From: Christophe Raffalli <christophe.raffalli@univ-savoie.fr>
To: Jon Harrop <jon@ffconsultancy.com>
Cc: peng.zang@gmail.com, caml-list <caml-list@inria.fr>
Subject: Re: [Caml-list] on objects, equality and playing nicely with the stdlib
Date: Thu, 29 May 2008 10:13:20 +0200 [thread overview]
Message-ID: <483E65A0.3060405@univ-savoie.fr> (raw)
In-Reply-To: <200805290813.18027.jon@ffconsultancy.com>
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Jon Harrop a écrit :
> On Thursday 29 May 2008 05:50:33 Peng Zang wrote:
>> Hello list,
>>
>> Recently, in building some toy games, I've started using objects. I've
>> been pleased with them and often find the subtyping convenient.
>>
>> However, I've run into some issues with the standard physical equality of
>> objects that the (=) operator performs. Namely, it makes several stdlib
>> modules difficult to use. Modules that provides a polymorphic (generic)
>> interface such as Hashtbl, List, Stack, etc.. rely on the (=) operator.
>> This is fine as long as it happens to be what I need. The minute I
>> introduce my own equality however, things break down.
>>
>> What is the best way to go about using the stdlib (or similar) when you
>> have your own equality function? This problem almost never shows up if you
>> just stick with basic data types, but with objects you quickly run into it.
>
> Hi Peng,
>
> You can use the Hashtbl.Make functor to create hash tables using your own
> equality (and hashing) function. Other than that, you're screwed: resort to
> cut and paste from the stdlib.
>
Via Custom block (section 18.9.1 of the manual), you could create a functor
with the following shape (I did not check my syntax):
module type Quotient =
type t
type qt
val class : t -> qt
val get_representent : qt -> t
end
module type Equivalence =
type t
val compare : t -> t -> int
end
module Make_Quotient (R : Equivalence) : Quotient with type t = R.t
Such that (class x) builds a custom block with
the given compare as comparison function ...
You can add hash and serialization to the Equivalence type too ...
It is a pity we have to do that from C, this should be the default way
to bridge the gap between polymorphic comparison and specific comparison.
(and PML will provide a buildin class constructor in the near future).
Hope this helps,
Christophe
--
Christophe Raffalli
Universite de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex
tel: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
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next prev parent reply other threads:[~2008-05-29 8:14 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-05-29 4:50 Peng Zang
2008-05-29 7:13 ` [Caml-list] " Jon Harrop
2008-05-29 8:13 ` Christophe Raffalli [this message]
2008-05-29 13:25 ` Peng Zang
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