foo.lua

local closureCache = setmetatable({}, {__mode="k"}) -- weak keys
function builtins.create_closure(obj, funcName)
	-- See if we have a closure cache for this object already
	local objClosures = closureCache[obj]
	if objClosures == nil then
		-- No cache for this object; create one
		objClosures = {}
		closureCache[obj] = objClosures
	end

	-- See if we have a closure created for this obj+funcName already
	local f = objClosures[funcName]
	if f ~= nil then
		return f
	end

	-- No closure created; create a new one
	f = function(...)
		return obj[funcName](obj, ...)
	end

	-- Store the new closure in the cache
	objClosures[funcName] = f
	return f
end

Example usage:

local typ = {
    Say = function(self, msg)
        print(self, "says", msg)
    end
}

-- obj has type "typ"
local obj = setmetatable({}, {__index=typ})

local objSay1 = builtins.create_closure(obj, "Say")
local objSay2 = builtins.create_closure(obj, "Say")
assert objSay1 == objSay2