Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!rpi!leah!bingvaxu!kym
From: kym@bingvaxu.cc.binghamton.edu (R. Kym Horsell)
Newsgroups: comp.lang.prolog
Subject: Re: Arrays in Prolog
Message-ID: <3996@bingvaxu.cc.binghamton.edu>
Date: 13 Sep 90 04:26:36 GMT
References: <90239.175243SCHMIED@DB0TUI11.BITNET> <1990Sep11.150245.26833@sics.se> <1990Sep12.170553.14414@cl.cam.ac.uk>
Reply-To: kym@bingvaxu.cc.binghamton.edu.cc.binghamton.edu (R. Kym Horsell)
Organization: SUNY Binghamton, NY
Lines: 22

In article <1990Sep12.170553.14414@cl.cam.ac.uk> ksh@cl.cam.ac.uk (Kish Shen) writes:
\\\
>Perhaps I am showing my ignorance, but can you not have a "special trail" to 
>store the those trailing that need value trailing? This is what I am doing for
>my own work with implementing dependent and-parallelism in Prolog -- I have a
>special trail which  grows in the opposite direction to the normal
>trail, towards
>the normal trail. This means that you only pay for the cost of value
>trailing for
>those variables that need to use it. Am I missing something? 
\\\

No you're not.

Although the trail _can_ grow to twice the size using naive value
trailing the cases where values _are_ trailed (and I don't have any
figures on this to hand on how _often_ this happens) can be ``special 
cased''. I prefer the using a single bit in the trail entries (you come 
across them during backtracking in sequence anyway, right)? Probably 
not too portable `tho.

-Kym Horsell