% if let
if let
permits [patterns][pattern] 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:
# 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
:
# 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:
# 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
:
# 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:
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);
}