Function futures_util::stream::repeat_with

pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F>

Creates a new stream that repeats elements of type A endlessly by applying the provided closure, the repeater, F: FnMut() -> A.

The repeat_with() function calls the repeater over and over again.

Infinite stream like repeat_with() are often used with adapters like [stream.take()], in order to make them finite.

If the element type of the stream you need implements [Clone], and it is OK to keep the source element in memory, you should instead use the [stream.repeat()] function.

Examples

Basic usage:

use futures::stream::{self, StreamExt};

// let's assume we have some value of a type that is not `Clone`
// or which don't want to have in memory just yet because it is expensive:
#[derive(PartialEq, Debug)]
struct Expensive;

// a particular value forever:
let mut things = stream::repeat_with(|| Expensive);

assert_eq!(Some(Expensive), things.next().await);
assert_eq!(Some(Expensive), things.next().await);
assert_eq!(Some(Expensive), things.next().await);

Using mutation and going finite:

use futures::stream::{self, StreamExt};

// From the zeroth to the third power of two:
let mut curr = 1;
let mut pow2 = stream::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
                    .take(4);

assert_eq!(Some(1), pow2.next().await);
assert_eq!(Some(2), pow2.next().await);
assert_eq!(Some(4), pow2.next().await);
assert_eq!(Some(8), pow2.next().await);

// ... and now we're done
assert_eq!(None, pow2.next().await);