fn

Declares a function, a method, or (written in an expression) an anonymous function.

A declaration lists typed parameters as Type name, an optional -> ReturnType and a body. Generic parameters go in <…> after the name. In an expression, fn (int x) => x + 1 is a lambda; it captures outer locals only when they are listed in use (…).

fn add(int a, int b) -> int {
    return a + b;
}

fn main() {
    let k = 10;
    let add_k = fn (int x) use (k) => x + k;
    let n = add_k(add(1, 2)); // 13
}

See also#

• let: Declares a mutable local variable.
• const: Declares a binding that cannot be reassigned: a local, a module constant, or a class field that is set only by new.
• static: Declares a module-level or class-level slot shared by the whole program, or a method without self.
• readonly: A read-only view of a value: nothing reachable through it can be changed.
• if: Runs a block when a bool condition is true.
• else: Introduces the branch of an if that runs when its condition is false (or its pattern does not match).
• while: Repeats a block while a bool condition is true.
• for: Iterates over the values of an array, Vec, range, coroutine or any IntoIterator.