Haskell solution for CodeChef - Ambiguous Permutations (improved)

permutations.hs

import Control.Monad
import qualified Data.IntMap as IM
import qualified Data.List as L
import Data.Maybe
import Data.Text
import qualified Data.Text.IO as TIO
import Data.Text.Read
import System.Exit

main = forever $ do
    -- get the number for this case, and exit if it's equal to 0
    n <- TIO.getLine >>= \x -> either (\_ -> exitFailure) (\(a,_) -> return a) (decimal x)
    when (n == 0) exitSuccess
    
    -- read the input and calculate the answer
    rawInput <- liftM (L.words . unpack) TIO.getLine
    let p = L.map read rawInput :: [Int]
    putStrLn $ solve p n
    
solve :: [Int] -> Int -> String
solve p n
    | p == inverse p n = "ambiguous"
    | otherwise        = "not ambiguous"

-- construct a map to speed up the process of finding
-- the correct index for a given value
inverseMap :: [Int] -> IM.IntMap (Int)
inverseMap p = f IM.empty p 1
    where f m [] _ = m 
          f m (x:xs) i = f (IM.insert x i m) xs (i+1)

-- convert a list of numbers into its inverse permutation
inverse :: [Int] -> Int -> [Int]
inverse p n = f [1..n]
    where m        = inverseMap p
          f []     = []
          f (x:xs) = m IM.! x : f xs