[tel]

Type classes are far less direct to encode. One also requires existential types to get the modularity properties of ML modules and they are a burden to deal with in Haskell.

A good example of something easy to do with ML modules and hard to do in Haskell is the Cohttp library which is a HTTP stack that works using the Lwt async backend, the Async async backend, and a Javascript Lwt backend. It is merely modularized over that interface and you can plug in whatever backend you like.

There are examples of doing similar things (Reflex is modeled this way, e.g.) but it's hairier in Haskell.