bug#65680: cl-print-to-string-with-limit erroneously imposes a maximum print-length of 50

[Alan Mackenzie <acm@muc.de>]

Hello, Stefan.

On Thu, Sep 21, 2023 at 12:16:57 -0400, Stefan Monnier wrote:
> Alan Mackenzie [2023-09-01 14:17:12] wrote:

> > Hello, Emacs.

> > In cl-print-to-string-with-limit appears the following binding:

> >     (print-length (cond
> >                     ((null limit) nil)
> >                     ((eq limit t) print-length)
> >                     (t (min limit 50))))
> >                         ^^^^^^^^^^^^

> > ..  This has the effect of ignoring the parameter LIMIT (unless it is
> > very small) and instead truncating the printed size to 50.  There is a
> > similar mechanism to limit print-level to 8.

> > Although the doc string doesn't explicitly say it won't truncate like
> > this, it kind of implies that LIMIT is the size it will truncate to.
> > This excessive truncation is a bug.

> See also bug#34183.

> > ..  I propose fixing this bug by removing these limits on print-length and
> > print-level in cl-print-to-string-with-limit.

> Sounds a bit drastic.  Strings can be obnoxiously long, so it's
> important for cl-print to be able to truncate them.

There is clearly no human-sized bound on string lengths, so they can
indeed be very long.  Most of the time they're not.  But they are very
frequently longer than 50 characters.

> [ IOW, I'm not happy with commit
>   761f8901fffdb155cbcc7f3b5a2329161c2c1826.  ]

Well I did post about it to emacs-devel on Sunday and Monday, asking if
anybody had any objection.  Nobody, not even you, responded.

I still believe that not truncating strings is better than truncating
them to the minute length of 50.  In fact, why truncate strings at all in
cl-prin1?  They're not truncated in prin1, etc.  The reason for
truncating lists and vectors is to prevent infinite printing when there's
a circular list or vector, something which cannot happen with a string.

If somebody doesn't want a string longer that 50 to get printed, then she
shouldn't call cl-prin1 with it.

The mistake in Emacs before my patch was a category error: wrongly
believing that print-length applies to a string length too.  It doesn't.
String lengths are a completely different kettle of fish from list
lengths.

To solve this problem properly, we need, as Eli has suggested, a separate
variable called something like print-string-length, to be set
independently of print-length (and print-level).  A sensible value for
this variable in printing backtraces might be, say, 500.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).