elixir-idioms

# 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 ## BEAM Architecture (Why Elixir Works This Way) - **Processes are cheap (2.6KB)** — Spawn liberally for concurrency/isolation - **Complete memory isolation** — No shared state, no locks needed - **Messages are copied** (except binaries >64 bytes) — Keep messages small - **Per-process GC** — No global GC pauses - **"Let it crash"** — Supervisors restart to known-good state ## Core Principles 1. **Pattern...