The Roman Numerals kata in Haskell

RomanNumeralsSpec.hs

import Test.Hspec
import Test.QuickCheck
import Control.Exception (evaluate)
import qualified Data.Map as Map
import Data.List

mapping :: Map.Map Int String
mapping = Map.fromList [
  (1, "I"),
  (4, "IV"),
  (5, "V"),
  (9, "IX"),
  (10, "X")]

keys :: [Int]
keys = (reverse . sort . Map.keys $ mapping)

largestReducer :: Int -> [Int] -> Int
largestReducer number = foldr1 (\x acc -> if x <= number then x else acc)

convert :: Int -> String
convert x
    | x == 0 = ""
    | otherwise = convertedString ++ convert (x - reducer)
    where reducer = largestReducer x keys
          convertedString = case (Map.lookup reducer mapping) of
                                Just value -> value
                                Nothing -> ""

main :: IO ()
main = hspec $ do
    describe "Roman Numerals" $ do
        it "finds the largest reducer" $ do
            largestReducer 11 keys `shouldBe` 10
            largestReducer 10 keys `shouldBe` 10
            largestReducer 9 keys `shouldBe` 9
            largestReducer 8 keys `shouldBe` 5
            largestReducer 4 keys `shouldBe` 4
        it "converts Arabic to Roman" $ do
            convert 1 `shouldBe` "I"
            convert 2 `shouldBe` "II"
            convert 4 `shouldBe` "IV"
            convert 8 `shouldBe` "VIII"
            convert 11 `shouldBe` "XI"
            convert 15 `shouldBe` "XV"