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From: dxf <dxforth@gmail.com>
Newsgroups: comp.lang.forth
Subject: Re: "Back & Forth" - Local variables
Date: Fri, 10 Jan 2025 10:54:42 +1100
Organization: i2pn2 (i2pn.org)
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On 10/01/2025 3:15 am, Hans Bezemer wrote:
> On 09-01-2025 13:42, dxf wrote:
>>> There is no guarantee that a saved interpreter pointer on the
>>> stack is an execution token.

Albert said that.  He's referring to the use of EXECUTE.  What surprised me
is half the forths on my desktop worked (though not DX-Forth). 

> 
> Nope - in ANS-Forth it is listed as:
> 
> nest-sys; definition calls; implementation dependent
> 
> So - that's obvious. But in 4tH it works out. And defining it as >R works out as well. BTW, I've tested the thing - and it holds up.
> 
> I got my work cut out for a next episode! On co-routines! ;-)

Great.  While I've seen co-routines mentioned, examples were rare so I
tended to ignore it.

> 
> Hans Bezemer
> 
> BTW, I've heard there are implementations where nest-sys aren't even on the return stack. The standard seems to confirm this:
> 
> return stack:  A stack that _MAY_BE_ used for program execution nesting, do-loop execution, temporary storage, and other purposes.
> 
> .. and sorry to spoil the fun, but what we're doing here is illegal anyways:
> 
> "A program shall _NOT_ access values on the return stack (using R@, R>, 2R@ or 2R>) that it _DID_NOT_ place there using >R or 2>R;"
> 
> In other words: your mileage may (be) very, very illegal.

The >R dependency is on what was (and perhaps still is) 'common practice'.
I was unsure how it would fare but all the popular forths seem to work.
If ANS-FORTH has issued one too many 'Thou shalt not's and in doing so has
excluded itself then that's too bad.