Propose pnpm support

text/0907-pnpm-support.md

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+---
+stage: accepted
+start-date: 2023-03-06T14:09:00.000Z
+release-date: # In format YYYY-MM-DDT00:00:00.000Z
+release-versions:
+teams: # delete teams that aren't relevant
+  - cli
+  - learning
+prs:
+  accepted: https://github.com/emberjs/rfcs/pull/907 
+project-link:
+suite: 
+---
+
+<!--- 
+Directions for above: 
+
+stage: Leave as is
+start-date: Fill in with today's date, 2032-12-01T00:00:00.000Z
+release-date: Leave as is
+release-versions: Leave as is
+teams: Include only the [team(s)](README.md#relevant-teams) for which this RFC applies
+prs:
+  accepted: Fill this in with the URL for the Proposal RFC PR
+project-link: Leave as is
+suite: Leave as is
+-->
+
+# `pnpm` support
+
+## Summary
+
+Enable Ember CLI users to opt into using `pnpm` for package management.
+
+## Motivation
+
+[`pnpm`](https://pnpm.io/) is a popular alternative to both `npm` and `yarn` that prioritizes correctness, especially for `peerDependencies` and monorepos. 
+
+`pnpm` has very active maintainance, support, and financial funding.
+Their [website](https://pnpm.io/) states that `pnpm` is:
+ - Fast: `pnpm` is up to 2x faster than the alternatives 
+ - Efficient: Files inside `node_modules` are cloned or hard linked from a single content-addressable storage 
+ - Supports monorepos: `pnpm` has built-in support for multiple packages in a repository
+ - Strict: `pnpm` creates a non-flat `node_modules` by default, so code has no access to arbitrary packages.
+
+For folks with lots of projects on their computers, `pnpm` is _extremely_ space-efficient.
+Where `npm` and `yarn` would duplicate `node_modules` per-project, `pnpm` will only download a package (at a specific version) once across your whole machine.
+
+Additionally, `npm` and `yarn` repeatedly have demonstrated that their strategies with `peerDependencies` are not correct, and it is vitally important we use and support a tool that can guide folks towards correctly creating addons. For example, `@embroider/macros` relies on node's resolution algorithm, so having `peerDependencies` resolved correctly is important for `dependencySatisfies` to work as expected in monorepos. 
+
+Ember CLI currently only supports `npm` (default) and `yarn` for project initialization as well as various commands. At present, projects work with `pnpm`, but the tooling is unaware.
+
+
+
+## Detailed design
+
+Enabling `pnpm` is designed as opt-in to prevent disruptions to developer's current workflow, much the same as `yarn`.
+
+There are a few integration points that we must support for `pnpm` (and any package manager):
+ - `ember install`
+ - `ember init`, `ember new`, `ember addon`
+ - `ember try:one`, `ember try:each`
+ - generated C.I. configs
+ - Documenation
+
+### `ember install`
+
+There are two mechanisms through which to opt-in.
+The first one is the presence of a `pnpm-lock.yaml` file in the project root.
+
+The `pnpm-lock.yaml` file is generated by `pnpm` when you run `pnpm install` (or the shorter `pnpm i`),
+so we assume that its presence means the developer intends to use `pnpm` to manage their dependencies.
+
+Alternatively you, you can force Ember CLI to use `pnpm` with the `--pnpm` flag, and symmetrically,
+you can force Ember CLI to not use `pnpm` with the `--no-pnpm` flag.
+
+To recap:
+
+- `ember install ember-resources` with `pnpm-lock.yaml` present will use `pnpm`
+- `ember install ember-resources` without `pnpm-lock.yaml` present will use npm
+- `ember install ember-resources --pnpm` will use `pnpm`
+- `ember install ember-resources --no-pnpm` will use npm
+
+### `ember init`, `ember new`, `ember addon`
+
+Since this triad of commands is generally ran before a project is set up, there is no `pnpm-lock.yaml` file presence to check.
+This means we are left with the `--pnpm`/`--no-pnpm` pair of flags, that will also be added to these commands:
+
+- `ember new my-app` will use npm
+- `ember new my-app --pnpm` will use `pnpm`
+
+The above also applies to `ember addon` and `ember init`, noting that `ember init` doesn't receive any arguments.
+
+#### `--skip-npm`
+
+The `--skip-npm` flag _actually means_ "skip installation of dependencies" when using `ember addon` and `ember new`, 
+including skipping installation with both `npm` and `yarn`. 
+This will need to be extended to also skip installation of dependencies when `pnpm` is used.
+
+For example:
+```bash
+ember new my-app --pnpm --skip-npm
+```
+
+
+### `ember try:one`, `ember try:each`
+
+At the time of writing this RFC, `ember-try` already supports `pnpm`, but it is undocumented in [the README](https://github.com/ember-cli/ember-try).
+Documentation will need to be added to the README, 
+as well as the relevant `ember-cli` blueprints will need to correctly configure `usePnpm: true` in the `ember-try.js` config file when the `pnpm` flag is present.
+
+### generated C.I. configs
+
+At the time of writing this RFC, `ember-cli` supports two C.I. environments: Travis and GitHub Actions. 
+
+Both the `.travis.yml` and `.github/workflows/ci.yml` config files for relevant blueprints will need to support the `pnpm` option such that C.I. passes on new projects using `pnpm`.
+
+### Documentation
+
+These pages presently mention `npm` / `yarn` and will need to be updated to include `pnpm` 
+ - https://cli.emberjs.com/release/basic-use/assets-and-dependencies/
+ - https://guides.emberjs.com/release/addons-and-dependencies/
+
+## How we teach this
+
+The Ember Guides should be updated to reflect the new flags, where applicable,
+as well as the new behavior of `ember install` in the presence of a `pnpm-lock.yaml` -- though most of the guides use `ember` as the CLI tool for managing packages. 
+
+We may want to consider updating the [tutorial](https://guides.emberjs.com/release/tutorial/part-1/orientation/) (and its automation) to use `pnpm`, as `npm` is very slow.
+
+
+In addition, the built-in instructions for `ember help` should be updated to reflect the new flags.
+
+
+## Drawbacks
+
+- `pnpm` is very strict about peers and what dependencies are allowed in a project and will error if a project's package.json is incorrect for a given scenario. `pnpm` is very clear about these errors and what to do for action items, but it means that we'll need to make sure that `pnpm` is tested in `ember-cli` when generating new projects so that we can be certain that folks' "first time experience" is smooth
+- There may be other package managers in the future, but we cannot see the future. There have been talks about making ember-cli somehow generically handle package-managers, but it is unknown how that would work, and is unneeded for now.
+
+
+## Alternatives
+
+- Figure out a way to generically handle any package manager 
+- Continue with the partial pnpm support we have today
+
+## Unresolved questions
+
+- Are there any other references in the guides to `npm` or `yarn`?
+  The only place I could find that _mentioned_ `yarn` is here: https://guides.emberjs.com/release/addons-and-dependencies/
+- Is there a `--package-manager` flag / option in `ember-cli`? for blueprint authors, that may be useful.
+- Should `--skip-npm` be aliased to `--skip-install`?
+
+