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-- Write a many parser, with type Parser a -> Parser [a]. It should apply a
-- parser until it fails.
-- Control.Monad.Error is deprecated. Control.Monad.Except should be used
-- instead. I modified the code from examples here to use it.
{-- From examples/examples/ch19/ParseInt.hs and modified --}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Module_19_b_1 where
import Control.Monad.Except
import Control.Monad
import Control.Monad.State
import qualified Data.ByteString.Char8 as B
data ParseError = NumericOverflow
| EndOfInput
| Chatty String
deriving (Eq, Ord, Show)
newtype Parser a = P {
runP :: ExceptT ParseError (State B.ByteString) a
} deriving (Functor, Applicative, Monad, MonadError ParseError)
liftP :: State B.ByteString a -> Parser a
liftP m = P (lift m)
satisfy :: (Char -> Bool) -> Parser Char
satisfy p = do
s <- liftP get
case B.uncons s of
Nothing -> throwError EndOfInput
Just (c, s')
| p c -> liftP (put s') >> return c
| otherwise -> throwError (Chatty "satisfy failed")
runParser :: Parser a -> B.ByteString
-> Either ParseError (a, B.ByteString)
runParser p bs = case runState (runExceptT (runP p)) bs of
(Left err, _) -> Left err
(Right r, bs) -> Right (r, bs)
optional :: Parser a -> Parser (Maybe a)
optional p = (Just `liftM` p) `catchError` \_ -> return Nothing
{-- End of code from examples --}
many :: Parser a -> Parser [a]
many p = ((:) <$> p <*> many p) `catchError` \_ -> return []
-- ghci> :l Module_19_b_1.hs
-- [1 of 1] Compiling Module_19_b_1 ( Module_19_b_1.hs, interpreted )
-- Ok, one module loaded.
-- ghci> :m +Data.Char
-- ghci> runParser (many (satisfy isDigit)) (B.pack "")
-- Right ("","")
-- ghci> runParser (many (satisfy isDigit)) (B.pack "abc")
-- Right ("","abc")
-- ghci> runParser (many (satisfy isDigit)) (B.pack "9abc")
-- Right ("9","abc")
-- ghci> runParser (many (satisfy isDigit)) (B.pack "987abc")
-- Right ("987","abc")
-- ghci> runParser (many (satisfy isDigit)) (B.pack "x987abc")
-- Right ("","x987abc")
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