Re: [Caml-list] First class modules sub-typing

[Jeremy Yallop <yallop@gmail.com>]

On 21 October 2014 08:49, Thomas Braibant <thomas.braibant@gmail.com> wrote:
> I am slightly puzzled by a type-error related to first class modules.
> I am basically defining two types (see full example below)
>
> ```
> type t = (module A)
> type 'a s = (module A with type I.t = 'a)
> ```
>
> and I expect a value of type `'a s` can be coerced to a value of type
> t. However, this is obviously not the case because I get the following
> error message, and I wonder why.

Conversions between first class module types need explicit coercions.
It's sufficient to change your test4 function as follows:

> let test4 (type a) : a s -> string = fun t -> test1 t

   let test4 (type a) : a s -> string = fun t -> test1 (t :> (module A))