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 <acow...@gmail.com>
> 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
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