The Difference Between Dot and Colon Syntax in Lua

Lua’s colon syntax is shorthand for passing the receiver as the first argument, conventionally named self.

These definitions are equivalent:

function MainScene:onCreate()
    print(self)
end

function MainScene.onCreate(self)
    print(self)
end

These calls are also equivalent:

scene:onCreate()
MainScene.onCreate(scene)

The definition style and call style must agree. If a method is defined with a colon but called with a dot and no explicit receiver, self is missing. If a plain function is called with a colon, it receives one unexpected extra argument.

local Calculator = {}

function Calculator.add(a, b)
    return a + b
end

function Calculator:addToTotal(value)
    self.total = (self.total or 0) + value
end

Use dot syntax for functions that do not operate on a particular object, and colon syntax for instance methods. The colon does not create a class or copy a function; it only adds the receiver argument at the call or definition site.

When saving a method as a callback, preserve its receiver explicitly:

local callback = function(...)
    return scene:onCreate(...)
end

Passing scene.onCreate by itself loses the object unless the callback API supplies it separately.