Rust
Fermetures et expressions lambda

Expressions lambda simples

// A simple adder function defined as a lambda expression. 
// Unlike with regular functions, parameter types often may be omitted because the
// compiler can infer their types
let adder = |a, b| a + b;
// Lambdas can span across multiple lines, like normal functions.
let multiplier = |a: i32, b: i32| {
    let c = b;
    let b = a;
    let a = c;
    a * b
};

// Since lambdas are anonymous functions, they can be called like other functions
println!("{}", adder(3, 5));
println!("{}", multiplier(3, 5));

Cela affiche:

8
15

Fermetures simples

Contrairement aux fonctions régulières, les expressions lambda peuvent capturer leurs environnements. Ces lambda sont appelées fermetures.

// variable definition outside the lambda expression...
let lucky_number: usize = 663;

// but the our function can access it anyway, thanks to the closures
let print_lucky_number = || println!("{}", lucky_number);

// finally call the closure
print_lucky_number();

Cela va imprimer:

663

Lambdas avec des types de retour explicites

// lambda expressions can have explicitly annotated return types
let floor_func = |x: f64| -> i64 { x.floor() as i64 };

Passer des lambdas autour

Comme les fonctions lambda sont des valeurs elles-mêmes, vous les stockez dans des collections, vous les transmettez à des fonctions, etc., comme vous le feriez avec d’autres valeurs.

// This function takes two integers and a function that performs some operation on the two arguments
fn apply_function<T>(a: i32, b: i32, func: T) -> i32 where T: Fn(i32, i32) -> i32 {
    // apply the passed function to arguments a and b
    func(a, b)
}

// let's define three lambdas, each operating on the same parameters
let sum = |a, b| a + b;
let product = |a, b| a * b;
let diff = |a, b| a - b;

// And now let's pass them to apply_function along with some arbitary values
println!("3 + 6 = {}", apply_function(3, 6, sum));
println!("-4 * 9 = {}", apply_function(-4, 9, product));
println!("7 - (-3) = {}", apply_function(7, -3, diff));

Cela va imprimer:

3 + 6 = 9
-4 * 9 = -36
7 - (-3) = 10

Retour de lambda de fonctions

Renvoyer des lambdas (ou des fermetures) à partir de fonctions peut être délicat car ils implémentent des traits et leur taille exacte est donc rarement connue.

// Box in the return type moves the function from the stack to the heap
fn curried_adder(a: i32) -> Box<Fn(i32) -> i32> {
    // 'move' applies move semantics to a, so it can outlive this function call
    Box::new(move |b| a + b)
}

println!("3 + 4 = {}", curried_adder(3)(4));

Cela affiche: 3 + 4 = 7