Supported GHC Versions

Déjà Fu supports the latest four GHC releases, at least. For testing purposes, we use Stackage snapshots as a proxy for GHC versions. The currently supported versions are:

9.6Nightly 2021-07-

In practice, we may compile with older versions of GHC, but keeping them working is not a priority.

Adding new GHC releases

When a new version of GHC is released, we need to make some changes for everything to go smoothly. In general, these changes should only cause a patch level version bump.

  1. Bump the upper bound of base and set up any needed conditional compilation
  2. Add the GHC and base versions to the table.
  3. Remove any unsupported versions from the table.
  4. Make a patch release.

A new GHC release won't get a Stackage snapshot for little while. When it does:

  1. Add the snapshot to the GitHub Actions configuration.
  2. Update the resolver in the stack.yaml.
  3. Put the snapshot in the table.

Dropping old GHC releases

When we want to drop an unsupported version of GHC, we need to bump the version bound on base to preclude it. This is a backwards-incompatible change which causes a major version bump.

  1. Remove the dropped GHC version from the GitHub Actions configuration.
  2. Bump the lower bound of base.
  3. Look through the other dependencies. Some may not work with our new lower bound on base, so we should bump those too.
  4. Remove any now-irrelevant conditional compilation (mostly CPP, but there may also be some cabal file bits).
  5. Make whatever change required the bump.
  6. Make a major release.

GHC versions shouldn't be dropped just because we can, but here are some good reasons to do it:

  • We want to bump the lower bounds of a dependency to a version which doesn't support that GHC.
  • We want to add a new dependency which doesn't support that GHC.
  • The conditional compilation needed to keep that GHC working is getting confusing.