Code shared from the Rust Playground

playground.rs

//! Examples for `unwrap_or_else()`
use std::process;


fn func(in_num: u8) -> Option<&'static str> {
    if in_num % 2 != 0 {
        return None;
    }
    Some("even")
}

fn func_result(in_num: u8) -> Result<&'static str, &'static str> {
    if in_num % 2 != 0 {
        return Err("Can't do this because input is odd...");
    }
    Ok("A good solid return...")
}

fn main() -> Result<(), &'static str> {
    // Deal with a function that returns an Option
    let a = func(3)
        .unwrap_or_else(|| { "default" });
    let b = func(2)
        .unwrap_or_else(|| { "default" });
    println!("a = {}, b = {}", a, b);
    
    // Deal with a function that returns a Result
    let c = func_result(3)
        .unwrap_or_else(|e| {
            println!("Error: {}", e);
            // Let's assume that an Err from func_result should end programme.
            process::exit(1);
            // You could return a default here...e.g. `return "default";`
            // It is expected that this closure returns the same type that it
            // received.
        });
    let d = func_result(2)
        .unwrap_or_else(|e| {
            println!("Error: {}", e);
            // Let's assume that an Err from func_result should end programme.
            process::exit(1);
            // return "default";
        });
    
    println!("c = {}, d = {}", c, d);
    Ok(())
}