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Re: Syntax tables for multiple modes [was: bug#22983: syntax-ppss return

From: Vitalie Spinu
Subject: Re: Syntax tables for multiple modes [was: bug#22983: syntax-ppss returns wrong result.]
Date: Mon, 21 Mar 2016 15:13:22 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.91 (gnu/linux)

>> On Mon, Mar 21 2016 08:26, Stefan Monnier wrote:

>> parse-partial-sexp should work between hard limits (at least the lower
>> bound). It should operate as if hard-narrowed buffer is the real buffer.

> You mean it should ignore the current (user)narrowing?  Why? I'd think that if
> something needs to ignore the (user)narrowing it'd be parse-partial-sexp's
> *caller* but not parse-partial-sexp itself.

Currently it just throws out-of-range errors. So in that sense it does ignore
user narrowing in a very inconvenient way.

parse-partial-sexp is called from code exclusively and it just happens that in
multi-modes it is called outside of narrow region quite often. That's a major
inconvenience. Why on earth one would need to take account in user narrowing for
syntax parsing? If parse-partial-sexp could be made to always widen to hard
limits it will automatically solve a bunch of problems. bug#22983 being one of
them, condition-case awkwardness in syntax-ppss being another one, and the
ubiquitous out-of-range errors in font-lock in multi-modes being the most
important one.

>> So ideally it should take (max FROM (car hard-widen-limits)) as the starting
>> position.

> You mean: as opposed to (max FROM (point-min))?


> I disagree.  Functions should usually not accept to talk about positions
> outside of the point-min/max range.

Depends on the function. point-max/min is mostly user level. Why wold syntax
parsing would need to respect that? Bug#22983 ilustrates that clearly. If user
narrows in the middles of a string, it creates huge problems.

Note that with Dmitry's new syntax-ppps-dont-widen proposal syntax-ppps widens

Can I ask you the reverse? What do you gain by respecting user narrowing in
syntax parsing?

> Notice how syntax-ppss is different in this regard: since it doesn't
> receive FROM, that same rule doesn't prevent syntax-ppss from widening
> to (car hard-widen-limits).

Well, not quite different. It has POS which might be outside of user narrowed

>> This will give the desired consistency between parse-partial-sexp and
>> syntax-ppss with the price of slightly modifying the semantics of
>> parse-partial-sexp in a backward compatible way.

> I'd be curious to know in which circumstances (i.e. specific code in specific
> packages) this would make a difference.  As mentioned above, I think these
> cases would be better fixed by changing the calling code to perform widening
> before calling parse-partial-sexp.

I think bug#22983 is illustrative enough. Multi-mode code is a nightmare because
of out-of-range errors in parsing. `syntax-ppss` is protected but that
condition-case is triggered in 99.99% of the times in multi-modes.

In multi modes you really want to keep narrowing because most of the major-mode
functionality works well on narrowed code. Pretty much all of it except
syntactic parsing and font-locking. Occasional property lockup outside of
narrowed region could be dealt with on case by case basis or, hopefully, with
new hard-narrowed-limits at the core of it.

>>>> A patch that would require hunting every single mode out there and
>>>> implementing multi-modes locally should have been more carefully
>>>> considered IMO.

>>   - Major mode authors won't need to know about multi-modes. That
>>     means not dealing with chunks/spans/headers etc.  These concepts are
>>     not even uniformly defined between existing multi-mode engines.

> I understand that's your claim, but I don't understand why/how this is
> different between the two proposals.

Major mode author has to deal with the span explicitly as defined in
previous-chunk in prog-indentation-context. Cognitively this is a more demanding
task. Ask a new person to go and read the doc of prog-indentation-context and
ask how much he or she understands of it. I read it and I think I understand
most of it, but looking at all the usages of prog-widen and prog-first-column in
python.el my brain gives up. Previous-chunk is not even used in python.el!

The prog-calculate-indent-function is more general. You can call it on any
buffer position (need not be last point in the previous span). It can be called
with whatever STRING-BEFORE and STRING-AFTER (these can, but need not be, actual
strings in the buffer). Current prog-indentation-context allows for possibility
of a string to be inserted before begging in of current chunk. STRING-BEFORE is
more more general than that because of the arbitrary POS that it can be applied

My claim is that we can achieve much higher generality and don't bother mode
authors with all those concepts like current/previous span/chunk, starting/end
position etc. Only multi-mode engine can take proper care of those anyways.

Here is a simple example when inner mode cannot decide by itself on the
indentation. Assume for concreteness a noweb header with some code immediately
following the header:

  <<foo, some_arg=4>>= some_call(blabla) 
      some_other_call(blabla) ## indented by offset 2 with respect to header or 

How do you indent the some_call(blabal) after the header? The most meaningful
way is to keep it untouched just as user defined it. If inner mode would indent
it by itself it would give offset of 4. This is a simple example of header

You can easily imagine more complex cases when not only one previous span need
to be considered but a range of previous spans of the same inner mode. Moreover
there might be nested inner chunks. Which chunk/span will you include in
prog-indentation-context? The entire previous code chunk or only the last
homogeneous span after the most recent inner-inner chunk?

Indentation of a span is commonly dependent on the header of the chunk (note the
terminology distinction). You can imagine having a parameter in the header that
would determine the indentation of the chunk's body. Header-dependence is a
simple and common case of inter-span dependence. It's not hard to imagine
complex cases when indentation in current span will depend not only on the
previous span of the same mode but on other spans of host mode or even other
inner (nested or not) modes.

IMO the best way is to leave all this complexities to multi-mode authors to deal
with on case by case basis. You never know what sort of complexities and chunk
dependencies new multi-modes will impose. Better keep things
generic. prog-calculate-indent-function seems like a multi-mode agnostic
solution. I am not sure if it will solve all problems, but it's surely solves
more than prog-indentation-context does in a cleaner way.

Note on terminology. I put quite some effort to sort things out in
polymode. Glossary of terms is here:


For many reasons it's important to distinguish between portions of code that
include header/tails and homogeneous portions of the same mode. Former portions
I call `chunks` and those can include other chunks of different sub-modes. The
latter, homogeneous portions, I call `spans`.

The fact that core emacs is now starting building pieces of multi-mode
functionality here and there and thus entrenching a somewhat naive
interpretation of a "chunk" doesn't make me happy. Not a big deal though.

> My hunch is that if STRING-BEFORE/AFTER don't matter,

It will actually matter for quite some modes in continuation chunks. I was too


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