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<main>
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<h1 id="frominto"><a class="header" href="#frominto">From/Into</a></h1>
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<p>The <code>From</code> trait allows for a type to define how to create itself from another type, hence providing a very simple mechanism for converting between several types.</p>
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<p>The <code>From</code> and <code>Into</code> traits are inherently linked, and this is actually part of its implementation. It means if we write something like this: <code>impl From<T> for U</code>, then we can use
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<code>let u: U = U::from(T)</code> or <code>let u:U = T.into()</code>.</p>
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<p>The <code>Into</code> trait is simply the reciprocal of the <code>From</code> trait. That is, if you have implemented the <code>From</code> trait for your type, then the <code>Into</code> trait will be automatically implemented for the same type.</p>
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<p>Using the <code>Into</code> trait will typically require the type annotations as the compiler is unable to determine this most of the time.</p>
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<p>For example, we can easily convert <code>&str</code> into <code>String</code> :</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let my_str = "hello";
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// three conversions below all depends on the fact: String implements From<&str>:
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let string1 = String::from(my_str);
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let string2 = my_str.to_string();
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// Explicit type annotation is required here
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let string3: String = my_str.into();
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}</code></pre></pre>
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<p>Because the standard library has already implemented this for us : <code>impl From<&'_ str> for String</code> .</p>
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<p>Some implementations of <code>From</code> trait can be found <a href="https://doc.rust-lang.org/stable/std/convert/trait.From.html#implementors">here</a>.</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">fn main() {
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// impl From<bool> for i32
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let i1: i32 = false.into();
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let i2: i32 = i32::from(false);
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assert_eq!(i1, i2);
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assert_eq!(i1, 0);
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// FIX the error in two ways
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/* 1. use a similar type which `impl From<char>`, maybe you
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should check the docs mentioned above to find the answer */
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// 2. a keyword from the last chapter
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let i3: i32 = 'a'.into();
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// FIX the error in two ways
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let s: String = 'a' as String;
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println!("Success!");
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}</code></pre></pre>
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<h3 id="implement-from-for-custom-types"><a class="header" href="#implement-from-for-custom-types">Implement <code>From</code> for custom types</a></h3>
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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">// From is now included in `std::prelude`, so there is no need to introduce it into the current scope
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// use std::convert::From;
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#[derive(Debug)]
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struct Number {
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value: i32,
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}
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impl From<i32> for Number {
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// IMPLEMENT `from` method
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}
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// FILL in the blanks
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fn main() {
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let num = __(30);
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assert_eq!(num.value, 30);
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let num: Number = __;
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assert_eq!(num.value, 30);
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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>🌟🌟🌟 When performing error handling it is often useful to implement <code>From</code> trait for our own error type. Then we can use <code>?</code> to automatically convert the underlying error type to our own error type.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::fs;
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use std::io;
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use std::num;
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enum CliError {
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IoError(io::Error),
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ParseError(num::ParseIntError),
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}
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impl From<io::Error> for CliError {
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// IMPLEMENT from method
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}
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impl From<num::ParseIntError> for CliError {
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// IMPLEMENT from method
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}
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fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
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// ? automatically converts io::Error to CliError
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let contents = fs::read_to_string(&file_name)?;
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// num::ParseIntError -> CliError
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let num: i32 = contents.trim().parse()?;
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Ok(num)
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}
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fn main() {
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println!("Success!");
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}</code></pre></pre>
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<h3 id="tryfromtryinto"><a class="header" href="#tryfromtryinto">TryFrom/TryInto</a></h3>
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<p>Similar to <code>From</code> and <code>Into</code>, <code>TryFrom</code> and <code>TryInto</code> are generic traits for converting between types.</p>
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<p>Unlike <code>From/Into</code>, <code>TryFrom</code> and <code>TryInto</code> are used for fallible conversions and return a <code>Result</code> instead of a plain value.</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">// TryFrom and TryInto are included in `std::prelude`, so there is no need to introduce it into the current scope
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// use std::convert::TryInto;
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fn main() {
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let n: i16 = 256;
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// Into trait has a method `into`,
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// hence TryInto has a method ?
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let n: u8 = match n.__() {
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Ok(n) => n,
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Err(e) => {
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println!("there is an error when converting: {:?}, but we catch it", e.to_string());
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0
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}
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};
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assert_eq!(n, __);
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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">#[derive(Debug, PartialEq)]
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struct EvenNum(i32);
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impl TryFrom<i32> for EvenNum {
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type Error = ();
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// IMPLEMENT `try_from`
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fn try_from(value: i32) -> Result<Self, Self::Error> {
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if value % 2 == 0 {
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Ok(EvenNum(value))
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} else {
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Err(())
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}
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}
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}
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fn main() {
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assert_eq!(EvenNum::try_from(8), Ok(EvenNum(8)));
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assert_eq!(EvenNum::try_from(5), Err(()));
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// FILL in the blanks
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let result: Result<EvenNum, ()> = 8i32.try_into();
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assert_eq!(result, __);
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let result: Result<EvenNum, ()> = 5i32.try_into();
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assert_eq!(result, __);
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println!("Success!");
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}</code></pre></pre>
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<blockquote>
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<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/type-conversions/from-into.md">here</a>(under the solutions path), but only use it when you need it</p>
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