GHC 9.4.1-rc1 released

bgamari - 2022-07-21

The GHC developers are happy to announce the availability of the first (and likely last) release candidate of GHC 9.4.1. Binary distributions, source distributions, and documentation are available at

This release will include:

  • A new profiling mode, -fprof-late, which adds automatic cost-center annotations to all top-level functions after Core optimisation has run. This provides informative profiles while interfering significantly less with GHC’s aggressive optimisations, making it easier to understand the performance of programs which depend upon simplification..

  • A variety of plugin improvements including the introduction of a new plugin type, defaulting plugins, and the ability for typechecking plugins to rewrite type-families.

  • An improved constructed product result analysis, allowing unboxing of nested structures, and a new boxity analysis, leading to less reboxing.

  • Introduction of a tag-check elision optimisation, bringing significant performance improvements in strict programs.

  • Generalisation of a variety of primitive types to be levity polymorphic. Consequently, the ArrayArray# type can at long last be retired, replaced by standard Array#.

  • Introduction of the \cases syntax from GHC proposal 0302.

  • A complete overhaul of GHC’s Windows support. This includes a migration to a fully Clang-based C toolchain, a deep refactoring of the linker, and many fixes in WinIO.

  • Support for multiple home packages, significantly improving support in IDEs and other tools for multi-package projects.

  • A refactoring of GHC’s error message infrastructure, allowing GHC to provide diagnostic information to downstream consumers as structured data, greatly easing IDE support.

  • Significant compile-time improvements to runtime and memory consumption.

  • On overhaul of our packaging infrastructure, allowing full traceability of release artifacts and more reliable binary distributions.

  • Reintroduction of deep subsumption (which was previously dropped with the simplified subsumption change) as a language extension.

  • … and much more. See the release notes for a full accounting.

Note that, as 9.4.1 is the first release for which the released artifacts will all be generated by our Hadrian build system, it is possible that there will be packaging issues. If you enounter trouble while using a binary distribution, please open a ticket. Likewise, if you are a downstream packager, do consider migrating to Hadrian to run your build; the Hadrian build system can be built using cabal-install, stack, or the in-tree bootstrap script. We will be publishing a blog post describing the migration process to Hadrian in the coming weeks.

We would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk stake pool, Tweag I/O, Serokell, Equinix, SimSpace, Haskell Foundation, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release.

As always, do give this release a try and open a ticket if you see anything amiss.

Happy testing,

  • Ben