Trinary logic

trinary_logic.lua

-- Set boolean metatable
debug.setmetatable(false, {
    __add = function(a, b)
        if b == nil then
            return b + a
        end
        return a or b
    end,
    __mul = function(a, b)
        if b == nil then
            return b * a
        end
        return a and b
    end,
    __unm = function(a)
        return not a
    end
})

-- Set nil metatable
debug.setmetatable(nil, {
    __add = function(a, b)
        if b == true then
            return true
        end
        return nil
    end,
    __mul = function(a, b)
        if b == false then
            return false
        end
        return nil
    end,
    __unm = function()
        return nil
    end
})

local neg = {
    {true, false},
    {false, true},
    {nil, nil}
}

for _, t in ipairs(neg) do
    local a, e = unpack(t)
    assert(-a == e)
end

local add = {
    {true, true, true},
    {true, false, true},
    {true, nil, true},

    {false, true, true},
    {false, false, false},
    {false, nil, nil},

    {nil, true, true},
    {nil, false, nil},
    {nil, nil, nil},
}

local mul = {
    {true, true, true},
    {true, false, false},
    {true, nil, nil},

    {false, true, false},
    {false, false, false},
    {false, nil, false},

    {nil, true, nil},
    {nil, false, false},
    {nil, nil, nil},
}

for i = 1, 9 do
    do
        local a, b, e = unpack(add[i])
        assert(a + b == e)
    end
    do
        local a, b, e = unpack(mul[i])
        assert(a * b == e)
    end
end