{-

- Solution to Project Euler problem 38

- Copyright (c) Project Nayuki. All rights reserved.

-

- https://www.nayuki.io/page/project-euler-solutions

- https://github.com/nayuki/Project-Euler-solutions

-}

import Data.List (sort)

main = putStrLn (show ans)

ans = foldl1 max $ concat [[number (concatProd n x) | x <- [1 .. 10 ^ (div 9 n) - 1], pandigital (concatProd n x)] | n <- [2..9]]

digits 0 = [0]

digits n = digits' n where

digits' 0 = []

digits' n = (digits' (div n 10)) ++ [mod n 10]

number d = number' d 0 where

number' [] acc = acc

number' (d:ds) acc = number' ds (acc * 10 + d)

pandigital d = (sort d) == [1..9]

concatProd n x = concat [digits (x * i) | i <- [1..n]]