Don&#x27;t get me wrong, types are an important part of Haskell. They&#x27;re probably a significant chunk of the reason Haskell exists, since that is what separates the ML family from Lisps (again, more-or-less).<p>Types are, however, irrelevant to Haskell&#x27;s relationship to category theory, which is more-or-less the topic under discussion. Haskell needs monads in order to do stateful things like IO, that are necessary for a programming language but a thorn in the side of lazy languages which don&#x27;t have an easily defined execution order.<p>Category theory&#x27;s importance to Haskell and vice-versa have nothing to do with Haskell&#x27;s type system and much more to do with the fact that it is a lazy functional programming language. In fact, one of the weaknesses of Haskell&#x27;s type system is that it can&#x27;t enforce the &quot;monad laws&quot; when you give an implementation of the monad typeclass.