Nested functors: Compiler can't determine type equality

Learning
4

Thank you, I will try to understand.

Interestingly, the following compiles:

module type S = sig end

module SomeModule = struct end

module Functor (Arg : S) = struct
  type t (* Note that [t] is abstract. *)
end

module Instance1 = Functor (SomeModule)
module Instance2 = Functor (SomeModule)

let foo : Instance1.t -> Instance2.t = fun x -> x

From the second linked thread:

So I think this is the minimal example of the issue I ran into:

module M = struct end

module Func (Arg : sig end) = struct
  type t
end

module Intermediate = Func (M)
module M1 = Func (Intermediate)
module M2 = Func (Func (M))

let foo : M1.t -> M2.t = fun x -> x

This results in:

File "./nestedfunctor.ml", line 11, characters 34-35:
11 | let foo : M1.t -> M2.t = fun x -> x
                                       ^
Error: The value x has type M1.t = Func(Intermediate).t
       but an expression was expected of type M2.t = Func(Func(M)).t

In my real-world use case, I think the problem is that when instantiating Set.Make, I need to make sure the exact same path is used. I think that will help me to avoid this problem in future.

Thank you so much!!

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