Iterators

#032 Mar 2026

32. iter::successors

Need to generate a sequence where each element depends on the previous one? std::iter::successors turns any “next from previous” logic into a lazy iterator.

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use std::iter::successors;

// Powers of 10 that fit in a u32
let powers: Vec<u32> = successors(Some(1u32), |&n| n.checked_mul(10))
    .collect();

// [1, 10, 100, 1_000, 10_000, 100_000, 1_000_000, 10_000_000, 100_000_000, 1_000_000_000]
assert_eq!(powers.len(), 10);

You give it a starting value and a closure that computes the next element from a reference to the current one. Return None to stop — here checked_mul naturally returns None on overflow, so the iterator terminates on its own.

It works great for any recurrence:

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use std::iter::successors;

// Collatz sequence starting from 12
let collatz: Vec<u64> = successors(Some(12u64), |&n| match n {
    1 => None,
    n if n % 2 == 0 => Some(n / 2),
    n => Some(3 * n + 1),
}).collect();

assert_eq!(collatz, vec![12, 6, 3, 10, 5, 16, 8, 4, 2, 1]);

Think of it as unfold for when your state is the yielded value. Simple, lazy, and zero-allocation until you collect.