[Haskell-cafe] flipped IO sequence
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 ? [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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] flipped IO sequence
2008/10/1 Cetin Sert [EMAIL PROTECTED]: 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 ? [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 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] flipped IO sequence
Hi Cetin, what you seem to want is warn :: String - IO Int warn = (return 1 ) . putStrLn Cetin Sert schrieb: 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 ? [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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] flipped IO sequence
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
Re: [Haskell-cafe] flipped IO sequence
2008/10/1 Cetin Sert [EMAIL PROTECTED]: 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 ? While this isn't directly what you're doing, you might be interested in the Kleisli composition operators in Control.Monad: (=) :: (Monad m) = (a - m b) - (b - m c) - (a - m c) (=) :: (Mnoad m) = (b - m c) - (a - m b) - (a - m c) Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
