[exercise] 4.8

Author: Dev-XYS

chapter04-Defining-functions/8.hs

@@ -0,0 +1,9 @@
+luhnDouble :: Int -> Int
+luhnDouble n | d > 9     = d - 9
+             | otherwise = d
+                   where d = n * 2
+
+luhn :: Int -> Int -> Int -> Int -> Bool
+luhn n1 n2 n3 n4 | sum `mod` 10 == 0 = True
+                 | otherwise         = False
+                       where sum = luhnDouble n1 + n2 + luhnDouble n3 + n4