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small Camlp4 code : get a free name
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Date: -- (:)
From: Martin Jambon <martin.jambon@e...>
Subject: Re: [Caml-list] small Camlp4 code : get a free name
On Wed, 23 Jan 2008, blue storm wrote:

>>> I deduce that there is no standard way of introducing
>>> `fresh' (w.r.t. the abstract syntax tree) variables
>>> within a camlp4 syntax extension? Wouldn't that be nice? =)
>
>>> Has anybody every implemented a solution to that problem?
>
>> That  would be nice, but doing it cleanly would require a large amount of work
>> and user visible changes.
>
> I'm not sure it's a "clean" way of doing it, but i think this may works :
> you can fold the AST to get every bound variable names, and then look
> for conflicts.
>
> I tried to do that. See my camlp4 code at the end of the message.
>
> The folder is very simple : it collects every identifier name, even
> not-really-bounded ones (eg. in patterns). I don't think it's a
> problem : the important thing is to get no false positive.
>
> Is there a problem with this method ?
> Do you have any comment or advice ?

Nice, but in theory, it may create identifiers that are bound to an 
incorrect value when the programmer forgot to define such a value.

For example, module A should provide A.x to module B (= another .ml file).
If the programmer forgets to define A.x, the compiler should report the 
error at the time of compiling b.ml. But if some camlp4 extension 
operating over a.ml introduces a variable A.x and by chance it has the 
same type as the expected A.x, then the compiler will remain silent and B 
will use the wrong value for A.x.


Martin

> ------------------
> open Camlp4;
>
> module Make (Syntax : Sig.Camlp4Syntax) = struct
>  open Syntax;
>
>  value get_bound_vars =
>    let folder = object (self)
>      inherit Ast.fold as super;
>
>      value binds = [];
>      method binds = binds;
>
>      method ident = fun
>        [ <:ident< $lid:id$ >> | <:ident< $uid:id$ >> ->
>          {< binds = [id::binds] >}
>        | other -> super#ident other ];
>    end in
>    fun expr -> (folder#expr expr)#binds;
>
>  value get_free_var suffix binds =
>    let rec generate var =
>      if List.mem var binds
>      then generate ("_" ^ var ^ "_")
>      else var
>    in generate suffix;
> end;
> ------------------
>
>
> You can test it with this example code (for example) :
> ------------------
> open Camlp4
> open PreCast
>
> module FV = Freevars.Make(Syntax)
>
> let _loc = Loc.ghost
> let expr = <:expr< match foo with bar -> foo2 | _ -> 1 + 2 >>
>
> let binds = FV.get_bound_vars expr
> let fv = FV.get_free_var "foo" binds
> (* fv is "_foo_" *)
> ------------------
>
> Thanks for your comments.
>
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