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I'm sure a number of readers have already seen something about the situation around Stack and Stackage Nightly/GHC 8.2. I tried to clarify how this happened on the relevant Github issue, plus the GHC trac ticket, but thought I'd reshare as a blog post for others who are interested.

EDIT Right after publishing, I saw that Stack 1.6.1 was released, so you should probably just run stack upgrade. Keep reading if you're curious on the bug.

The problem

When the first releases of Stackage Nightly for GHC 8.2.1 started coming out some months back, they did not work with Stack 1.5.0, due to an issue with the ghc.cabal file on Hackage. The reason for this is explained below. We made a point release (Stack 1.5.1) which worked around the issue temporarily, until Stack 1.6 was released with the complete fix.

In the interim, GHC 8.2.2 was released, and Stackage Nightly switched over to it. Contrary to my initial claims: this was a red herring and unrelated to anything.

On December 4, integer-gmp- was uploaded to Hackage, which reintroduced all of the breakage we had with Stack 1.5.0. Since our point release had a very targetted workaround (specifically for ghc.cabal), it did not work around the same bug occurring for integer-gmp.cabal. Therefore, all versions of Stack before 1.6 will fail to build a Stackage release with GHC 8.2.

The workaround

The best "workaround" is just a new release: Stack 1.6 was fortunately already in release candidate mode, and as I type this up it's going through the standard release process. By the time I hit publish, the workaround may be to run stack upgrade.

If that's not the case, you can upgrade to the release candidate by running:

stack upgrade --binary-version

Cabal background

In order to understand the explanation, you should be aware of a few different things that are all called Cabal:

  • cabal-install, the build tool. This is not relevant to the explanation below
  • Cabal the library. This is a normal Haskell library which Stack depends on, and is used for (among other things) parsing cabal files.
  • Cabal the file format. If you open up virtually any cabal file you'll see a cabal-version: >= 1.10 looking field. This is stating which version of the Cabal file format is being used. New versions of Cabal-the-library may add new features to the Cabal file format. The version of the format tracks the library version it was released with, so that a cabal file stating cabal-version: >= 1.24 can only be parsed by Cabal-the-library 1.24 or later.

There was an addition made to Cabal-the-file-format 2.0: a ^>= operator. This operator is not parseable by older versions of Cabal the library (meaning: Cabal 1.24 or earlier). Stack 1.5 was built against Cabal-the-library 1.24, and therefore cannot parse any Cabal files using this new operator.

The Stackage build process prevents any such Cabal files from being used yet to give tooling (like Stack) a chance to upgrade, something I've requested of Hackage as well. However, there are some packages which ship with GHC itself, and which Stackage has no control over in the creation of a snapshot. This includes packages like base, ghc, and integer-gmp.

Original breakage

There's a short explanation (and some code to demonstrate it!) for the original breakage with GHC 8.2.1 in the pull request:

Prior to Stack 1.6, there was a bug where Stack would try to get some metadata about libraries that shipped with GHC from their cabal files instead of directly from the package database. Historically, this has never been a problem, which is why it's survived in Stack for so long. The reason is that, historically, GHC-shipped packages did not use bleeding-edge features in their cabal files.

When GHC 8.2.1 was released, the ghc.cabal file uploaded to Hackage did something new: it used a feature of the newly released Cabal 2.0 file format (the ^>= operator) and required the new Cabal 2.0 library to parse it. This occurred before Stack had a chance to upgrade to Cabal-the-library 2.0, and for that matter before cabal-install 2.0 was released. In other words: at the time the file was placed on Hackage, no officially released version of any common tool supported it.

For unrelated reasons, I'd already fixed this bug on master as part of a refactoring. Strangely enough, that refactoring had to do with problems with revisions. Thanks to the revision system, it's not possible to rely on cabal files on Hackage to tell you anything about GHC-installed packages, since we can't know for certain which revision was used to build the package. (We'll get to integer-gmp in a moment, which is slightly worse in this regard.)

The behavior of Stack at this time with regard to GHC-shipped packages was the following (and this is a bug):

  • If the cabal file cannot be found: ignore the package entirely. This is necessary for packages like rts.
  • If the cabal file is found: try to parse it, and fail if the parse fails.

It was this second bullet which caused a problem. When we discovered this, we released an emergency patch release of Stack to work around this situation and simply ignore parse failures from ghc.cabal. We did not embark on a bigger fix because:

  1. A bigger fix would involve much more code change, introducing the chance for regressions
  2. We already had a fix on master, and knew that Stack 1.6 would be released before GHC 8.4

This went out the door, and all users who upgraded to Stack 1.5.1 were able to use the new Stackage Nightly snapshots based on GHC 8.2.2.

December 4, 2017

One of the packages that ships with GHC 8.2 is integer-gmp- Until December 4, this package was not uploaded to Hackage. As a result, Stack 1.5.1 simply ignored the package entirely, which worked fine. However, something we didn't anticipate happened:

  • Months after the GHC 8.2.1 release, integer-gmp- was uploaded to Hackage
  • The cabal file that was uploaded was manually modified to use Cabal-the-format 2.0 features (again, the ^>= operator).

You can compare the file on Hackage with the file on Github. It's unclear what the motivation was behind this modification, but this modification is what broke Stack 1.5.1 and GHC 8.2.

Before this upload, the missing integer-gmp.cabal file was simply ignored by Stack. Once it was uploaded, Stack (again, as a bug) tries to parse it, fails, and gives up.

The future

Obviously there was a bug in Stack that needed to be fixed, and has been fixed. However, the irregularities around the ghc.cabal and integer-gmp.cabal files are a little troubling, and make it difficult to predict future behavior. Hopefully some new policies from GHC HQ will address these concerns.

And while this case is a bug in Stack, I want to clarify a general point. It is entirely expected that over time, older releases of Stack will not be able to use newer Stackage snapshots. At some point in the future, Stackage will allow Cabal 2.0-formatted cabal files into snapshots, and then by design Stack 1.5 and earlier will be unable to parse those files. That's unfortunate, but expected. What's unexpected in this case was that

  1. These cabal files slipped into a snapshot through the back door (GHC's package database) so quickly, before Stack 1.6 was out the door
  2. That actions taken post-GHC release (a new upload of integer-gmp.cabal) could affect existing snapshots.

Both points will hopefully be hit both by the fixes that landed on Stack 1.6 ensuring less eager parsing of cabal files, and changes in GHC HQ policy.


  1. There's a bug in Stack, triggered by new behavior not seen before by GHC
  2. That bug affects reproducibility, because an upload to Hackage in the future (or a revision for that matter) can break existing build plans
  3. This bug is fixed on master fully (AFAICT, we've added an integration test to check for regressions)
  4. Instead of putting out another emergency Stack 1.5 patch for integer-gmp.cabal, we're going to get Stack 1.6 out the door ASAP

I hope that clarifies. This is definitely an unfortunate situation, and I know it's screwed up people's development, so my apologies on that front. I hope for all our sakes (mine included!) that the situation is more stable going forward.

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