This is the value restriction at work: only values are generalized
(often of the form "fun x -> ..."), and while (Int 3) is a value, (Int
(1 + 2)) is not.

On Mon, May 8, 2017 at 11:32 AM, Reed Wilson <cedilla@gmail.com> wrote:
> I've been working with some of the new features introduced with GADTs, and
> ran into a confusing instance of the "type variables that cannot be
> generalized" error.
>
> The simplified program is as follows:
> type _ field = Int : int -> 'a field
> let a = Int 3
> let b = Int (1 + 2)
> let inside = 1 + 2
> let c = Int inside
>
> ocamlc -i returns:
> type _ field = Int : int -> 'a field
> val a : 'a field
> val b : '_a field
> val inside : int
> val c : 'a field
>
> with b remaining non-generalized. The problem I'm having with it is that the
> type variable doesn't depend on the value at all, so I don't see how that
> can prevent generalization.
>
> Also, the manual says the reason some types aren't generalized is due to
> "polymorphic mutable data structures". Nothing I created is mutable, so why
> was generalization turned off in the first place?
>
> Finally, I'm confused why separating the function from the definition is
> enough to fix this; c is generalized simply by defining 1+2 in a separate
> value (which must be global, apparently).
>
> Thanks,
> Reed Wilson
>
> --
> ç