[Haskell-cafe] Golfing parsec code

[Jacek Generowicz]

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]