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[Caml-list] How can I check for the use of polymorphic equality?
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Date: -- (:)
From: Jason Hickey <jyh@c...>
Subject: Re: [Caml-list] How can I check for the use of polymorphic equality?
Pierre Weis wrote:
>>and more importantly b) means you can't use = anywhere, doesn't it?
> 
> No: it means you have to do something more than just writing = when
> you need to call the predefined generic equality. You first have to
> think if there is not another predicate that is not more appropriate
> to the situation, and if there is none, you just have to insert a mere
> 
>   let ( = ) = Pervasives.( = ) in
> 
> before each relevant occurrence of ( = ). Those single lines can be
> easily removed after proper debugging (use an emacs keyboard macro + a
> M-x grep -n -e ... *.ml).
 >
 > ...
 > Pierre Weis


One really nice thing about your solution is that we can get type 
inference to catch uses of =.  But it can be awkward because we still 
need relations on base types.  Imagine that we have a Set module that 
should use Set.equal, not =.  Consider the fixpoint code below.

type empty

let (=) (x : empty) (y : empty) = assert false
let (>) (x : empty) (y : empty) = assert false
...

let rec fixpoint s1 s2 =
    if Set.equal s1 s2 then
       s1
    (* Can't use >, so use Pervasives.(>) directly *)
    else if Pervasives.(>) (Set.cardinal s1) (Set.cardinal s2) then
       fixpoint (f s1) s2
    else
       fixpoint s1 (f s2)

It can get ugly if we have to use the Pervasives.(=) a lot.  Of course, 
we could be a little smarter, and define something like the following 
for each of the base types.

let (=@) : int -> int -> bool = Pervasives.(=)

Perhaps even

let (=$) : 'a set -> 'a set -> bool = Set.equal

This might be a compromise, though remembering all the equality suffixes 
would be a hassle.

Jason

-- 
Jason Hickey                  http://www.cs.caltech.edu/~jyh
Caltech Computer Science      Tel: 626-395-6568 FAX: 626-792-4257

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