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<h1 class='fqn'><span class='in-band'>Function <a href='../index.html'>futures</a>::<wbr><a href='index.html'>stream</a>::<wbr><a class="fn" href=''>unfold</a></span><span class='out-of-band'><span id='render-detail'>
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                       [<span class='inner'>&#x2212;</span>]
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               </span><a class='srclink' href='../../src/futures/stream/unfold.rs.html#49-57' title='goto source code'>[src]</a></span></h1>
<pre class='rust fn'>pub fn unfold&lt;T, F, Fut, It&gt;(init: T, f: F) -&gt; <a class="struct" href="../../futures/stream/struct.Unfold.html" title="struct futures::stream::Unfold">Unfold</a>&lt;T, F, Fut&gt; <span class="where fmt-newline">where<br>&nbsp;&nbsp;&nbsp;&nbsp;F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(T) -&gt; <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;Fut&gt;,<br>&nbsp;&nbsp;&nbsp;&nbsp;Fut: <a class="trait" href="../../futures/future/trait.IntoFuture.html" title="trait futures::future::IntoFuture">IntoFuture</a>&lt;Item = <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">(</a>It, T<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">)</a>&gt;,&nbsp;</span></pre><div class='docblock'><p>Creates a <code>Stream</code> from a seed and a closure returning a <code>Future</code>.</p>
<p>This function is the dual for the <code>Stream::fold()</code> adapter: while
<code>Stream::fold()</code> reduces a <code>Stream</code> to one single value, <code>unfold()</code> creates a
<code>Stream</code> from a seed value.</p>
<p><code>unfold()</code> will call the provided closure with the provided seed, then wait
for the returned <code>Future</code> to complete with <code>(a, b)</code>. It will then yield the
value <code>a</code>, and use <code>b</code> as the next internal state.</p>
<p>If the closure returns <code>None</code> instead of <code>Some(Future)</code>, then the <code>unfold()</code>
will stop producing items and return <code>Ok(Async::Ready(None))</code> in future
calls to <code>poll()</code>.</p>
<p>In case of error generated by the returned <code>Future</code>, the error will be
returned by the <code>Stream</code>.  The <code>Stream</code> will then yield
<code>Ok(Async::Ready(None))</code> in future calls to <code>poll()</code>.</p>
<p>This function can typically be used when wanting to go from the &quot;world of
futures&quot; to the &quot;world of streams&quot;: the provided closure can build a
<code>Future</code> using other library functions working on futures, and <code>unfold()</code>
will turn it into a <code>Stream</code> by repeating the operation.</p>
<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">futures</span>::<span class="ident">stream</span>::{<span class="self">self</span>, <span class="ident">Stream</span>};
<span class="kw">use</span> <span class="ident">futures</span>::<span class="ident">future</span>::{<span class="self">self</span>, <span class="ident">Future</span>};

<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">stream</span> <span class="op">=</span> <span class="ident">stream</span>::<span class="ident">unfold</span>(<span class="number">0</span>, <span class="op">|</span><span class="ident">state</span><span class="op">|</span> {
    <span class="kw">if</span> <span class="ident">state</span> <span class="op">&lt;=</span> <span class="number">2</span> {
        <span class="kw">let</span> <span class="ident">next_state</span> <span class="op">=</span> <span class="ident">state</span> <span class="op">+</span> <span class="number">1</span>;
        <span class="kw">let</span> <span class="ident">yielded</span> <span class="op">=</span> <span class="ident">state</span>  <span class="op">*</span> <span class="number">2</span>;
        <span class="kw">let</span> <span class="ident">fut</span> <span class="op">=</span> <span class="ident">future</span>::<span class="ident">ok</span>::<span class="op">&lt;</span>_, <span class="ident">u32</span><span class="op">&gt;</span>((<span class="ident">yielded</span>, <span class="ident">next_state</span>));
        <span class="prelude-val">Some</span>(<span class="ident">fut</span>)
    } <span class="kw">else</span> {
        <span class="prelude-val">None</span>
    }
});

<span class="kw">let</span> <span class="ident">result</span> <span class="op">=</span> <span class="ident">stream</span>.<span class="ident">collect</span>().<span class="ident">wait</span>();
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">result</span>, <span class="prelude-val">Ok</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="number">0</span>, <span class="number">2</span>, <span class="number">4</span>]));</pre>
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