qualified imports, PVP and so on (Was: add new Data.Bits.Bits(bitZero) method)

John Lato jwlato at gmail.com
Wed Feb 26 06:03:00 UTC 2014

On Tue, Feb 25, 2014 at 9:25 PM, Michael Snoyman <michael at snoyman.com>wrote:

> On Wed, Feb 26, 2014 at 1:28 AM, MightyByte <mightybyte at gmail.com> wrote:
>> On Tue, Feb 25, 2014 at 4:51 PM, Vincent Hanquez <tab at snarc.org> wrote:
>> >
>> > I'm not saying this is not painful, but i've done it in the past, and
>> using
>> > dichotomy and educated guesses (for example not using libraries released
>> > after a certain date), you converge pretty quickly on a solution.
>> >
>> > But the bottom line is that it's not the common use case. I rarely have
>> to
>> > dig old unused code.
>> And I have code that I would like to have working today, but it's too
>> expensive to go through this process.  The code has significant value
>> to me and other people, but not enough to justify the large cost of
>> getting it working again.
> I think we need to make these cases more concrete to have a meaningful
> discussion. Between Doug and Gregory, I'm understanding two different use
> cases:
> 1. Existing, legacy code, built again some historical version of Hackage,
> without information on the exact versions of all deep dependencies.
> 2. Someone starting a new project who wants to use an older version of a
> package on Hackage.
> If I've missed a use case, please describe it.
> For (1), let's start with the time machine game: *if* everyone had been
> using the PVP, then theoretically this wouldn't have happened. And *if* the
> developers had followed proper practice and documented their complete build
> environment, then PVP compliance would be irrelevant. So if we could go
> back in time and twist people's arms, no problems would exist. Hurray,
> we've established that 20/20 hindsight is very nice :).
> But what can be done today? Actually, I think the solution is a very
> simple tool, and I'll be happy to write it if people want:
> cabal-timemachine. It takes a timestamp, and then deletes all cabal files
> from our 00-index.tar file that represent packages uploaded after that
> date. Assuming you know the last date of a successful build, it should be
> trivial to get a build going again. And if you *don't* know the date, you
> can bisect until you get a working build. (For that matter, the tool could
> even *include* a bisecter in it.) Can anyone picture a scenario where this
> wouldn't solve the problem even better than PVP compliance?

This scenario is never better than PVP compliance.  First of all, the user
may want some packages that are newer than the timestamp, which this
wouldn't support.  As people have already mentioned, it's entirely possible
for valid install graphs to exist that cabal will fail to find if it
doesn't have upper bound information available, because it finds other
*invalid* graphs.

And even aside from that issue, this would push the work of making sure
that a library is compatible with its dependencies onto the library
*users*, instead of the developer, where it rightfully belongs (and your
proposal ends up pushing even more work onto users!).

Why do you think it's acceptable for users to do the testing to make sure
that your code works with other packages that your code requires?

> For (2), talking about older versions of a package is not relevant. I
> actively maintain a number of my older package releases, as I'm sure others
> do as well. The issue isn't about *age* of a package, but about
> *maintenance* of a package. And we simply shouldn't be encouraging users to
> start off with an unmaintained version of a package. This is a completely
> separate discussion from the legacy code base, where- despite the valid
> security and bug concerns Vincent raised- it's likely not worth updating to
> the latest and greatest.

Usually the case is not that somebody *wants* to use an older version of
package 'foo', it's that they're using some package 'bar' which hasn't yet
been updated to be compatible with the latest 'foo'.  There are all sorts
of reasons this may happen, including big API shifts (e.g. parsec2/parsec3,
openGL), poor timing in a maintenance cycle, and the usual worldly
distractions.  But if packages have upper bounds, the user can 'cabal
install', get a coherent package graph, and begin working.  At the very
worst, cabal will give them a clear lead as to what needs to be updated/who
to ping.  This is much better than the situation with no upper bounds,
where a 'cabal install' may fail miserably or even put together code that
produces garbage.

And again, it's the library *user* who ends up having to deal with these
problems.  Upper bounds lead to a better user experience.

> All of that said, I still think the only real solution is getting end
> users off of Hackage. We need an intermediate, stabilizing layer. That's
> why I started Stackage, and I believe that it's the only solution that will
> ultimately make library authors and end-users happy. Everything we're
> discussing now is window dressing.

A curated ecosystem can certainly function, but it seems like a lot more
work than just following the PVP and specifying upper bounds.  And upper
bounds are likely to work better with packages that, for whatever reason,
aren't in that curated ecosystem.
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