Elixir / Erlang: Idiomatic Efficiency Reference

When to Use

• Use this skill when the task matches this description: Language-specific super-code guidelines for elixir.

Table of Contents

1. Pattern Matching & Guards
2. Pipe Operator & Transforms
3. Processes & OTP
4. Error Handling
5. Collections & Enum
6. Structs & Protocols
7. Anti-patterns specific to Elixir/Erlang

1. Pattern Matching & Guards {#patterns}

# ❌ Extracting with Map.get then checking
value = Map.get(map, :key)
if value != nil do
  process(value)
end

# ✅ — pattern match directly
case map do
  %{key: value} -> process(value)
  _ -> :noop
end
# or with if:
if value = map[:key], do: process(value)

# ❌ Nested case for multiple conditions
case fetch_user(id) do
  {:ok, user} ->
    case validate(user) do
      {:ok, valid_user} -> save(valid_user)
      {:error, reason} -> {:error, reason}
    end
  {:error, reason} -> {:error, reason}
end

# ✅ — with clause
with {:ok, user} <- fetch_user(id),
     {:ok, valid_user} <- validate(user) do
  save(valid_user)
end

# ❌ if/else for known shapes
def area(shape) do
  if shape.type == :circle do
    :math.pi() * shape.radius * shape.radius
  else
    shape.width * shape.height
  end
end

# ✅ — multi-clause function with pattern match
def area(%{type: :circle, radius: r}), do: :math.pi() * r * r
def area(%{type: :rect, width: w, height: h}), do: w * h

# ❌ Checking type at runtime
def process(x) do
  if is_integer(x) and x > 0 do
    x * 2
  end
end

# ✅ — guard clause
def process(x) when is_integer(x) and x > 0, do: x * 2
def process(_), do: {:error, :invalid_input}

2. Pipe Operator & Transforms {#pipes}

# ❌ Nested function calls
String.trim(String.downcase(String.replace(input, ~r/\s+/, " ")))

# ✅
input
|> String.replace(~r/\s+/, " ")
|> String.downcase()
|> String.trim()

# ❌ Pipe into anonymous function awkwardly
data
|> (fn x -> x * 2 end).()

# ✅ — use then/1 or named function
data
|> then(&(&1 * 2))
# or better: extract a named function
data |> double()

# ❌ Single-step pipe (no gain in readability)
result = list |> Enum.count()

# ✅ — direct call for single operation
result = Enum.count(list)

Pipe when 2+ transforms. Direct call for single operation. First arg flows through pipe.

3. Processes & OTP {#otp}

# ❌ Raw spawn for stateful process
pid = spawn(fn -> loop(%{count: 0}) end)
send(pid, {:increment})

# ✅ — GenServer for stateful processes
defmodule Counter do
  use GenServer

  def start_link(init \\ 0), do: GenServer.start_link(__MODULE__, init)
  def increment(pid), do: GenServer.call(pid, :increment)

  @impl true
  def init(count), do: {:ok, count}

  @impl true
  def handle_call(:increment, _from, count), do: {:reply, count + 1, count + 1}
end

# ❌ Spawning without linking (orphan process on crash)
spawn(fn -> do_work() end)

# ✅ — Task for fire-and-forget with supervision
Task.start(fn -> do_work() end)
# or for awaitable result:
task = Task.async(fn -> do_work() end)
result = Task.await(task)

# ❌ Manual process registry
Process.register(self(), :my_worker)

# ✅ — use Registry or named GenServer
{:ok, _} = Registry.start_link(keys: :unique, name: MyRegistry)
GenServer.start_link(Worker, arg, name: {:via, Registry, {MyRegistry, :my_worker}})

# ❌ try/catch in GenServer (breaks supervision)
def handle_call(:work, _from, state) do
  try do
    result = risky_operation()
    {:reply, result, state}
  catch
    _ -> {:reply, :error, state}
  end
end

# ✅ — let it crash; supervisor restarts
def handle_call(:work, _from, state) do
  result = risky_operation()
  {:reply, result, state}
end

"Let it crash" — supervisors handle recovery. Don't defensively catch inside GenServers.

4. Error Handling {#errors}

# ❌ Raising for expected failures
def find_user(id) do
  case Repo.get(User, id) do
    nil -> raise "User not found"
    user -> user
  end
end

# ✅ — tagged tuples for expected outcomes
def find_user(id) do
  case Repo.get(User, id) do
    nil -> {:error, :not_found}
    user -> {:ok, user}
  end
end

# ❌ Ignoring error tuple
{:ok, result} = might_fail()  # crashes on {:error, _}

# ✅ — handle both cases
case might_fail() do
  {:ok, result} -> process(result)

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