Re: [Caml-list] Manipulating Modules Modularly

[Leo White <lpw25@cam.ac.uk>]

> Am I weird to run into this issue all the time or is it common?

Depends which issue you mean.

People run into the value restriction quite frequently but eta-expansion
is usually considered an acceptable solution, although it does require a
certain amount of boilerplate.

Note that your problem can also be worked around for a fixed number of
type parameters by defining your module types as:

    module type M = sig
        type 'a t
        val u : 'a t
        val p : 'a t -> 'a t -> 'a t
      end

    module type ME = sig
       type 'a t
       val ps: 'a t list -> 'a t
    end

which will now work for types with 0 or 1 parameters.

> PS: now that there are both applicative functors and generative functors, could applicative functors be restricted to
> being pure, so that this sort of manipulation become possible in a generic way?

That would break a lot of existing code and wouldn't work very well
without a full effect system in the core language.

Regards,

Leo