Staff-Level Elixir

Distilled staff-level judgment for Elixir on the BEAM — the design decisions Elixir, OTP, Ecto, and Phoenix force, and how an experienced engineer settles them, written so an agent applies them while writing or reviewing code. Each rule corrects a specific wrong default; there is no rule for things the model already gets right (syntax, basic idioms, the standard library).

When to Apply

• Writing or reviewing OTP code — deciding whether something needs a process, and which (GenServer, Task, Agent, ETS, supervisor)
• Handling failure — choosing between tagged tuples, exceptions, with, and letting a process crash
• Writing Ecto — queries, associations, transactions, changesets, large result sets
• Building Phoenix controllers and LiveViews — context boundaries, socket state, mount lifecycle
• Running concurrent work — parallel fan-out, fire-and-forget tasks, handling untrusted input
• Any request to make Elixir code more idiomatic, more fault-tolerant, or "staff-level"

Rule Categories

Quick Reference

1. Process & OTP Design

• otp-genserver-not-default — a GenServer is a serialization point, not the default abstraction
• otp-ets-for-shared-reads — read-heavy shared state belongs in ETS, not behind a GenServer.get
• otp-supervise-over-manual-restart — child specs + DynamicSupervisor/Registry, not hand-rolled restart logic
• otp-handle-continue-init — defer heavy startup to handle_continue; init/1 blocks the supervisor

2. Error Handling & Let-It-Crash

• err-tagged-tuples-vs-raise — {:ok/:error} for expected failures, raise only for invariant violations
• err-let-it-crash — don't rescue to mask bugs; let the supervisor restart clean state
• err-with-happy-path — chain fallible steps with with; keep the error term intact

3. Idioms & Design Choices

• data-pattern-match-over-conditionals — function clauses + guards over if/cond on argument shape
• data-stream-vs-enum — Stream for large/lazy/early-exit, Enum for small concrete lists
• data-iolists-over-concat — build large output as iolists; <> in a loop is O(n²)
• data-pipe-idioms — pipe a data subject through transformations, not to save a variable
• data-functions-over-macros — solve it with a function before reaching for a macro

4. Concurrency & the Scheduler

• conc-async-stream-bounded — Task.async_stream with max_concurrency + explicit timeout
• conc-task-supervised-nolink — supervised, unlinked Task for fire-and-forget work
• conc-atom-exhaustion — never build atoms from external input; use to_existing_atom

5. Ecto & Data Access

• ecto-preload-n-plus-one — preload up front, never inside an Enum loop
• ecto-multi-for-transactions — Ecto.Multi for atomic multi-step writes with failure attribution
• ecto-db-constraints-over-validation — DB constraint + unique_constraint, not a racy validation query
• ecto-stream-large-sets — Repo.stream/batch large sets; Repo.all OOMs at scale
• ecto-atomic-counters — update_all with inc: for counters; read-modify-write loses updates

6. Phoenix & LiveView

• phx-context-boundary — web layer calls contexts, never Repo or schemas directly
• phx-liveview-streams — streams for large/growing collections, not full lists in assigns
• phx-mount-twice-connected — gate side effects on connected?/1; mount runs twice

How to Use

Read a reference file when its decision comes up. Each rule names the wrong default it corrects, then shows the canonical way (with an incorrect/correct contrast only where the wrong way is a real trap).

• Section definitions — category structure and ordering
• Rule template — for adding new rules
• AGENTS.md — auto-built table of contents across all rules

Reference Files