# Closures

Multiple lines:

let closure = |num| {

Single line:

let closure = |num| num + 2;
// or
thread::spawn(|| { let data = 500; });

Closures in variables do not need type specification, because they are private to functions and are not exposes outside. Instead, we allow the compiler to infer all that information. However, we can add types if we want to.

let closure = |num: i32| -> i32 {


Unannotated closures have to be called for the program to compile. The compiler cannot infer the types without seeing how a closure is used


An unannotated closure cannot be used with different types, such a program will not compile.

let c = |x| x;
c("abc"); // WRONG! The closure was already compiled for i32

# Caching Results

Sometimes it might be useful to define a closure that calculates some result that we want to reuse multiple times. We use structs for that. It's analogical to Lazy<T> of .NET C#.

struct Cacher<T>
  where T: Fn(u32) -> u32,
  calculation: T,
  value: Option<u32>,

impl<T> Cacher<T>
where T: Fn(u32) -> u32,
  fn new(calculation: T) -> Cacher<T> {
    Cacher {
      value: None,

  fn value(&mut self, arg: u32) -> u32 {
    match self.value {
      Some(v) => v,
      None => {
        let v = (self.calculation)(arg); // why so many parentheses?
        self.value = Some(v);

When we ask for a value(), the first time we do it, closure will be executed. Consecutive value() calls will return a cached value of the closure's result.

When defining closure as a field of a struct, we need to specify its type. In the case above, the type is Fn(u32) -> u32.

# Capturing Environment

Closures can capture variables defined outside of the closure:

fn main() {
  let x = 4;
  let equal_to_x = |z| z == x; // captures x
  let y = 4;

Functions cannot do that. Captures can be done in three ways, depending on the Fn... trait being used:

  • FnOnce - Instances of FnOnce can be called, but might not be callable multiple times. Because of this, if the only thing known about a type is that it implements FnOnce, it can only be called once, because it takes ownership of passed arguments.
  • FnMut - it mutably borrows external values. Sutrait of FnOnce.
  • Fn - it immutably borrows external values. Sutrait of FnOnce.

Closure traits

  • Closure can implement one, two, or all of these traits.
  • By default, all closures implement FnOnce.
  • We can force closure to take ownership by using the move keyword in front of the closure
  • when choosing the trait, it's a good idea to start from Fn and change it only if necessary

# Function Pointers

Functions can also be passed in place of closures. Function pointers implement all three closure traits: FnOnce, FnMut, Fn.

Last Updated: 11/28/2022, 9:27:43 AM