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605 lines
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<main>
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<h1 id="closure"><a class="header" href="#closure">Closure</a></h1>
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<p>Closures can capture the enclosing environments. For example we can capture the <code>x</code> variable :</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let x = 1;
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let closure = |val| val + x;
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assert_eq!(closure(2), 3);
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}</code></pre></pre>
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<p>From the syntax, we can see that closures are very convenient for on the fly usage. Unlike functions, both the input and return types of a closure can be inferred by the compiler.</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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// Increment via closures and functions.
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fn function(i: i32) -> i32 { i + 1 }
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// Closures are anonymous, here we are binding them to references
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//
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// These nameless functions are assigned to appropriately named variables.
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let closure_annotated = |i: i32| -> i32 { i + 1 };
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let closure_inferred = |i | i + 1 ;
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let i = 1;
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// Call the function and closures.
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println!("function: {}", function(i));
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println!("closure_annotated: {}", closure_annotated(i));
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println!("closure_inferred: {}", closure_inferred(i));
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// A closure taking no arguments which returns an `i32`.
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// The return type is inferred.
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let one = || 1;
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println!("closure returning one: {}", one());
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}</code></pre></pre>
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<h2 id="capturing"><a class="header" href="#capturing">Capturing</a></h2>
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<p>Closures can capture variables by borrowing or moving. But they prefer to capture by borrowing and only go lower when required:</p>
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<ul>
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<li>By reference: <code>&T</code></li>
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<li>By mutable reference: <code>&mut T</code></li>
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<li>By value: <code>T</code></li>
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</ul>
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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">/* Make it work with least amount of changes*/
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fn main() {
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let color = String::from("green");
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let print = move || println!("`color`: {}", color);
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print();
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print();
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// `color` can be borrowed immutably again, because the closure only holds
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// an immutable reference to `color`.
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let _reborrow = &color;
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println!("{}",color);
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}</code></pre></pre>
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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">/* Make it work
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- Dont use `_reborrow` and `_count_reborrowed`
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- Dont modify `assert_eq`
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*/
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fn main() {
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let mut count = 0;
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let mut inc = || {
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count += 1;
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println!("`count`: {}", count);
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};
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inc();
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let _reborrow = &count;
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inc();
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// The closure no longer needs to borrow `&mut count`. Therefore, it is
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// possible to reborrow without an error
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let _count_reborrowed = &mut count;
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assert_eq!(count, 0);
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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">/* Make it work in two ways, none of them is to remove `take(movable)` away from the code
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*/
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fn main() {
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let movable = Box::new(3);
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let consume = || {
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println!("`movable`: {:?}", movable);
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take(movable);
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};
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consume();
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consume();
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}
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fn take<T>(_v: T) {}</code></pre></pre>
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<p>For comparison, the following code has no error:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let movable = Box::new(3);
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let consume = move || {
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println!("`movable`: {:?}", movable);
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};
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consume();
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consume();
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}</code></pre></pre>
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<h2 id="type-inferred"><a class="header" href="#type-inferred">Type inferred</a></h2>
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<p>The following four closures has no difference in input and return types.</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>fn add_one_v1 (x: u32) -> u32 { x + 1 }
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let add_one_v2 = |x: u32| -> u32 { x + 1 };
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let add_one_v3 = |x| { x + 1 };
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let add_one_v4 = |x| x + 1 ;
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<span class="boring">}</span></code></pre></pre>
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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">fn main() {
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let example_closure = |x| x;
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let s = example_closure(String::from("hello"));
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|
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/* Make it work, only change the following line */
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let n = example_closure(5);
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}</code></pre></pre>
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<h2 id="fn-fnmut-fnonce"><a class="header" href="#fn-fnmut-fnonce">Fn, FnMut, FnOnce</a></h2>
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<p>When taking a closure as an input parameter, the closure's complete type must be annotated using one of the following traits:</p>
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<ul>
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<li>Fn: the closure uses the captured value by reference (&T)</li>
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<li>FnMut: the closure uses the captured value by mutable reference (&mut T)</li>
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<li>FnOnce: the closure uses the captured value by value (T)</li>
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</ul>
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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">/* Make it work by changing the trait bound, in two ways*/
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fn fn_once<F>(func: F)
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where
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F: FnOnce(usize) -> bool,
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{
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println!("{}", func(3));
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println!("{}", func(4));
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}
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fn main() {
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let x = vec![1, 2, 3];
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fn_once(|z|{z == x.len()})
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}</code></pre></pre>
|
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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">fn main() {
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let mut s = String::new();
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let update_string = |str| s.push_str(str);
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exec(update_string);
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println!("{:?}",s);
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}
|
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|
|
/* Fill in the blank */
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|
fn exec<'a, F: __>(mut f: F) {
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f("hello")
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|
}</code></pre></pre>
|
|
<h4 id="which-trait-does-the-compiler-prefer-to-use"><a class="header" href="#which-trait-does-the-compiler-prefer-to-use">Which trait does the compiler prefer to use?</a></h4>
|
|
<ul>
|
|
<li>Fn: the closure uses the captured value by reference (&T)</li>
|
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<li>FnMut: the closure uses the captured value by mutable reference (&mut T)</li>
|
|
<li>FnOnce: the closure uses the captured value by value (T)</li>
|
|
</ul>
|
|
<p>On a variable-by-variable basis, the compiler will capture variables in the least restrictive manner possible.</p>
|
|
<p>For instance, consider a parameter annotated as FnOnce. This specifies that the closure may capture by <code>&T</code>, <code>&mut T</code>, or <code>T</code>, but the compiler will ultimately choose based on how the captured variables are used in the closure.
|
|
Which trait to use is determined by what the closure does with captured value.</p>
|
|
<p>This is because if a move is possible, then any type of borrow should also be possible. Note that the reverse is not true. If the parameter is annotated as <code>Fn</code>, then capturing variables by <code>&mut T</code> or <code>T</code> are not allowed.</p>
|
|
<ol start="7">
|
|
<li>🌟🌟</li>
|
|
</ol>
|
|
<pre><pre class="playground"><code class="language-rust editable edition2021">/* Fill in the blank */
|
|
|
|
// A function which takes a closure as an argument and calls it.
|
|
// <F> denotes that F is a "Generic type parameter"
|
|
fn apply<F>(f: F) where
|
|
// The closure takes no input and returns nothing.
|
|
F: __ {
|
|
|
|
f();
|
|
}
|
|
|
|
// A function which takes a closure and returns an `i32`.
|
|
fn apply_to_3<F>(f: F) -> i32 where
|
|
// The closure takes an `i32` and returns an `i32`.
|
|
F: Fn(i32) -> i32 {
|
|
|
|
f(3)
|
|
}
|
|
|
|
fn main() {
|
|
use std::mem;
|
|
|
|
let greeting = "hello";
|
|
// A non-copy type.
|
|
// `to_owned` creates owned data from borrowed one
|
|
let mut farewell = "goodbye".to_owned();
|
|
|
|
// Capture 2 variables: `greeting` by reference and
|
|
// `farewell` by value.
|
|
let diary = || {
|
|
// `greeting` is by reference: requires `Fn`.
|
|
println!("I said {}.", greeting);
|
|
|
|
// Mutation forces `farewell` to be captured by
|
|
// mutable reference. Now requires `FnMut`.
|
|
farewell.push_str("!!!");
|
|
println!("Then I screamed {}.", farewell);
|
|
println!("Now I can sleep. zzzzz");
|
|
|
|
// Manually calling drop forces `farewell` to
|
|
// be captured by value. Now requires `FnOnce`.
|
|
mem::drop(farewell);
|
|
};
|
|
|
|
// Call the function which applies the closure.
|
|
apply(diary);
|
|
|
|
// `double` satisfies `apply_to_3`'s trait bound
|
|
let double = |x| 2 * x;
|
|
|
|
println!("3 doubled: {}", apply_to_3(double));
|
|
}</code></pre></pre>
|
|
<p>Move closures may still implement <code>Fn</code> or <code>FnMut</code>, even though they capture variables by move. This is because the traits implemented by a closure type are determined by what the closure does with captured values, not how it captures them. The <code>move</code> keyword only specifies the latter.</p>
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
let s = String::new();
|
|
|
|
let update_string = move || println!("{}",s);
|
|
|
|
exec(update_string);
|
|
}
|
|
|
|
fn exec<F: FnOnce()>(f: F) {
|
|
f()
|
|
}</code></pre></pre>
|
|
<p>The following code also has no error:</p>
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
let s = String::new();
|
|
|
|
let update_string = move || println!("{}",s);
|
|
|
|
exec(update_string);
|
|
}
|
|
|
|
fn exec<F: Fn()>(f: F) {
|
|
f()
|
|
}</code></pre></pre>
|
|
<ol start="8">
|
|
<li>🌟🌟</li>
|
|
</ol>
|
|
<pre><pre class="playground"><code class="language-rust editable edition2021">/* Fill in the blank */
|
|
fn main() {
|
|
let mut s = String::new();
|
|
|
|
let update_string = |str| -> String {s.push_str(str); s };
|
|
|
|
exec(update_string);
|
|
}
|
|
|
|
fn exec<'a, F: __>(mut f: F) {
|
|
f("hello");
|
|
}</code></pre></pre>
|
|
<h2 id="input-functions"><a class="header" href="#input-functions">Input functions</a></h2>
|
|
<p>Since closure can be used as arguments, you might wonder can we use functions as arguments too? And indeed we can.</p>
|
|
<ol start="9">
|
|
<li>🌟🌟</li>
|
|
</ol>
|
|
<pre><pre class="playground"><code class="language-rust editable edition2021">
|
|
/* Implement `call_me` to make it work */
|
|
fn call_me {
|
|
f();
|
|
}
|
|
|
|
fn function() {
|
|
println!("I'm a function!");
|
|
}
|
|
|
|
fn main() {
|
|
let closure = || println!("I'm a closure!");
|
|
|
|
call_me(closure);
|
|
call_me(function);
|
|
}</code></pre></pre>
|
|
<h2 id="closure-as-return-types"><a class="header" href="#closure-as-return-types">Closure as return types</a></h2>
|
|
<p>Returning a closure is much harder than you may have thought of.</p>
|
|
<ol start="10">
|
|
<li>🌟🌟</li>
|
|
</ol>
|
|
<pre><pre class="playground"><code class="language-rust editable edition2021">/* Fill in the blank using two approaches,
|
|
and fix the error */
|
|
fn create_fn() -> __ {
|
|
let num = 5;
|
|
|
|
// How does the following closure capture the environment variable `num`
|
|
// &T, &mut T, T ?
|
|
|x| x + num
|
|
}
|
|
|
|
|
|
fn main() {
|
|
let fn_plain = create_fn();
|
|
fn_plain(1);
|
|
}</code></pre></pre>
|
|
<ol start="11">
|
|
<li>🌟🌟</li>
|
|
</ol>
|
|
<pre><pre class="playground"><code class="language-rust editable edition2021">/* Fill in the blank and fix the error*/
|
|
fn factory(x:i32) -> __ {
|
|
|
|
let num = 5;
|
|
|
|
if x > 1{
|
|
move |x| x + num
|
|
} else {
|
|
move |x| x + num
|
|
}
|
|
}</code></pre></pre>
|
|
<h2 id="closure-in-structs"><a class="header" href="#closure-in-structs">Closure in structs</a></h2>
|
|
<p><strong>Example</strong></p>
|
|
<pre><pre class="playground"><code class="language-rust edition2021">struct Cacher<T,E>
|
|
where
|
|
T: Fn(E) -> E,
|
|
E: Copy
|
|
{
|
|
query: T,
|
|
value: Option<E>,
|
|
}
|
|
|
|
impl<T,E> Cacher<T,E>
|
|
where
|
|
T: Fn(E) -> E,
|
|
E: Copy
|
|
{
|
|
fn new(query: T) -> Cacher<T,E> {
|
|
Cacher {
|
|
query,
|
|
value: None,
|
|
}
|
|
}
|
|
|
|
fn value(&mut self, arg: E) -> E {
|
|
match self.value {
|
|
Some(v) => v,
|
|
None => {
|
|
let v = (self.query)(arg);
|
|
self.value = Some(v);
|
|
v
|
|
}
|
|
}
|
|
}
|
|
}
|
|
fn main() {
|
|
|
|
}
|
|
|
|
#[test]
|
|
fn call_with_different_values() {
|
|
let mut c = Cacher::new(|a| a);
|
|
|
|
let v1 = c.value(1);
|
|
let v2 = c.value(2);
|
|
|
|
assert_eq!(v2, 1);
|
|
}</code></pre></pre>
|
|
<blockquote>
|
|
<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/functional-programing/closure.md">here</a>(under the solutions path), but only use it when you need it :)</p>
|
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