# Setup TS Deep Modules
Make every package in this repo a **deep module**: a lot of behaviour behind a small interface. A package's is its **entry points** (the files at the package root), and everything in its subfolders is hidden. This skill installs [dependency-cruiser](https://github.com/sverweij/dependency-cruiser) and the rules that make the entry points , then .
For the vocabulary (deep module, interface, seam, depth), call the Skill tool with "codebase-design" and 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 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. ****: 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; .
4. **No cycles**: no dependency cycles.
**Entry points, not a barrel.** Because the is *every* root file, a package can expose several small entry points (`index.ts`, `client.ts`, `server.ts`) instead of `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`](./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 , 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 , 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 :
- `index.ts` is 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 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, and 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, so 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 **** to it from the repo's agent-instructions file (`CLAUDE.md` if present, else `AGENTS.md`, creating `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 .
**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` (dependency-cruiser's group matching) are what let a package reach its own internals while outsiders can't. .
- 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.