[Haskell-cafe] Just 3 >>= (1+)?
Alexander Dunlap
alexander.dunlap at gmail.com
Sat May 9 16:42:29 EDT 2009
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>
>
The type of (>>=) is
(>>=) :: m a -> (a -> m b) -> m b
For the Maybe monad, that specializes to
(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b
But when you say
Just 3 >>= (+1)
this desugars to
(>>=) (Just 3) (\x -> x + 1)
but the second argument to (>>=) that you have given has the type (\x
-> x + 1) :: Num a => a -> a, whereas it needs to return a type of
Maybe a to fit the type signature.
What you probably want is
Just 3 >>= (Just . (+1))
so the second function returns a Maybe value. A nicer way of writing this is
fmap (+1) (Just 3), which uses the Functor class. Intuitively, the
fmap function applies a function to the inside of a container. All
monads can be defined as Functors as well; all Monads in the standard
libraries have their functor instances defined.
Hope that helps you.
Alex
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