[corethree]

> If I choose to add said side effect, now all my functions are marked with an io signature

You got the wrong idea. You're supposed to write FP in a way where the side effect is highly layered and segregated away from pure code. IO are singularities within your chains of pure function compositions. As soon as you hit a singularity you have to break out of it as soon as possible.

The main idea is the meat. Keep your bread tiny and keep it very very separate from the meat.

The pattern is called Imperative Shell, functional core. Think of your IO as two pieces of bread in a sandwich and your pure code is the meat that connects the read to the write.

The game you're playing with haskell is to avoid letting the IO monad pollute any of your logic as much as possible.

Anyway that being said in applications where IO is all over the place... this pattern becomes largely ineffective. You basically have more bread than meat.