Implement with instance chains

src/Data/Inject/Coproduct.purs

@@ -0,0 +1,23 @@
+module Data.Inject.Either 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 = id
+  prj = Just

src/Data/Inject/Either.purs

@@ -0,0 +1,22 @@
+module Data.Inject.Either where
+
+import Prelude
+
+import Data.Either (Either(..), either)
+import Data.Maybe (Maybe(..))
+
+class Inject a b where
+  inj :: a -> b
+  prj :: b -> Maybe a
+
+instance injectLeft :: Inject a b => Inject a (Either b c) where
+  inj = Left <<< inj
+  prj = either prj (const Nothing)
+
+else instance injectRight :: Inject a b => Inject a (Either c b) where
+  inj = Right <<< inj
+  prj = either (const Nothing) prj
+
+else instance injectReflexive :: Inject a a where
+  inj = id
+  prj = Just