ParserBox.hs

data ParserBox a = ParserBox ParserBoxInfo (Either String (Parser a))
data ParserBoxInfo = ParserBoxInfo (Maybe String)

defaultParserBoxInfo :: ParserBoxInfo
defaultParserBoxInfo = ParserBoxInfo Nothing

instance Functor ParserBox where
    fmap _ (ParserBox i (Left pe)) = ParserBox i $ Left pe
    fmap f (ParserBox i (Right x)) = ParserBox i $ Right $ fmap f x

instance Applicative ParserBox where
    pure = ParserBox defaultParserBoxInfo . Right . pure
    (ParserBox i (Left pe)) <*> _ = ParserBox i $ Left pe
    _ <*> (ParserBox i (Left pe)) = ParserBox i $ Left pe
    p1@(ParserBox i1 (Right f)) <*> p2@(ParserBox i2 (Right a)) =
        case mergeParserBoxInfos i1 i2 of
            (Left pe)       -> ParserBox i2 $ Left pe
            (Right mergedi) -> ParserBox mergedi $ Right $ (f <*> a)

instance Monad ParserBox where
    (ParserBox i (Left pe)) >>= _ = ParserBox i $ Left pe
    (ParserBox i1 (Right ba)) >>= k = ?

mergeParserBoxInfos :: ParserBoxInfo -> ParserBoxInfo -> (Either String ParserBoxInfo)
mergeParserBoxInfos p1@(ParserBoxInfo ind1) p2@(ParserBoxInfo ind2)
    | isNothing ind1 = Right p2
    | isNothing ind2 = Right p1
    | ind1 == ind2 = Right p1
    | otherwise = Left "different indentation styles mixed"