Is it possible to implement a generator function in a programming language without using the yield keyword?

Yes, a generator function in Python can be implemented without using the yield keyword, for example, by using a class that implements the iterator protocol. Such a class should have __iter__() and __next__() methods, which return the next element of the sequence on each call.

An example of implementing a Fibonacci number generator without yield:

class Fib:
    def __init__(self, n):
        self.n = n
        self.i = 0
        self.a, self.b = 0, 1

    def __iter__(self):
        return self

    def __next__(self):
        if self.i >= self.n:
            raise StopIteration
        self.i += 1
        a = self.a
        self.a, self.b = self.b, self.a + self.b
        return a

for num in Fib(5):
    print(num)

Thus, a generator can be manually implemented through an iterator, without yield. However, yield simplifies writing such functions.