From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: Fabrice Marchant <fabricemarchant@free.fr>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Building a Set module for elements of a Map
Date: Thu, 26 Jun 2008 11:26:27 +0200 [thread overview]
Message-ID: <486360C3.5020701@inria.fr> (raw)
In-Reply-To: <20080625153043.3bd7895c@free.fr>
Fabrice Marchant wrote:
> Please consider a functor F that takes as input the output signature of a Map.
>
> Here is the only way I see to declare a Set module whose elements type are
Map.S.key :
>
> module F ( M : Map.S ) = struct
> module MKeySet =
> Set.Make ( struct type t = M.key let compare = compare end )
> end
Jean-Christophe Filliâtre replied:
> Using Pervasives.compare instead of a user-defined comparison function
> may even be incorrect. (Suppose the intended comparison function should
> identify (x,y) and (y,x), for instance; obviously, Pervasives.compare
> will not.)
>
> A possible solution to your problem is to have functor F taking instead
> a module for keys as argument, and then to build module M inside F; thus
> it would look like:
>
> module F(K : OrderedType) = struct
> module M = Map.Make(K)
> module MKeySet = Set.Make(K)
> ...
> end
Jean-Christophe is right that Pervasives.compare is not the solution.
Yet another alternative is to parameterize F over both a Map and a Set,
adding a sharing constraint to ensure that they work on compatible
data types:
module F (M: Map.S) (MKeySet: Set.S with type elt = M.key) = struct
...
end
- Xavier Leroy
next prev parent reply other threads:[~2008-06-26 9:26 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-06-25 13:30 Fabrice Marchant
2008-06-26 9:21 ` [Caml-list] " Jean-Christophe Filliâtre
2008-06-26 9:26 ` Xavier Leroy [this message]
2008-06-27 18:10 ` Fabrice Marchant
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