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<h1 class="title">A Practical Intro to Macros in Rust 1.0</h1>
<nav id="TOC"><ul>
<li><a href="#a-little-context">1 A Little Context</a><ul></ul></li>
<li><a href="#macro-parsing-and-expansion">2 Macro Parsing and Expansion</a><ul></ul></li>
<li><a href="#construction">3 Construction</a><ul></ul></li>
<li><a href="#indexing-and-shuffling">4 Indexing and Shuffling</a><ul>
<li><a href="#substitution">4.1 Substitution</a><ul></ul></li>
<li><a href="#being-hygienic">4.2 Being Hygienic</a><ul></ul></li></ul></li>
<li><a href="#the-nuclear-option">5 The Nuclear Option</a><ul></ul></li>
<li><a href="#some-more-gotchas">6 Some More Gotchas</a><ul></ul></li>
<li><a href="#distribution">7 Distribution</a><ul></ul></li>
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<p>One feature of Rust that I'm rather enamoured with is its macro system. Known as "macro by example" (MBE) or sometimes referred to as <code>macro_rules</code>, it provides you with an impressive amount of power whilst not allowing for the sort of evil tricks you can pull in C/C++.</p>
<p>The purpose of this article is to go through the <em>process</em> by which I wrote one <em>particular</em> macro, in the hopes that showing off the process itself will prove useful. As a result, I will try to ensure that there are complete, unabridged code examples at reasonable intervals along the way for you to copy out and play with if you so desire. If you don't want to, the final product will also be available as a finished library.</p>
<p>If this isn't how you like to learn, or you simply want more information, there is also the <a href="http://doc.rust-lang.org/stable/book/macros.html">Macros chapter of the Rust Book</a>.</p>
<p>To set the stage, we will be going through the design and construction of a macro for quickly defining recurrence relations as iterators. Ideally, the following snippet of code should print the first ten numbers in the Fibonacci sequence:</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>
<h1 id="a-little-context" class='section-header'><a
href="#a-little-context">1 A Little Context</a></h1>
<blockquote>
<p><strong>Note</strong>: don't panic! What follows is the only time D or math will be talked about. You can quite safely skip this section if you just want to get to the meat of the article.</p>
</blockquote>
<p>This particular macro is inspired by a function in the D standard library. Specifically, <a href="http://dlang.org/phobos/std_range.html#recurrence"><code>std.range.recurrence</code></a> is a templated function which returns an iterator (called a "range" in D parlance) that yields successive elements of a recurrence relation. I wanted to see how close to the original D template I could get using Rust macros.</p>
<p>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>Thus, the first two numbers in the sequence are 0 and 1, with the third being <em>F<sub>0</sub></em> + <em>F<sub>1</sub></em> = 0 + 1 = 1, the fourth <em>F<sub>1</sub></em> + <em>F<sub>2</sub></em> = 1 + 1 = 2, and so on forever.</p>
<p>Now, <em>because</em> such a sequence can go on forever, that makes defining a <code>fibonacci</code> function a little tricky, since you obviously don't want to try returning a complete vector. What you <em>want</em> is to return something which will lazily compute elements of the sequence as needed.</p>
<p>In Rust, that means producing an <code>Iterator</code>. This is not especially <em>hard</em>, but there is a fair amount of boilerplate involved: you need to define a custom type, work out what state needs to be stored in it, then implement the <code>Iterator</code> trait for it.</p>
<p>However, recurrence relations are simple enough that almost all of these details can be abstracted out with a little macro-based code generation.</p>
<p>So, with all that having been said, let's get started.</p>
<h1 id="macro-parsing-and-expansion" class='section-header'><a
href="#macro-parsing-and-expansion">2 Macro Parsing and Expansion</a></h1>
<p>Before going into the construction of the <code>recurrence!</code> macro, it may be helpful to understand how Rust parses macros. 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 have one which expands to 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. This has the nice property that you cannot, for example, forget a closing paren or brace in a macro.</p>
<h1 id="construction" class='section-header'><a
href="#construction">3 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='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='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>
<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>. There is no support for "zero or one" or more specific numbers of repetitions.</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. The macro system will keep track of both possibilities, until it is able to decide which one to follow.
</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> ⌂ ⌂ <s>⌂</s></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 colspan="4" style="font-size:.7em;">
<em>Note</em>: the third, crossed-out marker indicates that the macro system has, as a consequence of the last token consumed, eliminated one of the previous possible branches.
</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> <s>⌂</s> ⌂</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),+ , ... , $recur:expr</code>
<code> ⌂</code></td>
<td></td>
<td><code>0</code>, <code>1</code></td>
<td><code>a[n-1] + a[n-2]</code></td>
</tr>
<tr>
<td colspan="4" style="font-size:.7em;">
<em>Note</em>: this particular step should make it clear that a binding like <tt>$recur:expr</tt> will consume an <em>entire expression</em>, using the compiler's knowledge of what constitutes a valid expression. As will be noted later, you can do this for other language constructs, too.
</td>
</tr>
</tbody>
</table>
<p></p>
<p>The key take-away from this is that the macro system will <em>try</em> to incrementally match the tokens provided as input to the macro against the provided rules. We'll come back to the "try" part.</p>
<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'>usize</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='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='kw'>type</span> <span class='ident'>Item</span> <span class='op'>=</span> <span class='ident'>u64</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. To summarise, 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'>usize</span>,
}
<span class='kw'>impl</span> <span class='ident'>Iterator</span> <span class='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='kw'>type</span> <span class='ident'>Item</span> <span class='op'>=</span> <span class='ident'>u64</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>
</ul>
<p>Source: <a href="https://github.com/rust-lang/rust/blob/1.0.0/src/libsyntax/ext/tt/macro_parser.rs#L511-L557"><code>libsyntax/ext/tt/macro_parser.rs</code></a></p>
<p>There's one other thing to be aware of: in the interests of future-proofing the language, the compiler restricts what tokens you're allowed to put <em>after</em> a matcher, depending on what kind it is. Typically, this comes up when trying to match expressions or statements; those can <em>only</em> be followed by one of <code>=></code>, <code>,</code>, and <code>;</code>.</p>
<p>Source <a href="https://github.com/rust-lang/rust/blob/1.0.0/src/libsyntax/ext/tt/macro_rules.rs#L430-L476"><code>libsyntax/ext/tt/macro_rules.rs</code></a></p>
</blockquote>
<h1 id="indexing-and-shuffling" class='section-header'><a
href="#indexing-and-shuffling">4 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 the user can access previous values in the sequence by indexing <code>a</code>; we want it to 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'>usize</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'>usize</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='kw'>type</span> <span class='ident'>Output</span> <span class='op'>=</span> <span class='ident'>u64</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='ident'>usize</span>) <span class='op'>-></span> <span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='ident'>u64</span> {
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>num</span>::<span class='ident'>Wrapping</span>;
<span class='kw'>let</span> <span class='ident'>index</span> <span class='op'>=</span> <span class='ident'>Wrapping</span>(<span class='ident'>index</span>);
<span class='kw'>let</span> <span class='ident'>offset</span> <span class='op'>=</span> <span class='ident'>Wrapping</span>(<span class='self'>self</span>.<span class='ident'>offset</span>);
<span class='kw'>let</span> <span class='ident'>window</span> <span class='op'>=</span> <span class='ident'>Wrapping</span>(<span class='number'>2</span>);
<span class='kw'>let</span> <span class='ident'>real_index</span> <span class='op'>=</span> <span class='ident'>index</span> <span class='op'>-</span> <span class='ident'>offset</span> <span class='op'>+</span> <span class='ident'>window</span>;
<span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>slice</span>[<span class='ident'>real_index</span>.<span class='number'>0</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::index</code> function (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='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.</p>
</blockquote>
<p>The working code thus far now looks like this:</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='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'>use</span> <span class='ident'>std</span>::<span class='ident'>ops</span>::<span class='ident'>Index</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'>usize</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'>usize</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'>usize</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='kw'>type</span> <span class='ident'>Output</span> <span class='op'>=</span> <span class='ident'>u64</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='ident'>usize</span>) <span class='op'>-></span> <span class='kw-2'>&</span><span class='lifetime'>'b</span> <span class='ident'>u64</span> {
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>num</span>::<span class='ident'>Wrapping</span>;
<span class='kw'>let</span> <span class='ident'>index</span> <span class='op'>=</span> <span class='ident'>Wrapping</span>(<span class='ident'>index</span>);
<span class='kw'>let</span> <span class='ident'>offset</span> <span class='op'>=</span> <span class='ident'>Wrapping</span>(<span class='self'>self</span>.<span class='ident'>offset</span>);
<span class='kw'>let</span> <span class='ident'>window</span> <span class='op'>=</span> <span class='ident'>Wrapping</span>(<span class='number'>2</span>);
<span class='kw'>let</span> <span class='ident'>real_index</span> <span class='op'>=</span> <span class='ident'>index</span> <span class='op'>-</span> <span class='ident'>offset</span> <span class='op'>+</span> <span class='ident'>window</span>;
<span class='kw-2'>&</span><span class='self'>self</span>.<span class='ident'>slice</span>[<span class='ident'>real_index</span>.<span class='number'>0</span>]
}
}
<span class='kw'>impl</span> <span class='ident'>Iterator</span> <span class='kw'>for</span> <span class='ident'>Recurrence</span> {
<span class='kw'>type</span> <span class='ident'>Item</span> <span class='op'>=</span> <span class='ident'>u64</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='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
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='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'>/*
What follows here is *literally* the code from before,
cut and pasted into a new position. No other changes
have been made.
*/</span>
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>ops</span>::<span class='ident'>Index</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'>usize</span>,
}