Re: [Caml-list] [OSR] Standard syntax extensions ?

[Richard Jones <rich@annexia.org>]

On Thu, Apr 24, 2008 at 04:52:50PM +0100, John Whitington wrote:
> On 24 Apr 2008, at 16:49, David Teller wrote:
> >* what kind of syntactic sugar is absolutely missing from the  
> >language ?
> 
> I'd like a keyword "matches", so I can write
> 
> map (matches (0, _, _)) l
> 
> rather than
> 
> map (function (0, _, _) -> true | _ -> false) l
> 
> Had anyone done that or similar in camlp4?

That's a good idea.  Attached is a trivial camlp4 implementation which
works to a first approximation.  ('when'-clauses don't seem to work
but no doubt would be very simple to fix/add).

Rich.

-------------------------------------------------- pa_matches.ml
(* matches keyword.
 * Copyright (C) 2008 Red Hat Inc., Richard W.M. Jones
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 *
 * $Id$
 *
 * Compile with:
 *   ocamlfind ocamlc -I +camlp4 camlp4lib.cma \
 *     -pp camlp4of.opt -c pa_matches.ml -o pa_matches.cmo
 * Test with:
 *   ocamlfind ocamlc -pp "camlp4o pa_matches.cmo" test.ml -o test
 *)

open Camlp4.PreCast
open Syntax
open Ast

let output_matches _loc patt =
  <:expr< function $patt$ -> true | _ -> false >>

EXTEND Gram
  GLOBAL: expr;

  expr: LEVEL ";" [
    [ "matches"; p = patt ->
	output_matches _loc p
    ]
  ];

END


-------------------------------------------------- test.ml
open Printf

let () =
  let xs =
    List.filter (matches 7)
      [ 1; 2; 3; 4; 5; 6; 7; 7; 7; 8; 9; 10 ] in
  printf "result = %s\n" (String.concat ";" (List.map string_of_int xs))


--------------------------------------------------

-- 
Richard Jones
Red Hat