bug#32793: 27.0.50; json-parse-string doesn't have the equivalent of json.el's json-array-type

[Dmitry Gutov]

On 12.04.2019 12:22, Eli Zaretskii wrote:

> Can't say I like this conversion of Lisp symbols into C enumerations.
> I'd rather we used symbols (Qarray, Qlist, etc.), but I can understand
> why you did this as json.c already did for the other keyword values.

I vaguely suspect it might help with performance. *shrug* Or not.

> > @@ -521,7 +527,7 @@ static void
> >   json_parse_args (ptrdiff_t nargs,
> >                    Lisp_Object *args,
> >                    struct json_configuration *conf,
> > -                 bool configure_object_type)
> > +                 bool configure_types)
>
> If we are renaming this argument, let's do a better job: I think its
> name should have been parse_object_types.

OK.

> Come to think of this: why do we need this boolean at all?  The
> callers which don't want :object-type parsed will ignore the result
> anyway, so it sounds like something we could just toss.

I'd rather we didn't accept argument we cannot handle, to avoid false 
expectations.

For example with this patch we can parse a JSON array into a Lisp list.

But there's no way to serialize a list back into a JSON array, yet.

I looked at implementing it, just for completeness (it's not necessary 
for my use case, for now). But there's an ambiguity between lists and 
alists which is not trivial to solve (i.e. when object-type is alist and 
array-type is list, how do we determinine, quickly and reliably, that a 
given list is actually an alist?). So maybe leave it for the future.

> > +          case json_array_list:
> > +            {
> > +              result = Qnil;
> > +              for (ptrdiff_t i = 0; i < size; ++i)
> > +                result = Fcons (json_to_lisp (json_array_get (json, i), conf),
> > +                                result);
> > +              result = Fnreverse (result);
>
> If you cons the list back to front, you can avoid the Fnreverse call,
> which will make this faster.

Done. No real performance impact that I can see, but it didn't hurt either.

> Also, please insert a call to rarely_quit into the loop, as JSON
> vectors could be quite large, AFAIU.

Also done. In the json_array_array case as well, where it was missing, 
it seems.

> Finally, this needs documentation update: the doc strings of
> json-parse-string and json-parse-buffer, NEWS, and the ELisp manual.

json-parse-buffer and NEWS don't need updating, I think.

Otherwise, done, see the new patch.

json-array-type.diff
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