2008/10/1 Cetin Sert :

warn :: String → IO Int warn = return 1 << putStrLn -- causes an error -- = \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

Why do I get this compile-time error?? How can one define << ?

cetin@linux-d312:~/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

This works for me (type signature added so GHCi doesn't choke) Prelude> let (<<) = flip (>>) :: IO b -> IO a -> IO b And thus: Prelude> return 1 << putStrLn "yo" yo 1 Prelude> You might be having problems with the point-free code: Prelude> let warn' = return 1 << putStrLn <interactive>:1:24: Couldn't match expected type `IO a' against inferred type `String -> IO ()' In the second argument of `(<<)', namely `putStrLn' In the expression: return 1 << putStrLn In the definition of `warn'': warn' = return 1 << putStrLn Adding in variable names straightens that out for me: Prelude> let warn s = return 1 << putStrLn s Prelude> warn "help" help 1 Prelude> Cheers, D

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 << ?

cetin@linux-d312:~/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.