The Elixir Style Guide

Table of Contents

• Prelude

• The Guide

  • Source Code Layout
• Syntax
• Naming

• Comments

  • Comment Annotations
• Modules
• Documentation
• Typespecs
• Structs
• Exceptions
• Collections
• Strings
• Regular Expressions
• Metaprogramming
• Testing
• Alternative Style Guides
• Tools

• Getting Involved

  • Contributing
• Spread the Word

• Copying

  • License
• Attribution

Prelude

Liquid architecture. It’s like jazz — you improvise, you work together, you play off each other, you make something, they make something.

—Frank Gehry

Style matters. Elixir has plenty of style but like all languages it can be stifled. Don’t stifle the style.

The Guide

This is our attempt at starting a community style guide for the Elixir programming language. Please feel free to make pull requests and contribute. We really want Elixir to have as vibrant of a community as any language that’s been around five times as long.

If you’re looking for other projects to contribute to please see the Hex package manager site.

Translations of the guide are available in the following languages:

• Chinese Traditional
• Japanese
• Korean

Source Code Layout

• Use two spaces per indentation level. No hard tabs. ^[link]

  # not preferred - four spaces
  def some_function do
      do_something
  end

  # preferred
  def some_function do
    do_something
  end

• Use Unix-style line endings (*BSD/Solaris/Linux/OSX users are covered by default, Windows users have to be extra careful). ^[link]

• If you’re using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in: ^[link]

  git config --global core.autocrlf true

• Use spaces around operators, after commas, colons and semicolons. Do not put spaces around matched pairs like brackets, parentheses, etc. Whitespace might be (mostly) irrelevant to the Elixir runtime, but its proper use is the key to writing easily readable code. ^[link]

  sum = 1 + 2
  {a, b} = {2, 3}
  [first | rest] = [1, 2, 3]
  Enum.map(["one", <<"two">>, "three"], fn num -> IO.puts num end)

• Do not use spaces after non-word operators that only take one argument; or around the range operator. ^[link]

  0 - 1 == -1
  ^pinned = some_func()
  5 in 1..10

• Use blank lines between defs to break up a function into logical paragraphs. ^[link]

  def some_function(some_data) do
    altered_data = Module.function(data)
  end

  def some_function do
    result
  end

  def some_other_function do
    another_result
  end

  def a_longer_function do
    one
    two

    three
    four
  end

• …but run single-line defs that match for the same function together. ^[link]

  def some_function(nil), do: {:err, "No Value"}
  def some_function([]), do: :ok
  def some_function([first | rest]) do
    some_function(rest)
  end

• If you use the do: syntax with functions and the line that makes up the function body is long, put the do: on a new line indented one level more than the previous line. ^[link]

  def some_function(args),
    do: Enum.map(args, fn(arg) -> arg <> " is on a very long line!" end)

When you use the convention above and you have more than one function clause using the do: syntax, put the do: on a new line for each function clause:

  # not preferred
  def some_function([]), do: :empty
  def some_function(_),
    do: :very_long_line_here

  # preferred
  def some_function([]),
    do: :empty
  def some_function(_),
    do: :very_long_line_here

• If you have more than one multi-line defs do not use single-line defs. ^[link]

  def some_function(nil) do
    {:err, "No Value"}
  end

  def some_function([]) do
    :ok
  end

  def some_function([first | rest]) do
    some_function(rest)
  end

  def some_function([first | rest], opts) do
    some_function(rest, opts)
  end

• Use the pipe operator (|>) to chain functions together. ^[link]

  # not preferred
  String.strip(String.downcase(some_string))

  # preferred
  some_string |> String.downcase |> String.strip

  # Multiline pipelines are not further indented
  some_string
  |> String.downcase
  |> String.strip

  # Multiline pipelines on the right side of a pattern match
  # should be indented on a new line
  sanitized_string =
    some_string
    |> String.downcase
    |> String.strip

While this is the preferred method, take into account that copy-pasting multiline pipelines into IEx might result in a syntax error, as IEx will evaluate the first line without realizing that the next line has a pipeline.

• Avoid using the pipe operator just once. ^[link]

  # not preferred
  some_string |> String.downcase

  # preferred
  String.downcase(some_string)

• Use bare variables in the first part of a function chain. ^[link]

  # THE WORST!
  # This actually parses as String.strip("nope" |> String.downcase).
  String.strip "nope" |> String.downcase

  # not preferred
  String.strip(some_string) |> String.downcase |> String.codepoints

  # preferred
  some_string |> String.strip |> String.downcase |> String.codepoints

• Avoid trailing whitespace. ^[link]

• End each file with a newline. ^[link]

Syntax

• Use parentheses when a def has arguments, and omit them when it doesn’t. ^[link]

  # not preferred
  def some_function arg1, arg2 do
    # body omitted
  end

  def some_function() do
    # body omitted
  end

  # preferred
  def some_function(arg1, arg2) do
    # body omitted
  end

  def some_function do
    # body omitted
  end

• Never use do: for multi-line if/unless. ^[link]

  # not preferred
  if some_condition, do:
    # a line of code
    # another line of code
    # note no end in this block

  # preferred
  if some_condition do
    # some
    # lines
    # of code
  end

• Use do: for single line if/unless statements. ^[link]

  # preferred
  if some_condition, do: # some_stuff

• Never use unless with else. Rewrite these with the positive case first. ^[link]

  # not preferred
  unless success? do
    IO.puts 'failure'
  else
    IO.puts 'success'
  end

  # preferred
  if success? do
    IO.puts 'success'
  else
    IO.puts 'failure'
  end

• Use true as the last condition of the cond special form when you need a clause that always matches. ^[link]

  # not preferred
  cond do
    1 + 2 == 5 ->
      "Nope"
    1 + 3 == 5 ->
      "Uh, uh"
    :else ->
      "OK"
  end

  # preferred
  cond do
    1 + 2 == 5 ->
      "Nope"
    1 + 3 == 5 ->
      "Uh, uh"
    true ->
      "OK"
  end

• Never put a space between a function name and the opening parenthesis. ^[link]

  # not preferred
  f (3 + 2) + 1

  # preferred
  f(3 + 2) + 1

• Use parentheses in function calls, especially inside a pipeline. ^[link]

  # not preferred
  f 3

  # preferred
  f(3)

  # not preferred and parses as rem(2, (3 |> g)), which is not what you want.
  2 |> rem 3 |> g

  # preferred
  2 |> rem(3) |> g

• Omit parentheses in macro calls when a do block is passed. ^[link]

  # not preferred
  quote(do
    foo
  end)

  # preferred
  quote do
    foo
  end

• Optionally omit parentheses in function calls (outside a pipeline) when the last argument is a function expression. ^[link]

  # preferred
  Enum.reduce(1..10, 0, fn x, acc ->
    x + acc
  end)

  # also preferred
  Enum.reduce 1..10, 0, fn x, acc ->
    x + acc
  end

• Use parentheses for calls to functions with zero arity, so they can be distinguished from variables. Starting in Elixir 1.4, the compiler will warn you about locations where this ambiguity exists. ^[link]

  defp do_stuff, do: ...

  # not preferred
  def my_func do
    do_stuff # is this a variable or a function call?
  end

  # preferred
  def my_func do
    do_stuff() # this is clearly a function call
  end

• Indent and align successive with clauses. Put the do: argument on a new line, indented normally. ^[link]

  with {:ok, foo} <- fetch(opts, :foo),
       {:ok, bar} <- fetch(opts, :bar),
    do: {:ok, foo, bar}

• If the with expression has a do block with more than one line, or has an else option, use multiline syntax. ^[link]

  with {:ok, foo} <- fetch(opts, :foo),
       {:ok, bar} <- fetch(opts, :bar) do
    {:ok, foo, bar}
  else
    :error ->
      {:error, :bad_arg}
  end

Naming

• Use snake_case for atoms, functions and variables. ^[link]

  # not preferred
  :"some atom"
  :SomeAtom
  :someAtom

  someVar = 5

  def someFunction do
    ...
  end

  def SomeFunction do
    ...
  end

  # preferred
  :some_atom

  some_var = 5

  def some_function do
    ...
  end

• Use CamelCase for modules (keep acronyms like HTTP, RFC, XML uppercase). ^[link]

  # not preferred
  defmodule Somemodule do
    ...
  end

  defmodule Some_Module do
    ...
  end

  defmodule SomeXml do
    ...
  end

  # preferred
  defmodule SomeModule do
    ...
  end

  defmodule SomeXML do
    ...
  end

• The names of predicate macros (compile-time generated functions that return a boolean value) that can be used within guards should be prefixed with is_. For a list of allowed expressions, see the Guard docs. ^[link]

  defmacro is_cool(var) do
    quote do: unquote(var) == "cool"
  end

• The names of predicate functions that cannot be used within guards should have a trailing question mark (?) rather than the is_ (or similar) prefix. ^[link]

  def cool?(var) do
    # Complex check if var is cool not possible in a pure function.
  end

• Private functions with the same name as public functions should start with do_. ^[link]

  def sum(list), do: do_sum(list, 0)

  # private functions
  defp do_sum([], total), do: total
  defp do_sum([head | tail], total), do: do_sum(tail, head + total)

Comments

• Write expressive code and try to convey your program’s intention through control-flow, structure and naming. ^[link]

• Use one space between the leading # character of the comment and the text of the comment. ^[link]

  String.first(some_string) #not preferred
  String.first(some_string) # preferred

• Comments longer than a word are capitalized, and sentences use punctuation. Use one space after periods. ^[link]

  # not preferred
  # these lowercase comments are missing punctuation

  # preferred
  # Capitalization example
  # Use punctuation for complete sentences.

Comment Annotations

• Annotations should usually be written on the line immediately above the relevant code. ^[link]

• The annotation keyword is uppercase, and is followed by a colon and a space, then a note describing the problem. ^[link]

  # TODO: Deprecate in v1.5.
  def some_function(arg), do: {:ok, arg}

• In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule. ^[link]

  start_task()
  Process.sleep(5000) # FIXME

• Use TODO to note missing features or functionality that should be added at a later date. ^[link]

• Use FIXME to note broken code that needs to be fixed. ^[link]

• Use OPTIMIZE to note slow or inefficient code that may cause performance problems. ^[link]

• Use HACK to note code smells where questionable coding practices were used and should be refactored away. ^[link]

• Use REVIEW to note anything that should be looked at to confirm it is working as intended. For example: REVIEW: Are we sure this is how the client does X currently? ^[link]

• Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project’s README or similar. ^[link]

Modules

• Use one module per file unless the module is only used internally by another module (such as a test). ^[link]

• Use snake_case file names for CamelCase module names. ^[link]

  # file is called some_module.ex

  defmodule SomeModule do
  end

• Represent each level of nesting within a module name as a directory. ^[link]

  # file is called parser/core/xml_parser.ex

  defmodule Parser.Core.XMLParser do
  end

• Don’t put a blank line after defmodule. ^[link]

• Put a blank line after module-level code blocks. ^[link]

• List module attributes and directives in the following order: ^[link]

1. @moduledoc
2. @behaviour
3. use
4. import
5. alias
6. require
7. defstruct
8. @type

9. @module_attribute

   Add a blank line between each grouping, and sort the terms (like module names) alphabetically. Here’s an overall example of how you should order things in your modules:

  defmodule MyModule do
    @moduledoc """
    An example module
    """

    @behaviour MyBehaviour

    use GenServer

    import Something
    import SomethingElse

    alias My.Long.Module.Name
    alias My.Other.Module.Name

    require Integer

    defstruct name: nil, params: []

    @type params :: [{binary, binary}]

    @module_attribute :foo
    @other_attribute 100

    ...
  end

• Use the __MODULE__ pseudo variable when a module refers to itself. This avoids having to update any self-references when the module name changes. ^[link]

  defmodule SomeProject.SomeModule do
    defstruct [:name]

    def name(%__MODULE__{name: name}), do: name
  end

• If you want a prettier name for a module self-reference, set up an alias. ^[link]

  defmodule SomeProject.SomeModule do
    alias __MODULE__, as: SomeModule

    defstruct [:name]

    def name(%SomeModule{name: name}), do: name
  end

Documentation

Documentation in Elixir (when read either in iex with h or generated with ExDoc) uses the Module Attributes @moduledoc and @doc.

• Always include a @moduledoc attribute in the line right after defmodule in your module. ^[link]

  # not preferred

  defmodule SomeModule do

    @moduledoc """
    About the module
    """
    ...
  end

  defmodule AnotherModule do
    use SomeModule
    @moduledoc """
    About the module
    """
    ...
  end

  # preferred

  defmodule SomeModule do
    @moduledoc """
    About the module
    """
    ...
  end

• Use @moduledoc false if you do not intend on documenting the module. ^[link]

  defmodule SomeModule do
    @moduledoc false
    ...
  end

• Separate code after the @moduledoc with a blank line. ^[link]

  # not preferred

  defmodule SomeModule do
    @moduledoc """
    About the module
    """
    use AnotherModule
  end

  # preferred
  defmodule SomeModule do
    @moduledoc """
    About the module
    """

    use AnotherModule
  end

• Use heredocs with markdown for documentation. ^[link]

  # not preferred

  defmodule SomeModule do
    @moduledoc "About the module"
  end

  defmodule SomeModule do
    @moduledoc """
    About the module

    Examples:
    iex> SomeModule.some_function
    :result
    """
  end

  # preferred
  defmodule SomeModule do
    @moduledoc """
    About the module

    ## Examples

        iex> SomeModule.some_function
        :result
    """
  end

Typespecs

Typespecs are notation for declaring types and specifications, for documentation or for the static analysis tool Dialyzer.

Custom types should be defined at the top of the module with the other directives (see Modules).

• Place @typedoc and @type definitions together, and separate each pair with a blank line. ^[link]

  defmodule SomeModule do
    @moduledoc false

    @typedoc "The name"
    @type name :: atom

    @typedoc "The result"
    @type result :: {:ok, term} | {:error, term}

    ...
  end

• If a union type is too long to fit on a single line, add a newline and indent with spaces to keep the return types aligned. ^[link]

  # not preferred - no indentation
  @type long_union_type :: some_type | another_type | some_other_type
  | a_final_type

  # preferred
  @type long_union_type :: some_type | another_type | some_other_type
                         | a_final_type

  # also preferred - one return type per line
  @type long_union_type :: some_type
                         | another_type
                         | some_other_type
                         | a_final_type

• Name the main type for a module t, for example: the type specification for a struct. ^[link]

  defstruct name: nil, params: []

  @type t :: %__MODULE__{
    name: String.t | nil,
    params: Keyword.t
  }

• Place specifications right before the function definition, without separating them by a blank line. ^[link]

  @spec some_function(term) :: result
  def some_function(some_data) do
    {:ok, some_data}
  end

Structs

• If all the struct’s fields default to nil, supply them as a list of atoms. ^[link]

  # not preferred
  defstruct name: nil, params: nil

  # preferred
  defstruct [:name, :params]

• Indent additional lines of the struct definition, keeping the first keys aligned. ^[link]

  defstruct foo: "test", bar: true, baz: false,
            qux: false, quux: nil

Exceptions

• Make exception names end with a trailing Error. ^[link]

  # not preferred
  defmodule BadHTTPCode do
    defexception [:message]
  end

  defmodule BadHTTPCodeException do
    defexception [:message]
  end

  # preferred
  defmodule BadHTTPCodeError do
    defexception [:message]
  end

• Use lowercase error messages when raising exceptions, with no trailing punctuation. ^[link]

  # not preferred
  raise ArgumentError, "This is not valid."

  # preferred
  raise ArgumentError, "this is not valid"

Collections

No guidelines for collections have been added yet.

Strings

• Match strings using the string concatenator rather than binary patterns: ^[link]

  # not preferred
  <<"my"::utf8, _rest>> = "my string"

  # preferred
  "my" <> _rest = "my string"

Regular Expressions

No guidelines for regular expressions have been added yet.

Metaprogramming

• Avoid needless metaprogramming. ^[link]

Testing

• When writing ExUnit assertions, be consistent with the order of the expected and actual values under testing. Prefer placing the expected result on the right, unless the assertion is a pattern match. ^[link]

  # preferred - expected result on the right
  assert actual_function(1) == true
  assert actual_function(2) == false

  # not preferred - inconsistent order
  assert actual_function(1) == true
  assert false == actual_function(2)

  # required - the assertion is a pattern match
  assert {:ok, expected} = actual_function(3)

Alternative Style Guides

• Aleksei Magusev’s Elixir Style Guide — An opinionated Elixir style guide stemming from the coding style practiced in the Elixir core libraries. Developed by Aleksei Magusev and Andrea Leopardi, members of Elixir core team. While the Elixir project doesn’t adhere to any specific style guide, this is the closest available guide to its conventions.

• Credo’s Elixir Style Guide — Style Guide for the Elixir language, implemented by Credo static code analysis tool.

Tools

Refer to Awesome Elixir for libraries and tools that can help with code analysis and style linting.

Getting Involved

Contributing

It’s our hope that this will become a central hub for community discussion on best practices in Elixir. Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

Check the contributing guidelines and code of conduct for more information.

Spread the Word

A community style guide is meaningless without the community’s support. Please tweet, star, and let any Elixir programmer know about this guide so they can contribute.

Copying

License

This work is licensed under a Creative Commons Attribution 3.0 Unported License

Attribution

The structure of this guide, bits of example code, and many of the initial points made in this document were borrowed from the Ruby community style guide. A lot of things were applicable to Elixir and allowed us to get some document out quicker to start the conversation.

Here’s the list of people who has kindly contributed to this project.