On Sat, May 9, 2009 at 12:31 PM, michael rice
Why doesn't this work?
Michael
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data Maybe a = Nothing | Just a
instance Monad Maybe where return = Just fail = Nothing Nothing >>= f = Nothing (Just x) >>= f = f x
instance MonadPlus Maybe where mzero = Nothing Nothing `mplus` x = x x `mplus` _ = x
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[michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> Just 3 >>= (1+)
<interactive>:1:0: No instance for (Num (Maybe b)) arising from a use of `it' at <interactive>:1:0-14 Possible fix: add an instance declaration for (Num (Maybe b)) In the first argument of `print', namely `it' In a stmt of a 'do' expression: print it Prelude>
The type of (>>=) is (>>=) :: m a -> (a -> m b) -> m b For the Maybe monad, that specializes to (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b But when you say Just 3 >>= (+1) this desugars to (>>=) (Just 3) (\x -> x + 1) but the second argument to (>>=) that you have given has the type (\x -> x + 1) :: Num a => a -> a, whereas it needs to return a type of Maybe a to fit the type signature. What you probably want is Just 3 >>= (Just . (+1)) so the second function returns a Maybe value. A nicer way of writing this is fmap (+1) (Just 3), which uses the Functor class. Intuitively, the fmap function applies a function to the inside of a container. All monads can be defined as Functors as well; all Monads in the standard libraries have their functor instances defined. Hope that helps you. Alex