The Elixir Style Guide

Prelude
About
Formatting
The Guide
- Expressions
- Naming
- Comments
  - Comment Annotations
- Modules
- Documentation
- Typespecs
- Structs
- Exceptions
- Collections
- Strings
- Regular Expressions
- Metaprogramming
- Testing
Resources
- 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.

About

This is community style guide for the Elixir programming language. Please feel free to make pull requests and suggestions, and be a part of Elixir's vibrant community.

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:

Formatting

Elixir v1.6 introduced a Code Formatter and Mix format task. The formatter should be preferred for all new projects and source code.

The rules in this section are applied automatically by the code formatter, but are provided here as examples of the preferred style.

Whitespace

Avoid trailing whitespace. ^[link]
End each file with a newline. ^[link]
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
```
Limit lines to 98 characters. Otherwise, set the :line_length option in your .formatter.exs file. ^[link]
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
  some_data |> other_function() |> List.first()
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

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

If the function head and do: clause are too long to fit on the same line, put do: on a new line, indented one level more than the previous line. ^[link]

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

When the do: clause starts on its own line, treat it as a multiline function by separating it with blank lines.

# 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

Add a blank line after a multiline assignment as a visual cue that the assignment is 'over'. ^[link]

# not preferred
some_string =
  "Hello"
  |> String.downcase()
  |> String.trim()
another_string <> some_string

# preferred
some_string =
  "Hello"
  |> String.downcase()
  |> String.trim()

another_string <> some_string

# also not preferred
something =
  if x == 2 do
    "Hi"
  else
    "Bye"
  end
String.downcase(something)

# preferred
something =
  if x == 2 do
    "Hi"
  else
    "Bye"
  end

String.downcase(something)

If a list, map, or struct spans multiple lines, put each element, as well as the opening and closing brackets, on its own line. Indent each element one level, but not the brackets. ^[link]
```
# not preferred
[:first_item, :second_item, :next_item,
:final_item]

# preferred
[
  :first_item,
  :second_item,
  :next_item,
  :final_item
]
```

When assigning a list, map, or struct, keep the opening bracket on the same line as the assignment. ^[link]

# not preferred
list =
[
  :first_item,
  :second_item
]

# preferred
list = [
  :first_item,
  :second_item
]

If any case or cond clause needs more than one line (due to line length, multiple expressions in the clause body, etc.), use multi-line syntax for all clauses, and separate each one with a blank line. ^[link]

# not preferred
case arg do
  true -> IO.puts("ok"); :ok
  false -> :error
end

# not preferred
case arg do
  true ->
    IO.puts("ok")
    :ok
  false -> :error
end

# preferred
case arg do
  true ->
    IO.puts("ok")
    :ok

  false ->
    :error
end

Place comments above the line they comment on. ^[link]

String.first(some_string) # not preferred

# preferred
String.first(some_string)

Use one space between the leading # character of the comment and the text of the comment. ^[link]
```
#not preferred
String.first(some_string)

# preferred
String.first(some_string)
```

Indentation

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

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

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

Parentheses

Use parentheses for one-arity functions when using the pipe operator (|>). ^[link]

# not preferred
some_string |> String.downcase |> String.trim

# preferred
some_string |> String.downcase() |> String.trim()

Never put a space between a function name and the opening parenthesis. ^[link]
```
# not preferred
f (3 + 2)

# preferred
f(3 + 2)
```

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 square brackets from keyword lists whenever they are optional. ^[link]

# not preferred
some_function(foo, bar, [a: "baz", b: "qux"])

# preferred
some_function(foo, bar, a: "baz", b: "qux")

The Guide

The rules in this section may not be applied by the code formatter, but they are generally preferred practice.

Expressions

Run single-line defs that match for the same function together, but separate multiline defs with a blank line. ^[link]

def some_function(nil), do: {:error, "No Value"}
def some_function([]), do: :ok

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

If you have more than one multiline def, do not use single-line defs. ^[link]

def some_function(nil) do
  {:error, "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.trim(String.downcase(some_string))

# preferred
some_string |> String.downcase() |> String.trim()

# Multiline pipelines are not further indented
some_string
|> String.downcase()
|> String.trim()

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

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. To avoid this, you can wrap the pasted code in parentheses.

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

# not preferred
some_string |> String.downcase()

System.version() |> Version.parse()

# preferred
String.downcase(some_string)

Version.parse(System.version())

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

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

# preferred
some_string |> String.trim() |> String.downcase() |> String.codepoints()

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

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

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
  # is this a variable or a function call?
  do_stuff
end

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

Naming

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

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

someVar = 5

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 name of macros suitable for use in guard expressions should be prefixed with is_. For a list of allowed expressions, see the Guard docs. ^[link]
```
defguard is_cool(var) when var == "cool"
defguardp is_very_cool(var) when var == "very cool"
```
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]

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.

Limit comment lines to 100 characters. ^[link]

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 with no note. This usage should be the exception and not the rule. ^[link]
```
start_task()

# FIXME
Process.sleep(5000)
```
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

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

@moduledoc
@behaviour
use
import
require
alias
@module_attribute
defstruct
@type
@callback
@macrocallback
@optional_callbacks
defmacro, defmodule, defguard, def, etc.

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

  require Integer

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

  @module_attribute :foo
  @other_attribute 100

  defstruct [:name, params: []]

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

  @callback some_function(term) :: :ok | {:error, term}

  @macrocallback macro_name(term) :: Macro.t()

  @optional_callbacks macro_name: 1

  @doc false
  defmacro __using__(_opts), do: :no_op

  @doc """
  Determines when a term is `:ok`. Allowed in guards.
  """
  defguard is_ok(term) when term == :ok

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

  # Define other functions here.
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

Avoid repeating fragments in module names and namespaces. This improves overall readability and eliminates ambiguous aliases. ^[link]
```
# not preferred
defmodule Todo.Todo do
  ...
end

# preferred
defmodule Todo.Item do
  ...
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 AnotherModule do
  use SomeModule

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

# preferred

defmodule AThirdModule do
  @moduledoc """
  About the module
  """

  use SomeModule
  ...
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, put each part of the type on a separate line, indented one level past the name of the type. ^[link]

# not preferred
@type long_union_type ::
        some_type | another_type | some_other_type | one_more_type | a_final_type

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

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

defstruct [:name, params: []]

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

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

@doc """
Some function description.
"""
@spec some_function(term) :: result
def some_function(some_data) do
  {:ok, some_data}
end

Structs

Use a list of atoms for struct fields that default to nil, followed by the other keywords. ^[link]

# not preferred
defstruct name: nil, params: nil, active: true

# preferred
defstruct [:name, :params, active: true]

Omit square brackets when the argument of a defstruct is a keyword list. ^[link]

# not preferred
defstruct [params: [], active: true]

# preferred
defstruct params: [], active: true

# required - brackets are not optional, with at least one atom in the list
defstruct [:name, params: [], active: true]

If a struct definition spans multiple lines, put each element on its own line, keeping the elements aligned. ^[link]

defstruct foo: "test",
          bar: true,
          baz: false,
          qux: false,
          quux: 1

If a multiline struct requires brackets, format it as a multiline list:

defstruct [
  :name,
  params: [],
  active: true
]

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

Always use the special syntax for keyword lists. ^[link]

# not preferred
some_value = [{:a, "baz"}, {:b, "qux"}]

# preferred
some_value = [a: "baz", b: "qux"]

Use the shorthand key-value syntax for maps when all of the keys are atoms. ^[link]

# not preferred
%{:a => 1, :b => 2, :c => 0}

# preferred
%{a: 1, b: 2, c: 3}

Use the verbose key-value syntax for maps if any key is not an atom. ^[link]

# not preferred
%{"c" => 0, a: 1, b: 2}

# preferred
%{:a => 1, :b => 2, "c" => 0}

Strings

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

# not preferred
<<"my"::utf8, _rest::bytes>> = "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, put the expression being tested to the left of the operator, and the expected result to the right, unless the assertion is a pattern match. ^[link]

# preferred
assert actual_function(1) == true

# not preferred
assert true == actual_function(1)

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

Resources

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 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 have kindly contributed to this project.

Glen-Cocoa/elixir_style_guide