Inject an abstract form when '$form' is seen

'$form' works much like '$var', only that it makes it possible to
inject ready-made abstract forms into the generated
functions/expressions.  The following call:

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

generates the following if called like this: gen(is_foo, {atom,0,foo})

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

doc/ct_expand.md

@@ -46,4 +46,4 @@ For example, the line
 
 `parse_transform(Forms, Options) -> any()`
 
-_Generated by EDoc, Oct 23 2010, 21:00:45._
+_Generated by EDoc, Oct 26 2010, 21:17:38._

doc/exprecs.md

@@ -125,4 +125,4 @@ Whenever record definitions need to be exported from a module,
 
 `parse_transform(Forms, Options) -> any()`
 
-_Generated by EDoc, Oct 23 2010, 21:00:45._
+_Generated by EDoc, Oct 26 2010, 21:17:38._

doc/parse_trans.md

@@ -273,4 +273,4 @@ Note that the Erlang forms are a subset of the Syntax Tools
 
 
    Makes one pass
-_Generated by EDoc, Oct 23 2010, 21:00:44._
+_Generated by EDoc, Oct 26 2010, 21:17:38._

doc/parse_trans_codegen.md

@@ -122,4 +122,31 @@ It is possible to do some limited expansion (importing a value
      lists:member(17, L).
   </pre>
 
-_Generated by EDoc, Oct 23 2010, 21:00:44._
+ 
+  
+
+##Form substitution##
+
+ 
+  
+It is possible to inject abstract forms, using the construct
+  `{'$form', F}`, where `F` is bound to a parsed form in
+  the scope of the call to `gen_function/2`.
+ 
+  Example:
+  
+<pre>
+  gen(Name, F) ->
+     codegen:gen_function(Name, fun(X) -> X =:= {'$form',F} end).
+  </pre>
+
+ 
+  After transformation, calling `gen(is_foo, {atom,0,foo})` will yield the
+  abstract form corresponding to:
+  
+<pre>
+  is_foo(X) ->
+     X =:= foo.
+  </pre>
+
+_Generated by EDoc, Oct 26 2010, 21:17:38._

doc/parse_trans_pp.md

@@ -83,4 +83,4 @@ a file could be pretty-printed using the following command:
 
 Pretty-prints the erlang source code corresponding to Forms into Out
  
-_Generated by EDoc, Oct 23 2010, 21:00:44._
+_Generated by EDoc, Oct 26 2010, 21:17:38._

examples/ex_codegen.erl

@@ -2,7 +2,7 @@
 
 -compile({parse_transform, parse_trans_codegen}).
 
--export([f/1, g/2, h/0, i/0, gen/2, fs/0]).
+-export([f/1, g/2, h/0, i/0, j/2, gen/2, fs/0]).
 
 
 f(Name) ->
@@ -39,6 +39,12 @@ i() ->
 			  end
 		  end).
 
+j(Name, Form) ->
+    codegen:gen_function(
+      Name,
+      fun(L) ->
+	      member({'$form',Form}, L)
+      end).
 
 gen(Name, X) ->
     codegen:gen_function(Name, fun(L) -> lists:member({'$var',X}, L) end).

src/parse_trans_codegen.erl

@@ -95,6 +95,25 @@
 %% contains_17(L) ->
 %%    lists:member(17, L).
 %% </pre>
+%%
+%% <h2>Form substitution</h2>
+%%
+%% It is possible to inject abstract forms, using the construct
+%% <code>{'$form', F}</code>, where `F' is bound to a parsed form in
+%% the scope of the call to `gen_function/2'.
+%%
+%% Example:
+%% <pre>
+%% gen(Name, F) ->
+%%    codegen:gen_function(Name, fun(X) -> X =:= {'$form',F} end).
+%% </pre>
+%%
+%% After transformation, calling `gen(is_foo, {atom,0,foo})' will yield the
+%% abstract form corresponding to:
+%% <pre>
+%% is_foo(X) ->
+%%    X =:= foo.
+%% </pre>
 %% @end
 %%
 parse_transform(Forms, Options) ->
@@ -184,6 +203,14 @@ substitute({tuple,L0,[{atom,_,tuple},
     {call, L0, {remote,L0,{atom,L0,erl_parse},
 			   {atom,L0,abstract}},
      [{var, L0, V}, {integer, L0, L}]};
+substitute({tuple,L0,[{atom,_,tuple},
+		      {integer,_,_},
+		      {cons,_,
+		       {tuple,_,[{atom,_,atom},{integer,_,_},{atom,_,'$form'}]},
+		       {cons,_,
+			{tuple,_,[{atom,_,var},{integer,_,_},{atom,_,F}]},
+			{nil,_}}}]}) ->
+    {var, L0, F};
 substitute([]) ->
     [];
 substitute([H|T]) ->