Lua style guide

Indentation and formatting

• 4 spaces instead of tabs. PIL suggests using two spaces, but a programmer looks at code from 4 to 8 hours a day, so it’s simpler to distinguish indentation with 4 spaces. Why spaces? Similar representation everywhere.

  You can use vim modelines:

  -- vim:ts=4 ss=4 sw=4 expandtab
  

• A file should ends w/ one newline symbol, but shouldn’t ends w/ blank line (two newline symbols).

• Every do/while/for/if/function should indent 4 spaces.

• Related or/and in if must be enclosed in the round brackets (). Example:

  if (a == true and b == false) or (a == false and b == true) then
      <...>
  end -- good
  
  if a == true and b == false or a == false and b == true then
      <...>
  end -- bad
  
  if a ^ b == true then
  end -- good, but not explicit
  

• Type conversion

  Do not use concatenation to convert to string or addition to convert to number (use tostring/tonumber instead):

  local a = 123
  a = a .. ''
  -- bad
  
  local a = 123
  a = tostring(a)
  -- good
  
  local a = '123'
  a = a + 5 -- 128
  -- bad
  
  local a = '123'
  a = tonumber(a) + 5 -- 128
  -- good
  

• Try to avoid multiple nested if’s with common body:

  if (a == true and b == false) or (a == false and b == true) then
      do_something()
  end
  -- good
  
  if a == true then
      if b == false then
          do_something()
      end
  if b == true then
      if a == false then
          do_something()
      end
  end
  -- bad
  

• Avoid multiple concatenations in one statement, use string.format instead:

  function say_greeting(period, name)
      local a = "good  " .. period .. ", " .. name
  end
  -- bad
  
  function say_greeting(period, name)
      local a = string.format("good %s, %s", period, name)
  end
  -- good
  
  local say_greeting_fmt = "good %s, %s"
  function say_greeting(period, name)
      local a = say_greeting_fmt:format(period, name)
  end
  -- best
  

• Use and/or for default variable values

  function(input)
      input = input or 'default_value'
  end -- good
  
  function(input)
      if input == nil then
          input = 'default_value'
      end
  end -- ok, but excessive
  

• if’s and return statements:

  if a == true then
      return do_something()
  end
  do_other_thing() -- good
  
  if a == true then
      return do_something()
  else
      do_other_thing()
  end -- bad
  

• Using spaces:

  • Don’t use spaces between function name and opening round bracket. Split arguments with one whitespace character:

    function name (arg1,arg2,...)
    end -- bad
    
    function name(arg1, arg2, ...)
    end -- good
    

  • Add a space after comment markers:

    while true do -- inline comment
    -- comment
    do_something()
    end
    --[[
    multiline
    comment
    ]]--
    

  • Surrounding operators:

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

  • Add a space after commas in tables:

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

  • Add a space in map definitions after equals signs and commas:

    return {1,2,3,4}
    return {
        key1 = val1,key2=val2
    }
    -- bad
    
    return {1, 2, 3, 4}
    return {
        key1 = val1, key2 = val2,
        key3 = vallll
    }
    -- good
    

    You can also use alignment:

    return {
        long_key  = 'vaaaaalue',
        key       = 'val',
        something = 'even better'
    }
    

  • Extra blank lines may be used (sparingly) to separate groups of related functions. Blank lines may be omitted between several related one-liners (for example, a set of dummy implementations).

    Use blank lines in functions (sparingly) to indicate logical sections:

    if thing ~= nil then
        -- ...stuff...
    end
    function derp()
        -- ...stuff...
    end
    local wat = 7
    -- bad
    
    if thing ~= nil then
        -- ...stuff...
    end
    
    function derp()
        -- ...stuff...
    end
    
    local wat = 7
    -- good
    

  • Delete whitespace at EOL (strongly forbidden. Use :s/\s\+$//gc in vim to delete them).

Avoid global variables

Avoid using global variables. In exceptional cases, start the name of such a variable with _G, add a prefix, or add a table instead of a prefix:

function bad_global_example()
end -- very, very bad

function good_local_example()
end
_G.modulename_good_local_example = good_local_example -- good
_G.modulename = {}
_G.modulename.good_local_example = good_local_example -- better

Always use a prefix to avoid name conflicts.

Naming

• Names of variables/”objects” and “methods”/functions: snake_case.
• Names of “classes”: CamelCase.
• Private variables/methods (future properties) of objects start with underscores <object>._<name>. Avoid syntax like local function private_methods(self) end.
• Boolean: naming is_<...>, isnt_<...>, has_, hasnt_ is good style.
• For “very local” variables:
  • t is for tables
  • i, j are for indexing
  • n is for counting
  • k, v is what you get out of pairs() (are acceptable, _ if unused)
  • i, v is what you get out of ipairs() (are acceptable, _ if unused)
  • k/key is for table keys
  • v/val/value is for values that are passed around
  • x/y/z is for generic math quantities
  • s/str/string is for strings
  • c is for 1-char strings
  • f/func/cb are for functions
  • status, <rv>.. or ok, <rv>.. is what you get out of pcall/xpcall
  • buf, sz is a (buffer, size) pair
  • <name>_p is for pointers
  • t0.. is for timestamps
  • err is for errors
• Abbreviations are acceptable if they’re very common or if they’re unambiguous and you’ve documented them.
• Global variables are spelled in ALL_CAPS. If it’s a system variable, it starts with an underscore (_G/_VERSION/..).
• Modules are named in snake_case (avoid underscores and dashes): for example, ‘luasql’, not ‘Lua-SQL’.
• *_mt and *_methods defines metatable and methods table.

Idioms and patterns

Always use round brackets in call of functions except multiple cases (common lua style idioms):

• *.cfg{ } functions (box.cfg/memcached.cfg/..)
• ffi.cdef[[ ]] function

Avoid the following constructions:

• <func>’<name>’. Strongly avoid require’..’.
• function object:method() end. Use function object.method(self) end instead.
• Semicolons as table separators. Only use commas.
• Semicolons at the end of line. Use semicolons only to split multiple statements on one line.
• Unnecessary function creation (closures/..).

Avoid implicit casting to boolean in if conditions like if x then or if not x then. Such expressions will likely result in troubles with box.NULL. Instead of those conditions, use if x ~= nil then and if x == nil then.

Modules

Don’t start modules with license/authors/descriptions, you can write it in LICENSE/AUTHORS/README files. To write modules, use one of the two patterns (don’t use modules()):

local M = {}

function M.foo()
...
end

function M.bar()
...
end

return M

or

local function foo()
...
end

local function bar()
...
end

return {
foo = foo,
bar = bar,
}

Commenting

Don’t forget to comment your Lua code. You shouldn’t comment Lua syntax (assume that the reader already knows the Lua language). Instead, tell about functions/variable names/etc.

Multiline comments: use matching (--[[ ]]--) instead of simple (--[[ ]]).

Public function comments:

--- Copy any table (shallow and deep version)
-- * deepcopy: copies all levels
-- * shallowcopy: copies only first level
-- Supports __copy metamethod for copying custom tables with metatables
-- @function gsplit
-- @table         inp  original table
-- @shallow[opt]  sep  flag for shallow copy
-- @returns            table (copy)

Testing

Use the tap module for writing efficient tests. Example of a test file:

#!/usr/bin/env tarantool

local test = require('tap').test('table')
test:plan(31)

do -- check basic table.copy (deepcopy)
    local example_table = {
        {1, 2, 3},
        {"help, I'm very nested", {{{ }}} }
    }

    local copy_table = table.copy(example_table)

    test:is_deeply(
        example_table,
        copy_table,
        "checking, that deepcopy behaves ok"
    )
    test:isnt(
        example_table,
        copy_table,
        "checking, that tables are different"
    )
    test:isnt(
        example_table[1],
        copy_table[1],
        "checking, that tables are different"
    )
    test:isnt(
        example_table[2],
        copy_table[2],
        "checking, that tables are different"
    )
    test:isnt(
        example_table[2][2],
        copy_table[2][2],
        "checking, that tables are different"
    )
    test:isnt(
        example_table[2][2][1],
        copy_table[2][2][1],
        "checking, that tables are different"
    )
end

<...>

os.exit(test:check() and 0 or 1)

When you test your code, the output will be something like this:

TAP version 13
1..31
ok - checking, that deepcopy behaves ok
ok - checking, that tables are different
ok - checking, that tables are different
ok - checking, that tables are different
ok - checking, that tables are different
ok - checking, that tables are different
...

Error handling

Be generous in what you accept and strict in what you return.

With error handling, this means that you must provide an error object as the second multi-return value in case of error. The error object can be a string, a Lua table, cdata, or userdata. In the latter three cases, it must have a __tostring metamethod defined.

In case of error, use nil for the first return value. This makes the error hard to ignore.

When checking function return values, check the first argument first. If it’s nil, look for error in the second argument:

local data, err = foo()
if data == nil then
    return nil, err
end
return bar(data)

Unless the performance of your code is paramount, try to avoid using more than two return values.

In rare cases, you may want to return nil as a legal return value. In this case, it’s OK to check for error first and then for return:

local data, err = foo()
if err == nil then
    return data
end
return nil, err

luacheck

To check the code style, Tarantool uses luacheck. It analyses different aspects of code, such as unused variables, and sometimes it checks more aspects than needed. So there is an agreement to ignore some warnings generated by luacheck:

"212/self",   -- Unused argument <self>.
"411",        -- Redefining a local variable.
"421",        -- Shadowing a local variable.
"431",        -- Shadowing an upvalue.
"432",        -- Shadowing an upvalue argument.