Proposal: Add Alternative adapter to Data.Monoid

[John Lato]

+1, I think I've defined one-off, specialized versions of this a dozen
times.

On Thu, Oct 30, 2014 at 9:46 AM, David Feuer <david.feuer at gmail.com> wrote:

> I found myself needing this type in my current overhaul of Data.Foldable,
> but it seems to be generally useful:
>
> newtype Alt f a = Alt { getAlt :: f a } deriving
>    (Eq,Ord,Show,Read,Typeable,Data,Generic,Num,Real,
>      Floating, Fractional, RealFrac, RealFloat, Integral, Enum,
>      Bounded,Ix,Functor,Foldable,Traversable,Applicative,
>      Alternative,Monad,MonadPlus,MonadFix, IsString, IsList)
> -- The laundry list of derived instances is as recommended by Edward
> Kmett, who would probably want more added if someone can think of more.
> Some of them would, of course, need to be derived in other modules. The key
> instance is this:
>
> instance Alternative t => Monoid (Alt t a) where
>   mempty = Alt empty
>   (Alt m) `mappend` (Alt n) = Alt (m <|> n)
>
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