Lovely! Perhaps a stylistic shift would encourage writing this sort of elegant, fusion-friendly code. -- Generalized version of "interact". Encapsulates data getter & putter. genInteract :: IO i -> (o -> IO ()) -> ((i -> o) -> IO ()) genInteract get put = \ f -> get >>= put . f The intention here is that i and o are pure value types (no IO). Solutions 2 and three use the following specialization. -- Lazy ByteString in and showable out lGetPrint :: Show o => (L.ByteString -> o) -> IO () lGetPrint = genInteract L.getContents print Then rewrite solution 2 as follows: main = lGetPrint f where f contents = foldl' (test k) 0 . map int . take n $ ls where (l:ls) = L.lines contents [n,k] = map int (L.split ' ' l) And solution 3 similarly. Plug: for additional examples and a more general approach to separating out IO, my blog post "separating IO from logic -- example http://conal-elliott.blogspot.com/2007/02/separating-io-from-logic-example.h...", which uses the TV http://www.haskell.org/haskellwiki/TV library.