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{-# LANGUAGE ConstraintKinds, GADTs, LambdaCase, RankNTypes #-}
module Parser where
import Control.Applicative
import Control.Monad
import Control.Selective
-- See Section 7.2 of the paper: https://dl.acm.org/doi/10.1145/3341694.
newtype Parser a = Parser { parse :: String -> [(a, String)] }
instance Functor Parser where
fmap f p = Parser $ \x -> [ (f a, s) | (a, s) <- parse p x ]
instance Applicative Parser where
pure a = Parser $ \s -> [(a, s)]
(<*>) = ap
instance Alternative Parser where
empty = Parser (const [])
p <|> q = Parser $ \s -> parse p s ++ parse q s
instance Selective Parser where
select = selectM
instance Monad Parser where
return = pure
p >>= f = Parser $ \x -> concat [ parse (f a) y | (a, y) <- parse p x ]
class MonadZero f where
zero :: f a
instance MonadZero Parser where
zero = Parser (const [])
item :: Parser Char
item = Parser $ \case
"" -> []
(c:cs) -> [(c,cs)]
sat :: (Char -> Bool) -> Parser Char
sat p = do { c <- item; if p c then pure c else zero }
char :: Char -> Parser Char
char c = sat (==c)
string :: String -> Parser String
string [] = pure ""
string (c:cs) = do
_ <- char c
_ <- string cs
pure (c:cs)
bin :: Parser Int
bin = undefined
hex :: Parser Int
hex = undefined
numberA :: Parser Int
numberA = (string "0b" *> bin) <|> (string "0x" *> hex)
numberS :: Parser Int
numberS = string "0" *> ifS (('b'==) <$> sat (`elem` "bx")) bin hex
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