gen

Declares a generator function (a coroutine): gen fn name(…) { … }.

Calling it allocates a suspended coroutine handle and runs nothing. Each resume runs it to the next yield (or its return), and for can iterate it. gen is only a keyword directly before fn (or static fn).

gen fn countdown(int n) {
    let i = n;
    while i > 0 {
        yield i;
        i = i - 1;
    }
}

fn main() {
    let total = 0;
    for v in countdown(3) {
        total = total + v;
    }
}

See also#

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