Code for damienzhang.com/numberplay

numberplay.hs

import Data.List
import Text.Printf

choose :: Int -> [a] -> [[a]]
choose 0 _ = [[]]
choose _ [] = []
choose n (x:xs) = map (x :) (choose (n - 1) xs) ++ choose n xs

perm :: Int -> [a] -> [[a]]
perm n = concatMap permutations . choose n

candidates :: [([Char], [Char])]
candidates = map (splitAt 3) . perm 6 $ ['0' .. '9']

solutions = filter f candidates
  where
    f (num, ber) =
      all (('0' /=) . head) [num, ber, play] &&
      allDistinct (num ++ ber ++ play)
      where
        play = printf "%04d" (read num + read ber :: Int)
        allDistinct xs = length xs == length (nub xs)

main = do
  putStrLn $
    let (num, ber) = head solutions
    in printf "One solution: %s + %s = %d" num ber (read num + read ber :: Int)
  putStrLn $ printf "Solutions: %d" (length solutions)