Lua Functions Default parameters


function sayHello(name)
    print("Hello, " .. name .. "!")

That function is a simple function, and it works well. But what would happen if we just called sayHello()?

stdin:2: attempt to concatenate local 'name' (a nil value)
stack traceback:
    stdin:2: in function 'sayHello'
    stdin:1: in main chunk
    [C]: in ?

That's not exactly great. There are two ways of fixing this:

  1. You immediately return from the function:

    function sayHello(name)
      if not (type(name) == "string") then
        return nil, "argument #1: expected string, got " .. type(name)
      end -- Bail out if there's no name.
      -- in lua it is a convention to return nil followed by an error message on error
      print("Hello, " .. name .. "!") -- Normal behavior if name exists.
  2. You set a default parameter.

    To do this, simply use this simple expression

function sayHello(name)
    name = name or "Jack" -- Jack is the default, 
                          -- but if the parameter name is given, 
                          -- name will be used instead
    print("Hello, " .. name .. "!")

The idiom name = name or "Jack" works because or in Lua short circuits. If the item on the left side of an or is anything other than nil or false, then the right side is never evaluated. On the other hand, if sayHello is called with no parameter, then name will be nil, and so the string "Jack" will be assigned to name. (Note that this idiom, therefore, will not work if the boolean false is a reasonable value for the parameter in question.)