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

[Neil Brown]

Hi,

(1+) :: Num a => a -> a

For the bind operator, you need something of type a -> Maybe b on the 
RHS, not simply a -> a.  You want one of these instead:

fmap (1+) (Just 3)
liftM (1+) (Just 3)

Alternatively, you may find it useful to define something like:

(>>*) = flip liftM

so that you can write:

Just 3 >>* (1+)

which bears a closer resemblance to the bind notation.

Thanks,

Neil.

michael rice 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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