1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
|
# `Option` & `unwrap`
In the last example, we showed that we can induce program failure at will.
We told our program to `panic` if we drink a sugary lemonade.
But what if we expect _some_ drink but don't receive one?
That case would be just as bad, so it needs to be handled!
We *could* test this against the null string (`""`) as we do with a lemonade.
Since we're using Rust, let's instead have the compiler point out cases
where there's no drink.
An `enum` called `Option<T>` in the `std` library is used when absence is a
possibility. It manifests itself as one of two "options":
* `Some(T)`: An element of type `T` was found
* `None`: No element was found
These cases can either be explicitly handled via `match` or implicitly with
`unwrap`. Implicit handling will either return the inner element or `panic`.
Note that it's possible to manually customize `panic` with [expect][expect],
but `unwrap` otherwise leaves us with a less meaningful output than explicit
handling. In the following example, explicit handling yields a more
controlled result while retaining the option to `panic` if desired.
```rust,editable,ignore,mdbook-runnable
// The adult has seen it all, and can handle any drink well.
// All drinks are handled explicitly using `match`.
fn give_adult(drink: Option<&str>) {
// Specify a course of action for each case.
match drink {
Some("lemonade") => println!("Yuck! Too sugary."),
Some(inner) => println!("{}? How nice.", inner),
None => println!("No drink? Oh well."),
}
}
// Others will `panic` before drinking sugary drinks.
// All drinks are handled implicitly using `unwrap`.
fn drink(drink: Option<&str>) {
// `unwrap` returns a `panic` when it receives a `None`.
let inside = drink.unwrap();
if inside == "lemonade" { panic!("AAAaaaaa!!!!"); }
println!("I love {}s!!!!!", inside);
}
fn main() {
let water = Some("water");
let lemonade = Some("lemonade");
let void = None;
give_adult(water);
give_adult(lemonade);
give_adult(void);
let coffee = Some("coffee");
let nothing = None;
drink(coffee);
drink(nothing);
}
```
[expect]: https://doc.rust-lang.org/std/option/enum.Option.html#method.expect
|
