Updated readme and added the haskell source code.

README.md

 # Credit Card Validator
 
-Haskell program to validate credit card numbers.
\ No newline at end of file
+This is an exercise from https://www.cis.upenn.edu/~cis194/spring13/hw/01-intro.pdf.
+
+I have written a rudimentary checksum formula which could be used to validate credit card numbers.
+
+To compile: ghc -o egOutputFile homework1.hs
+To run: ./egOutputFile

homework1.hs

+-- ****EX1****
+
+--toDigits 1234 returns [1,2,3,4]
+toDigits :: Int -> [Int]
+toDigits 0 = []
+toDigits n
+ | n >=0 = toDigits(n `div` 10) ++ [n `mod` 10]
+ | otherwise = []
+
+--toDigitsRev 1234 returns [4,3,2,1]
+toDigitsRev :: Int -> [Int]
+toDigitsRev 0 = []
+toDigitsRev n
+ | n >= 0 = n `mod` 10 : toDigitsRev(n `div` 10) 
+ | otherwise = []
+
+-- ****EX2****
+
+--doubles every other no. beginning from the rhs of list
+doubleEveryOther :: [Int] -> [Int]
+doubleEveryOther [] = []
+doubleEveryOther (x:y:z)
+ | (length (x:y:z)) `mod` 2 /= 0 = x : y*2 : doubleEveryOther z
+ | otherwise = x*2 : y : doubleEveryOther z
+
+-- ****EX3****
+
+-- calculates the sum of all digits
+-- e.g [16,7,12,5] = 1+6+7+1+2+5==22
+sumDigits :: [Int] -> Int
+sumDigits [] = 0
+sumDigits (x:xs)
+ | x < 10 = x + sumDigits xs
+ | otherwise = (x `mod` 10) + (x `div` 10) + sumDigits xs 
+
+
+-- ****EX4****
+--function confirms if the number is a valid credit card number
+--number is valid if remainder when div by 10 is 0
+validate :: Int -> Bool
+validate n
+ | (sumDigits (doubleEveryOther (toDigits n))) `mod` 10 == 0 = True
+ | otherwise = False
+
+-- main function
+main = do
+   putStrLn("Enter your credit card number")
+   a <- (readLn)
+   let b = validate a
+   if (b == True)
+       then putStrLn("Valid")
+       else putStrLn("Invalid")