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OCaml modules and Haskell type classes are quite separate, too. I would make more sense to compare OCaml and Haskell modules. Haskell type classes implement ad-hoc polymorphism and OCaml doesn't have that at all. You can't implement in OCaml something like a show function that works on integers and strings and list of integers and so on like in Haskell using the Show type class.

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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.

You sort of can with functors or GADTs.

Like you say, sort of. This would imply that you wrap all the basic types in a sum type. It's quite heavy. Using the PPX mechanism the compiler can also be extended to generate this code automatically but again, this isn't very natural at the moment.

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