Stackage HEAD is now live!

17 April 2018 — by Mark Karpov, Manuel Chakravarty

We are happy to announce that Stackage HEAD is now functional. For those who had missed the original blog post, or already forgot it, let’s refresh the idea behind Stackage HEAD.

Each Stackage Nightly is a build plan that includes a large set of real-world packages that are known to build and play nicely together. By picking a nightly plan and building it using development versions of GHC at different commits, we can detect changes in the build status of packages contained in the plan, which are caused by changes in GHC, thereby detecting potential regressions in the compiler.

Current workflow

The current workflow (which is slightly different from what was described in the original blog post) is the following:

  1. Our CI script runs in CircleCI inside a container instantiated from a Docker image derived from snoyberg/stackage:nightly. There are several reasons to use a separate image but all of them have to do with the same thing: hermeticity. The image snoyberg/stackage:nightly changes fairly regularly, but we want our results to be determined only by GHC and the current build plan, not by other factors. So we have to make sure that nothing else can affect our builds.

  2. To avoid re-compiling GHC (which takes some time), we fetch bindists and some associated metadata (such as commit, branch, tag, etc.) from S3. The new tool ghc-artifact-collector currently uploads the bindists and metadata from GHC’s CI processes run on CircleCI and AppVeyor. This tool was created to work around some limitations of AppVeyor and to provide a uniform interface to the artifact collection mechanism used on CircleCI and AppVeyor, but it came in handy for saving us time with Stackage HEAD, too.

    The tool probably deserves its own blog post. For example you can fetch the bindist created at the latest commit on GHC master, like so:

    curl --output latest-bindist.tar.xz

    You can also fetch metadata of this bindist:

    curl --output latest-metadata.json

    Which is literally what we do in our script.

  3. We pick a plan from the stackage-nightly repository. This plan should not change between tests, because we’re interested in catching changes introduced by GHC, not by packages themselves or their dependencies.

  4. We execute the build plan using stackage-curator and save the log.

  5. The build log is parsed by a helper app called stackage-head and transformed into a simple CSV file that we refer to as the build report. We’ll discuss its contents in the next section. Build reports are persisted in CircleCI caches at the moment. We accumulate more of them as more builds are attempted.

  6. stackage-head also maintains a history of build results. It has a command called diff that picks the two latest build reports and detects any changes, such as if a package stopped building or its test suites started to fail.

  7. If the changes are considered “suspicious” stackage-head diff fails with non-zero exit code and thus the whole CircleCI job fails and an email notification is sent (currently only to us). The stackage-head tool also automatically generates a Trac ticket with instructions on how to reproduce the issue and stores it as a build artifact on CircleCI. If we then decide that the failure is worth attention of GHC team, we just copy and paste the ticket.

  8. The CircleCI script is configured to run 4 times per day for a start.

Build results

Build results are currently modelled as follows:

-- | Results of building a build plan by Stackage curator typically obtained
-- through parsing of its log or by loading prepared build results file in
-- CSV format.

newtype BuildResults = BuildResults
  { unBuildResults :: HashMap Text BuildStatus
    -- ^ 'BuildStatus'es per package
  } deriving (Show, Eq)

-- | Build status.

data BuildStatus
  = BuildFailure
    -- ^ The package failed to build
  | BuildUnreachable
    -- ^ The package could not be built for some reason (e.g. its dependency
    -- failed to build—the most common case)
  | BuildSuccess !Int !Int
    -- ^ Success, 'Int's are:
    --     * the number of passing test suites
    --     * the number of failing test suites
  deriving (Show, Eq)

This is serialized into CSV files (the s tag means success, there is also f for build failures and u for unreachable packages):


Using nightly-2018-04-13, we got the following distribution:

  • Failing packages: 28
  • Unreachable packages: 826
  • Packages that build: 928

Some failing packages:

  • exceptions, blocking 118 packages
  • doctest, blocking 52 packages
  • cabal-doctest, blocking 36 packages
  • tagged, blocking 32 packages
  • generic-deriving, blocking 10 packages

In the future, we could perhaps open issues and ask maintainers of these packages to make them compile with the development version of GHC.

The full build takes about 1 hour. So if all packages start to build, we can expect the total time to be something about 2 hours with this plan.

Another interesting topic is what to consider a “suspicious” change. Quite conservatively, most changes are suspicious:

isChangeSuspicious :: Maybe BuildStatus -> Maybe BuildStatus -> Bool
isChangeSuspicious Nothing Nothing  = True -- this can't be, so suspicious
isChangeSuspicious (Just _) Nothing = True -- a package disappeared, not good
isChangeSuspicious Nothing (Just _) = True -- also strange
isChangeSuspicious (Just old) (Just new) =
  case (old, new) of
    -- There is no change, so this case won't be evaluated. But let's
    -- consider it not suspicious (nothing changes after all).
    (BuildFailure,     BuildFailure)     -> False
    -- A package became unreachable, this is suspicious.
    (BuildFailure,     BuildUnreachable) -> True
    -- Something was fixed, good. Still, we should look if any of its test
    -- suites fail.
    (BuildFailure,     BuildSuccess _ b) -> b > 0
    -- New failure, always suspicious.
    (BuildUnreachable, BuildFailure)     -> True
    -- There is no change, so this case won't be evaluated. Still
    -- unreachable, so not suspicious.
    (BuildUnreachable, BuildUnreachable) -> False
    -- This packages now builds, so really we should look if any of its test
    -- suites fail.
    (BuildUnreachable, BuildSuccess _ b) -> b > 0
    -- Now the package fails to build, suspicious.
    (BuildSuccess _ _, BuildFailure)     -> True
    -- A package became unreachable, suspicious.
    (BuildSuccess _ _, BuildUnreachable) -> True
    -- Here we should look carefully at the results of running test suites.
    (BuildSuccess p0 b0, BuildSuccess p1 b1) ->
      let moreTestSuitesFailToBuild = (p0 + b0) > (p1 + b1)
          moreTestSuitesFail        = b1 > b0
      in moreTestSuitesFailToBuild || moreTestSuitesFail

The Maybes are used to represent the possibility that a package has appeared/disappeared between in two build results which should not normally happen when we use the same build plan, but theoretically possible if we diff arbitrary build reports (which we should be able to do).

In action

Build diff is split into two sections: suspicious and innocent. Suspicious changes are those for which isChangeSuspicious returns True. A bit unexpectedly, the tool detected such a change even before we quite finished working on Stackage HEAD:


  at nightly-2018-04-05-5161609117c16cb7b29b2b8b1cd41e74341d4137.csv
    build succeeded, 2 test suites passed, 0 test suites failed
  at nightly-2018-04-05-3cfb12d8adac37e5565d66fd173e4648cc041e65.csv
    build succeeded, 1 test suites passed, 0 test suites failed

There are changes that need attention of GHC team.

Since the sum of succeeding and failing test suites decreased, this means that some test suites stopped to build. On further inspection, we determined that this change was not caused by GHC, and that forced us to improve hermeticity of the setup to prevent this from happening in the future.

Even if the change were caused by GHC, it’d not necessarily indicate a regression. Still, it’s good to be able to catch these changes instead of discovering them when a release candidate is out, or a new version of GHC is published.

CircleCI logs are publicly available here: The stackage-head project can be found here:

Running as a job in GHC’s CircleCI script

The script is currently run independently of GHC’s CI processes. There are some advantages to running Stackage HEAD as a job in GHC’s CircleCI script:

  1. We can test on a per-commit basis and precisely detect which commit introduces a change.

  2. We can see the effect of a commit before it’s merged into master.

Reasons to stick to running Stackage HEAD as a separate CircleCI script:

  1. Failure of Stackage HEAD is not necessarily a regression of GHC so it probably should not contribute to CI failures of GHC automatically.

  2. Build times may increase in the future as more packages start to build with the development version of GHC and more packages are added to Stackage.

  3. Right now: with separate setup we can configure email sending on failures whereas GHC CI is currently still always red due to failing tests.


Only time can tell how useful Stackage HEAD really is. But it looks like the experiment is worth it and may allow us to improve QA of the GHC development process. What do you think? Please let us know, or even better, please consider contributing to this new and exciting project.

About the authors

Mark Karpov

Mark is a build system expert with a particular focus on Bazel. As a consultant at Tweag he has worked with a number of large and well-known companies that use Bazel or decided to migrate to it. Other than build systems, Mark's background is in functional programming and in particular Haskell. His personal projects include high-profile Haskell libraries, tutorials, and a technical blog.

Manuel Chakravarty

If you enjoyed this article, you might be interested in joining the Tweag team.

This article is licensed under a Creative Commons Attribution 4.0 International license.


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