A closure is a short, function-like value written with `| |` instead of a function’s `( )`. It often has no name: you can write one directly where a method needs it. You can also bind one to a variable and call it later. Inputs go between the bars, as in `|x: i32|`; a longer body goes inside `{ }`. section 8.3
fn main() {
let number_one = 6;
let number_two = 10;
let my_closure = || println!("{}", number_one + number_two);
my_closure();
}
The closure prints the sum of two nearby variables, even though its `||` declares no inputs. This is called capturing variables from its environment. section 8.3
Closures are especially handy in iterator methods. In `.map(|x| x + 1)`, the method supplies each item as `x`, and the closure says what to do with it. `.for_each()` likewise runs a closure for each item without making a new iterator. If a method supplies an argument you do not need, write `|_|`, not `||`: the closure must still accept that argument. section 8.2 section 8.3.3
How a closure uses a captured variable matters. `Fn` uses a regular reference; `FnMut` can change the captured value through a mutable reference; `FnOnce` takes a value and may therefore be callable only once. When a function accepts a closure, its signature specifies the needed trait and the closure’s inputs and output. For example, `F: FnMut(Self::Item) -> bool` requires a closure that accepts an iterator item and returns a Boolean. section 11.6.3 section 12.1
Why two identical-looking closures can fail
Suppose a function declares both of its closure arguments as the same generic type `F`. Passing it two separately written `|| 9` closures fails: each closure has its own type, even though both return an `i32`. Give the arguments separate generic types, such as `F` and `G`, each with an `Fn() -> i32` bound. section 12.1.3
A function can return a closure too, using a return type such as `impl FnMut(i32) -> i32`. Start with the simple question—what inputs does the closure take, and what does it return?—then choose the closure trait that fits how it uses captured values. section 12.2.2 section 12.6
