Am Samstag 09 Mai 2009 21:31:20 schrieb michael rice:
Why doesn't this work?
Michael [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 Monad m => m a -> (a -> m b) -> m b, the type of (1 +) is Num n => n -> n. Using (1 +) as the second argument of (>>=), you must unify Num n => n -> n with Monad m => a -> m b the types of the arguments and results must match, so a = n m b = n = a , giving the type (Monad m, Num (m b)) => m b -> m b The first argument of (>>=) is Just 3 :: Num k => Maybe k. That must be unified with m a, giving m = Maybe and a = k. On the other hand we previously found a = m b, so in Just 3 >>= (1 +) the (1 +) has type Num (Maybe b) => Maybe b -> Maybe b and Just 3 has type Num (Maybe b) => Maybe (Maybe b). But ghci can't find an instance Num (Maybe b). You probably wanted fmap (1 +) (Just 3) ~> Just 4