an example of when metatables are useful

metatable_example.lua

ProcessorFlags = {
	Carry = false,    -- 0 | (C) Carry for arithmetic and compares
	Zero = false,     -- 1 | (Z) Set when operation results in 0
	IRQ = false,      -- 2 | (I) enable/disable IRQ flag
	Decimal = false,  -- 3 | (D) decimal mode
	Break = false,    -- 4 | (B) [only available in stack version]
	Unused = false,   -- 5 | (-) "not used" in regular 6502 (always 1?)
	Overflow = false, -- 6 | (V) Overflow for arithmetic and compares
	Sign = false,     -- 7 | (N) Negative value result
}
setmetatable(ProcessorFlags,{
	__call = function()
	-- combine the above flags into a single byte and return.
		local value = 0;
		value = value + (ProcessorFlags.Carry    and 0x01 or 0)
		value = value + (ProcessorFlags.Zero     and 0x02 or 0)
		value = value + (ProcessorFlags.IRQ      and 0x04 or 0)
		value = value + (ProcessorFlags.Decimal  and 0x08 or 0)
		value = value + (ProcessorFlags.Break    and 0x10 or 0)
		value = value + (ProcessorFlags.Unused   and 0x20 or 0)
		value = value + (ProcessorFlags.Overflow and 0x40 or 0)
		value = value + (ProcessorFlags.Sign     and 0x80 or 0)
		return value
	end,
	__tostring = function()
		return (ProcessorFlags.Sign and "N" or "n")..
			(ProcessorFlags.Overflow and "V" or "v")..
			(ProcessorFlags.Unused and "-" or "_")..
			(ProcessorFlags.Break and "B" or "b")..
			(ProcessorFlags.Decimal and "D" or "d")..
			(ProcessorFlags.IRQ and "I" or "i")..
			(ProcessorFlags.Zero and "Z" or "z")..
			(ProcessorFlags.Carry and "C" or "c")
	end,
})