Elixir Idioms

What it does

When your agent uses it

• Designing processes
• Debugging BEAM issues

Example prompts

• “/elixir-idioms”

Workflow steps

11 steps, taken from the first numbered list in SKILL.md.

1. 1NO PROCESS WITHOUT A RUNTIME REASON — Processes model concurrency, state, isolation—NOT code structure
2. 2MESSAGES ARE COPIED — Keep messages small (except binaries >64 bytes)
3. 3GUARDS USE and/or/not — Never use short-circuit operators in guards (guards require boolean operands)
4. 4CHANGESETS FOR EXTERNAL DATA — Use cast/4 for user input, change/2 for internal
5. 5RESCUE ONLY FOR EXTERNAL CODE — Never use rescue for control flow
6. 6NO DYNAMIC ATOM CREATION — String.to_atom(user_input) causes memory leak (atoms aren't GC'd)
7. 7@external_resource FOR COMPILE-TIME FILES — Modules reading files at compile time MUST declare @external_resource
8. 8SUPERVISE ALL LONG-LIVED PROCESSES — Never bare GenServer.start_link/Agent.start_link in production. Use supervision trees
9. 9WRAP THIRD-PARTY LIBRARY APIs — Always facade external deps behind a project-owned module. Enables swapping without touching callers
10. 10MIX TASKS START ONLY WHAT THEY NEED — Mix.Task.run("app.config") + Application.ensure_all_started/1, never Mix.Task.run("app.start")…
11. 11CAPTURE LOCALE BEFORE SPAWNING — Gettext/CLDR locale is process-local. Read it in the caller and pass explicitly; a spawned Task/GenServer…

What it can do on your machine

Read from SKILL.md and the folder at commit 9767a82. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are elixir).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  No URLs in SKILL.md.

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Elixir Idioms loads about 1.1k tokens when it runs, and up to ~12k if it reads all its reference files. Until then it costs about 46 tokens; SKILL.md has 400 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~46

When it runs · the whole SKILL.md, loaded when a task matches

~1.1k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~12k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from oliver-kriska/claude-elixir-phoenix at commit 9767a82, republished under its MIT licence (© oliver-kriska). 400 words, ~1,148 tokens.

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.

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

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

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

# 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)

© oliver-kriska, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 9 other files (references) in targets/pi/skills/elixir-idioms of oliver-kriska/claude-elixir-phoenix.

• SKILL.md
• references/anti-patterns.md
• references/elixir-118-features.md
• references/elixir-120-type-system.md
• references/error-handling.md
• references/mix-tasks.md
• references/otp-patterns.md
• references/pattern-matching.md
• references/troubleshooting.md
• references/with-and-pipes.md

Open the folder on GitHubat commit 9767a82