Ch4 isEven compatibility with negative inputs

exercises/chapter4/test/Main.purs

@@ -21,15 +21,28 @@ main =
     {-  Move this block comment starting point to enable more tests
 Note to reader: Delete this line to expand comment block -}
     suite "Exercise Group - Recursion" do
-      test "Exercise - isEven" do
-        assert "0 is even"
-          $ isEven 0
-        assertFalse "1 is odd"
-          $ isEven 1
-        assert "20 is even"
-          $ isEven 20
-        assertFalse "19 is odd"
-          $ isEven 19
+      suite "Exercise - isEven" do
+        test "0 is even" do
+          Assert.equal true
+            $ isEven 0
+        test "1 is odd" do
+          Assert.equal false
+            $ isEven 1
+        test "20 is even" do
+          Assert.equal true
+            $ isEven 20
+        test "19 is odd" do
+          Assert.equal false
+            $ isEven 19
+        test "-1 is odd" do
+          Assert.equal false
+            $ isEven (-1)
+        test "-20 is even" do
+          Assert.equal true
+            $ isEven (-20)
+        test "-19 is odd" do
+          Assert.equal false
+            $ isEven (-19)
       suite "Exercise - countEven" do
         test "[] has none" do
           Assert.equal 0

exercises/chapter4/test/no-peeking/Solutions.purs

@@ -9,14 +9,16 @@ import Data.Maybe (Maybe(..), fromMaybe, maybe)
 import Data.Path (Path, filename, isDirectory, ls, size)
 import Data.String.Common (split)
 import Data.String.Pattern (Pattern(..))
-import Data.Tuple (Tuple(..), snd)
+import Data.Tuple (Tuple(..))
 import Test.Examples
 
 isEven :: Int -> Boolean
-isEven n = case n of
-  0 -> true
-  1 -> false
-  _ -> isEven $ n - 2
+isEven n =
+  if n < 0
+    then isEven (-n)
+    else if n == 0
+      then true
+      else not (isEven (n - 1))
 
 oneIfEven :: Int -> Int
 oneIfEven n = if isEven n then 1 else 0
@@ -117,7 +119,7 @@ whereIs path fileName = head $ whereIs' $ allFiles path
     pure path
 
 largestSmallest :: Path -> Array Path
-largestSmallest path = 
+largestSmallest path =
   let files = onlyFiles path
       maybeSizes = map size files
       maybeMax = foldl (outlier (>)) Nothing maybeSizes