On Wed, Feb 26, 2014 at 10:36 AM, John Lato <jwlato@gmail.com> wrote:

On Tue, Feb 25, 2014 at 11:11 PM, Michael Snoyman <michael@snoyman.com> wrote:

On Wed, Feb 26, 2014 at 8:03 AM, John Lato <jwlato@gmail.com> wrote:

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

On Wed, Feb 26, 2014 at 1:28 AM, MightyByte <mightybyte@gmail.com> wrote:

On Tue, Feb 25, 2014 at 4:51 PM, Vincent Hanquez <tab@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-1.0.0.1 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.

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.