[Haskell-cafe] flipped IO sequence

[Daniel Fischer]

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 at 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.