[Haskell-cafe] Alternative versus Monoid

[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 <acowley at 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
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20111215/dda71e40/attachment.htm>