This first alpha comes quite a bit later than expected. However, we have
done a significant amount of testing on this pre-release and therefore
hope to be able to move forward quickly with a release candidate next
week and with a final release in mid-October.
GHC 9.0.1 will bring a number of new features:
* A first cut of the new LinearTypes language extension , allowing
use of linear function syntax and linear record fields.
* A new bignum library (ghc-bignum), allowing GHC to be more easily
used with integer libraries other than GMP.
* Improvements in code generation, resulting in considerable
performance improvements in some programs.
* Improvements in pattern-match checking, allowing more precise
detection of redundant cases and reduced compilation time.
* Implementation of the "simplified subsumption" proposal 
simplifying the type system and paving the way for QuickLook
impredicativity in GHC 9.2.
* Implementation of the QualifiedDo extension , allowing more
convenient overloading of `do` syntax.
* Improvements in compilation time.
And many more. See the release notes  for a full accounting of the
changes in this release.
Do note that there are a few things that we expect will change before
the final release:
* We expect to sort out a notarization workflow for Apple Darwin,
allowing our binary distributions to be used on macOS Catalina
Until this has been sorted out Catalina users can exempt the
current macOS binary distribution from the notarization requirement
themselves by running `xattr -cr .` on the unpacked tree before
running `make install`.
* We will likely transition the Alpine binary distribution to be fully
statically-linked, providing a convenient, distribution-independent
packaging option for Linux users.
* We will be merging a robust solution for #17760 which will introduce
a new primitive, `keepAlive#`, to the `base` library, subsuming
most uses of `touch#`.
As always, do test this release and open tickets for whatever issues you
encounter. To help with this, we will be publishing a blog post
describing use of our new `head.hackage` infrastructure to ease testing
of larger projects with Hackage dependencies later this week.
> Hi Ben,
> What are the current plans / schedule for 9.0.1?
At the moment things are blocked on a solution to #17760, which I am
currently in the process of working through. There have been several
false-starts on this ticket and while the solution we are ending up with
is indeed a compromise (in both performance impact and convenience), I
am fairly convinced it is the best we can do.
I am waiting for a version of the patch to validate as we speak. As soon
as it (and a few other relevant patches) have been backported I will
move ahead with cutting the release candidate. I hope this can happen by
the end of the week.