Types. (>>=) :: Monad m => m a -> (a -> m b) -> m b (1+) :: Num a => a -> a So, the typechecker deduces that 1) "a" is the same as "m b", and 2) "a" (and "m b", therefore) must be of class "Num" Now, Just 3 :: Num t => Maybe t and the typechecker learns from that that "m a" must be the same as "Maybe t", with "t" being of class "Num". This leads to two observations: 3) "m" is "Maybe", and 4) "a" is of class "Num" - the same as (2) above Now, from (1) and (3) it follows that "a" is the same as "Maybe b". (2) lead than to "Maybe b" being of class "Num" - but GHCi doesn't have this instance, and complains. What you've probably meant is something like Just 3 >>= \x -> return (x + 1) or, equivalently, liftM (+1) $ Just 3 On 9 May 2009, at 23:31, michael rice wrote:
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>
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