Let Control Flow

Rust has a few control flow constructs which differ from other languages. They are used for pattern matching:

• if let expressions
• while let expressions
• match expressions

if-let expressions

The if let expression lets you execute different code depending on whether a value matches a pattern:

use std::{time::Duration, thread::sleep};

fn sleep_for(secs: f32) {
    let dur = if let Ok(dur) = Duration::try_from_secs_f32(secs) {
        dur
    } else {
        Duration::from_millis(500)
    };
    sleep(dur);
    println!("slept for {:?}", dur);
}

fn main() {
    sleep_for(-10.0);
    sleep_for(0.8);
}

let-else

For the common case of matching a pattern and returning from the function, use let else. The “else” case must diverge (return, break, or panic - anything but falling off the end of the block).

use std::{time::Duration, thread::sleep};

fn sleep_for(secs: f32) {
    let Ok(dur) = Duration::try_from_secs_f32(secs) else {
        println!("Invalid seconds value!");
        return;
    };
    sleep(dur);
    println!("slept for {:?}", dur);
}

fn main() {
    sleep_for(-10.0);
    sleep_for(0.8);
}

while-let

Like with if let, there is a while let variant which repeatedly tests a value against a pattern:

fn main() {
    let mut name = String::from("Comprehensive Rust 🦀");
    while let Some(c) = name.pop() {
        println!("character: {c}");
    }
    // (There are more efficient ways to reverse a string!)
}

Here String::pop returns Some(c) until the string is empty, after which it will return None. The while let lets us keep iterating through all items.