parseMaybe :: (String -> Maybe b) -> Parser b
parseMaybe f = do
    ident <- identifier
    case f ident of
        Nothing -> parserZero
        Just x -> return x

-- Parser
parseMaybe :: (String -> Maybe b) -> Parser b
parseMaybe f = do
    ident <- identifier
    case f ident of
        Nothing -> parserZero
        Just x -> return x

-- Parser
parseMaybe f = do
    ident <- identifier
    case f ident of
        Nothing -> parserZero
        Just x -> return x

-- Parser