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semigroups via wrapped `At`/`FoldableWithIndex`
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| import Control.Monad.State | |
| import Control.Lens.At | |
| import Control.Lens.Indexed | |
| import Data.Functor.Identity | |
| unionWith | |
| :: (FoldableWithIndex (Index s) t, At s) | |
| => (a -> IxValue s) -> (a -> IxValue s -> IxValue s) | |
| -> s -> t a -> s | |
| unionWith a2b ab2b xs ys = execState (itraverse_ f ys) xs | |
| where | |
| f k y = at k %= Just . maybe (a2b y) (ab2b y) | |
| newtype Wrapped f = Wrapped f | |
| instance | |
| ( Eq k, Semigroup a, FoldableWithIndex k f | |
| , Index (f a) ~ k, IxValue (f a) ~ a , At (f a) | |
| ) => Semigroup (Wrapped (f a)) where | |
| Wrapped xs <> Wrapped ys = Wrapped $ unionWith id (flip (<>)) xs ys | |
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