Laziness, peekable and scan in Rust

Adapters do nothing until something pulls — which is what makes peeking and scanning cheap.

fn main() {
    let mut it = [1, 2, 3].iter().peekable();
    while let Some(n) = it.next() {
        if let Some(next) = it.peek() {
            println!("{n} then {next}");
        } else {
            println!("{n} is last");
        }
    }

    let running: Vec<i32> = [5, 10, 20]
        .iter()
        .scan(0, |total, n| {
            *total += n;
            Some(*total)
        })
        .collect();
    println!("{:?}", running);
}

How it works

1. Nothing runs until a terminal method asks for values.
2. peekable looks at the next item without consuming it.
3. scan carries state along, emitting one value per step.

Keywords and builtins used here

• Some
• Vec
• else
• fn
• i32
• if
• let
• main
• mut
• while

The run, in numbers

Lines

19

Characters to type

321

Tokens

128

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 77 seconds.

Type this snippet

Step 3 of 3 in Closures & folding, step 8 of 14 in Iterators & closures.

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