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Subject: |
26; Doc string of `region-extract-function' |
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Date: |
Fri, 26 Oct 2018 08:32:04 -0700 (PDT) |
I don't understand these parts of the description:
- nil: return the content as a string.
What does that mean, for a noncontiguous region? Are the buffer
substrings for all of the region segments (zones) concatenated together?
If so, say that.
- anything else: delete the region and return its content
as a string, after filtering it with `filter-buffer-substring', which
is called with METHOD as its 3rd argument.
So first the region is deleted, and the deleted content (see previous)
is filtered via `(filter-buffer-substring BEG END METHOD)' and then
returned.
Some of what I don't understand:
1. What are the BEG and END args passed to `filter-buffer-substring'?
Is BEG the smallest car of any of the zones in the noncontiguous
region, and END the largest cdr of any of the zones?
2. `filter-buffer-substring' calls the value of
`filter-buffer-substring-function' with the same 3 args. But what
can that function do with BEG and END (which are what?)? It's
presumably a function that expects to use a single stretch of buffer
text from BEG to END. But here we're talking about a noncontiguous
3. The 3rd arg to `filter-buffer-substring' just deletes the region from
BEG to END if it is non-nil, so it seems like passing that non-nil
3rd arg is useless, as the region gets deleted anyway, by
`region-extract-function'.
4. The use of `filter-buffer-substring' is also unclear. It is passed
BEG and END (and METHOD, but see #3, above). And it filters the
buffer text between BEG and END. But see #1 above - are BEG and END
buffer positions that make sense for the whole region text? Just
what happens here?
This is quite unclear to me. And following the rabbit hole from
`region-extract-function' down to `filter-buffer-substring' and then to
`filter-buffer-substring-function' does not make things more clear.
Or is what happens perhaps that EACH element of the noncontiguous
region, that is, each zone (BEG<N> . END<N>) of the list ((BEG1 . END1)
...) gets filtered by `filter-buffer-substring', passing its BEG END and
EMTHOD - so that a mapcar is applied? In that case, how are the
resulting buffer substrings assembled - are they concatenated to get the
return value?
In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
`configure --without-dbus --host=x86_64-w64-mingw32
--without-compress-install 'CFLAGS=-O2 -static -g3''
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--- Begin Message ---
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Subject: |
Re: bug#33167: 26; Doc string of `region-extract-function' |
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Date: |
Sat, 27 Oct 2018 14:20:36 +0300 |
> Date: Fri, 26 Oct 2018 08:32:04 -0700 (PDT)
> From: Drew Adams <address@hidden>
>
> - nil: return the content as a string.
>
> What does that mean, for a noncontiguous region? Are the buffer
> substrings for all of the region segments (zones) concatenated together?
> If so, say that.
No, the value is a list of strings in this case. I fixed the doc
string to say that.
> 1. What are the BEG and END args passed to `filter-buffer-substring'?
> Is BEG the smallest car of any of the zones in the noncontiguous
> region, and END the largest cdr of any of the zones?
They are the first and the last positions of the region for
filter-buffer-substring to act upon. That function is not supposed to
process non-contiguous regions.
> 2. `filter-buffer-substring' calls the value of
> `filter-buffer-substring-function' with the same 3 args. But what
> can that function do with BEG and END (which are what?)? It's
> presumably a function that expects to use a single stretch of buffer
> text from BEG to END. But here we're talking about a noncontiguous
No, we are talking about contiguous regions when this function is
concerned.
> 3. The 3rd arg to `filter-buffer-substring' just deletes the region from
> BEG to END if it is non-nil, so it seems like passing that non-nil
> 3rd arg is useless, as the region gets deleted anyway, by
> `region-extract-function'.
Not sure what is the problem here.
> 4. The use of `filter-buffer-substring' is also unclear. It is passed
> BEG and END (and METHOD, but see #3, above). And it filters the
> buffer text between BEG and END. But see #1 above - are BEG and END
> buffer positions that make sense for the whole region text? Just
> what happens here?
See above. It is not the job of filter-buffer-substring to DTRT when
the region is non-contiguous, it is the job of its callers. See
rect.el for one example.
> This is quite unclear to me. And following the rabbit hole from
> `region-extract-function' down to `filter-buffer-substring' and then to
> `filter-buffer-substring-function' does not make things more clear.
It isn't supposed to. If you want to see how non-contiguous regions
are used in this context, you need to look in places that do so.
> Or is what happens perhaps that EACH element of the noncontiguous
> region, that is, each zone (BEG<N> . END<N>) of the list ((BEG1 . END1)
> ...) gets filtered by `filter-buffer-substring', passing its BEG END and
> METHOD
Yes!
> - so that a mapcar is applied?
Not literally, but the result is a list, yes.
> In that case, how are the resulting buffer substrings assembled -
> are they concatenated to get the return value?
No, you get a list. Again, see rect.el.
Bottom line: I fixed the doc string of region-extract-function to say
what are the values when the region is non-contiguous, and how
filter-buffer-substring is invoked in that case. But I don't think
there's anything else that should be done here, because the details of
using this facility for non-contiguous regions is entirely up to the
Lisp program which implements such a feature.
Thanks.
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