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

[Thomas DuBuisson]

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 <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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