June 2022 - Glasgow-haskell-users

Heads up on mailing list stability
by Gershom B 29 Jun '22

29 Jun '22

Hi all! We're incrementally transitioning some of our mail sending to a new server. This list is one of the first being migrated to be relayed through the new server (we're also moving the ghc steering-committee list). If sending or receiving is dodgier than usual, please let me know! --Gershom

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Can I create a representation polymorphic datatype?
by Clinton Mead 29 Jun '22

29 Jun '22

Hi All I can make a list of unboxed ints like below: {-# LANGUAGE MagicHash #-} import GHC.Exts (Int#, Float#) data IntList = IntTail | IntNode Int# IntList intListLength :: IntList -> Int intListLength IntTail = 0 intListLength (IntNode _ rest) = 1 + intListLength rest I can then make a list of unboxed floats similarly: data FloatList = FloatTail | FloatNode Int# FloatList floatListLength :: FloatList -> Int floatListLength FloatTail = 0 floatListLength (FloatNode _ rest) = 1 + floatListLength rest But as you can see, this is getting a bit copy-pasta, which is not good. So instead, lets try this: newtype GeneralList (a :: l) = Tail | Node a (GeneralList a) This is not allowed here, I believe because `GeneralList` is expected to have one representation for all `a`, instead of a representation which depends on `a`. This is so that if one writes a function: generalListLength :: GeneralList a -> Int generalListLength Tail = 0 generalListLength (Node _ rest) = 1 + generalListLength rest You can't actually compile this into one function, because the relative location of the "next" pointer can change based on the size of `a` (assuming `a` is stored first). However, I can achieve what I want with copy pasting or Template Haskell hackery. Is there a way to get GHC to do the copy pasting for me? Or do I have to make a choice between extra runtime indirection and avoiding ugly code or having ugly code but avoiding the runtime indirection? A representation polymorphic list here is something that languages like C++, Rust, and even C# will handle happily, so Haskell seems behind here unless I'm missing something,

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GHC 9.4.1-alpha3 now available
by Ben Gamari 24 Jun '22

24 Jun '22

The GHC developers are happy to announce the availability of the third alpha release of the GHC 9.4 series. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org](https://downloads.haskell.org/ghc/9.4.1-alpha3). This major 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 incurs significantly less performance cost while still providing informative profiles. - 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. - ... 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's 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 would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk stake pool, Tweag I/O, Serokell, Equinix, SimSpace, 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 [GHC proposal 0302]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0302-c… [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [bootstrap script]: https://gitlab.haskell.org/ghc/ghc/-/blob/e2520df3fffa0cf22fb19c5fb872832d1… [Hadrian]: https://gitlab.haskell.org/ghc/ghc/-/blob/e2520df3fffa0cf22fb19c5fb872832d1… [release notes]: https://downloads.haskell.org/~ghc/9.4.1-alpha3/docs/users_guide/9.4.1-note…

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Two-factor authentication enforced on gitlab.haskell.org
by Ben Gamari 08 Jun '22

08 Jun '22

Hi all, Due to a recent up-tick in spam activity, we have started enforcing two-factor authentication on gitlab.haskell.org. We hope that this isn't too much of a burden, but do let us know if so and we can evaluate other options. Cheers, - Ben

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