Add remaining haskell book exercises

Haskell-book/28/DifferenceList/test/Spec.hs

@@ -0,0 +1,33 @@
+module Main where
+
+import Test.Hspec
+import Data.Queue
+
+main :: IO ()
+main = hspec $ do
+    describe "empty" $ do
+        it "returns an empty queue" $ do
+            (empty :: Queue Int) `shouldBe` (Queue [] [])
+
+    describe "push" $ do
+        it "puts an element into an empty queue" $ do
+            (push 5 empty) `shouldBe` (Queue [5] [])
+
+    describe "pop" $ do
+        it "takes the only element from the queue" $ do
+            (pop (Queue [5] [])) `shouldBe` (Just (5, Queue [] []))
+        it "returns nothing if the queue is empty" $ do
+            (pop ((Queue [] [])::Queue Int)) `shouldBe` Nothing
+        it "takes elements in the FIFO order" $ do
+            let queue = push 3 (push 5 empty)
+             in pop queue `shouldBe` Just (5, Queue [] [3])
+
+    describe "isEmpty" $ do
+        it "tells when the queue is empty" $ do
+            (isEmpty (empty :: Queue Int)) `shouldBe` True
+        it "tells when the enqueue part isn't empty" $ do
+            let queue = push 3 empty
+             in isEmpty queue `shouldBe` False
+        it "tells when the dequeue part isn't empty" $ do
+            let queue = fmap snd (pop $ push 3 (push 5 empty))
+             in fmap isEmpty queue `shouldBe` Just False