Path: utzoo!attcan!uunet!cs.utexas.edu!uwm.edu!gem.mps.ohio-state.edu!pacific.mps.ohio-state.edu!tut.cis.ohio-state.edu!att!cbnewsc!lgm
From: lgm@cbnewsc.ATT.COM (lawrence.g.mayka)
Newsgroups: comp.lang.lisp
Subject: Incremental worlds, on conventional processors
Keywords: layered saved images, disk space, Poplog, Harlequin, Lucid, Franz
Message-ID: <3391@cbnewsc.ATT.COM>
Date: 22 Sep 89 14:06:42 GMT
Organization: AT&T Bell Laboratories
Lines: 26


I am wondering about the Lisp system feature known as "incremental
worlds" (Symbolics) or "layered saved images" (Poplog).  For those
who haven't heard of it, this feature enables the programmer to
load a Lisp environment, make all manner of changes (e.g., defining
new functions, building data structures), then save *only those
changes* to a file.  Later, the programmer can bring up a Lisp
environment that includes that file, thereby exactly re-creating
the environment's state at the time of the save.  The primary
purpose of making an incremental world instead of a complete world
(i.e., an entire saved image) is to save disk space (so that
programmers need not feel guilty about saving a large number of
worlds).

What I would like to know is, Why is this feature not more common?
I used to assume that special hardware a la Symbolics was required,
but I've just found out that Poplog Common Lisp seems to be able
to pull it off on conventional processors.  But if I understand
correctly, Harlequin, Lucid, and Franz (I now consider these the
"Big Three" among Lisp implementations on Suns) don't have this
capability.  Perhaps they'll consider it in the future?


	Lawrence G. Mayka
	AT&T Bell Laboratories
	lgm@ihlpf.att.com