mirror of https://github.com/Dhghomon/easy_rust
You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
549 lines
34 KiB
HTML
549 lines
34 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="sidebar-visible no-js light">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title> Option and Result - Easy Rust</title>
|
|
|
|
|
|
|
|
|
|
<!-- Custom HTML head -->
|
|
|
|
|
|
|
|
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
|
<meta name="description" content="">
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
<meta name="theme-color" content="#ffffff" />
|
|
|
|
|
|
<link rel="icon" href="favicon.svg">
|
|
|
|
|
|
<link rel="shortcut icon" href="favicon.png">
|
|
|
|
<link rel="stylesheet" href="css/variables.css">
|
|
<link rel="stylesheet" href="css/general.css">
|
|
<link rel="stylesheet" href="css/chrome.css">
|
|
|
|
<link rel="stylesheet" href="css/print.css" media="print">
|
|
|
|
|
|
<!-- Fonts -->
|
|
<link rel="stylesheet" href="FontAwesome/css/font-awesome.css">
|
|
|
|
<link rel="stylesheet" href="fonts/fonts.css">
|
|
|
|
|
|
<!-- Highlight.js Stylesheets -->
|
|
<link rel="stylesheet" href="highlight.css">
|
|
<link rel="stylesheet" href="tomorrow-night.css">
|
|
<link rel="stylesheet" href="ayu-highlight.css">
|
|
|
|
<!-- Custom theme stylesheets -->
|
|
|
|
|
|
|
|
</head>
|
|
<body>
|
|
<!-- Provide site root to javascript -->
|
|
<script type="text/javascript">
|
|
var path_to_root = "";
|
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
|
|
</script>
|
|
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|
<script type="text/javascript">
|
|
try {
|
|
var theme = localStorage.getItem('mdbook-theme');
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|
}
|
|
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|
}
|
|
} catch (e) { }
|
|
</script>
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
<script type="text/javascript">
|
|
var theme;
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|
var html = document.querySelector('html');
|
|
html.classList.remove('no-js')
|
|
html.classList.remove('light')
|
|
html.classList.add(theme);
|
|
html.classList.add('js');
|
|
</script>
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
<script type="text/javascript">
|
|
var html = document.querySelector('html');
|
|
var sidebar = 'hidden';
|
|
if (document.body.clientWidth >= 1080) {
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|
sidebar = sidebar || 'visible';
|
|
}
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</script>
|
|
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|
<div class="sidebar-scrollbox">
|
|
<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Update</a></li><li class="chapter-item expanded "><a href="Chapter_1.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="Chapter_2.html"><strong aria-hidden="true">3.</strong> Who am I?</a></li><li class="chapter-item expanded "><a href="Chapter_3.html"><strong aria-hidden="true">4.</strong> Writing Rust in Easy English</a></li><li class="chapter-item expanded "><a href="Chapter_4.html"><strong aria-hidden="true">5.</strong> Rust Playground</a></li><li class="chapter-item expanded "><a href="Chapter_5.html"><strong aria-hidden="true">6.</strong> 🚧 and ⚠️</a></li><li class="chapter-item expanded "><a href="Chapter_6.html"><strong aria-hidden="true">7.</strong> Comments</a></li><li class="chapter-item expanded "><a href="Chapter_7.html"><strong aria-hidden="true">8.</strong> Types</a></li><li class="chapter-item expanded "><a href="Chapter_8.html"><strong aria-hidden="true">9.</strong> Type inference</a></li><li class="chapter-item expanded "><a href="Chapter_9.html"><strong aria-hidden="true">10.</strong> Printing 'hello, world!'</a></li><li class="chapter-item expanded "><a href="Chapter_10.html"><strong aria-hidden="true">11.</strong> Display and debug</a></li><li class="chapter-item expanded "><a href="Chapter_11.html"><strong aria-hidden="true">12.</strong> Mutability (changing)</a></li><li class="chapter-item expanded "><a href="Chapter_12.html"><strong aria-hidden="true">13.</strong> The stack, the heap, and pointers</a></li><li class="chapter-item expanded "><a href="Chapter_13.html"><strong aria-hidden="true">14.</strong> More about printing</a></li><li class="chapter-item expanded "><a href="Chapter_14.html"><strong aria-hidden="true">15.</strong> Strings</a></li><li class="chapter-item expanded "><a href="Chapter_15.html"><strong aria-hidden="true">16.</strong> const and static</a></li><li class="chapter-item expanded "><a href="Chapter_16.html"><strong aria-hidden="true">17.</strong> More on references</a></li><li class="chapter-item expanded "><a href="Chapter_17.html"><strong aria-hidden="true">18.</strong> Mutable references</a></li><li class="chapter-item expanded "><a href="Chapter_18.html"><strong aria-hidden="true">19.</strong> Giving references to functions</a></li><li class="chapter-item expanded "><a href="Chapter_19.html"><strong aria-hidden="true">20.</strong> Copy types</a></li><li class="chapter-item expanded "><a href="Chapter_20.html"><strong aria-hidden="true">21.</strong> Collection types</a></li><li class="chapter-item expanded "><a href="Chapter_21.html"><strong aria-hidden="true">22.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="Chapter_22.html"><strong aria-hidden="true">23.</strong> Tuples</a></li><li class="chapter-item expanded "><a href="Chapter_23.html"><strong aria-hidden="true">24.</strong> Control flow</a></li><li class="chapter-item expanded "><a href="Chapter_24.html"><strong aria-hidden="true">25.</strong> Structs</a></li><li class="chapter-item expanded "><a href="Chapter_25.html"><strong aria-hidden="true">26.</strong> Enums</a></li><li class="chapter-item expanded "><a href="Chapter_26.html"><strong aria-hidden="true">27.</strong> Loops</a></li><li class="chapter-item expanded "><a href="Chapter_27.html"><strong aria-hidden="true">28.</strong> Implementing structs and enums</a></li><li class="chapter-item expanded "><a href="Chapter_28.html"><strong aria-hidden="true">29.</strong> Destructuring</a></li><li class="chapter-item expanded "><a href="Chapter_29.html"><strong aria-hidden="true">30.</strong> References and the dot operator</a></li><li class="chapter-item expanded "><a href="Chapter_30.html"><strong aria-hidden="true">31.</strong> Generics</a></li><li class="chapter-item expanded "><a href="Chapter_31.html" class="active"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html"><strong aria-hidden="true">33.</strong> Other collections</a></li><li class="chapter-item expanded "><a href="Chapter_33.html"><strong aria-hidden="true">34.</strong> The ? operator</a></li><li class="chapter-item expanded "><a href="Chapter_34.html"><strong aria-hidden="true">35.</strong> Traits</a></li><li class="chapter-item expanded "><a href="Chapter_35.html"><strong aria-hidden="true">36.</strong> Chaining methods</a></li><li class="chapter-item expanded "><a href="Chapter_36.html"><strong aria-hidden="true">37.</strong> Iterators</a></li><li class="chapter-item expanded "><a href="Chapter_37.html"><strong aria-hidden="true">38.</strong> Closures</a></li><li class="chapter-item expanded "><a href="Chapter_38.html"><strong aria-hidden="true">39.</strong> The dbg! macro and .inspect</a></li><li class="chapter-item expanded "><a href="Chapter_39.html"><strong aria-hidden="true">40.</strong> Types of &str</a></li><li class="chapter-item expanded "><a href="Chapter_40.html"><strong aria-hidden="true">41.</strong> Lifetimes</a></li><li class="chapter-item expanded "><a href="Chapter_41.html"><strong aria-hidden="true">42.</strong> Interior mutability</a></li><li class="chapter-item expanded "><a href="Chapter_42.html"><strong aria-hidden="true">43.</strong> RefCell</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Mutex</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> RwLock</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_67.html"><strong aria-hidden="true">68.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_68.html"><strong aria-hidden="true">69.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_69.html"><strong aria-hidden="true">70.</strong> The end?</a></li></ol>
|
|
</div>
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|
</nav>
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
<div class="page">
|
|
|
|
<div id="menu-bar-hover-placeholder"></div>
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
|
<div class="left-buttons">
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|
<i class="fa fa-bars"></i>
|
|
</button>
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|
<i class="fa fa-paint-brush"></i>
|
|
</button>
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|
</ul>
|
|
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
|
<i class="fa fa-search"></i>
|
|
</button>
|
|
|
|
</div>
|
|
|
|
<h1 class="menu-title">Easy Rust</h1>
|
|
|
|
<div class="right-buttons">
|
|
|
|
<a href="print.html" title="Print this book" aria-label="Print this book">
|
|
<i id="print-button" class="fa fa-print"></i>
|
|
</a>
|
|
|
|
|
|
</div>
|
|
</div>
|
|
|
|
|
|
<div id="search-wrapper" class="hidden">
|
|
<form id="searchbar-outer" class="searchbar-outer">
|
|
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
|
</form>
|
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
|
<div id="searchresults-header" class="searchresults-header"></div>
|
|
<ul id="searchresults">
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
|
|
|
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
|
<script type="text/javascript">
|
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
|
});
|
|
</script>
|
|
|
|
<div id="content" class="content">
|
|
<main>
|
|
<h2><a class="header" href="#option-and-result" id="option-and-result">Option and Result</a></h2>
|
|
<p>We understand enums and generics now, so we can understand <code>Option</code> and <code>Result</code>. Rust uses these two enums to make code safer.</p>
|
|
<p>We will start with <code>Option</code>.</p>
|
|
<h3><a class="header" href="#option" id="option">Option</a></h3>
|
|
<p>You use <code>Option</code> when you have a value that might exist, or might not exist. When a value exists it is <code>Some(value)</code> and when it doesn't it's just <code>None</code>, Here is an example of bad code that can be improved with <code>Option</code>.</p>
|
|
<pre><pre class="playground"><code class="language-rust"> // ⚠️
|
|
fn take_fifth(value: Vec<i32>) -> i32 {
|
|
value[4]
|
|
}
|
|
|
|
fn main() {
|
|
let new_vec = vec![1, 2];
|
|
let index = take_fifth(new_vec);
|
|
}
|
|
</code></pre></pre>
|
|
<p>When we run the code, it panics. Here is the message:</p>
|
|
<pre><code class="language-text">thread 'main' panicked at 'index out of bounds: the len is 2 but the index is 4', src\main.rs:34:5
|
|
</code></pre>
|
|
<p>Panic means that the program stops before the problem happens. Rust sees that the function wants something impossible, and stops. It "unwinds the stack" (takes the values off the stack) and tells you "sorry, I can't do that".</p>
|
|
<p>So now we will change the return type from <code>i32</code> to <code>Option<i32></code>. This means "give me a <code>Some(i32)</code> if it's there, and give me <code>None</code> if it's not". We say that the <code>i32</code> is "wrapped" in an <code>Option</code>, which means that it's inside an <code>Option</code>. You have to do something to get the value out.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn take_fifth(value: Vec<i32>) -> Option<i32> {
|
|
if value.len() < 5 { // .len() gives the length of the vec.
|
|
// It must be at least 5.
|
|
None
|
|
} else {
|
|
Some(value[4])
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let new_vec = vec![1, 2];
|
|
let bigger_vec = vec![1, 2, 3, 4, 5];
|
|
println!("{:?}, {:?}", take_fifth(new_vec), take_fifth(bigger_vec));
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints <code>None, Some(5)</code>. This is good, because now we don't panic anymore. But how do we get the value 5?</p>
|
|
<p>We can get the value inside an option with <code>.unwrap()</code>, but be careful with <code>.unwrap()</code>. It's just like unwrapping a present: maybe there's something good inside, or maybe there's an angry snake inside. You only want to <code>.unwrap()</code> if you are sure. If you unwrap a value that is <code>None</code>, the program will panic.</p>
|
|
<pre><pre class="playground"><code class="language-rust">// ⚠️
|
|
fn take_fifth(value: Vec<i32>) -> Option<i32> {
|
|
if value.len() < 4 {
|
|
None
|
|
} else {
|
|
Some(value[4])
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let new_vec = vec![1, 2];
|
|
let bigger_vec = vec![1, 2, 3, 4, 5];
|
|
println!("{:?}, {:?}",
|
|
take_fifth(new_vec).unwrap(), // this one is None. .unwrap() will panic!
|
|
take_fifth(bigger_vec).unwrap()
|
|
);
|
|
}
|
|
</code></pre></pre>
|
|
<p>The message is: "thread 'main' panicked at 'called <code>Option::unwrap()</code> on a <code>None</code> value', src\main.rs:14:9".</p>
|
|
<p>But we don't need to use <code>.unwrap()</code>. We can use a <code>match</code>. Then we can print the value we have <code>Some</code>, and not touch it if we have <code>None</code>. For example:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn take_fifth(value: Vec<i32>) -> Option<i32> {
|
|
if value.len() < 4 {
|
|
None
|
|
} else {
|
|
Some(value[4])
|
|
}
|
|
}
|
|
|
|
fn handle_option(my_option: Vec<Option<i32>>) {
|
|
for item in my_option {
|
|
match item {
|
|
Some(number) => println!("Found a {}!", number),
|
|
None => println!("Found a None!"),
|
|
}
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let new_vec = vec![1, 2];
|
|
let bigger_vec = vec![1, 2, 3, 4, 5];
|
|
let mut option_vec = Vec::new(); // Make a new vec to hold our options
|
|
// The vec is type: Vec<Option<i32>>. That means a vec of Option<i32>.
|
|
|
|
option_vec.push(take_fifth(new_vec)); // This pushes "None" into the vec
|
|
option_vec.push(take_fifth(bigger_vec)); // This pushes "Some(5)" into the vec
|
|
|
|
handle_option(option_vec); // handle_option looks at every option in the vec.
|
|
// It prints the value if it is Some. It doesn't touch it if it is None.
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints:</p>
|
|
<pre><code class="language-text">Found a None!
|
|
Found a 5!
|
|
</code></pre>
|
|
<p>Because we know generics, we are able to read the code for <code>Option</code>. It looks like this:</p>
|
|
<pre><pre class="playground"><code class="language-rust">enum Option<T> {
|
|
None,
|
|
Some(T),
|
|
}
|
|
|
|
fn main() {}
|
|
</code></pre></pre>
|
|
<p>The important point to remember: with <code>Some</code>, you have a value of type <code>T</code> (any type). Also note that the angle brackets after the <code>enum</code> name around <code>T</code> is what tells the compiler that it's generic. It has no trait like <code>Display</code> or anything to limit it, so it can be anything. But with <code>None</code>, you don't have anything.</p>
|
|
<p>So in a <code>match</code> statement for Option you can't say:</p>
|
|
<pre><pre class="playground"><code class="language-rust">
|
|
<span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>// 🚧
|
|
Some(value) => println!("The value is {}", value),
|
|
None(value) => println!("The value is {}", value),
|
|
<span class="boring">}
|
|
</span></code></pre></pre>
|
|
<p>because <code>None</code> is just <code>None</code>.</p>
|
|
<p>Of course, there are easier ways to use Option. In this code, we will use a method called <code>.is_some()</code> to tell us if it is <code>Some</code>. (Yes, there is also a method called <code>.is_none()</code>.) In this easier way, we don't need <code>handle_option()</code> anymore. We also don't need a vec for the Options.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn take_fifth(value: Vec<i32>) -> Option<i32> {
|
|
if value.len() < 4 {
|
|
None
|
|
} else {
|
|
Some(value[4])
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let new_vec = vec![1, 2];
|
|
let bigger_vec = vec![1, 2, 3, 4, 5];
|
|
let vec_of_vecs = vec![new_vec, bigger_vec];
|
|
for vec in vec_of_vecs {
|
|
let inside_number = take_fifth(vec);
|
|
if inside_number.is_some() {
|
|
// .is_some() returns true if we get Some, false if we get None
|
|
println!("We got: {}", inside_number.unwrap()); // now it is safe to use .unwrap() because we already checked
|
|
} else {
|
|
println!("We got nothing.");
|
|
}
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints:</p>
|
|
<pre><code class="language-text">We got nothing.
|
|
We got: 5
|
|
</code></pre>
|
|
<h3><a class="header" href="#result" id="result">Result</a></h3>
|
|
<p>Result is similar to Option, but here is the difference:</p>
|
|
<ul>
|
|
<li>Option is about <code>Some</code> or <code>None</code> (value or no value),</li>
|
|
<li>Result is about <code>Ok</code> or <code>Err</code> (okay result, or error result).</li>
|
|
</ul>
|
|
<p>So <code>Option</code> is if you are thinking: "Maybe there will be something, and maybe there won't." But <code>Result</code> is if you are thinking: "Maybe it will fail."</p>
|
|
<p>To compare, here are the signatures for Option and Result.</p>
|
|
<pre><pre class="playground"><code class="language-rust">enum Option<T> {
|
|
None,
|
|
Some(T),
|
|
}
|
|
|
|
enum Result<T, E> {
|
|
Ok(T),
|
|
Err(E),
|
|
}
|
|
|
|
fn main() {}
|
|
</code></pre></pre>
|
|
<p>So Result has a value inside of <code>Ok</code>, and a value inside of <code>Err</code>. That is because errors usually (and should have) have information inside them.</p>
|
|
<p><code>Result<T, E></code> means you need to think of what you want to return for <code>Ok</code>, and what you want to return for <code>Err</code>. Actually, you can decide anything. Even this is okay:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn check_error() -> Result<(), ()> {
|
|
Ok(())
|
|
}
|
|
|
|
fn main() {
|
|
check_error();
|
|
}
|
|
</code></pre></pre>
|
|
<p><code>check_error</code> says "return <code>()</code> if we get <code>Ok</code>, and return <code>()</code> if we get <code>Err</code>". Then we return <code>Ok</code> with a <code>()</code>.</p>
|
|
<p>The compiler gives us an interesting warning:</p>
|
|
<pre><code class="language-text">warning: unused `std::result::Result` that must be used
|
|
--> src\main.rs:6:5
|
|
|
|
|
6 | check_error();
|
|
| ^^^^^^^^^^^^^^
|
|
|
|
|
= note: `#[warn(unused_must_use)]` on by default
|
|
= note: this `Result` may be an `Err` variant, which should be handled
|
|
</code></pre>
|
|
<p>This is true: we only returned the <code>Result</code> but it could have been an <code>Err</code>. So let's handle the error a bit, even though we're still not really doing anything.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn give_result(input: i32) -> Result<(), ()> {
|
|
if input % 2 == 0 {
|
|
return Ok(())
|
|
} else {
|
|
return Err(())
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
if give_result(5).is_ok() {
|
|
println!("It's okay, guys")
|
|
} else {
|
|
println!("It's an error, guys")
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints <code>It's an error, guys</code>. So we just handled our first error.</p>
|
|
<p>Remember, the four methods to easily check are <code>.is_some()</code>, <code>is_none()</code>, <code>is_ok()</code>, and <code>is_err()</code>.</p>
|
|
<p>Sometimes a function with Result will use a <code>String</code> for the <code>Err</code> value. This is not the best method to use, but it is a little better than what we've done so far.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn check_if_five(number: i32) -> Result<i32, String> {
|
|
match number {
|
|
5 => Ok(number),
|
|
_ => Err("Sorry, the number wasn't five.".to_string()), // This is our error message
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let mut result_vec = Vec::new(); // Create a new vec for the results
|
|
|
|
for number in 2..7 {
|
|
result_vec.push(check_if_five(number)); // push each result into the vec
|
|
}
|
|
|
|
println!("{:?}", result_vec);
|
|
}
|
|
</code></pre></pre>
|
|
<p>Our vec prints:</p>
|
|
<pre><code class="language-text">[Err("Sorry, the number wasn\'t five."), Err("Sorry, the number wasn\'t five."), Err("Sorry, the number wasn\'t five."), Ok(5),
|
|
Err("Sorry, the number wasn\'t five.")]
|
|
</code></pre>
|
|
<p>Just like Option, <code>.unwrap()</code> on <code>Err</code> will panic.</p>
|
|
<pre><pre class="playground"><code class="language-rust"> // ⚠️
|
|
fn main() {
|
|
let error_value: Result<i32, &str> = Err("There was an error"); // Create a Result that is already an Err
|
|
println!("{}", error_value.unwrap()); // Unwrap it
|
|
}
|
|
</code></pre></pre>
|
|
<p>The program panics, and prints:</p>
|
|
<pre><code class="language-text">thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "There was an error"', src\main.rs:30:20
|
|
</code></pre>
|
|
<p>This information helps you fix your code. <code>src\main.rs:30:20</code> means "inside main.rs in directory src, on line 30 and column 20". So you can go there to look at your code and fix the problem.</p>
|
|
<p>You can also create your own error types. Result functions in the standard library and other people's code usually do this. For example, this function from the standard library:</p>
|
|
<pre><pre class="playground"><code class="language-rust">
|
|
<span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>// 🚧
|
|
pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error>
|
|
<span class="boring">}
|
|
</span></code></pre></pre>
|
|
<p>This function takes a vector of bytes (<code>u8</code>) and tries to make a <code>String</code>. So the success case for the Result is a <code>String</code> and the error case is <code>FromUtf8Error</code>. You can give your error type any name you want.</p>
|
|
<p>Using a <code>match</code> with <code>Option</code> and <code>Result</code> sometimes requires a lot of code. For example, the <code>.get()</code> method returns an <code>Option</code> on a <code>Vec</code>.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let my_vec = vec![2, 3, 4];
|
|
let get_one = my_vec.get(0); // 0 to get the first number
|
|
let get_two = my_vec.get(10); // Returns None
|
|
println!("{:?}", get_one);
|
|
println!("{:?}", get_two);
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints</p>
|
|
<pre><code class="language-text">Some(2)
|
|
None
|
|
</code></pre>
|
|
<p>So now we can match to get the values. Let's use a range from 0 to 10 to see if it matches the numbers in <code>my_vec</code>.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let my_vec = vec![2, 3, 4];
|
|
|
|
for index in 0..10 {
|
|
match my_vec.get(index) {
|
|
Some(number) => println!("The number is: {}", number),
|
|
None => {}
|
|
}
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>This is good, but we don't do anything for <code>None</code> because we don't care. Here we can make the code smaller by using <code>if let</code>. <code>if let</code> means "do something if it matches, and don't do anything if it doesn't". <code>if let</code> is when you don't care about matching for everything.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let my_vec = vec![2, 3, 4];
|
|
|
|
for index in 0..10 {
|
|
if let Some(number) = my_vec.get(index) {
|
|
println!("The number is: {}", number);
|
|
}
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p><strong>Important to remember</strong>: <code>if let Some(number) = my_vec.get(index)</code> means "if you get <code>Some(number)</code> from <code>my_vec.get(index)</code>".</p>
|
|
<p>Also note: it uses one <code>=</code>. It is not a boolean.</p>
|
|
<p><code>while let</code> is like a while loop for <code>if let</code>. Imagine that we have weather station data like this:</p>
|
|
<pre><code class="language-text">["Berlin", "cloudy", "5", "-7", "78"]
|
|
["Athens", "sunny", "not humid", "20", "10", "50"]
|
|
</code></pre>
|
|
<p>We want to get the numbers, but not the words. For the numbers, we can use a method called <code>parse::<i32>()</code>. <code>parse()</code> is the method, and <code>::<i32></code> is the type. It will try to turn the <code>&str</code> into an <code>i32</code>, and give it to us if it can. It returns a <code>Result</code>, because it might not work (like if you wanted it to parse "Billybrobby" - that's not a number).</p>
|
|
<p>We will also use <code>.pop()</code>. This takes the last item off of the vector.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let weather_vec = vec![
|
|
vec!["Berlin", "cloudy", "5", "-7", "78"],
|
|
vec!["Athens", "sunny", "not humid", "20", "10", "50"],
|
|
];
|
|
for mut city in weather_vec {
|
|
println!("For the city of {}:", city[0]); // In our data, every first item is the city name
|
|
while let Some(information) = city.pop() {
|
|
// This means: keep going until you can't pop anymore
|
|
// When the vector reaches 0 items, it will return None
|
|
// and it will stop.
|
|
if let Ok(number) = information.parse::<i32>() {
|
|
// Try to parse the variable we called information
|
|
// This returns a result. If it's Ok(number), it will print it
|
|
println!("The number is: {}", number);
|
|
} // We don't write anything here because we do nothing if we get an error. Throw them all away
|
|
}
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>This will print:</p>
|
|
<pre><code class="language-text">For the city of Berlin:
|
|
The number is: 78
|
|
The number is: -7
|
|
The number is: 5
|
|
For the city of Athens:
|
|
The number is: 50
|
|
The number is: 10
|
|
The number is: 20
|
|
</code></pre>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
|
|
<a rel="prev" href="Chapter_30.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
|
|
|
|
<a rel="next" href="Chapter_32.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
|
|
|
|
<div style="clear: both"></div>
|
|
</nav>
|
|
</div>
|
|
</div>
|
|
|
|
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
|
|
|
<a rel="prev" href="Chapter_30.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
|
|
|
|
<a rel="next" href="Chapter_32.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
|
|
</nav>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<script type="text/javascript">
|
|
window.playground_copyable = true;
|
|
</script>
|
|
|
|
|
|
|
|
|
|
|
|
<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
|
|
|
|
|
|
<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="book.js" type="text/javascript" charset="utf-8"></script>
|
|
|
|
<!-- Custom JS scripts -->
|
|
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|