bug#33167: 26; Doc string of `region-extract-function'

[Eli Zaretskii]

> Date: Fri, 26 Oct 2018 08:32:04 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
>  - 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.