Avoid non-exhaustive pattern match.

src/Text/Pandoc/Parsing.hs

@@ -514,22 +514,16 @@ charsInBalanced open close parser = try $ do
 
 -- Auxiliary functions for romanNumeral:
 
-lowercaseRomanDigits :: [Char]
-lowercaseRomanDigits = ['i','v','x','l','c','d','m']
-
-uppercaseRomanDigits :: [Char]
-uppercaseRomanDigits = map toUpper lowercaseRomanDigits
-
 -- | Parses a roman numeral (uppercase or lowercase), returns number.
 romanNumeral :: Stream s m Char => Bool                  -- ^ Uppercase if true
              -> ParserT s st m Int
 romanNumeral upperCase = do
-    let romanDigits = if upperCase
-                         then uppercaseRomanDigits
-                         else lowercaseRomanDigits
-    lookAhead $ oneOf romanDigits
     let [one, five, ten, fifty, hundred, fivehundred, thousand] =
-          map char romanDigits
+          map char $
+            if upperCase
+               then ['I','V','X','L','C','D','M']
+               else ['i','v','x','l','c','d','m']
+    lookAhead $ choice [one, five, ten, fifty, hundred, fivehundred, thousand]
     thousands <- ((1000 *) . length) <$> many thousand
     ninehundreds <- option 0 $ try $ hundred >> thousand >> return 900
     fivehundreds <- option 0 $ 500 <$ fivehundred