Comment by dllthomas

It has an effect. Whether it's a "side effect" depends on how we've defined that.

One way of viewing Haskell is that you are lazily constructing the single composite "effect on the world" called main.

    helloWorld :: IO ()

then is a value representing an effect, but it only actually happens when it becomes a part of main.

Threads complicate this but don't completely destroy the model.