Comment by namanbharadwaj

In fact, OCaml modules do correspond to Haskell type class instances, and there are ways to extend the ML module system to support ad-hoc style polymorphism (http://www.mpi-sws.org/~dreyer/papers/mtc/main-long.pdf).

On the other hand, you can also use modules as a simple namespacing mechanism (as in Haskell modules). But this does not capture the full power of the module system.