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Subject: Re: [Caml-list] Constructors are not functions
From: =?ISO-8859-1?Q?J=E9r=E9mie?= Dimino <jeremie@dimino.org>
To: Chantal KELLER <chantal.keller@wanadoo.fr>
Cc: caml-list <caml-list@yquem.inria.fr>
In-Reply-To: <4ACB319A.1080608@wanadoo.fr>
References: <4ACB319A.1080608@wanadoo.fr>
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Date: Tue, 06 Oct 2009 17:50:59 +0200
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X-Spam: no; 0.00; constructors:01 0200,:01 ocaml:01 constructors:01 foo:01 foo:01 foobar:01 foobar:01 camlp:01 val:01 val:01 syntax:01 2009:98 wrote:01 partial:01 

Le mardi 06 octobre 2009 =C3=A0 14:01 +0200, Chantal KELLER a =C3=A9crit :
> Dear Ocaml users,
>=20
>   Is there a reason for constructors not to behave like functions? For
> instance, one cannot make a partial application from a constructor:
>=20
> # type foo =3D | Bar of int;;
> type foo =3D Bar of int
> # let foobar =3D Bar in foobar 17;;
> Error: The constructor Bar expects 1 argument(s),
>        but is applied here to 0 argument(s)

This does not answer the question, but you can have function generated
automatically for constructors using camlp4.

I wrote some time ago an extension which add a type-conv type processor
"constructor", so you can write:

  type t =3D Foo of string | Bar of int * int
    with constructor

and it automatically generates these two functions:

  val foo : string -> t
  val bar : int -> int -> t

The extension is available here:
http://darcs.ocamlcore.org/cgi-bin/darcsweb.cgi?r=3Dobus;a=3Dheadblob;f=3D/=
syntax/pa_constructor.ml

--=20
J=C3=A9r=C3=A9mie