bug#56799: (gnu services configuration) usage of *unspecified* is problematic

[bokr]

Hi,

On +2022-07-27 14:31:32 -0400, Maxim Cournoyer wrote:
> Hi,
> 
> Tobias Geerinckx-Rice <me@tobias.gr> writes:
> 
> > Hi Maxim,
> >
> > Maxim Cournoyer 写道：
> >> I'd suggest we revisit 8cb1a49a3998c39f315a4199b7d4a121a6d66449 to
> >> use
> >> 'unspecified (the symbol) instead of *unspecified*, which *can* be
> >> serialized without any fuss in gexps.
> >
> > Bah.  Could we provide our own reader?
> >
> > I'd much rather this be addressed in Guile (or failing that,
> > transparently by Guix) than have to deal with some magical
> > symbol. IIRC that was the argument for using *unspecified* in the
> > first place, and I think it makes sense.
> >
> > This looks more like an unexplored oversight than a well-reasoned
> > restriction to me.
> 
> This was my original impression, but thinking more about it, it became
> apparent that *unspecified* is well, unspecified and shouldn't be relied
> on by people to be something well defined.  For some background reading,
> see [0].  So it seems wrong in Scheme to actively set things to
> *unspecified*, and give a specific meaning to that.
>
> I think the semantic of the language is that it is to be used as the
> lack of a return value from a procedure or syntax, e.g.:
> 
> (unspecified? (if #f 'one-arm-if)) -> #t
> 
> Having 'unspecified?' even defined in Guile seems to go against that
> idea; perhaps because Wingo themselves seems to disagree in [0].
> 
> I'm also thinking 'unspecified being too close to *unspecified* is
> probably going to cause confusion down the line.  Reverting to the
> originally used 'disabled may be the lesser evil.
> 
> Other thoughts?
> 
> Thanks,
> 
> Maxim
> 
> [0]  
> https://scheme-reports.scheme-reports.narkive.com/QSQtJSAh/unspecified-values
> 
> 
>

Lots of systems are dealing with this issue, it seems, judging from
[1] https://en.wikipedia.org/wiki/Bottom_type

I think the problem is you really need a tuple to return both data and metadata
unambiguously from anything that produces a result (or not, which is a result).
Something like read-delimited with the 'split option, or using catch.

Personally, if I were designing a language :), my goal would be to have
nothing unspecified, and no undefined behaviour outside of physical failures ;-)

*unspecified* seems me like an ok word for the unasserted/high-impedance
state of tri-state memory address bus electronic logic,
but IMO the example above
--8<---------------cut here---------------start------------->8---
> (unspecified? (if #f 'one-arm-if)) -> #t
--8<---------------cut here---------------end--------------->8---

is not nice. (A nit, but For one thing "specified?" ought to be the question 
IMO,
if you are ging to have that concept, not "unspecified?" :)

What about using characters from some private upper unicode section
to represent various kinds of unspecified things? E.g.,
as guile named chars,

#\unspecified_function_retval
#\unspecified_function_error
#\unspecified_macro_err
#\unspecified_exception

#\nil or #\not_an_object -- or #\nao -- can't use #\n :)
#\paradox  -- e.g., (eval-nl-string "this sentence is lying")
#\nonsense -- e.g. when a question is based on false premises
             (eval-nl-string "Bob is bareheaded: Bob, is your hat too tight?")  
 

             Hm, one could argue that (+ "ab" "cd") could be based on the false
             premise that + was overloaded like
--8<---------------cut here---------------start------------->8---
scheme@(guile-user) [1]> (let*( (+ string-append) (sum (+ "ab" "cd"))) sum)
$8 = "abcd"
--8<---------------cut here---------------end--------------->8---
             and if it wasn't, should return #\nonsense :)
             (though maybe as part of the exception, which is practical for 
debugging etc :)
#\
#\guix_bottom -- a private unicode rather than U+22A5 which could be
                 returned as a valid character value by some functon.
--8<---------------cut here---------------start------------->8---
$ echo -en "\u22a5"|unicode-info
"⊥":
    glyph  codepoint .....int  name...
    _⊥_     +U0022a5     8869  UP TACK  
--8<---------------cut here---------------end--------------->8---

Well, hope you can extract something useful from the above :)

BTW, I didn't get far via the link [0] :(
--8<---------------cut here---------------start------------->8---
🤖 Hungry for data? 🤖

   As you guessed, this page is to confirm your affiliation to the human race.

   about - legalese

   Loading...
--8<---------------cut here---------------end--------------->8---
Ok machine, you identified me as human, and kept me out. Happy?
No, I know, machines can only fake that.
--
Regards,
Bengt Richter