Closure syntax

Define closures with the |param| body syntax.

Closure syntax

A closure is an anonymous function that can be assigned to a variable or passed to a function. Parameters go between vertical bars; a single-expression body needs no braces:

let kare = |x: i32| x * x;
let topla = |a: i32, b: i32| a + b;
let selam = || println!("merhaba");
let uzun = |x: i32| {
    let y = x * 2;
    y + 1
};

Closures are called like normal functions: kare(4). Their difference from functions is that they can capture variables from the environment where they are defined.

Comparison with functions

A function and a closure doing the same job:

fn kare_fn(x: i32) -> i32 { x * x }
let kare_cl = |x: i32| -> i32 { x * x };
let kare_kisa = |x| x * x;          // types are inferred

Types are mandatory in function signatures because they are part of an API; closures are usually short-lived and local, so their types can be inferred. Closures that capture nothing can also convert to a fn pointer.

Exercise First closures

Define the closures kare (|x: i32| x * x) and topla (|a: i32, b: i32| a + b). Print the results of kare(4) and topla(2, 3) separated by a space.

Show example solution

fn main() {
    let kare = |x: i32| x * x;
    let topla = |a: i32, b: i32| a + b;
    println!("{} {}", kare(4), topla(2, 3));
}

Test yourself

How is a closure without parameters written?
- () => expression
- fn() expression
- |_| expression
- || expression
Show answer

|| expression — || is an empty parameter list; |_| takes one parameter and ignores it.

Parameter list

let kare = ____ x * x;

Show answer

let kare = |x: i32| x * x;

Closure and result

(|x: i32| x * x)(4)

16
(|a: i32, b: i32| a + b)(2, 3)

5
(|| 7)()

7
(|x: i32| x - 1)(0)

-1

Next lesson: Type inference

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