On 7/20/06, Christophe TROESTLER <Christophe.Troestler@umh.ac.be> wrote:
> Hi,
>
> Is it possible to write a camlp4 syntax extension that "extract" the
> free variables of an expression?
You are quite lucky, the camlp4 version has one module to do that!

>  As an example, say I write
>
>   FUNCTION(let z = x + 2 in x + 2 * y * x * z)
>
> and it becomes
>
>   fun ~x ~y -> let z = x + 2 in x + 2 * y * x * z

#default_quotation "expr";

open Camlp4.PreCast;
open Format;

module FV = Camlp4.Struct.FreeVars.Make Ast;
module PP = Camlp4.Printers.OCaml.Make Syntax;
module S = FV.S;

value _loc = Loc.ghost;

value pervasives =
  let list =
    [ "+"; "-"; "/"; "*" (* ... *) ]
  in List.fold_right S.add list S.empty;

value f e =
  let fv = FV.free_vars pervasives e in
  S.fold (fun x acc -> << fun ~ $x$ -> $acc$ >>) fv e;

value print_expr = (new PP.printer ())#expr;

printf "%a@." print_expr (f <<let z = x + 2 in x + 2 * y * x * z>>);

(* end *)

# using an up to date CVS checkout of ocaml.

$ ocamlc -I +camlp4 Camlp4.cma -pp camlp4rf free_vars_test.ml
$ ./a.out
fun ~y ~x -> let z = x + 2 in x + (((2 * y) * x) * z)

Best regards,

-- 
Nicolas Pouillard