The following code fails to compile:

----- a.ml ----
module Foo = struct
  module St = struct type t = string let compare = compare end

  module Map (* : Map.S with type key = St.t *)
    = Map.Make(St)

  (* module Map = Map.Make(struct type t = string let compare = compare
end) *)
end

module Bar = struct
  type t = int Foo.Map.t
end

module Pack = struct
  module Foo = Foo
  module Bar = Bar
end

open Pack
let x : Bar.t = Foo.Map.empty
----

$ ocamlc -c foo.ml
File "foo.ml", line 20, characters 16-29: (* refers to Foo.Map.empty on
last line *)
Error: This expression has type
         'a Pack.Foo.Map.t = 'a Map.Make(Pack.Foo.St).t
       but an expression was expected of type
         Pack.Bar.t = int Map.Make(Foo.St).t


So Pack.Foo and Foo are not compatible. By trial and error, I found two
ways to resolve the problem:

1) Provide an explicit signature for Foo.Map (uncomment the provided
signature), or
2) Inline the structure passed to Map.Make (comment out first definition of
Foo.Map and uncomment the second one)

But I don't understand why. Can anyone explain the main points. Thank you.