elixir.cursorrules

# .cursorrules — Elixir / Phoenix (Senior / Staff)
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You are pairing with a senior engineer on a production Elixir/Phoenix system. Optimize for correctness,
observability, and predictable failure — not for looking productive.

## Baseline posture
- Read before you write. Match existing style, naming, and idioms. Do exactly what was asked — no
  unrequested abstractions or drive-by refactors.
- Every changed line traces to the request. Remove orphaned aliases/imports; don't touch unrelated code.
- Handle the error path, not just the happy path. Never claim "done" on uncompiled/untested code.

## Elixir idioms
- Pattern-match in function heads over conditional branching. Let unmatched cases fail loudly.
- Return tagged tuples (`{:ok, val}` / `{:error, reason}`); reserve `!` functions for "must succeed".
- Compose fallible steps with `with`; keep the happy path linear and the `else` explicit and small.
- Pipe `|>` for linear data transforms; don't pipe just to avoid a variable. Keep functions small and pure
  where you can — push side effects to the edges.
- `mix format` clean. Run `mix compile --warnings-as-errors` and Credo on touched files.

## OTP & failure
- Let it crash: don't `try/rescue` to paper over bugs. Rescue only genuinely expected, recoverable errors —
  never use a bare `rescue` for control flow.
- Put state in a supervised process (GenServer/Agent) with a deliberate restart strategy. Don't reach for a
  process when a pure function or ETS will do.
- Name the failure mode (mailbox overflow, partial writes, restart storms). Set timeouts on every `call`.
- Make state-mutating, retryable operations idempotent.

## Phoenix & Ecto
- Respect context boundaries: web layer calls contexts, contexts own the data. No `Repo` calls in
  controllers/LiveViews.
- All external input flows through an Ecto changeset (cast + validate). Never trust params.
- Enforce data integrity with DB constraints + matching changeset constraints (`unique_constraint`, FKs).
- Avoid N+1: `preload` associations; keep queries in the context. Use `Ecto.Multi` for multi-step writes.
- LiveView: minimize assigns, prefer `stream` for collections, keep `handle_event` thin.

## Tests
- ExUnit with `async: true` where there's no shared state. Use `Ecto.Adapters.SQL.Sandbox`.
- Cover the `{:error, _}` branches and boundary cases. Prefer real functions over heavy mocking; if you
  mock, mock a behaviour with Mox. No `Process.sleep` for synchronization.

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