upenn HW1

HW01.hs

-- Exercise 1
lastDigit :: Integer -> Integer
lastDigit n = n `mod` 10

dropLastDigit :: Integer -> Integer
dropLastDigit n = n `div` 10

--Exercise 2
toDigits :: Integer -> [Integer]
toDigits = reverse . toDigits'
toDigits' n 
	| n <= 0    = []
	| otherwise = lastDigit n : toDigits' (dropLastDigit n)

--Exercise 3
doubleEveryOther' :: [Integer] -> [Integer]
doubleEveryOther' (x:y:ys) = x : (y * 2) : doubleEveryOther' ys
doubleEveryOther' x = x

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther = reverse . doubleEveryOther' . reverse

--Exercise 4
sumDigits :: [Integer] -> Integer
sumDigits = sum . concat . map toDigits

--Exercise 5
validate :: Integer -> Bool
validate = (==0) . lastDigit . sumDigits . doubleEveryOther . toDigits

In validate you used an operator section (==0); you can also use operator sections with div and mod like so:

lastDigit = (`mod` 10)
dropLastDigit = (`div` 10)

You can also use concatMap toDigits in place of concat . map toDigits; it's part of the Prelude and will be more efficient (unless the compiler ends up doing the same optimization itself, anyway).