Packrat Parsing

Parser.hs

data Result v = Parsed v Derivs
                | NoParse

data Derivs = Derivs {
                dvAdditive :: Result Int,
                dvMultitive :: Result Int,
                dvPrimary :: Result Int,
                dvDecimal :: Result Int,
                dvChar :: Result Char}

parse :: String -> Derivs
parse s = d where
  d = Derivs add mult prim dec chr
  add = pAdditive d
  mult = pMultitive d
  prim = pPrimary d
  dec = pDecimal d
  chr = case s of
        (c:s') -> Parsed c (parse s')
        [] -> NoParse
        
pAdditive :: Derivs -> Result Int
pAdditive d = alt1 where
  alt1 = case dvMultitive d of
    Parsed vleft d' ->
      case dvChar d' of
        Parsed '+' d'' ->
          case dvAdditive d'' of
            Parsed vright d''' ->
              Parsed (vleft + vright) d'''
            _ -> alt2
        _ -> alt2
    _ -> alt2

  alt2 = dvMultitive d

pMultitive :: Derivs -> Result Int
pPrimary :: Derivs -> Result Int
pDecimal :: Derivs -> Result Int