Elixir Idioms

Name: elixir-idioms
Rating: 92
Author: oliver-kriska

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

Iron Laws — Never Violate These

•NO PROCESS WITHOUT A RUNTIME REASON — Processes model concurrency, state, isolation—NOT code structure
•MESSAGES ARE COPIED — Keep messages small (except binaries >64 bytes)
•GUARDS USE and/or/not — Never use short-circuit operators in guards (guards require boolean operands)
•CHANGESETS FOR EXTERNAL DATA — Use cast/4 for user input, change/2 for internal
•RESCUE ONLY FOR EXTERNAL CODE — Never use rescue for control flow
•NO DYNAMIC ATOM CREATION — String.to_atom(user_input) causes memory leak (atoms aren't GC'd)
•@external_resource FOR COMPILE-TIME FILES — Modules reading files at compile time MUST declare @external_resource
•SUPERVISE ALL LONG-LIVED PROCESSES — Never bare GenServer.start_link/Agent.start_link in production. Use supervision trees
•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

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

Quick Decision Trees

Control Flow

code

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

Error Handling

code

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

OTP

code

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

Quick Patterns

elixir

# 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)`

References

For detailed patterns, see:

•references/pattern-matching.md - Pattern matching, guards, binary matching
•references/otp-patterns.md - GenServer, Supervisor, Task, Registry
•references/error-handling.md - Tagged tuples, rescue, with
•references/with-and-pipes.md - When to use with and |> (idiomatic patterns)
•references/troubleshooting.md - Production BEAM debugging (memory, performance, crashes)
•references/anti-patterns.md - Common mistakes and fixes
•references/elixir-118-features.md - Duration module, dbg improvements (1.18+)