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<main>
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<h1 id="result-and-"><a class="header" href="#result-and-">result and ?</a></h1>
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<p><code>Result<T></code> is an enum to describe possible errors. It has two variants:</p>
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<ul>
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<li><code>Ok(T)</code>: A value T was found</li>
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<li><code>Err(e)</code>: An error was found with a value <code>e</code></li>
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</ul>
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<p>In short words, the expected outcome is <code>Ok</code>, while the unexpected outcome is <code>Err</code>.</p>
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<ol>
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">
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// FILL in the blanks and FIX the errors
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use std::num::ParseIntError;
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fn multiply(n1_str: &str, n2_str: &str) -> __ {
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let n1 = n1_str.parse::<i32>();
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let n2 = n2_str.parse::<i32>();
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Ok(n1.unwrap() * n2.unwrap())
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}
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fn main() {
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let result = multiply("10", "2");
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assert_eq!(result, __);
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let result = multiply("t", "2");
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assert_eq!(result.__, 8);
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println!("Success!");
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}</code></pre></pre>
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<h3 id=""><a class="header" href="#">?</a></h3>
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<p><code>?</code> is almost exactly equivalent to <code>unwrap</code>, but <code>?</code> returns instead of panic on <code>Err</code>.</p>
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<ol start="2">
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">
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use std::num::ParseIntError;
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// IMPLEMENT multiply with ?
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// DON'T use unwrap here
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fn multiply(n1_str: &str, n2_str: &str) -> __ {
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}
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fn main() {
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assert_eq!(multiply("3", "4").unwrap(), 12);
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println!("Success!");
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}</code></pre></pre>
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<ol start="3">
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">
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use std::fs::File;
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use std::io::{self, Read};
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fn read_file1() -> Result<String, io::Error> {
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let f = File::open("hello.txt");
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let mut f = match f {
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Ok(file) => file,
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Err(e) => return Err(e),
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};
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let mut s = String::new();
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match f.read_to_string(&mut s) {
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Ok(_) => Ok(s),
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Err(e) => Err(e),
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}
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}
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// FILL in the blanks with one code line
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// DON'T change any code lines
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fn read_file2() -> Result<String, io::Error> {
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let mut s = String::new();
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__;
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Ok(s)
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}
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fn main() {
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assert_eq!(read_file1().unwrap_err().to_string(), read_file2().unwrap_err().to_string());
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println!("Success!");
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}</code></pre></pre>
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<h3 id="map--and_then"><a class="header" href="#map--and_then">map & and_then</a></h3>
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<p><a href="https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.map">map</a> and <a href="https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.and_then">and_then</a> are two common combinators for <code>Result<T, E></code> (also for <code>Option<T></code>).</p>
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<ol start="4">
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::num::ParseIntError;
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// FILL in the blank in two ways: map, and then
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fn add_two(n_str: &str) -> Result<i32, ParseIntError> {
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n_str.parse::<i32>().__
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}
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fn main() {
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assert_eq!(add_two("4").unwrap(), 6);
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println!("Success!");
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}</code></pre></pre>
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<ol start="5">
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<li>🌟🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::num::ParseIntError;
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// With the return type rewritten, we use pattern matching without `unwrap()`.
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// But it's so Verbose...
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fn multiply(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
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match n1_str.parse::<i32>() {
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Ok(n1) => {
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match n2_str.parse::<i32>() {
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Ok(n2) => {
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Ok(n1 * n2)
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},
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Err(e) => Err(e),
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}
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},
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Err(e) => Err(e),
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}
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}
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// Rewriting `multiply` to make it succinct
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// You should use BOTH of `and_then` and `map` here.
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fn multiply1(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
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// IMPLEMENT...
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}
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fn print(result: Result<i32, ParseIntError>) {
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match result {
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Ok(n) => println!("n is {}", n),
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Err(e) => println!("Error: {}", e),
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}
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}
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fn main() {
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// This still presents a reasonable answer.
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let twenty = multiply1("10", "2");
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print(twenty);
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// The following now provides a much more helpful error message.
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let tt = multiply("t", "2");
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print(tt);
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println!("Success!");
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}</code></pre></pre>
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<h3 id="type-alias"><a class="header" href="#type-alias">Type alias</a></h3>
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<p>Using <code>std::result::Result<T, ParseIntError></code> everywhere is verbose and tedious, we can use alias for this purpose.</p>
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<p>At a module level, creating aliases can be particularly helpful. Errors found in a specific module often has the same <code>Err</code> type, so a single alias can succinctly defined all associated <code>Results</code>. This is so useful even the <code>std</code> library supplies one: <a href="https://doc.rust-lang.org/std/io/type.Result.html"><code>io::Result</code></a>.</p>
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<ol start="6">
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<li>🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::num::ParseIntError;
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// FILL in the blank
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type __;
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// Use the above alias to refer to our specific `Result` type.
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fn multiply(first_number_str: &str, second_number_str: &str) -> Res<i32> {
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first_number_str.parse::<i32>().and_then(|first_number| {
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second_number_str.parse::<i32>().map(|second_number| first_number * second_number)
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})
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}
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// Here, the alias again allows us to save some space.
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fn print(result: Res<i32>) {
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match result {
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Ok(n) => println!("n is {}", n),
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Err(e) => println!("Error: {}", e),
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}
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}
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fn main() {
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print(multiply("10", "2"));
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print(multiply("t", "2"));
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println!("Success!");
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}</code></pre></pre>
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<h3 id="using-result-in-fn-main"><a class="header" href="#using-result-in-fn-main">Using Result in <code>fn main</code></a></h3>
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<p>Typically <code>the</code> main function will look like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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println!("Hello World!");
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}</code></pre></pre>
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<p>However <code>main</code> is also able to have a return type of <code>Result</code>. If an error occurs within the <code>main</code> function it will return an error code and print a debug representation of the error( Debug trait ).</p>
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<p>The following example shows such a scenario:</p>
|
|
<pre><pre class="playground"><code class="language-rust editable edition2021">
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use std::num::ParseIntError;
|
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fn main() -> Result<(), ParseIntError> {
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|
let number_str = "10";
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let number = match number_str.parse::<i32>() {
|
|
Ok(number) => number,
|
|
Err(e) => return Err(e),
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};
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|
println!("{}", number);
|
|
Ok(())
|
|
}</code></pre></pre>
|
|
<blockquote>
|
|
<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/result-panic/result.md">here</a>(under the solutions path), but only use it when you need it :)</p>
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</blockquote>
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