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