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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8"/>
<title>C++ Identifiers using UAX 31</title>
<meta name="author" content="Steve Downey"/>
<meta name="description" content="
"/>
<meta name="keywords" content=" "/>
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<body>
<div class="reveal">
<div class="slides">
<section id="sec-title-slide">
<h1 class="title">C++ Identifiers using UAX 31</h1>
</section>
<section>
<section id="slide-org6746883">
<h2 id="org6746883">C++ Identifier Syntax using Unicode Standard Annex 31</h2>
<ul>
<li><p>
That C++ identifiers match the pattern
</p>
<blockquote>
<p>
(XID_Start + _ ) + XID_Continue*.
</p>
</blockquote></li>
<li>That portable source is required to be normalized as NFC.</li>
<li>That using unassigned code points be ill-formed.</li>
</ul>
</section>
<section id="slide-org39b5874">
<h3 id="org39b5874">Problem this fixes : NL 029</h3>
<blockquote>
<p>
Allowed characters include those from U+200b until U+206x; these are zero-width and control characters that lead to impossible to type names, indistinguishable names and unusable code & compile errors (such as those accidentally including RTL modifiers).
</p>
</blockquote>
</section>
<section id="slide-org86193c9">
<h3 id="org86193c9">Status Quo: we allow other "weird identifier code points"</h3>
<ul>
<li>The middle dot · which looks like an operator.</li>
<li>Many non-combining "modifiers" and accent marks, such as ´ and ¨ and ꓻ which don't really make sense on their own.</li>
<li>"Tone marks" from various languages, including ˫ (similar to a box-drawing character ├ which is an operator).</li>
<li>The "Greek question mark" ;</li>
<li>Symbols which are simply not linguistic, such as ۞ and ༒.</li>
</ul>
<p>
<a href="https://gist.github.com/jtbandes/c0b0c072181dcd22c3147802025d0b59#weird-identifier-code-points">https://gist.github.com/jtbandes/c0b0c072181dcd22c3147802025d0b59#weird-identifier-code-points</a>
</p>
</section>
<section id="slide-orgffbd8f8">
<h3 id="orgffbd8f8">UAX 31 - Unicode Identifier and Pattern Syntax</h3>
<ul>
<li>Follows the same principles as originally used for C++</li>
<li>Actively maintained</li>
<li>Stable</li>
</ul>
</section>
<section id="slide-org213e155">
<h3 id="org213e155">XID_Start and XID_Continue</h3>
<ul>
<li>Unicode database defined properties</li>
<li>Closed under normalization for all four forms</li>
<li>Once a code point has the property it is never removed</li>
<li>Roughly:
<ul>
<li>Start == letters</li>
<li>Continue == Start + numbers + some punctuation</li>
</ul></li>
</ul>
</section>
</section>
<section>
<section id="slide-org1774cc5">
<h2 id="org1774cc5">The Emoji Problem</h2>
<ul>
<li>The emoji-like code points that we knew about were excluded</li>
<li>We included all unassigned code points</li>
<li>Status Quo Emoji 'support' is an accident, incomplete, and broken</li>
</ul>
</section>
<section id="slide-orgfe900bb">
<h3 id="orgfe900bb">Status quo is broken</h3>
<div class="outline-text-3" id="text-orgfe900bb">
</div>
</section>
<section id="slide-org28d7c31">
<h4 id="org28d7c31">Some Status Quo examples</h4>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Not Valid</th>
<th scope="col" class="org-left">Valid</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left">int ⏰ = 0;</td>
<td class="org-left">int 🕐 = 0;</td>
</tr>
<tr>
<td class="org-left">int ☠️ = 0;</td>
<td class="org-left">int 💀 = 0;</td>
</tr>
<tr>
<td class="org-left">int ✋️ = 0;</td>
<td class="org-left">int 👊 = 0;</td>
</tr>
<tr>
<td class="org-left">int ✈️ = 0;</td>
<td class="org-left">int 🚀 = 0;</td>
</tr>
<tr>
<td class="org-left">int ☹️ = 0;</td>
<td class="org-left">int 😀 = 0;</td>
</tr>
</tbody>
</table>
<p>
When the character was added to Unicode controls validity
</p>
</section>
<section id="slide-org64948ae">
<h4 id="org64948ae">Status Quo: ♀ and ♂ are disallowed</h4>
<p>
Gendered variants of emoji are selected by using a zero width joiner together
with the male and female sign.
</p>
<div class="org-src-container">
<pre class="src src-C++"><span class="org-comment-delimiter">// </span><span class="org-comment">Valid</span>
<span class="org-type">bool</span> 👷 = <span class="org-constant">true</span>; <span class="org-comment-delimiter">// </span><span class="org-comment">Construction Worker</span>
<span class="org-comment-delimiter">// </span><span class="org-comment">Not valid</span>
<span class="org-type">bool</span> 👷‍♀ = <span class="org-constant">false</span>; <span class="org-comment-delimiter">// </span><span class="org-comment">Woman Construction Worker ({Construction Worker}{ZWJ}{Female Sign})</span>
</pre>
</div>
</section>
<section id="slide-org8578b9d">
<h3 id="org8578b9d">Problems adding Emoji as identifiers</h3>
<div class="outline-text-3" id="text-org8578b9d">
</div>
</section>
<section id="slide-orga19dfdd">
<h4 id="orga19dfdd">Emoji are complex</h4>
<ul>
<li>Not just code points</li>
<li>Need grapheme cluster analysis</li>
<li>May incur costs even for code not using emoji</li>
</ul>
</section>
<section id="slide-orgc99cd88">
<h4 id="orgc99cd88">Emoji are not "Stable" in Unicode</h4>
<p>
From the emoji spec
</p>
<blockquote>
<p>
isEmoji(♟)=false for Emoji Version 5.0, but true for Version 11.0.
</p>
</blockquote>
<p>
It is possible that the emoji property could be removed.
</p>
</section>
<section id="slide-org7945c40">
<h4 id="org7945c40">Identifying Emoji is difficult</h4>
<p>
The unicode standard provides a regex that will reject non-emoji, but does not guarantee a valid emoji sequence.
</p>
<pre class="example" id="org80bb79b">
\p{RI} \p{RI}
| \p{Emoji}
( \p{EMod}
| \x{FE0F} \x{20E3}?
| [\x{E0020}-\x{E007E}]+ \x{E007F} )?
(\x{200D} \p{Emoji}
( \p{EMod}
| \x{FE0F} \x{20E3}?
| [\x{E0020}-\x{E007E}]+ \x{E007F} )?
)*
</pre>
<p>
It's not clear how much of the unicode database would be required for complete support.
</p>
<p>
<a href="https://unicode.org/reports/tr51/">UNICODE EMOJI</a>
</p>
</section>
<section id="slide-org6efadf2">
<h4 id="org6efadf2">Some surprising things are emoji</h4>
<pre class="example" id="org1759999">
002A ; Emoji # E0.0 [1] (*️) asterisk
0030..0039 ; Emoji # E0.0 [10] (0️..9️) digit zero..digit nine
</pre>
<pre class="example" id="orgd59ae26">
{DIGIT ONE}{VARIATION SELECTOR-16}{COMBINING ENCLOSING KEYCAP} 1️⃣
{ASTERISK}{VARIATION SELECTOR-16}{COMBINING ENCLOSING KEYCAP} *️⃣
</pre>
<div class="org-src-container">
<pre class="src src-C++"><span class="org-comment-delimiter">/// </span><span class="org-comment">would this be valid?</span>
<span class="org-type">int</span> 1️⃣ = 1;
</pre>
</div>
</section>
<section id="slide-orgf1b632e">
<h4 id="orgf1b632e">Fixing the emoji problem would mean being inventive</h4>
<p>
Being inventive in an area outside our expertise is HARD
</p>
<p>
Adopting UAX31 as a base to move forward is conservative
</p>
<p>
UAX 31 is a known good state
</p>
</section>
</section>
<section>
<section id="slide-org2ce257a">
<h2 id="org2ce257a">Script Issues</h2>
<p>
Some scripts require characters to control display or require punctuation that are not in the identifier set.
</p>
</section>
<section id="slide-org36ba9d3">
<h3 id="org36ba9d3">This includes English</h3>
<ul>
<li>Apostrophe and dash
<ul>
<li><code>won't</code></li>
<li><code>can't</code></li>
<li><code>mustn't</code></li>
<li><code>mother-in-law</code></li>
</ul></li>
<li>Programmers are used to this and do not notice</li>
</ul>
</section>
<section id="slide-orgcb84853">
<h3 id="orgcb84853">Zero Width characters are excluded by UAX 31</h3>
<p>
Status quo allows these invisible characters
</p>
<div class="org-src-container">
<pre class="src src-C++"><span class="org-type">int</span> <span class="org-variable-name">tmp</span> = 0;
<span class="org-type">int</span> <span class="org-variable-name">t</span>‍‍mp = 0;
</pre>
</div>
<ul>
<li><p>
clang 10 warns
</p>
<p>
<source>:2:6: warning: identifier contains Unicode character <U+200D> that is invisible in some environments [-Wunicode-zero-width]
</p></li>
</ul>
<p>
int t<U+200D><U+200D>mp = 0;
</p>
</section>
<section id="slide-orga42b697">
<h3 id="orga42b697">ZWJ and ZWNJ</h3>
<p>
However zero width joiner and non joiner are used in some scripts
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
</colgroup>
<tbody>
<tr>
<td class="org-left">Farsi word "names"</td>
</tr>
<tr>
<td class="org-left">نامهای</td>
</tr>
<tr>
<td class="org-left">NOON + ALEF + MEEM + HEH + ALEF + FARSI YEH</td>
</tr>
<tr>
<td class="org-left"><img src="https://www.unicode.org/reports/tr31/images/uax31-figure-2-farsi-ex1-v1-web.jpg" alt="uax31-figure-2-farsi-ex1-v1-web.jpg" /></td>
</tr>
</tbody>
</table>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
</colgroup>
<tbody>
<tr>
<td class="org-left">Farsi word "a letter"</td>
</tr>
<tr>
<td class="org-left">نامهای</td>
</tr>
<tr>
<td class="org-left">NOON + ALEF + MEEM + HEH + <b>ZWNJ</b> + ALEF + FARSI YEH</td>
</tr>
<tr>
<td class="org-left"><img src="https://www.unicode.org/reports/tr31/images/uax31-figure-2-farsi-ex2-v1-web.jpg" alt="uax31-figure-2-farsi-ex2-v1-web.jpg" /></td>
</tr>
</tbody>
</table>
<p>
Anecdotally, these issues are understood and worked around
</p>
</section>
<section id="slide-orgd091436">
<h3 id="orgd091436">UAX 31 has an expensive solution</h3>
<p>
Identifiers can be checked for what script the code points in the identifier are used, and the rules for allowed characters can be tailored. This requires a Unicode database and would require extensive analysis during lexing.
</p>
<p>
SG 16 does not recommend this.
</p>
</section>
</section>
<section>
<section id="slide-org0ae771a">
<h2 id="org0ae771a">Other adopters</h2>
<ul>
<li>Java (<a href="https://docs.oracle.com/javase/specs/jls/se15/html/jls-3.html#jls-3.8">https://docs.oracle.com/javase/specs/jls/se15/html/jls-3.html#jls-3.8</a>)</li>
<li>Python 3 <a href="https://www.python.org/dev/peps/pep-3131/">https://www.python.org/dev/peps/pep-3131/</a></li>
<li>Erlang <a href="https://www.erlang.org/erlang-enhancement-proposals/eep-0040.html">https://www.erlang.org/erlang-enhancement-proposals/eep-0040.html</a></li>
<li>Rust <a href="https://rust-lang.github.io/rfcs/2457-non-ascii-idents.html">https://rust-lang.github.io/rfcs/2457-non-ascii-idents.html</a></li>
<li>JS <a href="https://tc39.es/ecma262/">https://tc39.es/ecma262/</a></li>
</ul>
</section>
</section>
</div>
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