Improve motivating example for Maybe

Inizia individuando la sezione della documentazione che contiene l’attuale esempio motivazionale di Maybe; nell’issue non sono indicati né un file né un test. Confronta l’esempio con le alternative Python dichiarate e chiarisci il caso d’uso previsto. Il lavoro è completato quando la documentazione presenta una motivazione credibile senza trattare i ritorni ambigui di None come una normale gestione degli errori.

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.