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try .. finally using new camlp4
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Date: -- (:)
From: Jon Harrop <jon@f...>
Subject: try .. finally using new camlp4

I'm just getting my toe wet with the new camlp4 and I can't get this simple 
syntax extension to work. AFAICT, the following code:

open Camlp4.PreCast;;

let expr : Camlp4.PreCast.Ast.expr Camlp4.PreCast.Gram.Entry.t = "expression"

module Example (Syntax : Camlp4.Sig.Camlp4Syntax) = struct
  open Camlp4.PreCast
  include Syntax

  let printer =
    let module P = Camlp4.Printers.OCaml.Make(Syntax) in
    new P.printer ()

    expr: LEVEL "top"
    [[ "try"; f=expr; "finally"; g=expr ->
	     | `Val v, g -> g(); v
	     | `Exn e, g -> g(); raise e)
	      ((try `Val($f$) with e -> `Exn e), (fun () -> $g$)))

module M = Camlp4.Register.OCamlSyntaxExtension(Id)(Example)

should provide the syntax extension. Compile with:

$ ocamlc -dtypes -pp camlp4of -I /usr/lib/ocaml/3.10.0/camlp4 -linkall 
camlp4lib.cma unix.cma Camlp4Parsers/Camlp4OCamlRevisedParser.cmo 
Camlp4Parsers/Camlp4OCamlParser.cmo Camlp4Printers/Camlp4AutoPrinter.cmo 
Camlp4Bin.cmo -o pa_tryfinally

Drop into a top-level with the extended syntax and the extension works but it 
seems to have replaced the original try .. with syntax rather than added a 
new one:

$ ocaml camlp4of.cma pa_tryfinally.cmo
        Objective Caml version 3.10.0

        Camlp4 Parsing version 3.10.0

# try raise Exit finally print_endline "Foo!";;
Exception: Pervasives.Exit.
# try raise Exit with _ -> print_endline "Foo!";;
Parse error: "finally" expected after [expr] (in [expr])

I've tried various things to correct this but I can't get it to work. What am 
I doing wrong?

Dr Jon D Harrop, Flying Frog Consultancy Ltd.
The OCaml Journal