Re: opaque type proposal

[gls@Think.COM]

   Date: Sat, 4 Jun 88 00:20:23 edt
   From: Juan Loaiza <jrl@vx.lcs.mit.edu>



	 You still need the length check in order to make sure that the slot containing
	 the ``tag'' object actually exists before trying to fetch its value.

		   Pavel

   Think again about the trick I mentioned.  It is a dirty, underhanded,
   shameful, slimy trick; but I think it works.  If a token for a type T
   is only ever present in the N'th slot of an object of type T, then
   looking at the N'th slot of any other object will never get you that
   token.  It does not matter how many slots the other object has.  If it
   has less than N slots you, will get a random slot out of another
   object, but never the N'th slot (assuming no objects have zero
   length).

   For our young listeners out there, please don't try this trick at home.

Some care is needed near the end of memory, to make sure that the start of
the last object is as far from the end of the legitimate address space as
the greatest distance of any tag from the origin of its object.  Ugh,
bletch.  Also, having any kind of "raw array" representation could screw
things up.  Otherwise, it's a great, wonderful, slimy crock.
Congratulations.
--Guy