| <!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/tantivy-fst-0.4.0/src/stream.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>stream.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="../../tantivy_fst/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="../../tantivy_fst/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="doccomment">/// Streamer describes a "streaming iterator." |
| /// |
| /// It provides a mechanism for writing code that is generic over streams |
| /// produced by this crate. |
| /// |
| /// Note that this is strictly less useful than `Iterator` because the item |
| /// associated type is bound to a specific lifetime. However, this does permit |
| /// us to write *some* generic code over streams that produce values tied |
| /// to the lifetime of the stream. |
| /// |
| /// Some form of stream abstraction is inherently required for this crate |
| /// because elements in a finite state transducer are produced *by iterating* |
| /// over the structure. The alternative would be to create a new allocation |
| /// for each element iterated over, which would be prohibitively expensive. |
| /// |
| /// # Usage & motivation |
| /// |
| /// Streams are hard to use because they don't fit into Rust's current type |
| /// system very well. They are so hard to use that this author loathes having a |
| /// publically defined trait for it. Nevertheless, they do just barely provide |
| /// a means for composing multiple stream abstractions with different concrete |
| /// types. For example, one might want to take the union of a range query |
| /// stream with a stream that has been filtered by a regex. These streams have |
| /// different concrete types. A `Streamer` trait allows us to write code that |
| /// is generic over these concrete types. (All of the set operations are |
| /// implemented this way.) |
| /// |
| /// A problem with streams is that the trait is itself parameterized by a |
| /// lifetime. In practice, this makes them very unergonomic because specifying |
| /// a `Streamer` bound generally requires a higher-ranked trait bound. This is |
| /// necessary because the lifetime can't actually be named in the enclosing |
| /// function; instead, the lifetime is local to iteration itself. Therefore, |
| /// one must assert that the bound is valid for *any particular* lifetime. |
| /// This is the essence of higher-rank trait bounds. |
| /// |
| /// Because of this, you might expect to see lots of bounds that look like |
| /// this: |
| /// |
| /// ```ignore |
| /// fn takes_stream<T, S>(s: S) |
| /// where S: for<'a> Streamer<'a, Item=T> |
| /// { |
| /// } |
| /// ``` |
| /// |
| /// There are *three* different problems with this declaration: |
| /// |
| /// 1. `S` is not bound by any particular lifetime itself, and most streams |
| /// probably contain a reference to an underlying finite state transducer. |
| /// 2. It is often convenient to separate the notion of "stream" with |
| /// "stream constructor." This represents a similar split found in the |
| /// standard library for `Iterator` and `IntoIterator`, respectively. |
| /// 3. The `Item=T` is invalid because `Streamer`'s associated type is |
| /// parameterized by a lifetime and there is no way to parameterize an |
| /// arbitrary type constructor. (In this context, `T` is the type |
| /// constructor, because it will invariably require a lifetime to become |
| /// a concrete type.) |
| /// |
| /// With that said, we must revise our possibly-workable bounds to a giant |
| /// scary monster: |
| /// |
| /// ```ignore |
| /// fn takes_stream<'f, I, S>(s: I) |
| /// where I: for<'a> IntoStreamer<'a, Into=S, Item=(&'a [u8], Output)>, |
| /// S: 'f + for<'a> Streamer<'a, Item=(&'a [u8], Output)> |
| /// { |
| /// } |
| /// ``` |
| /// |
| /// We addressed the above points correspondingly: |
| /// |
| /// 1. `S` is now bound by `'f`, which corresponds to the lifetime (possibly |
| /// `'static`) of the underlying stream. |
| /// 2. The `I` type parameter has been added to refer to a type that knows how |
| /// to build a stream. Notice that neither of the bounds for `I` or `S` |
| /// share a lifetime parameter. This is because the higher rank trait bound |
| /// specifies it works for *any* particular lifetime. |
| /// 3. `T` has been replaced with specific concrete types. Note that these |
| /// concrete types are duplicated. With iterators, we could use |
| /// `Item=S::Item` in the bound for `I`, but one cannot access an associated |
| /// type through a higher-ranked trait bound. Therefore, we must duplicate |
| /// the item type. |
| /// |
| /// As you can see, streams offer little flexibility, little ergonomics and a |
| /// lot of hard to read trait bounds. The situation is lamentable, but |
| /// nevertheless, without them, we would not be able to compose streams by |
| /// leveraging the type system. |
| /// |
| /// A redeemable quality is that these *same exact* trait bounds (modulo some |
| /// tweaks in the `Item` associated type) appear in many places in this crate |
| /// without much variation. Therefore, once you grok it, it's mostly easy to |
| /// pattern match it with "oh I need a stream." My hope is that clear |
| /// documentation and examples make these complex bounds easier to burden. |
| /// |
| /// Stretching this abstraction further with Rust's current type system is not |
| /// advised. |
| </span><span class="kw">pub trait </span>Streamer<<span class="lifetime">'a</span>> { |
| <span class="doccomment">/// The type of the item emitted by this stream. |
| </span><span class="kw">type </span>Item: <span class="lifetime">'a</span>; |
| |
| <span class="doccomment">/// Emits the next element in this stream, or `None` to indicate the stream |
| /// has been exhausted. |
| /// |
| /// It is not specified what a stream does after `None` is emitted. In most |
| /// cases, `None` should be emitted on every subsequent call. |
| </span><span class="kw">fn </span>next(<span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span><span class="self">self</span>) -> <span class="prelude-ty">Option</span><<span class="self">Self</span>::Item>; |
| } |
| |
| <span class="doccomment">/// IntoStreamer describes types that can be converted to streams. |
| /// |
| /// This is analogous to the `IntoIterator` trait for `Iterator` in |
| /// `std::iter`. |
| </span><span class="kw">pub trait </span>IntoStreamer<<span class="lifetime">'a</span>> { |
| <span class="doccomment">/// The type of the item emitted by the stream. |
| </span><span class="kw">type </span>Item: <span class="lifetime">'a</span>; |
| <span class="doccomment">/// The type of the stream to be constructed. |
| </span><span class="kw">type </span>Into: Streamer<<span class="lifetime">'a</span>, Item = <span class="self">Self</span>::Item>; |
| |
| <span class="doccomment">/// Construct a stream from `Self`. |
| </span><span class="kw">fn </span>into_stream(<span class="self">self</span>) -> <span class="self">Self</span>::Into; |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, S: Streamer<<span class="lifetime">'a</span>>> IntoStreamer<<span class="lifetime">'a</span>> <span class="kw">for </span>S { |
| <span class="kw">type </span>Item = S::Item; |
| <span class="kw">type </span>Into = S; |
| |
| <span class="kw">fn </span>into_stream(<span class="self">self</span>) -> S { |
| <span class="self">self |
| </span>} |
| } |
| </code></pre></div> |
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