Benjamin Pierce wrote:

For someone coming to Haskell from an OCaml background, one of the hardest things to get used to is the somewhat more bare bones module system that Haskell provides. ... This works fine as long as what you're exporting is just values, but it's awkward for types, since there is no way to *declare* a type (e.g., giving just its kind) without *defining* it.

Actually Haskell fully matches the module system of OCaml -- and then adds some. Haskell provides both generative and applicative (recursive) functors. The following two messages elucidate the correspondence http://www.haskell.org/pipermail/haskell/2004-August/014463.html http://www.haskell.org/pipermail/haskell/2004-September/014515.html The end of the first message gives the translation dictionary: from OCaml/ML module terminology to Haskell terminology. I have recently found out that the translation works the other way around, too. Both messages are doubly-literate code: messages can be loaded into OCaml or into a Haskell system. Regarding the specific question of hiding a type: oftentimes declaring a class and exporting it suffices. (forall a.C a => a) is an `abstract' type in Haskell. If the type to abstract has a kind *->*, we can write (forall a.C a => a b), etc. If we really want to prevent the user from ever knowing the details of the implementation, existential quantification (along with the typeclass) does the trick. That trick incidentally permits `self-tuned' data structures, the simple example of which is http://www.haskell.org/pipermail/haskell/2005-February/015356.html