[pypy-dev] about breaking RPython, RCTypes

[Martijn Faassen]

Hi there,

I posted this to a longer thread the other day but have received no 
answers to it whatsoever. Perhaps it got lost, so I'll do a repost. I 
spent a bit of time writing this post, and I ask some questions in it 
that I think should be answered. Perhaps by telling me I don't 
understand what I'm talking about or that my analysis is entirely 
incorrect. Perhaps it was simply too long?

Whatever it is, I do think the PyPy community needs to be aware of how 
technical decisions have an impact on attracting or repelling potential 
contributors. "answering by silence", which happened so far, is one way 
to make potential contributors worried. :)

I only understand a little of the technical discussion. I understand
that RCTypes has problems, and that people anticipate having to break it
and reimplement it in a different way in order to solve these problems.
I understand that one feature that RCTypes offers, namely the ability to
test your RPython code on CPython, might also go away.

There are currently, as far as I can see it, two reasons to use RPython
for CPython extensions:

a) benefit from performance gains by rewriting your CPython code into
RPython. This is what we want to do with the page template interpreter.
In case of language implementations, this might lead to the ability to
use other PyPy technology later (such as JIT generation and so on). Some
others might purely be interested in performance though and aren't
implementing a language.

b) better ways to write extensions that use C libraries in CPython. The
typical Pyrex use case, though it can do the former as well. This would
use RCTypes.

Use case a) probably has less interested people than use case b).
Eventually JIT technologies will help here, which will make use case a)
less important.

I happen to be interested in use case a) though. It is quite exciting to
me that we could have a faster template language interpreter
implementation that we can use in CPython today, with the potential
benefits of PyPy for interpreter translation in the future as well. We
are basically language implementors, one of your core target audiences.
We just feel more comfortable doing work if we know we have payoffs in
the near future, i.e. use in CPython. If we were implementing this
template language in isolation and had no hope we could deploy it in
production any time soon, we would be less interested.

I am actually quite interested in use case b) as well. While developing
lxml, I've done quite a bit of work with Pyrex and I can see how RPython
and RCTypes might potentially help projects like that. I've been taking
a wait and see attitude here though, as the technology needed more maturing.

As I understand it, the proposed changes wouldn't affect use case a). Or
is the ability to generate CPython extensions also going to be broken
due to this work? Please let us know, as if CPython support is going to
be broken, we would likely stop our efforts for the time being.

Use case b) would certainly be broken due to this work. What's more, I
understand that testing your extension code in CPython first wouldn't
work anymore (at least if it uses CTypes). This is a very nice feature
of RPython that Pyrex doesn't have. You mentioned that there
are potential ways to reimplement this feature again, but I'm not sure
whether you're committing to such implementation work.

If I were writing C extension modules with PyPy I'd like to know things
like:

* what are the reasons this change is going to be made?

* would this benefit just core PyPy or would it benefit extension module
writers too?

* when do you expect the new way to be finalized?

* will you make a real commitment to make the "Test in CPython first"
feature work again? Or is did you just sketched out possible ways to
implement this in the new system and you hope some contributor is going
to do the work?

* will you make a commitment to make this new way useful and supported
for extension module writers, or should they invest their time in some
other technology? I.e. what are the changes a future shift will make me
change things again?

* How well documented will this be for CPython extension developers?
Will you market this feature to developers? Things like this indicate
your commitment in another way. :)

Please note that I'm not actually *asking* for anything but information.
I'm not telling the PyPy developers they should make commitments and do
various kinds of work. That's up to you. I'm just trying to guess what
other developers might want to know.

Of course, the quality of your communication and the answers you
actually give will make people decide whether to continue to use PyPy or
move on. This will affect the way your open source community will grow
and develop.

I would urge the PyPy developers to consider investing in the open
source community by supporting features like this and promoting them.
This would mean serving admittedly self-interested developers with short
term goals. The promise of immediate payoff is a great way to attract
people to an open source project. If there is the hope of continuing
payoff (and PyPy has tremendous potential here), some of those new
people can be expected to become valuable contributors to the project as
a whole. And yes, I'm self-interested in saying all of this - that's my
point.

Regards,

Martijn