Path: utzoo!attcan!uunet!ogicse!ucsd!ucbvax!MITCH.ENG.SUN.COM!wmb
From: wmb@MITCH.ENG.SUN.COM
Newsgroups: comp.lang.forth
Subject: Re: interpreted control structures
Message-ID: <9009271317.AA13012@ucbvax.Berkeley.EDU>
Date: 27 Sep 90 03:14:06 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: wmb%MITCH.ENG.SUN.COM@SCFVM.GSFC.NASA.GOV
Organization: The Internet
Lines: 31

> Also to others who have used or created interpreted control structures:
>
> Would the following work:
>
>       CREATE crc-table  255 0 DO  I CRC-LOOKUP ,  LOOP
>
> If I have interpreted control structures, I would like to use them at compile
> time to initialize tables.  Are there systems that allow this?  And what are
> the complexities/tradeoffs involved?

The code that I published in the FORML proceedings did not allow this,
but my current implementation allows it, and the code the John Hayes
published in a recent Forth Dimensions allows it also.

To make it work, you have to switch the dictionary pointer to a separate
area (usually called a "compile buffer") while compiling the temporary
definition, and then switch the DP back to its original place just before
executing the temporary definition.

The tradeoffs are:

  1) The size of the compile buffer limits the maximum size of a temporary
     definition.

  2) You have to be careful that your ABORT mechanism puts the DP back to
     the right place if an ABORT happens during temporary compilation.
     (This is a good idea even if you don't use a separate compile buffer).

The compile buffer idea is very old.

Mitch Bradley, wmb@Eng.Sun.COM