elixir-idioms

Elixir Idioms and Patterns

Elixir rewards immutability, pattern matching, and the OTP supervision model. Idiomatic Elixir = functional, fault-tolerant, process-oriented.

Scope: Elixir coding idioms. Test naming: .agents/rules/testing-strategy.md.

Pattern Matching

1. Pattern match everything — function heads, case, with:

   def handle_result({:ok, task}), do: {:reply, task}
   def handle_result({:error, :not_found}), do: {:error, 404}
   def handle_result({:error, reason}), do: {:error, 500, reason}
   

2. with for happy-path pipelines:

   with {:ok, user} <- fetch_user(user_id),
        {:ok, task} <- create_task(user, params),
        :ok <- notify(user, task) do
     {:ok, task}
   end
   

OTP and Supervision

1. GenServer for stateful processes:

   defmodule TaskCache do
     use GenServer
   
     def start_link(opts), do: GenServer.start_link(__MODULE__, %{}, opts)
     def get(pid, id), do: GenServer.call(pid, {:get, id})
   
     @impl true
     def handle_call({:get, id}, _from, state) do
       {:reply, Map.get(state, id), state}
     end
   end
   

2. Supervision trees — "let it crash" philosophy. Supervisors restart failed children.

3. Never try/catch in normal flow — let processes crash and restart cleanly.

Pipe Operator

1. |> for data transformation pipelines:

   tasks
   |> Enum.filter(&(&1.active))
   |> Enum.sort_by(& &1.priority, :desc)
   |> Enum.map(&TaskView.render/1)
   

2. First argument must be the piped value. Design APIs pipe-friendly.

Naming

1. snake_case for functions, variables, atoms, module attributes.
2. PascalCase for module names.
3. ? suffix for boolean functions: active?/1.
4. ! suffix for functions that raise: get!/1 (vs get/1 returning {:ok, _}).

Testing

1. ExUnit:

   defmodule TaskServiceTest do
     use ExUnit.Case, async: true
   
     test "creates task with valid params" do
       {:ok, task} = TaskService.create(%{title: "Test", priority: :high})
       assert task.title == "Test"
     end
   end
   

2. Mox for mock-based testing (behaviour contracts).

Formatting and Static Analysis

Tool	Purpose	Command
mix format	Formatting	mix format
Credo	Linting	mix credo --strict
Dialyzer	Type checking	mix dialyzer
mix deps.audit	CVE scanning	mix deps.audit

Related

• Code Idioms and Conventions .agents/rules/code-idioms-and-conventions.md
• Error Handling Principles .agents/rules/error-handling-principles.md
• Testing Strategy .agents/rules/testing-strategy.md
• Concurrency and Threading Principles @.agents/rules/concurrency-and-threading-principles.md