[ufo]

It's not exactly the same thing because, for example, there is no subtyping or inheritance. In Ocaml you don't say that "type T1 is an Orderable".

However, it does serve a similar purpose. For example, if you want to create a datatype for an OrderedList then you'd create a higher-order-module (functor) that receives as an argument another module containing all the necessary comparison functions for the list elements. For example, if you apply the OrderedList functor to the IntOrdering module the result would be an OrderedListOfInt module that provides an abstract data type implementing an ordered list of integers.

In the Ocaml standard library the names would be different but that's the basic idea.

[svrb]

This makes it seem like modules are strictly inferior, if you can't assign names to common requirements.

[kdkeyser]

I don't see how you come to that conclusion. Assigning a name to a requirement (~ an interface) is the most simple usage of the module. E.g. to define something like IComparable, you could write:

  module type Comparable = sig
    type t
    val compare : t -> t -> int
  end

[ufo]

You can still give a name to the interface. However, interfaces apply to modules, not to types. In Ocaml you'd say "this module implements the Comparable interface" while in an OO languague you'd say "this type is a subtype of Comparable". Sorry for the confusion.

[svrb]

Ah, thanks for the clarification.