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From: wmb@MITCH.ENG.SUN.COM (Mitch Bradley)
Newsgroups: comp.lang.forth
Subject: TEXIT
Message-ID: <9102122127.AA04970@ucbvax.Berkeley.EDU>
Date: 12 Feb 91 19:09:12 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: Mitch Bradley <wmb%MITCH.ENG.SUN.COM@SCFVM.GSFC.NASA.GOV>
Organization: The Internet
Lines: 39

> Here's something else which (to my mind) cannot be implemented _efficiently_
> in a portable way using ANS Forth.  Even the inefficient ways are difficult.
>
> : ||   IF R> DROP -1 THEN ;     ( t - t)  ( f - )  ( exit on true)
> : &&   0= IF R> DROP 0 THEN ;   ( t - )  ( f - f)  ( exit on false)

: ||
   POSTPONE DUP  POSTPONE IF  POSTPONE EXIT  POSTPONE THEN
; IMMEDIATE
: &&
   POSTPONE DUP  POSTPONE 0=  POSTPONE IF  POSTPONE EXIT  POSTPONE THEN
; IMMEDIATE

This is efficient in terms of speed, but inefficient in terms of space.
I don't think it is particularly difficult.

The problem with R> DROP is that it implies detailed knowledge of how
the return stack is used for colon definitions.  For some Forth hardware
and some optimizing compilers, the assumptions made by R> DROP are
violated.  This is an existing problem that ANS Forth did not create;
it simply chose not invalidate those existing Forth implementations
by requiring something they could not provide.

It will still be possible to use R> DROP on most ANS Forth implementations.
From a de-facto standpoint, it will remain about as portable as it has
ever been (i.e. reasonably portable).

It is possible to use the portable implementations shown above for
the "completely-standard" version of your application, and substitute
the not-quite-portable versions when running on systems on which they
work.  This is not a new technique; people do it all the time with
CODE word "accelerators" for portable high-level definitions.

By the way, I sometimes use CATCH and THROW to implement multiple
"bail-out" points, which is what it looks like || and && are trying to
solve.  CATCH and THROW can be compiled inside colon definitions to
make new custom tools like the above.

Mitch