Am Mittwoch, 1. Oktober 2008 12:18 schrieb Cetin Sert: > warn :: String → IO Int > warn = return 1 << putStrLn -- causes an error
try warn = (return 1 <<) . putStrLn > -- = \msg → return 1 << putStrLn msg -- works just fine > -- = \msg → putStrLn msg >> return 1 -- works just fine > > (<<) :: Monad m ⇒ m b → m a → m b > b << a = a >>= \_ → b (<<) = flip (>>) > > Why do I get this compile-time error?? How can one define << ? > > [EMAIL PROTECTED]:~/lab/test/qths/p> ghc -fglasgow-exts -O2 -o d64x --make > demo2.hs system.hs > [1 of 2] Compiling Netman.System ( system.hs, system.o ) > > system.hs:23:14: > No instance for (Num (IO Int)) > arising from the literal `1' at system.hs:23:14 > Possible fix: add an instance declaration for (Num (IO Int)) > In the first argument of `return', namely `1' > In the first argument of `(<<)', namely `return 1' > In the expression: return 1 << putStrLn Okay warn = (return 1) << putStrLn putStrLn :: String -> IO () return 1 :: m b (<<) :: m b -> m a -> m b warn :: String -> IO Int so we must have (String -> IO ()) === m a (String -> IO Int) === m b So the monad is ((->) String), a === IO () b === IO Int, hence in return 1 :: String -> IO Int the 1 must have type IO Int. Now 1 is actually fromInteger 1, fromInteger :: (Num a) => Integer -> a, so the compiler looks for the instance Num (IO Int) where ... which it doesn't find. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe