I think you're looking for fmap/liftM here. The type of >>= is: (>>=) :: (Monad m) => m a -> (a -> m b) -> m b
so it's trying to make your function (1+) return m b, which in this case should be a Maybe. Clearly, (1+) doesn't return a Maybe, so it breaks. Another options is to do return . (1+) to lift the function into Maybe, but that might be a little much. On Sat, May 9, 2009 at 3:31 PM, michael rice <[email protected]> wrote: > 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 > > ================ > > [mich...@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> > > > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
