Monday, December 30, 2013

Dynamic-cabal to solve dependency hell in BuildWrapper

As I've explained in several places, we have an issue in BuildWrapper (the underlying Haskell program for the EclipseFP plugins): BuildWrapper depends on both the GHC API and the Cabal API. Since GHC itself has a dependency on Cabal, once you've installed GHC, BuildWrapper can only use the same version of the Cabal API that was used by that GHC. Moreover, some Cabal files (setup-config in the dist directory for example) are cabal version dependent (since they only serialize Haskell structures, this is for safety). The net result is that if you install a newer version of Cabal and cabal-install, BuildWrapper still uses the old version, and hence the Cabal library BuildWrapper uses and the cabal-install executable have different versions, and hell breaks loose.

I have looked at solving the issue in GHC itself: GHC does not need to have a dependency to the Cabal API. I had started investigating with help from GHC devs, but it was end of October, and I had understood that GHC 7.8 was to be released in November, so there was no time to do such a change. So I thought I would wait for the next release. But it's now end of December, and GHC 7.8 still hasn't been released. So should we have to wait another year to solve that issue? Gasp!

But then, out of the blue, help came! Benno Fünfstück released dynamic-cabal, with the promise that it would solve exactly this issue! I actually should have had that idea myself, but hey... Basically, dynamic-cabal provides simpler structures than the full Cabal API, and extracts them from the Cabal API via dynamic code generation. The code is generated and ran dynamically using the GHC API. Since the running code itself doesn't depend on GHC, it can use the latest version of the Cabal library, and hence read the setup-config file properly!

I have now rewritten parts of BuildWrapper to use dynamic-cabal, and I only had to add minor enhancements to dynamic-cabal. I have upgraded my own cabal-install to 1.18 and everything works (my GHC still uses Cabal 1.16)!! I'm not releasing that version yet because it will provide some new features for EclipseFP 2.6, but if really you want to give it a try just build it from Github.

So my thanks go to Benno! A big hurdle has been cleared on EclipseFP's path to world domination!

Sunday, December 29, 2013

EclipseFP 2.5.6 released

It's my pleasure to announce a new version of the Eclipse plugins for Haskell development, version 2.5.6! This is a minor, bug fixing release. The release notes can be found here.

To install, just point Eclipse to the update site:

I expect that the next version will be a 2.6 with more features, as hopefully I'll have more free time.

Happy Haskell Hacking!