if let

if let allows you to combine if and let together to reduce the overhead of certain kinds of pattern matches.

For example, let’s say we have some sort of Option<T>. We want to call a function on it if it’s Some<T>, but do nothing if it’s None. That looks like this:

match option {
    Some(x) => { foo(x) },
    None => {},
}

We don’t have to use match here, for example, we could use if:

if option.is_some() {
    let x = option.unwrap();
    foo(x);
}

Neither of these options is particularly appealing. We can use if let to do the same thing in a nicer way:

if let Some(x) = option {
    foo(x);
}

If a pattern matches successfully, it binds any appropriate parts of the value to the identifiers in the pattern, then evaluates the expression. If the pattern doesn’t match, nothing happens.

If you want to do something else when the pattern does not match, you can use else:

if let Some(x) = option {
    foo(x);
} else {
    bar();
}

while let

In a similar fashion, while let can be used when you want to conditionally loop as long as a value matches a certain pattern. It turns code like this:

let mut v = vec![1, 3, 5, 7, 11];
loop {
    match v.pop() {
        Some(x) =>  println!("{}", x),
        None => break,
    }
}

Into code like this:

let mut v = vec![1, 3, 5, 7, 11];
while let Some(x) = v.pop() {
    println!("{}", x);
}