| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.28/src/stream/unfold.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>unfold.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="../../../source-script.js"></script><script defer src="../../../source-files.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../futures_util/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../futures_util/index.html"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="kw">use </span><span class="kw">super</span>::assert_stream; |
| <span class="kw">use </span><span class="kw">crate</span>::unfold_state::UnfoldState; |
| <span class="kw">use </span>core::fmt; |
| <span class="kw">use </span>core::pin::Pin; |
| <span class="kw">use </span>futures_core::future::Future; |
| <span class="kw">use </span>futures_core::ready; |
| <span class="kw">use </span>futures_core::stream::{FusedStream, Stream}; |
| <span class="kw">use </span>futures_core::task::{Context, Poll}; |
| <span class="kw">use </span>pin_project_lite::pin_project; |
| |
| <span class="doccomment">/// Creates a `Stream` from a seed and a closure returning a `Future`. |
| /// |
| /// This function is the dual for the `Stream::fold()` adapter: while |
| /// `Stream::fold()` reduces a `Stream` to one single value, `unfold()` creates a |
| /// `Stream` from a seed value. |
| /// |
| /// `unfold()` will call the provided closure with the provided seed, then wait |
| /// for the returned `Future` to complete with `(a, b)`. It will then yield the |
| /// value `a`, and use `b` as the next internal state. |
| /// |
| /// If the closure returns `None` instead of `Some(Future)`, then the `unfold()` |
| /// will stop producing items and return `Poll::Ready(None)` in future |
| /// calls to `poll()`. |
| /// |
| /// This function can typically be used when wanting to go from the "world of |
| /// futures" to the "world of streams": the provided closure can build a |
| /// `Future` using other library functions working on futures, and `unfold()` |
| /// will turn it into a `Stream` by repeating the operation. |
| /// |
| /// # Example |
| /// |
| /// ``` |
| /// # futures::executor::block_on(async { |
| /// use futures::stream::{self, StreamExt}; |
| /// |
| /// let stream = stream::unfold(0, |state| async move { |
| /// if state <= 2 { |
| /// let next_state = state + 1; |
| /// let yielded = state * 2; |
| /// Some((yielded, next_state)) |
| /// } else { |
| /// None |
| /// } |
| /// }); |
| /// |
| /// let result = stream.collect::<Vec<i32>>().await; |
| /// assert_eq!(result, vec![0, 2, 4]); |
| /// # }); |
| /// ``` |
| </span><span class="kw">pub fn </span>unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut> |
| <span class="kw">where |
| </span>F: FnMut(T) -> Fut, |
| Fut: Future<Output = <span class="prelude-ty">Option</span><(Item, T)>>, |
| { |
| assert_stream::<Item, <span class="kw">_</span>>(Unfold { f, state: UnfoldState::Value { value: init } }) |
| } |
| |
| <span class="macro">pin_project! </span>{ |
| <span class="doccomment">/// Stream for the [`unfold`] function. |
| </span><span class="attribute">#[must_use = <span class="string">"streams do nothing unless polled"</span>] |
| </span><span class="kw">pub struct </span>Unfold<T, F, Fut> { |
| f: F, |
| <span class="attribute">#[pin] |
| </span>state: UnfoldState<T, Fut>, |
| } |
| } |
| |
| <span class="kw">impl</span><T, F, Fut> fmt::Debug <span class="kw">for </span>Unfold<T, F, Fut> |
| <span class="kw">where |
| </span>T: fmt::Debug, |
| Fut: fmt::Debug, |
| { |
| <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, f: <span class="kw-2">&mut </span>fmt::Formatter<<span class="lifetime">'_</span>>) -> fmt::Result { |
| f.debug_struct(<span class="string">"Unfold"</span>).field(<span class="string">"state"</span>, <span class="kw-2">&</span><span class="self">self</span>.state).finish() |
| } |
| } |
| |
| <span class="kw">impl</span><T, F, Fut, Item> FusedStream <span class="kw">for </span>Unfold<T, F, Fut> |
| <span class="kw">where |
| </span>F: FnMut(T) -> Fut, |
| Fut: Future<Output = <span class="prelude-ty">Option</span><(Item, T)>>, |
| { |
| <span class="kw">fn </span>is_terminated(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="kw">if let </span>UnfoldState::Empty = <span class="self">self</span>.state { |
| <span class="bool-val">true |
| </span>} <span class="kw">else </span>{ |
| <span class="bool-val">false |
| </span>} |
| } |
| } |
| |
| <span class="kw">impl</span><T, F, Fut, Item> Stream <span class="kw">for </span>Unfold<T, F, Fut> |
| <span class="kw">where |
| </span>F: FnMut(T) -> Fut, |
| Fut: Future<Output = <span class="prelude-ty">Option</span><(Item, T)>>, |
| { |
| <span class="kw">type </span>Item = Item; |
| |
| <span class="kw">fn </span>poll_next(<span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>, cx: <span class="kw-2">&mut </span>Context<<span class="lifetime">'_</span>>) -> Poll<<span class="prelude-ty">Option</span><<span class="self">Self</span>::Item>> { |
| <span class="kw">let </span><span class="kw-2">mut </span>this = <span class="self">self</span>.project(); |
| |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(state) = this.state.as_mut().take_value() { |
| this.state.set(UnfoldState::Future { future: (this.f)(state) }); |
| } |
| |
| <span class="kw">let </span>step = <span class="kw">match </span>this.state.as_mut().project_future() { |
| <span class="prelude-val">Some</span>(fut) => <span class="macro">ready!</span>(fut.poll(cx)), |
| <span class="prelude-val">None </span>=> <span class="macro">panic!</span>(<span class="string">"Unfold must not be polled after it returned `Poll::Ready(None)`"</span>), |
| }; |
| |
| <span class="kw">if let </span><span class="prelude-val">Some</span>((item, next_state)) = step { |
| this.state.set(UnfoldState::Value { value: next_state }); |
| Poll::Ready(<span class="prelude-val">Some</span>(item)) |
| } <span class="kw">else </span>{ |
| this.state.set(UnfoldState::Empty); |
| Poll::Ready(<span class="prelude-val">None</span>) |
| } |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="futures_util" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
