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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li class="expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li class="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li class="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li class="expanded "><a href="ch10-00-generics.html" class="active"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li class="expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li class="expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li class="expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li class="expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li class="expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li class="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li class="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li class="expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li class="expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li class="expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li class="expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li class="expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li class="expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects That Allow for Values of Different Types</a></li><li class="expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li class="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li class="expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li class="expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning Our Single-Threaded Server into a Multithreaded Server</a></li><li class="expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li class="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<h1><a class="header" href="#generic-types-traits-and-lifetimes" id="generic-types-traits-and-lifetimes">Generic Types, Traits, and Lifetimes</a></h1>
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<p>Every programming language has tools for effectively handling the duplication
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of concepts. In Rust, one such tool is <em>generics</em>. Generics are abstract
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stand-ins for concrete types or other properties. When we’re writing code, we
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can express the behavior of generics or how they relate to other generics
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without knowing what will be in their place when compiling and running the code.</p>
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<p>Similar to the way a function takes parameters with unknown values to run the
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same code on multiple concrete values, functions can take parameters of some
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generic type instead of a concrete type, like <code>i32</code> or <code>String</code>. In fact, we’ve
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already used generics in Chapter 6 with <code>Option<T></code>, Chapter 8 with <code>Vec<T></code>
|
||
and <code>HashMap<K, V></code>, and Chapter 9 with <code>Result<T, E></code>. In this chapter, you’ll
|
||
explore how to define your own types, functions, and methods with generics!</p>
|
||
<p>First, we’ll review how to extract a function to reduce code duplication. Next,
|
||
we’ll use the same technique to make a generic function from two functions that
|
||
differ only in the types of their parameters. We’ll also explain how to use
|
||
generic types in struct and enum definitions.</p>
|
||
<p>Then you’ll learn how to use <em>traits</em> to define behavior in a generic way. You
|
||
can combine traits with generic types to constrain a generic type to only
|
||
those types that have a particular behavior, as opposed to just any type.</p>
|
||
<p>Finally, we’ll discuss <em>lifetimes</em>, a variety of generics that give the
|
||
compiler information about how references relate to each other. Lifetimes allow
|
||
us to borrow values in many situations while still enabling the compiler to
|
||
check that the references are valid.</p>
|
||
<h2><a class="header" href="#removing-duplication-by-extracting-a-function" id="removing-duplication-by-extracting-a-function">Removing Duplication by Extracting a Function</a></h2>
|
||
<p>Before diving into generics syntax, let’s first look at how to remove
|
||
duplication that doesn’t involve generic types by extracting a function. Then
|
||
we’ll apply this technique to extract a generic function! In the same way that
|
||
you recognize duplicated code to extract into a function, you’ll start to
|
||
recognize duplicated code that can use generics.</p>
|
||
<p>Consider a short program that finds the largest number in a list, as shown in
|
||
Listing 10-1.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let number_list = vec![34, 50, 25, 100, 65];
|
||
|
||
let mut largest = number_list[0];
|
||
|
||
for number in number_list {
|
||
if number > largest {
|
||
largest = number;
|
||
}
|
||
}
|
||
|
||
println!("The largest number is {}", largest);
|
||
<span class="boring"> assert_eq!(largest, 100);
|
||
</span>}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 10-1: Code to find the largest number in a list
|
||
of numbers</span></p>
|
||
<p>This code stores a list of integers in the variable <code>number_list</code> and places
|
||
the first number in the list in a variable named <code>largest</code>. Then it iterates
|
||
through all the numbers in the list, and if the current number is greater than
|
||
the number stored in <code>largest</code>, it replaces the number in that variable.
|
||
However, if the current number is less than or equal to the largest number seen
|
||
so far, the variable doesn’t change, and the code moves on to the next number
|
||
in the list. After considering all the numbers in the list, <code>largest</code> should
|
||
hold the largest number, which in this case is 100.</p>
|
||
<p>To find the largest number in two different lists of numbers, we can duplicate
|
||
the code in Listing 10-1 and use the same logic at two different places in the
|
||
program, as shown in Listing 10-2.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let number_list = vec![34, 50, 25, 100, 65];
|
||
|
||
let mut largest = number_list[0];
|
||
|
||
for number in number_list {
|
||
if number > largest {
|
||
largest = number;
|
||
}
|
||
}
|
||
|
||
println!("The largest number is {}", largest);
|
||
|
||
let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8];
|
||
|
||
let mut largest = number_list[0];
|
||
|
||
for number in number_list {
|
||
if number > largest {
|
||
largest = number;
|
||
}
|
||
}
|
||
|
||
println!("The largest number is {}", largest);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 10-2: Code to find the largest number in <em>two</em>
|
||
lists of numbers</span></p>
|
||
<p>Although this code works, duplicating code is tedious and error prone. We also
|
||
have to update the code in multiple places when we want to change it.</p>
|
||
<p>To eliminate this duplication, we can create an abstraction by defining a
|
||
function that operates on any list of integers given to it in a parameter. This
|
||
solution makes our code clearer and lets us express the concept of finding the
|
||
largest number in a list abstractly.</p>
|
||
<p>In Listing 10-3, we extracted the code that finds the largest number into a
|
||
function named <code>largest</code>. Unlike the code in Listing 10-1, which can find the
|
||
largest number in only one particular list, this program can find the largest
|
||
number in two different lists.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn largest(list: &[i32]) -> i32 {
|
||
let mut largest = list[0];
|
||
|
||
for &item in list.iter() {
|
||
if item > largest {
|
||
largest = item;
|
||
}
|
||
}
|
||
|
||
largest
|
||
}
|
||
|
||
fn main() {
|
||
let number_list = vec![34, 50, 25, 100, 65];
|
||
|
||
let result = largest(&number_list);
|
||
println!("The largest number is {}", result);
|
||
<span class="boring"> assert_eq!(result, 100);
|
||
</span>
|
||
let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8];
|
||
|
||
let result = largest(&number_list);
|
||
println!("The largest number is {}", result);
|
||
<span class="boring"> assert_eq!(result, 6000);
|
||
</span>}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 10-3: Abstracted code to find the largest number
|
||
in two lists</span></p>
|
||
<p>The <code>largest</code> function has a parameter called <code>list</code>, which represents any
|
||
concrete slice of <code>i32</code> values that we might pass into the function. As a
|
||
result, when we call the function, the code runs on the specific values that we
|
||
pass in.</p>
|
||
<p>In sum, here are the steps we took to change the code from Listing 10-2 to
|
||
Listing 10-3:</p>
|
||
<ol>
|
||
<li>Identify duplicate code.</li>
|
||
<li>Extract the duplicate code into the body of the function and specify the
|
||
inputs and return values of that code in the function signature.</li>
|
||
<li>Update the two instances of duplicated code to call the function instead.</li>
|
||
</ol>
|
||
<p>Next, we’ll use these same steps with generics to reduce code duplication in
|
||
different ways. In the same way that the function body can operate on an
|
||
abstract <code>list</code> instead of specific values, generics allow code to operate on
|
||
abstract types.</p>
|
||
<p>For example, say we had two functions: one that finds the largest item in a
|
||
slice of <code>i32</code> values and one that finds the largest item in a slice of <code>char</code>
|
||
values. How would we eliminate that duplication? Let’s find out!</p>
|
||
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