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<p class='location'>Trait Fn</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.call">call</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>ops</a></p><script>window.sidebarCurrent = {name: 'Fn', ty: 'trait', relpath: ''};</script><script defer src="sidebar-items.js"></script></div>
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<h1 class='fqn'><span class='in-band'>Trait <a href='../index.html'>core</a>::<wbr><a href='index.html'>ops</a>::<wbr><a class="trait" href=''>Fn</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><span id='render-detail'>
<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
[<span class='inner'>−</span>]
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</span><a class='srclink' href='../../src/core/ops/function.rs.html#70-74' title='goto source code'>[src]</a></span></h1>
<pre class='rust trait'><div class="docblock attributes">#[lang = "fn"]
</div>pub trait Fn<Args>: <a class="trait" href="../../core/ops/trait.FnMut.html" title="trait core::ops::FnMut">FnMut</a><Args> {
extern "rust-call" fn <a href='#tymethod.call' class='fnname'>call</a>(&self, args: Args) -> Self::<a class="type" href="../../core/ops/trait.FnOnce.html#associatedtype.Output" title="type core::ops::FnOnce::Output">Output</a>;
}</pre><div class='docblock'><p>The version of the call operator that takes an immutable receiver.</p>
<p>Instances of <code>Fn</code> can be called repeatedly without mutating state.</p>
<p><em>This trait (<code>Fn</code>) is not to be confused with <a href="../../std/primitive.fn.html">function pointers</a>
(<code>fn</code>).</em></p>
<p><code>Fn</code> is implemented automatically by closures which only take immutable
references to captured variables or don't capture anything at all, as well
as (safe) <a href="../../std/primitive.fn.html">function pointers</a> (with some caveats, see their documentation
for more details). Additionally, for any type <code>F</code> that implements <code>Fn</code>, <code>&F</code>
implements <code>Fn</code>, too.</p>
<p>Since both <a href="trait.FnMut.html"><code>FnMut</code></a> and <a href="trait.FnOnce.html"><code>FnOnce</code></a> are supertraits of <code>Fn</code>, any
instance of <code>Fn</code> can be used as a parameter where a <a href="trait.FnMut.html"><code>FnMut</code></a> or <a href="trait.FnOnce.html"><code>FnOnce</code></a>
is expected.</p>
<p>Use <code>Fn</code> as a bound when you want to accept a parameter of function-like
type and need to call it repeatedly and without mutating state (e.g. when
calling it concurrently). If you do not need such strict requirements, use
<a href="trait.FnMut.html"><code>FnMut</code></a> or <a href="trait.FnOnce.html"><code>FnOnce</code></a> as bounds.</p>
<p>See the <a href="../../book/second-edition/ch13-01-closures.html">chapter on closures in <em>The Rust Programming Language</em></a> for
some more information on this topic.</p>
<p>Also of note is the special syntax for <code>Fn</code> traits (e.g.
<code>Fn(usize, bool) -> usize</code>). Those interested in the technical details of
this can refer to <a href="../../nomicon/hrtb.html">the relevant section in the <em>Rustonomicon</em></a>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1><h2 id="calling-a-closure" class="section-header"><a href="#calling-a-closure">Calling a closure</a></h2>
<pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">square</span> <span class="op">=</span> <span class="op">|</span><span class="ident">x</span><span class="op">|</span> <span class="ident">x</span> <span class="op">*</span> <span class="ident">x</span>;
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">square</span>(<span class="number">5</span>), <span class="number">25</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20square%20%3D%20%7Cx%7C%20x%20*%20x%3B%0Aassert_eq!(square(5)%2C%2025)%3B%0A%7D">Run</a></pre>
<h2 id="using-a-fn-parameter" class="section-header"><a href="#using-a-fn-parameter">Using a <code>Fn</code> parameter</a></h2>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">call_with_one</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="ident">func</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">usize</span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">Fn</span>(<span class="ident">usize</span>) <span class="op">-></span> <span class="ident">usize</span> {
<span class="ident">func</span>(<span class="number">1</span>)
}
<span class="kw">let</span> <span class="ident">double</span> <span class="op">=</span> <span class="op">|</span><span class="ident">x</span><span class="op">|</span> <span class="ident">x</span> <span class="op">*</span> <span class="number">2</span>;
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">call_with_one</span>(<span class="ident">double</span>), <span class="number">2</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20call_with_one%3CF%3E(func%3A%20F)%20-%3E%20usize%0A%20%20%20%20where%20F%3A%20Fn(usize)%20-%3E%20usize%20%7B%0A%20%20%20%20func(1)%0A%7D%0A%0Alet%20double%20%3D%20%7Cx%7C%20x%20*%202%3B%0Aassert_eq!(call_with_one(double)%2C%202)%3B%0A%7D">Run</a></pre>
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<h2 id='required-methods' class='small-section-header'>
Required Methods<a href='#required-methods' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='tymethod.call' class='method'><span id='call.v' class='invisible'><code>extern "rust-call" fn <a href='#tymethod.call' class='fnname'>call</a>(&self, args: Args) -> Self::<a class="type" href="../../core/ops/trait.FnOnce.html#associatedtype.Output" title="type core::ops::FnOnce::Output">Output</a></code></span></h3><div class='stability'><div class='stab unstable'><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>fn_traits </code><a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</div></div><div class='docblock'><p>Performs the call operation.</p>
</div></div>
<h2 id='implementors' class='small-section-header'>
Implementors<a href='#implementors' class='anchor'></a>
</h2>
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<li><div class='out-of-band'><a class='srclink' href='../../src/core/ops/function.rs.html#228-234' title='goto source code'>[src]</a></div><code>impl<'a, A, F: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> Fn<A> for &'a F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../core/ops/trait.Fn.html" title="trait core::ops::Fn">Fn</a><A>, </span></code></li>
</ul><script type="text/javascript" async
src="../../implementors/core/ops/trait.Fn.js">
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