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{-# LANGUAGE GADTs #-}
module Opt where
import Control.Arrow
import Control.Category
import Prelude hiding (id, (.))
data Opt arr a b where
Lift :: arr a b -> Opt arr a b
First :: Opt arr a b -> Opt arr (a,c) (b,c)
runOpt :: Arrow arr => Opt arr a b -> arr a b
runOpt (Lift f) = f
runOpt (First f) = first (runOpt f)
instance Arrow arr => Category (Opt arr) where
id = Lift id
First f . First g = First (f . g)
f . g = Lift (runOpt f . runOpt g)
instance Arrow arr => Arrow (Opt arr) where
arr = Lift . arr
first = First
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