[Proposal] Add instance Monoid a => Monoid (Const a b)

[Edward Kmett]

A decent argument in favor is there really isn't another viable instance
for it. Being able to glue together folds with (<>) never occurred to me.

+1 from me

-Edward


On Sun, Sep 29, 2013 at 12:01 PM, <haskell at nand.wakku.to> wrote:

> Hello,
>
> I think this a Monoid instance on Const would be useful. The motivation
> comes
> from ‘lens’, where folds come out as (a -> Const a a) -> s -> Const a s.
> This
> instance, together with the Monoid a => Monoid (r -> a) instance, allows
> folds
> themselves to be an instance of Monoid, allowing for some really pretty
> code.
>
> Example of the instance body:
>
> > instance Monoid a => Monoid (Const a b) where
> >   mempty = Const mempty
> >   mappend (Const a) (Const b) = Const (mappend a b)
>
> Examples of it being used together with lens:
>
> >>> "Hello, world!" ^.. mconcat [ix 2, ix 7, ix 1]
> "lwe"
>
> Discussion Period: 2 weeks
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