identifier :: Parser String
identifier = lexeme $ do
    x <- letter <|> char '_'
    xs <- many (alphaNum <|> char '_')
    return $ x:xs
identifier :: Parser String
identifier = lexeme $ do
    x <- letter <|> char '_'
    xs <- many (alphaNum <|> char '_')
    return $ x:xs
identifier = lexeme $ do
    x <- letter <|> char '_'
    xs <- many (alphaNum <|> char '_')
    return $ x:xs