bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Lars Ingebrigtsen]

(Looks like the debbugs address in the Cc was mangled, so I'm just
resending the entire email to have it end up in the bug tracker.)

Alan Mackenzie <acm@muc.de> writes:

> Hello, Lars.
>
> In article <mailman.2415.1564240928.2688.bug-gnu-emacs@gnu.org> you wrote:
>> Michael Heerdegen <michael_heerdegen@web.de> writes:
>
>>> in the docstring of `parse-sexp-ignore-comments' (or at least in the
>>> manual), I miss a description about what a value of nil exactly means:
>>> How are comments treated then?  Are they treated as separate units that
>>> can then be parsed as well, but separately from code, or are they treated
>>> as indistinguishable from code?
>>>
>>> For example, if parsing starts from within a comment, and parsing finds
>>> the end of the comment and is not yet finished, is parsing just
>>> continued inside the following code, or does it fail?
>
>> After doing some testing, it seems that if it's nil, the commands
>> affected by the setting just treat the commented-out text as if it
>> wasn't commented out.
>
>> So the answer to your last question seems to be "yes".
>
>> ;; (foo
>
>> (bar zot))
>
>> However, pretty much the same thing happens with a non-nil value, too --
>> with point before (foo C-M-f will advance past zot)).
>
>> So it doesn't treat comments as whitespace, really -- it only does that
>> if point is outside (before, at the end of a line, etc) the comment to
>> begin with.  Seems like you could write an essay about it, but perhaps
>> it's not worth listing the eccentricities here which I guess could change.
>
> parse-sexp-ignore-comments (or, rather, parse_sexp_ignore_comments) is
> tested, and acted upon, by precisely one C function, scan_lists in
> syntax.c.  scan_lists is used by just two primitives, scan-lists and
> scan-sexps.  These, in turn, are called by forward-list, etc.
>
> When this variable is nil, scan_lists fails to recognise comment
> delimiters - it just goes past them as though they were random
> punctuation characters.  By contrast, when parse-sexp-ignore-comments is
> t, scan_lists calls a comment scanning function when it encounters a
> comment delimiter.
>
> Noteworthy is that parse-partial-sexp doesn't respect the setting of
> parse-sexp-ignore-comments.  This might be considered a bug.
>
> All in all, this variable seems not to be a good idea.  It is not tested
> consistently by the syntax routines (see above), must be set explicitly
> to t by any major mode with comments, the nil value is rarely used, and
> it is not clear whether this nil value is actually ever useful.
>
> It would seem that if a major mode does not want comments to be
> recognised, it would be better not to give any character comment syntax
> in its syntax table.

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

bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Michael Heerdegen]

Hello,

in the docstring of `parse-sexp-ignore-comments' (or at least in the
manual), I miss a description about what a value of nil exactly means:
How are comments treated then?  Are they treated as separate units that
can then be parsed as well, but separately from code, or are they treated
as indistinguishable from code?

For example, if parsing starts from within a comment, and parsing finds
the end of the comment and is not yet finished, is parsing just
continued inside the following code, or does it fail?


Thanks in advance,

Michael.



In GNU Emacs 25.1.1 (x86_64-pc-linux-gnu, GTK+ Version 3.20.6)
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bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Lars Ingebrigtsen]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> in the docstring of `parse-sexp-ignore-comments' (or at least in the
> manual), I miss a description about what a value of nil exactly means:
> How are comments treated then?  Are they treated as separate units that
> can then be parsed as well, but separately from code, or are they treated
> as indistinguishable from code?
>
> For example, if parsing starts from within a comment, and parsing finds
> the end of the comment and is not yet finished, is parsing just
> continued inside the following code, or does it fail?

After doing some testing, it seems that if it's nil, the commands
affected by the setting just treat the commented-out text as if it
wasn't commented out.

So the answer to your last question seems to be "yes".

;; (foo

(bar zot))

However, pretty much the same thing happens with a non-nil value, too --
with point before (foo C-M-f will advance past zot)).

So it doesn't treat comments as whitespace, really -- it only does that
if point is outside (before, at the end of a line, etc) the comment to
begin with.  Seems like you could write an essay about it, but perhaps
it's not worth listing the eccentricities here which I guess could change.

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

bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Stefan Kangas]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Michael Heerdegen <michael_heerdegen@web.de> writes:
>
>> in the docstring of `parse-sexp-ignore-comments' (or at least in the
>> manual), I miss a description about what a value of nil exactly means:
>> How are comments treated then?  Are they treated as separate units that
>> can then be parsed as well, but separately from code, or are they treated
>> as indistinguishable from code?
>>
>> For example, if parsing starts from within a comment, and parsing finds
>> the end of the comment and is not yet finished, is parsing just
>> continued inside the following code, or does it fail?
>
> After doing some testing, it seems that if it's nil, the commands
> affected by the setting just treat the commented-out text as if it
> wasn't commented out.
>
> So the answer to your last question seems to be "yes".
>
> ;; (foo
>
> (bar zot))
>
> However, pretty much the same thing happens with a non-nil value, too --
> with point before (foo C-M-f will advance past zot)).
>
> So it doesn't treat comments as whitespace, really -- it only does that
> if point is outside (before, at the end of a line, etc) the comment to
> begin with.  Seems like you could write an essay about it, but perhaps
> it's not worth listing the eccentricities here which I guess could change.

So is there anything more to do here, or could this be closed?  I can't
see anything actionable from reading this thread.

bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Lars Ingebrigtsen]

Stefan Kangas <stefan@marxist.se> writes:

> So is there anything more to do here, or could this be closed?  I can't
> see anything actionable from reading this thread.

Yes, I tried adding some more to the doc string there, and it just ended
up restating the first line, but negatively, which doesn't seem to help
any:

--
If nil, text that has been commented out is taken into consideration
when doing commands like `forward-sexp'.
--

So I'm closing this bug report without doing that.

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

bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Alan McKenzie]

Hello, Lars.

In article <mailman.2415.1564240928.2688.bug-gnu-emacs@gnu.org> you wrote:
> Michael Heerdegen <michael_heerdegen@web.de> writes:

>> in the docstring of `parse-sexp-ignore-comments' (or at least in the
>> manual), I miss a description about what a value of nil exactly means:
>> How are comments treated then?  Are they treated as separate units that
>> can then be parsed as well, but separately from code, or are they treated
>> as indistinguishable from code?

>> For example, if parsing starts from within a comment, and parsing finds
>> the end of the comment and is not yet finished, is parsing just
>> continued inside the following code, or does it fail?

> After doing some testing, it seems that if it's nil, the commands
> affected by the setting just treat the commented-out text as if it
> wasn't commented out.

> So the answer to your last question seems to be "yes".

> ;; (foo

> (bar zot))

> However, pretty much the same thing happens with a non-nil value, too --
> with point before (foo C-M-f will advance past zot)).

> So it doesn't treat comments as whitespace, really -- it only does that
> if point is outside (before, at the end of a line, etc) the comment to
> begin with.  Seems like you could write an essay about it, but perhaps
> it's not worth listing the eccentricities here which I guess could change.

parse-sexp-ignore-comments (or, rather, parse_sexp_ignore_comments) is
tested, and acted upon, by precisely one C function, scan_lists in
syntax.c.  scan_lists is used by just two primitives, scan-lists and
scan-sexps.  These, in turn, are called by forward-list, etc.

When this variable is nil, scan_lists fails to recognise comment
delimiters - it just goes past them as though they were random
punctuation characters.  By contrast, when parse-sexp-ignore-comments is
t, scan_lists calls a comment scanning function when it encounters a
comment delimiter.

Noteworthy is that parse-partial-sexp doesn't respect the setting of
parse-sexp-ignore-comments.  This might be considered a bug.

All in all, this variable seems not to be a good idea.  It is not tested
consistently by the syntax routines (see above), must be set explicitly
to t by any major mode with comments, the nil value is rarely used, and
it is not clear whether this nil value is actually ever useful.

It would seem that if a major mode does not want comments to be
recognised, it would be better not to give any character comment syntax
in its syntax table.

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

-- 
Alan Mackenzie (Nuremberg, Germany).

bug#24172: 25.1; Doc of parse-sexp-ignore-comments: what does a value of nil mean?

[Michael Heerdegen]

Alan McKenzie <MAILER-DAEMON@mail.muc.de> writes:

> All in all, this variable seems not to be a good idea.  It is not
> tested consistently by the syntax routines (see above), must be set
> explicitly to t by any major mode with comments, the nil value is
> rarely used, and it is not clear whether this nil value is actually
> ever useful.

My question mainly arised from pondering whether this variable could be
useful for el-search, a package for searching Elisp code (with
patterns).  It could be of minor use if it allowed to search commented
code.  But I can also implement this in other ways.  Apart from that, I
have no use cases, so I wrote the bug report only because of the unclear
documentation.

But the variable seems to be used.  I have 75 matches in my load-path.

Michael.