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/s... [2]: https://hackage.haskell.org/package/base-4.16.3.0/docs/Data-...

Everything does not have to be immutable in Haskell. It's only that mutability can only exist in IO or some other context that deals with it. The language enforces purity, not immutability.