Things Haskell Has Taught Me Or Helped Me Learn About

Monads, functors, categories, limits, the Yoneda Lemma, domain theory, denotational semantics, abstract algebra, the theory of combinatorial species, type theory, the lambda calculi, intuitionistic logic, referential transparency, natural transformations, comonads, currying/partial application, lazyness/eagerness, strictness/non-strictness, and adjunctions.

And much more, but thats all I can think of at the moment. I don't completely understand all of these things (actually I'm not sure I would really say I understand any of them). The important thing is that Haskell has taught me about math, logic and computer science. Also, knowing Haskell has given me a way of understanding new material in math. What a cool language.