  64  bits  machine.  Example
       example 0203.als illustrates the computation of a hash value for our favorite test string.

       # test our favorite string
       const hello "hello world"
       hello:hashid # 1054055120

       The  algorithm  used  by  the  engine  is shown as an example below. As a side note, it is
       recommended to print the shift amount in the program.  One  may  notice,  that  the  value
       remains  bounded  by  24. Since we are xoring the final value, it does illustrate that the
       algorithm is design for a 32 bits machine.  With  a  64  bits  machine  the  algorithm  is
       slightly  modified  to  use  the extra space. This also means that the hashid value is not
       portable across platforms.

       # compute string hashid
       const hashid (s) {
         const len (s:length)
         trans cnt 0
         trans val 0
         trans sht 17
         do {
           # compute the hash value
           trans i (Integer (s:get cnt))
           val:= (val:xor (i:shl sht))
           # adjust shift index
           if (< (sht:-= 7) 0) (sht:+= 24)
         } (< (cnt:++) len)
         eval val
       }

       Regular expression
       A regular expression or regex is a special literal object designed to describe a character
       string in a compact form with regular patterns. A regular expression provides a convenient
       way to perform pattern matching and filed extraction within a character string.

       Regex syntax
       A regular expression is defined with a special Regex object. A regular expression  can  be
       built implicitly or explicitly with the use of the Regex object. The regex syntax uses the
       [ and ]  characters  as  block  delimiters.  When  used  in  a  source  file,  the  parser
       automatically  recognizes  a regex and built the object accordingly. The following example
       shows two equivalent methods for the same regex expression.

       # syntax built-in regex
       (== [$d+] 2000)         # true
       # explicit built-in regex
       (== (Regex "$d+") 2000) # true

       In its first form, the [ and ] characters are  used  as  syntax  delimiters.  The  lexical
       analyzer  automatically  recognizes  this  token as a regex and built the equivalent Regex
       object. The second form is the explicit construction of the Regex object. Note  also  that
       the [ and ] characters are also used as regex block delimiters.

       Regex characters and meta-characters
       Any character, except the one used as operators can be used in a regex. The $ character is
       used as a meta-character -- or control character --  to  represent  a  particular  set  of
       characters.  For  example,  [hello  world]  is  a regex which match only the "hello world"
       string. The [$d+] regex matches one or more digits.  The  following  meta  characters  are
       built-in in the regex engine.

       Character   Description
       $a          matches any letter or digit
       $b          matches any blank characters
       $c          matches any combining alphanumeric
       $d          matches any digit
       $e          matches eol, cr and eos
       $l          matches any lower case letter
       $n          matches eol or cr
       $s          matches any letter
       $u          matches any upper case letter
       $v          matches any valid afnix constituent
       $w          matches any word constituent

       $x          matches any hexadecimal characters

       The  uppercase  version is the complement of the corresponding lowercase character set.  A
       character which follows a $ character and that is not a meta character  is  treated  as  a
       normal character. For example $[ is the [ character. A quoted string can be used to define
       character matching which could otherwise be interpreted as control characters or operator.
       A quoted string also interprets standard escaped sequences but not meta characters.

       (== [$d+]   2000) # true
       (== ["$d+"] 2000) # false

       Combining  alphanumerical characters can generate surprising result when used with Unicode
       string. Combining alphanumeric characters are  alphanumeric  characters  and  non  spacing
       combining  mark as defined by the Unicode consortium. In practice, the combining marks are
       the diacritics used with regular letter, such  like  the  accents  found  in  the  western
       languages.  Because the writing system uses a canonical decomposition for representing the
       Unicode string, it turns out that the printed string is generally  represented  with  more
       bytes, making the string length longer than it appears.

       Regex character set
       A  character  set is defined with the < and > characters. Any enclosed character defines a
       character set. Note that meta characters are also interpreted inside a character set.  For
       example,  <$d+->  represents any digit or a plus or minus. If the first character is the ^
       character in the character set, the character set is  complemented  with  regards  to  its
       definition.

       Regex blocks and operators
       The  [  and  ]  characters  are the regex sub-expressions delimiters. When used at the top
       level of a regex definition, they can identify an implicit object. Their use  at  the  top
       level for explicit construction is optional. The following example is strictly equivalent.

       # simple real number check
       const real-1 (Regex "$d*.$d+")
       # another way with [] characters
       const real-2 (Regex "[$d*.$d+]")

       Sub-expressions  can  be nested -- that's their role -- and combined with operators. There
       is no limit in the nesting level.

       # pair of digit testing
       (== [$d$d[$d$d]+] 2000)  # true
       (== [$d$d[$d$d]+] 20000) # false

       The following unary operators can be used with single character,  control  characters  and
       sub-expressions.

       Operator   Description
       *          match 0 or more times
       +          match 1 or more times
       ?          match 0 or 1 time
       |          alternation

       Alternation  is  an  operator which work with a secondary expression. Care should be taken
       when writing the right sub-expression. For  example  the  following  regex  [$d|hello]  is
       equivalent  to  [[$d|h]ello]. In other word, the minimal first sub-expression is used when
       compiling the regex.

       Grouping
       Groups of sub-expressions are created with the  (  and  )  characters.  When  a  group  is
       matched,  the  resulting  sub-string  is  placed on a stack and can be used later. In this
       respect, the regex engine can be  used  to  extract  sub-strings.  The  following  example
       extracts    the    month,    day    and    year    from    a   particular   date   format:
       [($d$d):($d$d):($d$d$d$d)]. This regex assumes a date in the form mm:dd:yyyy.

       if (== (const re [($d$d):($d$d)]) "12:31") {
         trans hr (re:get 0)
         trans mn (re:get 1)
       }

       Grouping is the mechanism to retrieve sub-strings when a match is successful. If the regex
       is bound to a symbol, the get method can be used to get the sub-string by index.

       Regex object
       Although a regex can be built implicitly, the Regex object can also be used to build a new
       regex. The argument is a string which is compiled during the object construction. A  Regex
       object  is  a literal object. This means that the to-string method is available and that a
       call to the println special form will work directly.

       const   re (Regex "$d+")
       println re           # $d+
       println re:to-string # [$d+]

       Regex operators
       The == and != operators are the primary  operators  to  perform  a  regex  match.  The  ==
       operator  returns  true if the regex matches the string argument from the beginning to the
       end of string. Such operator implies the begin and end of string anchoring. The < operator
       returns true if the regex matches the string or a sub-string or the string argument.

       Regex methods
       The  primary  regex  method is the get method which returns by index the sub-string when a
       group has been matched. The length method returns the number of group match.

       if (== (const re [($d$d):($d$d)]) "12:31") {
         re:length # 2
         re:get 0  # 12
         re:get 1  # 31
       }

       The match method returns the first string which is matched by the regex.

       const regex [$d+]
       regex:match "Happy new year 2000" # 2000

       The replace method any occurrence of the matching string with the string argument.

       const regex [$d+]
       regex:replace "Hello year 2000" "3000" # hello year 3000

       Argument conversion
       The use of the Regex operators implies that the arguments are evaluated as literal object.
       For  this  reason,  an  implicit  string conversion is made during such operator call. For
       example, passing the integer 12 or the string "12" is strictly equivalent. Care should  be
       taken when using this implicit conversion with real numbers.

CONTAINER OBJECTS

       This chapter covers the standard container objects and more specifically, iterable objects
       such like Cons, List and Vector. Special objects like Fifo, Queue and Bitset  are  treated
       at  the  end of this chapter. Although the name container is sufficient enough to describe
       the object functionality, it is clear that a  container  is  more  than  a  simple  object
       reservoir.  In  particular,  the  choice  of a container object is often associated to the
       underlying algorithm used to store the object. For example, a vector is  appropriate  when
       storing  by  index  is  important.  If the order of storage must be preserved, then a fifo
       object might be more appropriate. In any case, the choice  of  a  container  is  always  a
       question of compromise, so is the implementation.

       Cons object
       Originally,  a  Cons  object  or cons cell have been the fundamental object of the Lisp or
       Scheme machine. The cons cell is the building block  for  list  and  is  similar  in  some
       respect  to  the  cons  cell  found in traditional functional programming language. A Cons
       object is a simple element used to build linked list. The cons cell holds an object and  a
       pointer  to  the next cons cell. The cons cell object is called car and the next cons cell
       is called the cdr. This original  Lisp  notation  is  maintained  here  for  the  sake  of
       tradition.  Although  a  cons  cell is the building block for single linked list, the cell
       itself is not a list object. When a list object is needed, the  List  double  linked  list
       object might be more appropriate.

       Cons cell constructors
       The  default  constructor  creates a cons cell those car is initialized to the nil object.
       The constructor can also take one or several objects.

       const nil-cons (Cons)
       const lst-cons (Cons 1 'a' "hello")

       The constructor can take any kind of objects. When all objects have  the  same  type,  the
       result  list  is  said  to  be  homogeneous. If all objects do not have the same type, the
       result list is said to be heterogeneous. List can also  be  constructed  directly  by  the
       parser.  Since  all  internal  forms  are  built  with  cons cell, the construction can be
       achieved by simply protecting the form from being interpreted.

       const blist (protect ((1) ((2) ((3)))))

       Cons cell methods
       A Cons object provides several methods to access the car and the cdr of a cons cell. Other
       methods allows access to a list by index.

       const c (Cons "hello" "world")
       c:length   # 2
       c:get-car  # "hello"
       c:get-cadr # "world"
       c:get 0    # "hello"
       c:get 1    # "world"

       The  set-car  method  set the car of the cons cell. The add method adds a new cons cell at
       the end of the cons list and set the car with the specified object.

       List object
       The List object provides the facility of a double-link list. The List  object  is  another
       example  of  iterable  object.  The  List object provides support for forward and backward
       iteration.

       List construction
       A list is constructed like a cons cell with zero or more arguments. Unlike the cons  cell,
       the List can have a null size.

       const nil-list (List)
       const dbl-list (List 1 'a' "hello")

       List methods
       The  List object methods are similar the Cons object. The add method adds an object at the
       end of the list. The insert method inserts an object at the beginning of the list.

       const list (List "hello" "world")
       list:length      # 2
       list:get 0       # "hello"
       list:get 1       # "world"
       list:add "folks" # "hello" "world" "folks"

       Vector object
       The Vector object provides the facility of an index array of objects. The Vector object is
       another  example  of  iterable  object. The Vector object provides support for forward and
       backward iteration.

       Vector construction
       A vector is constructed like a cons cell or a list.  The  default  constructor  creates  a
       vector with 0 objects.

       const nil-vector (Vector)
       const obj-vector (Vector 1 'a' "hello")

       Vector methods
       The Vector object methods are similar to the List object. The add method appends an object
       at the end of the vector. The set method set a vector position by index.

       const vec (Vector "hello" "world")
       vec:length          # 2
       vec:get 0           # "hello"
       vec:get 1           # "world"
       vec:add "folks"     # "hello" "world" "folks"
       vec:set 0 "bonjour" # "bonjour" "world" "folks"

       Set object
       The Set object provides the facility of an object container. The  Set  object  is  another
       example  of iterable object. The Set object provides support for forward iteration. One of
       the property of a set is that there is only one object representation per set. Adding  two
       times the same object results in one object only.

       Set construction
       A set is constructed like a vector. The default constructor creates a set with 0 objects.

       const nil-set (Set)
       const obj-set (Set 1 'a' "hello")

       Set methods
       The  Set object methods are similar to the Vector object. The add method adds an object in
       the set. If the object is already in the set, the object is not added. The  length  method
       returns the number of elements in the set.

       const set       (Set "hello" "world")
       set:get-size    # 2
       set:add "folks" # "hello" "world" "folks"

       Iteration
       When  an object is iterable, it can be used with the reserved keyword for. The for keyword
       iterates on one or several objects and binds associated symbols during each  step  of  the
       iteration  process.  All  iterable  objects  provides  also  the method get-iterator which
       returns an iterator for a given object. The use of iterator is justified  during  backward
       iteration, since for only perform forward iteration.

       Function mapping
       Given  a  function func, it is relatively easy to apply this function to all objects of an
       iterable object. The result is a list of successive calls with the function. Such function
       is called a mapping function and is generally called map.

       const map (obj func) {
         trans result (Cons)
         for (car) (obj) (result:link (func car))
         eval result
       }

       The  link method differs from the add method in the sense that the object to append is set
       to the cons cell car if the car and cdr is nil.

       Multiple iteration
       Multiple iteration can be done with one call to for. The computation of a  scalar  product
       is a simple but illustrative example.

       # compute the scalar product of two vectors
       const scalar-product (u v) {
         trans result 0
         for (x y) (u v) (result:+= (* x y))
         eval result
       }

       Note  that  the  function  scalar-product does not make any assumption about the object to
       iterate. One could compute the scalar product between a vector a list for example.

       const u (Vector 1 2 3)
       const v (List   2 3 4)
       scalar-product u v

       Conversion of iterable objects
       The use of an iterator is suitable for direct conversion between one object  and  another.
       The conversion to a vector can be simply defined as indicted below.

       # convert an iterable object to a vector
       const to-vector (obj) {
         trans result (Vector)
         for (i) (obj) (result:add i)
         eval result
       }

       Explicit iterator
       An  explicit  iterator  is  constructed with the get-iterator method. At construction, the
       iterator is reset to the beginning position. The get-object method returns the  object  at
       the  current  iterator  position. The next advances the iterator to its next position. The
       valid-p method returns true if the iterator is in a  valid  position.  When  the  iterator
       supports  backward operations, the prev method move the iterator to the previous position.
       Note that Cons objects do not support backward  iteration.  The  begin  method  reset  the
       iterator  to  the  beginning.  The  end  method moves the iterator the last position. This
       method is available only with backward iterator.

       # reverse a list
       const reverse-list (obj) {
         trans result (List)
         trans itlist (obj:get-iterator)
         itlist:end
         while (itlist:valid-p) {
           result:add (itlist:get-object))
         itlist:prev
       }
       eval result
       }

       Special Objects
       The engine incorporates other container objects. To name  a  few,  such  objects  are  the
       Queue, Bitset or Fifo objects.

       Queue object
       A  queue is a special object which acts as container with a fifo policy. When an object is
       placed in the queue, it remains there until it has  been  dequeued.  The  Fifo  and  Queue
       objects  are somehow similar, with the fundamental difference that the queue resize itself
       if needed.

       # create a queue with objects
       const q (Queue "hello" "world")
       q:empty-p # false
       q:length  # 2
       # dequeue some object
       q:dequeue # hello
       q:dequeue # world
       q:empty-p # true

       Bitset object
       A bit set is a special container for bit. A bit set can be  constructed  with  a  specific
       size.  When  the  bit  set  is  constructed,  each  bit can be marked and tested by index.
       Initially, the bitset size is null.

       # create a bit set by size
       const bs (Bitset 8)
       bitset-p bs # true
       # check, mark and clear
       assert false (bs:marked-p 0)
       bs:mark 0
       assert true  (bs:marked-p 0)
       bs:clear 0
       assert false (bs:marked-p 0)

CLASSES

       This chapter covers the class model and its associated  operations.  The  class  model  is
       slightly  different  compared  to  traditional  one because dynamic symbol bindings do not
       enforce to declare the class data members. A class is an object which can  be  manipulated
       by  itself.  Such  class is said to belongs to a group of meta class as described later in
       this chapter. Once the class concept has been detailed, the chapter moves to  the  concept
       of  instance  of that class and shows how instance data members and functions can be used.
       The chapter terminates with a description of dynamic class programming.

       Class object
       A class object is simply a nameset which can be replicated via a construction mechanism. A
       class  is created with the special form class. The result is an object of type Class which
       supports various symbol binding operations.

       Class declaration and bindings
       A new class is an object created with the reserved keyword class. Such class is an  object
       which can be bound to a symbol.

       const Color (class)

       Because  a  class  acts  like  a nameset, it is possible to bind directly symbols with the
       qualified name notation.

       const Color (class)
       const Color:RED-FACTOR   0.75
       const Color:BLUE-FACTOR  0.75
       const Color:GREEN-FACTOR 0.75

       When a data is defined in the class nameset, it is common to refer  it  as  a  class  data
       member.  A  class  data member is invariant over the instance of that class. When the data
       member is declared with the const reserved keyword, the symbol binding  is  in  the  class
       nameset.

       Class closure binding
       A  lambda  or gamma expression can be define for a class. If the class do not reference an
       instance of that class, the resulting closure is called a  class  method  of  that  class.
       Class methods are invariant among the class instances. The standard declaration syntax for
       a lambda or gamma expression is still valid with a class.

       const Color:get-primary-by-string (color value) {
         trans val "0x"
         val:+= (switch color (
             ("red"   (value:substr 1 3))
             ("green" (value:substr 3 5))
             ("blue"  (value:substr 5 7))
           ))
         Integer val
       }

       The invocation of a class method is done with the standard qualified name notation.

       Color:get-primary-by-string "red"   "#23c4e5"
       Color:get-primary-by-string "green" "#23c4e5"
       Color:get-primary-by-string "blue"  "#23c4e5"

       Class symbol access
       A class acts as a nameset and therefore provides the mechanism to evaluate any symbol with
       the qualified name notation.

       const Color:RED-VALUE "#ff0000"
       const Color:print-primary-colors (color) {
         println "red   color " (
           Color:get-primary-color "red"   color)
         println "green color " (
           Color:get-primary-color "green" color)
         println "blue  color " (
           Color:get-primary-color "blue"  color)
       }
       # print the color components for the red color
       Color:print-primary-colors Color:RED-VALUE

       Instance
       An  instance of a class is an object which is constructed by a special class method called
       a constructor. If an instance constructor does not exist, the instance is said to  have  a
       default  construction.  An  instance  acts  also  as a nameset. The only difference with a
       class, is that a symbol resolution is done first in the instance nameset and then  in  the
       instance  class.  As a consequence, creating an instance is equivalent to define a default
       nameset hierarchy.

       Instance construction
       By default, a instance of the class is an object which defines an  instance  nameset.  The
       simplest way to define an anonymous instance is to create it directly.

       const i     ((class))
       const Color (class)
       const red   (Color)

       The  example above define an instance of an anonymous class. If a class object is bound to
       a symbol, such symbol can be used to create an instance of that class. When an instance is
       created,  the special symbol named this is defined in the instance nameset. This symbol is
       bounded to the instance object and can be used  to  reference  in  an  anonymous  way  the
       instance itself.

       Instance initialization
       When  an  instance  is  created,  the  engine looks for a special lambda expression called
       preset. This lambda expression, if it exists, is executed after the default  instance  has
       been constructed. Such lambda expression is a method since it can refer to the this symbol
       and bind some instance symbols.  The  arguments  which  are  passed  during  the  instance
       construction are passed to the preset method.

       const Color (class)
       trans Color:preset (red green blue) {
         const this:red   (Integer red)
         const this:green (Integer green)
         const this:blue  (Integer blue)
       }
       # create some default colors
       const Color:RED   (Color 255   0   0)
       const Color:GREEN (Color   0 255   0)
       const Color:BLUE  (Color   0   0 255)
       const Color:BLACK (Color   0   0   0)
       const Color:WHITE (Color 255 255 255)

       In  the example above, each time a color is created, a new instance object is created. The
       constructor is invoked with the this symbol bound to the newly created instance. Note that
       the  qualified  name this:red defines a new symbol in the instance nameset. Such symbol is
       sometimes referred as an instance data member. Note as well that there is no ambiguity  in
       resolving  the  symbol  red.  Once  the symbol is created, it shadows the one defined as a
       constructor argument.

       Instance symbol access
       An instance acts as a nameset. It is therefore possible to bind locally to an  instance  a
       symbol.  When  a  symbol needs to be evaluated, the instance nameset is searched first. If
       the symbol is not found, the class nameset is searched. When  an  instance  symbol  and  a
       class  symbol  have the same name, the instance symbol is said to shadow the class symbol.
       The simple example below illustrates this property.

       const c   (class)
       const c:a 1
       const i   (c)
       const j   (c)
       const i:a 2
       # class symbol access
       println   c:a
       # shadow symbol access
       println   i:a
       # non shadow access
       println   j:a

       When the instance is created, the special symbol meta is bound  in  the  instance  nameset
       with  the  instance  class  object.  This  symbol can therefore be used to access a shadow
       symbol.

       const c   (class)
       const i   (c)
       const c:a 1
       const i:a 2
       println   i:a
       println   i:meta:a

       The symbol meta must be used carefully, especially inside an initialization since it might
       create an infinite recursion as shown below.

       const c (class)
       trans c:preset nil (const i (this:meta))
       const i (c)

       Instance method
       When  lambda  expression  is defined within the class or the instance nameset, that lambda
       expression is callable from the instance itself. If the lambda expression  uses  the  this
       symbol,  that  lambda is called an instance method since the symbol this is defined in the
       instance nameset. If the instance method is defined in the  class  nameset,  the  instance
       method  is  said  to  be  global,  that is, callable by any instance of that class. If the
       method is defined in the instance nameset, that method is said to be local and is callable
       by  the  instance  only.  Due  to  the  nature  of the nameset parent binding, only lambda
       expression can be used. Gamma expressions will not work since the gamma nameset has always
       the top level nameset as its parent one.

       const Color (class)
       # class constructor
       trans Color:preset (red green blue) {
         const this:red   (Integer red)
         const this:green (Integer green)
         const this:blue  (Integer blue)
       }
       const Color:RF 0.75
       const Color:GF 0.75
       const Color:BF 0.75
       # this method returns a darker color
       trans Color:darker nil {
         trans lr (Integer (max (this:red:*   Color:RF) 0))
         trans lg (Integer (max (this:green:* Color:GF) 0))
         trans lb (Integer (max (this:blue:*  Color:BF) 0))
         Color lr lg lb
       }
       # get a darker color than yellow
       const yellow      (Color 255 255 0)
       const dark-yellow (yellow:darker)

       Instance operators
       Any  operator  can  be defined at the class or the instance level. Operators like == or !=
       generally requires the ability to assert if the argument  is  of  the  same  type  of  the
       instance.  The  global  operator == will return true if two classes are the same. With the
       use of the meta symbol, it is possible to assert such equality.

       # this method checks that two colors are equals
       trans Color:== (color) {
         if (== Color color:meta) {
           if (!= this:red   color:red)   (return false)
           if (!= this:green color:green) (return false)
           if (!= this:blue  color:blue)  (return false)
           eval true
         } false
       }
       # create a new yellow color
       const  yellow (Color 255 255 0)
       (yellow:== (Color 255 255 0)) # true

       The global operator == returns true if both arguments are  the  same,  even  for  classes.
       Method operators are left open to the user.

       Complex number example
       As  a  final  example,  a  class  simulating  the  behavior  of  a complex number is given
       hereafter. The interesting point to note is the  use  of  the  operators.  As  illustrated
       before, the class uses uses a default method method to initialize the data members.

       # class declaration
       const Complex (class)
       # constructor
       trans Complex:preset (re im) {
         trans this:re (Real re)
         trans this:im (Real im)
       }

       The  constructor creates a complex object with the help of the real part and the imaginary
       part. Any object type which can be bound to a Real object is acceptable.

       # class mutators
       trans Complex:set-re (x) (trans this:re (Real re))
       trans Complex:set-im (x) (trans this:im (Real im))
       # class accessors
       trans Complex:get-re nil (Real this:re)
       trans Complex:get-im nil (Real this:im)

       The accessors and the mutators  simply  provides  the  interface  to  the  complex  number
       components and perform a cloning of the calling or returned objects.

       # complex number module
       trans Complex:module nil {
         trans result (Real (+ (* this:re this:re)
             (* this:im this:im)))
         result:sqrt
       }
       # complex number formatting
       trans Complex:format nil {
         trans result (String this:re)
         result:+= "+i"
         result:+= (String this:im)
       }

       The  module  and  format  are  simple  methods.  Note the the complex number formatting is
       arbitrary here.

       # complex predicate
       const complex-p (c) (
         if (instance-p c) (== Complex c:meta) false)

       The complex-p predicate is the perfect illustration  of  the  use  of  the  meta  reserved
       symbol.  However,  it  shall  be noted that the meta-comparison is done if and only if the
       calling argument is an instance.

       # operators
       trans Complex:== (c) (
         if (complex-p c) (and (this:re:== c:re)
           (this:im:== c:im)) (
           if (number-p c)  (and (this:re:== c)
             (this:im:zero-p)) false))
       trans Complex:= (c) {
         if (complex-p c) {
           this:re:= (Real c:re)
           this:im:= (Real c:im)
           return this
         }
         this:re:= (Real c)
         this:im:= 0.0
         return this
       }
       trans Complex:+ (c) {
         trans result (Complex this:re this:im)
         if (complex-p c) {
           result:re:+= c:re
           result:im:+= c:im
           return result
         }
         result:re:+= (Real c)
         eval result
       }

       The operators are a little tedious to write. The comparison can be  done  with  a  complex
       number  or  a built-in number object. The assignation operator creates a copy for both the
       real and imaginary part. The summation operator is given here for illustration purpose.

       Inheritance
       Inheritance is the mechanism by which a class or an instance  inherits  methods  and  data
       member  access  from  a  parent  object.  The class model is based on a single inheritance
       model. When an instance object defines a parent object, such  object  is  called  a  super
       instance.  The  instance which has a super instance is called a derived instance. The main
       utilization of inheritance is the ability to reuse methods for that super instance.

       Derivation construction
       A derived object is generally defined within the preset method of that instance by setting
       the  super  data  member.  The  super  reserved  keyword  is  set  to  nil at the instance
       construction. The good news is that any  object  can  be  defined  as  a  super  instance,
       including built-in object.

       const c (class)
       const c:preset nil {
         trans this:super 0
       }

       In the example above, an instance of class c is constructed. The super instance is with an
       integer object. As a consequence, the instance  is  derived  from  the  Integer  instance.
       Another  consequence  of this scheme is that derived instance do not have to be built from
       the same base class.

       Derived symbol access
       When an instance is derived from another one,  any  symbol  which  belongs  to  the  super
       instance  can  be  access  with  the  use of the super data member. If the super class can
       evaluate a symbol, that symbol is resolved automatically by the derived instance.

       const c       (class)
       const i       (c)
       trans i:a     1
       const j       (c)
       trans j:super i
       println j:a

       When a symbol is evaluated, a set of  search  rules  is  applied.  The  engine  gives  the
       priority to the class nameset vs the super instance. As a consequence, a class data member
       might shadow a super instance data member. The rule associated with  a  symbol  evaluation
       can be summarized as follow.
              Look in the instance nameset.
              Look in the class nameset.
              Look in the super instance if it exists.
              Look in the base object.

       Instance re-parenting
       The  ability  to  set  dynamically  the  parent  instance  make  the object model an ideal
       candidate to support instance re-parenting. In this model, a change in the parent instance
       is automatically reflected at the instance method call.

       const c (class)
       const i (c)
       trans i:super 0
       println (i:to-string) # 0
       trans i:super "hello world"
       println (i:to-string) # hello world

       In  this example, the instance is originally set with an Integer instance parent. Then the
       instance is re-parented with a String instance parent. The call to  the  to-string  method
       illustrates this behavior.

       Instance re-binding
       The ability to set dynamically the instance class is another powerful feature of the class
       model. In this approach, the instance meta class can be changed dynamically with the  mute
       method. Furthermore, it is also possible to create initially an instance without any class
       binding, which is later muted.

       # create a point class
       const  point (class)
       # point class
       trans point:preset (x y) {
         trans this:x x
         trans this:y y
       }
       # create an empty instance
       const p (Instance)
       # bind the point class
       p:mute point 1 2

       In this example, when the instance is muted, the preset  method  is  called  automatically
       with the extra arguments.

       Instance inference
       The ability to instantiate dynamically inferred instance is offered by the instance model.
       An instance b is said to be inferred by the instance a when the instance a  is  the  super
       instance of the instance b. The instance inference is obtained by binding the infer symbol
       to a class. When an instance of that class is  created,  the  inferred  instance  is  also
       created.

       # base class A
       const A  (class)
       # inferred class B
       const B  (class)
       const A:infer B
       # create an instance from A
       const  x (A)
       assert B (x:meta)
       assert A (x:super:meta)

       In  this  example, when the instance is created, the inferred instance is also created and
       returned by the instantiation process. The preset method is only called for  the  inferred
       instance if possible or the base instance if there is no inferring class. Because the base
       preset preset method is not called automatically, the inferred method is responsible to do
       such call.

       trans B:preset (x y) {
         trans this:xb x
         trans this:yb y
         if (== A this:super:meta) (this:super:preset x y)
       }

       Because  the  class  can  mute  from  one call to another and also the inferred class, the
       preset method call must be used after a discrimination of the meta class has been made  as
       indicated by the above example.

       Instance deference
       In  the  process  of creating instances, one might have a generic class with a method that
       attempts to access a data member which is bound to another class.  The  concept  of  class
       deference is exactly designed for this purpose. With the help of reserved keyword defer, a
       class with virtual data member accessors can be bound to a base class as indicated in  the
       example below.

       # create the base and defer class
       const bc (class)
       const dc (class)
       # bind the base preset method
       trans bc:preset nil (const this:y 2)
       # bind the defer accessor to the base data member
       trans dc:get-y nil (eval this:y)
       # bind the defer class in the base class
       const bc:defer dc
       # create an instance from the base class
       const i (bc)
       # access to the base member with the defer method
       assert 2 (i:get-y)

       It  is worth to note that the class deference is made at the class level. When an instance
       of the base class is created, all methods associated with the deferent class  are  visible
       from the base class, thus making the deferent class a virtual interface to the base class.

ADVANCED CONCEPTS

       This  chapter  covers  advanced  concepts  of the writing system. The first subject is the
       exception model. The second subject covers some properties of the namesets in the  context
       of  the  interpreter  object.  The  thread  sub-system  is  then  described along with the
       synchronization mechanism. Finally, some notes related to the functional system are  given
       at the end of this chapter.

       Exception
       An  exception  is an unexpected change in the execution flow. The exception model is based
       on a mechanism which throws the exception to be caught by a handler. The mechanism is also
       designed to be compatible with the native "C++" implementation.

       Throwing an exception
       An  exception  is thrown with the reserved keyword throw. When an exception is thrown, the
       normal flow of execution is  interrupted  and  an  object  used  to  carry  the  exception
       information  is  created.  Such  exception object is propagated backward in the call stack
       until an exception handler catch it.

       if (not (number-p n))
       (throw "type-error" "invalid object found" n)

       The example above is the general form to throw an exception. The first argument is the the
       exception  id.  The  second  argument  is  the exception reason. The third argument is the
       exception object. The exception id and reason are always a string.  The  exception  object
       can  be any object which is carried by the exception. The reserved keyword throw accepts 0
       or more arguments.

       throw
       throw "type-error"
       throw "type-error" "invalid argument"

       With 0 argument, the exception is thrown with the exception id  set  to  "user-exception".
       With  one  argument,  the argument is the exception id. With 2 arguments, the exception id
       and reason are set. Within a try block, an exception can be  thrown  again  by  using  the
       exception object represented with the what symbol.

       try {
         ...
       } {
         println "exception caught and re-thrown"
         throw what
       }

       Exception handler
       The  special  form try executes a form and catch an exception if one has been thrown. With
       one argument, the form is executed and the result is the  result  of  the  form  execution
       unless  an  exception  is  caught.  If an exception is caught, the result is the exception
       object. If the exception is a native one, the result is nil.

       try (+ 1 2)
       try (throw)
       try (throw "hello")
       try (throw "hello" "world")
       try (throw "hello" "world" "folks")

       In its second form, the try reserved keyword can accept a second form  which  is  executed
       when an exception is caught. When an exception is caught, a new nameset is created and the
       special symbol what is bounded  with  the  exception  object.  In  such  environment,  the
       exception can be evaluated.

       Symbol   Description
       eid      Exception id
       name     Exception file name
       line     Exception line number
       about    Exception extended reason
       reason   Exception reason
       object   Exception object

       try (throw "hello")
       (eval what:eid)
       try (throw "hello" "world")
       (eval what:reason)
       try (throw "hello" "world" 2000)
       (eval what:object)

       Exceptions  are  useful  to  notify  abruptly  that  something went wrong. With an untyped
       language, it is also a convenient mechanism to abort an expression call if some  arguments
       do not match the expected types.

       # protected factorial
       const fact (n) {
         if (not (integer-p n))
         (throw "number-error" "invalid argument in fact")
         if (== n 0) 1 (* n (fact (- n 1)))
       }
       try (fact 5) 0
       try (fact "hello") 0

       Nameset
       A  nameset  is  created  with  the reserved keyword nameset. Without argument, the nameset
       reserved keyword creates a nameset without  setting  its  parent.  With  one  argument,  a
       nameset is created and the parent set with the argument.

       const nset (nameset)
       const nset (nameset ...)

       Default namesets
       When  a  nameset  is created, the symbol . is automatically created and bound to the newly
       created nameset. If a parent nameset exists, the symbol .. is also automatically  created.
       The  use  of the current nameset is a useful notation to resolve a particular name given a
       hierarchy of namesets.

       trans a 1 # 1
       block {
         trans   a (+ a 1) # 2
         println ..:a 1    # 1
       }
       println a           # 1

       Nameset and inheritance
       When a nameset is set as the super object of an instance,  some  interesting  results  are
       obtained. Because symbols are resolved in the nameset hierarchy, there is no limitation to
       use a  nameset  to  simulate  a  kind  of  multiple  inheritance.  The  following  example
       illustrates this point.

       const   cls (class)
       const   ins (cls)
       const   ins:super (nameset)
       const   ins:super:value 2000
       const   ins:super:hello "hello world "
       println ins:hello ins:value # hello world 2000

       Delayed Evaluation
       The  engine  provides  a  mechanism  called delayed evaluation. Such mechanism permits the
       encapsulation of a form to be evaluated inside an object called a promise.

       Creating a promise
       The reserved keyword delay creates a promise. When the promise is created, the  associated
       object is not evaluated. This means that the promise evaluates to itself.

       const a (delay (+ 1 2))
       promise-p a # true

       The  previous  example  creates a promise and store the argument form. The form is not yet
       evaluated. As a consequence, the symbol a evaluates to the promise object.

       Forcing a promise
       The reserved keyword force the evaluation of a promise. Once the promise has been  forced,
       any  further call will produce the same result. Note also that, at this stage, the promise
       evaluates to the evaluated form.

       trans   y 3
       const   l ((lambda (x) (+ x y)) 1)
       assert  4 (force l)
       trans   y 0
       assert  4 (force l)

       Enumeration
       Enumeration, that is, named constant bound to an object, can be declared with the reserved
       keyword enum. The enumeration is built with a list of literal and evaluated as is.

       const  e    (enum E1 E2 E3)
       assert true (enum-p e)

       The  complete  enumeration  evaluates  to  an  Enum  object.  Once built, enumeration item
       evaluates by literal and returns an Item object.

       assert true   (item-p e:E1)
       assert "Item" (e:E1:repr)

       Items are comparable objects. Only items can be compared. For a  given  item,  the  source
       enumeration can be obtained with the get-enum method.

       # check for item equality
       const i1 e:E1
       const i2 e:E2
       assert true  (i1:== i1)
       assert false (== i1 i2)
       # get back the enumeration
       assert true (enum-p (i1:get-enum))

       Logger
       The  Logger  class  is a message logger that stores messages in a buffer with a level. The
       default level is the level 0. A negative level generally indicates a warning or  an  error
       message  but  this  is  just a convention which is not enforced by the class. A high level
       generally indicates a less important message.  The  messages  are  stored  in  a  circular
       buffer.  When  the  logger  is full, a new message replace the oldest one. By default, the
       logger is initialized with a 256 messages capacity that can be re-sized.

       const log    (Logger)
       assert true  (logger-p log)

       When a message is added, the message is stored with a time-stamp and a  level.  The  time-
       stamp  is  used  later to format a message. The length method returns the number of logged
       messages. The get-message method returns a message by index. Because the  system  operates
       with  a  circular  buffer, the get-message method manages the indexes in such way that the
       old messages are accessible with the oldest  index.  For  example,  even  after  a  buffer
       circulation,  the  index 0 will point to the oldest message. The get-message-level returns
       the message level and the get-message-time returns the message posted time.

       const mesg (log:get-message 0)

       In term of usage, the logger facility can be conveniently used with other derived classes.
       The standard i/o module provides several classes that permits to manage logging operations
       in a convenient way.

       Interpreter
       The interpreter is by itself a special object with specialized methods which do  not  have
       equivalent  using  the  standard  notation.  The  interpreter  is always referred with the
       special symbol interp. The following table is a summary of the symbols and  methods  bound
       to the interpreter.

       Symbol                   Description
       argv                     Command arguments vector
       os-name                  Operating system name
       os-type                  Operating system type
       version                  Full version
       program-name             Interpreter program name
       major-version            Major version number
       minor-version            Minor version number
       patch-version            Patch version number
       afnix-uri                Official uri name
       load                     Load a file and execute it
       launch                   Launch a normal thread
       library                  Load and initialize a library
       set-absolute-precision   Set absolute precision
       set-relative-precision   Set relative precision
       get-absolute-precision   Get absolute precision
       get-relative-precision   Get relative precision

       Arguments vector
       The  interp:argv  qualified name evaluates to a vector of strings. Each argument is stored
       in the vector during the interpreter initialization.

       zsh> axi hello world
       (axi) println (interp:argv:length) # 2
       (axi) println (interp:argv:get 0)  # hello

       Interpreter version
       Several symbols can be used to track the interpreter version and the operating system. The
       full  version  is bound to the interp:version qualified name. The full version is composed
       of the major, minor and patch number. The operating system name is bound to the  qualified
       name interp:os-name. The operating system type is bound to the interp:os-type.

       println "major number   : " interp:major-version
       println "minor number   : " interp:minor-version
       println "patch number   : " interp:patch-version
       println "version number : " interp:version
       println "system name    : " interp:os-name
       println "system type    : " interp:os-type
       println "official uri   : " interp:afnix-uri

       File loading
       The  interp:load  method  loads  and  execute  a file. The interpreter interactive command
       session is suspended during the execution of the file. In case of error or if an exception
       is  raised,  the file execution is terminated. The process used to load a file is governed
       by the file resolver. Without extension, a compiled file is  searched  first  and  if  not
       found a source file is searched.

       Library loading
       The  interp:library  method  loads  and initializes a library. The interpreter maintains a
       list of opened library. Multiple execution of  this  method  for  the  same  library  does
       nothing. The method returns the library object.

       interp:library "afnix-sys"
       println "random number: " (afnix:sys:get-random)

       Interpreter duplication
       The  interpreter  can  be  duplicated with the help of the dup method. Without argument, a
       clone of the current interpreter is made and a terminal object is  attached  to  it.  When
       used  in  conjunction with the roll method, this approach permits to create an interactive
       interpreter. The dup method also accepts a terminal object.

       # duplicate the interpreter
       const si (interp:dup)
       # change the primary prompt
       si:set-primary-prompt "(si)"

       Interpreter loop
       The interpreter loop can  be  run  with  the  roll.  The  loop  operates  by  reading  the
       interpreter  input  stream.  If  the  interpreter has been cloned with the help of the dup
       method, this method provides a convenient way to operate in interactive mode.  The  method
       is not called loop because it is a reserved keyword and starting a loop is like having the
       ball rolling.

       # duplicate the interpreter
       const si (interp:dup)
       # loop with this interpreter
       si:roll

       Librarian object
       A librarian file is a special file  that  acts  as  a  containers  for  various  files.  A
       librarian file is created with the axl -- cross librarian --utility. Once a librarian file
       is created, it can be added  to  the  interpreter  resolver.  The  file  access  is  later
       performed automatically by name with the standard interpreter load method.

       Creating a librarian
       The  axl  utility  is  the  preferred way to create a librarian. Given a set of files, axl
       combines them into a single one.

       zsh: axl -h
       usage: axl [options] [files]
       [h]      print this help message
       [v]      print version information
       [c]      create a new librarian
       [x]      extract from the librarian
       [s]      get file names from the librarian
       [t]      report librarian contents
       [f] lib  set the librarian file name

       The c option creates a new librarian. The librarian file name  is  specified  with  the  f
       option.

       zsh: axl -c -f librarian.axl file-1.als file-2.als

       The  previous  command  combines  file-1.als  and  file-2.als  into  a  single file called
       librarian.axl . Note that any file can be included in a librarian.

       Using the librarian
       Once a librarian is created, the interpreter -i option can be used to specify it.  The  -i
       option  accepts  either  a directory name or a librarian file. Once the librarian has been
       opened, the interpreter load method can be used as usual.

       zsh> axi -i librarian.axl
       (axi) interp:load "file-1.als"
       (axi) interp:load "file-2.als"

       The librarian acts like a file archive.  The  interpreter  file  resolver  takes  care  to
       extract the file from the librarian when the load method is invoked.

       Librarian contents
       The  axl  utility provides the -t and -s options to look at the librarian contents. The -s
       option returns all file  name  in  the  librarian.  The  -t  option  returns  a  one  line
       description for each file in the librarian.

       zsh: axl -t -f librarian.axl
       --------       1234 file-1.als
       --------       5678 file-2.als

       The  one  line  report  contains the file flags, the file size and the file name. The file
       flags are not used at this time. One possible use in the future is for example,  an  auto-
       load bit or any other useful things.

       Librarian extraction
       The  -x  option permits to extract file from the librarian. Without any file argument, all
       files are extracted. With some file arguments, only those specified files are extracted.

       zsh: axl -x -f librarian.axl
       zsh: axl -x -f librarian.axl file-1.als

       Librarian object
       The Librarian object can be used as a convenient way to create a collection of files or to
       extract some of them.

       Output librarian
       The Librarian object is a standard object. Its predicate is librarian-p. Without argument,
       a librarian is created in output mode. With a string argument, the librarian is opened  in
       input mode, with the file name argument. The output mode is used to create a new librarian
       by adding file into it. The input mode is created to read file from the librarian.

       # create a new librarian
       const lbr (Librarian)
       # add a file into it
       lbr:add "file-1.als"
       # write it
       lbr:write "librarian.axl"

       The add method adds a new file into the librarian. The write method the full librarian  as
       a single file those name is write method argument.

       Input librarian
       With an argument, the librarian object is created in input mode. Once created, file can be
       read or extracted. The length method -- which  also  work  with  an  output  librarian  --
       returns  the  number of files in the librarian. The exists-p predicate returns true if the
       file name argument exists in the librarian. The get-names method returns a vector of  file
       names  in  this  librarian.  The  extract  method  returns  an input stream object for the
       specific file name.

       # open a librarian for reading
       const lbr (Librarian "librarian.axl")
       # get the number of files
       println (lbr:length)
       # extract the first file
       const is (lbr:extract "file-1.als")
       # is is an input stream - dump each line
       while (is:valid-p) (println (is:readln))

       Most of the time, the librarian object is used to  extract  file  dynamically.  Because  a
       librarian  is  mapped  into  the  memory at the right offset, there is no worry to use big
       librarian, even for a small file. Note that  any  type  of  file  can  be  used,  text  or
       binaries.

       File resolver
       The  file  resolver is a special object used by the interpreter to resolve file path based
       on the search path. The resolver uses a mixed list of directories and librarian  files  in
       its search path. When a file path needs to be resolved, the search path is scanned until a
       matched is found. Because the librarian resolution  is  integrated  inside  the  resolver,
       there  is  no need to worry about file extraction. That process is done automatically. The
       resolver can also be used to perform any kind of file path resolution.

       Resolver object
       The resolver object is created without argument. The add method adds a directory path or a
       librarian  file  to the resolver. The valid method checks for the existence of a file. The
       lookup method returns an input stream object associated with the object.

       # create a new resolver
       const rslv (Resolver)
       assert true (resolver-p rslv)
       # add the local directory on the search path
       rslv:add "."
       # check if file test.als exists
       # if this is ok - print its  contents
       if (rslv:valid-p "test.als") {
         const is (rslv:lookup "test.als")
         while (is:valid-p) (println (is:readln))
       }

       Thread operations
       The interpreter is a  multi-threaded  engine  with  a  native  implementation  of  objects
       locking.  A thread is started with the reserved keyword launch. The execution is completed
       when all threads have terminated. This means that the master thread (i.e the first thread)
       is suspended until all other threads have completed their execution.

       Starting a thread
       A  thread  is started with the reserved keyword launch. The form to execute in a thread is
       the argument. The simplest thread to execute is the nil thread.

       launch (nil)

       There exists an alternate mechanism to start a thread with the reserved keyword launch and
       a  thread  object.  Such mechanism is used when using deferred thread object creation or a
       thread generator object known as a thread set.

       Thread object and result
       When a thread terminate, the thread object holds the result of the last executed form. The
       thread  object  is returned by the launch command. The thread-p predicates returns true if
       the object is a thread descriptor.

       const thr (launch (nil))
       println   (thread-p thr) # true

       The thread result can be obtained with the help of the result method. Although the  result
       can  be accessed at any time, the returned value will be nil until the thread as completed
       its execution.

       const thr (launch (nil))
       println   (thr:result)   # nilp

       Although the engine will ensure that the result is nil until the thread has completed  its
       execution, it does not mean that it is a reliable approach to test until the result is not
       nil. The engine provides various mechanisms to synchronize a thread  and  eventually  wait
       for its completion.

       Shared objects
       The  whole  purpose  of  using  a  multi-threaded  environment  is to provide a concurrent
       execution with some shared variables. Although, several threads can  execute  concurrently
       without  sharing  data,  the most common situation is that one or more global variable are
       accessed -- and even changed -- by one or more threads. Various  scenarios  are  possible.
       For  example,  a  variable is changed by one thread, the other thread just read its value.
       Another scenario is one read, multiple write, or even more complicated, multiple read  and
       multiple  write.  In any case, the interpreter subsystem must ensure that each objects are
       in a  good  state  when  such  operation  do  occur.  The  engine  provides  an  automatic
       synchronization  mechanism for global objects, where only one thread can modify an object,
       but several thread can read it. This mechanism known as read-write locking guarantees that
       there  is  only one writer, but eventually multiple reader. When a thread starts to modify
       an object, no other thread are allowed to read or write this object until the  transaction
       has  been  completed.  On  the  opposite,  no  thread is allowed to change (i.e. write) an
       object, until all thread which access (i.e. read) the  object  value  have  completed  the
       transaction. Because a context switch can occur at any time, the object read-write locking
       will ensure a safe protection during each concurrent access.

       Shared protection access
       We illustrate the previous discussion with an  interesting  example  and  some  variations
       around  it.  Let's consider a form which increase an integer object and another form which
       decrease the same integer object. If the integer is initialized to 0, and  the  two  forms
       run  in  two  separate  threads,  we  might  expect  to  see the value bounded by the time
       allocated for each thread. In other word, this simple example is a very good  illustration
       of your machine scheduler.

       # shared variable access
       const var 0
       # increase method
       const incr nil {
         while true (println "increase: " (var:= (+ var 1)))
       }
       # decrease method
       const decr nil {
         while true (println "decrease: " (var:= (- var 1)))
       }
       # start both threads
       launch (decr)
       launch (incr)

       In the previous example, var is initialized to 0. The incr thread increments var while the
       decr thread decrements var. Depending on the operating system, the  result  stays  bounded
       within  a certain range. The previous example can be changed by using the main thread or a
       third thread to print the variable value. The end result is the same, except that there is
       more threads competing for the shared variable.

       # shared variable access
       const var 0
       # incrementer, decrementer and printer
       const incr nil (while true (var:= (+ var 1)))
       const decr nil (while true (var:= (- var 1)))
       const prtv nil (while true (println "value = " var))
       # start all threads
       launch (decr)
       launch (incr)
       launch (prtv)

       Synchronization
       Although,  there  is  an  automatic  synchronization  mechanism  for reading or writing an
       object, it is sometimes necessary to control the execution flow. There are  basically  two
       techniques to do so. First, protect a form from being executed by several threads. Second,
       wait for one or several threads to complete their task before going to the next  execution
       step.

       Form synchronization
       The reserved keyword sync can be used to synchronize a form. When a form, is synchronized,
       the engine guarantees that only one thread will execute this form.

       const print-message (code mesg) (
         sync {
           errorln "error  : " code
           errorln "message: " mesg
         }
       )

       The previous example creates a gamma expression which make sure that both the  error  code
       and error message are printed in one group, when several threads call it.

       Thread completion
       The  other  piece  of  synchronization  is  the  thread  completion  indicator. The thread
       descriptor contains a method called wait which suspend the calling thread until the thread
       attached  to  the  descriptor  has been completed. If the thread is already completed, the
       method returns immediately.

       # simple flag
       const flag false
       # simple tester
       const ftest (bval) (flag:= bval)
       # run the thread and wait
       const thr (launch (ftest true))
       thr:wait
       assert true flag

       This example is taken from the test suites. It checks that a boolean variable is set  when
       started in a thread. Note the use of the wait method to make sure the thread has completed
       before checking for the flag value. It is also worth to note  that  wait  is  one  of  the
       method  which guarantees that a thread result is valid. Another use of the wait method can
       be made with a vector of thread descriptors when one wants to wait until all of them  have
       completed.

       # shared vector of threads descriptors
       const thr-group (Vector)
       # wait until all threads in the group are finished
       const wait-all nil (for (thr) (thr-group) (thr:wait))

       Complete example
       We  illustrate  the  previous discussion with a complete example. The idea is to perform a
       matrix multiplication. A thread is launched  when  when  multiplying  one  line  with  one
       column.  The  result  is stored in the thread descriptor. A vector of thread descriptor is
       used to store the result.

       # initialize the shared library
       interp:library "afnix-sys"
       # shared vector of threads descriptors
       const thr-group (Vector)
       # waits until all threads in the group are finished
       const wait-all nil (for (thr) (thr-group) (thr:wait))

       The group of threads is represented as a vector. Based on the the previous  discussion,  a
       simple  loop  that  blocks  until  all threads are completed is designed as a simple gamma
       expression.

       # initializes a matrix with random numbers
       const init-matrix (n) {
         trans i (Integer 0)
         const m (Vector)
         do {
           trans v (m:add (Vector))
           trans j (Integer)
           do {
             v:add (afnix:sys:get-random)
           } (< (j:++) n)
         } (< (i:++) n)
         eval m
       }

       The matrix initialization is quite straightforward. The matrix is represented as a  vector
       of  lines.  Each  line is also a vector of random integer number. It is here worth to note
       that the standard math module provides a native implementation of real matrix.

       # this procedure multiply one line with one column
       const mult-line-column (u v) {
         assert (u:length) (v:length)
         trans result 0
         for (x y) (u v) (result:+= (* x y))
         eval result
       }
       # this procedure multiply two vectors assuming one
       # is a line and one is a column from the matrix
       const mult-matrix (mx my) {
         for (lv) (mx) {
           assert true (vector-p lv)
           for (cv) (my) {
             assert true (vector-p cv)
             thr-group:add (launch (mult-line-column lv cv))
           }
         }
       }

       The matrix vector multiplication  is  at  the  heart  of  the  example.  Each  line-column
       multiplication  is  started  into a thread and the thread object is placed into the thread
       group vector.

       # check for some arguments
       # note the use of errorln method
       if (== 0 (interp:argv:length)) {
         errorln "usage: axi 0607.als size"
         afnix:sys:exit 1
       }
       # get the integer and multiply
       const n (Integer (interp:argv:get 0))
       mult-matrix (init-matrix n) (init-matrix n)
       # wait for all threads to complete
       wait-all
       # make sure we have the right number
       assert (* n n) (thr-group:length)

       The main execution is started with the matrix size  as  the  first  argument.  Two  random
       matrices  are  then  created  and  the multi-threaded multiplication is launched. The main
       thread is blocked until all threads in the thread group are completed.

       Condition variable
       A condition variable is another mechanism to  synchronize  several  threads.  A  condition
       variable  is  modeled  with the Condvar object. At construction, the condition variable is
       initialized to false. A thread calling the wait method  will  block  until  the  condition
       becomes  true.  The mark method can be used by a thread to change the state of a condition
       variable and eventually awake some threads which are blocked on it. The following  example
       shows how the main thread blocks until another change the state of the condition.

       # create a condition variable
       const cv (Condvar)
       # this function runs in a thread - does some
       # computation and mark the condition variable
       const do-something nil {
         # do some computation
         ....
         # mark the condition
         cv:mark
       }
       # start some computation in a thread
       launch (do-something)
       # block until the condition is changed
       cv:wait-unlock
       # continue here

       In this example, the condition variable is created at the beginning. The thread is started
       and the main thread blocks until the thread change the state of the condition variable. It
       is important to note the use of the wait-unlock method. When the main thread is re-started
       (after the condition variable has been marked), the main thread owns the  lock  associated
       with the condition variable. The wait-unlock method unlocks that lock when the main thread
       is restarted. Note also that the wait-unlock method reset the condition variable.  if  the
       wait  method  was  used  instead  of wait-unlock the lock would still be owned by the main
       thread. Any attempt by other thread to call the mark method would result  in  the  calling
       thread  to  block  until the lock is released. The Condvar class has several methods which
       can be used to control the behavior of the condition variable. Most of them are related to
       lock  control.  The reset method reset the condition variable. The lock and unlock control
       the condition variable locking.  The  mark,  wait  and  wait-unlock  method  controls  the
       synchronization among several threads.

       Function expression
       A lambda expression or a gamma expression can be seen like a function object with no name.
       During the evaluation process, the expression object is evaluated as well as the arguments
       --  from  left  to  right  --  and a result is produced by applying those arguments to the
       function object. An expression can be built dynamically as part of the evaluation process.

       (axi) println ((lambda (n) (+n 1)) 1)
       2

       The difference between a lambda expression and a gamma expression is only in  the  nameset
       binding  during  the  evaluation process. The lambda expression nameset is linked with the
       calling one, while the gamma expression nameset is linked with the top level nameset.  The
       use  of  gamma  expression  is particularly interesting with recursive functions as it can
       generate a significant execution speedup. The previous example will behaves the same  with
       a gamma expression.

       (axi) println ((gamma (n) (+n 1)) 1)
       2

       Self reference
       When  combining  a  function  expression with recursion, the need for the function to call
       itself is becoming a problem since that function expression does not have a name. For this
       reason,  the  writing system provides the reserved keyword self that is a reference to the
       function  expression.  We  illustrate  this  capability  with  the  well-known   factorial
       expression written in pure functional style.

       (axi) println ((gamma (n)
           (if (<= n 1) 1 (* n (self (- n 1))))) 5)
       120

       The  use  of a gamma expression versus a lambda expression is a matter of speed. Since the
       gamma expression does not have free variables, the symbol  resolution  is  not  a  concern
       here.

       Closed variables
       One of the writing system characteristic is the treatment of free variables. A variable is
       said to be free if it is not bound in the expression environment or its  children  at  the
       time  of  the  symbol  resolution.  For  example,  the expression ((lambda (n) (+ n x)) 1)
       computes the sum of the argument n with the free variable x. The evaluation will  succeeds
       if  x  is  defined  in  one  of  the  parent  environment.  Actually this example can also
       illustrates the difference between a lambda  expression  and  a  gamma  expression.  Let's
       consider the following forms.

       trans x 1
       const do-print nil {
         trans x 2
         println ((lambda (n) (+ n x)) 1)
       }

       The gamma expression do-print will produce 3 since it sums the argument n bound to 1, with
       the free variable x which is defined in the calling environment as 2. Now  if  we  rewrite
       the  previous example with a gamma expression the result will be one, since the expression
       parent will be the top level environment that defines x as 1.

       trans x 1
       const do-print nil {
         trans x 2
         println ((gamma (n) (+ n x)) 1)
       }

       With this example, it is easy to see that there is a  need  to  be  able  to  determine  a
       particular  symbol  value during the expression construction. Doing so is called closing a
       variable. Closing a variable is a mechanism that binds into the  expression  a  particular
       symbol with a value and such symbol is called a closed variable, since its value is closed
       under the current environment  evaluation.  For  example,  the  previous  example  can  be
       rewritten to close the symbol x.

       trans x 1
       const do-print nil {
         trans x 2
         println ((gamma (n) (x) (+ n x)) 1)
       }

       Note  that  the  list  of  closed  variable  immediately follow the argument list. In this
       particular case, the function do-print will print 3 since x has been closed with the value
       2 has defined in the function do-print.

       Dynamic binding
       Because  there  is  a dynamic binding symbol resolution, it is possible to have under some
       circumstances a free or closed  variable.  This  kind  of  situation  can  happen  when  a
       particular symbol is defined under a condition.

       lambda (n) {
         if (<= n 1) (trans x 1)
         println (+ n x)
       }

       With  this  example,  the symbol x is a free variable if the argument n is greater than 1.
       While this mechanism can be powerful, extreme caution  should  be  made  when  using  such
       feature.

       Lexical and qualified names
       The  basic forms elements are the lexical and qualified names. Lexical and qualified names
       are constructed by the parser. Although the evaluation process make  that  lexical  object
       transparent, it is possible to manipulate them directly.

       (axi) const sym (protect lex)
       (axi) println   (sym:repr)
       Lexical

       In  this  example,  the  protect  reserved  keyword is used to avoid the evaluation of the
       lexical object named lex. Therefore the symbol sym refers to a  lexical  object.  Since  a
       lexical  --  and  a  qualified  -- object is a also a literal object, the println reserved
       function will work and print the object name. In fact, a literal object provides  the  to-
       string method that returns the string representation of a literal object.

       (axi) const sym (protect lex)
       (axi) println   (sym:to-string)
       lex

       Symbol and argument access
       Each  nameset  maintains  a  table of symbols. A symbol is a binding between a name and an
       object. Eventually, the symbol carries the  const  flag.  During  the  lexical  evaluation
       process,  the lexical object tries to find an object in the nameset hierarchy. Such object
       can be either a symbol or an argument. Again, this process  is  transparent,  but  can  be
       controlled  manually.  Both  lexical  and  qualified named object have the map method that
       returns the first object associated in the nameset hierarchy.

       (axi) const obj 0
       (axi) const lex (protect obj)
       (axi) const sym (lex:map)
       (axi) println   (sym:repr)
       Symbol

       A symbol is also a literal object, so the to-string and to-literal methods will return the
       symbol name. Symbol methods are provided to access or modify the symbol values. It is also
       possible to change the const symbol flag with the set-const method.

       (axi) println (sym:get-const)
       true
       (axi) println (sym:get-object)
       0
       (axi) sym:set-object true
       (axi) println (sym:get-object)
       true

       A symbol name cannot be modified, since the name must be  synchronized  with  the  nameset
       association. On the other hand, a symbol can be explicitly constructed. As any object, the
       = operator can be used to assign a symbol value. The operator will behaves like  the  set-
       object method.

       (axi) const sym (Symbol "symbol")
       (axi) println sym
       symbol
       (axi) sym:= 0
       (axi) println (eval sym)
       0

       Closure
       As  an  object,  the Closure can be manipulated outside the traditional declarative way. A
       closure is a special object that holds an argument list, a set of closed variables  and  a
       form  to execute. The mechanic of a closure evaluation has been described earlier. What we
       are interested here is the ability to manipulate a closure as  an  object  and  eventually
       modify  it.  Note  that by default a closure is constructed as a lambda expression. With a
       boolean argument set to true the same result is obtained. With false, a  gamma  expression
       is created.

       (axi) const f (Closure)
       (axi) println (closure-p f)
       true

       This  example  creates  an empty closure. The default closure is equivalent to the trans f
       nil nil. The same can be obtained with const f (Closure true). For a gamma expression, the
       following forms are equivalent, const f (Closure false) and const f nil nil. Remember that
       it is trans and const that differentiate between a lambda and a gamma expression. Once the
       closure object is defined, the set-form method can be used to bind a form.

       # the simple way
       trans f nil (println "hello world")
       # the complex way
       const f    (Closure)
       f:set-form (protect (println "hello world"))

       There  are  numerous  situations  where  it  is  desirable  to  mute dynamically a closure
       expression. The simplest one is the closure that mute itself based on some  context.  With
       the  use  of  self,  a  new  form can be set to the one that is executed. Another use is a
       mechanism call  advice,  where  some  new  computation  are  inserted  prior  the  closure
       execution.  Note  that  appending  to  a  closure  can lead to some strange results if the
       existing closure expression uses return special forms. In  a  multi-threaded  environment,
       the  ability  to  change a closure expression is particularly handy. For example a special
       thread could be used to monitor some context. When a particular situation  develops,  that
       threads  might  trigger  some  closure  expression  changes.  Note that changing a closure
       expression does not affect the one that is  executed.  If  such  change  occurs  during  a
       recursive call, that change is seen only at the next call.