From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <dbueno@gmail.com>
X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on yquem.inria.fr
X-Spam-Level: *
X-Spam-Status: No, score=1.4 required=5.0 tests=AWL,DNS_FROM_RFC_ABUSE,
	SPF_NEUTRAL autolearn=disabled version=3.1.3
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39])
	by yquem.inria.fr (Postfix) with ESMTP id 128AEBC69
	for <caml-list@yquem.inria.fr>; Sun, 15 Oct 2006 05:29:06 +0200 (CEST)
Received: from nf-out-0910.google.com (nf-out-0910.google.com [64.233.182.186])
	by concorde.inria.fr (8.13.6/8.13.6) with ESMTP id k9F3T5ce016901
	for <caml-list@inria.fr>; Sun, 15 Oct 2006 05:29:05 +0200
Received: by nf-out-0910.google.com with SMTP id y38so1955006nfb
        for <caml-list@inria.fr>; Sat, 14 Oct 2006 20:29:05 -0700 (PDT)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
        s=beta; d=gmail.com;
        h=received:message-id:date:from:to:subject:mime-version:content-type:content-transfer-encoding:content-disposition;
        b=OyyjkHJrzu+3CHcYpkMIKjaV5Cq7YaCensita8j1UBYNCYqtAKb1dJOytbItQ0Nn8WG0jEDCvw+ynoC+kCDSX8EpAcRKmP9XvI6r3pxaB0cdA20oIkRVTxpk+AubhgTjAYMWUnEYbSGezuPPBTI4i0pnjp5LK4RVA5PLAl7M8dM=
Received: by 10.78.188.19 with SMTP id l19mr5919565huf;
        Sat, 14 Oct 2006 20:29:04 -0700 (PDT)
Received: by 10.78.100.6 with HTTP; Sat, 14 Oct 2006 20:29:04 -0700 (PDT)
Message-ID: <6dbd4d000610142029m92d7005v65e95f031e7eae9b@mail.gmail.com>
Date: Sat, 14 Oct 2006 23:29:04 -0400
From: "Denis Bueno" <dbueno@gmail.com>
To: "OCaml Mailing List" <caml-list@inria.fr>
Subject: Type error
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
X-j-chkmail-Score: MSGID : 4531AB01.000 on concorde : j-chkmail score : X : 0/20 1
X-Miltered: at concorde with ID 4531AB01.000 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)!
X-Spam: no; 0.00; val:01 val:01 bool:01 cmi:01 compilable:01 compile:01 match:02 match:02 objects:02 similarly:03 let:03 library:03 fix:04 interface:05 interface:05 

I'm writing a simple (stupid) de-functorised Set library. Its skeleton is:

,----
| type 'a t
|   (** The type of sets. *)
|
| val empty: 'a t
|   (** The empty set. *)
|
| val mem: 'a -> 'a t -> bool
|   (** [mem x s] tests whether [x] belongs to the set [s]. *)
|
| val add: 'a -> 'a t -> 'a t
|   (** [add x s] returns a set containing all elements of [s],
|       plus [x]. If [x] was already in [s], [s] is returned unchanged. *)
`----

In my implementation, sets are lists.

One of the things I'd like to do is write an of_array to create an
array from a set. However, it is often the case that I have an array
of objects from which I'd like to pull out one field & make a set of
those fields. So, my of_array looks like this:

Interface:
,----
| val of_array: ?getter : ('b -> 'a) -> 'b array -> 'a t
|   (** [of_array xs] returns a set containing the elements in [xs]. *)
`----

Implementation:
,----
| let of_array ?(getter = fun x -> x) xs =
|   Array.fold_right (fun x set -> add (getter x) set) xs empty;;
`----

I get a type error when trying to compile this. I don't know how to
fix it, or even if it's possible without wrecking my nice of_array
signature. The type error:

,----
| The implementation pSet.ml does not match the interface pSet.cmi:
| Values do not match:
|   val of_array : ?getter:('a -> 'a) -> 'a array -> 'a list
| is not included in
|   val of_array : ?getter:('a -> 'b) -> 'a array -> 'b t
`----

I understand why this shouldn't be compilable, but, is it possible to
do something similarly elegant (without creating a separate function)?

-Denis