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

1. Pattern match over conditionals — Function heads first, then case, then cond
2. Tagged tuples for expected failures — {:ok, _}/{:error, _} for expected errors, raise for bugs
3. Pipe operator for data transformation — Start with data, never pipe single calls
4. Let it crash — Handle expected errors, crash on unexpected ones
5. Explicit over implicit — Be clear about intentions

Quick Decision Trees

Control Flow

Need patterns? → case (or function heads)
Multiple operations? → with
Boolean conditions? → cond (multiple) or if (single)

Error Handling

Expected failure? → {:ok, _}/{:error, _} tuples
Unexpected/bug? → raise exception (let supervisor handle)
External library? → rescue (only here!)

OTP

Need state?
├─ No → Plain functions
├─ Simple get/update → Agent or ETS
├─ Complex messages/timeouts → GenServer
└─ One-off async → Task

Quick Patterns

# Pattern match in function head
def process(%{status: :active} = user), do: activate(user)
def process(%{status: :inactive} = user), do: deactivate(user)

# with for happy path
with {:ok, user} <- get_user(id),
     {:ok, order} <- create_order(user) do
  {:ok, order}
end

# Task for async
Task.Supervisor.async_nolink(TaskSup, fn -> work() end)
|> Task.yield(5000) || Task.shutdown(task)

Common Pitfalls

Wrong	Right
length(list) == 0	list == [] or Enum.empty?(list)
list ++ [item]	[item | list] |> Enum.reverse()
String.to_atom(input)	String.to_existing_atom(input)
spawn(fn -> log(conn) end)	ip = conn.ip; spawn(fn -> log(ip) end)
unless condition	if !condition (unless deprecated in 1.18)

References

For detailed patterns, see:

• ${CLAUDE_SKILL_DIR}/references/pattern-matching.md - Pattern matching, guards, binary matching
• ${CLAUDE_SKILL_DIR}/references/otp-patterns.md - GenServer, Supervisor, Task, Registry
• ${CLAUDE_SKILL_DIR}/references/error-handling.md - Tagged tuples, rescue, with
• ${CLAUDE_SKILL_DIR}/references/with-and-pipes.md - When to use with and |> (idiomatic patterns)
• ${CLAUDE_SKILL_DIR}/references/troubleshooting.md - Production BEAM debugging (memory, performance, crashes)
• ${CLAUDE_SKILL_DIR}/references/anti-patterns.md - Common mistakes and fixes
• ${CLAUDE_SKILL_DIR}/references/mix-tasks.md - Mix task naming, option parsing, shell output
• ${CLAUDE_SKILL_DIR}/references/elixir-118-features.md - Duration module, dbg improvements (1.18+)
• ${CLAUDE_SKILL_DIR}/references/elixir-120-type-system.md - Gradual type checker, dynamic(), verified bugs as compile warnings (1.20+, OTP 27+)