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

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

Le Monday 21 Feb 2011 à 07:56:43 (-0800), Dario Teixeira a écrit :
> Hi,
> 
> In a signature FOO, I'm trying to constrain an abstract type kind_t to be a
> subtype of Kind.t (please see code below).  However, I get a compiler error
> when I declare an actual struct Foo1 which supposedly obeys signature FOO:
> 
>   Type declarations do not match:
>     type 'a kind_t = [ `A ]
>   is not included in
>     type 'a kind_t = 'a constraint 'a = [< Kind.t ]
> 
> 
> Any hints as to what is wrong here?  Thanks in advance for your help!

Well, I clearly see to what 'a is bound to in

	type 'a kind_t = 'a constraint 'a = [< Kind.t ]

but I do not see to what 'a is bound to in

	type 'a kind_t = [ `A ]

As 'a kind_t is declared to be 'a in the first type declaration, it
should also be the case in the second type declaration. However, 'a is a
phantom type parameter there.

> Best regards,
> Dario Teixeira

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.

-- 
     Guillaume Yziquel