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NAME

       generator - Procedures for creating and using generators.

SYNOPSIS

       package require Tcl  8.6

       package require generator  ?0.1?

       generator define name params body

       generator yield arg ?args..?

       generator foreach varList generator varList generator ?...? body

       generator next generator ?varName..?

       generator exists generator

       generator names

       generator destroy ?generator..?

       generator finally cmd ?arg..?

       generator from format value

       generator to format generator

       generator map function generator

       generator filter predicate generator

       generator reduce function zero generator

       generator foldl function zero generator

       generator foldr function zero generator

       generator all predicate generator

       generator and generator

       generator any generator

       generator concat generator ?generator..?

       generator concatMap function generator

       generator drop n generator

       generator dropWhile predicate generator

       generator contains element generator

       generator foldl1 function generator

       generator foldli function zero generator

       generator foldri function zero generator

       generator head generator

       generator tail generator

       generator init generator

       generator takeList n generator

       generator take n generator

       generator iterate function init

       generator last generator

       generator length generator

       generator or predicate generator

       generator product generator

       generator repeat n value..

       generator sum generator

       generator takeWhile predicate generator

       generator splitWhen predicate generator

       generator scanl function zero generator

_________________________________________________________________________________________________

DESCRIPTION

       The  generator package provides commands to define and iterate over generator expressions.
       A generator is a command that returns a sequence of values. However,  unlike  an  ordinary
       command  that  returns  a  list, a generator yields each value and then suspends, allowing
       subsequent values to be fetched on-demand. As such, generators can be used to  efficiently
       iterate  over  a  set  of  values,  without  having  to  generate  all  answers in-memory.
       Generators can be used to iterate over elements of a data structure, or rows in the result
       set of a database query, or to decouple producer/consumer software designs such as parsers
       and  tokenizers,  or  to  implement  sophisticated  custom  control  strategies  such   as
       backtracking search. Generators reduce the need to implement custom control structures, as
       many such structures can be recast as generators, leading to both a simpler implementation
       and  a more standardised interface. The generator mechanism is built on top of the Tcl 8.6
       coroutine mechanism.

       The package exports a single ensemble command, generator. All functionality is provided as
       subcommands  of  this  command. The core subcommands of the package are define, yield, and
       foreach. The define command works  like  Tcl's  proc  command,  but  creates  a  generator
       procedure;  that  is,  a  procedure  that  returns a generator when called.  The generator
       itself is a command that can be called multiple times: each time it returns the next value
       in the generated series. When the series has been exhausted, the generator command returns
       an empty list and then destroys itself. Rather than manually call  a  generator,  however,
       the  package also provides a flexible foreach command that loops through the values of one
       or more generators. This loop construct mimicks the  functionality  of  the  built-in  Tcl
       foreach  command,  including  handling  multiple  return  values and looping over multiple
       generators at once. Writing a generator is also a simple task, much like writing a  normal
       procedure:  simply  use  the  define  command to define the generator, and then call yield
       instead of return.  For example, we  can  define  a  generator  for  looping  through  the
       integers in a particular range:

                  generator define range {n m} {
                      for {set i $n} {$i <= $m} {incr i} { generator yield $i }
                  }
                  generator foreach x [range 1 10] {
                      puts "x = $x"
                  }

       The  above  example  will print the numbers from 1 to 10 in sequence, as you would expect.
       The difference from a normal loop over a list is that the numbers are  only  generated  as
       they  are  needed.  If  we  insert a break into the loop then any remaining numbers in the
       sequence would never be generated. To illustrate, we can define a generator that  produces
       the sequence of natural numbers: an infinite series. A normal procedure would never return
       trying to produce this series as a list. By using a generator we  only  have  to  generate
       those values which are actually used:

                  generator define nats {} {
                      while 1 { generator yield [incr nat] }
                  }
                  generator foreach n [nats] {
                      if {$n > 100} { break }
                  }

COMMANDS

       generator define name params body
              Creates  a  new  generator procedure. The arguments to the command are identical to
              those for proc: a name, a list of parameters, and a body. The parameter list format
              is  identical  to  a procedure. In particular, default values and the ?args? syntax
              can be used as usual. Each time the resulting  generator  procedure  is  called  it
              creates  a  new  generator  command (coroutine) that will yield a list of values on
              each call. Each result from a generator is guaranteed to be  a  non-empty  list  of
              values.  When  a  generator is exhausted it returns an empty list and then destroys
              itself to free up resources. It is  an  error  to  attempt  to  call  an  exhausted
              generator as the command no longer exists.

       generator yield arg ?args..?
              Used  in the definition of a generator, this command returns the next set of values
              to the consumer. Once the yield command has been called the generator will  suspend
              to  allow the consumer to process that value. When the next value is requested, the
              generator will resume as if the yield command had just returned, and  can  continue
              processing to yield the next result. The yield command must be called with at least
              one argument, but can be called with multiple arguments,  in  which  case  this  is
              equivalent to calling yield once for each argument.

       generator foreach varList generator varList generator ?...? body
              Loops  through  one  or more generators, assigning the next values to variables and
              then executing the loop body. Works much like the  built-in  foreach  command,  but
              working with generators rather than lists. Multiple generators can be iterated over
              in parallel, and multiple results can be retrieved from a single generator at once.
              Like  the built-in foreach, the loop will continue until all of the generators have
              been exhausted: variables for generators that are exhausted early will  be  set  to
              the empty string.

              The foreach command will automatically clean-up all of the generators at the end of
              the loop, regardless of whether the loop terminated early or not.   This  behaviour
              is  provided as a convenience to avoid having to explicitly clean up a generator in
              the usual cases. Generators can however be destroyed before the end of the loop, in
              which  case  the  loop  will continue as normal until all the other generators have
              been destroyed or exhausted.

              The foreach command does not take a snapshot of the generator. Any changes  in  the
              state  of the generator made inside the loop or by other code will affect the state
              of the loop. In particular, if the code  in  the  loop  invokes  the  generator  to
              manually  retrieve  the  next  element, this element will then be excluded from the
              loop, and the next iteration will continue from the element after  that  one.  Care
              should  be taken to avoid concurrent updates to generators unless this behaviour is
              required (e.g., in argument processing).

       generator next generator ?varName..?
              Manually retrieves the next values from a generator. One  value  is  retrieved  for
              each variable supplied and assigned to the corresponding variable. If the generator
              becomes exhausted at any time then any remaining variables are  set  to  the  empty
              string.

       generator exists generator
              Returns 1 if the generator (still) exists, or 0 otherwise.

       generator names
              Returns a list of all currently existing generator commands.

       generator destroy ?generator..?
              Destroys one or more generators, freeing any associated resources.

       generator finally cmd ?arg..?
              Used  in  the  definition  of  a  generator  procedure, this command arranges for a
              resource to be cleaned up whenever the generator is destroyed, either explicitly or
              implicitly when the generator is exhausted. This command can be used like a finally
              block in the try command, except that it is tied to the life-cycle of the generator
              rather than to a particular scope. For example, if we create a generator to iterate
              over the lines in a text file, we can use finally to ensure that the file is closed
              whenever the generator is destroyed:

                  generator define lines file {
                      set in [open $file]
                      # Ensure file is always closed
                      generator finally close $in
                      while {[gets $in line] >= 0} {
                          generator yield $line
                      }
                  }
                  generator foreach line [lines /etc/passwd] {
                      puts "[incr count]: $line"
                      if {$count > 10} { break }
                  }
                  # File will be closed even on early exit

       If  you  create  a generator that consumes another generator (such as the standard map and
       filter generators defined later), then you should use a finally  command  to  ensure  that
       this  generator is destroyed when its parent is. For example, the map generator is defined
       as follows:

                  generator define map {f xs} {
                      generator finally generator destroy $xs
                      generator foreach x $xs { generator yield [{*}$f $x] }
                  }

       generator from format value
              Creates a generator from a data structure. Currently, supported formats  are  list,
              dict,  or  string.  The  list format yields each element in turn. For dictionaries,
              each key and  value  are  yielded  separately.   Finally,  strings  are  yielded  a
              character at a time.

       generator to format generator
              Converts  a  generator  into a data structure. This is the reverse operation of the
              from command, and supports the same data structures. The two  operations  obey  the
              following identity laws (where = is interpreted appropriately):

                  [generator to $fmt [generator from $fmt $value]] = $value
                  [generator from $fmt [generator to $fmt $gen]]   = $gen

PRELUDE

       The  following  commands  are  provided as a standard library of generator combinators and
       functions that perform convenience operations on generators. The functions in this section
       are  loosely  modelled on the equivalent functions from the Haskell Prelude. Warning: most
       of the functions in this prelude destroy any generator arguments  they  are  passed  as  a
       side-effect. If you want to have persistent generators, see the streams library.

       generator map function generator
              Apply  a function to every element of a generator, returning a new generator of the
              results. This is the classic map function from functional programming,  applied  to
              generators. For example, we can generate all the square numbers using the following
              code (where nats is defined as earlier):

                  proc square x { expr {$x * $x} }
                  generator foreach n [generator map square [nats]] {
                      puts "n = $n"
                      if {$n > 1000} { break }
                  }

       generator filter predicate generator
              Another classic functional programming gem. This command returns a  generator  that
              yields  only  those  items  from  the argument generator that satisfy the predicate
              (boolean function). For example, if we had a generator employees  that  returned  a
              stream  of  dictionaries  representing  people,  we  could  filter  all those whose
              salaries are above 100,000 dollars (or  whichever  currency  you  prefer)  using  a
              simple filter:

                  proc salary> {amount person} { expr {[dict get $person salary] > $amount} }
                  set fat-cats [generator filter {salary> 100000} $employees]

       generator reduce function zero generator
              This  is the classic left-fold operation. This command takes a function, an initial
              value, and a generator of values. For each element in the generator it applies  the
              function  to  the  current accumulator value (the zero argument initially) and that
              element, and then uses the result as the new accumulator  value.  This  process  is
              repeated  through the entire generator (eagerly) and the final accumulator value is
              then returned. If we consider the function to be a binary operator,  and  the  zero
              argument  to  be  the left identity element of that operation, then we can consider
              the reduce command as folding the operator between each successive pair  of  values
              in  the generator in a left-associative fashion. For example, the sum of a sequence
              of numbers can be calculated by folding a + operator between them, with  0  as  the
              identity:

                  # sum xs          = reduce + 0 xs
                  # sum [range 1 5] = reduce + 0 [range 1 5]
                  #                 = reduce + [+ 0 1] [range 2 5]
                  #                 = reduce + [+ 1 2] [range 3 5]
                  #                 = ...
                  #                 = reduce + [+ 10 5] <empty>
                  #                 = ((((0+1)+2)+3)+4)+5
                  #                 = 15
                  proc + {a b} { expr {$a + $b} }
                  proc sum gen { generator reduce + 0 $gen }
                  puts [sum [range 1 10]]

       The  reduce  operation  is  an  extremely  useful  one,  and  a great variety of different
       operations can be defined using it. For example, we can define a factorial function as the
       product  of  a  range  using generators. This definition is both very clear and also quite
       efficient (in both memory and running time):

                  proc * {x y} { expr {$x * $y} }
                  proc prod gen { generator reduce * 0 $gen }
                  proc fac n { prod [range 1 $n] }

       However, while the reduce operation is efficient for finite  generators,  care  should  be
       taken not to apply it to an infinite generator, as this will result in an infinite loop:

                  sum [nats]; # Never returns

       generator foldl function zero generator
              This is an alias for the reduce command.

       generator foldr function zero generator
              This  is  the  right-associative  version  of  reduce.  This operation is generally
              inefficient, as the entire generator needs to be evaluated into memory (as a  list)
              before  the reduction can commence. In an eagerly evaluated language like Tcl, this
              operation has limited use, and should be avoided if possible.

       generator all predicate generator
              Returns true if all elements of the generator satisfy the given predicate.

       generator and generator
              Returns true if all elements of the generator are true  (i.e.,  takes  the  logical
              conjunction of the elements).

       generator any generator
              Returns  true  if  any  of  the  elements  of the generator are true (i.e., logical
              disjunction).

       generator concat generator ?generator..?
              Returns a generator which is the concatenation of each of the argument generators.

       generator concatMap function generator
              Given a function which maps a value to a series  of  values,  and  a  generator  of
              values  of that type, returns a generator of all of the results in one flat series.
              Equivalent to concat applied to the result of map.

       generator drop n generator
              Removes the given number of elements from the front of the  generator  and  returns
              the resulting generator with those elements removed.

       generator dropWhile predicate generator
              Removes all elements from the front of the generator that satisfy the predicate.

       generator contains element generator
              Returns  true  if  the  generator  contains  the given element. Note that this will
              destroy the generator!

       generator foldl1 function generator
              A version of foldl that takes the zero argument  from  the  first  element  of  the
              generator. Therefore this function is only valid on non-empty generators.

       generator foldli function zero generator
              A  version  of  foldl  that supplies the integer index of each element as the first
              argument to the function. The first element in the generator at this point is given
              index 0.

       generator foldri function zero generator
              Right-associative version of foldli.

       generator head generator
              Returns the first element of the generator.

       generator tail generator
              Removes the first element of the generator, returning the rest.

       generator init generator
              Returns  a new generator consisting of all elements except the last of the argument
              generator.

       generator takeList n generator
              Returns the next n elements of the generator as a list. If not enough elements  are
              left in the generator, then just the remaining elements are returned.

       generator take n generator
              Returns  the next n elements of the generator as a new generator. The old generator
              is destroyed.

       generator iterate function init
              Returns an infinite generator formed by repeatedly applying  the  function  to  the
              initial argument. For example, the Fibonacci numbers can be defined as follows:

                  proc fst pair { lindex $pair 0 }
                  proc snd pair { lindex $pair 1 }
                  proc nextFib ab { list [snd $ab] [expr {[fst $ab] + [snd $ab]}] }
                  proc fibs {} { generator map fst [generator iterate nextFib {0 1}] }

       generator last generator
              Returns the last element of the generator (if it exists).

       generator length generator
              Returns the length of the generator, destroying it in the process.

       generator or predicate generator
              Returns 1 if any of the elements of the generator satisfy the predicate.

       generator product generator
              Returns the product of the numbers in a generator.

       generator repeat n value..
              Returns  a  generator  that consists of n copies of the given elements. The special
              value Inf can be used to generate an infinite sequence.

       generator sum generator
              Returns the sum of the values in the generator.

       generator takeWhile predicate generator
              Returns a generator of the first elements in the argument  generator  that  satisfy
              the predicate.

       generator splitWhen predicate generator
              Splits  the  generator  into  lists  of  elements  using  the predicate to identify
              delimiters. The resulting lists are returned as a generator. Elements matching  the
              delimiter  predicate  are discarded. For example, to split up a generator using the
              string "|" as a delimiter:

                  set xs [generator from list {a | b | c}]
                  generator split {string equal "|"} $xs ;# returns a then b then c

       generator scanl function zero generator
              Similar to foldl, but returns a generator of all of the intermediate values for the
              accumulator  argument.  The  final element of this generator is equivalent to foldl
              called on the same arguments.

BUGS, IDEAS, FEEDBACK

       Please report any errors in this document, or in the package it describes, to Neil  Madden
       [mailto:nem@cs.nott.ac.uk].

KEYWORDS

       control  structure,  coroutine,  filter,  foldl, foldr, foreach, generator, iterator, map,
       reduce, scanl