Improve motivating example for Maybe

Bắt đầu bằng cách xác định phần tài liệu chứa ví dụ minh họa động lực hiện tại cho Maybe; issue không nêu tên file hoặc test nào. So sánh ví dụ với các phương án thay thế bằng Python đã nêu và làm rõ trường hợp sử dụng dự kiến. Công việc được hoàn tất khi tài liệu đưa ra một động lực thuyết phục mà không coi các giá trị trả về None mơ hồ là cách xử lý lỗi thông thường.

The current example motivating the use of Maybe is somewhat misleading because it solves a made-up problem:

Alleged original "python" code:

if user is not None:
     balance = user.get_balance()
     if balance is not None:
         credit = balance.credit_amount()
         if credit is not None and credit > 0:
             discount_program = choose_discount(credit)

Alleged "better" solution using Maybe:

discount_program: Maybe['DiscountProgram'] = Maybe.from_optional(
    user,
).bind_optional(  # This won't be called if `user is None`
    lambda real_user: real_user.get_balance(),
).bind_optional(  # This won't be called if `real_user.get_balance()` is None
    lambda balance: balance.credit_amount(),
).bind_optional(  # And so on!
    lambda credit: choose_discount(credit) if credit > 0 else None,
)

Usual python code solving this exact problem:

try:
    discount_program = choose_discount(user.get_balance().credit_amount())
except AttributeError:
    pass

The example is based on the very bad habit of signaling errors by return values, e.g. returning None.

No sane (python) developer would write a function that returns None in case of an error unless there is good reason for it, it is properly documented and returning None immediately and unambiguously tells the caller what went wrong. When exceptions occur, exceptions should be raised.

For example, credit_amount() returning None conveys no meaning at all. No credit? Credit amount == 0? Credit amount < 0? Raccoons taking over the world?

And even if one had to use flawed 3rd party code like this, there is a shorter and more concise version to handle this without Maybe.

I believe there is a legitimate use case for Maybe, but this is not it.