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Rust Programming for Smart Contract Development

Error handling

Introduction to Error Handling in Rust

Welcome aboard the Rust train, folks! If you're thinking, "I've never made a mistake in my life," then firstly, congratulations, you might just be the first perfect programmer in history. But for the rest of us mere mortals, errors are a fact of life. They're like the in-laws of programming - not always welcome, but we have to learn how to deal with them gracefully!

In Rust, we don't just "deal" with errors, we handle them. It's like juggling flaming swords, except in Rust, we've got a super safe suit of armor on, so we're less likely to lose a limb.

Now, error handling in Rust might seem a bit different if you're coming from other languages. You see, Rust doesn't like to sweep errors under the carpet. There's no sweeping of exceptions into a "catch" block where they may be forgotten. No, sir! In Rust, we face our errors head-on. It's a bit like that school bully you can't avoid forever; at some point, you've got to face the music.

Rust's approach to error handling revolves around some specific tools - namely, the Result and Option types. These guys are the bread and butter of safe, effective error handling. Result is used when a function could Ok-ay-ly end or, well, result in an error, while Option is used when a function could return something or nothing at all. Spoiler alert: these aren't just types, they're superheroes in disguise, saving our code from the villainous clutches of unexpected errors.

Here's a simple example:

fn divide(numerator: f64, denominator: f64) -> Option<f64> {
    if denominator == 0.0 {
    } else {
        Some(numerator / denominator)

In this function, we're dividing two numbers. But we all know that dividing by zero is a big no-no. It's like ordering pineapple on pizza - you just don't do it (unless you're into that sort of thing). So, if the denominator is zero, we return None, and if it's not, we carry on with the division and return Some(result). Simple, right?

But hold on to your hats, because we're just scratching the surface. In the coming sections, we're going to dive deeper into the world of Result, Option, and even the infamous panic! macro. So stay tuned, and remember, in Rust we trust!

The ? Operator

Alright folks, buckle up! We're about to dive into the magic world of the ? operator. This little symbol might look innocent, just minding its own business at the end of your expressions, but don't let its size fool you. It’s a powerful tool in the Rust universe that can transform your error handling from a grueling task into a walk in the park.

First things first, what is this ? operator all about? In essence, it’s Rust’s way of simplifying error propagation. In other words, it's a handy-dandy shortcut for handling errors in your functions. Imagine you've got a function that could potentially go belly-up (i.e., return an Err). You could use a match expression to handle the Ok and Err variants separately, but if you have a lot of these, your code can quickly start to look like a plate of spaghetti.

Enter the ? operator, Rust's little magic wand. When placed at the end of an expression that returns a Result, it'll do one of two things:

  1. If the Result is Ok, it takes the value out of the Ok variant and returns it.
  2. If the Result is Err, it returns from the function early and gives the error.

Let's look at an example. Suppose we have a function that reads a number from a file:

use std::fs;

fn read_number_from_file() -> Result<i32, std::io::Error> {
    let contents = fs::read_to_string("my_number.txt")?;
    let number: i32 = contents.trim().parse()?;

In this function, we're using ? twice. When we try to read the file, if everything goes smoothly, we get our file's content. But if there's an error (file not found, aliens took it, etc.), ? will immediately return this error from read_number_from_file.

Same goes for the parsing. If the parsing goes well, we get our number. If it fails (for example, because someone put "potato" in the file), ? returns the error immediately.

And there you have it! The ? operator, a small and mighty tool that saves you from getting lost in a maze of match expressions. But be aware, with great power comes great responsibility. The ? can only be used in functions that return a Result (or an Option), because when an error occurs, it needs a place to go!

Combining Result and Option Types

Hello there, brave Rustacean! Let's step into the world of combining Result and Option types, where Rust's elegance and precision really shines. Picture this: you're not only dealing with a situation that can fail, but also an optional value that may or may not exist. Sounds like a recipe for a headache, right? Not with Rust!

Before we start, let me explain this with an analogy. Imagine you're trying to bake a cake (stick with me here). The Result type is like checking whether you have all the ingredients in your pantry. If you don't have eggs, your baking endeavor fails with an Err. However, the Option type is more like checking if you have any sprinkles left. You might have them, you might not (Some or None), but either way, you can still bake a cake.

Now, back to Rust. Let's dive into some code:

fn get_ingredient(name: &str) -> Result<Option<String>, &'static str> {
    let pantry: HashMap<String, Option<String>> = // ... your pantry

    match pantry.get(name) {
        Some(ingredient) => {
            match ingredient {
                Some(details) => Ok(Some(details.to_string())),
                None => Ok(None),
        None => Err("Ingredient not found!"),

let eggs = get_ingredient("eggs");

match eggs {
    Ok(Some(details)) => println!("We have eggs: {}", details),
    Ok(None) => println!("No")
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