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Building a Set module for elements of a Map
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Date: 2008-06-26 (09:19)
From: Jean-Christophe_Filliâtre <Jean-Christophe.Filliatre@l...>
Subject: Re: [Caml-list] Building a Set module for elements of a Map
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
>   The problem is to access the OrderedType that permitted to build the Map M. It seems to have disappeared inside the output Map signature S. So the trial of regeneration :
> struct type t = M.key let compare = compare end
>   It maybe exists a straightforward signature we could pick somewhere inside Map related modules ?
>   Moreover I'm afraid this "Ersatz" of signature based on _Pervasives_.compare is in some cases deficient, because different from original OrderedType of Map elements...

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)

Hope this helps,
Jean-Christophe Filliâtre