Credit card number validation

Lecture 01/CreditCardValidation/CreditCard.hs

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+
+
+lastDigit :: Integer -> Integer
+lastDigit n = n `mod` 10
+
+dropLastDigit :: Integer -> Integer
+dropLastDigit n = n `div` 10
+
+toDigits :: Integer -> [Integer]
+toDigits n
+  | n <= 0 = []
+  | otherwise = toDigits(dropLastDigit(n)) ++ [lastDigit(n)]
+
+doubleEveryOtherLeft :: [Integer] -> [Integer]
+doubleEveryOtherLeft [] = []
+doubleEveryOtherLeft (x:[]) = [x]
+doubleEveryOtherLeft (x:y:zs) = x : (y * 2) : doubleEveryOtherLeft(zs)
+
+doubleEveryOther :: [Integer] -> [Integer]
+doubleEveryOther x = reverse(doubleEveryOtherLeft(reverse(x)))
+
+crossSum :: Integer -> Integer
+crossSum x
+  | x < 10 = x
+  | otherwise = lastDigit(x) + crossSum(dropLastDigit(x))
+
+sumDigits :: [Integer] -> Integer
+sumDigits [] = 0
+sumDigits (x:xs) = crossSum(x) + sumDigits(xs)
+
+validate :: Integer -> Bool
+validate x = lastDigit(sumDigits(doubleEveryOther(toDigits(x)))) == 0

Lecture 01/scratch/isEven.hs

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+isEven :: Integer -> Bool
+isEven n = n `mod` 2 == 0
+

Lecture 01/scratch/sumFun.hs

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+sumFun :: Integer -> Integer
+sumFun 0 = 0
+sumFun n
+  | n < 0 = 0
+  | otherwise = sumFun(n - 1) + n
+
+sumList :: Integer -> [Integer]
+sumList 0 = []
+sumList n = sumFun(n) : sumList(n - 1)
+