| <!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/try_stream/try_unfold.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>try_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>core::fmt; |
| <span class="kw">use </span>core::pin::Pin; |
| <span class="kw">use </span>futures_core::future::TryFuture; |
| <span class="kw">use </span>futures_core::ready; |
| <span class="kw">use </span>futures_core::stream::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 `TryStream` from a seed and a closure returning a `TryFuture`. |
| /// |
| /// This function is the dual for the `TryStream::try_fold()` adapter: while |
| /// `TryStream::try_fold()` reduces a `TryStream` to one single value, |
| /// `try_unfold()` creates a `TryStream` from a seed value. |
| /// |
| /// `try_unfold()` will call the provided closure with the provided seed, then |
| /// wait for the returned `TryFuture` 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(TryFuture)`, then the |
| /// `try_unfold()` will stop producing items and return `Poll::Ready(None)` in |
| /// future calls to `poll()`. |
| /// |
| /// In case of error generated by the returned `TryFuture`, the error will be |
| /// returned by the `TryStream`. The `TryStream` will then yield |
| /// `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 |
| /// `TryFuture` using other library functions working on futures, and |
| /// `try_unfold()` will turn it into a `TryStream` by repeating the operation. |
| /// |
| /// # Example |
| /// |
| /// ``` |
| /// # #[derive(Debug, PartialEq)] |
| /// # struct SomeError; |
| /// # futures::executor::block_on(async { |
| /// use futures::stream::{self, TryStreamExt}; |
| /// |
| /// let stream = stream::try_unfold(0, |state| async move { |
| /// if state < 0 { |
| /// return Err(SomeError); |
| /// } |
| /// |
| /// if state <= 2 { |
| /// let next_state = state + 1; |
| /// let yielded = state * 2; |
| /// Ok(Some((yielded, next_state))) |
| /// } else { |
| /// Ok(None) |
| /// } |
| /// }); |
| /// |
| /// let result: Result<Vec<i32>, _> = stream.try_collect().await; |
| /// assert_eq!(result, Ok(vec![0, 2, 4])); |
| /// # }); |
| /// ``` |
| </span><span class="kw">pub fn </span>try_unfold<T, F, Fut, Item>(init: T, f: F) -> TryUnfold<T, F, Fut> |
| <span class="kw">where |
| </span>F: FnMut(T) -> Fut, |
| Fut: TryFuture<<span class="prelude-val">Ok </span>= <span class="prelude-ty">Option</span><(Item, T)>>, |
| { |
| assert_stream::<<span class="prelude-ty">Result</span><Item, Fut::Error>, <span class="kw">_</span>>(TryUnfold { f, state: <span class="prelude-val">Some</span>(init), fut: <span class="prelude-val">None </span>}) |
| } |
| |
| <span class="macro">pin_project! </span>{ |
| <span class="doccomment">/// Stream for the [`try_unfold`] function. |
| </span><span class="attribute">#[must_use = <span class="string">"streams do nothing unless polled"</span>] |
| </span><span class="kw">pub struct </span>TryUnfold<T, F, Fut> { |
| f: F, |
| state: <span class="prelude-ty">Option</span><T>, |
| <span class="attribute">#[pin] |
| </span>fut: <span class="prelude-ty">Option</span><Fut>, |
| } |
| } |
| |
| <span class="kw">impl</span><T, F, Fut> fmt::Debug <span class="kw">for </span>TryUnfold<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">"TryUnfold"</span>).field(<span class="string">"state"</span>, <span class="kw-2">&</span><span class="self">self</span>.state).field(<span class="string">"fut"</span>, <span class="kw-2">&</span><span class="self">self</span>.fut).finish() |
| } |
| } |
| |
| <span class="kw">impl</span><T, F, Fut, Item> Stream <span class="kw">for </span>TryUnfold<T, F, Fut> |
| <span class="kw">where |
| </span>F: FnMut(T) -> Fut, |
| Fut: TryFuture<<span class="prelude-val">Ok </span>= <span class="prelude-ty">Option</span><(Item, T)>>, |
| { |
| <span class="kw">type </span>Item = <span class="prelude-ty">Result</span><Item, Fut::Error>; |
| |
| <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.take() { |
| this.fut.set(<span class="prelude-val">Some</span>((this.f)(state))); |
| } |
| |
| <span class="kw">match </span>this.fut.as_mut().as_pin_mut() { |
| <span class="prelude-val">None </span>=> { |
| <span class="comment">// The future previously errored |
| </span>Poll::Ready(<span class="prelude-val">None</span>) |
| } |
| <span class="prelude-val">Some</span>(future) => { |
| <span class="kw">let </span>step = <span class="macro">ready!</span>(future.try_poll(cx)); |
| this.fut.set(<span class="prelude-val">None</span>); |
| |
| <span class="kw">match </span>step { |
| <span class="prelude-val">Ok</span>(<span class="prelude-val">Some</span>((item, next_state))) => { |
| <span class="kw-2">*</span>this.state = <span class="prelude-val">Some</span>(next_state); |
| Poll::Ready(<span class="prelude-val">Some</span>(<span class="prelude-val">Ok</span>(item))) |
| } |
| <span class="prelude-val">Ok</span>(<span class="prelude-val">None</span>) => Poll::Ready(<span class="prelude-val">None</span>), |
| <span class="prelude-val">Err</span>(e) => Poll::Ready(<span class="prelude-val">Some</span>(<span class="prelude-val">Err</span>(e))), |
| } |
| } |
| } |
| } |
| } |
| </code></pre></div> |
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