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Vincent Hanquez and I have been playing a proverbial game of hot potato over the past few months about writing this blog post. A cabal-install bug recently came to my attention that finally pushed me over the edge into writing this myself. I'll discuss that a bit later.
The basic claim here is simple:
- Supporting older GHCs together with new ones adds a real maintenance cost to a library.
- This is even higher if your goal is to make the code compile without warnings across all supported versions. You'll often end up resorting to CPP or weird import hacks to work around redundant imports.
- There's nothing wrong with maintaining a code base using an old version of tools and libraries. However, you should make sure that you are pinning your dependencies in any such project, and therefore missing out on the latest updates to a library is not a critical issue, excluding security patches.
Usually, when I drop support for an older version of GHC in a library, I receive no complaints. Occasionally, if I do receive a concern about it, it's from someone trying to maintain their own CI matrix with older GHC support. I rarely, if ever, receive a complaint from someone trying to actually use an older version of GHC and the newest version of my code.
Instead of spinning wheels to maintain compatibility on the off chance that someone may desire bleeding-edge libraries with years-old compilers, I recommend cutting out support for older GHCs, updating your cabal files to reflect this decision, and keeping your CI build matrix curated appropriately. Only if a user has a specific request for a feature to work with an older GHC would I consider changing direction on this.
A generally accepted rule of thumb is three major GHC releases. At the time of writing, that would mean supporting GHC 8.0, 8.2, and 8.4. I recommend only supporting the latest minor version of each line, which would mean GHC 8.0.2, 8.2.2, and 8.4.3.
Updating cabal files
The most common method for specifying which GHC versions you support
is to use the version of the
base library that ships with GHC. You
can use this handy lookup table I put
together, which I made because I can never remember this
information. Using that table, if you decided "I want to support GHC
8.0.2 and later", you'd look up the corresponding base version, find
that it's 220.127.116.11, and add the following to your cabal file:
build-depends: base >= 18.104.22.168
(Or equivalent if using hpack/package.yaml.)
Even though this is the standard approach today, there are a few problems with it:
- The meaning isn't immediately clear. Most people in my experience
think about GHC versions, not
baseversions. Someone reading that line will need to go look up what GHC version that corresponds to.
- Newer users may not understand that
baseis a special, non-upgradeable package, and not realize that this is pinning the GHC version. (Many bug reports and support requests about both cabal-install and Stack back up this claim.)
- At some point in the future, in theory,
basemay in fact be upgradeable, at which point this approach won't work.
baselibrary versions don't always bump with new versions of GHC. For example, both GHC 8.4.2 and 8.4.3 ship with
base-22.214.171.124. If you wanted to state that you only support GHC 8.4.3, you can't use the base approach.
- It's quite possible to write some code which is compatible with an
baselibrary version, but depends on newer features of GHC, like a new language extension.
Therefore, I recommend taking a two-pronged approach for dropping support for older GHC versions:
- Add a lower bound on
baseas mentioned above. It's the standard approach, and some tooling may depend on it.
- Add a stanza to your cabal file using cabal conditionals based on the GHC version, e.g:
if impl(ghc < 8.0.2) build-depends: unsupported-ghc-version > 1 && < 1
Originally, Vincent and I had discussed using
buildable: False, but
that's not an option, because...
Recently uncovered cabal-install bug
I received a bug report on Friday about my new release of
causing Travis build breakages for GHC 7.10.3. Later this was
opened as a Hackage Trustee issue,
where I found out that the same bug was being triggered by an earlier
It turns out that with all released versions of cabal-install, there's
a bug that works something like this: if a component is
buildable: False, then all of its dependencies are ignored. However, the
dependency solver will still select such a package as a library
dependency, even though the library component is marked as
non-buildable. Then, when trying to build this dependency,
cabal-install will (rightfully) complain that it cannot build a
package with no library or executables.
This bug has been fixed on cabal-install HEAD (and maybe the 2.2
maintenance branch? I'm not 100% sure). However, it's unreasonable to
expect people to upgrade to bleeding-edge versions of tooling. So
instead of the
buildable: False approach, I've adapted the cabal
files in question to use an impossible-to-satisfy constraint, which
the dependency solver does better with:
if impl(ghc < 8.0.2) -- Disable building with GHC before 8.0.2. -- Due to a cabal bug, do not use buildable: False, -- but instead give it an impossible constraint. -- See: https://github.com/haskell-infra/hackage-trustees/issues/165 build-depends: unsupported-ghc-version > 1 && < 1
The dependency solver realizes that it cannot find a version of the
unsupported-ghc-version package which is both greater
than and less than version 1, and so ignores this version of the
package when using GHC before 8.0.2. Problem solved!
I strongly recommend added a comment explaining this unusual
formulation, as it's not clear to a reader why such a constraint is
being added instead of the arguably clearer