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Simplify and clarify match guards

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I feel like the precedence issues are a bit too nuanced and not worth
spending time on. Match guards can be useful, though.

Fixes #115 at this point, in my view.
This commit is contained in:
Carol (Nichols || Goulding) 2016-06-29 21:58:00 -06:00
parent 9e292dfa6d
commit b113cc9014
1 changed files with 6 additions and 25 deletions
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31
src/ch06-04-patterns.md
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@ -175,7 +175,8 @@ println!("name is: {:?}", name);
## Guards ## Guards
You can introduce ‘match guards’ with `if`: You can introduce "match guards" with `if`. This adds an extra condition that
often uses a value that the pattern has bound to:
```rust ```rust
let x = Some(5); let x = Some(5);
@ -187,30 +188,10 @@ match x {
} }
``` ```
If you’re using if with multiple patterns, the if applies to both sides: In this case, we bound the inner value of a `Some` to `x` and then "guarded" the
first match arm with an additional condition that `x` must be less than 5. In
```rust this case, `Some(5)` does not have an inner value that is less than 5, so this
let x = 4; code will just print out `5`.
let y = false;
match x {
4 | 5 if y => println!("yes"),
_ => println!("no"),
}
```
This prints `no`, because the if applies to the whole of `4 | 5`, and not to only
the `5`. In other words, the precedence of if behaves like this:
```text
(4 | 5) if y => ...
```
not this:
```text
4 | (5 if y) => ...
```
Whew! That’s a lot of different ways to match things. Let's cover one more place Whew! That’s a lot of different ways to match things. Let's cover one more place
you can use your newfound knowledge of patterns: `if let`. you can use your newfound knowledge of patterns: `if let`.