Most languages, sure, but not all; Haskell doesn’t have NULL. You can express the absense of a value with a Maybe:

    Just “value” -> Maybe String
    Nothing      -> Maybe a

If your function returns a “Maybe String”, it can either be just a value, or Nothing. However, if a function returns a “String”, you can’t return Nothing, only an actual String.

There’s no null in Haskell.