[Haskell-cafe] Just 3 >>= (1+)?
pumpkingod at gmail.com
Sat May 9 16:21:01 EDT 2009
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 <nowgate at yahoo.com> wrote:
> Why doesn't this work?
> 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 at 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+)
> 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
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
More information about the Haskell-Cafe