Option
& unwrap
In the last example, we showed that we can induce program failure at will.
We told our program to panic
if the princess received an inappropriate
gift - a snake. But what if the princess expected a gift and didn’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 snake.
Since we’re using Rust, let’s instead have the compiler point out cases
where there’s no gift.
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 typeT
was foundNone
: No element was found
These cases can either be explicitly handled via match
or implicitly withunwrap
. Implicit handling will either return the inner element or panic
.
Note that it’s possible to manually customize panic
with 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.
// The commoner has seen it all, and can handle any gift well.
// All gifts are handled explicitly using `match`.
fn give_commoner(gift: Option<&str>) {
// Specify a course of action for each case.
match gift {
Some("snake") => println!("Yuck! I'm throwing that snake in a fire."),
Some(inner) => println!("{}? How nice.", inner),
None => println!("No gift? Oh well."),
}
}
// Our sheltered princess will `panic` at the sight of snakes.
// All gifts are handled implicitly using `unwrap`.
fn give_princess(gift: Option<&str>) {
// `unwrap` returns a `panic` when it receives a `None`.
let inside = gift.unwrap();
if inside == "snake" { panic!("AAAaaaaa!!!!"); }
println!("I love {}s!!!!!", inside);
}
fn main() {
let food = Some("cabbage");
let snake = Some("snake");
let void = None;
give_commoner(food);
give_commoner(snake);
give_commoner(void);
let bird = Some("robin");
let nothing = None;
give_princess(bird);
give_princess(nothing);
}