Lua Style Guide

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Introduction

Before applying these Lua style guidelines, consider the words of warning taken from the [Python style guide], which applies just as well to Lua. Code is read much more often than it is written, and this style guide intends to improve the readability of code through consistency. Consistency is important, in increasing measure, with other projects, within a project, and within a single module or function. But most importantly: know when to be inconsistent, and use your best judgment -- sometimes the style guide just doesn't apply. It can be advisable to break a rule when applying the rule would make the code less readable.

Lua has its own syntax and idioms and therefore warrants its own style guide, which we attempt to formulate here. Nevertheless, there is no need repeating many of the lessons learned from decades of experience by programmers in other more established languages, especially related scripting languages, so we can take some inspiration from the style guides of these other languages:

Programming style is an art. There is some arbitrariness to the rules, but there are sound rationales for them. It is useful not only to provide sound advice on style but to understand the underlying rationale and human aspect of why the style recommendations are formed:

Now, on to Lua...In defining a recommending style, we'll take input from a few sources of Lua code that are quasi-authoritative because they are from official documentation, original Lua authors, or other reputable sources:

Formatting

Indentation - Indenting often uses two spaces. This is followed in Programming in Lua, the Lua Reference Manual, Beginning Lua Programming, and the Lua users wiki. (Why this is the case, I don't know, but perhaps it is because Lua statements can tend to be deeply nested, even in a LISP or functional manner, or perhaps it is affected by the fact that the code in these examples is small and pedagogical.) You'll see other common conventions (e.g. 3-4 spaces or tabs).

for i,v in ipairs(t) do
  if type(v) == "string" then
    print(v)
  end
end

Naming

Variable name length - It's a general rule (not necessarily a Lua one) that variable names with larger scope should be more descriptive than those with smaller scope. For example, i is probably a poor choice for a global variable in a large program, but it's perfectly fine as a counter in a small loop.

Value and object variable naming - Variables holding values or objects are typically lowercase and short (e.g. color). The book Beginning Lua Programming uses CamelCase for user variables, but this seems to largely be for pedagogical reasons to clearly distinguish user-defined variables from built-in ones.

Function naming - Function often follows similar rules to value and object variable naming (functions being first class objects). A function name consisting of multiple words may run together as in (getmetatable), as done in the standard Lua functions, though you may choose to use underscores (print_table). Some from other programming backgrounds use CamelCase, as in obj::GetValue().

Lua internal variable naming - The [Lua 5.1 Reference Manual - Lexical Conventions] says, "As a convention, names starting with an underscore followed by uppercase letters (such as _VERSION) are reserved for internal global variables used by Lua." These are often constants but not necessarily, e.g. _G.

Constants naming - Constants, particularly ones that are simple values, are often given in ALL_CAPS, with words optionally separated by underscores (e.g. MAXLINES in PiL2, 4.3 and also the Markov example in PiL2, Listing 10.6).

Module/package naming - Module names are often nouns with names that are short and lowercase, with nothing between words. At least, that's the general pattern noticed in Kepler: luasql.postgres (not Lua-SQL.Postgres). Examples: "lxp", "luasql", "luasql.postgres", "luasql.mysql", "luasql.oci8", "luasql.sqlite", "luasql.odbc", "socket", "xmlrpc", "xmlrpc.http", "soap", "lualdap", "logging", "md5", "zip", "stable", "copas", "lxp", "lxp.lom", "stable", "lfs", "htk". However, see comments in the Modules section below concerning modules used as classes.

The variable consisting of only an underscore "_" is commonly used as a placeholder when you want to ignore the variable:

for _,v in ipairs(t) do print(v) end

Note: This resembles the use of "_" in Haskell, Erlang, Ocaml, and Prolog languages, where "_" takes the special meaning of anonymous (ignored) variables in pattern matches. In Lua, "_" is only a convention with no inherent special meaning though. Semantic editors that normally flag unused variables may avoid doing so for variables named "_" (e.g. LuaInspect is such a case).

i, k, v, and t are often used as follows:

for k,v in pairs(t) ... end
for i,v in ipairs(t) ... end
mt.__newindex = function(t, k, v) ... end

M is sometimes used as the "current module table" (e.g., see PIL2, 15.3).

self refers to the object a method is invoked on (like this in C++ or Java). In fact, this is enforced by the : syntactic sugar:

  function Car:move(distance)
    self.position = self.position + distance
  end

Class names (or at least metatables representing classes) may be mixed case (BankAccount), or they might not be. If so, acronyms (e.g. XML) might only uppercase the first letter (XmlDocument).

Hungarian notations ([Hungarian notations]). Encoding semantic information into variable names can be helpful to readers of your code, particularly if that information cannot otherwise be easily deduced, though overdoing it may be redundant and reduce code readability. In some more static languages (e.g. C), where data types are known to the compiler, it is redundant to encode the data type into the variable name, but, even there, data type is not the only nor necessarily the most useful form of semantic information.

local function paint(canvas, ntimes)
  for i=1,ntimes do
    local hello_str_asc_lc_en_const = "hello world"
    canvas:draw(hello_str_asc_lc_en_const:toupper())
  end
end

The variable naming conventions in the above example imply the following to the reader. canvas is a canvas object, perhaps one explicitly derived from something like Canvas, which is information possibly not otherwise easy to deduce from the code. ntimes is an integer number of times to draw something. i is conventionally an integer index, which though obvious, even if called something else, also keeps the code short. The _str_asc_lc_en_const is redundant, perhaps even confusing, and can be removed: obviously this variable is a constant, English, lowercase, ASCII string.

Libraries and Features

Avoid using the debug library unless necessary, especially if trusted code is being run. (Using the debug library is sometimes a hack: StringInterpolation.)

Avoid deprecated features. In 5.1 these include table.getn, table.setn, table.foreach[i], and gcinfo. Refer to the [Lua 5.1 Reference Manual - 7 - Incompatibilities with the Previous Version] for alternatives.

Scope

Use locals rather than globals whenever possible.

local x = 0
local function count()
  x = x + 1
  print(x)
end

Globals have larger scopes and lifetimes and therefore increase [coupling] and complexity. [1] Don't pollute the environment. In Lua, access to locals is also faster than globals [PIL 4.2] since globals require a table lookup at run-time, while locals exist as registers [ ScopeTutorial ].

A useful method of detecting inadvertent use of globals is given in DetectingUndefinedVariables. In Lua, globals are sometimes the result of mispellings and other lurking errors in your code.

At times it is useful to further limit the scope of local variables with do-blocks [PIL 4.2]:

local v
do
  local x = u2*v3-u3*v2
  local y = u3*v1-u1*v3
  local z = u1*v2-u2*v1
  v = {x,y,z}
end

local count
do
  local x = 0
  count = function() x = x + 1; return x end
end

Global variables scope can be reduced as well via the Lua module system [PIL2 15] or [setfenv].

Modules

The Lua 5.1 module system is often recommended. However, there have been some criticisms of the Lua 5.1 module system. For details, see LuaModuleFunctionCritiqued, but in summary, you might think of writing a module like this:

-- hello/mytest.lua
module(..., package.seeall)
local function test() print(123) end
function test1() test() end
function test2() test1(); test1() end

and use it like this:

require "hello.mytest"
hello.mytest.test2()

The criticisms are that this creates a global variable hello in all modules (which is a side-effect), and the global environment is exposed through the hello table, e.g. hello.mytest.print == _G.print (which among various things could be detrimental to sandboxing and is just plain weird).

These problems can be avoided by not using the module function but instead simply defining modules in the following simple way:

-- hello/mytest.lua
local M = {}

local function test() print(123) end
function M.test1() test() end
function M.test2() M.test1(); M.test1() end

return M

and importing modules this way:

local MT = require "hello.mytest"
MT.test2()

A module containing a class with constructor (in the object-oriented sense) can be packaged in a number of ways in a module. Here is one reasonably good approach.[*2]

-- file: finance/BankAccount.lua

local M = {}; M.__index = M

local function construct()
  local self = setmetatable({balance = 0}, M)
  return self
end
setmetatable(M, {__call = construct})

function M:add(value) self.balance = self.balance + 1 end

return M

A module defined in this way typically only contains a single class (or at least a single public one), which is the module itself.

It can be used like this:

local BankAccount = require "finance.BankAccount"
local account = BankAccount()

or even like this:

local new = require
local account = new "finance.BankAccount" ()

The above followed somewhat the Java convention of the package "finance" being in all lowercase, while the class BankAccount being in mixed (Camel) case and objects being lower-case. Notice the advantage. The classes are easy to spot and differentiate from instantiations of classes (i.e. objects), which are lower-case. If you spot something like BankAccount:add(1), it is almost certainly an error since : is a method call on an object, but you'll notice that BankAccount is obviously a class due to the case convention.

The above is not the only approach used. You will see many other styles:

account = finance.newBankAccount()
account = finance.create_bank_account()
account = finance.bankaccount.create()
account = finance.BankAccount.new()

It can be argued that variety, without good justification, is not a good thing.

Commenting

Use a space after --.

return nil  -- not found    (suggested)
return nil  --not found     (discouraged)

(The above follows luarefman, PiL, luagems minus chap 21, BLP, and Kepler/LuaRocks.)

There is no standard convention for commenting.

Docstrings can be simulated (see DecoratorsAndDocstrings). POD format has also been advocated (see LuaSearch). There is also [LuaDoc].

Kepler sometimes uses this doxygen/Javadoc-like style:

-- taken from cgilua/src/cgilua/session.lua
-------------------------------------
-- Deletes a session.
-- @param id Session identification.
-------------------------------------
function delete (id)
        assert (check_id (id))
        remove (filename (id))
end

End terminator

Because "end" is a terminator for many different constructs, it can help the reader (especially in a large block) if a comment is used to clarify which construct is being terminated: [*3]

  for i,v in ipairs(t) do
    if type(v) == "string" then
      ...lots of code here...
    end -- if string
  end -- for each t

If your editor lacks Lua block matching but does have generic parenthesis matching, then this hack might even be used[*4] (or just get a better editor):

  for i,v in ipairs(t) do --(
    if type(v) == "string" then --(
      ...lots of code here...
    end --) if string
  end --) for each t

Software Licensing

The choice of software license for your Lua code depends on your goals and what type of code it is (e.g. module or application). Licensing choice is particularly significant for modules, which are often distributed with other modules and applications.

There are advantages to licensing Lua modules, or at least those intended for the general Lua community, under the same terms as Lua[2] itself, which as of version 5.0 is the MIT license[3]. Not only is the MIT license a very simple to understand and unrestrictive license (in fact, no more restrictive than Lua itself), but consistency in licensing between modules and with Lua allows simplified distribution of bundles of modules and Lua together, such as for distributions and embedded versions of Lua. (This basic approach has worked very well in the past for the Perl language, which has thousands of modules most entirely under an MIT-like license called the Artistic License, under which Perl is also distributed. Perl modules often indicate their licensing simply with the statement "Licensed under the same terms as Perl itself.") Those advantages of using an MIT license are reduced, though not eliminated, if your Lua module acts as a binding to some C library released under some very different license such (L)GPL or a closed source one since the latter code impose stronger restrictions on distribution anyway. Avoid, whenever possible, writing your own license or adding additional clauses but rather consider strongly the words of warning about consistency at the top of this document since inconsistency is a detriment to reusability, and reusability is a main advantage of modules.

Lua Idioms

To test whether a variable is not nil in a conditional, it is terser to just write the variable name rather than explicitly compare against nil. Lua treats nil and false as false (and all other values as true) in a conditional:

local line = io.read()
if line then  -- instead of line ~= nil
  ...
end
...
if not line then  -- instead of line == nil
  ...
end

However, if the variable tested can ever contain false as well, then you will need to be explicit if the two conditions must be differentiated: line == nil v.s. line == false.

and and or may be used for terser code:

local function test(x)
  x = x or "idunno"
    -- rather than if x == false or x == nil then x = "idunno" end
  print(x == "yes" and "YES!" or x)
    -- rather than if x == "yes" then print("YES!") else print(x) end
end

Clone a small table t (warning: this has a system dependent limit on table size; it was just over 2000 on one system):

u = {unpack(t)}

Determine if a table t is empty (including non-integer keys, which #t ignores):

if next(t) == nil then ...

To append to an array, it can be terser and more efficient to do t[#t+1] = 1 rather than table.insert(t, 1).

Design Patterns

Lua is small language with a small number of simple building blocks that can be combined in a vast number of powerful ways. With this freedom comes the need for self-discipline in the form of design patterns. Often there is an idiomatic way or design pattern to achieving a certain effect in Lua that can be reused frequently (e.g. ObjectOrientationTutorial or ReadOnlyTables). See the LuaTutorial and SampleCode for common solutions to such problems.

Coding Standards

Here is a list of coding standards used in various Lua projects:

Personal Preferences

This section contains less refined or more subjective content not yet incorporated into the main text above.

[*2] (Advocates of the style described above include DavidManura, and others like RiciLake have mentioned similar things. (add your name here))

[*3] Noted by GavinWraith

[*4] JulioFernandez


PAGE COMMENTS

I think "Lua style" is too subjective to reach consensus on, therefore this page is not workable. Suggest page rename to DavidsLuaStyleGuide?. --JohnBelmonte

The purpose of this document is to help users improve the style of their coding. Lua has some conventions that are commonly acknowledged in some form or even stated in the standard Lua references (e.g. avoiding globals, limiting scope, avoiding debug library, using and/or to shorten conditions, etc.) Indeed, Lua style is more diverse than in other languages, and there are many cases with an absence of consensus. However, consensus is not required: it is useful merely to describe the various approaches commonly used, and indicate where they are used, so that users can more effectively choose conventions in their own projects. A "Personal Preferences" section is provided for more subjective styles or personal opinions. --DavidManura

An attempt at a style guide should probably limit focus to the Lua language itself (i.e. exclude C API usage). --JohnBelmonte The style of how you design a binding, and the style of how you write binding in C and C++, are legitimate discussions for a style guide for a language designed for embedding. -- RiciLake


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Last edited September 16, 2012 8:36 pm GMT (diff)