[dan-robertson]

Module signatures may contain types.

So you can write a signature like:

  module type VectorSpace = sig
    module Field : Field
    type t
    val zero : t
    val (+) : t * t -> t
    val (*) : Field.t * t -> t
  end

In OOP it becomes hard to write an interface like this simple example, and more complex examples become harder still.

[horsawlarway]

Can you give an example of the value this would provide that isn't provided by generics?

[spopejoy]

One thing the example shows is the classic problem with OO's mechanism, namely binary functions. For a given class, something like `+` is impossible to implement elegantly, it has to bias one operand over the other -- after any syntactic sugar, it's always `arg1.op(arg2)`. Module functors resemble true ADTs more, in that they do not dispatch from a live instance of the datatype.