elixir-essentials

Elixir Essentials

RULES — Follow these with no exceptions

1. Use pattern matching over if/else for control flow and data extraction
2. Add @impl true before every callback function (mount, handle_event, handle_info, etc.)
3. Return {:ok, result} | {:error, reason} tuples for fallible operations
4. Use with for 2+ sequential fallible operations instead of nested case
5. Use the pipe operator for 2+ chained transformations
6. Never nest if/else statements — use case, cond, or multi-clause functions
7. Predicate functions end with ?, dangerous functions end with !
8. Let it crash — don't write defensive code for impossible states

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Pattern Matching

Pattern matching is the primary control flow mechanism in Elixir. Prefer it over conditional statements.

Prefer Pattern Matching Over if/else

Bad:

def process(result) do
  if result.status == :ok do
    result.data
  else
    nil
  end
end

Good:

def process(%{status: :ok, data: data}), do: data
def process(_), do: nil

Use Case for Multiple Patterns

Bad:

# Nested if inside else — hard to read and not how Elixir expresses multi-way branching
def handle_response(response) do
  if response.status == 200 do
    {:ok, response.body}
  else
    if response.status == 404 do
      {:error, :not_found}
    else
      {:error, :unknown}
    end
  end
end

Good:

def handle_response(%{status: 200, body: body}), do: {:ok, body}
def handle_response(%{status: 404}), do: {:error, :not_found}
def handle_response(_), do: {:error, :unknown}

Pipe Operator

Use the pipe operator |> to chain function calls for improved readability.

Basic Piping

Bad:

String.upcase(String.trim(user_input))

Good:

user_input
|> String.trim()
|> String.upcase()

Pipe into Function Heads

Bad:

def process_user(user) do
  validated = validate_user(user)
  transformed = transform_user(validated)
  save_user(transformed)
end