I'd find it hard to justify nowadays understanding that there's a lot useful in Type theory but then ignoring Category theory. Even Object Orientation can be related to CT as coalgebras.

The type system in Haskell is not complex and that's a reason to like it. Everything derives from 6 simple rules:

http://en.wikipedia.org/wiki/Hindley%E2%80%93Milner_type_sys...

Simplicity in a type system makes it very expressive - contrast with complex, concrete type systems which do not allow e.g. Church numerals or linked lists to be represented within it.