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

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Elixir Idioms

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

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Iron Laws — Never Violate These

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  1. NO PROCESS WITHOUT A RUNTIME REASON — Processes model concurrency, state, isolation—NOT code structure
    2. \n
  2. MESSAGES ARE COPIED — Keep messages small (except binaries >64 bytes)
    3. \n
  3. GUARDS USE and/or/not — Never use short-circuit operators in guards (guards require boolean operands)
    4. \n
  4. CHANGESETS FOR EXTERNAL DATA — Use cast/4 for user input, change/2 for internal
    5. \n
  5. RESCUE ONLY FOR EXTERNAL CODE — Never use rescue for control flow
    6. \n
  6. NO DYNAMIC ATOM CREATION — String.to_atom(user_input) causes memory leak (atoms aren't GC'd)
    7. \n
  7. @external_resource FOR COMPILE-TIME FILES — Modules reading files at compile time MUST declare @external_resource
    8. \n
  8. SUPERVISE ALL LONG-LIVED PROCESSES — Never bare GenServer.start_link/Agent.start_link in production. Use supervision trees
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  9. WRAP THIRD-PARTY LIBRARY APIs — Always facade external deps behind a project-owned module. Enables swapping without touching callers
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  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. \n
  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
    12. \n
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BEAM Architecture (Why Elixir Works This Way)

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Core Principles

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

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Control Flow

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Need patterns? → case (or function heads)\nMultiple operations? → with\nBoolean conditions? → cond (multiple) or if (single)\n
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Error Handling

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Expected failure? → {:ok, _}/{:error, _} tuples\nUnexpected/bug? → raise exception (let supervisor handle)\nExternal library? → rescue (only here!)\n
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OTP

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

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# Pattern match in function head\ndef process(%{status: :active} = user), do: activate(user)\ndef process(%{status: :inactive} = user), do: deactivate(user)\n\n# with for happy path\nwith {:ok, user} <- get_user(id),\n     {:ok, order} <- create_order(user) do\n  {:ok, order}\nend\n\n# Task for async\nTask.Supervisor.async_nolink(TaskSup, fn -> work() end)\n|> Task.yield(5000) || Task.shutdown(task)\n
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Common Pitfalls

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WrongRight
length(list) == 0list == [] 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 conditionif !condition (unless deprecated in 1.18)
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References

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For detailed patterns, see:

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