Like the article here states, though, you don't need to learn category theory to learn Haskell. Most math is useful in driving intuition and insight, and Category Theory is no different in that respect, but learning Haskell (in particular enough to use it and write normal applications and libraries, not necessarily create the neatest cutting-edge type-trickery libraries - some of which are amazing) requires much more of a concrete "here's how you use these things" than any knowledge of Category Theory.