Lua nil-condition support module for Defold game engine

--[[

 nil-condition support module by alprog.
 Provide two form of expression syntax. First one is more readable
 but use debug.getlocal and may be unacceptable for some reason

 Unsafe call:         Safe call (form 1):       Safe call (form 2):

 a.b.c.d              __.a.b.c.d.__             __(a).b.c.d.__
 foo:bar(1, 2, 3)     __.foo:bar(1, 2, 3).__    __(foo):bar(1, 2, 3).__
 a.b:c('text').d.e    __.a.b:c('text').d.e.__   __(a).b:c('text').d.e.__

 if such expressions acts as single call without using returned value
  the closing tag is not written:

 __.foo:bar(1, 2, 3)
 __(foo):bar(1, 2, 3)

  also this form may be convenient:

 Wrap(foo):bar(1, 2, 3)

--]]

local Wrapper = {}

function Wrapper:__index(key)
  local object = rawget(self, 'object')
  if key == '__' then
    return object -- unwrap
  end

  if object then
    self.parent = object
    self.object = object[key]
  end
  return self
end

function Wrapper:__call(arg0, ...)
  local object = rawget(self, 'object')
  if object then
    if arg0 == self then
      local parent = rawget(self, 'parent')
      self.parent = object
      self.object = object(parent, ...)
    else
      self.parent = object
      self.object = object(arg0, ...)
    end
  end
  return self
end

function Wrap(object)
  local wrapper = { object = object }
  setmetatable(wrapper, Wrapper)
  return wrapper
end

local NilCondition = {}

function NilCondition:__index(key)
  local index = 1
  while true do
    local name, value = debug.getlocal(2, index)
    if name then
      if name == key then
        return Wrap(value)
      end
    else
      break
    end
    index = index + 1
  end
  return _G[key]
end

function NilCondition:__call(object)
  return Wrap(object)
end

_G['__'] = {}
setmetatable(__, NilCondition)