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#Module parse_trans_codegen#

Parse transform for code generation pseduo functions.

Authors: : Ulf Wiger (ulf.wiger@erlang-solutions.com).

##Description##

...

##Function Index##

parse_transform/2 Searches for calls to pseudo functions in the module codegen, and converts the corresponding erlang code to a data structure representing the abstract form of that code.

##Function Details##

###parse_transform/2##

parse_transform(Forms, Options) -> NewForms



Searches for calls to pseudo functions in the module codegen,
and converts the corresponding erlang code to a data structure
representing the abstract form of that code.

The purpose of these functions is to let the programmer write
the actual code that is to be generated, rather than manually
writing abstract forms, which is more error prone and cannot be
checked by the compiler until the generated module is compiled.

Supported functions:

##gen_function/2##

Usage: codegen:gen_function(Name, Fun)

Substitutes the abstract code for a function with name Name and the same behaviour as Fntun.

Fun can either be a anonymous fun, which is then converted to a named function. It can also be an implicit fun, e.g. fun is_member/2. In this case, the referenced function is fetched and converted to an abstract form representation. It is also renamed so that the generated function has the name Name.

##gen_functions/1##

Takes a list of {Name, Fun} tuples and produces a list of abstract data objects, just as if one had written [codegen:gen_function(N1,F1),codegen:gen_function(N2,F2),...].

##exprs/1##

Usage: codegen:exprs(Fun)

Fun is either an anonymous function, or an implicit fun with only one
function clause. This "function" takes the body of the fun and produces
a data type representing the abstract form of the list of expressions in
the body. The arguments of the function clause are ignored, but can be
used to ensure that all necessary variables are known to the compiler.

##Variable substitution##

It is possible to do some limited expansion (importing a value bound at compile-time), using the construct {'$var', V}, where V is a bound variable in the scope of the call to gen_function/2.

Example:

  gen(Name, X) ->
     codegen:gen_function(Name, fun(L) -> lists:member({'$var',X}, L) end).

After transformation, calling gen(contains_17, 17) will yield the abstract form corresponding to:

  contains_17(L) ->
     lists:member(17, L).

##Form substitution##

It is possible to inject abstract forms, using the construct {'$form', F}, where F is bound to a parsed form in the scope of the call to gen_function/2.

Example:

  gen(Name, F) ->
     codegen:gen_function(Name, fun(X) -> X =:= {'$form',F} end).

After transformation, calling gen(is_foo, {atom,0,foo}) will yield the abstract form corresponding to:

  is_foo(X) ->
     X =:= foo.