operatorparser.hs

import Control.Monad
import Control.Monad.Error
import Control.Applicative ((<$>))
import Text.ParserCombinators.Parsec hiding (spaces)

main :: IO ()
main = do
  text <- getLine
  evaled <- return $ readExpr text >>= eval >>= return . show
  -- evaled     :: ThrowsError LispVal
  -- right hand :: IO (ThrowsError LispVal)
  putStrLn . extractValue $ trapError evaled

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

instance Show LispVal where show = showVal

showVal :: LispVal -> String
showVal (String contents) = "\"" ++ contents ++ "\""
showVal (Atom name)       = name
showVal (Number number)   = show number
showVal (Bool True)       = "#t"
showVal (Bool False)      = "#f"
showVal (List contents)   = "(" ++ (unwords $ map showVal contents) ++ ")"
showVal (DottedList h t)  = "(" ++ (unwords $ map showVal h) ++
                            " . " ++ showVal t ++ ")"
-- LispVal type end

-- LispError type begin
data LispError = NumArgs Integer [LispVal]
               | TypeMismatch String LispVal
               | Parser ParseError
               | BadSpecialForm String LispVal
               | NotFunction String String
               | UnboundVar String String
               | Default String

showError :: LispError -> String
showError (UnboundVar message varname) = message ++ ": " ++ varname
showError (BadSpecialForm message form) = message ++ ": " ++ show form
showError (NotFunction message func) = message ++ ": " ++ show func
showError (NumArgs expected found) = "Expected " ++ show expected ++
  " args: found values " ++ (unwords $ map showVal found)
showError (TypeMismatch expected found) = "Invalid type: expected " ++
  expected ++ ", found " ++ show found
showError (Parser parseErr) = "Parser error at " ++ show parseErr

instance Show LispError where show = showError

instance Error LispError where
  noMsg = Default "An error has occurred"
  strMsg = Default

type ThrowsError = Either LispError

trapError action = action `catchError` (return . show)

extractValue :: ThrowsError a -> a
extractValue (Right val) = val
-- LispError type end

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

spaces :: Parser ()
spaces = skipMany1 space

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 = Number . read <$> many1 digit

parseList :: Parser LispVal
parseList = 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

readExpr :: String -> ThrowsError LispVal
readExpr input = case parse parseExpr "lisp" input of
  Left err  -> throwError $ Parser err
  Right val -> return val
-- parser end

-- evaluation begin
eval :: LispVal -> ThrowsError LispVal
eval val@(String _) = return val
eval val@(Number _) = return val
eval val@(Bool _)   = return val
eval (List [Atom "quote", val]) = return val
eval (List (Atom func : args)) = mapM eval args >>= apply func
eval badForm = throwError $
  BadSpecialForm "Unrecognized special form" badForm

apply :: String -> [LispVal] -> ThrowsError LispVal
apply func args = case lookup func primitives of
  Nothing -> throwError $ NotFunction
    "Unrecognized primitive function args" func
  Just f  -> f args

primitives :: [(String, [LispVal] -> ThrowsError LispVal)]
primitives = [("+", numericBinop (+)),
              ("-", numericBinop (-)),
              ("*", numericBinop (*)),
              ("/", numericBinop div),
              ("mod", numericBinop mod),
              ("quotient", numericBinop quot),
              ("remainder", numericBinop rem),
              ("=", numBoolBinop (==)),
              ("<", numBoolBinop (<)),
              (">", numBoolBinop (>)),
              ("/=", numBoolBinop (/=)),
              (">=", numBoolBinop (>=)),
              ("<=", numBoolBinop (<=)),
              ("&&", boolBoolBinop (&&)),
              ("||", boolBoolBinop (||)),
              ("string=?", strBoolBinop (==)),
              ("string<?", strBoolBinop (<)),
              ("string>?", strBoolBinop (>)),
              ("string<=?", strBoolBinop (<=)),
              ("string>=?", strBoolBinop (>=))]

numericBinop :: (Integer -> Integer -> Integer) -> [LispVal]
                -> ThrowsError LispVal
numericBinop op singleVal@[_] = throwError $ NumArgs 2 singleVal
numericBinop op params = Number . foldl1 op <$> mapM unpackNum params

boolBinop :: (LispVal -> ThrowsError a) -> (a -> a -> Bool)
              -> [LispVal] -> ThrowsError LispVal
boolBinop unpacker op args = if length args /= 2
  then throwError $ NumArgs 2 args
  else do
    left <- unpacker $ args !! 0
    right <- unpacker $ args !! 1
    return . Bool $ op left right

numBoolBinop = boolBinop unpackNum
boolBoolBinop = boolBinop unpackBool
strBoolBinop = boolBinop unpackStr

unpackNum :: LispVal -> ThrowsError Integer
unpackNum (Number n) = return n
unpackNum (String n) = let parsed = reads n in
  if null parsed then throwError . TypeMismatch "number" $ String n
    else return . fst . head $ parsed
unpackNum (List [n]) = unpackNum n
unpackNum notNum = throwError $ TypeMismatch "number" notNum

unpackBool :: LispVal -> ThrowsError Bool
unpackBool (Bool b) = return b
unpackBool notBool = throwError $ TypeMismatch "boolean" notBool

unpackStr :: LispVal -> ThrowsError String
unpackStr (String s)  = return s
unpackStr (Number s)  = return $ show s
unpackStr (Bool b)    = return $ show b
unpackStr notString   = throwError $ TypeMismatch "string" notString
-- evaluation end