Someone who knows #Haskell and ML: is there a writeup somewhere explaining how first-class modules (a la ML) can do similar things to Haskell type classes? I'm finding it hard to figure that out.

To give a more concrete case: suppose I wanted to write something like Control.Applicative, which provides an 'interface' Applicative with some methods, as well as functions that work for _any_ Applicative. How would I do this with ML-style first-class modules?