To me, "pure" means referential transparency: same input, same output. So an `Int -> Int` function will return same result on same argument. So, similar to `Int

To me, "pure" means referential transparency: same input, same output. So an `Int -> Int` function will return same result on same argument. So, similar to `Int -> IO Int`, the function (action) will return an Int after interacting with outside world, `IO` tracking the fact that this is the case.