Interest check: implement __bool__ for Result and Maybe

この issue では、具体的なファイルやテストは指定されていません。まず Result と Maybe の実装、およびそれらに既存する型の絞り込みテストを見つけ、次に Success と Failure がどのように区別されているかを確認してください。Done には、両方の型について合意された bool の動作、明らかになった型のカバレッジ、既存の真偽値に関する前提との互換性の検討を含める必要があります。

Long before I found Returns, I implemented my own Result monad from scratch. It was a lot of fun, and right now it coexists with Returns as the library it's a part of is being incrementally deprecated. At some point, I decided it would be nice if I could do things like this:

result = some_function()  # Result[S, F]
if result:
    reveal_type(result)  # Success[S]
else:
    reveal_type(result)  # Failure[F]

It's worked great. I have my own analogue to is_successful, but with __bool__ I don't even have to import it. Also, it actually works better than my functions: my analogue is a pair of TypeGuard functions (TypeIs didn't exist yet), so even after narrowing the container type in the first if, the else would have no type information about the container. Cf., __bool__(self) -> Literal[False] and __bool__(self) -> Literal[True] make type narrowing work perfectly, and it feels very pythonic, imo.

As I update more and more code to use Returns, instead, I find this to be the one thing about my library that I actually miss. There are a lot of cases where I could reach for match for something as ergonomic, but most of the time I'm only interested in Success vs. Failure; match is overkill if I don't need to peak at the contained value.

The one drawback I anticipate to adding this feature would be the possibility of breaking any code that's relying on the default object.__bool__ behavior to differentiate a Failure from eg. a None... but hopefully, anybody using this library would have their Optionals wrapped in a Maybe already?