My immediate reaction here is that because you are interacting with something 'global', is it 'correct' to do this? Or would you better to pass toWorld into the function. Learn4Haskell is a GitHub-located course that will get you into the Haskell Functional Programming world in just 4 Pull Requests. This function relies on a toWorld function that would have been defined elsewhere. One thing I'm not currently sure of is, say you have something like the following: worlds :: -> The compiler simply refuses to build a program which incorrectly mixes pure and impure functions.

However I'm trying to get my head around best practices. More incredibly, the compiler also knows exactly which functions are impure (putStrLn, getLine) and which are pure ((++), tail, reverse), so it can prevent us from, say, accidentally mutating global state in a pure list-reversing function. I am currently learning Haskell, and loving the way it balances flexibility allowing rapid prototyping with functional purity. I'm sure this must have been asked before, but I can't find anywhere that actually answers my question, so apologies if I have simply overlooked this.

Learn haskell functional programming