bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Visuwesh]

The notion of "last sexp" is different for eval-last-sexp and
pp-eval-last-sexp as seen by the following,

    ,(list 1 2 3)

eval-last-sexp echoes (1 2 3) in the echo area but pp-eval-last-sexp
signals an error.  I am not sure which one is more correct, but
eval-last-sexp behaviour is more convenient (and backward-sexp when
after the closing parenthesis places the point after , too...).

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bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Lars Ingebrigtsen]

Visuwesh <visuweshm@gmail.com> writes:

> The notion of "last sexp" is different for eval-last-sexp and
> pp-eval-last-sexp as seen by the following,
>
>     ,(list 1 2 3)
>
> eval-last-sexp echoes (1 2 3) in the echo area but pp-eval-last-sexp
> signals an error.  I am not sure which one is more correct, but
> eval-last-sexp behaviour is more convenient (and backward-sexp when
> after the closing parenthesis places the point after , too...).

The pp function uses (forward-sexp -1) to determine where the preceding
expression begins, which seems natural.  (And it skips to before the
comma.)

The non-pp function uses (elisp--preceding-sexp), which basically does
the same, but then explicitly skips past the comma.

I think the non-pp function has the most convenient behaviour, so I'd be
in favour of changing it to just use (elisp--preceding-sexp).  Any opinions?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Drew Adams]

> I think the non-pp function has the most convenient behaviour, so I'd
> be in favour of changing it to just use (elisp--preceding-sexp).  Any
> opinions?

Whether it's better ("more convenient") or not,
I don't know.  But it's a backward-incompatible
change.  That might be one thing to consider -
what users expect will break/change.

bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Visuwesh]

[புதன், மார்ச் 23 2022] Drew Adams wrote:

>> I think the non-pp function has the most convenient behaviour, so I'd
>> be in favour of changing it to just use (elisp--preceding-sexp).  Any
>> opinions?
>
> Whether it's better ("more convenient") or not,
> I don't know.  But it's a backward-incompatible
> change.  That might be one thing to consider -
> what users expect will break/change.

Sure, but the current (inconsistent) behaviour is surprising.  I would
be personally okay with such a backwards-incompatible change, but I am
biased since I filed the bug report.

bug#54537: Re: bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Drew Adams]

> > Whether it's better ("more convenient") or not,
> > I don't know.  But it's a backward-incompatible
> > change.  That might be one thing to consider -
> > what users expect will break/change.
> 
> Sure, but the current (inconsistent) behaviour is surprising.

Is it?  The `pp-*' commands have different uses.

FWIW, I even add separate variables for PP, for
controlling print level and length.  Force-using
the same values for the `pp-*' commands doesn't
make sense to me.

Similarly, I'm not shocked by the difference this
bug report wants to get rid of.  I don't necessarily
object to the change.  I'm just not convinced that
such consistency is important.

What matters most wrt consistency is _local_
consistency - being consistent within a given
context/scope/set of rules.  PP is a different
world (context).

(That's not an argument why PP's last-sexp should
be different.  It's an argument that just a "more
convenient" argument that it should be the same
isn't a strong one.)

> I would be personally okay with such a
> backwards-incompatible change, but I am
> biased since I filed the bug report.

It's a reasonable proposal to examine.  And it
might be the right thing to do.

My contribution is just to point out that PP eval
is not non-PP eval, and "more convenient" isn't a
strong argument when the increment of "more" isn't
large.  And when introducing backward-incompatible
behavior maybe other, stronger arguments would help.

Maybe bring this up on emacs-devel, to see if more
and better arguments for and against the change
are available?  (Not a requirement, of course.)

bug#54537: Re: bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Michael Heerdegen]

Drew Adams <drew.adams@oracle.com> writes:

> > Sure, but the current (inconsistent) behaviour is surprising.
>
> Is it?  The `pp-*' commands have different uses.
>
> FWIW, I even add separate variables for PP, for
> controlling print level and length.  Force-using
> the same values for the `pp-*' commands doesn't
> make sense to me.
>
> Similarly, I'm not shocked by the difference this
> bug report wants to get rid of.  I don't necessarily
> object to the change.  I'm just not convinced that
> such consistency is important.

Could as well be that it's just only different because nobody cared
enough about pp all the time.  I would not be shocked if the different
behavior is only an accident.

Michael.

bug#54537: Re: bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Drew Adams]

> Could as well be that it's just only different because nobody cared
> enough about pp all the time.  I would not be shocked if the different
> behavior is only an accident.

Could be.  Dunno.

Still calls out for a better argument than a vague
opinion that such a change adding incompatibility
would be "more convenient".  That's all.

The commands have different uses.  Their behavior
has long been different in this regard.  What's a
great argument for making this change?  That's the
question.

As I said:

  It's a reasonable proposal to examine.  And it
  might be the right thing to do.

  PP eval is not non-PP eval, and "more convenient"
  isn't a strong argument when the increment of
  "more" isn't large.

  When introducing backward-incompatible behavior
  maybe other, stronger arguments would help.

bug#54537: Re: bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Michael Heerdegen]

Drew Adams <drew.adams@oracle.com> writes:

> Still calls out for a better argument than a vague
> opinion that such a change adding incompatibility
> would be "more convenient".  That's all.

Ok - let's become concrete now.

About what kinds of use cases are we a talking about?  Use cases like
this:

#+begin_src emacs-lisp
`(,(+ 2 3)
  ,(+ 4 5))
#+end_src

The non-pp behavior is useful (you are able to eval the inserted
subexpressions).  The pp behavior is not useful (error).

This behaves identically in both versions:

#+begin_src emacs-lisp
',(+ 4 5)
#+end_src

(you get the expected `,(+ 4 5)`.)

So we only talk about plain naked `unquote` expressions.

Do you see any concrete advantages of the pp-version behavior?  Or some
concrete hints that the pp version must be like this for more
consistency in the pp package?

OTOH, a concrete problem I see is that people avoid pp due to such things.
As far as I recall the history of the pp package, I don't expect much
logic behind the behavior.  Maybe it really just...sucks?

Michael.

bug#54537: Re: bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Visuwesh]

[புதன், மார்ச் 23 2022] Drew Adams wrote:

>> > Whether it's better ("more convenient") or not,
>> > I don't know.  But it's a backward-incompatible
>> > change.  That might be one thing to consider -
>> > what users expect will break/change.
>> 
>> Sure, but the current (inconsistent) behaviour is surprising.
>
> Is it?  The `pp-*' commands have different uses.
>

They do, and I don't object to that.  But when I see pp-eval-last-sexp,
I think it would do the same thing as eval-last-sexp but pretty prints
the result in a separate buffer instead.  In this regard, which I think
is a reasonable interpretation, pp-eval-last-sexp is not inconsistent
and hence behaves in an unexpected manner.

> FWIW, I even add separate variables for PP, for
> controlling print level and length.  Force-using
> the same values for the `pp-*' commands doesn't
> make sense to me.
>
> Similarly, I'm not shocked by the difference this
> bug report wants to get rid of.  I don't necessarily
> object to the change.  I'm just not convinced that
> such consistency is important.
>

I would agree with you if this change removed an ability to do a certain
thing in the name of consistency but here, it merely changes the
behaviour to something more convenient.  I see no point in bikeshedding
further however, so I will stop here.  The final decision will be up to
the maintainers.

> What matters most wrt consistency is _local_
> consistency - being consistent within a given
> context/scope/set of rules.  PP is a different
> world (context).
>
> (That's not an argument why PP's last-sexp should
> be different.  It's an argument that just a "more
> convenient" argument that it should be the same
> isn't a strong one.)
>
>> I would be personally okay with such a
>> backwards-incompatible change, but I am
>> biased since I filed the bug report.
>
> It's a reasonable proposal to examine.  And it
> might be the right thing to do.
>
> My contribution is just to point out that PP eval
> is not non-PP eval, and "more convenient" isn't a
> strong argument when the increment of "more" isn't
> large.  And when introducing backward-incompatible
> behavior maybe other, stronger arguments would help.
>
> Maybe bring this up on emacs-devel, to see if more
> and better arguments for and against the change
> are available?  (Not a requirement, of course.)

bug#54537: Re: bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Drew Adams]

> `(,(+ 2 3)
>   ,(+ 4 5))
> 
> The non-pp behavior is useful (you are able to eval the inserted
> subexpressions).  The pp behavior is not useful (error).

OK, I'm convinced.  I didn't (and I don't) see a good
reason for the `pp-*' behavior is that regard.  Just
wanted people to take a look.

What's needed is a fixed version of `pp-last-sexp'.
It's not about `pp-eval-last-sexp'.

pp-last-sexp' has several differences from
`elisp--preceding-sexp'.  Should some of those
differences be kept?  Dunno.  E.g.,  it uses `read',
and it ignores leading comments.

Regardless of what changes are made to `pp-last-sexp',
I think those changes should allow the functions that
call `pp-last-sexp' to remain unchanged; IOW, fix
`pp-last-sexp', but don't change how it's used/called.

E.g., `pp-eval-last-sexp' should remain with this code:

(defun pp-eval-last-sexp (arg)
  "..."
  (interactive "P")
  (if arg
      (insert (pp-to-string (eval (pp-last-sexp)
                                  lexical-binding)))
    (pp-eval-expression (pp-last-sexp))))

Similarly `pp-macroexpand-last-sexp'.  There's no doubt
other code out there (I have some) that makes use of
`pp-last-sexp'.  Let's please have just a simple fix of
`pp-last-sexp'.

And besides allowing no changes in calling, preferably
no other behavior would be changed than to fix this bug.

bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Lars Ingebrigtsen]

Visuwesh <visuweshm@gmail.com> writes:

> The notion of "last sexp" is different for eval-last-sexp and
> pp-eval-last-sexp as seen by the following,
>
>     ,(list 1 2 3)
>
> eval-last-sexp echoes (1 2 3) in the echo area but pp-eval-last-sexp
> signals an error.  I am not sure which one is more correct, but
> eval-last-sexp behaviour is more convenient (and backward-sexp when
> after the closing parenthesis places the point after , too...).

I've now fixed this in Emacs 29.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#54537: 29.0.50; Last sexp notion is different for eval-last-sexp and pp-eval-last-sexp

[Visuwesh]

[வெள்ளி, மார்ச் 25, 2022] Lars Ingebrigtsen wrote:

> Visuwesh <visuweshm@gmail.com> writes:
>
>> The notion of "last sexp" is different for eval-last-sexp and
>> pp-eval-last-sexp as seen by the following,
>>
>>     ,(list 1 2 3)
>>
>> eval-last-sexp echoes (1 2 3) in the echo area but pp-eval-last-sexp
>> signals an error.  I am not sure which one is more correct, but
>> eval-last-sexp behaviour is more convenient (and backward-sexp when
>> after the closing parenthesis places the point after , too...).
>
> I've now fixed this in Emacs 29.

Great, thanks!