Setup TS Deep Modules

Setup TS Deep Modules

Make every package in this repo a deep module: a lot of behaviour behind a small interface. A package's public surface is its entry points — the files at the package root — and everything in its subfolders is hidden. This skill installs dependency-cruiser and the rules that make the entry points the only way in, then proves the rules bite.

For the vocabulary (deep module, interface, seam, depth), run the /codebase-design skill — use its language throughout.

The shape this enforces

src/packages/
  <name>/
    index.ts        ← an entry point (public). Import this from outside.
    client.ts       ← another entry point. Packages may expose SEVERAL.
    lib/            ← implementation: hidden from outside, free to import each other.
    tests/          ← co-located tests + fixtures (a subfolder, so private).

The public surface is the package's root files — not one designated index.ts. By convention implementation lives in lib/ and tests in tests/, giving every package the same two-folder shape. The rule itself is general, though: anything in any subfolder is private, so you never extend the config to add a folder.

Four rules, all error:

1. Entry-point boundary — code outside a package (app code or another package) may import only that package's entry points (its root files), never anything in its subfolders.
2. Intra-package freedom — a package's own files import each other freely.
3. Tests through the entry points — files under <pkg>/tests/ may import any package's entry points and their own tests/ fixtures, but never any package's subfolder internals (not even their own). Integration tests across packages are fine; deep imports are not.
4. No cycles — no dependency cycles.

Entry points, not a barrel. Because the public surface is every root file, a package can expose several small entry points (index.ts, client.ts, server.ts) instead of funnelling everything through one giant index.ts. Barrel files that re-export a whole subtree are discouraged — keep entry points small and hide implementation in subfolders.

Layering (which packages may depend on which) is a different concern and is left as a commented stub in the config for this repo to fill in.

Steps

1. Detect the environment

• Package manager — pnpm-lock.yaml → pnpm, yarn.lock → yarn, bun.lockb → bun, else npm. Use it for every command below (pnpm/yarn/npm run/bunx).
• Packages root — if src/ exists use src/packages, else packages. Confirm the choice with the user if the repo already has a different obvious convention.
• Existing config — check for a .dependency-cruiser.* file. If one exists, do not overwrite it: merge the four rules and the options in, and tell the user what you added.

Done when: package manager, packages root, and existing-config status are all known.

2. Install dependency-cruiser

Install dependency-cruiser as a devDependency with the detected package manager.

Done when: dependency-cruiser is in devDependencies.

3. Write the config

Copy dependency-cruiser.config.cjs to the repo root as .dependency-cruiser.cjs. Set PACKAGES_ROOT to the root detected in step 1. The rules are path-depth based and extension-agnostic, so nothing else needs adapting.

Done when: .dependency-cruiser.cjs exists with the correct PACKAGES_ROOT, and the four forbidden rules are present.

4. Wire it into the checks

• Add a lint:boundaries script: depcruise <packages-root> (or depcruise src).
• Fold it into the repo's umbrella check command — the one that already runs typecheck (e.g. a check / ci / validate script). Do not touch tsconfig or add path aliases.
• If there is no umbrella script, add lint:boundaries and tell the user to include it in CI.

Done when: lint:boundaries exists and runs as part of the same command as typecheck.

5. Scaffold the example package

Create a committed <packages-root>/example/ as a copy-me template:

• index.ts — an entry point. Export one function that delegates to an internal file (so the package is visibly deep, not a pass-through).
• lib/impl.ts — an internal file in a subfolder, imported by index.ts, not reachable from outside.
• tests/example.test.ts — imports only ../index (an entry point), and asserts against the public function.

Tell the user this is a starter template to copy or delete.

Done when: the example package exists, exposes its behaviour through a root entry point, and hides impl in a subfolder.

6. Prove the rules bite

This is the completion criterion for the whole skill — a config that doesn't fail on a violation is worthless.

1. Run lint:boundaries. It must pass on the clean example.
2. Temporarily add a deep import to tests/example.test.ts (e.g. import { thing } from "../lib/impl"). Run lint:boundaries again — it must fail with tests-through-entrypoints.
3. Revert the deep import. Run once more — it must pass.

Done when: you have observed a pass, then a fail on the deep import, then a pass again. If step 2 does not fail, the rules are not wired correctly — fix before finishing.

7. Document the convention

Write a README.md in the packages folder (<packages-root>/README.md) — next to the packages it governs — covering: the src/packages/<name>/ layout (entry points at the root, lib/ for implementation, tests/ for tests), "import only through a package's entry points (its root files)", and how to run lint:boundaries. Discourage barrel files explicitly — expose several small entry points instead of re-exporting a whole subtree through one index. Keep it to the copy-me snippet plus the four rules in one paragraph each.

Then add a context pointer to it from the repo's agent-instructions file — CLAUDE.md if present, else AGENTS.md (create AGENTS.md if neither exists). One line is enough, e.g. Packages are deep modules — see [src/packages/README.md](./src/packages/README.md) before adding or importing one. This is what makes an agent discover the boundary rule instead of tripping over it.

Done when: <packages-root>/README.md exists and discourages barrels, and the repo's CLAUDE.md/AGENTS.md links to it.

Notes

• The config's $1 back-references (dependency-cruiser's group matching) are what let a package reach its own internals while outsiders can't — don't flatten them into separate per-package rules.
• Public vs private is decided by depth: a package's root files are entry points; anything in a subfolder is private. The conventional subfolders are lib/ (implementation) and tests/, but the rule doesn't hardcode them — any subfolder is private, so a new folder never needs a config change. Adding an entry point is just adding a root file — no barrel.
• Packages are flat: one tier of immediate children under the root. A package's internals may nest as deep as you like; a package may not contain another package.
• Use .cjs (not .js) so the config's module.exports works even in "type": "module" repos.

Source: mattpocock/skills → skills/in-progress/setup-ts-deep-modules/SKILL.md