Re: new "empty" value/type in Lua?

[Scott Fial <scott@...>]

On 7/3/2013 9:46 PM, Scott Fial wrote:
> What if there was a formal way to set/get the internal "default value"
> of a table?  For example, if I were to call setdefault(t, -1), then t
> could contain any value except -1.  Reading a non-existent key would now
> return -1.  Assigning -1 to a key would delete it.
> Scott

Continuing this thought.  For convenience, "table" could provide the 
basic functions and a pre-made "undef" atom for general use as an 
alternative default:

  function table.getdefault(t) --[[ magic ]] end

  function table.setdefault(t, d) --[[ magic ]] end

  function table.has(t, k) return t[k] ~= table.getdefault(t) end

  function table.delete(t, k) t[k] = table.getdefault(t) end

  table.undef = {__tostring = function () return "undef" end}
  setmetatable(table.undef, table.undef)


Scott