I was astonished that `typing.Maybe[T]` doesn't already exist, but if it
did exist, surely it would have to be `Union[T, None]`. Ah wait, that
would be spelled "Optional".
As for the pseudo-type `False` meaning "any falsey value", I don't think
that tracking the *value* of variables is something that the current
generation of type checkers are capable of. Consider:
text = input()
flag = "x" in text
We can infer that text is a string, but not whether it is a truthy
string or a falsey string; we can infer that flag is a bool, but not
whether it is True or False.
Adding variable annotations is no help, because the annotations have to
be added when we edit the code, but whether or not the values are truthy
or falsey isn't generally known until the code is run. And the type
checker runs at the intermediate time where the code is compiled:
1. Edit time.
2. Compile time (includes linters, the parser, etc).
3. Run time.
We can only annotate information known at stage 1; the type checker can
only check things known at stage 2; but the value of most things are
only known at stage 3.
--
Steve
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