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[Caml-list] How can I check for the use of polymorphic equality?
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 Date: 2003-04-04 (19:05) From: Jason Hickey 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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