Writing documentation

Conventions for hand-written prose about this project. For which pages are generated and must not be edited, see Documentation toolchain.

Where these rules apply

Most of what follows is about precision and economy, which do not change with the surface. They apply to the pages under docs/, to docblocks in src/, and to pull request descriptions and commit messages. Docblocks are spliced into the generated reference pages, so an imprecise docblock ships as published documentation.

Three parts are about pages only and do not transfer: Structure, Spelling, and the generated-page items in the review checklist.

Pull request descriptions have one extra constraint. They describe the change, and include context only where the change cannot be understood without it. How the work was found, what else was investigated and what it might lead to are not part of the diff. CONTRIBUTING has the template and the title format.

Commit message bodies have the opposite constraint. They document what the diff does, not why it exists. A sentence explaining a motive belongs in the pull request description. Routine verification is not a fact about the diff either. That the change is tested is assumed, so a body noting it says nothing.

State a fact once

Do not restate something the page's own structure already establishes.

The beta status of the spec pipeline, for example, is carried by the Status row in guide/modes.md, by (beta) in the section headings, by the ::: warning Beta callouts, by the version timeline, and by the 🧪 in the page titles. A sentence saying "this is the only mode not marked beta" adds nothing and rots independently of the places that do.

The question is whether the reader learns anything they could not already see. Reading well is not enough. This applies to maturity, version requirements, which namespace to use, and what a mode does.

Marketing filler and restatement filler are the same defect. Replacing "the stable, production-ready mode" with "the only mode not marked beta" is not an improvement.

A third form is the clause explaining why the fact matters: "which is what makes X possible", "which matters because…", "which is the point". It carries no fact of its own. State the fact and stop. Where the significance is not visible from the fact, the fact is the wrong one.

Detail belongs in exactly one place

Orientation may appear in several places. Detail may not.

Two pages may introduce the same subject, and often need to, because they are read by different people arriving from different directions. The same specifics must not be spelled out twice. The two copies drift, and nothing shows which one is stale.

The split that works is by depth, not by topic. A page aimed at users says what a thing does and why it matters, then links onward. The page aimed at contributors carries the mechanism, the rules and the exact lists. Neither repeats the other's half.

reference/architecture.md and dev/pipeline.md are the worked example. The reference page describes the Assembler in a paragraph and links onward. The internals page owns slot maps, the resolution algorithm and the root-attribute list. Before this split, both described slot maps, augmenter ordering and the class hierarchy, in different words.

Writing something a second time is the signal to link instead.

Reuse the content, do not copy it

When the same text has to appear in more than one output, extract it and pull it in. Copying is what rots.

Two mechanisms are already in use here:

• docs/snippets/preamble_*.md: markdown fragments the reference generators splice into their pages, via DocGenerator::snippetContent(). Editing the fragment updates the generated page.
• <<< @/snippets/path.php: transcludes a real, test-executed source file into a page, so a documented example cannot drift from code that runs.

A third, <!--@include: ./path.md-->, is supported by Vitepress but not yet used here. It suits prose shared between hand-written pages.

Prefer extracting a fragment over pasting. Where the content can be derived at all, prefer generating it over both. The toolchain notes say what is already generated.

Write about the subject, not about the documentation

A page describes its subject. It does not explain how the docs are produced or why a cross-reference can be trusted, unless the documentation is the subject, as on this page or the toolchain notes.

No page, of either kind, needs to announce its own audience or justify its existence. The title and the link that brought the reader here have already done that.

Three real examples, all from one page:

The Augmenters reference — generated from the pipeline itself, so it cannot fall out of step with the code

see Spec pipeline internals, which is written for people changing the pipeline rather than using it

This page stays at the level of "what happens and in what order"

Each is true, and none answers the reader's only question, which is how the thing works. Whether a reference is generated matters to whoever maintains it. The reader just follows the link.

The usual tell is a clause attached to a link, explaining the link. Let the link text do that work: "see [Spec pipeline internals]" is complete. Anything after the comma is the page talking about itself.

Enumerate closed sets only

A list someone has to remember to update is a list that will be wrong.

Enumerate a set only when it is closed, meaning it is defined somewhere in code and changes only when that code changes. The generated pages are closed. tools/docgen.php names them, so a table listing them holds until someone edits the generator. "Everything else under docs/" is open. It grows whenever anyone adds a page, so it gets described rather than listed.

The same test applies to a count attached to a list. If the set can change, so can the number, so a count like "six pages" goes when the enumeration it summarised goes.

Do not hand-write what can be captured

Command output, default configuration, and API listings should be copied from the real thing, not composed. A hand-written openapi -h block once carried the wrong format, a wrong --mode description, a --version default that did not exist, and four missing options, for as long as nobody ran it.

./bin/openapi -h            # capture this
./bin/openapi --mode spec -D src

Do not cite line numbers

Class::method() survives refactoring. Foo.php:110 does not. Of three line references in ADR-001, two had already rotted.

Do not claim what you have not verified

If a page compares spec mode to classic mode, run both. A hedged comparison such as "classic mode may emit the array form" is a sign the comparison was reasoned about rather than tested. A reader cannot act on it.

Either verify and state it plainly, or document only the behavior you know.

Do not contradict the page you are on

Two examples from the same page:

• attributes described as "immutable" while the augmenter section describes writing to them
• "all modes produce the same output", immediately above a table of differences between modes

If a claim needs the reader to ignore the next paragraph, cut the claim.

Voice

Short, plain sentences, one idea each. A sentence carrying three qualifications is two or three sentences.

• No em dashes. Use a comma, parentheses, a colon or a new sentence.
• No semicolons joining clauses. Split the sentence.
• No contractions: "does not", "it is".
• Understate. A feature is described by what it does, not by how good it is.
• Explanations describe the code: "swagger-php will automatically register the @OA alias", "can be used to". Instructions address the reader as "you".
• No "not X but Y" turns, and no closing recap such as "In short".

Spelling

The codebase is mixed. normalise sits alongside serialization, and both spellings of behavio(u)r appear in src/ and in docs/. There is no single convention to appeal to.

Match whatever the file you are editing already uses, and leave the rest alone. A bulk correction would churn far more than it fixes.

Structure

• Code snippets under docs/snippets/ are executed by the test suite. Testing covers what a new snippet requires.
• The <codeblock> component renders annotation, attribute and spec variants as tabs. Keep the three implementations in sync.
• Internal links use site-absolute paths (/reference/architecture), not relative file paths, so they survive being rendered at a different depth.
• Prose describing behaviour that differs between classic and spec goes in an ::: info 🧪 Mode difference container, so the divergences can be listed again when the pipelines converge. A page whose subject is modes carries this in its structure and does not mark individual paragraphs.

Reviewing documentation changes

A pass to run over a documentation diff, your own or someone else's, and over a pull request description or commit message. Every item below caught a real defect in this codebase at least once.

Where a check fails, fix the source rather than the symptom. A wrong generated page means a wrong docblock or generator, not a page to hand-edit.

Checks you can run

• [ ] Generated pages untouched. composer docs:gen leaves no diff on the pages listed in the toolchain notes.
• [ ] Every composer <script> mentioned exists in composer.json.
• [ ] Every code reference resolves. Check class names, Class::method(), file paths, CLI flags and config keys against the source. This catches the most damaging class of error, where confident prose describes an API that was renamed or never existed.
• [ ] Command output was captured, not composed. Any --help text, default config dump or API listing should come from a real run.
• [ ] No line-number citations. Foo.php:123 rots silently. Cite Class::method().
• [ ] Links resolve. Relative links point at files that exist. The site build fails on dead internal links, so composer docs:build covers the rest.
• [ ] Section anchors verified, or dropped. A #heading-slug link is only safe when the heading is plain words, as punctuation and dashes make the generated slug ambiguous. If you have not confirmed it, link to the page.
• [ ] No volatile values. Counts ("six pages", "around 27 test files"), ports, versions, anything that shifts without anyone editing the doc. Derive it, describe it qualitatively, or, where the tool announces it, say that instead. A dev server that prints its own URL should be documented as printing its URL, not as a port number.
• [ ] Version and requirement claims match composer.json.
• [ ] No marketing or stock filler: "production-ready", "seamless", "powerful", "clean", "simply", "robust", "comprehensive", "leverage", "crucial", "genuinely".
• [ ] No stock phrase repeated across the diff. If the same formulation appears twice, one of them is padding.
• [ ] No em dashes, and no semicolons joining clauses. Both show up in a search of the diff.

Checks that need reading

• [ ] No open-ended set enumerated. A list is for a set defined in code, which changes only when that code changes. Anything that grows when someone adds a file gets described instead.
• [ ] Detail appears exactly once. Two pages may introduce the same subject. Only one may carry the specifics. If you are writing something a second time, link instead.
• [ ] Claims are verified, not reasoned. Hedges such as "may", "should" or "is expected to" usually mark a claim nobody tested. Run it, then state it plainly, or cut it.
• [ ] The page does not contradict itself. A claim the reader must ignore two paragraphs later is worse than no claim.
• [ ] The level matches the audience. User-facing pages say what something does and link onward. Contributor pages carry the mechanism. Neither should hold the other's half.
• [ ] No commentary about the documentation itself, unless the docs are the page's subject. That includes how a page is generated and why a link is trustworthy. Watch for a clause hanging off a link that explains the link.
• [ ] No page announcing its own audience or purpose. "This page is for…", "these notes exist because…". Applies to contributor pages too, not just user-facing ones.
• [ ] No clause explaining the significance of the sentence before it, such as "which is what makes X possible" or "which matters because…". State the fact and stop.
• [ ] Nothing restates what the page's own structure already shows. A status table, a heading or a callout already told the reader. A sentence repeating it only rots.
• [ ] Spelling follows the file it is in. Neither spelling is worth correcting in bulk.
• [ ] Shared content is reused, not copied. See above.
• [ ] One idea per sentence. A sentence with three qualifications gets split.
• [ ] Explanations describe the code. Instructions address the reader as "you".