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 => {}, }Run
We don’t have to use match here, for example, we could use if:
if option.is_some() { let x = option.unwrap(); foo(x); }Run
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); }Run
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(); }Run
while letIn 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, } }Run
Into code like this:
let mut v = vec![1, 3, 5, 7, 11]; while let Some(x) = v.pop() { println!("{}", x); }Run