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

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