>if you don't understand category theory and type theory well, using Haskell will be either hard or impossible

Absolutely not true. I never completed a single credit of university and struggled with high school math. I don't know any type theory or category theory.

Not only am I comfortable in Haskell, I teach Haskell.

Here's my guide for learning Haskell: https://gist.github.com/bitemyapp/8739525