import Data.Char

-- some examples of recursive functions over lists

-- "length"
size :: [a] -> Int
size []     = 0
size (x:xs) = 1 + size xs

-- "maximum"
mest :: Ord a => [a] -> a
mest [x]    = x
mest (x:xs) = max x (mest xs)

-- "++"
(+++) :: [a] -> [a] -> [a]
[]     +++ ys = ys
(x:xs) +++ ys = x : (xs +++ ys)

-- "reverse"
rev :: [a] -> [a]
rev []     = []
rev (x:xs) = rev xs +++ [x]

-- value

value :: String -> Integer
value s = calculate (reverse s)

calculate :: String -> Integer
calculate ""    = 0
calculate (c:s) = toInteger (digitToInt c) + 10 * calculate s