Add remaining haskell book exercises

Haskell-book/15/optional/test/Spec.hs

@@ -0,0 +1,21 @@
+import Data.Monoid
+import Optional
+import Test.Hspec
+
+main :: IO ()
+main = hspec $ do
+    describe "Sum" $ do
+        it "1 + 1 is 2" $ do
+            Only (Sum 1) `mappend` Only (Sum 1) `shouldBe` Only (Sum {getSum = 2})
+    describe "Product" $ do
+        it "4 * 2 is 8" $ do
+            Only (Product 4) `mappend` Only (Product 2) `shouldBe` Only (Product {getProduct = 8})
+    describe "Sum with Nada" $ do
+        it "1 + Nada is 1" $ do
+            Only (Sum 1) `mappend` Nada `shouldBe` Only (Sum {getSum = 1})
+    describe "List" $ do
+        it "[1] <> Nada is [1]" $ do
+            Only [1] `mappend` Nada `shouldBe` Only [1]
+    describe "Nada with sum" $ do
+        it "Nada + 1 is 1" $ do
+            Nada `mappend` Only (Sum 1) `shouldBe` Only (Sum {getSum = 1})