gistfile1.hsc

f :: Monad m => m Int -> m Bool
f a = do
    b <- a
    return $ b == 0

f' :: Functor f => f Int -> f Bool
f' = fmap helper
    where
        helper = (== 0)

main :: IO ()
main = do
    dump $ Just 10
    dump $ Just 0
    dump Nothing
    dump []
    dump [-2, -1 .. 2]

    where
        dump :: (Monad m, Functor m, Show (m Bool), Show (m Int)) => m Int -> IO ()
        dump a = do
            putStrLn ("f(" ++ show a ++ "): " ++ show (f a))
            putStrLn ("f'(" ++ show a ++ "): " ++ show (f' a))