gistfile1.hs

import Control.Monad.ST
import Control.Monad
import Data.Array
import Data.Array.ST
import Data.List (elemIndex)
import Data.Maybe (fromMaybe)

newSTArray :: (Ix i) => (i,i) -> e -> ST s (STArray s i e)
newSTArray = newArray

minFree :: [Int] -> Int
minFree ns = fromMaybe 0 $ elemIndex True seen
  where
    bnd = (0, length ns)
    safeWrite arr n = when (inRange bnd n) $ writeArray arr n False
    setTrues arr = mapM_ (safeWrite arr) ns >> return arr
    seen = elems $ runSTArray $ newSTArray bnd False >>= setTrues

main :: IO ()
main = interact id