Add cochoice instances for Joker and Star

Author: masaeedu

src/Data/Profunctor/Joker.purs

@@ -2,10 +2,13 @@ module Data.Profunctor.Joker where
 
 import Prelude
 
-import Data.Either (Either(..))
+import Control.Alternative (empty)
+import Control.MonadPlus (class MonadZero)
+import Data.Either (Either(..), either)
 import Data.Newtype (class Newtype, un)
 import Data.Profunctor (class Profunctor)
 import Data.Profunctor.Choice (class Choice)
+import Data.Profunctor.Cochoice (class Cochoice)
 
 -- | Makes a trivial `Profunctor` for a covariant `Functor`.
 newtype Joker f a b = Joker (f b)
@@ -38,5 +41,10 @@ instance bindJoker :: Bind f => Bind (Joker f a) where
 
 instance monadJoker :: Monad m => Monad (Joker m a)
 
+instance cochoiceJoker :: MonadZero f => Cochoice (Joker f)
+  where
+  unleft  (Joker fa) = Joker $ fa >>= either pure (const empty)
+  unright (Joker fb) = Joker $ fb >>= either (const empty) pure
+
 hoistJoker :: forall f g a b. (f ~> g) -> Joker f a b -> Joker g a b
 hoistJoker f (Joker a) = Joker (f a)

src/Data/Profunctor/Star.purs

@@ -15,6 +15,7 @@ import Data.Newtype (class Newtype)
 import Data.Profunctor (class Profunctor)
 import Data.Profunctor.Choice (class Choice)
 import Data.Profunctor.Closed (class Closed)
+import Data.Profunctor.Cochoice (class Cochoice)
 import Data.Profunctor.Strong (class Strong)
 import Data.Tuple (Tuple(..))
 
@@ -75,6 +76,10 @@ instance choiceStar :: Applicative f => Choice (Star f) where
   left  (Star f) = Star $ either (map Left <<< f) (pure <<< Right)
   right (Star f) = Star $ either (pure <<< Left) (map Right <<< f)
 
+instance cochoiceStar :: MonadZero f => Cochoice (Star f) where
+  unleft  (Star f) = Star $ \a -> (=<<) (either pure (const empty)) $ f (Left a)
+  unright (Star f) = Star $ \a -> (=<<) (either (const empty) pure) $ f (Right a)
+
 instance closedStar :: Distributive f => Closed (Star f) where
   closed (Star f) = Star \g -> distribute (f <<< g)