What do four consecutive vertical lines mean?

Question

In the following code snippet:

let read_pool = ReadPool::new(
    "readpool",
    &Config::default_for_test(),
    || || Context {}
);

What are the four vertical lines || ||?

Answer 1

|| represents a lambda taking no arguments.

fn main() {
    let a = || println!("hello");
    let b = || { println!("world") };

    a(); // hello
    b(); // world
}

Two together is a lambda returning another lambda:

fn main() {
    let a = || || println!("hello world");

    let c = a();
    c(); // hello world
}

Answer 2

In Rust, || introduces a lambda (an unnamed function).

The arguments of the lambda are specified in between the two:

• |a, b| a + b: a lambda taking 2 arguments a and b and returning their sum,
• || println!("Hello, World"): a lambda taking no argument and printing "Hello, World!".

Since the body of a lambda is just an expression, it can be another lambda:

• || || println!("Hello, World"): a lambda taking no argument and returning a lambda taking no argument and printing "Hello, World!".

Therefore, || || Context {} is simply:

• a lambda taking no argument and returning || Context {},
• which is a lambda taking no argument and returning an instance of Context.