From the article "String interpolation in Lua" ( http://hisham.hm/2016/01/04/string-interpolation-in-lua ).

string-interpolation.lua

function f(str)
  local outer_env = _ENV

  return (str:gsub("%b{}", function(block)
    local code = block:match("{(.*)}")
    local exp_env = {}

    setmetatable(exp_env, { __index = function(_, k)
      local stack_level = 5

      while debug.getinfo(stack_level, "") ~= nil do
        local i = 1

        repeat
          local name, value = debug.getlocal(stack_level, i)

          if name == k then
            return value
          end

          i = i + 1
        until name == nil

        stack_level = stack_level + 1
      end

      return rawget(outer_env, k)
    end })

    local fn, err = load("return "..code, "expression `"..code.."`", "t", exp_env)

    if fn then
      return tostring(fn())
    else
      error(err, 0)
    end
  end))
end