NineDigitProblem

NineDigitProblem.hs

-- http://csokavar.hu/blog/2010/04/20/problem-of-the-week-9-digit-problem/

import Data.List (delete)

main :: IO ()
main = print $ solve 0 1 [1..9]

solve :: Integral a => a -> a -> [a] -> [a]
solve curr _ [] = [curr]
solve curr divisor possibilities = concatMap solveFor possibilities
  where solveFor n = let curr' = curr `appendDigit` n
                     in if curr' `divisibleBy` divisor
                           then solve curr' (divisor+1) (delete n possibilities)
                           else []
        appendDigit a b = (a*10) + b
        divisibleBy a b = a `mod` b == 0