gistfile1.lua

local _mtg = getmetatable(_G) or {}
function _mtg.__index(self, k)
	return rawget(self,k.."___macro")
end
setmetatable(_G,_mtg)

function string.escape(text)
	local str = ""
	for i=1, #text do
		local chr = text:sub(i,i)
		local escape = ""
		if isIn(chr, {"(", ")"}) then escape = "%" end
		str = str .. escape..chr
	end
	return str
end

function isIn(obj, table)
	for _,v in ipairs(table) do
		if obj == v then return true end
	end
end

function get_locals(func)
     local n = 1
     local locals = {}
     func = (type(func) == "number") and func + 1 or func
     while true do
          local lname, lvalue = debug.getlocal(func, n)
          if lname == nil then break end
          if lvalue == nil then lvalue = mynil end
          locals[lname] = lvalue
          n = n + 1
     end
     return locals
end

Macro = setmetatable({}, {__call=function(self, ...) return self:init(...) end})
function Macro:init(...)
	--We want to retain processing capability even after declaration
	local args = {...}
	local __string = ""
	for _,obj in ipairs(args) do
		__string = __string .. tostring(obj)
	end
	local members = {
		__string = __string,
		type="Macro"
	}
	local methods = {
		__tostring__ = function(self, n)
			--Actual Processing Logic goes here
			local str = self.__string
			local match = "${.-[^\\]}"
			--Check if str has ${statement}
			local indices = {str:find(match)}
			while #indices > 0 do
				local sub = str:sub(indices[1]+2, indices[2]-1)
				local repl = str:sub(unpack(indices))
				local final = ""
				local locals = get_locals(3+n)
				for k,_ in pairs(locals) do
					rawset(_G,k.."___macro",_)
				end
				--Evaluate
				local _fn, e = loadstring(sub)
				if not _fn then
					_fn, e = loadstring("return "..sub)
				end
				if not _fn then
					final = " "..e.." "
				else
					final = tostring(_fn() or " ")
				end
				for k,_ in pairs(locals) do
					rawset(_G,k.."___macro",nil)
				end
				str = str:gsub(repl:escape(), final)
				indices = {str:find(match)}
			end
			return str
		end,
		__tostring = function(self)
			return getmetatable(self).__tostring__(self, 2)
		end,
		__call = function(self)
			return getmetatable(self).__tostring__(self, 0)
		end
	}
	return setmetatable(members, methods)
end

test = Macro "Hello ${jack} ${'Bauer'}"

function test1()
	local jack = "Jack"
	print(test)
end

test1()

function test2()
	local jack = "No One"
	print(test)
end

test2()

function test3()
	local test = Macro "Hello ${world}"
	local world = "World"
	print(test())
end

test3()

function test4()
	local test = Macro "Hello ${print('asdf'); return 'World'}"
	local world = "World"
	print(test)
end

test4()