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<h2 id="option"><a class="header" href="#option">Option</a></h2>
<p>O tipo <code>Option</code> é de extrema importância para o Rust garantir o comportamento de <em>null safe</em>.</p>
<p>Alguns casos de uso:</p>
<ul>
<li>Valores iniciais</li>
<li>Argumentos de funções opcionais</li>
<li>Retorno de funções parciais (que podem não retornar nada)</li>
<li>Campos que podem ser retirados/movidos</li>
</ul>
<p>Funciona de forma muito semelhante ao <code>Optional</code> do Java.</p>
<p>Na verdade <code>Option</code> é um <code>enum</code> de variantes <code>None</code> e <code>Some(T)</code>. <code>None</code> indica nenhum conteúdo e <code>Some</code> é uma variante algébrica.</p>
<p>Para declarar usamos a sintaxe semelhante ao Java:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let a: Option<u32> = None;
<span class="boring">}
</span></code></pre></pre>
<p>No exemplo acima vemos o uso da notação de <code>generic</code>, também muito semelhante ao Java.</p>
<p>Ao declarar um <code>Option</code> com valor válido podemos declarar das seguintes formas, com tipo explícito e inferido:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let a: Option<u32> = Some(1);
let b = Some(1);
<span class="boring">}
</span></code></pre></pre>
<p>Comumente <code>Option</code> é utilizado com <em>pattern match</em>, por exemplo:</p>
<pre><pre class="playground"><code class="language-rust">fn divide(numerator: f64, denominator: f64) -> Option<f64> {
if denominator == 0.0 {
None
} else {
Some(numerator / denominator)
}
}
fn main() {
// The return value of the function is an option
let result = divide(2.0, 3.0);
// Pattern match to retrieve the value
match result {
// The division was valid
Some(x) => println!("Result: {}", x),
// The division was invalid
None => println!("Cannot divide by 0"),
}
}
</code></pre></pre>
<p>A forma mais segura de obter um valor de um <code>Option</code> é através de <code>match</code> e <code>if let</code>.
A forma menos segura, equivalente ao <code>get</code> do Java , é através do <code>unwrap</code>:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let a: Option<u32> = Some(1);
let n = a.unwrap();
<span class="boring">}
</span></code></pre></pre>
<p>Esse método é perigoso, devemos usar somente quando termos certeza do que estamos fazendo.
Se o <code>Option</code> possuir <code>None</code> irá levantar uma erro <code>panic</code> e interromper a execução do programa.</p>
<p>Para retornar um <code>default</code> quando o valor não existe podemos usar o <code>unwrap_or</code>:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let opt_none: Option<i32> = None;
let result = opt_none.unwrap_or(1);
<span class="boring">}
</span></code></pre></pre>
<p>Porém observe que nesse caso o valor 1 é sempre passado como parâmetro, mesmo se não for usado.
Eventualmente é interessante o valor ser obtido por callback( <code>closure</code> ou lambda), principalmente
quando queremos passar uma função. Nesse caso usamos <code>unwrap_or_else</code>:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let opt_none: Option<i32> = None;
let result = opt_none.unwrap_or_else(|| 1);
<span class="boring">}
</span></code></pre></pre>
<p>Os caracteres indicam <code>||</code> passagem de parâmetro de lambda, semelhante como no Ruby.</p>
<p>Observe que <code>unwrap_or</code> e <code>unwrap_or_else</code> não modificam o conteúdo do <code>Option</code>,
apenas retorna o valor <code>default</code>.</p>
<p>Caso o nosso desejo for de realmente alterar o conteúdo do <code>Option</code>, como um <em>lazyload</em>,
então podemos usar <code>get_or_insert_with</code>. Nesse caso devemos declarar o <code>Option</code> com <code>mut</code>:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let mut opt_none: Option<i32> = None;
let result = opt_none.get_or_insert_with(|| 1);
<span class="boring">}
</span>
Também podemos encadear vários `Option`, com `or`, `and` e `and_then`:
```rust
fn double_opt(i: i32) -> Option<i32> {
Some(i * 2)
}
<span class="boring">fn main() {
</span> let opt_some_1 = Some::<i32>(1);
let opt_some_2 = Some::<i32>(2);
let opt_some_3 = Some::<i32>(3);
let result_or = opt_some_1.or(opt_some_2).or(opt_some_3);
assert_eq!(result_or, Some(1));
let result_and = opt_some_1.and(opt_some_2).and(opt_some_3);
assert_eq!(result_and, Some(3));
let result_or = opt_some_1.and_then(double_opt).and_then(double_opt);
assert_eq!(result_or, Some(1));
}
</code></pre></pre>
<h3 id="obtendo-a-referência-do-valor-com-referência"><a class="header" href="#obtendo-a-referência-do-valor-com-referência">Obtendo a referência do valor com referência</a></h3>
<p>Quando o valor do <code>Option</code> contém um tipo que não faz <em>copy</em> implícito, tal como tipo <code>String</code>,
o código abaixo causa um erro de compilação:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span>let opt = Some(String::from("Rech"));
match opt {
Some(n) => {
println!("{}", n)
}
None => {
println!("none")
}
}
println!("{:?}", opt);
<span class="boring">}
</span></code></pre></pre>
<p>É retornada a mensagem:</p>
<pre><code>borrow of partially moved value: `opt`
partial move occurs because value has type `std::string::String`, which does not implement the `Copy` trait
</code></pre>
<p>Na linha <code>Some(n)</code> ocorre o <em>move</em> para a variável <code>n</code>, é como se ela fosse "consumida".
Para evitar isso temos que fazer o match na referência da variável. Temos que fazer <code>match &opt</code> ou <code>match opt.as_ref()</code>:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span><span class="boring">let opt = Some(String::from("Rech"));
</span>match &opt {
Some(n) => {
println!("{}", n)
}
None => {
println!("none")
}
}
<span class="boring">println!("{:?}", opt);
</span><span class="boring">}
</span></code></pre></pre>
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