i have this in a Monad IO:

forM_ fltrdNamesBDs $ \(name,bdSidonie,bdWDS) -> if (bdWDS /= bdSidonie) then putStrLn $ name ++ " " ++ (show (bdSidonie :: Maybe Float)) ++ " " ++ show (bdWDS :: Maybe Float) ++ " " ++ show (bdWDS == bdSidonie) else putStr ""

is there a way to remove the silly putStr "" that output an empty string, i tried with when.... but as when return Nothing in case of False it fails to compile

I don't see the problem with when? This (in the expanded version) doesn't work? forM_ fltrdNamesBDs $ \(name,bdSidonie,bdWDS) -> when (bdWDS /= bdSidonie) $ showStuff name bdSidonie bdWDS As an extra remark, whenever I want to output a whole bunch of stuff in a row like this, I like to refactor into a concat, just to make the code more readable. If I can factor out the types, all the better: where showStuff :: String -> Maybe Float -> Maybe Float -> IO () showStuff name bdSidonie bdWDS = putStrLn $ concat [name," ",show bdSidonie," ",show bdWDS," False"] Or even showStuff :: String -> Maybe Float -> Maybe Float -> IO () showStuff name bdSidonie bdWDS = putStrLn $ Data.List.intercalate " " [name,show bdSidonie,show bdWDS,show False] Cheers.