Teal implementation of https://doc.rust-lang.org/rust-by-example/trait.html

traits.tl

local interface Animal
    -- Associated function signature; `Self` refers to the implementor type.
    new: function(string): self

    -- Method signatures; these will return a string.
    name: function(self): string
    noise: function(self): string

    talk: function(self)
end

local record Sheep is Animal
    _naked: boolean
    _name: string
end

-- impl Sheep
function Sheep:is_naked(): boolean
    return self._naked
end

function Sheep:shear()
    if self:is_naked() then
        -- Implementor methods can use the implementor's trait methods.
        print(self:name(), "is already naked...");
    else
        print(self._name, "gets a haircut!");

        self._naked = true;
    end
end

-- Implement the `Animal` trait for `Sheep`.
-- impl Animal for Sheep
    -- `Self` is the implementor type: `Sheep`.
function Sheep.new(name: string): Sheep
    local res: Sheep = { _name=name, _naked=false }
    return setmetatable(res, {__index=Sheep})
end

function Sheep:name(): string
    return self._name
end

function Sheep:noise(): string
    if self:is_naked() then
        return "baaaaah?"
    else
        return "baaaaah!"
    end
end

function Sheep:talk()
    -- For example, we can add some quiet contemplation.
    print(self._name, "pauses briefly... ", self:noise());
end


-- fn main()

-- Type annotation is necessary in this case.
local dolly = Sheep.new("Dolly")

dolly:talk();
dolly:shear();
dolly:talk();