[Haskell-cafe] Just 3 >>= (1+)?

[Daniel Peebles]

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?
>
> 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 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+)
>
> <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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