Have also wondered how Haskell would be. From my limited understanding it’s one of the few languages whose compiler enforces functional purity. I’ve always liked that idea in theory but never tried the language
Any side effect has to be performed inside `IO<T>` type, which means impure functions need to be marked as `IO<T>` return. And any function that tries to "execute" `IO<T>` side effect has to mark itself as returning `IO<T>` as well.
You basically compose a description of the side effects and pass this value representing those to the main handler which is special in that it can execute the side effects.
For the rest of the codebase this is simply an ordinary value you can pass on/store etc.