ch1 hw - std and tail-call recursion

gistfile1.hs

-- module cis194.ch1 where

-- std
toDigits' :: Integer -> [Integer]
toDigits' = reverse . f
  where
    f 0 = []
    f x = (x `rem` 10) : f (x `div` 10)

-- tail-call
toDigits :: Integer -> [Integer]
toDigits = f []
  where
    f rs 0 = rs
    f rs x = f (x `rem` 10 : rs) (x `div` 10)

-- std
toDigitsRev' :: Integer -> [Integer]
toDigitsRev' 0 = []
toDigitsRev' x = (x `rem` 10) : toDigitsRev' (x `div` 10)

-- tail-call
toDigitsRev :: Integer -> [Integer]
toDigitsRev = f []
  where
    f rs 0 = rs
    f rs x = f (x `rem` 10 : rs) (x `div` 10)

-- std
doubleEveryOther' :: [Integer] -> [Integer]
doubleEveryOther' = reverse . f . reverse
  where
    f :: [Integer] -> [Integer]
    f []       = []
    f (x:[])   = [x]
    f (x:y:xs) = x : (y + y) : f xs

-- tail-call
doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther = f [] . reverse
  where
    f rs []       = rs
    f rs (x:[])   = x : rs
    f rs (x:y:xs) = f (y + y : x : rs) xs

-- std
sumDigits' :: [Integer] -> Integer
sumDigits' []     = 0
sumDigits' (x:xs) = sum (toDigits x) + sumDigits' xs

-- tail-call
sumDigits :: [Integer] -> Integer
sumDigits = f 0
  where
    f r []     = r
    f r (x:xs) = f (r + sum (toDigits x)) xs

validate :: Integer -> Bool
validate = (== 0) . (`mod` 10) . sumDigits . doubleEveryOther . toDigits

type Peg  = String
type Move = (Peg, Peg)
hanoi :: Integer -> Peg -> Peg -> Peg -> [Move]
hanoi 0 _ _ _        = []
hanoi n src dest tmp = (hanoi (n - 1) src tmp dest) ++
                       [(src, dest)] ++
                       (hanoi (n - 1) tmp dest src)