elixir-conventions

name	elixir-conventions
description	Elixir-specific coding patterns and conventions.

Elixir Conventions

Control Flow

• Pattern match in function heads rather than branching in the body.
• Prefer with chains over nested case.
• Keep iteration flat: Enum.flat_map, Enum.filter, or split into sequential steps (gather then process). Never nest Enum.reduce_while or Enum.reduce — if you need two levels, decompose into a flat_map + single reduce, or use recursion.
• Enum.reduce_while is fine for a single-level early-exit loop, but reach for simpler constructs first (flat_map, Enum.find, Enum.any?, multi-clause recursion).
• Map-then-combine over stateful reduce (Guy Steele pattern): instead of threading an accumulator through Enum.reduce, map each element into monoid space then combine with a monoidal op:

  # Instead of:
  Enum.reduce(items, %{}, fn {k, v}, acc -> Map.put(acc, k, v) end)
  # Prefer:
  Map.new(items, fn {k, v} -> {k, v} end)
  # Or when Map.new doesn't fit:
  items |> Enum.map(&to_map/1) |> Enum.reduce(%{}, &Map.merge/2)
  

  Common monoids: Map.merge/2, MapSet.union/2, ++, +. Shorthand: Map.new/2, MapSet.new/2, Enum.flat_map/2.

Types & Structs

• Use typedstruct for struct definitions.
• Prefer union types for option lists over keyword defaults:

  @type startup_options ::
          {:name, atom()}
          | {:timeout, pos_integer()}
  
  @spec start_link(list(startup_options())) :: GenServer.on_start()
  

• Keep type definitions near the top, after use/alias blocks.

GenServer

• handle_call body: {:reply, do_foo(...), state} — core logic in the do_ function.
• Never call a callback from another callback.
• Organize GenServer modules into clear sections:

  ############################################################
  #                        Public API                        #
  ############################################################
  
  ############################################################
  #                    GenServer Callbacks                   #
  ############################################################
  
  ############################################################
  #                   Private Implementation                 #
  ############################################################
  

State Boundaries

Choose where state lives based on two questions:

1. Should users or external code be able to query it? If yes → persistent storage (DB on disk).
2. Can we tolerate losing it on crash or shutdown? If yes → process state is fine for pure implementation details. If we can tolerate loss on shutdown but not on crash → an ETS/Mnesia table (survives process crashes, lost on VM stop) may be the right middle ground.

Documentation

• First-person voice: "I am the X module." / "I return the Y."
• @moduledoc starts with one-sentence purpose.
• ### Public API section listing all public functions.
• All public functions need @doc and @spec.

Examples

Examples use the ExExample library for caching and composition.

• use ExExample in the module, import ExUnit.Assertions for assertions.
• Define examples with the example macro:

  @spec create_upload() :: Upload.t()
  example create_upload do
    {:ok, upload} = Uploads.store_new_file(...)
    assert upload.stored_name != nil
    upload  # return a useful object
  end
  

• Examples are cached — calling one from another reuses the result instead of re-running. This enables composition:

  example add_comment do
    upload = EUpload.create_upload()  # cached, not re-run
    {:ok, comment} = Comments.create(...)
    comment
  end
  

• Override rerun?/1 to control caching per example: def rerun?(_), do: true to always re-run (e.g., examples that spawn processes or depend on runtime state). Pattern match on specific examples for selective control.
• Tests: use ExExample.ExUnit, for: Examples.EModule to auto-generate one test per example.
• Live in lib/examples/e_<module>.ex.
• Module name: E<Module> (e.g., EUpload, EEventStore).
• Every example gets @spec — they are typed public functions.
• Run with: mix run -e 'EModule.example_name()'
• Build examples incrementally: verify each layer in IEX before writing the next. The examples ARE the verification.

Interactive Testing

• Run one-off expressions: timeout 60 mix run -e 'code' (never use --no-halt, it hangs the VM).
• Inspect process state: :sys.get_state(pid)

Formatting

• 98 character line length.
• Run mix format before finalizing.
• Section headers use banner comments:

  ############################################################
  #                      Section Name                        #
  ############################################################