How is generator typing implemented in programming languages?

In Python, generator typing is done using the typing module, specifically through the Generator or Iterator types. The Generator type allows you to specify the types of values that the generator yields when called with yield, the type of value that can be sent to the generator via send(), and the type of value returned when the generator terminates.

Example of typing a generator that yields integers and does not accept values via send:

from typing import Generator

def count_up_to(max_value: int) -> Generator[int, None, None]:
    count = 0
    while count < max_value:
        yield count
        count += 1

Here, Generator[int, None, None] means the generator yields int, does not accept values via send (type None), and returns None upon completion.

If the generator accepts values via send, for example:

def echo() -> Generator[str, str, None]:
    received = yield "start"
    while True:
        received = yield received

Here, Generator[str, str, None] means the generator yields strings, accepts strings via send, and returns None.