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

[Scott Fial <scott@...>]

Right now the default value of a table is nil.  It is the one value that 
can never be *in* a table.  Reading a non-existent key returns this 
value . Assigning this value to a key removes it from the table.

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.

It would be generally useful as a convenience.  Plus if I were to set 
the default value of a table to some atom, I could make an "array" which 
could contain any other lua value, including nil.

Of course, you'd now have to test return values against a table's 
default value rather than nil

  function exists(t, key) return t[k] ~= getdefault(t) end

Calling setdefault() on a non-empty table would raise an error.

Its a half-baked idea.  I haven't really thought through the 
implications.  Is it just trading one problem for another?  Could it be 
implemented efficiently?  But I thought maybe it could lead somewhere.

Scott