Adding Methods to Shapes

From your Lua OOP chapter you know tables can hold functions just like any other value. Shapes can describe those function fields too: turning a shape into a small contract of data and behavior.

A method field uses the function-type syntax you learned in the functions chapter. Its first parameter is conventionally named self, the table the method belongs to, and the shape can even reference itself:

type Greeter = {
    name: string,
    greet: (self: Greeter) -> string,
}

To implement the shape, store a function in the field and give it that self parameter:

local greeter: Greeter = {
    name = "Ana",
    greet = function(self: Greeter): string
        return `Hello, I'm {self.name}`
    end,
}

Calling it means passing the table as self yourself: greeter.greet(greeter). Through self the method reads the fields of whichever table it was called on.

Writing the table twice feels clumsy, and you already know Lua's cure: the colon. Next lesson connects : syntax to these typed shapes.