Try to further understanding the interface/module of OCaml

Question

I understand in OCaml there are concepts of interfaces and module.

And I understand how to use them now.

However, what I don\'t under

Answer 1

Here's an example that shows what I think you're asking for:

# module type HASH = sig type t val hash : t -> int end ;;
module type HASH = sig type t val hash : t -> int end
# module I = struct type t = int let hash i = i end ;;
module I : sig type t = int val hash : 'a -> 'a end
# module J = struct type t = int end ;;
module J : sig type t = int end
# module M : HASH = I ;;
module M : HASH
# module N : HASH = J ;;
Error: Signature mismatch:
       Modules do not match: sig type t = int end is not included in HASH
       The field `hash' is required but not provided

The extra ": HASH" specifies that the module must match the HASH signature (and it also restricts it to that signature).

Just as a side comment, I believe the OCaml module system is world famous for its expressivity (at least in module system circles). I'm still a beginner at it, but it is worth studying.