A lua metatable demo.

set.lua

Set = {}

-- create a new set with the values of the
-- given list

local mt = {}


function Set.new(l)
  local set = {}
	setmetatable(set, mt)
	for _, v in ipairs(l) do set[v] = true end
	return set
end

function Set.union( a, b )
	if getmetatable(a) ~= mt or getmetatable(b) ~= mt then
		error("attempt to 'add' a set with a non-set value", 2)
	end
	local res = Set.new{}
	for k in pairs(a) do res[k] = true end
	for k in pairs(b) do res[k] = true end
	return res
end

function Set.intersection( a, b )
	local res = Set.new{}
	for k in pairs(a) do
		res[k] = b[k]
	end
	return res
end

function Set.tostring( set )
	local l = {}
	for e in pairs(set) do
		l[#l + 1] = e
	end	
	return "{" .. table.concat( l, ", ") .. "}"
end

function Set.print ( s )
	print(Set.tostring(s))
end


mt.__add = Set.union
mt.__mul = Set.intersection
mt.__le  = function ( a, b )	-- set containment
	for k in pairs(a) do
		if not b[k] then return false end
	end
	return true
end
mt.__lt  = function ( a, b )
	return a <= b and not (b <= a)
end

mt.__eq = function ( a, b )
	return a <=b and b <= a
end

s1 = Set.new{10, 20, 30, 50}
s2 = Set.new{30, 1}
print(getmetatable(s1))
print(getmetatable(s2))


s3 = s1 + s2
Set.print(s3)
Set.print((s1+s2)*s1)


s4 = Set.new{2, 4}
s5 = Set.new{4, 10, 2}

print(s4 <= s5)
print(s4 < s5)
print(s4 >= s4)
print(s4 > s4)
print(s4 == s5*s4)

s = {1, 2, 3, 4}
t = {1, 2, 3}
for k, v in pairs(s) do
	print (t[k])
	if t[k] then 
		print "yes"
	else
		print "no"
	end
end