setup-ts-deep-modules

Files (2)

• SKILL.md
• dependency-cruiser.config.cjs

SKILL.md (first lines)

# 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](https://github.com/sverweij/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), 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 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.

Five rules, all `error`:

1. **Entry-point boundary from app**: code outside a package (app code or root code) may import only that package's entry points (its root files), never anything in its subfolders.
2. **Entry-point boundary across packages**: a package's own files import each other freely, but may reach other packages only through their entry points, never their subfolder internals.
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. **Tests folder is private**: files under `<pkg>/tests/` are reachable only from test files; application code may not import test fixtures.
5. **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`).
- **TypeScript and module setup**:
  - Check `package.json` for `"type": "module"` (ESM).
  - Inspect `tsconfig.json` (or root solution / base tsconfig) for `moduleResolution` (`NodeNext`, `Node16`, `Bundler`, `Node`) and path mappings (`compilerOptions.paths`).
  - Check if project references (`references: [...]`) or monorepo workspaces are used.
- **Packages root**: if `src/packages` exists use `src/packages`, else `packages` or `src/`. 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 five rules and resolver options in, and tell the user what you added.
- **Upstream verification**: When encountering non-standard bundler plugins, path aliases, or experimental TypeScript options, use `search_web` and `read_url_content` to inspect upstream `dependency-cruiser` documentation and release notes for the exact `enhancedResolveOptions` syntax.

**Done when:** package manager, TypeScript module setup, 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.
- Set `options.tsConfig.fileName` to point to the repo's `tsconfig.json` (or the root solution config that resolves path aliases and references).
- Ensure `options.enhancedResolveOptions` includes modern TypeScript extensions (`.ts`, `.tsx`, `.d.ts`, `.js`, `.jsx`, `.mjs`, `.cjs`, `.json`) and export conditions (`exportsFields: ["exports"]`, `conditionNames: ["import", "require", "node", "default", "types"]`).

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`, resolver options, and the five forbidden rules.

### 4. Wire it into the checks

- Add a `lint:boundaries` script in `package.json`: `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 solely for boundary linting.
- 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` is an entry point. Export one function that delegates to an internal file (so the package is visibly *deep*, not a pass-through). If using NodeNext ESM resolution, use explicit relative extensions (such as `./lib/impl.js`).
- `lib/impl.ts`: an internal file in a **subfolder**, imported by `index.ts`, not reachable from outside.
- `tests/example.test.ts` imports **only** `../index` (or `../index.js` under NodeNext) 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"` or `../lib/impl.js`). 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 rules in one paragraph each.

Then add a **context pointer** to it from the repo's agent-instructions file (`AGENTS.md` if present, else `CLAUDE.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 tripping over it.

**Done when:** `<packages-root>/README.md` exists and discourages barrels, and the repo's `AGENTS.md`/`CLAUDE.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 cannot. Do not 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 does not 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.
- In TypeScript ESM projects using `NodeNext` or `Node16`, source code imports write `.js` extensions while files on disk are `.ts`. Dependency-cruiser resolves these specifiers to the underlying TypeScript files via `tsConfig` and `enhancedResolveOptions`.

From fderuiter/agy-skills, MIT.

More from fderuiter/agy-skills