Syntax highlighting

Syntax highlighting is colour coding whose categories come from the language rather than from whoever is reading. A tokeniser walks the characters, decides that this run is a keyword and that one is a string literal, and each class is handed a colour. What the reader gets back is structure at a glance: the shape of a block before any of it has been read, a string that never closed running green to the end of the file, a name misspelt into a colour it should not be. None of that is a message the code contains. It is the editor showing its parse, which is why a line that highlights wrongly is usually a line that is wrong.

There are two passes and they do not always agree. The first is lexical: a grammar of patterns, TextMate style, matching runs of characters without understanding them, which is fast, needs no project and works on a file that does not compile. The second is semantic, and it asks a language server what each name actually resolves to, so a bare identifier can be told apart as a parameter, a class, a property, or an unused import. The second pass runs later and is allowed to overrule the first, which is the part worth knowing: a name that changes colour half a second after a file opens has not been re-coloured for effect, it has been resolved. Where no server is available, the grammar’s answer is the only answer, and it is usually good enough.

The convention is not the widget. A code editor has highlighting among its features, along with a gutter, folding and a caret that understands indentation, but highlighting long ago escaped the editor: it runs in read-only documentation blocks, in terminal output, in code review and diff views, in chat messages, in a static site generator that ships pre-coloured markup with no JavaScript at all. Naming the convention separately is what lets a design system specify it once for surfaces that have no editor in them.

Colour is not the only channel a theme has, and the good ones do not lean on it alone. Comments go italic, keywords take weight, a deprecated symbol gets a strikethrough, and an unreachable branch is faded rather than tinted. That redundancy matters more here than in most colour coding, because a theme runs out of separable hues at roughly the same dozen everything else does, while a language hands it far more token classes than that. The way out is not more hues: it is grouping the classes that can share one, and spending weight, slant and opacity on the distinctions colour can no longer carry. Highlighting is also usually the first thing a reader tunes, so a set that only works at one contrast level and one background is a set that stops working the moment somebody switches themes.