pe_36.hs

-- Project Euler Problem 36
-- Double-base palindromes
--
-- The decimal number, 585 = 1001001001 base 2 (binary), is palindromic in both bases.
--
-- Find the sum of all numbers, less than one million, which are palindromic in
-- base 10 and base 2.
--
-- (Please note that the palindromic number, in either base, may not include
-- leading zeros.)

import Data.Digits

isBinDecPalindrome :: Integer -> Bool
isBinDecPalindrome n = isBinPalindrome n && isDecPalindrome n
  where
    isDecPalindrome n = (reverse . show $ n) == show n
    isBinPalindrome n = (reverse . showBinary $ n) == showBinary n
    showBinary = foldr (\x s -> (show x) ++ s) [] . digits 2

main = print . sum . filter isBinDecPalindrome $ [1..1000000]