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

[Jeremy Yallop <yallop@gmail.com>]

On 21 October 2014 09:12, Jeremy Yallop <yallop@gmail.com> wrote:
> 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))

or you can, of course, use the alias 't' to coerce:

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