[tathougies]

Honestly, none. I'm not a category theorist, and have no more understanding of monads than anyone who's used a javascript promise library, and I've been employeed professionally as a Haskell programmer for the past year, contributed libraries back to the community, and given talks in my local area.

Haskell is a language like any other. Many people would like to complicate it, but if you spend the time learning its syntax and semantics, there is very little need to learn the theory.