Proposal: newtype for applicatives of monoids

[Christopher Allen]

Might as well capture all of 'em in one go.

+1 w/ aforementioned mappend/mplus fix.


On Sun, Jul 26, 2015 at 10:34 PM, David Feuer <david.feuer at gmail.com> wrote:

> Sorry for the terrible title, but I think we should add something like the
> following to Data.Monoid (I already mentioned this in the discussion of a
> Monoid instance for ZipList).
>
> newtype Appl f m = Appl (f m)
>
> instance (Applicative f, Monoid m) => Monoid (Appl f m) where
>   mempty = Appl $ pure mempty
>   mplus (Appl x) (Appl y) = Appl $ liftA2 mplus x y
>
> It may be possible to use coercions to improve mplus; I haven't tried it.
>
> This type is the same as WrappedApplicative from semigroupoids, do it
> might be best just to give it that name.
>
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-- 
Chris Allen
Currently working on http://haskellbook.com
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