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       ms_transform - Parse_transform that translates fun syntax into match specifications.


       This  module  implements  the  parse_transform  that  makes  calls to ets and dbg:fun2ms/1
       translate into literal match specifications. It also implements the back end for the  same
       functions when called from the Erlang shell.

       The  translations from fun's to match_specs is accessed through the two "pseudo functions"
       ets:fun2ms/1 and dbg:fun2ms/1.

       Actually this introduction is more or less an introduction to the whole concept  of  match
       specifications.  Since  everyone  trying  to use ets:select or dbg seems to end up reading
       this page, it seems in good place to explain a little more  than  just  what  this  module

       There  are  some caveats one should be aware of, please read through the whole manual page
       if it's the first time you're using the transformations.

       Match specifications are used more or less as filters. They resemble usual Erlang matching
       in  a  list comprehension or in a fun used in conjunction with lists:foldl etc. The syntax
       of pure match specifications is somewhat awkward though, as they are  made  up  purely  by
       Erlang  terms and there is no syntax in the language to make the match specifications more

       As the match specifications execution and structure is quite like that of a fun, it  would
       for  most  programmers  be more straight forward to simply write it using the familiar fun
       syntax and having that translated into a match specification automatically.  Of  course  a
       real  fun  is  more  powerful  than  the match specifications allow, but bearing the match
       specifications in mind, and what they can do, it's still more convenient to write  it  all
       as  a  fun.  This  module  contains  the  code  that simply translates the fun syntax into
       match_spec terms.

       Let's start with an ets example. Using ets:select  and  a  match  specification,  one  can
       filter out rows of a table and construct a list of tuples containing relevant parts of the
       data in these rows. Of course one could use ets:foldl instead, but the select call is  far
       more  efficient.  Without  the  translation,  one  has  to  struggle  with  writing  match
       specifications terms to accommodate this, or one  has  to  resort  to  the  less  powerful
       ets:match(_object)  calls,  or  simply  give  up  and  use  the more inefficient method of
       ets:foldl. Using the ets:fun2ms transformation, a ets:select call is at least as  easy  to
       write as any of the alternatives.

       As an example, consider a simple table of employees:

       -record(emp, {empno,     %Employee number as a string, the key
                     surname,   %Surname of the employee
                     givenname, %Given name of employee
                     dept,      %Department one of {dev,sales,prod,adm}
                     empyear}). %Year the employee was employed

       We create the table using:


       Let's also fill it with some randomly chosen data for the examples:


       Now,  the  amount  of  data  in the table is of course to small to justify complicated ets
       searches, but on real tables, using select to get exactly the data you want will  increase
       efficiency remarkably.

       Lets  say  for  example  that  we'd  want  the  employee  numbers of everyone in the sales
       department. One might use ets:match in such a situation:

       1> ets:match(emp_tab, {'_', '$1', '_', '_', sales, '_'}).

       Even though ets:match does not require a full match specification,  but  a  simpler  type,
       it's  still somewhat unreadable, and one has little control over the returned result, it's
       always a list of lists. OK, one might use ets:foldl or ets:foldr instead:

       ets:foldr(fun(#emp{empno = E, dept = sales},Acc) -> [E | Acc];
                    (_,Acc) -> Acc

       Running that would result in ["011103","076324"] , which at least gets rid  of  the  extra
       lists.  The  fun  is  also quite straightforward, so the only problem is that all the data
       from the table has to be transferred from the table to the calling process for  filtering.
       That's inefficient compared to the ets:match call where the filtering can be done "inside"
       the emulator and only the result is transferred to the process. Remember that  ets  tables
       are  all  about efficiency, if it wasn't for efficiency all of ets could be implemented in
       Erlang, as a process receiving requests and sending answers back. One uses ets because one
       wants  performance,  and  therefore  one wouldn't want all of the table transferred to the
       process for filtering. OK, let's look at  a  pure  ets:select  call  that  does  what  the
       ets:foldr does:

       ets:select(emp_tab,[{#emp{empno = '$1', dept = sales, _='_'},[],['$1']}]).

       Even though the record syntax is used, it's still somewhat hard to read and even harder to
       write. The first element of the tuple, #emp{empno = '$1', dept = sales, _='_'} tells  what
       to  match,  elements  not  matching  this will not be returned at all, as in the ets:match
       example. The second element, the empty list is a list of guard expressions, which we  need
       none,  and  the third element is the list of expressions constructing the return value (in
       ets this almost always is a list containing one single term). In our case '$1' is bound to
       the  employee  number  in  the head (first element of tuple), and hence it is the employee
       number that is returned. The result is  ["011103","076324"],  just  as  in  the  ets:foldr
       example, but the result is retrieved much more efficiently in terms of execution speed and
       memory consumption.

       We have one efficient but hardly readable way of doing it and one inefficient  but  fairly
       readable  (at  least  to  the  skilled Erlang programmer) way of doing it. With the use of
       ets:fun2ms, one could have something that is as efficient as possible but still is written
       as a filter using the fun syntax:


       % ...

       ets:select(emp_tab, ets:fun2ms(
                             fun(#emp{empno = E, dept = sales}) ->

       This  may  not be the shortest of the expressions, but it requires no special knowledge of
       match specifications to read. The fun's head should simply match what you want  to  filter
       out and the body returns what you want returned. As long as the fun can be kept within the
       limits of the match specifications, there is no need to transfer all data of the table  to
       the  process  for  filtering as in the ets:foldr example. In fact it's even easier to read
       then the ets:foldr example, as the select call in itself discards  anything  that  doesn't
       match,  while the fun of the foldr call needs to handle both the elements matching and the
       ones not matching.

       It's  worth  noting  in  the  above  ets:fun2ms  example  that  one   needs   to   include
       ms_transform.hrl  in the source code, as this is what triggers the parse transformation of
       the  ets:fun2ms  call  to  a  valid  match  specification.  This  also  implies  that  the
       transformation  is  done at compile time (except when called from the shell of course) and
       therefore will take no resources at all in runtime. So although you use the more intuitive
       fun syntax, it gets as efficient in runtime as writing match specifications by hand.

       Let's  look at some more ets examples. Let's say one wants to get all the employee numbers
       of any employee hired before the year 2000. Using ets:match isn't an alternative  here  as
       relational  operators  cannot  be  expressed  there.  Once again, an ets:foldr could do it
       (slowly, but correct):

       ets:foldr(fun(#emp{empno = E, empyear = Y},Acc) when Y < 2000 -> [E | Acc];
                         (_,Acc) -> Acc

       The result will be ["052341","076324","535216","789789","989891"], as  expected.  Now  the
       equivalent  expression  using  a handwritten match specification would look something like

       ets:select(emp_tab,[{#emp{empno = '$1', empyear = '$2', _='_'},
                            [{'<', '$2', 2000}],

       This gives the same result, the [{'<', '$2', 2000}] is in the  guard  part  and  therefore
       discards anything that does not have a empyear (bound to '$2' in the head) less than 2000,
       just as the guard in the foldl example. Lets jump on to writing it using ets:fun2ms


       % ...

       ets:select(emp_tab, ets:fun2ms(
                             fun(#emp{empno = E, empyear = Y}) when Y < 2000 ->

       Obviously readability is gained by using the parse transformation.

       I'll show some more examples without the tiresome comparing-to-alternatives  stuff.  Let's
       say  we'd  want  the whole object matching instead of only one element. We could of course
       assign a variable to every part of the record and build it up once again in  the  body  of
       the fun, but it's easier to do like this:

       ets:select(emp_tab, ets:fun2ms(
                             fun(Obj = #emp{empno = E, empyear = Y})
                                when Y < 2000 ->

       Just  as  in ordinary Erlang matching, you can bind a variable to the whole matched object
       using a "match in then match", i.e. a =.  Unfortunately  this  is  not  general  in  fun's
       translated  to  match  specifications,  only  on  the "top level", i.e. matching the whole
       object arriving to be matched into a separate variable, is it allowed. For the one's  used
       to  writing  match  specifications  by hand, I'll have to mention that the variable A will
       simply be translated into '$_'. It's not general, but it has very common usage, why it  is
       handled  as  a  special, but useful, case. If this bothers you, the pseudo function object
       also returns the whole matched object, see the part about caveats and limitations below.

       Let's do something in the fun's body too: Let's say that someone realizes that there are a
       few  people  having  an  employee  number  beginning  with  a zero (0), which shouldn't be
       allowed. All those should have their numbers changed to begin with a one (1)  instead  and
       one wants the list [{<Old empno>,<New empno>}] created:

       ets:select(emp_tab, ets:fun2ms(
                             fun(#emp{empno = [$0 | Rest] }) ->

       As a matter of fact, this query hits the feature of partially bound keys in the table type
       ordered_set, so that not the whole table need be searched, only  the  part  of  the  table
       containing keys beginning with 0 is in fact looked into.

       The  fun  of  course  can have several clauses, so that if one could do the following: For
       each employee, if he or she is hired prior to 1997, return the tuple {inventory, <employee
       number>},  for  each  hired  1997  or  later,  but  before 2001, return {rookie, <employee
       number>}, for all others return {newbie, <employee number>}. All except for the ones named
       Smith  as  they  would  be  affronted by anything other than the tag guru and that is also
       what's returned for their numbers; {guru, <employee number>}:

       ets:select(emp_tab, ets:fun2ms(
                             fun(#emp{empno = E, surname = "Smith" }) ->
                                (#emp{empno = E, empyear = Y}) when Y < 1997  ->
                                     {inventory, E};
                                (#emp{empno = E, empyear = Y}) when Y > 2001  ->
                                     {newbie, E};
                                (#emp{empno = E, empyear = Y}) -> % 1997 -- 2001
                                     {rookie, E}

       The result will be:


       and so the Smith's will be happy...

       So, what more can you do? Well, the simple answer would be; look in the  documentation  of
       match specifications in ERTS users guide. However let's briefly go through the most useful
       "built in functions" that you can use when the fun  is  to  be  translated  into  a  match
       specification  by ets:fun2ms (it's worth mentioning, although it might be obvious to some,
       that calling other functions than the one's allowed  in  match  specifications  cannot  be
       done.  No  "usual"  Erlang code can be executed by the fun being translated by fun2ms, the
       fun is after all limited exactly to the  power  of  the  match  specifications,  which  is
       unfortunate,  but  the  price  one  has  to  pay  for the execution speed of an ets:select
       compared to ets:foldl/foldr).

       The head of the fun is obviously a head  matching  (or  mismatching)  one  parameter,  one
       object of the table we select from. The object is always a single variable (can be _) or a
       tuple, as that's what's in ets, dets and mnesia tables (the match  specification  returned
       by  ets:fun2ms  can  of  course be used with dets:select and mnesia:select as well as with
       ets:select). The use of = in the head is allowed (and encouraged) on the top level.

       The guard section can contain any guard expression of Erlang. Even the "old" type test are
       allowed  on  the  toplevel  of the guard (integer(X) instead of is_integer(X)). As the new
       type tests (the is_ tests) are in practice just guard bif's they can also be  called  from
       within  the  body of the fun, but so they can in ordinary Erlang code. Also arithmetics is
       allowed, as well as ordinary guard bif's. Here's a list of bif's and expressions:

         * The type tests: is_atom, is_float, is_integer, is_list,  is_number,  is_pid,  is_port,
           is_reference, is_tuple, is_binary, is_function, is_record

         * The boolean operators: not, and, or, andalso, orelse

         * The relational operators: >, >=, <, =<, =:=, ==, =/=, /=

         * Arithmetics: +, -, *, div, rem

         * Bitwise operators: band, bor, bxor, bnot, bsl, bsr

         * The guard bif's: abs, element, hd, length, node, round, size, tl, trunc, self

         * The  obsolete  type  test  (only  in guards): atom, float, integer, list, number, pid,
           port, reference, tuple, binary, function, record

       Contrary to the fact with "handwritten" match specifications, the is_record guard works as
       in ordinary Erlang code.

       Semicolons  (;)  in  guards are allowed, the result will be (as expected) one "match_spec-
       clause" for each semicolon-separated part of the guard. The semantics being  identical  to
       the Erlang semantics.

       The  body  of the fun is used to construct the resulting value. When selecting from tables
       one usually just construct a suiting term here, using ordinary Erlang  term  construction,
       like  tuple  parentheses, list brackets and variables matched out in the head, possibly in
       conjunction with the occasional constant. Whatever expressions are allowed in  guards  are
       also  allowed  here,  but  there  are no special functions except object and bindings (see
       further down), which returns the whole matched object  and  all  known  variable  bindings

       The  dbg  variants  of  match  specifications  have  an  imperative  approach to the match
       specification body, the ets dialect hasn't. The fun body for ets:fun2ms returns the result
       without  side  effects, and as matching (=) in the body of the match specifications is not
       allowed  (for  performance  reasons)  the  only  thing  left,  more  or  less,   is   term

       Let's  move  on to the dbg dialect, the slightly different match specifications translated
       by dbg:fun2ms.

       The same reasons for using the parse transformation applies to dbg, maybe even more so  as
       filtering  using Erlang code is simply not a good idea when tracing (except afterwards, if
       you trace to file). The concept is similar to that of ets:fun2ms except that  you  usually
       use it directly from the shell (which can also be done with ets:fun2ms).

       Let's manufacture a toy module to trace on


       -export([start/1, store/2, retrieve/1]).

       start(Args) ->
           toy_table = ets:new(toy_table,Args).

       store(Key, Value) ->

       retrieve(Key) ->
           [{Key, Value}] = ets:lookup(toy_table,Key),

       During model testing, the first test bails out with a {badmatch,16} in {toy,start,1}, why?

       We  suspect  the  ets  call,  as  we  match  hard  on  the return value, but want only the
       particular new call with toy_table as first parameter. So we start a default tracer on the

       1> dbg:tracer().

       And  so  we  turn  on  call  tracing  for  all  processes,  we  are going to make a pretty
       restrictive trace pattern, so there's no need to call  trace  only  a  few  processes  (it
       usually isn't):

       2> dbg:p(all,call).

       It's   time   to   specify   the   filter.   We   want   to   view   calls  that  resemble

       3> dbg:tp(ets,new,dbg:fun2ms(fun([toy_table,_]) -> true end)).

       As can be seen, the fun's used with dbg:fun2ms takes a single list as parameter instead of
       a single tuple. The list matches a list of the parameters to the traced function. A single
       variable may also be used of course. The body of the fun expresses in  a  more  imperative
       way  actions to be taken if the fun head (and the guards) matches. I return true here, but
       it's only because the body of a fun cannot be empty, the return value will be discarded.

       When we run the test of our module now, we get the following trace output:

       (<0.86.0>) call ets:new(toy_table,[ordered_set])

       Let's play we haven't spotted the problem yet, and want to see what ets:new returns. We do
       a slightly different trace pattern:

       4> dbg:tp(ets,new,dbg:fun2ms(fun([toy_table,_]) -> return_trace() end)).

       Resulting in the following trace output when we run the test:

       (<0.86.0>) call ets:new(toy_table,[ordered_set])
       (<0.86.0>) returned from ets:new/2 -> 24

       The  call  to  return_trace,  makes  a  trace message appear when the function returns. It
       applies only to the  specific  function  call  triggering  the  match  specification  (and
       matching  the  head/guards of the match specification). This is the by far the most common
       call in the body of a dbg match specification.

       As the test now fails with {badmatch,24}, it's obvious that the badmatch  is  because  the
       atom  toy_table does not match the number returned for an unnamed table. So we spotted the
       problem, the table should be named and the arguments supplied by our test program does not
       include named_table. We rewrite the start function to:

       start(Args) ->
           toy_table = ets:new(toy_table,[named_table |Args]).

       And with the same tracing turned on, we get the following trace output:

       (<0.86.0>) call ets:new(toy_table,[named_table,ordered_set])
       (<0.86.0>) returned from ets:new/2 -> toy_table

       Very  well.  Let's say the module now passes all testing and goes into the system. After a
       while someone realizes that the table toy_table grows while the system is running and that
       for  some  reason  there  are a lot of elements with atom's as keys. You had expected only
       integer keys and so does the rest of the system. Well, obviously not all  of  the  system.
       You turn on call tracing and try to see calls to your module with an atom as the key:

       1> dbg:tracer().
       2> dbg:p(all,call).
       3> dbg:tpl(toy,store,dbg:fun2ms(fun([A,_]) when is_atom(A) -> true end)).

       We  use  dbg:tpl  here  to  make sure to catch local calls (let's say the module has grown
       since the smaller version and  we're  not  sure  this  inserting  of  atoms  is  not  done
       locally...). When in doubt always use local call tracing.

       Let's  say  nothing  happens  when we trace in this way. Our function is never called with
       these parameters. We make the conclusion that someone else (some other module) is doing it
       and  we realize that we must trace on ets:insert and want to see the calling function. The
       calling function may be retrieved using the match specification function caller and to get
       it into the trace message, one has to use the match spec function message. The filter call
       looks like this (looking for calls to ets:insert):

       4> dbg:tpl(ets,insert,dbg:fun2ms(fun([toy_table,{A,_}]) when is_atom(A) ->

       The caller will now appear in the "additional message" part of the trace  output,  and  so
       after a while, the following output comes:

       (<0.86.0>) call ets:insert(toy_table,{garbage,can}) ({evil_mod,evil_fun,2})

       You have found out that the function evil_fun of the module evil_mod, with arity 2, is the
       one causing all this trouble.

       This  was  just  a  toy  example,  but  it  illustrated  the  most  used  calls  in  match
       specifications  for  dbg  The  other, more esotheric calls are listed and explained in the
       Users guide of the ERTS application, they really are beyond the scope of this document.

       To end this chatty introduction with something more precise, here follows some parts about
       caveats  and  restrictions  concerning  the  fun's used in conjunction with ets:fun2ms and

       To use the pseudo functions triggering the translation, one has to include the header file
       ms_transform.hrl  in  the  source  code.  Failure to do so will possibly result in runtime
       errors rather than compile time, as the expression may be valid as a plain Erlang  program
       without translation.

       The fun has to be literally constructed inside the parameter list to the pseudo functions.
       The fun cannot be bound to a variable first and then passed to ets:fun2ms  or  dbg:fun2ms,
       i.e  this  will  work:  ets:fun2ms(fun(A)  ->  A  end)  but not this: F = fun(A) -> A end,
       ets:fun2ms(F). The later will result in a compile time error if the  header  is  included,
       otherwise  a  runtime  error. Even if the later construction would ever appear to work, it
       really doesn't, so don't ever use it.

       Several restrictions apply to the fun that is being translated into a match_spec.  To  put
       it  simple  you  cannot  use anything in the fun that you cannot use in a match_spec. This
       means that, among others, the following restrictions apply to the fun itself:

         * Functions written  in  Erlang  cannot  be  called,  neither  local  functions,  global
           functions or real fun's

         * Everything  that  is  written  as a function call will be translated into a match_spec
           call to a builtin function,  so  that  the  call  is_list(X)  will  be  translated  to
           {'is_list',  '$1'}  ('$1' is just an example, the numbering may vary). If one tries to
           call a function that is not a match_spec builtin, it will cause an error.

         * Variables occurring in the head of the fun will be replaced by match_spec variables in
           the  order of occurrence, so that the fragment fun({A,B,C}) will be replaced by {'$1',
           '$2', '$3'} etc. Every occurrence of such a variable later in the match_spec  will  be
           replaced  by  a  match_spec  variable in the same way, so that the fun fun({A,B}) when
           is_atom(A) -> B end will be translated into [{{'$1','$2'},[{is_atom,'$1'}],['$2']}].

         * Variables that are not appearing in the head are imported  from  the  environment  and
           made into match_spec const expressions. Example from the shell:

         1> X = 25.
         2> ets:fun2ms(fun({A,B}) when A > X -> B end).

         * Matching  with  =  cannot be used in the body. It can only be used on the top level in
           the head of the fun. Example from the shell again:

         1> ets:fun2ms(fun({A,[B|C]} = D) when A > B -> D end).
         2> ets:fun2ms(fun({A,[B|C]=D}) when A > B -> D end).
         Error: fun with head matching ('=' in head) cannot be translated into
         3> ets:fun2ms(fun({A,[B|C]}) when A > B -> D = [B|C], D end).
         Error: fun with body matching ('=' in body) is illegal as match_spec

           All variables are bound in the head of a match_spec, so the translator can  not  allow
           multiple  bindings.  The special case when matching is done on the top level makes the
           variable bind to '$_' in the resulting match_spec, it  is  to  allow  a  more  natural
           access  to  the  whole  matched  object.  The  pseudo  function object() could be used
           instead, see below. The following expressions are translated equally:

         ets:fun2ms(fun({a,_} = A) -> A end).
         ets:fun2ms(fun({a,_}) -> object() end).

         * The special match_spec variables '$_' and '$*' can  be  accessed  through  the  pseudo
           functions  object()  (for  '$_')  and  bindings() (for '$*'). as an example, one could
           translate the following ets:match_object/2 call to a ets:select call:

         ets:match_object(Table, {'$1',test,'$2'}).

  the same as...

         ets:select(Table, ets:fun2ms(fun({A,test,B}) -> object() end)).

           (This was just an example, in this simple  case  the  former  expression  is  probably
           preferable in terms of readability). The ets:select/2 call will conceptually look like
           this in the resulting code:

         ets:select(Table, [{{'$1',test,'$2'},[],['$_']}]).

           Matching on the top level of the fun head might feel like a more natural way to access
           '$_', see above.

         * Term constructions/literals are translated as much as is needed to get them into valid
           match_specs, so that tuples are  made  into  match_spec  tuple  constructions  (a  one
           element  tuple  containing the tuple) and constant expressions are used when importing
           variables  from  the  environment.  Records  are  also  translated  into  plain  tuple
           constructions,  calls  to  element  etc. The guard test is_record/2 is translated into
           match_spec code using the three parameter version that's built  into  match_specs,  so
           that is_record(A,t) is translated into {is_record,'$1',t,5} given that the record size
           of record type t is 5.

         * Language constructions like case, if, catch etc that are not  present  in  match_specs
           are not allowed.

         * If  the  header  file  ms_transform.hrl  is not included, the fun won't be translated,
           which may result in a runtime error (depending on if the fun is valid in a pure Erlang
           context).  Be  absolutely  sure  that  the  header  is  included  when  using  ets and
           dbg:fun2ms/1 in compiled code.

         * If the pseudo function triggering the translation is ets:fun2ms/1, the fun's head must
           contain  a  single  variable or a single tuple. If the pseudo function is dbg:fun2ms/1
           the fun's head must contain a single variable or a single list.

       The translation from fun's to match_specs is done at compile time, so runtime  performance
       is not affected by using these pseudo functions. The compile time might be somewhat longer

       For more information about match_specs, please read about them in ERTS users guide.


       parse_transform(Forms, Options) -> Forms


                 Forms = [erl_parse:abstract_form()]
                 Options = term()
                   Option list, required but not used.

              Implements the actual transformation at compile time. This function  is  called  by
              the  compiler to do the source code transformation if and when the ms_transform.hrl
              header file is included in your source code. See the ets and dbg:fun2ms/1  function
              manual  pages  for  documentation  on  how  to  use  this  parse_transform, see the
              match_spec chapter in ERTS users guide for a description of match specifications.

       transform_from_shell(Dialect, Clauses, BoundEnvironment) -> term()


                 Dialect = ets | dbg
                 Clauses = [erl_parse:abstract_clause()]
                 BoundEnvironment = erl_eval:binding_struct()
                   List of variable bindings in the shell environment.

              Implements the actual transformation when the fun2ms functions are called from  the
              shell.  In  this case the abstract form is for one single fun (parsed by the Erlang
              shell), and all imported variables should  be  in  the  key-value  list  passed  as
              BoundEnvironment. The result is a term, normalized, i.e. not in abstract format.

       format_error(Error) -> Chars


                 Error = {error, module(), term()}
                 Chars = io_lib:chars()

              Takes  an  error  code  returned  by  one  of the other functions in the module and
              creates a textual description of the error. Fairly uninteresting function actually.