Re: "Back & Forth" - Local variables

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.