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

Michael Snoyman michael at snoyman.com
Wed Feb 26 20:13:54 UTC 2014

On Wed, Feb 26, 2014 at 10:01 PM, John Lato <jwlato at gmail.com> wrote:

> On Wed, Feb 26, 2014 at 1:56 AM, Michael Snoyman <michael at snoyman.com>wrote:
>> On Wed, Feb 26, 2014 at 10:36 AM, John Lato <jwlato at gmail.com> wrote:
>>> On Tue, Feb 25, 2014 at 11:11 PM, Michael Snoyman <michael at snoyman.com>wrote:
>>>> On Wed, Feb 26, 2014 at 8:03 AM, John Lato <jwlato at gmail.com> wrote:
>>>>> 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?
>>>>  You're not at all addressing the case I described. The case was a
>>>> legacy project that someone is trying to rebuild. I'm not talking about any
>>>> other case in this scenario. To repeat myself:
>>>> > 1. Existing, legacy code, built again some historical version of
>>>> Hackage, without information on the exact versions of all deep dependencies.
>>>> In *that specific case*, why wouldn't having a tool to go back in time
>>>> and build against a historical version of Hackage be *exactly* what you'd
>>>> need to rebuild the project?
>>> I had understood people talking about "legacy projects" to mean
>>> something other than how you read it.  In which case, I would suggest that
>>> there is a third use case, which IMHO is more important than either of the
>>> use cases you have identified.  Here's an example:
>>> 1.  package foo-0.1 appears on hackage
>>> 2.  package bar-0.1 appears on hackage with a dependency on foo >= 0.1
>>> 3.  awesomeApp-0.1 appears on hackage, which depends on bar-0.1 and
>>> text>=1.0
>>> 4.  users install awesomeApp
>>> 5.  package foo-0.2 appears on hackage, with lots of breaking changes
>>> 6.  awesomeApp users notice that it sometimes breaks with Hungarian
>>> characters, and the problem is traced to an error in text
>>> 6.  text- is released with some bug fixes
>>> 7.  awesomeApp users attempt to do cabal update; cabal install, which
>>> fails inscrutably (because it tries to mix foo-0.2 with bar-0.1)
>>> There's nothing in this situation that requires any of these packages be
>>> unmaintained.  The problem is that, rather than wanting to reproduce a
>>> fixed set of package versions (which cabal already allows for if that's
>>> really desired), sometimes it's desirable that updates be held back in
>>> active code bases.  Replace "foo" with "QuickCheck" for example (where for
>>> a long time users stayed with quickcheck2 because version 3 had major
>>> performance regressions in certain use cases).
>>> This sort of conflict used to happen *all the time*, and it's very
>>> frustrating to users (because something worked before, now it's not
>>> working, and they're not generally in a good position to know why).  It's
>>> annoying to reproduce because the install graph cabal produces depends in
>>> part on the user's installed packages.  So just because something builds on
>>> a developer's box doesn't mean that it would build on the user's box, or it
>>> would work for some users but not others (sandboxing has at least helped
>>> with that problem).
>> IIUC, this is *exactly* the case of an unmaintained package. I'm not
>> advocating leaving a package like bar-0.1 on Hackage without an upper bound
>> on foo, if it's known that it breaks in that case. In order for the package
>> to be properly maintained, the maintainer would have to (1) make bar work
>> with foo-0.2, or (2) add an upper bound. So to me, this falls squarely into
>> the category of unmaintained.
> I disagree.  I think it's unreasonable to expect that maintainers provide
> 24/7 availability and near-immediate maintenance releases when updated deps
> are released.  And in the meantime (which may be days, or even a couple
> weeks for a single-maintainer who might be on holiday), there's plenty of
> time for this to bite hard.  In the past, this meant that broken packages
> would remain available on hackage for a long time.  At least now the
> package maintainers can do so themselves, but it still means that broken
> packages have escaped into the wild, which is also bad.
>> Let me relax my position just a bit. If package maintainers are not going
>> to be responsive to updates in the Hackage ecosystem, then I agree that
>> they should use the PVP. I also think they should advertise their packages
>> as not being actively maintained, and people should try to avoid using them
>> if possible. But if an author is giving quick updates to packages, I don't
>> see a huge benefit to the PVP for users, and instead see some downsides
>> (inability to test against newer dependencies), not to mention the much
>> higher maintenance burden for library authors.
> What do you consider "responsive"?  2 hours?  24?  1 week?  I suspect that
> you have IMO unrealistic expectations of maintainers, so I think it would
> be good to get into specifics.

I think I just realized the assumption I've been making which skews my view
of things a bit here. I think it's a terrible idea that we're telling new
users to go onto Hackage, type cabal install foo, and hope for the best.
Not just because of build plan issues. Who knows if the newest version of a
package actually works as advertised? There's been no testing period at
all, no vetting. This is one of the primary reasons for yesod-platform: it
gives users a starting point with a sane build plan and at least some
degree of integration testing (though I'll admit that the level of testing
isn't nearly to what I'd like it to be).

If the constraints on the system are:

1. Any user at any time must be able to go onto Hackage and cabal install
2. There can be no point at which foo doesn't install.

Then requiring preemptive upper bounds is the only solution. I simply don't
like those constraints, and think we're approaching the problem wrong as a

There's one other constraint that I'm trying to optimize for, which is
"when a user runs `cabal install foo bar`, there's a high probability that
a sane build plan exists." This is the constraint which the PVP hinders.

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