Hi, Below is code I’m building up for simple monadic and applicative parsers from first principles. I’ve just added code for single line Haskell style comments. -- comment being '--' comment :: Parser () comment = do string "--" many (sat ( /='\n') ) return () — or as applicative comment' = string "--" *> many (sat (/='\n') ) I’d really appreciate help on writing a function for multi line comments! “{-“ and end with “-}” I know I need some sort of look ahead but can’t quite get the feel of how! Thanks Mike ====================================== import Control.Applicative import Data.Char newtype Parser a = P (String -> [(a,String)]) parse :: Parser a -> String -> [(a,String)] parse (P p) = p item :: Parser Char item = P (\s -> case s of [] -> [] (x:xs) -> [(x, xs)]) instance Functor Parser where -- fmap :: (a -> b) -> f a -> f b fmap g p = P (\inp -> case parse p inp of [] -> [] [(x, xs)] -> [(g x, xs)]) instance Applicative Parser where -- pure :: a -> Parser a -- (<*>) :: Parser (a -> b) -> Parser a -> Parser b pure x = P (\inp -> [(x, inp)]) pab <*> pa = P (\inp -> case parse pab inp of [] -> [] [(aTob, out)] -> parse (fmap aTob pa) out) three :: Parser (Char, Char) three = pure g <*> item <*> item <*> item where g x y z = (x, z) instance Monad Parser where -- return :: a -> Parser a return x = P (\inp -> [(x, inp)]) -- (>>=) :: Parser a -> (a -> Parser b) -> Parser b pa >>= f = P (\inp -> case parse pa inp of [] -> [] [(a, out)] -> parse (f a) out) instance Alternative Parser where -- empty :: Parser a empty = P (\inp -> []) -- (<|>) :: Parser a-> Parser a -> Parser a -- if p1 works the that one otherwise p2 p1 <|> p2 = P (\inp -> case parse p1 inp of [] -> parse p2 inp out -> out) sat :: (Char -> Bool) -> Parser Char sat p = do x <- item if p x then return x else empty 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 s@(x:xs) = do char x *> string xs *> return s ident :: Parser String ident = do x <- lower xs <- many alphanum return (x:xs) ident' :: Parser String ident' = pure (:) <*> lower <*> many alphanum nat :: Parser Int nat = do xs <- some digit return (read xs) nat' :: Parser Int nat' = pure read <*> some digit space :: Parser () space = do many (sat isSpace ) return () space' :: Parser () space' = many (sat isSpace ) *> return () int :: Parser Int int = do char '-' n <- nat return (-n) <|> nat int' :: Parser Int int' = char '-' *> pure ((-1)*) <*> nat <|> nat token :: Parser a -> Parser a token p = do space v <- p space return v token' :: Parser a -> Parser a token' p = space *> p <* space 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) nats :: Parser [Int] nats = do symbol "[" n <- natural ns <- many ( do symbol "," natural ) symbol "]" return (n:ns) -- comment being '--' comment :: Parser () comment = do string "--" many (sat ( /='\n') ) return () comment' = string "--" *> many (sat (/='\n') ) nats' :: Parser [Int] -- sequence and ignore "[" and the rest -- the rest is fmap list cons into result of natural and the apply that to many others and -- finally ignore the closing "]" nats' = symbol "[" >> (:) <$> natural <*> many (symbol "," >> natural) <* symbol "]"