Excerpts from michael rice's message of Sat May 09 14:31:20 -0500 2009:
Why doesn't this work?
Michael
================
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
================
[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>
Look at the types: Prelude> :t (>>=) (>>=) :: (Monad m) => m a -> (a -> m b) -> m b Prelude> :t (+1) (+1) :: (Num a) => a -> a Prelude> The return type of '(+1)' in this case should be 'm b' but it instead only returns 'b'. If we tag a return on there, it will work fine: Prelude> Just 3 >>= return . (+1) Just 4 Prelude> Austin