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</pre><pre class="rust ">
<span class="kw">use</span> <span class="ident">core</span>::<span class="ident">mem</span>;
<span class="kw">use</span> {<span class="ident">Future</span>, <span class="ident">IntoFuture</span>, <span class="ident">Async</span>, <span class="ident">Poll</span>};
<span class="kw">use</span> <span class="ident">stream</span>::<span class="ident">Stream</span>;
<span class="doccomment">/// Creates a `Stream` from a seed and a closure returning a `Future`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This function is the dual for the `Stream::fold()` adapter: while</span>
<span class="doccomment">/// `Stream:fold()` reduces a `Stream` to one single value, `unfold()` creates a</span>
<span class="doccomment">/// `Stream` from a seed value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `unfold()` will call the provided closure with the provided seed, then wait</span>
<span class="doccomment">/// for the returned `Future` to complete with `(a, b)`. It will then yield the</span>
<span class="doccomment">/// value `a`, and use `b` as the next internal state.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the closure returns `None` instead of `Some(Future)`, then the `unfold()`</span>
<span class="doccomment">/// will stop producing items and return `Ok(Async::Ready(None))` in future</span>
<span class="doccomment">/// calls to `poll()`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// In case of error generated by the returned `Future`, the error will be</span>
<span class="doccomment">/// returned by the `Stream`. The `Stream` will then yield</span>
<span class="doccomment">/// `Ok(Async::Ready(None))` in future calls to `poll()`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This function can typically be used when wanting to go from the "world of</span>
<span class="doccomment">/// futures" to the "world of streams": the provided closure can build a</span>
<span class="doccomment">/// `Future` using other library functions working on futures, and `unfold()`</span>
<span class="doccomment">/// will turn it into a `Stream` by repeating the operation.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Example</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```rust</span>
<span class="doccomment">/// use futures::stream::{self, Stream};</span>
<span class="doccomment">/// use futures::future::{self, Future};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut stream = stream::unfold(0, |state| {</span>
<span class="doccomment">/// if state <= 2 {</span>
<span class="doccomment">/// let next_state = state + 1;</span>
<span class="doccomment">/// let yielded = state * 2;</span>
<span class="doccomment">/// let fut = future::ok::<_, u32>((yielded, next_state));</span>
<span class="doccomment">/// Some(fut)</span>
<span class="doccomment">/// } else {</span>
<span class="doccomment">/// None</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// });</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let result = stream.collect().wait();</span>
<span class="doccomment">/// assert_eq!(result, Ok(vec![0, 2, 4]));</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">unfold</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span>, <span class="ident">Fut</span>, <span class="ident">It</span><span class="op">></span>(<span class="ident">init</span>: <span class="ident">T</span>, <span class="ident">f</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">Unfold</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span>, <span class="ident">Fut</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Fut</span><span class="op">></span>,
<span class="ident">Fut</span>: <span class="ident">IntoFuture</span><span class="op"><</span><span class="ident">Item</span> <span class="op">=</span> (<span class="ident">It</span>, <span class="ident">T</span>)<span class="op">></span>,
{
<span class="ident">Unfold</span> {
<span class="ident">f</span>: <span class="ident">f</span>,
<span class="ident">state</span>: <span class="ident">State</span>::<span class="ident">Ready</span>(<span class="ident">init</span>),
}
}
<span class="doccomment">/// A stream which creates futures, polls them and return their result</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This stream is returned by the `futures::stream::unfold` method</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="attribute">#[<span class="ident">must_use</span> <span class="op">=</span> <span class="string">"streams do nothing unless polled"</span>]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Unfold</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span>, <span class="ident">Fut</span><span class="op">></span> <span class="kw">where</span> <span class="ident">Fut</span>: <span class="ident">IntoFuture</span> {
<span class="ident">f</span>: <span class="ident">F</span>,
<span class="ident">state</span>: <span class="ident">State</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">Fut</span>::<span class="ident">Future</span><span class="op">></span>,
}
<span class="kw">impl</span> <span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span>, <span class="ident">Fut</span>, <span class="ident">It</span><span class="op">></span> <span class="ident">Stream</span> <span class="kw">for</span> <span class="ident">Unfold</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span>, <span class="ident">Fut</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Fut</span><span class="op">></span>,
<span class="ident">Fut</span>: <span class="ident">IntoFuture</span><span class="op"><</span><span class="ident">Item</span> <span class="op">=</span> (<span class="ident">It</span>, <span class="ident">T</span>)<span class="op">></span>,
{
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">It</span>;
<span class="kw">type</span> <span class="ident">Error</span> <span class="op">=</span> <span class="ident">Fut</span>::<span class="ident">Error</span>;
<span class="kw">fn</span> <span class="ident">poll</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="ident">Poll</span><span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">It</span><span class="op">></span>, <span class="ident">Fut</span>::<span class="ident">Error</span><span class="op">></span> {
<span class="kw">loop</span> {
<span class="kw">match</span> <span class="ident">mem</span>::<span class="ident">replace</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>.<span class="ident">state</span>, <span class="ident">State</span>::<span class="ident">Empty</span>) {
<span class="comment">// State::Empty may happen if the future returned an error</span>
<span class="ident">State</span>::<span class="ident">Empty</span> <span class="op">=></span> { <span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">Async</span>::<span class="ident">Ready</span>(<span class="prelude-val">None</span>)); }
<span class="ident">State</span>::<span class="ident">Ready</span>(<span class="ident">state</span>) <span class="op">=></span> {
<span class="kw">match</span> (<span class="self">self</span>.<span class="ident">f</span>)(<span class="ident">state</span>) {
<span class="prelude-val">Some</span>(<span class="ident">fut</span>) <span class="op">=></span> { <span class="self">self</span>.<span class="ident">state</span> <span class="op">=</span> <span class="ident">State</span>::<span class="ident">Processing</span>(<span class="ident">fut</span>.<span class="ident">into_future</span>()); }
<span class="prelude-val">None</span> <span class="op">=></span> { <span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">Async</span>::<span class="ident">Ready</span>(<span class="prelude-val">None</span>)); }
}
}
<span class="ident">State</span>::<span class="ident">Processing</span>(<span class="kw-2">mut</span> <span class="ident">fut</span>) <span class="op">=></span> {
<span class="kw">match</span> <span class="macro">try</span><span class="macro">!</span>(<span class="ident">fut</span>.<span class="ident">poll</span>()) {
<span class="ident">Async</span>:: <span class="ident">Ready</span>((<span class="ident">item</span>, <span class="ident">next_state</span>)) <span class="op">=></span> {
<span class="self">self</span>.<span class="ident">state</span> <span class="op">=</span> <span class="ident">State</span>::<span class="ident">Ready</span>(<span class="ident">next_state</span>);
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">Async</span>::<span class="ident">Ready</span>(<span class="prelude-val">Some</span>(<span class="ident">item</span>)));
}
<span class="ident">Async</span>::<span class="ident">NotReady</span> <span class="op">=></span> {
<span class="self">self</span>.<span class="ident">state</span> <span class="op">=</span> <span class="ident">State</span>::<span class="ident">Processing</span>(<span class="ident">fut</span>);
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">Async</span>::<span class="ident">NotReady</span>);
}
}
}
}
}
}
}
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">enum</span> <span class="ident">State</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span> <span class="kw">where</span> <span class="ident">F</span>: <span class="ident">Future</span> {
<span class="doccomment">/// Placeholder state when doing work, or when the returned Future generated an error</span>
<span class="ident">Empty</span>,
<span class="doccomment">/// Ready to generate new future; current internal state is the `T`</span>
<span class="ident">Ready</span>(<span class="ident">T</span>),
<span class="doccomment">/// Working on a future generated previously</span>
<span class="ident">Processing</span>(<span class="ident">F</span>),
}
</pre>
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