Christian Maeder wrote:

-- a filter program process an entire input to yield some output
type FilterProgram = [Line] -> [Line]


Forget this, if it's not an (old) exercise

Yes, people don't write lazy functional programs in Haskell any more. In the Era of Monadic Enlightenment, obfuscated imperative programming is the Way To Go. :-\ :-)

However, for those who like to indulge the odd moment of nostalgia, in the Haskell 98 Prelude there is:

interact :: (String -> String) -> IO ()

If this function does not work correctly with the Haskell implementation you use, do report the fault to the implementors.

Colin R
(purveyor of old exercises)