{-
A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 * 99.
Find the largest palindrome made from the product of two 3-digit numbers.
-}

rev_list :: [Char] -> [Char]
rev_list []     = []
rev_list [x]    = [x]
rev_list (x:xs) = (rev_list xs) ++ [x]

bigger :: Int -> Int -> Int
bigger x y = if (x > y) then x else y

is_palindrome :: Int -> Bool
is_palindrome x = (xs == (rev_list xs)) where xs = show x

solution' :: Int -> Int -> Int -> Int
solution' x    1000 res = res
solution' 1000 y    res = solution' 100 (y + 1) res
solution' x y res = solution' (x + 1) y $! (if (is_palindrome (x * y))
                                            then (bigger (x * y) res)
                                            else res)

solution :: Int
solution = solution' 100 100 0

main :: IO ()
main = putStrLn ("Solution: " ++ show solution)