Monoid and Alternative are not the same. There is a very important
difference between them:
class Alternative f where
(<|>) :: f a -> f a -> f a
...
class Monoid a where
mappend :: a -> a -> a
...
The equivalent to Alternative is MonadPlus, not Monoid. The kinds
matter. In Alternative, you are guaranteed that the type that f is
applied to cannot affect the semantics of (<|>). As has been already
demonstrated aptly, the type a in the instance Monoid a => Monoid
(Maybe a) matters quite a lot.
Carl
On Thu, Dec 15, 2011 at 8:04 AM, Yves Parès
So why don't we use First and Last with the Alternative interface too?
It's indeed weird the Maybe doesn't react the same way with Alternative and Monoid.
2011/12/15 Anthony Cowley
On Dec 15, 2011, at 10:19 AM, Brent Yorgey wrote:
On Thu, Dec 15, 2011 at 06:49:13PM +1000, Gregory Crosswhite wrote:
So at the end of the day... what is the point of even making Maybe and [] instances of Alternative?
The Alternative and Monoid instances for [] are equivalent. However, the Alternative and Monoid instances for Maybe are not. To wit:
(Just (Sum 4)) <|> (Just (Sum 3)) Just (Sum {getSum = 4})
(Just (Sum 4)) `mappend` (Just (Sum 3)) Just (Sum {getSum = 7})
We already have,
First (Just (Sum 4)) `mappend` First (Just (Sum 3)) First {getFirst = Just (Sum {getSum = 4})}
So the overlap of apparent Alternative and Monoid functionality remains. This just represents an opportunity for the caller to select the monoid they want.
Anthony _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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