Project Euler Problem 038

import Data.List

build :: (Enum a, Num a, Show a) => a -> String
build n = imp r "" where
  r :: [String]
  r = map show (n:map (*n) [2..])
  imp :: [String] -> String -> String
  imp [] acc = acc
  imp (x:xs) acc | (length acc) < 9 = imp xs (acc++x)
  imp (_:_) acc | otherwise = acc
ispan :: [Char] -> Bool
ispan n = (length n) == 9 && (nub $ sort $  n)=="123456789"
ansl :: String
ansl = maximum [r | x<-[1..9999::Integer],let r=build x,ispan r]

-- "932718654"
main :: IO ()
main = print ansl