[Caml-list] Applicative Functor Madness

[Arlen Cox <arlencox@gmail.com>]

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Hi everyone,

I'm having some trouble getting some code that relies heavily on
applicative functors to type check.  Does anyone know what I'm doing wrong
with this?

module type S = sig
  module T : Set.OrderedType
  module ST : module type of Set.Make(T)
end

module Make(T_in : Set.OrderedType) : S (* <- ERROR *)
  with module T = T_in
   and module ST = Set.Make(T_in)
= struct
  module T = T_in
  module ST = Set.Make(T_in)
end

I get the following error message referencing the above point in the
program.

Error: In this `with' constraint, the new definition of ST
       does not match its original definition in the constrained signature:
       ...
       Type declarations do not match:
         type t = Set.Make(T_in).t
       is not included in
         type t = Set.Make(T).t
       File "set.mli", line 68, characters 4-10: Expected declaration
       File "set.mli", line 68, characters 4-10: Actual declaration

It seems to me that since T = T_in, but applicative functors should make
the type of Set.Make(T) = Set.Make(T_in).  Does this not work this way?

Note that if I change the definition of S slightly, the same definition of
Make now type checks:

module type S = sig
  module T : Set.OrderedType
  module ST : Set.S with type elt = T.t
end

This solution is undesirable because I have a number of modules whose types
would require an excessive number of "with module ... = ..." constraints to
constrain in this way.  Is there a better way of getting this to type check?

Thank you,
Arlen

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