module SExprParser (SExpr(..), parseSExpr) where

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

-- representation of an s-expr
-- (improper lists are not supported)
data SExpr = Atom String
           | List [SExpr]
           | Number Integer
           | Bool Bool
           | String String
             deriving Show

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

spaces :: Parser ()
spaces = skipMany1 space

parseExpr = parseAtom
            <|> parseNumber
            <|> parseString
            <|> do char '('
                   x <- parseList
                   char ')'
                   return x

parseAtom :: Parser SExpr
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

parseList = liftM List $ sepBy parseExpr spaces

parseNumber = liftM (Number . read) $ many1 digit

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

parseSExpr :: String -> SExpr
parseSExpr prog = case (parse parseExpr "error" prog) of 
                    Left e -> error $ show e
                    Right sexpr -> sexpr