It's a trade-off. You can either use direct mutation using some sort of impure reference type, or use a pure mutation using first class accessor labels. Impure

It's a trade-off. You can either use direct mutation using some sort of impure reference type, or use a pure mutation using first class accessor labels. Impure references are probably easier to use, but are harder to reason about and allow for more subtle bugs in your code. Accessor labels require a bit more trickery, but give a more principled approach and result in code that is less error-prone and easier to compose. Haskell doesn't force you in any of those directions. Pick the approach that bests fits your domain and likings. Personally I'm convinced pure code is a way better default, however for certain domains I certainly resort to using `IORef`s, `TVar`s, etc.