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Date: Wed, 16 Sep 2009 18:40:49 +0200
From: Matthias Puech <puech@cs.unibo.it>
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To: caml-list@yquem.inria.fr
Subject: Sets and home-made ordered types
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X-Spam: no; 0.00; unibo:01 home-made:01 elt:01 bool:01 elt:01 off-topic:02 interaction:02 matthias:02 equivalence:03 equivalence:03 seems:03 module:03 element:03 element:03 types:05 

Dear Camlists,

This is my first post on this list, so please excuse me if it is too 
vague/off-topic/already discussed... I've been asking myself this 
question about the standard library's Set for quite a while.

The ability of the module Set to take an order (compare) as input, 
possibly larger than Pervasive's one, allows for some valuable tricks, 
i.e. assure that a set contains at most one element of each equivalence 
class. This element is actually stored in the structure, but then it 
seems not possible to retreive it (efficiently): the only interaction is 
to test if some element of the same equivalence class is present, with 
mem : elt -> t -> bool.

My question is : why isn't there a function find : elt -> t -> elt, 
where find e s would return the stored element  e' s.t compare e e' = 1 
? These two elements can be interestingly distinct though...

Thanks in advance for all enlightenment.

    --Matthias

PS: Of course I don't want to fold, filter or such, for efficiency 
reasons...