Notes by Lex Toumbourou

In Lua, since everything is a table, metatables are a paradigm that lets you change the behaviour of a table, using another table. For example, you can define the behaviour when 2 tables are added together using the + operator by defining a metatable with an __add method. This is similar to the concept of magic methods (Python), though in Python magic methods are defined on instance's class.

In this example, I'm defining addition as the sum of all keys in the left-most table

local metatable = {
    __add = function(self, other)
        local output = {}
        for k, v in pairs(self) do
            output[k] = self[k] + other[k]
        end
        return output
    end
}

local t = {A=100, B=200}

setmetatable(t, metatable)

local result = t + {A=50, B=100}

print(result['A'])  -- 150
print(result['B'])  -- 300

__add is an arithmetic metamethod, alongside __mul (multiplication behaviour of * between 2 tables), __div (division), __sub (subtraction), __unm (negation) and __pow (exponential).

There are also relational metamethods for comparison: __eq, __lt and __le

Lastly, Lua Table-Access Metamethods allow for defining behaviour when missing keys are looked up.

References:

• Programming in Lua - Chapter 13 - Metatables and Metamethods