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<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"><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 &amp;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> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_55.html" class="active"><strong aria-hidden="true">56.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> The end?</a></li></ol>
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<h1 class="menu-title">Easy Rust</h1>
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<h2 id="default-and-the-builder-pattern"><a class="header" href="#default-and-the-builder-pattern">Default and the builder pattern</a></h2>
<p>You can implement the <code>Default</code> trait to give values to a <code>struct</code> or <code>enum</code> that you think will be most common. The builder pattern works nicely with this to let users easily make changes when they want. First let's look at <code>Default</code>. Actually, most general types in Rust already have <code>Default</code>. They are not surprising: 0, &quot;&quot; (empty strings), <code>false</code>, etc.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let default_i8: i8 = Default::default();
let default_str: String = Default::default();
let default_bool: bool = Default::default();
println!(&quot;'{}', '{}', '{}'&quot;, default_i8, default_str, default_bool);
}
</code></pre></pre>
<p>This prints <code>'0', '', 'false'</code>.</p>
<p>So <code>Default</code> is like the <code>new</code> function except you don't have to enter anything. First we will make a <code>struct</code> that doesn't implement <code>Default</code> yet. It has a <code>new</code> function which we use to make a character named Billy with some stats.</p>
<pre><pre class="playground"><code class="language-rust">struct Character {
name: String,
age: u8,
height: u32,
weight: u32,
lifestate: LifeState,
}
enum LifeState {
Alive,
Dead,
NeverAlive,
Uncertain
}
impl Character {
fn new(name: String, age: u8, height: u32, weight: u32, alive: bool) -&gt; Self {
Self {
name,
age,
height,
weight,
lifestate: if alive { LifeState::Alive } else { LifeState::Dead },
}
}
}
fn main() {
let character_1 = Character::new(&quot;Billy&quot;.to_string(), 15, 170, 70, true);
}
</code></pre></pre>
<p>But maybe in our world we want most of the characters to be named Billy, age 15, height 170, weight 70, and alive. We can implement <code>Default</code> so that we can just write <code>Character::default()</code>. It looks like this:</p>
<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
struct Character {
name: String,
age: u8,
height: u32,
weight: u32,
lifestate: LifeState,
}
#[derive(Debug)]
enum LifeState {
Alive,
Dead,
NeverAlive,
Uncertain,
}
impl Character {
fn new(name: String, age: u8, height: u32, weight: u32, alive: bool) -&gt; Self {
Self {
name,
age,
height,
weight,
lifestate: if alive {
LifeState::Alive
} else {
LifeState::Dead
},
}
}
}
impl Default for Character {
fn default() -&gt; Self {
Self {
name: &quot;Billy&quot;.to_string(),
age: 15,
height: 170,
weight: 70,
lifestate: LifeState::Alive,
}
}
}
fn main() {
let character_1 = Character::default();
println!(
&quot;The character {:?} is {:?} years old.&quot;,
character_1.name, character_1.age
);
}
</code></pre></pre>
<p>It prints <code>The character &quot;Billy&quot; is 15 years old.</code> Much easier!</p>
<p>Now comes the builder pattern. We will have many Billys, so we will keep the default. But a lot of other characters will be only a bit different. The builder pattern lets us chain very small methods to change one value each time. Here is one such method for <code>Character</code>:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn height(mut self, height: u32) -&gt; Self { // 🚧
self.height = height;
self
}
<span class="boring">}
</span></code></pre></pre>
<p>Make sure to notice that it takes a <code>mut self</code>. We saw this once before, and it is not a mutable reference (<code>&amp;mut self</code>). It takes ownership of <code>Self</code> and with <code>mut</code> it will be mutable, even if it wasn't mutable before. That's because <code>.height()</code> has full ownership and nobody else can touch it, so it is safe to be mutable. Then it just changes <code>self.height</code> and returns <code>Self</code> (which is <code>Character</code>).</p>
<p>So let's have three of these builder methods. They are almost the same:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn height(mut self, height: u32) -&gt; Self { // 🚧
self.height = height;
self
}
fn weight(mut self, weight: u32) -&gt; Self {
self.weight = weight;
self
}
fn name(mut self, name: &amp;str) -&gt; Self {
self.name = name.to_string();
self
}
<span class="boring">}
</span></code></pre></pre>
<p>Each one of those changes one variable and gives <code>Self</code> back: this is what you see in the builder pattern. So now we can write something like this to make a character: <code>let character_1 = Character::default().height(180).weight(60).name(&quot;Bobby&quot;);</code>. If you are building a library for someone else to use, this can make it easy for them. It's easy for the end user because it almost looks like natural English: &quot;Give me a default character but with height of 180, weight of 60, and name of Bobby.&quot; So far our code looks like this:</p>
<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
struct Character {
name: String,
age: u8,
height: u32,
weight: u32,
lifestate: LifeState,
}
#[derive(Debug)]
enum LifeState {
Alive,
Dead,
NeverAlive,
Uncertain,
}
impl Character {
fn new(name: String, age: u8, height: u32, weight: u32, alive: bool) -&gt; Self {
Self {
name,
age,
height,
weight,
lifestate: if alive {
LifeState::Alive
} else {
LifeState::Dead
},
}
}
fn height(mut self, height: u32) -&gt; Self {
self.height = height;
self
}
fn weight(mut self, weight: u32) -&gt; Self {
self.weight = weight;
self
}
fn name(mut self, name: &amp;str) -&gt; Self {
self.name = name.to_string();
self
}
}
impl Default for Character {
fn default() -&gt; Self {
Self {
name: &quot;Billy&quot;.to_string(),
age: 15,
height: 170,
weight: 70,
lifestate: LifeState::Alive,
}
}
}
fn main() {
let character_1 = Character::default().height(180).weight(60).name(&quot;Bobby&quot;);
println!(&quot;{:?}&quot;, character_1);
}
</code></pre></pre>
<p>One last method to add is usually called <code>.build()</code>. This method is a sort of final check. When you give a user a method like <code>.height()</code> you can make sure that they only put in a <code>u32()</code>, but what if they enter 5000 for height? That might not be okay in the game you are making. We will use a final method called <code>.build()</code> that returns a <code>Result</code>. Inside it we will check if the user input is okay, and if it is, we will return an <code>Ok(Self)</code>.</p>
<p>First though let's change the <code>.new()</code> method. We don't want users to be free to create any kind of character anymore. So we'll move the values from <code>impl Default</code> to <code>.new()</code>. And now <code>.new()</code> doesn't take any input.</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span> fn new() -&gt; Self { // 🚧
Self {
name: &quot;Billy&quot;.to_string(),
age: 15,
height: 170,
weight: 70,
lifestate: LifeState::Alive,
}
}
<span class="boring">}
</span></code></pre></pre>
<p>That means we don't need <code>impl Default</code> anymore, because <code>.new()</code> has all the default values. So we can delete <code>impl Default</code>.</p>
<p>Now our code looks like this:</p>
<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
struct Character {
name: String,
age: u8,
height: u32,
weight: u32,
lifestate: LifeState,
}
#[derive(Debug)]
enum LifeState {
Alive,
Dead,
NeverAlive,
Uncertain,
}
impl Character {
fn new() -&gt; Self {
Self {
name: &quot;Billy&quot;.to_string(),
age: 15,
height: 170,
weight: 70,
lifestate: LifeState::Alive,
}
}
fn height(mut self, height: u32) -&gt; Self {
self.height = height;
self
}
fn weight(mut self, weight: u32) -&gt; Self {
self.weight = weight;
self
}
fn name(mut self, name: &amp;str) -&gt; Self {
self.name = name.to_string();
self
}
}
fn main() {
let character_1 = Character::new().height(180).weight(60).name(&quot;Bobby&quot;);
println!(&quot;{:?}&quot;, character_1);
}
</code></pre></pre>
<p>This prints the same thing: <code>Character { name: &quot;Bobby&quot;, age: 15, height: 180, weight: 60, lifestate: Alive }</code>.</p>
<p>We are almost ready to write the method <code>.build()</code>, but there is one problem: how do we make the user use it? Right now a user can write <code>let x = Character::new().height(76767);</code> and get a <code>Character</code>. There are many ways to do this, and maybe you can imagine your own. But we will add a <code>can_use: bool</code> value to <code>Character</code>.</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[derive(Debug)] // 🚧
struct Character {
name: String,
age: u8,
height: u32,
weight: u32,
lifestate: LifeState,
can_use: bool, // Set whether the user can use the character
}
\\ Cut other code
fn new() -&gt; Self {
Self {
name: &quot;Billy&quot;.to_string(),
age: 15,
height: 170,
weight: 70,
lifestate: LifeState::Alive,
can_use: true, // .new() always gives a good character, so it's true
}
}
<span class="boring">}
</span></code></pre></pre>
<p>And for the other methods like <code>.height()</code>, we will set <code>can_use</code> to <code>false</code>. Only <code>.build()</code> will set it to <code>true</code> again, so now the user has to do a final check with <code>.build()</code>. We will make sure that <code>height</code> is not above 200 and <code>weight</code> is not above 300. Also, in our game there is a bad word called <code>smurf</code> that we don't want characters to use.</p>
<p>Our <code>.build()</code> method looks like this:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn build(mut self) -&gt; Result&lt;Character, String&gt; { // 🚧
if self.height &lt; 200 &amp;&amp; self.weight &lt; 300 &amp;&amp; !self.name.to_lowercase().contains(&quot;smurf&quot;) {
self.can_use = true;
Ok(self)
} else {
Err(&quot;Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)&quot;
.to_string())
}
}
<span class="boring">}
</span></code></pre></pre>
<p><code>!self.name.to_lowercase().contains(&quot;smurf&quot;)</code> makes sure that the user doesn't write something like &quot;SMURF&quot; or &quot;IamSmurf&quot; . It makes the whole <code>String</code> lowercase (small letters), and checks for <code>.contains()</code> instead of <code>==</code>. And the <code>!</code> in front means &quot;not&quot;.</p>
<p>If everything is okay, we set <code>can_use</code> to <code>true</code>, and give the character to the user inside <code>Ok</code>.</p>
<p>Now that our code is done, we will create three characters that don't work, and one character that does work. The final code looks like this:</p>
<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
struct Character {
name: String,
age: u8,
height: u32,
weight: u32,
lifestate: LifeState,
can_use: bool, // Here is the new value
}
#[derive(Debug)]
enum LifeState {
Alive,
Dead,
NeverAlive,
Uncertain,
}
impl Character {
fn new() -&gt; Self {
Self {
name: &quot;Billy&quot;.to_string(),
age: 15,
height: 170,
weight: 70,
lifestate: LifeState::Alive,
can_use: true, // .new() makes a fine character, so it is true
}
}
fn height(mut self, height: u32) -&gt; Self {
self.height = height;
self.can_use = false; // Now the user can't use the character
self
}
fn weight(mut self, weight: u32) -&gt; Self {
self.weight = weight;
self.can_use = false;
self
}
fn name(mut self, name: &amp;str) -&gt; Self {
self.name = name.to_string();
self.can_use = false;
self
}
fn build(mut self) -&gt; Result&lt;Character, String&gt; {
if self.height &lt; 200 &amp;&amp; self.weight &lt; 300 &amp;&amp; !self.name.to_lowercase().contains(&quot;smurf&quot;) {
self.can_use = true; // Everything is okay, so set to true
Ok(self) // and return the character
} else {
Err(&quot;Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)&quot;
.to_string())
}
}
}
fn main() {
let character_with_smurf = Character::new().name(&quot;Lol I am Smurf!!&quot;).build(); // This one contains &quot;smurf&quot; - not okay
let character_too_tall = Character::new().height(400).build(); // Too tall - not okay
let character_too_heavy = Character::new().weight(500).build(); // Too heavy - not okay
let okay_character = Character::new()
.name(&quot;Billybrobby&quot;)
.height(180)
.weight(100)
.build(); // This character is okay. Name is fine, height and weight are fine
// Now they are not Character, they are Result&lt;Character, String&gt;. So let's put them in a Vec so we can see them:
let character_vec = vec![character_with_smurf, character_too_tall, character_too_heavy, okay_character];
for character in character_vec { // Now we will print the character if it's Ok, and print the error if it's Err
match character {
Ok(character_info) =&gt; println!(&quot;{:?}&quot;, character_info),
Err(err_info) =&gt; println!(&quot;{}&quot;, err_info),
}
println!(); // Then add one more line
}
}
</code></pre></pre>
<p>This will print:</p>
<pre><code class="language-text">Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)
Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)
Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)
Character { name: &quot;Billybrobby&quot;, age: 15, height: 180, weight: 100, lifestate: Alive, can_use: true }
</code></pre>
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