expr.hs

module Expression where

import Text.ParserCombinators.Parsec
import Text.Parsec.Token
import Control.Monad.Error


data Expr = LispInt Integer |
            LispSym String |
            LispLi [Expr]

instance Show Expr where
  show (LispInt x) = show x
  show (LispSym x) = x
  -- show (LispFun _) = "<fun>"
  show (LispLi x) = "(" ++ unwords (map show x) ++ ")"

parseInteger = do
  num <- many1 digit
  return (LispInt (read num))

parseSymbol = do
  pre <- oneOf "*-+" <|> letter
  suf <- many (letter <|> digit)
  return $ LispSym ( [pre] ++ suf)

parseList = do
  char '('
  skipMany space
  x <- parseExpAux `sepEndBy` (many1 space)
  skipMany space
  char ')'
  return $ LispLi x

parseExpAux = (try parseInteger) <|> (try parseList) <|> (try parseSymbol)

parseExp :: Parser Expr
parseExp = do skipMany space
              x <- parseExpAux
              skipMany space; eof
              return x