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

[Dario Teixeira <darioteixeira@yahoo.com>]

Hi,

> Abstraction is about hiding type representation. Variant subtyping is
> a kind of structural subtyping and is thus about type representation.
> So constraining an abstract type to be of a given structural subtype
> seems contradictory.

Yes, you're right, I abused the term "abstract type", when I meant to
say that the type exposed by the interface was not your run-of-the-mill
concrete type but instead any type that satisfied a given constraint.
What would be the proper terminology in this case?


> The only way I see to type what you want is to make FOO.kind_t
> abstract. This doesn't ensure that the implementation of 'a kind_t is
> a subtype of Kind.t itself. However if your goal is some kind of
> phantom type constraints, it will do the job. And you can still
> require a function to convert 'a kind to Kind.t. See the code below.

Which does work quite nicely, thanks!

Cheers,
Dario