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From: dxf <dxforth@gmail.com>
Newsgroups: comp.lang.forth
Subject: Re: QUIT and ABORT
Date: Mon, 5 May 2025 12:15:14 +1000
Organization: i2pn2 (i2pn.org)
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On 4/05/2025 11:33 pm, Anton Ertl wrote:
> dxf <dxforth@gmail.com> writes:
>> On 4/05/2025 2:02 am, Anton Ertl wrote:
>>> dxf <dxforth@gmail.com> writes:
>>>> Checking the doc it says:
>>>>
>>>>  9.3.5 Possible actions on an ambiguous condition
>>>>
>>>>  A system choosing to execute THROW when detecting one of the ambiguous
>>>>  conditions listed in table 9.3.6 shall use the throw code listed there.
>>>
>>> Calling ABORT or QUIT is not an ambiguous condition, so that section
>>> plays no role there.
>>
>> Yet ABORT ABORT" and QUIT are present in the table and assigned codes.
> 
> For ABORT the section 9.6.2.0670 specifies that it performs a -1
> THROW.  While -1 happens to be the value shown in table 9.1 as being
> reserved for ABORT, there is no ambiguous condition involved here.
> Likewise for -2, ABORT" and section 9.6.2.0680.

If and only if EXCEPTION EXT is implemented are those mandatory.  Otherwise
they are optionally CATCHable at the implementer's discretion per section
9.3.5 above.

> So table 9.1 is useful beyond the purpose of assigning throw codes to
> ambiguous conditions.  However, for QUIT there is no redefinition in
> chapter 9.

Again only because ANS hadn't specified QUIT as mandatory.  For all I know
it was because it would have killed the QUIT-based exception handler ANS
offered.  To not include QUIT in Table 9.2 would have raised questions as
to why not.

>> It is clear to me all three are candidates for CATCHing and that this
>> was the intent.
> 
> It may have been the intent of adding the -56 throw code to also have
> a corresponding redefinition of QUIT in Chapter 9, but they did not
> follow through on it.  Maybe because -56 THROW changes the data stack
> depth while QUIT does not.

That's a bundle of maybe's.  I believe I've given an explanation that
requires no suppositions.

>> This is confirmed by the EXCEPTION EXT wordset in which
>> ABORT and ABORT" are required to be CATCHable irrespective of their
>> category.
> 
> Category?

ABORT ABORT" QUIT not being exceptions
 
> A system that implements 9.6.2.0670 makes ABORT catchable, and a
> system that implements 9.6.2.0680 makes ABORT" catchable.  I find it
> perverse that these words are in Exception ext, so you could have a
> system that has CATCH, but an uncatchable ABORT and/or an uncatchable
> ABORT".  And for QUIT, the standard as it is does not have a catchable
> variant at all.

In fact all of the conditions listed in Table 9.2 are optional.  Only
CATCH and THROW are mandatory.  I read that to mean a standard program
can only rely on positive codes it has specified itself.
 
>>>>  : QUIT -56 THROW ;
>>>
>>> That would not be a standard system, because in a standard system QUIT
>>> must do what 6.1.2050 QUIT says, and "-56 THROW" is not a correct
>>> implementation of that.
>>
>> No for the fact QUIT is CATCHable by virtue of its inclusion in the table
>> of codes in Section 9.
> 
> There is nothing in the standard that makes such a connection (unlike
> for ABORT and ABORT").  I don't know for what purpose -56 was added to
> the table, > but I know (and we have the proposal text) for what purpose
> -59 (ALLOCATE) was added to the table, and it was not with the intent
> of redefining ALLOCATE as
> 
> : ALLOCATE -59 THROW ;
> 
> So obviously it's just your interpretation of the table that if the
> description of a throw code C contains a word name W, systems
> implementing the exception wordset are free to redefine W as throwing
> C.

Again it's an interpretation that fits what is present in ANS.

> ...