Re: [Caml-list] small Camlp4 code : get a free name

["Nicolas Pouillard" <nicolas.pouillard@gmail.com>]

Excerpts from bluestorm.dylc's message of Wed Jan 23 20:59:59 +0100 2008:
> >> 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 ?

Apart  performance  issues  if used too naively, another problem could be that
we  need  to  find/have the roots of all the sources in order to get all bound
variable.

This  approach  is  very  nice because clear, simple and also commonly used in
formal developments.

However  if  someone  want  to  use  this method, it would be nice to define a
"fresh"  function  on top get_free_var that hold the set bound variables as an
internal state.

Best regards,

> ------------------
> 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_" *)

-- 
Nicolas Pouillard aka Ertai