June 2022 - Haskell-Cafe

[ANN] cabal-clean (new tool)
by Andreas Abel 19 Aug '22

19 Aug '22

=== Wondering where your disk space goes with too many Haskell projects compiled with too many GHC versions? === https://hackage.haskell.org/package/cabal-clean https://github.com/andreasabel/cabal-clean `cabal-clean` is a small executable that removes outdated cabal build artifacts from `dist-newstyle`. A build tree is considered outdated if there is one with either - a newer version of the package you are building, or - a newer minor version of the GHC compiler. In contrast to `cabal clean`, the tool `cabal-clean` retains the latest build trees for each GHC major version. Thus, it is geared towards multi-GHC development. Example: (self-applied to cabal-clean's build directory) --- 3.7M ghc-8.8.4 /cabal-clean-0.1.20210815 --- 3.5M ghc-8.10.6/cabal-clean-0.1.20210815 --- 3.7M ghc-9.0.1 /cabal-clean-0.1.20210815 --- 3.5M ghc-8.10.6/cabal-clean-0.1.20210924 +++ 3.6M ghc-8.10.7/cabal-clean-0.1.20210924 +++ 3.7M ghc-9.0.1 /cabal-clean-0.1.20210924 The directories prefixed with --- will be deleted, the ones with +++ retained. Disclaimers: 1. cabal-clean does not check whether deletion will cause dangling references like broken symlinks etc. (Could happen if you created a symlink to an executable that resides in one of the deleted build directories.) 2. cabal-clean does not read nor utilize information from the .cabal file. 3. Calls `du` ("disk usage"), so, might not work on Windows. Enjoy, Andreas

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Where does η-equivalence stop?
by Hécate 29 Jun '22

29 Jun '22

Hi Café! I was wondering if anyone knew of a centralised list of occurrences where GHC Haskell stops upholding η-equivalence. I am interested in both type changes and operational semantics. So far I've collected the following things: * Rank-2 types * Monomorphism restriction * The presence of seq * Non-pedantic bottoms One source is the GHC Manual¹, and Neil Mitchell pointed me to the list of bugs and limitations of HLint². If you have other examples, or explanations of the mechanisms at play here, I would be very interested, and intend to upstream those in the GHC manual. Cheers, Hécate [¹] https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#e… [²] https://github.com/ndmitchell/hlint#bugs-and-limitations -- Hécate ✨ 🐦: @TechnoEmpress IRC: Hecate WWW: https://glitchbra.in RUN: BSD

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Munich Haskell Meeting, 2022-06-28 @ 19:30
by Leah Neukirchen 26 Jun '22

26 Jun '22

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, June 28 at Hirschgarten (Biergarten, self-service area) at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we'll look out for you. It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jun-2022/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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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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A pattern does not admit a type signature.
by Ignat Insarov 23 Jun '22

23 Jun '22

Hello Café! I cannot understand why the following code does not compile. ```Haskell {-# LANGUAGE UnicodeSyntax #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} module Arity where import GHC.TypeNats data Vektor length value where Sektor ∷ value → Vektor length value → Vektor (1 + length) value Mektor ∷ Vektor 0 value pattern (:>) ∷ value → Vektor length value → Vektor (1 + length) value pattern value :> values = Sektor value values infixr 5 :> ``` The error: ``` • Could not deduce: length1 ~ length from the context: (1 + length) ~ (1 + length1) bound by a pattern with constructor: Sector :: forall value (length :: Natural). value -> Vector length value -> Vector (1 + length) value, in a pattern synonym declaration at Arity.hs:16:27-45 Expected: Vector length value Actual: Vector length1 value ‘length1’ is a rigid type variable bound by a pattern with constructor: Sector :: forall value (length :: Natural). value -> Vector length value -> Vector (1 + length) value, in a pattern synonym declaration at Arity.hs:16:27-45 ‘length’ is a rigid type variable bound by the signature for pattern synonym ‘:>’ at Arity.hs:15:16-70 • In the declaration for pattern synonym ‘:>’ • Relevant bindings include values :: Vector length1 value (bound at Arity.hs:16:40) | 16 | pattern value :> values = Sector value values | ^^^^^^ ``` If I omit the pattern's type signature, the code compiles. What is going on?

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Onward! Papers 2022 - CfP
by Andreea Costea 21 Jun '22

21 Jun '22

*** Onward! Papers 2022 Call for Papers *** Onward! is a premier multidisciplinary conference focused on everything to do with programming and software: including processes, methods, languages, communities and applications. Onward! is more radical, more visionary and more open than other conferences to ideas that are well-argued but not yet proven. We welcome different ways of thinking about, approaching and reporting on programming language and software engineering research. Onward! Papers is looking for grand visions and new paradigms that could make a big difference in how we will one day build software. But it is not looking for research-as-usual papers—conferences like OOPSLA are the place for that. Those conferences require rigorous validation such as theorems or empirical experiments, which are necessary for scientific progress, but which typically preclude discussion of early-stage ideas. Onward! papers must also supply some degree of validation because mere speculation is not a good basis for progress. However, Onward! accepts less rigorous methods of validation such as compelling arguments, exploratory implementations, and substantial examples. The use of worked-out examples to support new ideas is strongly encouraged. https://2022.splashcon.org/track/splash-2022-Onward-papers The submission deadline is July 10th, 2022 AOE.

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Re: [Haskell-cafe] traversal with an arrow
by Olaf Klinke 19 Jun '22

19 Jun '22

> On Fri, Jun 17, 2022 at 05:01:55PM +0200, Olaf Klinke wrote: > > Is there prior art to the following generalisation? > > > traverseArrow :: Traversable t => a x y -> a (t x) (t y) > > Perhaps you are looking for this: > > https://github.com/tomjaguarpaw/Arrows2/issues/3#issuecomment-561973678 > Thanks Tom, can you explain what's behind this implementation, for those unfamiliar with arrow syntax? I guess [*] that the Traversal type used is a manifestation of the fact that one can transform any traversable structure (t a) into ([a],t ()) and sort the contents of the linked list back into their original places? This way, arbitrary traversals can be reduced to traversals over linked lists. Then the ArrowChoice is needed to pattern match on the list constructors? E. Kmett's profunctor implementation wanderA, linked to from the end of the discussion, seems to be missing from the current Data.Profunctor.Traversing module. Olaf [*] Structurally, Traversal a r b ~ Free (Const a :*: ((->) r)) b and also their Applicative instances seem to match on first glance. In particular, Traversal a () b ~ ([a],b) whence traversal :: Traversable t => t a -> Traversal a b (t b) can be specialized to traversal :: Traversable t => t a -> ([a],t ())

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traversal with an arrow
by Olaf Klinke 17 Jun '22

17 Jun '22

Dear Café, Is there prior art to the following generalisation? A Hoogle search for the type signatures did not turn up anything. import Control.Arrow import Control.Monad (foldM) -- | Categories which every Traversable is a functor in. -- For Traversable t, the instance should satisfy -- -- @ -- foldMapArrow f = foldMapArrow id . traverseArrow f -- @ class Arrow a => ArrowTraverse a where traverseArrow :: Traversable t => a x y -> a (t x) (t y) foldArrow :: Foldable t => a (y,x) y -> a (y,t x) y instance ArrowTraverse (->) where traverseArrow = fmap foldArrow f = uncurry ((foldl.curry) f) instance Monad m => ArrowTraverse (Kleisli m) where traverseArrow (Kleisli k) = Kleisli (mapM k) foldArrow (Kleisli k) = (Kleisli . uncurry) (foldM (curry k)) -- | Generalizes foldMap. -- For Kleisli m, this function is also known as foldMapM. foldMapArrow :: (ArrowTraverse a, Foldable f, Monoid y) => a x y -> a (f x) y foldMapArrow f = (arr (const mempty) &&& id) >>> foldArrow ((id *** f) >>> arr (uncurry mappend)) The thing is that there are more instances for this class, for example by using Ross Paterson's arrow transformers [1]. I found myself implementing an arrow that is a Kleisli arrow with a reader context. Neither the standard arrow machinery nor the arrows package seem to grant me the power to write a traverseArrow for it. Either it can't be done, or my arrow-fu is not strong enough. Olaf [1] https://hackage.haskell.org/package/arrows

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邀请： Deadline Extended: 26th Brazilian Symposiu... @ 2022年6月17日 (周五) 下午6:30 - 下午7:30 (CST) (haskell-cafe@haskell.org)
by rippletc＠gmail.com 17 Jun '22

17 Jun '22

[Haskell-cafe] Deadline Extended: 26th Brazilian Symposium on Programming Languages (SBLP 2022) 2022年6月17日 (星期五) ⋅ 下午6:30 – 下午7:30 台北标准时间通过 Google Meet 加入视频会议 https://meet.google.com/wvx-gvgo-mmj?hs=224 SBLP 2022 is the 26th edition of the Brazilian Symposium on Programming Languages. It is promoted by the Brazilian Computer Society (SBC) and constitutes a forum for researchers, students, and professionals to present and discuss ideas and innovations in the design, definition, analysis, implementation, and practical use of programming languages. SBLP's first edition was in 1996. Since 2010, it has been part of CBSoft, the Brazilian Conference on Software: Theory and Practice. The Conference is planned to be held on October 3-7, 2022, fully online. Submission Guidelines Papers can be written in Portuguese or English. Submissions in English are encouraged considering that only accepted papers written in English will appear in the proceedings, usually indexed in the ACM Digital Library (to be confirmed). The acceptance of a paper implies the registration of at lea... 组织者 rippletc(a)gmail.com rippletc(a)gmail.com 邀请对象 rippletc(a)gmail.com- 组织者 cdvasconcellos(a)hotmail.com haskell-cafe(a)haskell.org 查看所有邀请对象信息 https://calendar.google.com/calendar/event?action=VIEW&eid=M3RxajQ0a2pkamF2… 回复 haskell-cafe(a)haskell.org 的邀请并查看更多详细信息 https://calendar.google.com/calendar/event?action=VIEW&eid=M3RxajQ0a2pkamF2… 您可以选择是否参加。 ~~//~~ 来自 Google 日历的邀请：https://calendar.google.com/calendar/ 您之所以收到这封电子邮件，是因为您是活动的参加者。如要停止接收有关此活动的后续更新，请拒绝此活动。转发此邀请后，任何收件人都可以向组织者发送回复，也可以被添加到邀请对象列表；无论其邀请状态为何，他们都可以邀请其他人；他们还可以修改您的回复。了解详情https://support.google.com/calendar/answer/37135#forwarding

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Post Doc Vacancy at St Andrews
by Christopher Brown 17 Jun '22

17 Jun '22

The Programming Languages Group at the University of St Andrews seeks a post-doctoral research fellow for a fixed 12-month position. Details of the position can be found here: https://www.vacancies.st-andrews.ac.uk/Vacancies/W/4238/0/352367/889/resear… Vacancy Description School of Computer Science Salary: £34,304 per annum Start date: 1 October 2022 or as soon as possible thereafter Fixed-term: 12 months Applications are sought for a committed Post-doctoral Research Fellow to work with Dr Chris Brown conducting research for a EPSRC funded project entitled Energise: Refactorings and Skeletons for Energy-Aware Applications on High-Performance Embedded Systems. The primary duties will be to implementing refactorings for target languages such as C; proving general soundness of the refactorings using e.g. dependent types; developing new programming abstractions (skeletons) for abstracting common energy-reducing programming patterns; running experiments; writing proofs of correctness. The successful applicant will have (or be near to completion of) a PhD in Programming Languages with expertise in formal semantics of programming languages, compilers, program transformation, lightweight formal methods (including, e.g. dependent types) and/or knowledge of parallelism and non-functional properties, including energy. The post is available on a fixed-term basis for 12 months starting 1st October or as soon as possible thereafter. Further details of the project can be found by contacting Dr Christopher Brown (cmb21(a)st-andrews.ac.uk<mailto:cmb21@st-andrews.ac.uk>). Applications are particularly welcome from women, people from the Black, Asian and Minority Ethnic (BAME) community, and other protected characteristics who are under-represented in research posts at the University. Equality, diversity and inclusion are at the heart of the St Andrews experience. We strive to create a fair and inclusive culture demonstrated through our commitment to diversity awards (Athena Swan, Carer Positive, LGBT Charter, Race Charters and Stonewall). We celebrate diversity by promoting profiles of BAME, LGBTIQ+ staff and supporting networks including the Staff BAME Network; Staff with Disabilities Network; Staff LGBTIQ+ Network; and the Staff Parents & Carers Network. Full details available online: https://www.st-andrews.ac.uk/hr/edi/ Closing Date: 29 July 2022 Interview Date: 23 August 2022 Please quote ref:AR2699GB School of Computer Science Salary: £34,304 per annum Start date: 1 October 2022 or as soon as possible thereafter Fixed-term: 12 months

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