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 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

length(list) == 0

list == []

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

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+)