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module Parser where
import Control.Applicative
newtype Parser a = Parser { runParser :: String -> Maybe (a, String) }
instance Functor Parser where
fmap f (Parser p) = Parser new_p
where new_p s = do
(x, s') <- p s
return (f x, s')
instance Applicative Parser where
pure x = Parser (\s -> Just (x, s))
(Parser p1) <*> (Parser p2) = Parser new_p
where new_p s = do
(f, s') <- p1 s
(x, s'') <- p2 s'
return (f x, s'')
-- (Parser p1) *> (Parser p2) = Parser new_p
-- (Parser p1) <* (Parser p2) = Parser new_p
instance Alternative Parser where
empty = Parser (\_ -> Nothing)
(Parser p1) <|> (Parser p2) = Parser new_p
where new_p s = p1 s <|> p2 s
data Equation
= EquationAdd
| EquationSub
| EquationMul
-- | EquationDiv ?
| EquationExp
| EquationEqu
| EquationNum Float
charP :: Char -> Parser Char
charP c = Parser f
where f "" = Nothing
f (c:cs) = Just (c, cs)
-- equationAddP :: Parser Equation
-- equationAddP = charP '+' *> EquationAdd
-- equationP :: Parser Equation
-- equationP =
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