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% if let | ||
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COMING SOON | ||
`if let` allows you to combine `if` and `let` together to reduce the overhead | ||
of certain kinds of pattern matches. | ||
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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: | ||
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```rust | ||
# let option = Some(5); | ||
# fn foo(x: i32) { } | ||
match option { | ||
Some(x) => { foo(x) }, | ||
None => {}, | ||
} | ||
``` | ||
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We don’t have to use `match` here, for example, we could use `if`: | ||
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```rust | ||
# let option = Some(5); | ||
# fn foo(x: i32) { } | ||
if option.is_some() { | ||
let x = option.unwrap(); | ||
foo(x); | ||
} | ||
``` | ||
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Neither of these options is particularly appealing. We can use `if let` to | ||
do the same thing in a nicer way: | ||
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```rust | ||
# let option = Some(5); | ||
# fn foo(x: i32) { } | ||
if let Some(x) = option { | ||
foo(x); | ||
} | ||
``` | ||
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If a [pattern][patterns] 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. | ||
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If you’d rather to do something else when the pattern does not match, you can | ||
use `else`: | ||
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```rust | ||
# let option = Some(5); | ||
# fn foo(x: i32) { } | ||
# fn bar() { } | ||
if let Some(x) = option { | ||
foo(x); | ||
} else { | ||
bar(); | ||
} | ||
``` | ||
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## `while let` | ||
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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: | ||
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```rust | ||
# let option: Option<i32> = None; | ||
loop { | ||
match option { | ||
Some(x) => println!("{}", x), | ||
_ => break, | ||
} | ||
} | ||
``` | ||
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Into code like this: | ||
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```rust | ||
# let option: Option<i32> = None; | ||
while let Some(x) = option { | ||
println!("{}", x); | ||
} | ||
``` | ||
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[patterns]: patterns.html |