OCaml has first-class modules & functors (functions on modules). I can write something like:

  let f (type t) some_arg (module M : Stringable with type t = t) = M.to_string some_arg

This will have the type signature

  val f : 'a -> (module Stringable with type t = 'a) -> string

In this case the module type Stringable looks like:

  module type Stringable = sig
    type t
    val to_string : t -> string

Functors are just module types so can be used in the same way. Module types fit in as normal values, you don't have to pattern match on them. This works:

  let id x = x
  
  id (module Int)