Control Flow: The "Expression Engine"

In JavaScript, if/else is a Statement (it does something). In Rust, if/else and match are Expressions (they evaluate to a value).

Think of Rust’s control flow as a Logic Engine that processes conditions and returns direct results, acting more like a “Turn-Key” switch than a simple “Yes/No” gate.

🏗️ The 3-Step Narrative

1. if/else as Data

In JS, you use const x = condition ? a : b. In Rust, the entire if block is the value itself.

fn main() {
    let condition = true;
    let number = if condition { 5 } else { 6 }; // 🪄 if/else is the value

    println!("The value is: {number}");
}

IMPORTANT: The Rule

Because it’s a typed contract, both the if and the else blocks must return the same data type. You can’t return a number in one and a string in the other.

2. match: The “Exhaustive” Engine

While JS uses switch, Rust uses match. This is a strict Exhaustiveness Contract. The compiler literally forces you to handle every possible case (including _ for “anything else”).

fn main() {
    let number = 3;

    match number {
        1 => println!("One!"),
        2 | 3 | 5 | 7 | 11 => println!("This is a prime"),
        13..=19 => println!("A teen"),
        _ => println!("Something else"), // 🔮 Exhaustive default
    }
}

NOTE: The Magic

This exhaustiveness is a superpower when combined with Structs and Enums. It prevents the “Unhandled State” bugs that often crash frontend apps (like forgetting a Loading or Error state).

3. Loop Power: Returning Data from Loops

Rust takes loops to a whole new level. You can actually return a value directly from a loop once it breaks!

fn main() {
    let mut counter = 0;

    let result = loop {
        counter += 1;
        if counter == 10 {
            break counter * 2; // 🪄 loop evaluates to 20
        }
    };
}

🔬 From the Official Book

Pattern Matching with if let

Sometimes a full match is too much boilerplate. Use if let as a “Shorthand Contract” when you only care about one specific pattern.

if let Some(3) = some_value {
    println!("Found three!");
}

💡 What Control Flow Unlocks

• Unbreakable Logic: By forcing exhaustiveness in match, Rust eliminates the “Ghost States” that cause logic bugs in your application.
• Syntactic Sugars: Expression-based if/else means cleaner, more declarative code without needing ternary operators or temporary variables.
• Infinite Flexibility: Loops that return values allow for sophisticated “Retry” or “Calculation” patterns that are impossible in standard JS loops.