Hi Matej,

I'm no expert, but I'm pretty sure the problem here is related to the value
restriction [0]. If you change the last line to:

# let _ = f { enter = fun x -> (+) 1 x } ;;
- : int = 3

there is no typing error anymore. Note that with ocaml 4.04.0 and 4.04.1,
your original definition is accepted.

hope this helps,
  Philippe.

[0] https://caml.inria.fr/pub/papers/garrigue-value_restriction-fiwflp04.pdf



2017-05-26 10:08 GMT+02:00 Matej Kosik <
5764c029b688c1c0d24a2e97cd764f@gmail.com>:

> Hi,
>
> There's an Ocaml language feature I do not understand.
>
> E.g. in this example:
>
>   type 'a phantom = int
>   type t = { enter : 'a. 'a phantom -> int }
>   let f g = g.enter 2
>   let _ = f { enter = fun x -> 1 + x } (* ok *)
>   let _ = f { enter = (+) 1 }          (* fails to typecheck *)
>
> I don't get why, if we have:
>
>   type 'a phantom = int
>
> i.e. "'a phantom" is defined as an alias of "int",
> how is it possible that the following types:
>
>   { enter : 'a. 'a phantom -> int }
>
> is not equal to
>
>   { enter : 'a. int -> int }
>
> I've tried to find a corresponding section in the Reference Manual, but I
> failed.
> Where is this explained?
>
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