Elixir Idioms

---
name: elixir-idioms
description: "OTP/BEAM patterns and Elixir idioms — GenServer, Supervisor, Task, Registry, pattern matching, with chains, pipes. Use when designing processes or debugging BEAM issues."
effort: medium
user-invocable: false
---

# Elixir Idioms

Reference for writing idiomatic Elixir code with BEAM-aware patterns.

## Iron Laws — Never Violate These

1. **NO PROCESS WITHOUT A RUNTIME REASON** — Processes model concurrency, state, isolation—NOT code structure
2. **MESSAGES ARE COPIED** — Keep messages small (except binaries >64 bytes)
3. **GUARDS USE `and`/`or`/`not`** — Never use short-circuit operators in guards (guards require boolean operands)
4. **CHANGESETS FOR EXTERNAL DATA** — Use `cast/4` for user input, `change/2` for internal
5. **RESCUE ONLY FOR EXTERNAL CODE** — Never use rescue for control flow
6. **NO DYNAMIC ATOM CREATION** — `String.to_atom(user_input)` causes memory leak (atoms aren't GC'd)
7. **@external_resource FOR COMPILE-TIME FILES** — Modules reading files at compile time MUST declare `@external_resource`
8. **SUPERVISE ALL LONG-LIVED PROCESSES** — Never bare `GenServer.start_link`/`Agent.start_link` in production. Use supervision trees
9. **WRAP THIRD-PARTY LIBRARY APIs** — Always facade external deps behind a project-owned module. Enables swapping without touching callers
10. **MIX TASKS START ONLY WHAT THEY NEED** — `Mix.Task.run("app.config")` + `Application.ensure_all_started/1`, never `Mix.Task.run("app.start")` (boots the FULL tree: endpoint port, Oban consuming)
11. **CAPTURE LOCALE BEFORE SPAWNING** — Gettext/CLDR locale is process-local. Read it in the caller and pass explicitly; a spawned Task/GenServer starts with the default locale
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