Closures give us a way to quickly define small, anonymous functions, which optionally grab some of the variables defined in the outer scope. That's where the name comes from: the variables from the outer scope are "closed over".

Threads are often launched via closures due to their terser syntax and features. The syntax should be familiar to any Ruby programmers, but there are a few Rusty additions.

In a simple form, closures are semantically identical to functions. Here are four similar functions or closures that take and return a value, and two that take no parameters:

fn square(x: u32) -> u32 {
 x * x
}

fn function_without_vars() {
 println!("Entered function without variables");
}

fn main() {
 let square_c1 = |x: u32| x*x;
 let square_c2 = |x: u32| { x*x };
 let square_c3 = |x: u32| -> u32 { x*x };

 let closure_without_vars = || println!("Entered closure without variables");

 println!("square of 4 = {}", square(4));
 println!("square of 4 = {}", square_c1(4));
 println...