Re: Bug #25608 and the comment-cache branch

[Dmitry Gutov <dgutov@yandex.ru>]

On 04.02.2017 12:24, Alan Mackenzie wrote:

> You want comment-cache to be wholly abandoned.

At least the part that maintains a separate cache. I'm not sure if 
there's anything else there.

It's not because I enjoy disagreeing with you, though.

>> And then we should seek the simplest solution that satisfies all of our
>> requirements.
> 
> As simple as possible, but definitely not simpler.  The "solution" you
> favour is too simple.  It doesn't work all the time.

I concede it's not ideal. However, I strongly believe "fixing" the 
narrowing problem in syntax-ppss with take care of this example, *and* 
will result in lower overall complexity and maintenance burden.

Consider the problems you've had merging master into the comment-cache 
branch. If there were conflicts, that means the new code touches a 
changing area, and it will need to be considered and taken care of by 
the maintainers, probably on an ongoing basis. The AP, on the other 
hand, still applies cleanly.

>> "It introduces a second source of truth" seems like a concise summary.
> 
> So what?  There are any number of "sources of truth" in Emacs.  If one
> of them turns out to be a "source of untruth" we call that a bug, and we
> fix it.

One normally adds an alternative source of truth (i.e. a "cache") to fix 
a significant performance problem, when one really can't do so otherwise.

It seems we agree now that comment-cache's existence can't be justified 
by performance considerations.

Cache invalidation is a known hard problem in CS, so we generally don't 
want to have extra caches.

>> At best, it'll use more memory than it has to.
> 
> The thing to do here is measure this extra memory.  I did this back in
> spring last year, and the number of extra conses used for the cache was
> not inordinately high.  Especially not for a 64-bit machine with several
> gigabytes of RAM.

Maybe it's not bad, without a direct link it's hard for me to comment on 
that now. But "no extra memory usage" would be a better outcome anyway.

> I think you're seeing something that's not there.  You're picturing some
> imagined process where two alternative ways of storing information have
> great difficulty staying together, and somehow, over time, are destined
> to drift apart.  Sort of like two national currencies trying to stay
> pegged to eachother, or something like that.

I'm picturing weird syntax highlighting/defun navigation/etc behavior 
that comes and goes seemingly randomly, and which forces us to debug 
both cache mechanisms to see which one is getting something wrong.

They don't even have to drift far apart functionality-wise, as long as 
their implementations are largely independent.

> That's not how computer programs work.  If those two ways end up
> differing, we have a bug, which can be fixed like any other bug.  Heck,
> even a single "source of truth" can be buggy, with just as severe
> consequences.  We get bugs, we fix them.

And the more sources of truth we have, we more places we might end up 
having to fix.

> Note, in this context, that syntax-ppss is broken (bug #22983) and
> doesn't look like getting fixed any time soon, yet the world hasn't come
> to an end.

A consistently "wrong" behavior is better than having some standard 
library functions work "correctly", and some otherwise.

Consider this again: as long as syntax-ppss continues to have problems 
in the cases you imagine, the caches _will_ diverge in those cases.

Honestly, my head hurts when I start thinking up problem examples, but 
I'm sure the users and authors of modes that define 
syntax-propertize-function and/or use syntax-ppss won't like them.

>>> Note that there has been NO constructive criticism of comment-cache.
> 
>> That's insulting, Alan.
> 
> It might be, but I think it's true.  You want comment-cache to be wholly
> abandoned.  You are not suggesting ways to make it better.  You haven't
> tried it, that I'm aware of.  You haven't looked for flaws, with the
> intention of getting them fixed.

You seem to argue that a high-level criticism can't be constructive, and 
that any good one has to discuss lower-level implementation details.

 > Instead you are putting forward
 > reasons, not all of them good, for abandoning comment-cache.

Aside from "two sources of truth", the other reason is that we have a 
much-simpler patch that gives us (or will eventually give) the same 
benefits.