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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../translations.html"><strong aria-hidden="true">1.1.</strong> Translations</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">2.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html"><strong aria-hidden="true">2.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">2.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">2.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">2.4.</strong> 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aria-hidden="true">2.14.</strong> Temporary mutability</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">3.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/intro.html"><strong aria-hidden="true">3.1.</strong> Behavioural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/command.html"><strong aria-hidden="true">3.1.1.</strong> Command</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/interpreter.html"><strong aria-hidden="true">3.1.2.</strong> Interpreter</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/newtype.html"><strong aria-hidden="true">3.1.3.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/RAII.html"><strong aria-hidden="true">3.1.4.</strong> RAII Guards</a></li><li class="chapter-item expanded 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aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li><li class="chapter-item expanded "><a href="../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</a></li></ol></li><li class="chapter-item expanded "><a href="../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
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<h1 id="pass-variables-to-closure"><a class="header" href="#pass-variables-to-closure">Pass variables to closure</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>By default, closures capture their environment by borrowing. Or you can use
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<code>move</code>-closure to move whole environment. However, often you want to move just
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some variables to closure, give it copy of some data, pass it by reference, or
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perform some other transformation.</p>
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<p>Use variable rebinding in separate scope for that.</p>
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<h2 id="example"><a class="header" href="#example">Example</a></h2>
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<p>Use</p>
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<pre><pre class="playground"><code class="language-rust edition2018">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::rc::Rc;
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let num1 = Rc::new(1);
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let num2 = Rc::new(2);
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let num3 = Rc::new(3);
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let closure = {
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// `num1` is moved
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let num2 = num2.clone(); // `num2` is cloned
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let num3 = num3.as_ref(); // `num3` is borrowed
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move || {
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*num1 + *num2 + *num3;
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}
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};
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<span class="boring">}
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</span></code></pre></pre>
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<p>instead of</p>
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<pre><pre class="playground"><code class="language-rust edition2018">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::rc::Rc;
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let num1 = Rc::new(1);
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let num2 = Rc::new(2);
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let num3 = Rc::new(3);
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let num2_cloned = num2.clone();
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let num3_borrowed = num3.as_ref();
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let closure = move || {
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*num1 + *num2_cloned + *num3_borrowed;
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};
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<span class="boring">}
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</span></code></pre></pre>
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<h2 id="advantages"><a class="header" href="#advantages">Advantages</a></h2>
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<p>Copied data are grouped together with closure definition, so their purpose is
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more clear, and they will be dropped immediately even if they are not consumed
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by closure.</p>
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<p>Closure uses same variable names as surrounding code whether data are copied or
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moved.</p>
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<h2 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h2>
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<p>Additional indentation of closure body.</p>
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