Re: [Caml-list] Constraining abstract type to be of a given subtype

[Guillaume Yziquel <guillaume.yziquel@citycable.ch>]

Le Monday 21 Feb 2011 à 10:42:43 (-0800), Dario Teixeira a écrit :
> Hi,
> 
> > But I think that you will not be able to do such type narrowing in a
> > module interface / module interface fashion. At least not without an
> > indication of covariance or contravariance. And even then, I'm not sure
> > if that is possible.
> 
> Perhaps I am indeed pushing the module language beyond its design
> intentions, but my requirement does not strike me as completely
> unreasonable...
> 
> If I get rid of the constraint altogether, then I lose the guarantee that
> kind_t is a subtype of Kind.t.  But if I declare kind_t = Kind.t in the
> signature FOO, then I lose the ability to declare particular instances
> of FOO to be more *tightly* constrained than the generic Kind.t.
> 
> Any further ideas on how I may be able to have my cake and eat it too?
> 
> Cheers,
> Dario Teixeira

I think it's going to be tough. The closest I've come to is:

# module type Q = sig type 'a t end;;
module type Q = sig type 'a t end
# module type W = Q with type 'a t = [< `A | `B] as 'a;;
Error: In this `with' constraint, the new definition of t
       does not match its original definition in the constrained
signature:
       Type declarations do not match:
         type 'a t = 'a constraint 'a = [< `A | `B ]
       is not included in
         type 'a t
       Their constraints differ.

While it is possible to 'specialise' a type in a module type signature,
it seems unlikely that it is possible to 'specialise' constraints to
narrower constraints.


-- 
     Guillaume Yziquel