In the Stackage maintainer's agreement, there's a section about keeping your package compatible with the newest versions of all dependencies. What the maintainer's agreement doesn't (yet) discuss is when it's important to be compatible with old versions of a package. The reasons for this are not immediately obvious, especially as it affects a smaller subset of the Hackage author population. This blog post will cover some of the reasons for this goal.
The original impetus for writing this was to get one specific message across: please continue supporting transformers-0.3.0.0! For the explanation of why, please keep reading.
The simplest case to discuss is packages like base and template-haskell. Not only are these packages shipped with GHC, but they cannot be upgraded. As a result, if you have a package that says
base >= 4.7, it will only work with GHC 7.8 and later. Users who are still using 7.6 (or 7.4... or earlier... yes, those people do in fact exist) will have no means of using your package.
That of course brings up a question of how many versions of GHC you want to support. I'd highly recommend always supporting the most recent Haskell Platform release, as many users (especially Windows users) stick to that. Going back an extra version as well isn't a bad idea either, especially as some distributions (e.g., Ubuntu) tend to ship relatively old GHC versions.
This issue is more subtle. In addition to non-upgradeable packages, GHC includes a number of packages which can be installed separately, resulting in one copy of the package in your global database, and one in your user database. (Yes, you can also install into the global database, but I'm covering the common case here.) Examples of these packages are bytestring, binary, and containers.
The first problem with this is that it can lead to end-user confusion. How many of you have tried working in GHCi, or just compiling code with
ghc --make, and gotten a message along the lines of "Could not match type ByteString with ByteString"? That usually comes from two versions of a package being available.
Now that's just a bit of an annoyance, and building your code with cabal will almost always avoid it. But there's a second, more serious problem. Some of these upgradeable packages are in turn depended upon by non-upgradeable packages. For example, template-haskell depends on containers. As a result, imagine if you try to use containers 0.5 and template-haskell when on GHC 7.4. Since template-haskell depends on containers-0.4.2.1, you'll run into issues.
Another problem is the ghc package (aka GHC-the-library). With GHC 7.8.2, I have the following dependencies for the installed ghc package:
depends: Cabal-220.127.116.11-9a922a1eb7c28f3b842ec080141cce40 array-0.5.0.0-9f212a0e8caa74d931af75060b4de2ab base-18.104.22.168-018311399e3b6350d5be3a16b144df9b bin-package-db-0.0.0.0-1742af7e25e78544d39ad66b24fbcc26 bytestring-0.10.4.0-7de5230c6d895786641a4de344336838 containers-0.5.5.1-19036437a266c66c860794334111ee93 directory-22.214.171.124-a0555efb610606fd4fd07cd3bba0eac2 filepath-126.96.36.199-15473fd51668a6d87ee97211624eb8aa hoopl-188.8.131.52-2477f10040d16e4625a4a310015c7bb6 hpc-0.6.0.1-6b2f98032f6f0d7ac5618b78a349a835 process-184.108.40.206-eaf7dde3bcb1e88fafb7f0f02d263145 template-haskell-220.127.116.11-dcc8c210fb02937e104bc1784d7b0f06 time-1.4.2-b47642c633af921325b5eb4d5824b9de transformers-0.3.0.0-7df0c6cd5d27963d53678de79b98ed66 unix-18.104.22.168-23f79f72106a0fbca2437feb33a4e846
So if I try to use- for example- transformers 0.4.1.0 and a package requiring ghc at the same time, I'll run into a conflict. And there are actually a large number of such packages; just doctest has over 100 dependencies.
The last reason is the one I hear the most pushback about from package authors. The Haskell Platform pegs users at specific versions of dependencies. For example, the most recent HP release pegs text at 0.11.3.1. Now imagine that you write a package that depends on
text >= 1.0. A user with the Haskell Platform installed will likely get warnings from cabal when installing your package about conflicting versions of text, and possibly breaking other packages that depend on it.
I can tell you what I've personally done about this situation. For my open source packages, I make sure to keep compatibility with the Haskell Platform released version of a package. Sometimes this does lead to some ugliness. Two examples are:
In the Stackage project, I run versions of the build both with and without Haskell Platform constraints. There are actually a whole slew of conditionals in the version selection which say "if you're using HP, then use this older version of a dependency." However, as time goes on, more and more packages are simply not supporting the HP-pegged versions of packages anymore.
I'm not commenting here on the value of HP-pegged versions, but simply pointing out a reality: if you want your users to have a good experience, especially Windows users, it's probably a good idea to keep compatibility with the older HP-provided versions. I also think the ramifications of the HP approach really need to be discussed by the community, it seems like there's not much discussion going on about the impact of the HP.
Also, regarding the packages shipped with GHC: there have certainly been discussions about improving this situation. I know that removing the Cabal dependency from ghc has been discussed, and would certainly improve the situation somewhat. If others want to kick off a conversation on improving things, I'd be happy to participate, but I frankly don't have any concrete ideas on how to make things better right now.