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<!DOCTYPE html>
<html lang="en">
<head>
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<title>A Quick Intro to Rust Macros</title>
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<h1 class="title">A Quick Intro to Rust Macros</h1>
<nav id="TOC"><ul>
<li><a href="#macro-mechanics">1 Macro Mechanics</a><ul></ul></li>
<li><a href="#construction">2 Construction</a><ul></ul></li>
<li><a href="#indexing-and-shuffling">3 Indexing and Shuffling</a><ul>
<li><a href="#substitution">3.1 Substitution</a><ul></ul></li>
<li><a href="#being-hygienic">3.2 Being Hygienic</a><ul></ul></li></ul></li>
<li><a href="#the-nuclear-option">4 The Nuclear Option</a><ul></ul></li>
<li><a href="#some-more-gotchas">5 Some More Gotchas</a><ul></ul></li>
<li><a href="#distribution">6 Distribution</a><ul></ul></li>
<li><a href="#postscript">7 Postscript</a><ul></ul></li></ul></nav><!-- DO-NOT-TEST -->
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<blockquote>
<p><strong>Note</strong>: This article is for an obsolete version of Rust, and has been superceded by the slightly less misleadingly named <a href="https://danielkeep.github.io/practical-intro-to-macros.html">"A Practical Intro to Macros in Rust 1.0"</a>.</p>
</blockquote>
<p></p>
<blockquote>
<p><strong>Note</strong>: This article exists as an additional resource for learning about macros in Rust. There is also the <a href="http://doc.rust-lang.org/guide-macros.html">official Rust Macros Guide</a>.</p>
</blockquote>
<p>I think I might be weird. It seems like almost the first thing I do when encountering a new language is go hunting for metaprogramming facilities. This is usually followed by working out how to abuse them to do hideous, evil things.</p>
<p>I've become quite enamoured with Rust lately, due in no small part to its interesting macro system. On a whim the other night, I decided to see about duplicating the functionality of D's <code>std.range.recurrence</code> function in Rust. I was rather chuffed with the result, and thought that it might serve as a nice little, non-trivial example of how to construct macros in Rust.</p>
<p>The TLDR of this article, then, is that we will construct a macro that lets us easily define recurrence relation iterators in Rust, with the following syntax:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='kw'>let</span> <span class='ident'>fib</span> <span class='op'>=</span> <span class='macro'>recurrence</span><span class='macro'>!</span>(<span class='ident'>a</span>[<span class='ident'>n</span>]: <span class='ident'>u64</span> <span class='op'>=</span> <span class='number'>0</span>, <span class='number'>1</span> ... <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>1</span>] <span class='op'>+</span> <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>2</span>]);
<span class='kw'>for</span> <span class='ident'>e</span> <span class='kw'>in</span> <span class='ident'>fib</span>.<span class='ident'>take</span>(<span class='number'>10</span>) { <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}"</span>, <span class='ident'>e</span>) }
</pre>
<blockquote>
<p><strong>Note</strong>: don't panic! What follows is the only time D or math will be talked about.</p>
</blockquote>
<p>For context, <code>std.range.recurrence</code> is a templated function which returns an iterator (called a "range" in D parlance) that yields successive elements of a recurrence relation. If you aren't familiar, a recurrence relation is a sequence where each value is defined in terms of one or more <em>previous</em> values, with one or more initial values to get the whole thing started. For example, the <a href="https://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci sequence</a> can be defined by the relation:</p>
<div class="katex" style="font-size: 100%; text-align: center;">
<span class="katex"><span class="katex-inner"><span style="height: 0.68333em;" class="strut"></span><span style="height: 0.891661em; vertical-align: -0.208331em;" class="strut bottom"></span><span class="base textstyle uncramped"><span class="reset-textstyle displaystyle textstyle uncramped"><span class="mord displaystyle textstyle uncramped"><span class="mord"><span class="mord mathit" style="margin-right: 0.13889em;">F</span><span class="vlist"><span style="top: 0.15em; margin-right: 0.05em; margin-left: -0.13889em;" class=""><span class="fontsize-ensurer reset-size5 size5"><span style="font-size: 0em;" class=""></span></span><span class="reset-textstyle scriptstyle cramped"><span class="mord mathit">n</span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size: 0em;" class=""></span></span></span></span></span><span class="mrel">=</span><span class="mord">0</span><span class="mpunct">,</span><span class="mord">1</span><span class="mpunct">,</span><span class="mpunct">…</span><span class="mpunct">,</span><span class="mord"><span class="mord mathit" style="margin-right: 0.13889em;">F</span><span class="vlist"><span style="top: 0.15em; margin-right: 0.05em; margin-left: -0.13889em;" class=""><span class="fontsize-ensurer reset-size5 size5"><span style="font-size: 0em;" class=""></span></span><span class="reset-textstyle scriptstyle cramped"><span class="mord scriptstyle cramped"><span class="mord mathit">n</span><span class="mbin">−</span><span class="mord">1</span></span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size: 0em;" class=""></span></span></span></span></span><span class="mbin">+</span><span class="mord"><span class="mord mathit" style="margin-right: 0.13889em;">F</span><span class="vlist"><span style="top: 0.15em; margin-right: 0.05em; margin-left: -0.13889em;" class=""><span class="fontsize-ensurer reset-size5 size5"><span style="font-size: 0em;" class=""></span></span><span class="reset-textstyle scriptstyle cramped"><span class="mord scriptstyle cramped"><span class="mord mathit">n</span><span class="mbin">-</span><span class="mord">2</span></span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size: 0em;" class=""></span></span></span></span></span></span></span></span></span></span>
</div>
<p>The way it works in D is that the user provides the template with a string literal which contains an expression defining the recurrence. The expression uses the fixed names <code>a</code> and <code>n</code> to refer to the sequence and the value currently being computed. The template expands to a function which is then called with the initial elements of the sequence. So, the Fibonacci sequence would be written, with <code>recurrence</code> in D as:</p>
<pre><code class="language-{d}">// a[0] = 0, a[1] = 1, and compute a[n+1] = a[n-1] + a[n]
auto fib = recurrence!("a[n-1] + a[n-2]")(0, 1);
// print the first 10 Fibonacci numbers
foreach (e; take(fib, 10)) { writeln(e); }
</code></pre>
<h1 id="macro-mechanics" class='section-header'><a
href="#macro-mechanics">1 Macro Mechanics</a></h1>
<p>Before going into the construction of the <code>recurrence!</code> macro, it may be helpful to understand how macros in Rust work. If you're already comfortable with this, feel free to skip this section.</p>
<p>A macro invocation in Rust is, in contrast to something like C, not a wholly separate pass over the source code. Macro invocations are actually a normal part of the compiler's AST representation. This means that invocations can <em>only</em> appear in positions where they're explicitly supported. Currently, they can appear in place of items, methods, statements, expressions, and patterns. Note that, as a consequence of this, there are some things you can't do with macros, such as generate the identifier for a function declaration.</p>
<p>However, the status of macro invocations as first-class members of the AST means that the Rust parser has to be able to parse them into something sensible, even when they use syntax that Rust itself doesn't support. The way this is done is by parsing the contents of an invocation into "token trees". If we take the <code>fib</code> example above, given the invocation:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='macro'>recurrence</span><span class='macro'>!</span>(<span class='ident'>a</span>[<span class='ident'>n</span>]: <span class='ident'>u64</span> <span class='op'>=</span> <span class='number'>0</span>, <span class='number'>1</span> ... <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>1</span>] <span class='op'>+</span> <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>2</span>])
</pre>
<p>the invocation arguments stored in the AST look something like:</p>
<pre><code class="language-{text}">[ `a` `[ ]` `:` `u64` `=` `0` `,` `1` `...` `a` `[ ]` `+` `a` `[ ]` ]
^ ^ |
[ `n` ] [ `n` `-` `1` ] ^
[ `n` `-` `2` ]
</code></pre>
<p>Sequences enclosed by parentheses, braces, or brackets become a single logical "token tree" node. This is how the parser keeps track of how deep into a macro invocation it is, and when to stop parsing it. This is <em>also</em> why your macro syntax must have balanced parens, braces, and brackets.</p>
<p>When it comes time to expand a macro invocation, the compiler feeds the parsed token trees into the macro, which must expand to a new sequence of token trees which can be parsed as an AST node that matches the invocation's position. In other words, if you have a macro invocation in expression position, the token trees which it expands to <em>must</em> be parseable as an expression.</p>
<p>This means that not only is <em>where</em> you can use a macro restricted, you also <em>cannot</em> have a macro which expands to something that isn't a complete, valid Rust construct.</p>
<h1 id="construction" class='section-header'><a
href="#construction">2 Construction</a></h1>
<p>Usually, when working on a new macro, the first thing I do is decide what the macro invocation should look like. In this specific case, my first attempt looked like this:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='comment'>/*
let fib = recurrence![a[n] = 0, 1, ..., a[n-1] + a[n-2]];
for e in fib.take(10) { println!("{}", e) }
*/</span>
</pre>
<p>From that, we can take a stab at how the macro should be defined, even if we aren't sure of the actual expansion. This is useful because if you can't figure out how to parse the input syntax, then <em>maybe</em> you need to change it.</p>
<pre id='rust-example-rendered' class='rust '>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>recurrence</span> {
( <span class='ident'>a</span>[<span class='ident'>n</span>] <span class='op'>=</span> $(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>inits</span>:<span class='ident'>expr</span>),<span class='op'>+</span> , ... , <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>recur</span>:<span class='ident'>expr</span> ) <span class='op'>=></span> { <span class='comment'>/* ... */</span> };
}
</pre>
<p>Assuming you aren't familiar with the syntax, allow me to elucidate. This is defining a macro using the <code>macro_rules</code> machinery (there is one other way to define macros, but we'll come back to that) called <code>recurrence</code>. This macro has a single parsing rule. That rule says the input to the macro must match:</p>
<ul>
<li>the literal token sequence <code>a [ n ] =</code>,</li>
<li>a repeating (the <code>$( ... )</code>) sequence, using <code>,</code> as a separator, and one or more (<code>+</code>) repeats of:
<ul>
<li>a valid <em>expression</em> captured into the variable <code>inits</code> (<code>$inits:expr</code>)</li>
</ul></li>
<li>the literal token sequence <code>, ... ,</code>,</li>
<li>a valid <em>expression</em> captured into the variable <code>recur</code> (<code>$recur:expr</code>).</li>
</ul>
<p>Finally, the rule says that <em>if</em> the input matches this rule, then the macro invocation should be replaced by the token sequence <code>/* ... */</code>.</p>
<p>It's worth noting that <code>inits</code>, as implied by the name, actually contains <em>all</em> the expressions that match in this position, not just the first or last. What's more, it captures them <em>as a sequence</em> as opposed to, say, irreversibly pasting them all together. Also note that you can do "zero or more" with a repetition by using <code>*</code> instead of <code>+</code>.</p>
<p>As an exercise, let's take the proposed input and feed it through the rule, to see how it is processed. The "Position" column will show which part of the syntax pattern needs to be matched against next, denoted by a "⌂". Note that in some cases, there might be more than one possible "next" element to match against. "Input" will contain all of the tokens that have <em>not</em> been consumed yet. <code>inits</code> and <code>recur</code> will contain the contents of those bindings.</p>
<table class="parse-table">
<thead>
<tr>
<th>Position</th>
<th>Input</th>
<th><code>inits</code></th>
<th><code>recur</code></th>
</tr>
</thead>
<tbody class="small-code">
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code>⌂</code></td>
<td><code>a[n] = 0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>[n] = 0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>n] = 0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>] = 0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>= 0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>0, 1, ..., a[n-1] + a[n-2]</code></td>
<td></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂ ⌂</code></td>
<td><code>, 1, ..., a[n-1] + a[n-2]</code></td>
<td><code>0</code></td>
<td></td>
</tr>
<tr>
<td colspan="4" style="font-size:.7em;"><em>Note</em>: there are two ⌂ here, because
a comma could mean <em>either</em> another element in the
repetition, <em>or</em> the comma <em>after</em> the
repetition.</td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂ ⌂</code></td>
<td><code>1, ..., a[n-1] + a[n-2]</code></td>
<td><code>0</code></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂ ⌂</code></td>
<td><code>, ..., a[n-1] + a[n-2]</code></td>
<td><code>0</code>, <code>1</code></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂ ⌂</code></td>
<td><code>..., a[n-1] + a[n-2]</code></td>
<td><code>0</code>, <code>1</code></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>, a[n-1] + a[n-2]</code></td>
<td><code>0</code>, <code>1</code></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td><code>a[n-1] + a[n-2]</code></td>
<td><code>0</code>, <code>1</code></td>
<td></td>
</tr>
<tr>
<td><code>a[n] = $($inits:expr),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td></td>
<td><code>0</code>, <code>1</code></td>
<td><code>a[n-1] + a[n-2]</code></td>
</tr>
</tbody>
</table>
<p>Now, let's begin writing the final, fully expanded form. For this expansion, I was looking for something like:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='kw'>let</span> <span class='ident'>fib</span> <span class='op'>=</span> {
<span class='kw'>struct</span> <span class='ident'>Recurrence</span> {
<span class='ident'>mem</span>: [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>pos</span>: <span class='ident'>uint</span>,
}
</pre>
<p>This will be the actual iterator type. <code>mem</code> will be the memo buffer to hold the last few values so the recurrence can be computed. <code>pos</code> is to keep track of the value of <code>n</code>.</p>
<blockquote>
<p><strong>Aside</strong>: I've chosen <code>u64</code> as a "sufficiently large" type for the elements of this sequence. Don't worry about how this will work out for <em>other</em> sequences; we'll come to it.</p>
</blockquote>
<pre id='rust-example-rendered' class='rust '>
<span class='kw'>impl</span> <span class='ident'>Iterator</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>next</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>) <span class='op'>-></span> <span class='prelude-ty'>Option</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> {
<span class='kw'>if</span> <span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'><</span> <span class='number'>2</span> {
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='self'>self</span>.<span class='ident'>pos</span>];
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
</pre>
<p>We need a branch to yield the initial values of the sequence; nothing tricky.</p>
<pre id='rust-example-rendered' class='rust '>
} <span class='kw'>else</span> {
<span class='kw'>let</span> <span class='ident'>a</span> <span class='op'>=</span> <span class='comment'>/* something */</span>;
<span class='kw'>let</span> <span class='ident'>n</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>pos</span>;
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> (<span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>1</span>] <span class='op'>+</span> <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>2</span>]);
<span class='self'>self</span>.<span class='ident'>mem</span>.<span class='ident'>TODO_shuffle_down_and_append</span>(<span class='ident'>next_val</span>);
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
}
}
}
</pre>
<p>This is a bit harder; we'll come back and look at <em>how</em> exactly to define <code>a</code>. Also, <code>TODO_shuffle_down_and_append</code> is another placeholder; I want something that places <code>next_val</code> on the end of the array, shuffling the rest down by one space, dropping the 0th element.</p>
<pre id='rust-example-rendered' class='rust '>
<span class='ident'>Recurrence</span> { <span class='ident'>mem</span>: [<span class='number'>0</span>, <span class='number'>1</span>], <span class='ident'>pos</span>: <span class='number'>0</span> }
};
<span class='kw'>for</span> <span class='ident'>e</span> <span class='kw'>in</span> <span class='ident'>fib</span>.<span class='ident'>take</span>(<span class='number'>10</span>) { <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}"</span>, <span class='ident'>e</span>) }
</pre>
<p>Lastly, return an instance of our new structure, which can then be iterated over. The complete expansion is:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='kw'>let</span> <span class='ident'>fib</span> <span class='op'>=</span> {
<span class='kw'>struct</span> <span class='ident'>Recurrence</span> {
<span class='ident'>mem</span>: [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>pos</span>: <span class='ident'>uint</span>,
}
<span class='kw'>impl</span> <span class='ident'>Iterator</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>next</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>) <span class='op'>-></span> <span class='prelude-ty'>Option</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> {
<span class='kw'>if</span> <span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'><</span> <span class='number'>2</span> {
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='self'>self</span>.<span class='ident'>pos</span>];
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
} <span class='kw'>else</span> {
<span class='kw'>let</span> <span class='ident'>a</span> <span class='op'>=</span> <span class='comment'>/* something */</span>;
<span class='kw'>let</span> <span class='ident'>n</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>pos</span>;
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> (<span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>1</span>] <span class='op'>+</span> <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>2</span>]);
<span class='self'>self</span>.<span class='ident'>mem</span>.<span class='ident'>TODO_shuffle_down_and_append</span>(<span class='ident'>next_val</span>.<span class='ident'>clone</span>());
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
}
}
}
<span class='ident'>Recurrence</span> { <span class='ident'>mem</span>: [<span class='number'>0</span>, <span class='number'>1</span>], <span class='ident'>pos</span>: <span class='number'>0</span> }
};
<span class='kw'>for</span> <span class='ident'>e</span> <span class='kw'>in</span> <span class='ident'>fib</span>.<span class='ident'>take</span>(<span class='number'>10</span>) { <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}"</span>, <span class='ident'>e</span>) }
</pre>
<blockquote>
<p><strong>Aside</strong>: Yes, this <em>does</em> mean we're defining a different <code>Recurrence</code> struct and its implementation for each macro invocation. Most of this will optimise away in the final binary, with some judicious use of <code>#[inline]</code> attributes.</p>
</blockquote>
<p>It's also useful to check your expansion as you're writing it. If you see anything in the expansion that needs to vary with the invocation, but <em>isn't</em> in the actual macro syntax, you should work out where to introduce it. In this case, we've added <code>u64</code>, but that's not neccesarily what the user wants, nor is it in the macro syntax. So let's fix that.</p>
<pre id='rust-example-rendered' class='rust '>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>recurrence</span> {
( <span class='ident'>a</span>[<span class='ident'>n</span>]: <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>sty</span>:<span class='ident'>ty</span> <span class='op'>=</span> $(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>inits</span>:<span class='ident'>expr</span>),<span class='op'>+</span> , ... , <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>recur</span>:<span class='ident'>expr</span> ) <span class='op'>=></span> { <span class='comment'>/* ... */</span> };
}
<span class='comment'>/*
let fib = recurrence![a[n]: u64 = 0, 1, ..., a[n-1] + a[n-2]];
for e in fib.take(10) { println!("{}", e) }
*/</span>
</pre>
<p>Here, I've added a new capture: <code>sty</code> which should be a type.</p>
<blockquote>
<p><strong>Aside</strong>: if you're wondering, the bit after the colon in a capture can be any of the following:</p>
<ul>
<li><code>item</code>: an item, like a function, struct, module, etc.</li>
<li><code>block</code>: a block (<em>i.e.</em> a block of statments and/or an expression, surrounded by braces)</li>
<li><code>stmt</code>: a statement</li>
<li><code>pat</code>: a pattern</li>
<li><code>expr</code>: an expression</li>
<li><code>ty</code>: a type</li>
<li><code>ident</code>: an identifier</li>
<li><code>path</code>: a path (<em>e.g.</em> <code>foo</code>, <code>::std::mem::replace</code>, <code>transmute::<_, int></code>, ...)</li>
<li><code>meta</code>: a meta item; the things that go inside <code>#[...]</code> and <code>#![...]</code> attributes</li>
<li><code>tt</code>: a single token tree</li>
<li><code>matchers</code>: a matching pair of tokens; <code>(...)</code>, <code>{...}</code>, or <code>[...]</code></li>
</ul>
</blockquote>
<h1 id="indexing-and-shuffling" class='section-header'><a
href="#indexing-and-shuffling">3 Indexing and Shuffling</a></h1>
<p>I will skim a bit over this part, since it's effectively tangential to the macro stuff. We want to make it so that whatever <code>a</code> is, we can index it directly, and it will act as a sliding window keeping the last few (in this case, 2) elements of the sequence.</p>
<p>We can do this pretty easily with a wrapper type:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='kw'>struct</span> <span class='ident'>IndexOffset</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
<span class='ident'>slice</span>: <span class='kw-2'>&</span><span class='lifetime'>'a</span> [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>offset</span>: <span class='ident'>uint</span>,
}
<span class='kw'>impl</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> <span class='ident'>Index</span><span class='op'><</span><span class='ident'>uint</span>, <span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>IndexOffset</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
<span class='attribute'>#[<span class='ident'>inline</span>(<span class='ident'>always</span>)]</span>
<span class='kw'>fn</span> <span class='ident'>index</span><span class='op'><</span><span class='lifetime'>'b</span><span class='op'>></span>(<span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='self'>self</span>, <span class='ident'>index</span>: <span class='kw-2'>&</span><span class='ident'>uint</span>) <span class='op'>-></span> <span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='ident'>u64</span> {
<span class='kw'>let</span> <span class='ident'>real_index</span> <span class='op'>=</span> <span class='op'>*</span><span class='ident'>index</span> <span class='op'>-</span> <span class='self'>self</span>.<span class='ident'>offset</span> <span class='op'>+</span> <span class='number'>2</span>;
<span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>slice</span>[<span class='ident'>real_index</span>]
}
}
</pre>
<blockquote>
<p><strong>Aside</strong>: since lifetimes come up <em>a lot</em> with people new to Rust, a quick explanation: <code>'a</code> and <code>'b</code> are lifetime parameters that are used to track where a reference (<em>i.e.</em> a borrowed pointer to some data) is valid. In this case, <code>IndexOffset</code> borrows a reference to our iterator's data, so it needs to keep track of how long it's allowed to hold that reference for, using <code>'a</code>.</p>
<p><code>'b</code> is used because the <code>Index</code> trait (which is how subscript syntax is actually implemented) is <em>also</em> parameterised on a lifetime, on account of returning a borrowed reference. <code>'a</code> and <code>'b</code> are not necessarily the same thing in all cases. The borrow checker will make sure that even though we don't explicitly relate <code>'a</code> and <code>'b</code> to one another, we don't accidentally violate memory safety.</p>
</blockquote>
<p>This changes the definition of <code>a</code> to:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='kw'>let</span> <span class='ident'>a</span> <span class='op'>=</span> <span class='ident'>IndexOffset</span> { <span class='ident'>slice</span>: <span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>mem</span>, <span class='ident'>offset</span>: <span class='ident'>n</span> };
</pre>
<p>The only remaining question is what to do about <code>TODO_shuffle_down_and_append</code>. I wasn't able to find a method in the standard library with exactly the semantics I wanted, but it isn't hard to do by hand.</p>
<pre id='rust-example-rendered' class='rust '>
{
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>mem</span>::<span class='ident'>swap</span>;
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>swap_tmp</span> <span class='op'>=</span> <span class='ident'>next_val</span>;
<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='ident'>range</span>(<span class='number'>0</span>, <span class='number'>2</span>).<span class='ident'>rev</span>() {
<span class='ident'>swap</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>swap_tmp</span>, <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='ident'>i</span>]);
}
}
</pre>
<p>This swaps the new value into the end of the array, swapping the other elements down one space.</p>
<blockquote>
<p><strong>Aside</strong>: doing it this way means that this code will work for non-copyable types, as well, since they <em>can</em> be swapped.</p>
</blockquote>
<p>The complete expansion now looks like this:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='attribute'>#<span class='op'>!</span>[<span class='ident'>feature</span>(<span class='ident'>macro_rules</span>)]</span>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>recurrence</span> {
( <span class='ident'>a</span>[<span class='ident'>n</span>]: <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>sty</span>:<span class='ident'>ty</span> <span class='op'>=</span> $(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>inits</span>:<span class='ident'>expr</span>),<span class='op'>+</span> , ... , <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>recur</span>:<span class='ident'>expr</span> ) <span class='op'>=></span> { <span class='comment'>/* ... */</span> };
}
<span class='kw'>fn</span> <span class='ident'>main</span>() {
<span class='comment'>/*
let fib = recurrence![a[n]: u64 = 0, 1, ..., a[n-1] + a[n-2]];
for e in fib.take(10) { println!("{}", e) }
*/</span>
<span class='kw'>let</span> <span class='ident'>fib</span> <span class='op'>=</span> {
<span class='kw'>struct</span> <span class='ident'>Recurrence</span> {
<span class='ident'>mem</span>: [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>pos</span>: <span class='ident'>uint</span>,
}
<span class='kw'>struct</span> <span class='ident'>IndexOffset</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
<span class='ident'>slice</span>: <span class='kw-2'>&</span><span class='lifetime'>'a</span> [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>offset</span>: <span class='ident'>uint</span>,
}
<span class='kw'>impl</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> <span class='ident'>Index</span><span class='op'><</span><span class='ident'>uint</span>, <span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>IndexOffset</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
<span class='attribute'>#[<span class='ident'>inline</span>(<span class='ident'>always</span>)]</span>
<span class='kw'>fn</span> <span class='ident'>index</span><span class='op'><</span><span class='lifetime'>'b</span><span class='op'>></span>(<span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='self'>self</span>, <span class='ident'>index</span>: <span class='kw-2'>&</span><span class='ident'>uint</span>) <span class='op'>-></span> <span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='ident'>u64</span> {
<span class='kw'>let</span> <span class='ident'>real_index</span> <span class='op'>=</span> <span class='op'>*</span><span class='ident'>index</span> <span class='op'>-</span> <span class='self'>self</span>.<span class='ident'>offset</span> <span class='op'>+</span> <span class='number'>2</span>;
<span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>slice</span>[<span class='ident'>real_index</span>]
}
}
<span class='kw'>impl</span> <span class='ident'>Iterator</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>next</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>) <span class='op'>-></span> <span class='prelude-ty'>Option</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> {
<span class='kw'>if</span> <span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'><</span> <span class='number'>2</span> {
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='self'>self</span>.<span class='ident'>pos</span>];
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
} <span class='kw'>else</span> {
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> {
<span class='kw'>let</span> <span class='ident'>n</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>pos</span>;
<span class='kw'>let</span> <span class='ident'>a</span> <span class='op'>=</span> <span class='ident'>IndexOffset</span> { <span class='ident'>slice</span>: <span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>mem</span>, <span class='ident'>offset</span>: <span class='ident'>n</span> };
(<span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>1</span>] <span class='op'>+</span> <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>2</span>])
};
{
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>mem</span>::<span class='ident'>swap</span>;
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>swap_tmp</span> <span class='op'>=</span> <span class='ident'>next_val</span>;
<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='ident'>range</span>(<span class='number'>0</span>, <span class='number'>2</span>).<span class='ident'>rev</span>() {
<span class='ident'>swap</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>swap_tmp</span>, <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='ident'>i</span>]);
}
}
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
}
}
}
<span class='ident'>Recurrence</span> { <span class='ident'>mem</span>: [<span class='number'>0</span>, <span class='number'>1</span>], <span class='ident'>pos</span>: <span class='number'>0</span> }
};
<span class='kw'>for</span> <span class='ident'>e</span> <span class='kw'>in</span> <span class='ident'>fib</span>.<span class='ident'>take</span>(<span class='number'>10</span>) { <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}"</span>, <span class='ident'>e</span>) }
}
</pre>
<p>Note that I've changed the order of the declarations of <code>n</code> and <code>a</code>, as well as wrapped them (along with the recurrence expression) in a block. The reason for the first should be obvious (<code>n</code> needs to be defined first so I can use it for <code>a</code>). The reason for the second is that the borrowed reference <code>&self.mem</code> will prevent the swaps later on from happening (you cannot mutate something that is aliased elsewhere). The block ensures that the <code>&self.mem</code> borrow expires before then.</p>
<p>Incidentally, the only reason the code that does the <code>mem</code> swaps is in a block is to narrow the scope in which <code>std::mem::swap</code> is available, for the sake of being tidy.</p>
<p>If we take this code and run it, we get:</p>
<pre><code class="language-{text}">0
1
1
2
3
5
8
13
21
34
</code></pre>
<p>Success! Now, let's copy & paste this into the macro expansion, and replace the expanded code with an invocation. This gives us:</p>
<pre id='rust-example-rendered' class='rust '>
<span class='attribute'>#<span class='op'>!</span>[<span class='ident'>feature</span>(<span class='ident'>macro_rules</span>)]</span>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>recurrence</span> {
( <span class='ident'>a</span>[<span class='ident'>n</span>]: <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>sty</span>:<span class='ident'>ty</span> <span class='op'>=</span> $(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>inits</span>:<span class='ident'>expr</span>),<span class='op'>+</span> , ... , <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>recur</span>:<span class='ident'>expr</span> ) <span class='op'>=></span> {
{
<span class='kw'>struct</span> <span class='ident'>Recurrence</span> {
<span class='ident'>mem</span>: [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>pos</span>: <span class='ident'>uint</span>,
}
<span class='kw'>struct</span> <span class='ident'>IndexOffset</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
<span class='ident'>slice</span>: <span class='kw-2'>&</span><span class='lifetime'>'a</span> [<span class='ident'>u64</span>, ..<span class='number'>2</span>],
<span class='ident'>offset</span>: <span class='ident'>uint</span>,
}
<span class='kw'>impl</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> <span class='ident'>Index</span><span class='op'><</span><span class='ident'>uint</span>, <span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>IndexOffset</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
<span class='attribute'>#[<span class='ident'>inline</span>(<span class='ident'>always</span>)]</span>
<span class='kw'>fn</span> <span class='ident'>index</span><span class='op'><</span><span class='lifetime'>'b</span><span class='op'>></span>(<span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='self'>self</span>, <span class='ident'>index</span>: <span class='kw-2'>&</span><span class='ident'>uint</span>) <span class='op'>-></span> <span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='ident'>u64</span> {
<span class='kw'>let</span> <span class='ident'>real_index</span> <span class='op'>=</span> <span class='op'>*</span><span class='ident'>index</span> <span class='op'>-</span> <span class='self'>self</span>.<span class='ident'>offset</span> <span class='op'>+</span> <span class='number'>2</span>;
<span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>slice</span>[<span class='ident'>real_index</span>]
}
}
<span class='kw'>impl</span> <span class='ident'>Iterator</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>next</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>) <span class='op'>-></span> <span class='prelude-ty'>Option</span><span class='op'><</span><span class='ident'>u64</span><span class='op'>></span> {
<span class='kw'>if</span> <span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'><</span> <span class='number'>2</span> {
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='self'>self</span>.<span class='ident'>pos</span>];
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
} <span class='kw'>else</span> {
<span class='kw'>let</span> <span class='ident'>next_val</span> <span class='op'>=</span> {
<span class='kw'>let</span> <span class='ident'>n</span> <span class='op'>=</span> <span class='self'>self</span>.<span class='ident'>pos</span>;
<span class='kw'>let</span> <span class='ident'>a</span> <span class='op'>=</span> <span class='ident'>IndexOffset</span> { <span class='ident'>slice</span>: <span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>mem</span>, <span class='ident'>offset</span>: <span class='ident'>n</span> };
(<span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>1</span>] <span class='op'>+</span> <span class='ident'>a</span>[<span class='ident'>n</span><span class='op'>-</span><span class='number'>2</span>])
};
{
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>mem</span>::<span class='ident'>swap</span>;
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>swap_tmp</span> <span class='op'>=</span> <span class='ident'>next_val</span>;
<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='ident'>range</span>(<span class='number'>0</span>, <span class='number'>2</span>).<span class='ident'>rev</span>() {
<span class='ident'>swap</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>swap_tmp</span>, <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>.<span class='ident'>mem</span>[<span class='ident'>i</span>]);
}
}
<span class='self'>self</span>.<span class='ident'>pos</span> <span class='op'>+=</span> <span class='number'>1</span>;
<span class='prelude-val'>Some</span>(<span class='ident'>next_val</span>)
}
}
}
<span class='ident'>Recurrence</span> { <span class='ident'>mem</span>: [<span class='number'>0</span>, <span class='number'>1</span>], <span class='ident'>pos</span>: <span class='number'>0</span> }
}
};