Add injection tests

test/Main.purs

@@ -2,14 +2,22 @@ module Test.Main where
 
 import Prelude
 
+import Data.Const (Const)
 import Data.Functor.Compose (Compose(..))
+import Data.Functor.Coproduct (Coproduct, left, right)
+import Data.Functor.Coproduct.Inject (inj)
 import Data.Identity (Identity(..))
 import Data.Maybe (Maybe(..))
 import Effect (Effect)
 import Test.Assert (assertEqual)
 
 main :: Effect Unit
 main = do
+  testComposeOrdering
+  testInjections
+
+testComposeOrdering :: Effect Unit
+testComposeOrdering = do
   assertEqual
     { expected: Compose (Identity (Just true))
     , actual: Compose (Identity (Just true))
@@ -18,3 +26,18 @@ main = do
     { expected: Compose (Identity (Just true)) > Compose (Identity (Just false))
     , actual: true
     }
+
+testInjections :: Effect Unit
+testInjections = do
+  assertEqual
+    { expected: Identity unit
+    , actual: inj (Identity unit)
+    }
+  assertEqual
+    { expected: left (Identity unit) :: Coproduct Identity (Const Void) Unit
+    , actual: inj (Identity unit)
+    }
+  assertEqual
+    { expected: right (Identity unit) :: Coproduct (Const Void) Identity Unit
+    , actual: inj (Identity unit)
+    }