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.

Types
Tables
Strings
Functions
Properties
Variables
Conditional Expressions & Equality
Blocks
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Constructors
Modules
File Structure
Testing
Performance
Resources
In the Wild
Contributors
License

Types

Primitives: When you access a primitive type you work directly on its value
- string
- number
- boolean
- nil
```
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

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

[⬆]

Tables

Use the constructor syntax for table property creation where possible.

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

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

Define functions externally to table definition.

-- 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)
```
-- 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.

-- 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
}

[⬆]

Strings

Use single quotes '' for strings.

-- 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.

-- 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.'

[⬆]

Functions

Prefer lots of small functions to large, complex functions. Smalls Functions Are Good For The Universe.

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

-- 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.

-- 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.

-- 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

[⬆]

Properties

Use dot notation when accessing known properties.

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.

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

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

local isJedi = getProp('jedi')

[⬆]

Variables

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

-- 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

[⬆]

Conditional Expressions & Equality

False and nil are falsy in conditional expressions. All else is true.
```
local str = ''

if str then
  -- true
end
```

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

-- 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.

--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.

--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.

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

[⬆]

Blocks

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

-- 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

[⬆]

Whitespace

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

-- 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.

-- 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.

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

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

Surround operators with spaces.

-- 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.

--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.

--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.

[⬆]

Commas

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

-- 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,
}

[⬆]

Semicolons

Nope. Separate statements onto multiple lines.

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

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

[⬆]

Type Casting & Coercion

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.

-- bad
local totalScore = reviewScore .. ''

-- good
local totalScore = tostring(reviewScore)

Use tonumber for Numbers.

local inputValue = '4'

-- bad
local val = inputValue * 1

-- good
local val = tonumber(inputValue)

[⬆]

Naming Conventions

Avoid single letter names. Be descriptive with your naming. You can get away with single-letter names when they are variables in loops.
```
-- bad
local function q() 
  -- ...stuff...
end

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

Use underscores for ignored variables in loops.

--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.

-- 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.

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

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

[⬆]

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

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

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

Modules

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.

-- 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 and therefore should usually be factories (a function returning a new instance of a table) unless static (like utility libraries.)

[⬆]

File Structure

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

Testing

Use 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

[⬆]