Lychrel.hs

module Lychrel where

import System.Environment
import Data.Foldable


iterateUntil f v =
  maybe
    []
    ((:) <$> id <*> iterateUntil f)
    (f v)


reverseInt n = aux n 0
  where
    aux 0 y = y
    aux x y =
      let
        (x',y') = x `quotRem` 10
      in
       aux x' (10*y+y')


reverseAndAdd x =
  if x == x'
    then Nothing
    else return (x + x')
  where
    x' = reverseInt x


main = do
  [number] <- getArgs
  let l = iterateUntil reverseAndAdd $ read number

  traverse_ print l

  putStrLn $ "Palindrome found in " ++ show (length l) ++ " steps"