Lazy sequence generation in Rust

How can I create what other languages call a lazy sequence or a "generator" function?

In Python, I can use yield as in the following example (from Python's docs) to lazily generate a sequence that is iterable in a way that does not use the memory of an intermediary list:

# a generator that yields items instead of returning a list
def firstn(n):
    num = 0
    while num < n:
        yield num
        num += 1

sum_of_first_n = sum(firstn(1000000))

How can I do something similar in Rust?


  • Rust 1.0 does not have generator functions, so you'd have to do it manually with explicit iterators.

    First, rewrite your Python example as a class with a next() method, since that is closer to the model you're likely to get in Rust. Then you can rewrite it in Rust with a struct that implements the Iterator trait.

    You might also be able to use a function that returns a closure to achieve a similar result, but I don't think it would be possible to have that implement the Iterator trait (since it would require being called to generate a new result).