This is not true, Haskell of course allows you to mutate state. It just forces you to declare it: For example, there's a MutableByteArray type whose operations

This is not true, Haskell of course allows you to mutate state. It just forces you to declare it: For example, there's a MutableByteArray type whose operations only work in a specific Monad (because Haskell...) [1]. There's also more basic stuff like IORef which represents an assignable variable [2]. Again, you're constrained to use this in IO contexts. Etc. Haskell basically just forces you to do that thing someone else in this thread mentioned: Write functional APIs, allow side effects but make them explicit and confined to very specific places. [1]: https://hackage.haskell.org/package/primitive-0.7.4.0/docs/src/Data.Primitive.ByteArray.html#newByteArray https://hackage.haskell.org/package/primitive-0.7.4.0/docs/s... [2]: https://hackage.haskell.org/package/base-4.16.3.0/docs/Data-IORef.html https://hackage.haskell.org/package/base-4.16.3.0/docs/Data-...