Many people saw Ruby for the first time when they looked at a Rails tutorial. Many people see Python for the first time when they look at a Django tutorial. In both cases, you'd still need to learn more about the language to do something useful. I think the disclaimer "You shouldn't need to know Haskell" just means that the same thing works for this tutorial: you don't need to have seen Haskell before going through the tutorial, because you'll learn enough to follow the tutorial, even though you'll need to learn more later.