<!DOCTYPE HTML> <html lang="en"> <head> <meta charset="UTF-8"> <title>Higher-Rank Trait Bounds - </title> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <meta name="description" content=""> <meta name="viewport" content="width=device-width, initial-scale=1"> <base href=""> <link rel="stylesheet" href="book.css"> <link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css"> <link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css"> <link rel="shortcut icon" href="favicon.png"> <!-- Font Awesome --> <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css"> <link rel="stylesheet" href="highlight.css"> <link rel="stylesheet" href="tomorrow-night.css"> <!-- MathJax --> <script type="text/javascript" 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For instance, we can write the following code:</p> <pre><pre class="playpen"><code class="language-rust">struct Closure<F> { data: (u8, u16), func: F, } impl<F> Closure<F> where F: Fn(&(u8, u16)) -> &u8, { fn call(&self) -> &u8 { (self.func)(&self.data) } } fn do_it(data: &(u8, u16)) -> &u8 { &data.0 } fn main() { let clo = Closure { data: (0, 1), func: do_it }; println!("{}", clo.call()); } </code></pre></pre> <p>If we try to naively desugar this code in the same way that we did in the lifetimes section, we run into some trouble:</p> <pre><code class="language-rust ignore">struct Closure<F> { data: (u8, u16), func: F, } impl<F> Closure<F> // where F: Fn(&'??? (u8, u16)) -> &'??? u8, { fn call<'a>(&'a self) -> &'a u8 { (self.func)(&self.data) } } fn do_it<'b>(data: &'b (u8, u16)) -> &'b u8 { &'b data.0 } fn main() { 'x: { let clo = Closure { data: (0, 1), func: do_it }; println!("{}", clo.call()); } } </code></pre> <p>How on earth are we supposed to express the lifetimes on <code>F</code>'s trait bound? We need to provide some lifetime there, but the lifetime we care about can't be named until we enter the body of <code>call</code>! Also, that isn't some fixed lifetime; <code>call</code> works with <em>any</em> lifetime <code>&self</code> happens to have at that point.</p> <p>This job requires The Magic of Higher-Rank Trait Bounds (HRTBs). The way we desugar this is as follows:</p> <pre><code class="language-rust ignore">where for<'a> F: Fn(&'a (u8, u16)) -> &'a u8, </code></pre> <p>(Where <code>Fn(a, b, c) -> d</code> is itself just sugar for the unstable <em>real</em> <code>Fn</code> trait)</p> <p><code>for<'a></code> can be read as "for all choices of <code>'a</code>", and basically produces an <em>infinite list</em> of trait bounds that F must satisfy. Intense. There aren't many places outside of the <code>Fn</code> traits where we encounter HRTBs, and even for those we have a nice magic sugar for the common cases.</p> </div> <!-- Mobile navigation buttons --> <a href="unbounded-lifetimes.html" class="mobile-nav-chapters previous"> <i class="fa fa-angle-left"></i> </a> <a href="subtyping.html" class="mobile-nav-chapters next"> <i class="fa fa-angle-right"></i> </a> </div> <a href="unbounded-lifetimes.html" class="nav-chapters previous" title="You can navigate through the chapters using the arrow keys"> <i class="fa fa-angle-left"></i> </a> <a href="subtyping.html" class="nav-chapters next" title="You can navigate through the chapters using the arrow keys"> <i class="fa fa-angle-right"></i> </a> </div> <!-- Local fallback for Font Awesome --> <script> if ($(".fa").css("font-family") !== "FontAwesome") { $('<link rel="stylesheet" type="text/css" href="_FontAwesome/css/font-awesome.css">').prependTo('head'); } </script> <!-- Livereload script (if served using the cli tool) --> <script src="highlight.js"></script> <script src="book.js"></script> </body> </html>