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Type constraints
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 Date: 2004-12-07 (18:13) From: William Lovas Subject: Re: [Caml-list] Type constraints
```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

```