interpolation.lua

local function locals(n)
  n = n + 1
  local result = {}
  for i=1,math.huge do
    name, value = debug.getlocal(n, i)
    if not name then break end
    result[name] = {value}
  end
  return result
end

local function quote(str)
  return "'"..str:gsub("\\", "\\\\"):gsub("'", "\\'").."'"
end

local function I(str)
  local locals = locals(2)
  local env = setmetatable({}, {__index = function(self, idx)
    if locals[idx] then
      return locals[idx][1]
    else
      return _ENV[idx]
    end
  end})

  -- For simplicity, only finds a single interpolation for now

  local buffer = {}

  local start = 1
  while true do
    local a = str:find('#{', start)
    local b = str:find('}', a)
    if not a or not b then
      table.insert(buffer, quote(str:sub(start, -1)))
      break
    end
    table.insert(buffer, quote(str:sub(start, a-1)))
    table.insert(buffer, "tostring("..str:sub(a+2, b-1)..")")
    start = b+1
  end

  return load("return "..table.concat(buffer, ".."), "interp", "bt", env)()
end

local x = 100
print(I"Hello world #{x} :)")