I'm trying to write a utility which is able to read Ratios from two distinct formats "2 / 3" -> 2 % 3 "4" -> 4 % 1 I'm sure that a skilled Haskell programmer could vastly improve on my code (do I even need Parsec?), so I've come to solicit advice. import Text.ParserCombinators.Parsec import qualified Text.ParserCombinators.Parsec.Token as P import Text.Parsec.Language (haskellDef) import Data.Ratio readFraction :: String -> Either ParseError (Ratio Integer) readFraction = parse ratio "-" lexer = P.makeTokenParser haskellDef integer = P.integer lexer natural = P.natural lexer whiteSpace = P.whiteSpace lexer ratio = try frac <|> integer' integer' = do i <- integer return $ fromInteger i frac = do whiteSpace num <- natural char '/' den <- integer return (num % den) f = readFraction " 3 / 2 " i = readFraction " 6 " main = print [f,i]