How to Use Closures for Lazy Evaluation in Rust

Use a closure with unwrap_or_else to defer expensive computation until the value is actually needed. This pattern ensures the closure body only executes if the Option is None, achieving lazy evaluation.

let result: Option<i32> = None;
let value = result.unwrap_or_else(|| {
    println!("Computing...");
    42
});

In this example, println! only runs if result is None. If result were Some(10), the closure would never execute.

Lazy evaluation means you only do work when you absolutely have to. Think of it like a vending machine that only starts the motor to drop a snack after you've inserted money and pressed a button, rather than dropping snacks immediately when you walk by. In Rust, closures let you write code that waits to run until a specific condition triggers it.