> module Main where
> import System.IO
> import Data.Char
> import Control.Monad
> import System.Environment (getArgs)
> infixr 5 +++
>
> newtype Parser a = P (String -> [(a,String)])
>
> instance Monad Parser where
> return v = P (\inp -> [(v,inp)])
> p >>= f = P (\inp -> case parse p inp of
> [] -> []
> [(v,out)] -> parse (f v) out)
>
> instance MonadPlus Parser where
> mzero = P (\inp -> [])
> p `mplus` q = P (\inp -> case parse p inp of
> [] -> parse q inp
> [(v,out)] -> [(v,out)])
Basic parsers
-------------
> failure :: Parser a
> failure = mzero
>
> item :: Parser Char
> item = P (\inp -> case inp of
> [] -> []
> (x:xs) -> [(x,xs)])
>
> parse :: Parser a -> String -> [(a,String)]
> parse (P p) inp = p inp
Choice
------
> (+++) :: Parser a -> Parser a -> Parser a
> p +++ q = p `mplus` q
Derived primitives
------------------
> sat :: (Char -> Bool) -> Parser Char
> sat p = do x <- item
> if p x then return x else failure
>
> digit :: Parser Char
> digit = sat isDigit
>
> lower :: Parser Char
> lower = sat isLower
>
> upper :: Parser Char
> upper = sat isUpper
>
> letter :: Parser Char
> letter = sat isAlpha
>
> alphanum :: Parser Char
> alphanum = sat isAlphaNum
>
> char :: Char -> Parser Char
> char x = sat (== x)
>
> string :: String -> Parser String
> string [] = return []
> string (x:xs) = do char x
> string xs
> return (x:xs)
>
> many :: Parser a -> Parser [a]
> many p = many1 p +++ return []
>
> many1 :: Parser a -> Parser [a]
> many1 p = do v <- p
> vs <- many p
> return (v:vs)
>
> ident :: Parser String
> ident = do x <- lower
> xs <- many alphanum
> return (x:xs)
>
> nat :: Parser Int
> nat = do xs <- many1 digit
> return (read xs)
>
> int :: Parser Int
> int = do char '-'
> n <- nat
> return (-n)
> +++ nat
>
> space :: Parser ()
> space = do many (sat isSpace)
> return ()
Ignoring spacing
----------------
> token :: Parser a -> Parser a
> token p = do space
> v <- p
> space
> return v
>
> identifier :: Parser String
> identifier = token ident
>
> natural :: Parser Int
> natural = token nat
>
> integer :: Parser Int
> integer = token int
>
> symbol :: String -> Parser String
> symbol xs = token (string xs)
> expr :: Parser Int
> expr = do t <- term
> do symbol "+"
> e <- expr
> return (t+e)
> +++ return t
>
> term :: Parser Int
> term = do f <- factor
> do symbol "*"
> t <- term
> return (f * t)
> +++ return f
>
> factor :: Parser Int
> factor = do symbol "("
> e <- expr
> symbol ")"
> return e
> +++ natural
>
> eval :: String -> Int
> eval xs = case (parse expr xs) of
> [(n,[])] -> n
> [(_,out)] -> error ("unused input " ++ out)
> [] -> error "invalid input"
>
> main :: IO()
> main = do
> putStr "Calculate:"
> cal <- getLine;
> --result <- eval cal;
> putStrLn ( cal ++ " = " ++ show(eval( cal )))
