Re: [Caml-list] Type constraints

[William Lovas <wlovas@stwing.upenn.edu>]

On Tue, Dec 07, 2004 at 06:44:36PM +0100, Damien Doligez wrote:
> 
> On 7 Dec 2004, at 15:57, Andreas Rossberg wrote:
> 
> >Is this really a counter-example? I don't see any problem with making 
> >it polymorphic - it evaluates to ref, and ref can happily be 
> >polymorphic.
> 
> Yes, well I simplified it a bit too much.  Try this instead:
> 
>   let module M = struct let v = ref [] end in M.v;;

I'm still not convinced.  Yes, the type variable should not be generalized
in the above, by analogy with:

    # ref [];;
    - : '_a list ref = {contents = []}

But the `let module' in question -- or one similar in spirit, at least --

    # let module M = struct let v = fun x -> x end in M.v;;
    - : '_a -> '_a = <fun>

is analogous to the expression

    # fun x -> x
    - : 'a -> 'a = <fun>

in which the type variable *is* generalized.

The following behavior confuses me, too:

    # let module M = struct let v = fun x -> x end in (M.v 5, M.v true);;
    - : int * bool = (5, true)
    This expression has type bool but is here used with type int
    # let v =
        let module M = struct let v = fun x -> x end in M.v
      in
        (v 5, v true);;
                ^^^^
    This expression has type bool but is here used with type int

Why is the type variable generalized inside the `let module's body but not
generalized if we pass it to the outside?

So the `ref' example above as a counterexample is at the very least hiding
some of the story.  What's really going on here?

William