Given a signature with two abstract types, define a subsignature that makes them equal

Question

I have a very large signature with two abstract types:

signature GENERAL_FOO_BAR =
sig
  type foo
  type bar
  (* lots of stuff here *)
end

I want to declare a subsignature of GENERAL_FOO_BAR, where the two abstract types are identified, but none of the following attempts works:

signature SPECIAL_FOO_BAR = GENERAL_FOO_BAR
  where type foo = bar

signature SPECIAL_FOO_BAR = GENERAL_FOO_BAR
  sharing type foo = bar

It would be very cumbersome to use a dummy type as follows:

signature SPECIAL_FOO_BAR =
sig
  type both
  include GENERAL_FOO_BAR
    where type foo = both
    where type bar = both
end

Since it forces anyone who wants to implement SPECIAL_FOO_BAR to, well, define the dummy type. Is there a better alternative?

Answer 1

You don't need to be so explicit about "subsignatures". Here is a simple example of "subsignatures":

signature F =
sig
    type foo
    type bar
end

signature G =
sig
    type foo = int
    type bar = string
end

structure C : G =
struct
    type foo = int
    type bar = string
end

structure D : F = C