Re: [Caml-list] User-defined equality on types?

[Brian Rogoff <bpr@best.com>]

On Mon, 23 Apr 2001, Xavier Leroy wrote:
> > I'd like to suggest allowing the user to define a chosen interpretation
> > of the equality symbol, and perhaps the polymorphic orderings too, on
> > each new (maybe just abstract) data type. This seems natural in the 
> > context of abstract data types with non-canonical representation, giving
> > a kind of quotient type. Has this ever been considered? 
> 
> Yes.  This was one of the first motivations for Haskell type classes,
> I believe.

Would the proposed generic polymorphism extension solve this problem? It
seems that it would, and it would be a clean solution. John, in case you 
haven't seen it it is a typeclass like approach where instead of defining 
a type class you define generic functions which have a typecase and
dispatch to the right function (like CLOS). 

> > Are there good reasons against it?
> 
> It's not easy to implement.  One can do it the Haskell way, by passing
> around compiler-generated functions as extra arguments, but the
> language extensions needed to declare ad-hoc polymorphic operations
> and define implementations for these operations at particular types 
> are fairly complex.  I'd rather not add Haskell's type classes to
> OCaml :-)

That's too bad. I was hoping that one day the nice features of Haskell
would get stol^H^H^H^Hused in some future ML and I've always envied 
Haskeller's their type classes. 

-- Brian


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