Elixir Idioms

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Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha 492f17636999 · 18 Oct 2026Report an issue or request a re-scan

• Publisheroliver-kriska GitHub userOwner of the repository this skill is read from
• Source folderoliver-kriska/claude-elixir-phoenix/plugins/elixir-phoenix/skills/elixir-idiomsThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdict492f17636999A new audit runs when the source changes
• Repository statusNot archivedLast push 2 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: OTP/BEAM patterns and Elixir idioms — GenServer, Supervisor, Task, Registry, pattern matching, with chains, pipes. Use when designing processes or debugging BEAM issues.

---
name: elixir-idioms
description: "OTP/BEAM patterns and Elixir idioms — GenServer, Supervisor, Task, Registry, pattern matching, with chains, pipes. Use when designing processes or debugging BEAM issues."
effort: medium
user-invocable: false
---

# 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

```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)` |
| `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+)

Excerpt, first 4,805 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/oliver-kriska/claude-elixir-phoenix
git -C claude-elixir-phoenix checkout 492f17636999

Works in

1. 2026-10-18
   ✓SAFE B 89 / 100

   first audit · source 492f17636999

Questions about Elixir Idioms

• github.com/oliver-kriska/claude-elixir-phoenix/tree/main/plugins — the audited source at commit 492f17636999 and every finding aboveObserved · 18 Oct 2026

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