#13702 error messages: specialized error messages for non-functors

[Octachron]

As reported in #13702, functors appearing in a context where non-functor where expected, for instance

module Int_set: sig include Set.S end = Set.Make

are handled by the generic functor parameters mismatch error message:

Error: Signature mismatch:
      Modules do not match:
        (Ord : Set.OrderedType) -> ...
      is not included in
         -> ...
      An extra argument is provided of module type Set.OrderedType

which is not easy to decipher.

This PR proposes to specialize the error message to reduce it to a simpler

Error: Signature mismatch:
       This module should not be a functor, a signature was expected.
       Hint: Did you forget to apply the functor?

with a variant when the expected type is incompatible with the body of the functor

module Int_set: sig type x end = Set.Make

      This module should not be a functor, a signature was expected.
      Moreover, the type of the functor body is incompatible with the
      expected module type.

The reverse situation of structure appearing where a functor was expected is handled symmetrically

module M: () -> sig end = Int

Error: Signature mismatch:
      This module should not be structure, a functor was expected.

without the symmetrical hint of suggesting adding abstractions, which does not seems likely to be correct.

[gasche]

I think we should iterate a bit on the proposed message in the first case.

This module should not be a functor, a signature was expected.

First, I think that you mean that a structure was expected, right?

Second, I'm worried that the wording "This module should not be a functor" could be confusing to users which use "module" as a synonym for "structure". I would write simply:

You provided a functor, but a structure was expected.

or maybe a "plain structure" or a "(non-functor) structure" or a "(non-parametrized) structure".

[Octachron]

You are right that structure will be clearer.

For your second point, I believe that

This module should not be a functor, but a structure

is clear even for people that doesn't make a clear distinction between functors and structures, or use module as a convenient abbreviation for structure. After the all, the important information is should not be a functor.

[Octachron]

I have updated the wording; and removed an unreachable code path for abstract module types.