Hello Simon,
If you changed your perspective, you would realize that all functions in haskell are pure.
A function is pure if it returns the same output if given the same input. Every monadic function (including functions returning IO) is also pure.
For example,
putStrLn :: String -> IO ()
-- A function that takes a string, and returns an impure computation
-- which, when executed will print the given String.
For any string, putStrLn applied to that same string always describes the same impure computation, thus the function is actually pure.
I am not familiar with any other functional language, but there are not many purely functional ones out there [1].
I guess the impure ones get around this issue by giving in to impurity, but I'm not sure.
I'll be interested in hearing more about the other languages too.