# Lua Style Guide

This style guide contains a list of guidelines that we try to follow for our
projects. It does not attempt to make arguments for the styles; its goal is
to provide consistency across projects.

Feel free to fork this style guide and change to your own
liking, and file issues / pull requests if you have questions, comments, or if
you find any mistakes or typos.


## <a name='TOC'>Table of Contents</a>

  1. [Types](#types)
  1. [Tables](#tables)
  1. [Strings](#strings)
  1. [Functions](#functions)
  1. [Properties](#properties)
  1. [Variables](#variables)
  1. [Conditional Expressions & Equality](#conditionals)
  1. [Blocks](#blocks)
  1. [Whitespace](#whitespace)
  1. [Commas](#commas)
  1. [Semicolons](#semicolons)
  1. [Type Casting & Coercion](#type-coercion)
  1. [Naming Conventions](#naming-conventions)
  1. [Accessors](#accessors)
  1. [Constructors](#constructors)
  1. [Modules](#modules)
  1. [File Structure](#file-structure)
  1. [Testing](#testing)
  1. [Performance](#performance)
  1. [Resources](#resources)
  1. [In the Wild](#in-the-wild)
  1. [Contributors](#contributors)
  1. [License](#license)

## <a name='types'>Types</a>

  - **Primitives**: When you access a primitive type you work directly on its value

    + `string`
    + `number`
    + `boolean`
    + `nil`

    ```lua
    local foo = 1
    local bar = foo

    bar = 9

    print(foo, bar) -- => 1	9
    ```

  - **Complex**: When you access a complex type you work on a reference to its value

    + `table`
    + `function`
    + `userdata`

    ```lua
    local foo = { 1, 2 }
    local bar = foo

    bar[0] = 9
    foo[1] = 3

    print(foo[0], bar[0]) -- => 9   9
    print(foo[1], bar[1]) -- => 3   3
    print(foo[2], bar[2]) -- => 2   2		
    ```

    **[[⬆]](#TOC)**

## <a name='tables'>Tables</a>

  - Use the constructor syntax for table property creation where possible.

    ```lua
    -- bad
    local player = {}
    player.name = 'Jack'
    player.class = 'Rogue'

    -- good
    local player = {
      name = 'Jack',
      class = 'Rogue'
    }
    ```

  - Define functions externally to table definition.

    ```lua
    -- bad
    local player = {
      attack = function() 
      -- ...stuff...
      end
    }

    -- good
    local function attack()
    end

    local player = {
      attack = attack
    }
    ```

  - Consider `nil` properties when selecting lengths.
    A good idea is to store an `n` property on lists that contain the length
    (as noted in [Storing Nils in Tables](http://lua-users.org/wiki/StoringNilsInTables))

    ```lua
    -- nils don't count
    local list = {}
    list[0] = nil
    list[1] = 'item'

    print(#list) -- 0
    print(select('#', list)) -- 1
    ```

  - When tables have functions, use `self` when referring to itself.

    ```lua
    -- bad
    local me = {
      fullname = function(this)
        return this.first_name + ' ' + this.last_name
      end
    }

    -- good
    local me = {
      fullname = function(self)
        return self.first_name + ' ' + self.last_name
      end
    }
    ```

    **[[⬆]](#TOC)**

## <a name='strings'>Strings</a>

  - Use single quotes `''` for strings.

    ```lua
    -- bad
    local name = "Bob Parr"

    -- good
    local name = 'Bob Parr'

    -- bad
    local fullName = "Bob " .. self.lastName

    -- good
    local fullName = 'Bob ' .. self.lastName
    ```

  - Strings longer than 80 characters should be written across multiple lines 
    using concatenation. This allows you to indent nicely.

    ```lua
    -- bad
    local errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.'

    -- bad
    local errorMessage = 'This is a super long error that \
    was thrown because of Batman. \
    When you stop to think about \
    how Batman had anything to do \
    with this, you would get nowhere \
    fast.'


    -- bad
    local errorMessage = [[This is a super long error that
      was thrown because of Batman.
      When you stop to think about
      how Batman had anything to do
      with this, you would get nowhere
      fast.]]

    -- good
    local errorMessage = 'This is a super long error that ' ..
      'was thrown because of Batman. ' ..
      'When you stop to think about ' ..
      'how Batman had anything to do ' ..
      'with this, you would get nowhere ' ..
      'fast.'
    ```

    **[[⬆]](#TOC)**


## <a name='functions'>Functions</a>
  - Prefer lots of small functions to large, complex functions. [Smalls Functions Are Good For The Universe](http://kikito.github.io/blog/2012/03/16/small-functions-are-good-for-the-universe/).

  - Prefer function syntax over variable syntax. This helps differentiate
    between named and anonymous functions.

    ```lua
    -- bad
    local nope = function(name, options)
      -- ...stuff...
    end

    -- good
    local function yup(name, options)
      -- ...stuff...
    end
    ```

  - Never name a parameter `arg`, this will take precendence over the `arg` object that is given to every function scope in older versions of Lua.

    ```lua
    -- bad
    local function nope(name, options, arg) 
      -- ...stuff...
    end

    -- good
    local function yup(name, options, ...)
      -- ...stuff...
    end
    ```

  - Perform validation early and return as early as possible.

    ```lua
    -- bad
    local is_good_name = function(name, options, arg)
      local is_good = #name > 3
      is_good = is_good and #name < 30

      -- ...stuff...

      return is_bad
    end

    -- good
    local is_good_name = function(name, options, args)
      if #name < 3 or #name > 30 then return false end

      -- ...stuff...

      return true
    end
    ```

  **[[⬆]](#TOC)**


## <a name='properties'>Properties</a>

  - Use dot notation when accessing known properties.

    ```lua
    local luke = {
      jedi = true,
      age = 28
    }

    -- bad
    local isJedi = luke['jedi']

    -- good
    local isJedi = luke.jedi
    ```

  - Use subscript notation `[]` when accessing properties with a variable
    or if using a table as a list.

    ```lua
    local luke = {
      jedi = true,
      age = 28
    }

    local function getProp(prop) 
      return luke[prop]
    end

    local isJedi = getProp('jedi')
    ```

    **[[⬆]](#TOC)**


## <a name='variables'>Variables</a>

  - Always use `local` to declare variables. Not doing so will result in
    global variables to avoid polluting the global namespace.

    ```lua
    -- bad
    superPower = SuperPower()

    -- good
    local superPower = SuperPower()
    ```

  - Assign variables at the top of their scope where possible. This makes it
    easier to check for existing variables.

    ```lua
    -- bad
    local bad = function()
      test()
      print('doing stuff..')

      //..other stuff..

      local name = getName()

      if name == 'test' then
        return false
      end

      return name
    end

    -- good
    local function good()
      local name = getName()

      test()
      print('doing stuff..')

      //..other stuff..

      if name == 'test' then
        return false
      end

      return name
    end
    ```

    **[[⬆]](#TOC)**


## <a name='conditionals'>Conditional Expressions & Equality</a>

  - False and nil are *falsy* in conditional expressions. All else is true.

    ```lua
    local str = ''

    if str then
      -- true
    end
    ```

  - Use shortcuts when you can, unless you need to know the difference between
    false and nil.

    ```lua
    -- bad
    if name ~= nil then
      -- ...stuff...
    end

    -- good
    if name then
      -- ...stuff...
    end
    ```

  - Prefer *true* statements over *false* statements where it makes sense. 
    Prioritize truthy conditions when writing multiple conditions.

    ```lua
    --bad
    if not thing then
      -- ...stuff...
    else
      -- ...stuff...
    end

    --good
    if thing then
      -- ...stuff...
    else
      -- ...stuff...
    end
    ```

  - Prefer defaults to `else` statements where it makes sense. This results in
    less complex and safer code at the expense of variable reassignment, so
    situations may differ.

    ```lua
    --bad
    local function full_name(first, last)
      local name

      if first and last then
        name = first .. ' ' .. last
      else
        name = 'John Smith'
      end

      return name
    end

    --good
    local function full_name(first, last)
      local name = 'John Smith'

      if first and last then
        name = first .. ' ' .. last
      end

      return name
    end
    ```

  - Short ternaries are okay.

    ```lua
    local function default_name(name)
      -- return the default 'Waldo' if name is nil
      return name or 'Waldo'
    end

    local function brew_coffee(machine)
      return machine and machine.is_loaded and 'coffee brewing' or 'fill your water'
    end
    ```


    **[[⬆]](#TOC)**


## <a name='blocks'>Blocks</a>

  - Single line blocks are okay for *small* statements. Try to keep lines to 80 characters.
    Indent lines if they overflow past the limit.

    ```lua
    -- good
    if test then return false end

    -- good
    if test then
      return false
    end

    -- bad
    if test < 1 and do_complicated_function(test) == false or seven == 8 and nine == 10 then do_other_complicated_function()end

    -- good
    if test < 1 and do_complicated_function(test) == false or
        seven == 8 and nine == 10 then

      do_other_complicated_function() 
      return false 
    end
    ```

    **[[⬆]](#TOC)**


## <a name='whitespace'>Whitespace</a>

  - Use soft tabs set to 2 spaces. Tab characters and 4-space tabs result in public flogging.

    ```lua
    -- bad
    function() 
    ∙∙∙∙local name
    end

    -- bad
    function() 
    ∙local name
    end

    -- good
    function() 
    ∙∙local name
    end
    ```

  - Place 1 space before opening and closing braces. Place no spaces around parens.

    ```lua
    -- bad
    local test = {one=1}

    -- good
    local test = { one = 1 }

    -- bad
    dog.set('attr',{
      age = '1 year',
      breed = 'Bernese Mountain Dog'
    })

    -- good
    dog.set('attr', {
      age = '1 year',
      breed = 'Bernese Mountain Dog'
    })
    ```

  - Place an empty newline at the end of the file.

    ```lua
    -- bad
    (function(global) 
      -- ...stuff...
    end)(self)
    ```

    ```lua
    -- good
    (function(global) 
      -- ...stuff...
    end)(self)

    ```

  - Surround operators with spaces.

    ```lua
    -- bad
    local thing=1
    thing = thing-1
    thing = thing*1
    thing = 'string'..'s'

    -- good
    local thing = 1
    thing = thing - 1
    thing = thing * 1
    thing = 'string' .. 's'
    ```

  - Use one space after commas.

    ```lua
    --bad
    local thing = {1,2,3}
    thing = {1 , 2 , 3}
    thing = {1 ,2 ,3}

    --good
    local thing = {1, 2, 3}
    ```

  - Add a line break after multiline blocks.

    ```lua
    --bad
    if thing then
      -- ...stuff...
    end
    function derp()
      -- ...stuff...
    end
    local wat = 7

    --good
    if thing then
      -- ...stuff...
    end

    function derp()
      -- ...stuff...
    end

    local wat = 7
    ```

  - Delete unnecessary whitespace at the end of lines.

    **[[⬆]](#TOC)**

## <a name='commas'>Commas</a>

  - Leading commas aren't okay. An ending comma on the last item is okay but discouraged.

    ```lua
    -- bad
    local thing = {
      once = 1
    , upon = 2
    , aTime = 3
    }

    -- good
    local thing = {
      once = 1,
      upon = 2,
      aTime = 3
    }

    -- okay
    local thing = {
      once = 1,
      upon = 2,
      aTime = 3,
    }
    ```

    **[[⬆]](#TOC)**


## <a name='semicolons'>Semicolons</a>

  - **Nope.** Separate statements onto multiple lines.

    ```lua
    -- bad
    local whatever = 'sure';
    a = 1; b = 2

    -- good
    local whatever = 'sure'
    a = 1
    b = 2
    ```

    **[[⬆]](#TOC)**


## <a name='type-coercion'>Type Casting & Coercion</a>

  - Perform type coercion at the beginning of the statement. Use the built-in functions. (`tostring`, `tonumber`, etc.)

  - Use `tostring` for strings if you need to cast without string concatenation.

    ```lua
    -- bad
    local totalScore = reviewScore .. ''

    -- good
    local totalScore = tostring(reviewScore)
    ```

  - Use `tonumber` for Numbers.

    ```lua
    local inputValue = '4'

    -- bad
    local val = inputValue * 1

    -- good
    local val = tonumber(inputValue)
    ```

    **[[⬆]](#TOC)**


## <a name='naming-conventions'>Naming Conventions</a>

  - Avoid single letter names. Be descriptive with your naming. You can get
    away with single-letter names when they are variables in loops.

    ```lua
    -- bad
    local function q() 
      -- ...stuff...
    end

    -- good
    local function query() 
      -- ..stuff..
    end
    ```

  - Use underscores for ignored variables in loops.

    ```lua
    --good
    for _, name in pairs(names) do
      -- ...stuff...
    end
    ```

  - Use snake_case when naming objects, functions, and instances. Tend towards
    verbosity if unsure about naming.

    ```lua
    -- bad
    local OBJEcttsssss = {}
    local thisIsMyObject = {}
    local this-is-my-object = {}

    local c = function()
      -- ...stuff...
    end

    -- good
    local this_is_my_object = {}

    local function do_that_thing()
      -- ...stuff...
    end
    ```

  - Use PascalCase for factories.

    ```lua
    -- bad
    local player = require('player')

    -- good
    local Player = require('player')
    local me = Player({ name = 'Jack' })
    ```

    **[[⬆]](#TOC)**

  - Use `is` or `has` for boolean-returning functions that are part of tables.

    ```lua
    --bad
    local function evil(alignment)
      return alignment < 100
    end

    --good
    local function is_evil(alignment)
      return alignment < 100
    end
    ```

## <a name='modules'>Modules</a>

  - The module should return a table or function.
  - The module should not use the global namespace for anything ever. The
    module should be a closure.
  - The file should be named like the module.

    ```lua
    -- thing.lua
    local thing = { }

    local meta = {
      __call = function(self, key, vars)
        print key
      end
    }


    return setmetatable(thing, meta)
    ```

  - Note that modules are [loaded as singletons](http://lua-users.org/wiki/TheEssenceOfLoadingCode)
    and therefore should usually be factories (a function returning a new instance of a table)
    unless static (like utility libraries.)

  **[[⬆]](#TOC)**

## <a name='file-structrure'>File Structure</a>

  - Files should be named in all lowercase.
  - Lua files should be in a top-level `src` folder. The main library file should 
    be called `modulename.lua`.
  - Rockspecs, license, readme, etc should be in the top level.
  - Tests should be in a top-level `spec` folder.
  - Executables should be in a top-level `bin` folder.
  - Example:

    ```
    ./my_module
      bin/
        script.sh

      spec/
        my_module_spec.lua
        some_file.lua

      src/
        my_module.lua
        some_file.lua

      README.md
      LICENSE.md
    ```

## <a name='testing'>Testing</a>

  - Use [busted](http://olivinelabs.com/busted) and write lots of tests in a /spec 
    folder. Separate tests by module.
  - Use descriptive `describe` and `it` blocks so it's obvious to see what
    precisely is failing.
  - Test interfaces. Don't test private methods. If you need to test something
    that is private, it probably shouldn't be private in the first place.
  - Example:

    ```
    ./my_module
      bin/
        script.sh

      spec/
        my_module_spec.lua

        util/
          formatters_spec.lua

      src/
        my_module.lua

        util/
          formatters.lua

      README.md
      LICENSE.md
    ```

    **[[⬆]](#TOC)**

## <a name='contributors'>Contributors</a>

  - [View contributors](https://github.com/Olivine-Labs/lua-style-guide/graphs/contributors)

    **[[⬆]](#TOC)**

## <a name='license'>License</a>

  - Released under CC0 (Public Domain).
    Information can be found at [http://creativecommons.org/publicdomain/zero/1.0/](http://creativecommons.org/publicdomain/zero/1.0/).

**[[⬆]](#TOC)**
