As Erik Meijer said, there are many ways to be dirty, but only one way to be pure. Haskell allows you to annotate classes of side effects such as "changes stat

As Erik Meijer said, there are many ways to be dirty, but only one way to be pure. Haskell allows you to annotate classes of side effects such as "changes state" or "might throw exception", not necessarily the full IO monad, so annotating as pure doesn't make sense.