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,
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"
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