if let

if let permits patterns matching within the condition of an if statement. This allows us to reduce the overhead of certain kinds of pattern matches and express them in a more convenient way.

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:

# #![allow(unused_variables)]
#fn main() {
# let option = Some(5);
# fn foo(x: i32) { }
match option {
    Some(x) => { foo(x) },
    None => {},
}
#}

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

# #![allow(unused_variables)]
#fn main() {
# let option = Some(5);
# fn foo(x: i32) { }
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:

# #![allow(unused_variables)]
#fn main() {
# let option = Some(5);
# fn foo(x: i32) { }
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:

# #![allow(unused_variables)]
#fn main() {
# let option = Some(5);
# fn foo(x: i32) { }
# fn bar() { }
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:

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

Into code like this:

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