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Lua Style Guide

This style guides lists the coding conventions used in the LuaRocks project. It does not claim to be the best Lua coding style in the planet, but it is used successfully in a long-running project, and we do provide rationale for many of the design decisions listed below.

The list of recommendations in this document was based on the ones mentioned in the following style guides: (sometimes copying material verbatim, sometimes giving the opposite advice! :) )

• https://github.com/Olivine-Labs/lua-style-guide/
• https://github.com/zaki/lua-style-guide
• http://lua-users.org/wiki/LuaStyleGuide
• http://sputnik.freewisdom.org/en/Coding_Standard
• https://gist.github.com/catwell/b3c01dbea413aa78675740546dfd5ce2

Indentation and formatting

• Let's address the elephant in the room first. LuaRocks is indented with 3 spaces.

for i, pkg in ipairs(packages) do
   for name, version in pairs(pkg) do
      if name == searched then
         print(version)
      end
   end
end

Rationale: There is no agreement in the Lua community as for indentation, so 3 spaces lies nicely as a middle ground between the 2-space camp and the 4-space camp. Also, for a language that nests with do/end blocks, it produces pleasant-looking block-closing staircases, as in the example above.

• One should not use tabs for indentation, or mix it with spaces.

• Use LF (Unix) line endings.

Documentation

• Document function signatures using LDoc. Specifying typing information after each parameter or return value is a nice plus.

--- Load a local or remote manifest describing a repository.
-- All functions that use manifest tables assume they were obtained
-- through either this function or load_local_manifest.
-- @param repo_url string: URL or pathname for the repository.
-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
-- @return table or (nil, string, [string]): A table representing the manifest,
-- or nil followed by an error message and an optional error code.
function manif.load_manifest(repo_url, lua_version)
   -- code
end

• Use TODO and FIXME tags in comments. TODO indicates a missing feature to be implemented later. FIXME indicates a problem in the existing code (inefficient implementation, bug, unnecessary code, etc).

-- TODO: implement method
local function something()
   -- FIXME: check conditions
end

• Prefer LDoc comments over the function that explain what the function does than inline comments inside the function that explain how it does it. Ideally, the implementation should be simple enough so that comments aren't needed. If the function grows complex, split it into multiple functions so that their names explain what each part does.

Variable names

• Variable names with larger scope should be more descriptive than those with smaller scope. One-letter variable names should be avoided except for very small scopes (less than ten lines) or for iterators.

• i should be used only as a counter variable in for loops (either numeric for or ipairs).

• Prefer more descriptive names than k and v when iterating with pairs, unless you are writing a function that operates on generic tables.

• Use _ for ignored variables (e.g. in for loops:)

for _, item in ipairs(items) do
   do_something_with_item(item)
end

• Variables and function names should use snake_case.

-- bad
local OBJEcttsssss = {}
local thisIsMyObject = {}
local c = function()
   -- ...stuff...
end

-- good
local this_is_my_object = {}

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

Rationale: The standard library uses lowercase APIs, with joinedlowercase names, but this does not scale too well for more complex APIs. snake_case tends to look good enough and not too out-of-place along side the standard APIs.

• When doing OOP, classes should use CamelCase. Acronyms (e.g. XML) should only uppercase the first letter (XmlDocument). Methods use snake_case too. In LuaRocks, this is used in the Api object in the luarocks.upload.api module.

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

• Prefer using is_ when naming boolean functions:

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

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

• UPPER_CASE is to be used sparingly, with "constants" only.

Rationale: "Sparingly", since Lua does not have real constants. This notation is most useful in libraries that bind C libraries, when bringing over constants from C.

• Do not use uppercase names starting with _, they are reserved by Lua.

Tables

• When creating a table, prefer populating its fields all at once, if possible:

local player = {
   name = "Jack",
   class = "Rogue",
}

• You can add a trailing comma to all fields, including the last one.

Rationale: This makes the structure of your tables more evident at a glance. Trailing commas make it quicker to add new fields and produces shorter diffs.

• Use plain key syntax whenever possible, use ["key"] syntax when using names that can't be represented as identifiers and avoid mixing representations in a declaration:

table = {
   ["1394-E"] = val1,
   ["UTF-8"] = val2,
   ["and"] = val2,
}

Strings

• Use "double quotes" for strings; use 'single quotes' when writing strings that contain double quotes.

local name = "LuaRocks"
local sentence = 'The name of the program is "LuaRocks"'

Rationale: Double quotes are used as string delimiters in a larger number of programming languages. Single quotes are useful for avoiding escaping when using double quotes in literals.

Line lengths

• There are no hard or soft limits on line lengths. Line lengths are naturally limited by using one statement per line. If that still produces lines that are too long (e.g. an expression that produces a line over 256-characters long, for example), this means the expression is too complex and would do better split into subexpressions with reasonable names.

Rationale: No one works on VT100 terminals anymore. If line lengths are a proxy for code complexity, we should address code complexity instead of using line breaks to fit mind-bending statements over multiple lines.

Function declaration syntax

• 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

• Perform validation early and return as early as possible.

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

   -- ...stuff...

   return is_good
end

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

   -- ...stuff...

   return true
end

Function calls

• Even though Lua allows it, do not omit parenthesis for functions that take a unique string literal argument.

-- bad
local data = get_data"KRP"..tostring(area_number)
-- good
local data = get_data("KRP"..tostring(area_number))
local data = get_data("KRP")..tostring(area_number)

Rationale: It is not obvious at a glace what the precedence rules are when omitting the parentheses in a function call. Can you quickly tell which of the two "good" examples in equivalent to the "bad" one? (It's the second one).

• You should not omit parenthesis for functions that take a unique table argument on a single line. You may do so for table arguments that span several lines.

local an_instance = a_module.new {
   a_parameter = 42,
   another_parameter = "yay",
}

Rationale: The use as in a_module.new above occurs alone in a statement, so there are no precedence issues.

Table attributes

• Use dot notation when accessing known properties.

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

-- bad
local is_jedi = luke["jedi"]

-- good
local is_jedi = luke.jedi

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

local vehicles = load_vehicles_from_disk("vehicles.dat")

if vehicles["Porsche"] then
   porsche_handler(vehicles["Porsche"])
   vehicles["Porsche"] = nil
end
for name, cars in pairs(vehicles) do
   regular_handler(cars)
end

Rationale: Using dot notation makes it clearer that the given key is meant to be used as a record/object field.

Functions in tables

• When declaring modules and classes, declare functions external to the table definition:

local my_module = {}

function my_module.a_function(x)
   -- code
end

• When declaring metatables, declare function internal to the table definition.

local version_mt = {
   __eq = function(a, b)
      -- code
   end,
   __lt = function(a, b)
      -- code
   end,
}

Rationale: Metatables contain special behavior that affect the tables they're assigned (and are used implicitly at the call site), so it's good to be able to get a view of the complete behavior of the metatable at a glance.

This is not as important for objects and modules, which usually have way more code, and which don't fit in a single screen anyway, so nesting them inside the table does not gain much: when scrolling a longer file, it is more evident that check_version is a method of Api if it says function Api:check_version() than if it says check_version = function() under some indentation level.

Variable declaration

• Always use local to declare variables.

-- bad
superpower = get_superpower()

-- good
local superpower = get_superpower()

Rationale: Not doing so will result in global variables to avoid polluting the global namespace.

Variable scope

• Assign variables with the smallest possible scope.

-- bad
local function good()
   local name = get_name()

   test()
   print("doing stuff..")

   --...other stuff...

   if name == "test" then
      return false
   end

   return name
end

-- good
local bad = function()
   test()
   print("doing stuff..")

   --...other stuff...

   local name = get_name()

   if name == "test" then
      return false
   end

   return name
end

Rationale: Lua has proper lexical scoping. Declaring the function later means that its scope is smaller, so this makes it easier to check for the effects of a variable.

Conditional expressions

• False and nil are falsy in conditional expressions. 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

• Avoid designing APIs which depend on the difference between nil and false.

• Use the and/or idiom for the pseudo-ternary operator when it results in more straightforward code. When nesting expressions, use parentheses to make it easier to scan visually:

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

Note that the x and y or z as a substitute for x ? y : z does not work if y may be nil or false so avoid it altogether for returning booleans or values which may be nil.

Blocks

• Use single-line blocks only for then return, then break and function return (a.k.a "lambda") constructs:

-- good
if test then break end

-- good
if not ok then return nil, "this failed for this reason: " .. reason end

-- good
use_callback(x, function(k) return k.last 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

• Separate statements onto multiple lines. Do not use semicolons as statement terminators.

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

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

Spacing

• Use a space after --.

--bad
-- good

• Always put a space after commas and between operators and assignment signs:

-- bad
local x = y*9
local numbers={1,2,3}
numbers={1 , 2 , 3}
numbers={1 ,2 ,3}
local strings = { "hello"
                , "Lua"
                , "world"
                }
dog.set( "attr",{
  age="1 year",
  breed="Bernese Mountain Dog"
})

-- good
local x = y * 9
local numbers = {1, 2, 3}
local strings = {
   "hello",
   "Lua",
   "world",
}
dog.set("attr", {
   age = "1 year",
   breed = "Bernese Mountain Dog",
})

• Indent tables and functions according to the start of the line, not the construct:

-- bad
local my_table = {
                    "hello",
                    "world",
                 }
using_a_callback(x, function(...)
                       print("hello")
                    end)

-- good
local my_table = {
   "hello",
   "world",
}
using_a_callback(x, function(...)
   print("hello")
end)

Rationale: This keep indentation levels aligned at predictable places. You don't need to realign the entire block if something in the first line changes (such as replacing x with xy in the using_a_callback example above).

• The concatenation operator gets a pass for avoiding spaces:

-- okay
local message = "Hello, "..user.."! This is your day # "..day.." in our platform!"

Rationale: Being at the baseline, the dots already provide some visual spacing.

• No spaces after the name of a function in a declaration or in its arguments:

-- bad
local function hello ( name, language )
   -- code
end

-- good
local function hello(name, language)
   -- code
end

• Add blank lines between functions:

-- bad
local function foo()
   -- code
end
local function bar()
   -- code
end

-- good
local function foo()
   -- code
end

local function bar()
   -- code
end

• Avoid aligning variable declarations:

-- bad
local a               = 1
local long_identifier = 2

-- good
local a = 1
local long_identifier = 2

Rationale: This produces extra diffs which add noise to git blame.

• Alignment is occasionally useful when logical correspondence is to be highlighted:

-- okay
sys_command(form, UI_FORM_UPDATE_NODE, "a",      FORM_NODE_HIDDEN,  false)
sys_command(form, UI_FORM_UPDATE_NODE, "sample", FORM_NODE_VISIBLE, false)

Typing

• In non-performance critical code, it can be useful to add type-checking assertions for function arguments:

function manif.load_manifest(repo_url, lua_version)
   assert(type(repo_url) == "string")
   assert(type(lua_version) == "string" or not lua_version)

   -- ...
end

Rationale: This is a practice adopted early on in the development of LuaRocks that has shown to be beneficial in many occasions.

• Use the standard functions for type conversion, avoid relying on coercion:

-- bad
local total_score = review_score .. ""

-- good
local total_score = tostring(review_score)

Errors

• Functions that can fail for reasons that are expected (e.g. I/O) should return nil and a (string) error message on error, possibly followed by other return values such as an error code.

• On errors such as API misuse, an error should be thrown, either with error() or assert().

Modules

Follow these guidelines for writing modules. In short:

• Always require a module into a local variable named after the last component of the module’s full name.

local bar = require("foo.bar") -- requiring the module

bar.say("hello") -- using the module

• Don’t rename modules arbitrarily:

-- bad
local skt = require("socket")

Rationale: Code is much harder to read if we have to keep going back to the top to check how you chose to call a module.

• Start a module by declaring its table using the same all-lowercase local name that will be used to require it. You may use an LDoc comment to identify the whole module path.

--- @module foo.bar
local bar = {}

• Try to use names that won't clash with your local variables. For instance, don't name your module something like “size”.

• Use local function to declare local functions only: that is, functions that won’t be accessible from outside the module.

That is, local function helper_foo() means that helper_foo is really local.

• Public functions are declared in the module table, with dot syntax:

function bar.say(greeting)
   print(greeting)
end

Rationale: Visibility rules are made explicit through syntax.

• Do not set any globals in your module and always return a table in the end.

• If you would like your module to be used as a function, you may set the __call metamethod on the module table instead.

Rationale: Modules should return tables in order to be amenable to have their contents inspected via the Lua interactive interpreter or other tools.

• Requiring a m