Re: single-xt approach in the standard

On 2024-09-18 03:04, Stephen Pelc wrote:

On 17 Sep 2024 at 21:12:08 CEST, "Anthony Howe" <achowe@snert.com> wrote:
 

On 2024-09-17 06:54, Ruvim wrote:

Do you think that the Forth standard should recognize the classic single-xt
approach as possible for implementing a standard Forth system?
>
The classic single-xt approach implies that only one execution token (xt) is
associated with a name token (nt), and only one name token is associated with a
word (a named Forth definition). And words whose compilation semantics differ
form default compilation semantics are implemented as immediate words.

>
Given I prefer single-xt (currently), what I don't get is how for a dual-xt
system you define a single word with two actions for interpret and compile. I
don't see a word to distinguish between the two, except maybe IMMEDIATE.

 VFX Forth 64 is a single xt system (with sub-xts) .

If VFX Forth allows to obtain sub-xts from the main xt, then FVX Forth effectively has multiple xts associated with an nt.
Moreover, in VFX Forth, the execution semantics of `FIND` depends on STATE (at least, for some arguments). Therefore, VFX Forth does not follow the classic single-xt approach.

It manages to separate
compilation  and interpret actions for a word cleanly. S" is defined as below:
 : S"            \ Comp: "ccc<quote>" -- ; Run: -- c-addr u      6.1.2165
\ *G Describe a string. Text is taken up to the next double-quote
\ ** character. The address and length of the string are returned.
   [char] " parse >syspad
;
ndcs: ( -- )  discard-sinline  compile (s") ",  ;
 NDCS: describes the non-default compilation sematics of the word.

This implementation is not standard compliant, because `compile,`, which shall has stack effect ( xt -- ), consumes and produces some other stack parameters for some xts, e.g.
   ' if     compile, \ actual effect( xt -- x )
   ' then   compile, \ actual effect ( x xt -- )
Or, in a standard program:
   : test-compile(
     state @ 0= -14 and throw
     bl word find if compile, else -13 throw then
     parse-name s" )" compare abort" ')' expected"
   ; immediate
   : foo test-compile( if ) ;
   \ Err# -22 ERR: Control structure mismatch.
--
Ruvim