Comment by themk

Passing an interface as a parameter is a monad. (Io -> _) is an instance of Monad in Haskell.

Haskell just has syntax to make using (any) monad much nicer. In this case, it let's you elide the `Io` parameter in the syntax if you are just going to be passing the same Io to a bunch of other functions. But it still is there.