Syntax Highlighting

2020/06/27

Themes and Parsing

There are two dimensions to syntax highlighting: parsing and themes.

Parsing are the tokens identified by the highligher. The theme defines how those individual tokens are styled.

Parsing

Parsing is the foundation of syntax highlighting, because it defines the space of possible highlighting.

For examples, here are the tokens identified by some of the popular syntax highlighters:

highlighter type tokens
highlight.js client string, keyword
rainbow.js client function call, string, keyword operator, namespace
prism.js client operator, punctation, string
Hugo server strings, keywords, functions, operators
sublime n/a strings, keywords, arguments, functions, operators
Github n/a strings, arguments, some keywords
RStudio n/a strings, keywords, operators, parentheses

Parsing annotates tokens in the code with specific CSS classes. Below, I’ll show some examples from my R Sytnax Highlighting Gallery.

Here’s how highlight.js parses the following:

<pre class="r language-r"><code class="hljs"><span class="hljs-keyword">library</span>(tidyverse)

urchins &lt;- read_csv(<span class="hljs-string">"https://tidymodels.org/start/models/urchins.csv"</span>) %&gt;%
  setNames(c(<span class="hljs-string">"food_regime"</span>, <span class="hljs-string">"initial_volume"</span>, <span class="hljs-string">"width"</span>)) %&gt;%
  mutate(food_regime = factor(food_regime, levels = c(<span class="hljs-string">"Init"</span>, <span class="hljs-string">"Low"</span>, <span class="hljs-string">"High"</span>)))

urchins %&gt;%
  group_by(food_regime) %&gt;%
  summarise(
    across(everything(), mean), n = n()
  )

mean(urchins$width)
ggplot2::cut_interval(urchins$initial_volume)

lm(width ~ initial_volume * food_regime, data = urchins)</code></pre>
highlighter tokens example
highlight.js keyword library
highlight.js string “Init”, “food_regime”

There really aren’t that many words that highlight.js is even able to highlight. This drastically limits the potential for highlighting.

Rainbow.js parsing includes more tokens:

<pre class="r language-r rainbow-show" data-trimmed="true"><code class="rainbow rainbow-show"><span class="function call">library</span>(tidyverse)

urchins <span class="keyword operator">&lt;-</span> <span class="function call">read_csv</span>(<span class="string">"https://tidymodels.org/start/models/urchins.csv"</span>) %<span class="keyword operator">&gt;</span>%
  <span class="function call">setNames</span>(<span class="function call">c</span>(<span class="string">"food_regime"</span>, <span class="string">"initial_volume"</span>, <span class="string">"width"</span>)) %<span class="keyword operator">&gt;</span>%
  <span class="function call">mutate</span>(food_regime <span class="keyword operator">=</span> <span class="function call">factor</span>(food_regime, levels <span class="keyword operator">=</span> <span class="function call">c</span>(<span class="string">"Init"</span>, <span class="string">"Low"</span>, <span class="string">"High"</span>)))

urchins %<span class="keyword operator">&gt;</span>%
  <span class="function call">group_by</span>(food_regime) %<span class="keyword operator">&gt;</span>%
  <span class="function call">summarise</span>(
    <span class="function call">across</span>(<span class="function call">everything</span>(), mean), n <span class="keyword operator">=</span> <span class="function call">n</span>()
  )

<span class="function call">mean</span>(urchins<span class="keyword operator">$</span>width)
<span class="namespace">ggplot2</span><span class="keyword operator">::</span><span class="function call">cut_interval</span>(urchins<span class="keyword operator">$</span>initial_volume)

<span class="function call">lm</span>(width <span class="keyword operator">~</span> initial_volume <span class="keyword operator">*</span> food_regime, data <span class="keyword operator">=</span> urchins)</code><div class="preloader"><div></div><div></div><div></div><div></div><div></div><div></div><div></div></div></pre>```
highlighter tokens example
Rainbow.js function call library, setNames, mutate
Rainbow.js string “Init”, “food_regime”
Rainbow.js keyword operator <-, =, >, ~, *, $
Rainbow.js namespace ggplot2

Prism.js takes a different approach:

<pre class="r  language-r"><code class=" language-r">library<span class="token punctuation">(</span>tidyverse<span class="token punctuation">)</span>

urchins <span class="token operator">&lt;-</span> read_csv<span class="token punctuation">(</span><span class="token string">"https://tidymodels.org/start/models/urchins.csv"</span><span class="token punctuation">)</span> <span class="token percent-operator operator">%&gt;%</span>
  setNames<span class="token punctuation">(</span>c<span class="token punctuation">(</span><span class="token string">"food_regime"</span><span class="token punctuation">,</span> <span class="token string">"initial_volume"</span><span class="token punctuation">,</span> <span class="token string">"width"</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token percent-operator operator">%&gt;%</span>
  mutate<span class="token punctuation">(</span>food_regime <span class="token operator">=</span> factor<span class="token punctuation">(</span>food_regime<span class="token punctuation">,</span> levels <span class="token operator">=</span> c<span class="token punctuation">(</span><span class="token string">"Init"</span><span class="token punctuation">,</span> <span class="token string">"Low"</span><span class="token punctuation">,</span> <span class="token string">"High"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>

urchins <span class="token percent-operator operator">%&gt;%</span>
  group_by<span class="token punctuation">(</span>food_regime<span class="token punctuation">)</span> <span class="token percent-operator operator">%&gt;%</span>
  summarise<span class="token punctuation">(</span>
    across<span class="token punctuation">(</span>everything<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> mean<span class="token punctuation">)</span><span class="token punctuation">,</span> n <span class="token operator">=</span> n<span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">)</span>

mean<span class="token punctuation">(</span>urchins<span class="token operator">$</span>width<span class="token punctuation">)</span>
ggplot2<span class="token operator">::</span>cut_interval<span class="token punctuation">(</span>urchins<span class="token operator">$</span>initial_volume<span class="token punctuation">)</span>

lm<span class="token punctuation">(</span>width <span class="token operator">~</span> initial_volume <span class="token operator">*</span> food_regime<span class="token punctuation">,</span> data <span class="token operator">=</span> urchins<span class="token punctuation">)</span></code></pre>
highlighter tokens example
Prism.js operator <-, =, %>%, ~, *, $
Prism.js punctuation (, ), ,,
Prism.js string “Init”, “food_regime”

All of the JavaScript parsers are lm

Themes

Themes determine the final appearance of the code. While parsing defines the space of possibilities, the theme may not live up to it.

In the last section, we showed that the Rainbow.js parse is more complete than highlight.js, since it labels more tokens. The parsing, however, only defines the space of possibilities. It’s the theme that’s responsible for the final appearance.

For the Github theme they look almost identical, although neither look anything like the actual Github syntax highlighting for R.

On the left, we have Highlight.js Github and on the right Rainbow.js Github. If you look really closely (or follow the links to high-quality Gallery renders) you’ll see that Rainbow.js operators are slightly darker than the ones in Highlight.js.

Even if a parser identifies the token, it’s the theme’s choice whether to emphasize certain tokens.