Because you're looking for: Just 3 >>= return . (+1) or more simply Just 3 >>= Just . (+1)
or more generally: return 3 >>= return . (+1) The second argument of (>>=) is supposed to be of type (Monad m => a -> m b) but (+1) ishe of type (Num a => a -> a). Wre is the monad in that? Thomas On Sat, May 9, 2009 at 12:31 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
