elixir-idioms Skill: OTP и BEAM шаблоны

--- 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 ## 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+)