module Text.ParserCombinators.Parsec.Lisp
 (parseExpr, LispVal(..))
where

import Text.ParserCombinators.Parsec hiding (spaces)

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
 deriving (Read, Show, Eq)

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
                          otherwise -> Atom atom

parseNumber :: Parser LispVal
parseNumber = fmap (Number . read) $ many1 digit

parseList :: Parser LispVal
parseList = fmap List $ sepBy parseExpr spaces

parseDottedList :: Parser LispVal
parseDottedList = do
    head <- endBy parseExpr spaces
    tail <- char '.' >> spaces >> parseExpr
    return $ DottedList head tail

parseQuoted :: Parser LispVal
parseQuoted = do
    char '\''
    x <- parseExpr
    return $ List [Atom "quote", x]

parseExpr :: Parser LispVal
parseExpr = parseAtom
        <|> parseString
        <|> parseNumber
        <|> parseQuoted
        <|> do char '('
               x <- (try parseList) <|> parseDottedList
               char ')'
               return x