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elixir

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Use when writing or refactoring Elixir/OTP on the BEAM — GenServers, supervision trees and restart strategies, pattern matching, mix projects and releases — or when processes misbehave (restart loops, mailbox growth, call timeouts). NOT a Phoenix web app, LiveView, Ecto or channels (that is `phoenix`).

skills/elixir/SKILL.md

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# Elixir

You are writing Elixir on the BEAM. The runtime gives you cheap isolated processes, preemptive scheduling, and supervision. The single mental shift that separates idiomatic Elixir from ported imperative code: **let it crash and supervise it, do not defend every call**. A process that hits an impossible state should die and be restarted clean by its supervisor — that is more correct than a `try/rescue` that limps on with corrupt state.

Default to **pure functions**. Most of your code is data transformation and needs no process at all. Reach for a process only when you need state, concurrency, or fault isolation. Target Elixir **v1.19.5** (stable, requires Erlang/OTP 28.1+) or **v1.20-rc** (full type inference, OTP 27+/29). Use the modern stdlib: built-in `JSON`, set-theoretic type warnings, `mix format`.

## Decision: do you even need a process?

A process is not "an object": spawning one to hold a value you could pass as an argument is the most common beginner mistake, and it adds a serialization bottleneck and a failure mode for nothing.

| Situation | Use | Why |
|---|---|---|
| Pure transform of input -> output | plain function / module | No state, no concurrency: a process only adds overhead and a mailbox |
| Hold mutable state behind an API | `GenServer` (or `Agent` for trivial state) | Serializes access, owns a lifecycle, supervisable |