[FIXED] What is | | symbol in function call?

Issue

While going through rust documentation I found below code

let k = 21;
let x : Result<_, &str> = Ok("foo");
assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);

What exactly is |e| & |v| in last line ? why it is required?

Solution

The .map_or_else functions take a closure as a parameter. A closure is an anonymous function that can optionally capture the environment where they are defined. The syntax to define a closure is

let c = |x| println!(x + 2);

This takes 1 parameters which is mapped to the x variable. Calling c(2) will print 4, calling c(4) will print 6.

Similarly |e| k * 2 and |v| v.len() is also a closure. They also take 1 parameter, e and k which are the parameter names whose values will be filled by the .map_or_else function.

Answered By - Arijit Dey

Answer Checked By - Katrina (PHPFixing Volunteer)