simple_parser.hs

import Text.ParserCombinators.Parsec hiding (spaces)
import System.Environment
import Control.Monad

symbol::Parser Char
symbol = oneOf "!#$%&|*+-/:<=>?@^_~"

spaces :: Parser ()
spaces = skipMany1 space

data LispVal = Atom String
             | List [LispVal]
             | DottedList [LispVal] LispVal
             | Number Integer
             | String String
             | Bool Bool

--parseExpr :: Parser LispVal
--parseExpr = parseAtom
--         <|> parseString
--         <|> parseNumber

parseString :: Parser LispVal
parseString = do
                char '"'
                x <- many (noneOf "\"")
                char '"'
                return $ String x

parseAtom :: Parser LispVal
parseAtom = do
              first <- letter <|> symbol
              rest <- many (letter <|> digit <|> symbol)
              let atom = first:rest 
              return $ case atom of 
                        "#t" -> Bool True
                        "#f" -> Bool False
                        _    -> Atom atom 

parseNumber :: Parser LispVal
parseNumber = liftM (Number . read) $ many1 digit
                
readExpr :: String -> String
readExpr input = case parse parseExpr "lisp" input of 
    Left err -> "No match: " ++ show err
    Right val -> "Found value"


parseExpr :: Parser LispVal
parseExpr = parseAtom
         <|> parseString
         <|> parseNumber

main :: IO()
main = do 
          (expr:_) <- getArgs
          putStrLn (readExpr expr)