Examples of different syntaxes for function application in validators

lib/Homework/Ch01.hs

@@ -1,39 +1,48 @@
 module Homework.Ch01 where
 
--- This calls reverse on toDigitsRev because I somehow did this backwards by default.
 toDigits :: Integer -> [Integer]
-toDigits = rev [] . toDigitsRev
+toDigits = go []
   where
-    rev acc (x : xs) = rev (x : acc) xs
-    rev acc [] = acc
+    go acc 0 = acc
+    go acc n = go (n `mod` 10 : acc) (n `div` 10)
+
+revDigits :: [Integer] -> [Integer]
+revDigits = go []
+  where
+    go acc (x : xs) = go (x : acc) xs
+    go acc [] = acc
 
--- I don't know how I managed it, but turning the number into an array already reversed it?
 toDigitsRev :: Integer -> [Integer]
-toDigitsRev n
-  | n < 1 = []
-  | otherwise =
-    let leftDigit = n `mod` 10
-        shiftedDigits = n `div` 10
-     in leftDigit : toDigitsRev shiftedDigits
+toDigitsRev = revDigits . toDigits
 
 doubleEveryOther :: [Integer] -> [Integer]
-doubleEveryOther (odd' : even' : rest) = odd' : (even' * 2) : doubleEveryOther rest
-doubleEveryOther rest = rest
+doubleEveryOther = revDigits . go []
+  where
+    go acc (first : second : rest) = go ((second * 2) : first : acc) rest
+    go acc [last'] = last' : acc
+    go acc [] = acc
 
 sumDigits :: [Integer] -> Integer
-sumDigits = sumDigits' 0 . flattenDigits [] . eachToDigits []
+sumDigits = sumDigits' . flattenDigits . eachToDigits
   where
-    eachToDigits acc (x : xs) = eachToDigits (toDigits x : acc) xs
-    eachToDigits acc [] = acc
+    eachToDigits = foldDigits [] (\n acc -> toDigits n : acc)
+    flattenDigits = foldDigits [] (++)
+    sumDigits' = foldDigits 0 (+)
 
-    flattenDigits acc ((x : xs) : rest) = flattenDigits (x : acc) (xs : rest)
-    flattenDigits acc ([] : rest) = flattenDigits acc rest
-    flattenDigits acc [] = acc
+    foldDigits acc f (x : xs) = foldDigits (f x acc) f xs
+    foldDigits acc _ [] = acc
 
-    sumDigits' acc (x : xs) = sumDigits' (acc + x) xs
-    sumDigits' acc [] = acc
+validateWithParens :: Integer -> Bool
+validateWithParens n = f n `mod` 10 == 0
+  where
+    f n' = sumDigits (doubleEveryOther (toDigitsRev n'))
 
-validate :: Integer -> Bool
-validate n = f n `mod` 10 == 0
+validateWithDollars :: Integer -> Bool
+validateWithDollars n = f n `mod` 10 == 0
+  where
+    f n' = sumDigits $ doubleEveryOther $ toDigitsRev n'
+
+validateWithCompose :: Integer -> Bool
+validateWithCompose n = f n `mod` 10 == 0
   where
     f = sumDigits . doubleEveryOther . toDigitsRev

test/Homework/Ch01Spec.hs

@@ -44,10 +44,38 @@ spec = describe "Credit Card Validation" $ do
     it "splits doubled digits into single digits and sums all single digits together" $ do
       sumDigits [16, 7, 12, 5] `shouldBe` 22
 
-  describe "validate" $ do
-    it "returns True for a valid credit card number" $ do
-      validate 5105105105105100 `shouldBe` True
-      validate 2223577120017656 `shouldBe` True
-    it "returns False for invalid credit card number" $ do
-      validate 5105105105105101 `shouldBe` False
-      validate 2223573420017656 `shouldBe` False
+  context "validators" $ do
+    let validators =
+          [ ("validateWithParens", validateWithParens),
+            ("validateWithDollars", validateWithDollars),
+            ("validateWithCompose", validateWithCompose)
+          ]
+
+        accept num = (num, True)
+        reject num = (num, False)
+        labelExpectation num expectIsValid
+          | expectIsValid = "accepts " ++ show num
+          | otherwise = "rejects " ++ show num
+
+        nums =
+          [ accept 5105105105105100,
+            accept 2223577120017656,
+            accept 371449635398431,
+            accept 6011000990139424,
+            accept 30569309025904,
+            accept 3566002020360505,
+            reject 5105105105105101,
+            reject 5420933878724339,
+            reject 5506923616306249,
+            reject 3999292939485618
+          ]
+
+        runExpectation validate' num expectedResult =
+          it (labelExpectation num expectedResult) $ do
+            validate' num `shouldBe` expectedResult
+
+        runValidator label validate' =
+          describe label $ do
+            sequence_ (uncurry (runExpectation validate') <$> nums)
+
+    sequence_ (uncurry runValidator <$> validators)