Define default values for function arguments

Question

In the Lua wiki I found a way to define default values for missing arguments:

function myfunction(a,b,c)
    b = b or 7
    c = c or 5
    print (a,b,c)
end

Is that the only way? The PHP style myfunction (a,b=7,c=5) does not seem to work. Not that the Lua way doesn't work, I am just wondering if this is the only way to do it.

Answer 1

As always, "Lua gives you the power, you build the mechanisms". The first distinction to make here is that between named parameters and the commonly used parameter list.

The parameter list

Assuming all your args are given in the parameter list as follows, they will all be initialized. At this point, you can't distinguish between "wasn't passed" and "was passed as nil" - both will simply be nil. Your options for setting defaults are:

1. Using the or operator if you expect a truthy value (not nil or false). Defaulting to something even if false is given might be a feature in this case.
2. Using an explicit nil check param == nil, used either as if param == nil then param = default end or the typical Lua ternary construct param == nil and default or param.

If you find yourself frequently repeating the patterns from point (2), you might want to declare a function:

function default(value, default_value)
    if value == nil then return default_value end
    return value
end

(whether to use global or local scope for this function is another issue I won't get into here).

I've included all three ways the following example:

function f(x, y, z, w)
    x = x or 1
    y = y == nil and 2 or y
    if z == nil then z = 3 end
    w = default(w, 4)
    print(x, y, z, w)
end
f()
f(1)
f(1, 2)
f(1, 2, 3)
f(1, 2, 3, 4)

note that this also allows omitting arguments inbetween; trailing nil arguments will also be treated as absent:

f(nil)
f(nil, 2, 3)
f(nil, 2, nil, 4)
f(1, 2, 3, nil)

Varargs

A lesser known feature of Lua is the ability to actually determine how many arguments were passed, including the ability to distinguish between explicitly passed nil arguments and "no argument" through the select function. Let's rewrite our function using this:

function f(...)
    local n_args = select("#", ...) -- number of arguments passed
    local x, y, z, w = ...
    if n_args < 4 then w = 4 end
    if n_args < 3 then z = 3 end
    if n_args < 2 then y = 2 end
    if n_args < 1 then x = 1 end
    print(x, y, z, w)
end
f() -- prints "1 2 3 4"
f(nil) -- prints "nil 2 3 4"
f(1, nil) -- prints "1 nil 3 4"
f(1, nil, 3) -- prints "1 nil 3 4"
f(nil, nil, nil, nil) -- prints 4x nil

Caveat: (1) the argument list got dragged into the function, hurting readability (2) this is rather cumbersome to write manually, and should probably be abstracted away, perhaps time using a wrapper function wrap_defaults({1, 2, 3, 4}, f) that supplies the defaults as appropriate. Implementation of this is left up to the reader as an exercise (hint: the straightforward way would first collect the args into a garbage table, then unpack that after setting the defaults).

Table calls

Lua provides syntactic sugar for calling functions with a single table as the only argument: f{...} is equivalent to f({...}). Furthermore, {f(...)} can be used to capture a vararg returned by f (caveat: if f returns nils, the table will have holes in it's list part).

Tables also allow implementing named "arguments" as table fields: Tables allow mixing a list and a hash part, making f{1, named_arg = 2} perfectly valid Lua.

In terms of limitations, the advantage of table call is that it only leaves a single argument - the table - on the stack rather than multiple arguments. For recursive functions, this allows hitting the stack overflow later. Since PUC Lua drastically increased the stack limit to ~1M this isn't much of an issue anymore; LuaJIT still has a stack limit of ~65k however, and PUC Lua 5.1 is even lower at around 15k.

In terms of performance & memory consumption, the table call is obviously worse: It requires Lua to build a garbage table, which will then waste memory until the GC gets rid of it. Garbage parameter tables should therefore probably not be used in hotspots where plenty of calls happen. Indexing a hashmap is also obviously slower than getting values straight off the stack.

That said, let's examine the ways to implement defaults for tables:

Unpacking / Destructuring

unpack (table.unpack in later versions (5.2+)) can be used to convert a table into a vararg, which can be treated like a parameter list; note however that in Lua the list part can't have trailing nil values, not allowing you to distinguish "no value" and nil. Unpacking / destructuring to locals also helps performance since it gets rid of repeated table indexing.

function f(params)
    local x, y, z, w = unpack(params)
    -- use same code as if x, y, z, w were regular params
end
f{1, 2, nil}

if you use named fields, you'll have to explicitly destructure those:

function f(params)
    local x, y, z, w = params.x, params.y, params.z, params.w
    -- use same code as if x, y, z, w were regular params
end
f{x = 1, w = 4}

mix & match is possible:

function f(params)
    local x, y, z = unpack(params)
    local w = params.w
    -- use same code as if x, y, z, w were regular params
end
f{1, 2, w = 4}

Metatables

The __index metatable field can be used to set a table which is indexed with name if params.name is nil, providing defaults for nil values. One major drawback of setting a metatable on a passed table is that the passed table's metatable will be lost, perhaps leading to unexpected behavior on the caller's end. You could use getmetatable and setmetatable to restore the metatable after you're done operating with the params, but that would be rather dirty, hence I would recommend against it.

Bad

function f(params)
    setmetatable(params, {__index = {x = 1, y = 2, z = 3, w = 4}})
    -- use params.[xyzw], possibly unpacking / destructuring
end
f{x = 1}

in addition to the presumably garbage params table, this will create (1) a garbage metatable and (2) a garbage default table every time the function is called. This is pretty bad. Since the metatable is constant, simply drag it out of the function, making it an upvalue:

Okay

local defaults_metatable = {__index = {x = 1, y = 2, z = 3, w = 4}}
function f(params)
    setmetatable(params, defaults_metatable)
    -- use params.[xyzw], possibly unpacking / destructuring
end

Avoiding metatables

If you want a default table without the hackyness of metatables, consider once again writing yourself a helper function to complete a table with default values:

local function complete(params, defaults)
    for param, default in pairs(defaults) do
        if params[param] == nil then
            params[param] = default
        end
    end
end

this will change the params table, properly setting the defaults; use as params = complete(params, defaults). Again, remember to drag the defaults table out of the function.

Answer 2

Short answer is that it's simplest and best way . in lua , variables by default equal with nil . this means if we don't pass argument to lua functions ,the argument is exits but is nil and lua programmers uses of this lua attribute for set the default value .

also it's not a way for set default value but you can use following function

this function create a error is you don't pass values to arguments

function myFn(arg1 , arg2)
err = arg1 and arg2
if not err then error("argument") end
-- or
if not arg1 and arg2 then error("msg") end

but it's not a good way and better is don't use of this function

and in diagrams shows optional argument in [,arg]

function args(a1 [,a2])
-- some
end
function args ( a1 [,a2[,a3]])
-- some
end

Answer 3

If your function expects neither Boolean false nor nil to be passed as parameter values, your suggested approach is fine:

function test1(param)
  local default = 10
  param = param or default
  return param
end

--[[
test1(): [10]
test1(nil): [10]
test1(true): [true]
test1(false): [10]
]]

If your function allows Boolean false, but not nil, to be passed as the parameter value, you can check for the presence of nil, as suggested by Stuart P. Bentley, as long as the default value is not Boolean false:

function test2(param)
  local default = 10
  param = (param == nil and default) or param
  return param
end

--[[
test2(): [10]
test2(nil): [10]
test2(true): [true]
test2(false): [false]
]]

The above approach breaks when the default value is Boolean false:

function test3(param)
  local default = false
  param = (param == nil and default) or param
  return param
end

--[[
test3(): [nil]
test3(nil): [nil]
test3(true): [true]
test3(false): [false]
]]

Interestingly, reversing the order of the conditional checks does allow Boolean false to be the default value, and is nominally more performant:

function test4(param)
  local default = false
  param = param or (param == nil and default)
  return param
end

--[[
test4(): [false]
test4(nil): [false]
test4(true): [true]
test4(false): [false]
]]

This approach works for reasons that seem counter-intuitive until further examination, upon which they are discovered to be kind of clever.

If you want default parameters for functions that do allow nil values to be passed, you'll need to do something even uglier, like using variadic parameters:

function test5(...)
  local argN = select('#', ...)
  local default = false
  local param = default
  if argN > 0 then
    local args = {...}
    param = args[1]
  end
  return param
end

--[[
test5(): [false]
test5(nil): [nil]
test5(true): [true]
test5(false): [false]
]]

Of course, variadic parameters completely thwart auto-completion and linting of function parameters in functions that use them.

Answer 4

If you want named arguments and default values like PHP or Python, you can call your function with a table constructor:

myfunction{a,b=3,c=2}

(This is seen in many places in Lua, such as the advanced forms of LuaSocket's protocol modules and constructors in IUPLua.)

The function itself could have a signature like this:

function myfunction(t)
    setmetatable(t,{__index={b=7, c=5}})
    local a, b, c =
      t[1] or t.a, 
      t[2] or t.b,
      t[3] or t.c
    -- function continues down here...
end

Any values missing from the table of parameters will be taken from the __index table in its metatable (see the documentation on metatables).

Of course, more advanced parameter styles are possible using table constructors and functions- you can write whatever you need. For example, here is a function that constructs a function that takes named-or-positional argument tables from a table defining the parameter names and default values and a function taking a regular argument list.

As a non-language-level feature, such calls can be changed to provide new behaviors and semantics:

• Variables could be made to accept more than one name
• Positional variables and keyword variables can be interspersed - and defining both can give precedence to either (or cause an error)
• Keyword-only positionless variables can be made, as well as nameless position-only ones
• The fairly-verbose table construction could be done by parsing a string
• The argument list could be used verbatim if the function is called with something other than 1 table

Some useful functions for writing argument translators are unpack (moving to table.unpack in 5.2), setfenv (deprecated in 5.2 with the new _ENV construction), and select (which returns a single value from a given argument list, or the length of the list with '#').

Answer 5

In my opinion there isn't another way. That's just the Lua mentality: no frills, and except for some syntactic sugar, no redundant ways of doing simple things.

Answer 6

Technically, there's b = b == nil and 7 or b (which should be used in the case where false is a valid value as false or 7 evaluates to 7), but that's probably not what you're looking for.

Answer 7

The only way i've found so far that makes any sense is to do something like this:

function new(params)
  params = params or {}
  options = {
    name = "Object name"
  }

  for k,v in pairs(params) do options[k] = v end

  some_var = options.name
end

new({ name = "test" })
new()