15.6.2.1909.20 NAME>INTERPRET name-to-interpret TOOLS EXT

For a standard program, this is usually a restriction rather than an entitlement: It has to cope with the possibility of NAME>INTERPRET returning 0. Anyway, here is an example of how to deal with that: A user-defined text interpreter:

... parse-name 2dup find-name if nip nip
  state @ if
    name>compile
  else
    name>interpret dup 0= -14 and throw
  then
  execute
else
  ... deal with numbers
then

My point is that this would only be useful in a system that has some way to explicitly mark definitions as having no interpretation semantics.

The standard defines words with undefined interpretation semantics, and some systems implement this in some way; that's the reason for allowing NAME>INTERPRET to return 0. If you know your system never returns 0 from NAME>INTERPRET, you do not need to write the program in that way, but for portable programs, you have to take this possibility into account.

I agree, the possibility of the zero result in name>interpret is only a restriction for programs (see also my rationale in comment #1111). But what does this possibility provide for systems? Nothing! Because in any case a system can have an internal word that returns xt|0 if it needs that.

It seems, a better choice was to make name>interpret always return some xt.

Some systems (e.g., Gforth-0.7) implement words (e.g., if) without interpretation semantics; what should these systems do when the nt of if is passed to name>interpret? The standard says that it returns 0. If it did not say that, what else should it say for that case?

If a system implements interpretation semantics for if (e.g., Gforth-1.0, which makes them the same as compilation semantics), name>interpret of course returns an xt for those interpretation semantics.

So, as a program, you cannot use name>interpret to detect that the standard does not define the interpretation semantics, but you can use it to detect that the system at hand does not define interpretation semantics. What it does with that information is up to the program; in the text interpreter example above a -14 throw was performed, but, e.g., an implementation of find will probably not do that.

Some systems (e.g., Gforth-0.7) implement words (e.g., if) without interpretation semantics; what should these systems do when the nt of if is passed to name>interpret? The standard says that it returns 0.

The standard says "If nt has no interpretation semantics, NAME>INTERPRET returns 0". This is an unclear formulation, since it's impossible for a Forth system not to have interpretation semantics for a word (see below); and if one means the standard, one should say "interpretation semantics are not defined by this standard".

Anyway, the standard allows to return 0 in this case, but it also allows to return an xt of a system-specific behavior.

If it did not say that, what else should it say for that case?

I think the standard could say that in this case name>interpret shall return an xt for a system-defined behavior (the system anyway shows some behavior).

An example of wording (based on my proposal 1110):

• xt identifies the execution semantics of the word identified by nt. When this xt is executed in interpretation state, the interpretation semantics for the word are performed. If interpretation semantics for the word are not defined by this standard, the system-defined interpretation semantics are performed.
  An ambiguous condition exists in any of the following conditions:
  • interpretation semantics for the word are not defined by this standard and xt is executed;
  • execution semantics of the word are not defined by this standard and xt is executed in compilation state;

What does Gforth-0.7 do when its text interpreter encounters if in interpretation state? This behavior is the implemented interpretation semantics for if. And name>interpret may return an xt that shows this behavior in interpretation state, also it may return [: -14 throw ;] regardless what behavior is shown.

And no standard program can have any problem due to this behavior of name>interpret. Right?

but you can use it to detect that the system at hand does not define interpretation semantics.

I would say it does not define an execution token for the word (and for that semantics). Because some interpretation semantics are actually defined by the system anyway (for example, it shows an error message and does abort).

What it does with that information is up to the program; in the text interpreter example above a -14 throw was performed, but, e.g., an implementation of find will probably not do that.

find in interpretation state may return the same xt that name>interpret returns.

If you want to propose words that are not standardized, the way to do it is to write a proposal. But note that words for which there is little common practice are very unlikely to become standard eventually.

So the first thing you should do is to look at common practice. Gforth (development version) has words that work on the most recent definition:

• SET-EXECUTE ( ca -- ), but it takes a machine-code address and is unlikely to be standardizable.

• SET-DOES> ( xt -- ), which changes the most recent definition like DOES> does.

• DEFINER! ( definer xt -- ), but it is obsolete (will be removed in a future version of Gforth); it goes with >DEFINER ( xt -- definer ) for getting a definer.

• SET-TO ( xt -- ), but it takes the xt of a to-table that you have to define with TO-TABLE: (for also supporting +TO etc.). These words are still under discussion among Gforth developers.

You can make a definition "most recent" with MAKE-LATEST, but note that if you change a definition after it has already been used, the behaviour coming out of earlier uses may or may not change. For the thinking behind these words, you may want to read The new Gforth Header and/or watch the video.

VFX has a word for defining the interpretation semantics, but AFAIK not for TO.

For other Forth systems I am not aware that they have words for this kind of stuff, and it probably requires significant changes to add something like SET-TO (probably in VFX, too).

Thank you Anton. I've thought proposals, but as you said I'd need to check common practice, plus I want to make sure I'm not talking nonsense.

I've implemented this in my (Scheme-based) Forth, and it appears to ... just work. I kind of like it, as it allows the users to define TO-handlers in pure FORTH:

\ NAME>TO ( nt -- to-xt )  \ implementation-specific
\ NAME<TO ( to-xt nt -- )  \ implementation-specific, see do_to_value below for to-xt stack signature
: TO
  PARSE-NAME DUP 0= IF -16 THROW THEN
  FIND-NAME  DUP 0= IF -13 THROW THEN
  DUP NAME>TO  \ ( nt to-xt -- )
  STATE @ IF SWAP POSTPONE LITERAL COMPILE, ELSE EXECUTE THEN ;
IMMEDIATE

: do_to_value ( x nt -- ) NAME>INTERPRET >BODY ! ;
: VALUE >IN @ PARSE-NAME 2>R
              >IN ! CREATE , 
              ['] do_to_value 2R> FIND-NAME NAME<TO
              DOES> @ ;

(sorry for spamming an unrelated word... this should have been in the comment section for TO, but I don't know how to move it)