Add inject class for coproducts

Author: garyb

src/Data/Functor/Coproduct/Inject.purs

@@ -0,0 +1,23 @@
+module Data.Functor.Coproduct.Inject where
+
+import Prelude
+
+import Data.Either (Either(..))
+import Data.Functor.Coproduct (Coproduct(..), coproduct)
+import Data.Maybe (Maybe(..))
+
+class Inject f g where
+  inj :: forall a. f a -> g a
+  prj :: forall a. g a -> Maybe (f a)
+
+instance injectLeft :: Inject f g => Inject f (Coproduct g h) where
+  inj = Coproduct <<< Left <<< inj
+  prj = coproduct prj (const Nothing)
+
+else instance injectRight :: Inject f g => Inject f (Coproduct h g) where
+  inj = Coproduct <<< Right <<< inj
+  prj = coproduct (const Nothing) prj
+
+else instance injectReflexive :: Inject f f where
+  inj = identity
+  prj = Just