Re: [Haskell-cafe] Platform Versioning Policy: upper bounds are not our friends

Of course if you wish or need to upgrade a package then you can just upgrade it -- I am not suggesting anyone should forgo upgrades! It is just that there is no need to make the stability of a build process dependent on new package releases. To upgrade a package I would fork my sandbox, hack away at the package database (removing, upgrading, installing packages) until I have a candidate combination of packages and swap in the new sandbox into my work tree and test it, reverting to the tried and tested environment if things don't work out. If the new configuration works then then I would dump the new package configuration and check it in. Subsequent updates and builds on other work trees should pick up the new environment. The key thing I was looking for was control of when your build environment gets disrupted and stability in between -- even when building from the repo. Chris -----Original Message----- From: Erik Hesselink [mailto:hesselink@gmail.com] Sent: 20 August 2012 14:35 To: Chris Dornan Cc: Bryan O'Sullivan; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Platform Versioning Policy: upper bounds are not our friends Hub looks interesting, I'll have to try it out (though I'm not on an RPM based distro). But isn't this the goal of things like semantic versioning [0] and the PVP? To know that you can safely upgrade to a bugfix release, and relavily safely to a minor release, but on a major release, you have to take care? Haskell makes it much easier to see if you can use a new major (or minor) version of a library, since the type checker catches many (but not all!) problems for you. However, this leads to libraries breaking their API's much more easily, and that in turn causes the problems voiced in this thread. However, fixing all versions seems like a bit of a blunt instrument, as it means I'll have to do a lot of work to bring even bug fixes in. Erik [0] http://semver.org/ On Mon, Aug 20, 2012 at 3:13 PM, Chris Dornan wrote:

I think we should encourage stable build environments to know precisely which package versions they have been using and to keep using them until told otherwise. Even when the types and constraints all work out there is a risk that upgraded packages will break. Everybody here wants cabal to just install the packages without problem, but if you want to insulate yourself from package upgrades surely sticking with proven combinations is the way to go.

Chris

-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Erik Hesselink Sent: 20 August 2012 08:33 To: Bryan O'Sullivan Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Platform Versioning Policy: upper bounds are not our friends

I am strongly against this, especially for packages in the platform.

If you fail to specify an upper bound, and I depend on your package, your dependencies can break my package! For example, say I develop executable A and I depend on library B == 1.0. Library B depends on library C >= 0.5 (no upper bound). Now C 0.6 is released, which is incompatible with B. This suddenly breaks my build, even though I have not changed anything about my code or dependencies. This goes against the 'robust' aspect mentioned as one of the properties of the Haskell platform, and against the Haskell philosophy of correctness in general.

This is not an imaginary problem. At my company, we've run into these problems numerous times already. Since we also have people who are not experts at Cabal and the Haskell ecosystem building our software, this can be very annoying. The fix is also not trivial: we can add a dependency on a package we don't use to all our executables or we can fork the library (B, in the example above) and add an upper bound/fix the code. Both add a lot of complexity that we don't want. Add to that the build failures and associated emails from CI systems like Jenkins.

I can see the maintenance burder you have, since we have to do the same for our code. But until some Cabal feature is added to ignore upper bounds or specify soft upper bounds, please follow the PVP, also in this regard. It helps us maintain a situation where only our own actions can break our software.

Erik

On Wed, Aug 15, 2012 at 9:38 PM, Bryan O'Sullivan wrote:

...

Hi, folks -

I'm sure we are all familiar with the phrase "cabal dependency hell" at this point, as the number of projects on Hackage that are intended to hack around the problem slowly grows.

I am currently undergoing a fresh visit to that unhappy realm, as I try to rebuild some of my packages to see if they work with the GHC 7.6 release candidate.

A substantial number of the difficulties I am encountering are related to packages specifying upper bounds on their dependencies. This is a recurrent problem, and its source lies in the recommendations of the PVP itself (problematic phrase highlighted in bold):

...

When publishing a Cabal package, you should ensure that your dependencies in the build-depends field are accurate. This means specifying not only lower bounds, but also upper bounds on every dependency.

I understand that the intention behind requiring tight upper bounds was good, but in practice this has worked out terribly, leading to depsolver failures that prevent a package from being installed, when everything goes smoothly with the upper bounds relaxed. The default response has been for a flurry of small updates to packages in which the upper bounds are loosened, thus guaranteeing that the problem will recur in a year or less. This is neither sensible, fun, nor sustainable.

In practice, when an author bumps a version of a depended-upon package, the changes are almost always either benign, or will lead to compilation failure in the depending-upon package. A benign change will obviously have no visible effect, while a compilation failure is actually better than a depsolver failure, because it's more informative.

This leaves the nasty-but-in-my-experience-rare case of runtime failures caused by semantic changes. In these instances, a downstream package should reactively add an upper bound once a problem is discovered.

I propose that the sense of the recommendation around upper bounds in the PVP be reversed: upper bounds should be specified only when there is a known problem with a new version of a depended-upon package.

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