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