prettier/prettier

Why this case study matters

prettier is the cleanest “structural floor on top” data point in the Wave 1+2 inventory so far: prettier is itself a code-formatter (it dogfoods), its existing tooling is mature and tightly curated (ESLint flat-config + 9 internal prettier-internal-rules/* + the self-prettier --check + cspell + knip + tsc + 5 custom node validation scripts), and yet the plugin-shape conventions that the entire codebase architecture rests on are encoded only in code review and folder-name memory.

For any plugin-architecture project, webpack, rollup, vite, babel, postcss, the “every plugin has shape X” rule is universal and rarely enforced. prettier shows what the structural floor looks like when it lands cleanly under a mature tool stack.

Headline catch

prettier ships eight plugins under src/language-*/ (js, css, html, markdown, yaml, json, graphql, handlebars). Each one must export index.js (parser/printer/options) plus languages.evaluate.js (linguist-languages lookup table consumed at build time). A plugin that drops languages.evaluate.js would silently be omitted from the production bundle. Yet nothing on disk asserts this contract. Same thing for the 1:1 mapping between plugins and changelog_unreleased/<lang>/ category directories: a new plugin landing without its category dir ships fine, and PR notes for it have nowhere to go.

Alint’s for_each_dir over src/language-* is the missing structural floor. Five net-new gates that prettier’s existing ESLint + prettier-itself + cspell + knip + tsc stack does not enforce today:

• prettier-each-language-plugin-has-index
• prettier-each-language-plugin-has-languages-evaluate
• prettier-each-plugin-package-shape
• prettier-plugin-package-prettier-scope
• prettier-changelog-categories-exist

Where alint earns its keep here

• Single declarative gate catches both directions of the plugin ↔ changelog-category drift in one rule.
• Script consolidation: prettier has 5 custom node scripts (lint-changelog.js, check-deps.js, format-test-lint.js, ensure-no-files-changed.js, clean-cspell.js) totalling ~330 LoC, plus 8 yarn lint scripts, plus a 16-step GitHub workflow. Five of six invariants in lint-changelog.js and one of two in check-deps.js collapse to declarative alint rules. That’s ~80 LoC of bespoke JS replaced by ~50 lines of declarative YAML, easier to read in PR review, easier to extend, schema-validated at config-load time.
• Speed delta: yarn lint runs the 8 lint scripts in parallel via npm-run-all2 -p. Each script does its own fs walk; cspell
  • prettier + eslint + tsc each pay startup cost per process. Typical wall-clock on a clean checkout: ~30-45 s on a developer laptop. alint runs the alint-replaceable subset in ~0.5-1 s expected.
• Cross-cutting v0.10 candidate driver: prettier’s ensure-no-files-changed.js is the canonical command_idempotent shape (most-demanded primitive in the inventory: prettier, ruff, kubernetes, airflow, turbo). format-test-lint.js is the canonical for_each_leaf_dir shape (prettier, rust-lang/rust, ruff, any test-corpus tree).

Future story angles

• Plugin-architecture launch tile: generalise from prettier to webpack, rollup, vite, babel, postcss. Every plugin-architecture project has the “every plugin has shape X” universal-and-unenforced rule. prettier is the canonical demo.
• “Adopt alint to consolidate the orchestration layer” angle, pair with cpython for the case where mature tooling already exists but the structural floor is the missing piece.
• command_idempotent v0.10 launch post: prettier’s idempotency check is the cleanest example of the most-demanded primitive in the inventory.
• for_each_leaf_dir v0.10 launch post: prettier’s format-test-lint pattern recurs in every test-corpus tree (rust-ui, ruff snapshots, deno specs).