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TRPL editing: match

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Steve Klabnik 2015-04-18 15:32:26 -04:00
parent 213708867e
commit 836c8a826b
1 changed files with 16 additions and 15 deletions
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src/doc/trpl/match.md
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@ -1,10 +1,8 @@
% Match % Match
Often, a simple `if`/`else` isn’t enough, because you have more than two Often, a simple [`if`][if]/`else` isn’t enough, because you have more than two
possible options. Also, `else` conditions can get incredibly complicated, so possible options. Also, conditions can get quite complex. Rust
what’s the solution? has a keyword, `match`, that allows you to replace complicated `if`/`else`
Rust has a keyword, `match`, that allows you to replace complicated `if`/`else`
groupings with something more powerful. Check it out: groupings with something more powerful. Check it out:
```rust ```rust
@ -20,16 +18,18 @@ match x {
} }
``` ```
`match` takes an expression and then branches based on its value. Each *arm* of [if]: if.html
`match` takes an expression and then branches based on its value. Each ‘arm’ of
the branch is of the form `val => expression`. When the value matches, that arm’s the branch is of the form `val => expression`. When the value matches, that arm’s
expression will be evaluated. It’s called `match` because of the term ‘pattern expression will be evaluated. It’s called `match` because of the term ‘pattern
matching’, which `match` is an implementation of. There’s an [entire section on matching’, which `match` is an implementation of. There’s an [entire section on
patterns][patterns] coming up next, that covers all the options that fit here. patterns][patterns] that covers all the patterns that are possible here.
[patterns]: patterns.html [patterns]: patterns.html
So what’s the big advantage here? Well, there are a few. First of all, `match` So what’s the big advantage? Well, there are a few. First of all, `match`
enforces *exhaustiveness checking*. Do you see that last arm, the one with the enforces ‘exhaustiveness checking’. Do you see that last arm, the one with the
underscore (`_`)? If we remove that arm, Rust will give us an error: underscore (`_`)? If we remove that arm, Rust will give us an error:
```text ```text
@ -37,11 +37,12 @@ error: non-exhaustive patterns: `_` not covered
``` ```
In other words, Rust is trying to tell us we forgot a value. Because `x` is an In other words, Rust is trying to tell us we forgot a value. Because `x` is an
integer, Rust knows that it can have a number of different values – for example, integer, Rust knows that it can have a number of different values – for
`6`. Without the `_`, however, there is no arm that could match, and so Rust refuses example, `6`. Without the `_`, however, there is no arm that could match, and
to compile. `_` acts like a ‘catch-all arm’. If none of the other arms match, so Rust refuses to compile the code. `_` acts like a ‘catch-all arm’. If none
the arm with `_` will, and since we have this catch-all arm, we now have an arm of the other arms match, the arm with `_` will, and since we have this
for every possible value of `x`, and so our program will compile successfully. catch-all arm, we now have an arm for every possible value of `x`, and so our
program will compile successfully.
`match` is also an expression, which means we can use it on the right-hand `match` is also an expression, which means we can use it on the right-hand
side of a `let` binding or directly where an expression is used: side of a `let` binding or directly where an expression is used:
@ -59,4 +60,4 @@ let numer = match x {
}; };
``` ```
Sometimes, it’s a nice way of converting things. Sometimes it’s a nice way of converting something from one type to another.