a little modification of https://gist.github.com/ildar/e5e9676f62a6b96ef2ac7b81be74ca12

traits2.tl

local record Sheep
    _naked: boolean
    _name: string
    __index: Sheep
end
Sheep.__index = Sheep

-- impl Sheep
function Sheep:shear()
    print(self._name, "gets a haircut!");

    self._naked = true;
end

function Sheep:create(x: Sheep): self
    return setmetatable(x, { __index = Sheep })
end

-- fn main()
local dolly = Sheep:create { _name = "Dolly" }

dolly:shear();

traits3.lua

local interface Object is self.__name
    __index: self
    __name: string
end

local record Sheep is Object where self.__name == "Sheep"
    _naked: boolean
    _name: string
end
Sheep.__index = Sheep
Sheep.__name = "Sheep"

-- impl Sheep
function Sheep:shear()
    print(self._name, "gets a haircut!");

    self._naked = true;
end

function Sheep:create(x: Sheep): self
    x.__name = "Sheep"
    return setmetatable(x, { __index = Sheep })
end

-- fn main()
local dolly = Sheep:create { _name = "Dolly" }

dolly:shear();