- Haskell-Cafe - Haskell.org

OverloadedStrings, but with compile-time invariants
by Max Ulidtko 10 Dec '23

10 Dec '23

Greetings @cafe, Transparent wrapping of string literals into custom types, driven automatically by type inference, is of course very useful. I'm getting huge mileage off GHC's OverloadedStrings when working with e.g. the Text type: class IsString a where fromString :: String -> a instance IsString Text {- ... -} greeting :: Text greeting = "Hello" But I hope there's possibly room for improvement. I've got a bag of several use-cases; perhaps the simplest is the following. Consider a type called NonEmptyText which is a (wrapper around) Text, but with an added invariant that the contained text is never the empty string "". Depending on codebase, such a type can be very useful too, allowing to offload the burden of keeping track of the invariant onto machine; the typechecker will show me all the spots to handle, current or future ones. IOW, this type enables writing a non-boolean-blind null check, toNonEmptyText :: ToText str => str -> Maybe NonEmptyText — which makes not only the programmer, but also the compiler, aware of the null-check result. I'm focusing the example on Text to somewhat maintain generality of the idea. NonEmpty Char can be an alternative formulation of "non-empty String"; length-indexed vectors could possibly handle this too. But there're all sorts of invariants besides non-emptiness one may wish to express. I'll omit other examples; but to summarize, I've got plenty of practical motivation to get "smart literals". And to the point: IsString as it stands, forces us to write much unsatisfactory instances: instance IsString NonEmptyText where fromString (c:cs) = NonEmptyText.new c cs fromString [] = error "unavoidable runtime panic" -- argh ... despite fromString being invoked, presumably with OverloadedStrings, on a string literal — which is statically known at compile-time. I'd imagine upgrading the interface with GHC's own TypeLits: class KnownSymbol lit => IsString' str lit where fromString' :: proxy lit -> str This 2-parameter formulation of IsString enables pretty straightforward user code, along the lines of: type family MyInvariant (s :: Symbol) :: Constraint where MyInvariant "" = TypeError ('Text "Empty string literal") MyInvariant _s = () instance (MyInvariant lit, KnownSymbol lit) => IsString' NonEmptyText lit where fromString' = nothingImpossible . NeT.fromText . toText . symbolVal where nothingImpossible (Just x) = x nothingImpossible Nothing = error "typelevel invariant violated" text1, text2 :: NonEmptyText text1 = fromString' $ Proxy @"hello, types" text2 = fromString' $ Proxy @"" -- compile error here, as expected! With the primes, this is possible to write today. With AllowAmbiguousTypes, the unwieldy syntax can be abbreviated somewhat, e.g. smartLiteral @"overloaded string literal with compiler-checked invariant" — but is still rather awkward... in the same way that ubiquitous T.pack is annoying enough to want to enable OverloadedStrings and stop seeing it all over the place. The question is, could this IsString' somehow become the IsString that OverloadedStrings is friends with? I guess, not soon, with compatibility in mind... But in principle? Is it even worth trying to pursue the GHC Proposal path? Perhaps a new extension OverloadedTypelevelStrings? Relatedly... Can a GHC Plugin do this? And overall, to zoom out of "XY Problem" pitfalls: am I missing a neater way to have "partial" IsString instances, those that can reject some literals at compile-time? Having this code work is the goal: text1, text2 :: NonEmptyText text1 = "hello, types" text2 = "" -- compile error Best regards, Max

7 6

[ANN] mig-2.1 minimal server library
by Anton Kholomiov 10 Dec '23

10 Dec '23

I'm happy to announce a new release of the mig library. It's a minimal and simple library to build servers. This release adds some goodies for servers that produce HTML: * cookies (new [input Cookie]( https://hackage.haskell.org/package/mig-0.2.1.0/docs/Mig-Core-Types-Route.h…) type and [function to set]( https://hackage.haskell.org/package/mig-0.2.1.0/docs/Mig-Core-Class-Respons…) the cookies) * [type-safe stable URLs]( https://hackage.haskell.org/package/mig-0.2.1.0/docs/Mig-Core-Class-Url.html) from server definition * an example [HtmlTemplate]( https://github.com/anton-k/mig/tree/main/examples/mig-example-apps/HtmlTemp…) on how to use template engine and type-safe URLs with the library Also it adds a stack template to create new hello-world server with stack (on how to use it see the section on how to start [a new mig project]( https://anton-k.github.io/mig/#how-to-start-a-new-project) in the tutorial). Links: * [github](https://github.com/anton-k/mig) * [tutorial](https://anton-k.github.io/mig/) * [hackage for the server library]( https://hackage.haskell.org/package/mig-server) * [hackage for the core library](https://hackage.haskell.org/package/mig) * [examples]( https://github.com/anton-k/mig/tree/main/examples/mig-example-apps#mig-exam… )

1 0

PLDI'24 Artifact Evaluation Committee: Call for nominations
by Ningning Xie 08 Dec '23

08 Dec '23

Dear all, We are looking for motivated students and researchers to be members of the PLDI 2024 Artifact Evaluation Committee (AEC). The artifact evaluation process aims to promote, share and catalog the research artifacts of papers accepted to the PLDI research track. This year, we are accepting self-nominations for the AEC. The self-nomination form is available at: https://forms.gle/wWGfW33qMY2gUqqz7 The first round of nomination ends on Dec 22nd, 2023. As a committee member, the primary responsibilities would be to review the artifacts submitted corresponding to the already accepted papers in the main research track. In particular, you may have to run the associated tool, check whether the results in the main paper can be reproduced, and inspect the data. We expect the bulk of the review work to take place between early February and mid April. Each artifact will take about 8h to review, and reviewers will be assigned about 3 artifacts to review. Come join us in improving the quality of research in our field! Best regards, Manuel Rigger and Ningning Xie

1 0

CGO'24 - Student Research Competition (SRC) and more
by L. Cicolini 08 Dec '23

08 Dec '23

Dear all, The International Symposium on Code Generation and Optimization (CGO) is hosting a Student Research Competition (SRC). Our SRC is a unique opportunity for undergraduate and graduate students to present their original research before a panel of judges and attendees at CGO'24. The winner will be invited to the Grand 2024 ACM SRC competition. If you are an undergraduate or postgraduate student doing research in compiler-related topics, consider applying. We look forward to meeting you! CGO'24 is a compiler conference happening in Edinburgh, March 2nd to 6th 2024. This conference is a great opportunity for anyone interested in compilers to connect with great researchers and professionals and will host four existing workshops: LLVM [1], the BuildIt Framework [2], LATHC [3], and C4ML [4]. Find more information on https://conf.researchr.org/home/cgo-2024 and share this message with anyone who might be interested! Best, Luisa Cicolini [1] LLVM Performance Workshop: https://llvm.org/devmtg/2024-03/ [2] BuildIt: Building DSLs made easy with the BuildIt Framework: https://buildit.so/tutorial/ [3] LATHC: Languages, Architectures, and Tools for Heterogeneous Computing Workshop: https://jnamaral.github.io/LATHC24/ [4] C4ML - Compilers For Machine Learning: https://www.c4ml.org/c4ml-2024

1 0

Haskell.org Call for Nominations (for the terms 2024-2027)
by Ida Bzo 05 Dec '23

05 Dec '23

Dear Haskellers, We are pleased to announce that nominations are now open for the Haskell.org committee. You can nominate yourself or a friend for a three-year term (2024-2027) by sending an email to [committee at haskell.org] by January 10, 2024. Self-nominations and re-nominations are also welcome. Please include any relevant information about yourself or your nominee that you think will help us make our decision. Committee members do not have to be technical experts in Haskell. We are looking for people who are enthusiastic about improving the Haskell community and come from a variety of backgrounds, such as academia, industry, open-source development, and community building. Our goal is to represent the various facets of the Haskell world, including gender, race, location, and industry or research. The committee's responsibilities include setting policies, providing guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with Haskell.org funds and donations. Being a committee member does not require a significant amount of time, but members should be responsive during discussions and should be able to attend monthly calls and participate in the Haskell.org Slack and mailing lists. Candidates for the committee should possess strong leadership, communication, and judgment skills. They must be able to set aside personal or business-related biases and make decisions with the good of the open-source Haskell community in mind. For more information about the committee's roles and responsibilities, please visit https://www.haskell.org/haskell-org-committee/ If you have any questions about the nomination process, please feel free to email us at [committee at haskell.org] or contact one of us individually. λCheers, Ida Bzo

1 0

Download stats not updated on Hackage
by Harendra Kumar 05 Dec '23

05 Dec '23

Hi, It seems the download count for packages on Hackage is stuck for quite some time now. Is this issue known to the Hackage maintainers? -harendra

1 0

Why Choose Digitology.co as Your Digital Marketing Agency in Egypt
by Ihab Mohsen 05 Dec '23

05 Dec '23

Looking for an exceptional [digital marketing agency in Egypt](https://digitology.co)? Look no further than [Digitology.co](https://digitology.co)! Renowned as Egypt's premier [digital marketing agency](https://digitology.co), we specialize in enhancing online presence and fostering unparalleled success for your business. What Sets [Digitology.co](https://digitology.co) Apart as Egypt's Best [Digital Marketing Agency](https://digitology.co)? 1. Unmatched Expertise in [Digital Marketing](https://digitology.co) At [Digitology.co](https://digitology.co), our team comprises seasoned professionals excelling in various digital marketing facets. From tailored [SEO strategies for the Egyptian market](https://digitology.co) to impactful social media campaigns, our experts craft personalized solutions to meet your unique business needs. 2. Proven Track Record of Success As Egypt's [top digital marketing agency](https://digitology.co), our consistent delivery of outstanding results speaks volumes. We've significantly enhanced visibility, traffic, and conversions for numerous clients. Our success stories testify to our unwavering commitment to excellence. 3. Comprehensive [SEO and Online Marketing](https://digitology.co) Approach Understanding the importance of a holistic approach, our strategies encompass diverse techniques like content optimization, link building, and technical SEO. This ensures prominent search engine rankings for your website. Why [Digitology.co](https://digitology.co) Stands Out Among Egypt's [SEO Agencies](https://digitology.co) As a leading [SEO agency in Egypt](https://digitology.co), we prioritize driving organic growth and maximizing online presence. Our tailored strategies aim to improve website visibility, increase organic traffic, and enhance conversions by resonating with Egyptian audiences. Choose [Digitology.co](https://digitology.co) for Unparalleled [Digital Marketing Solutions](https://digitology.co) Partnering with us grants access to cutting-edge strategies, personalized solutions, and a dedicated team committed to your success. We aim to propel your business to new heights through innovative, results-oriented digital marketing strategies. Take the Next Step Towards Success Ready to elevate your digital presence? Partner with [Digitology.co](https://digitology.co), Egypt's [best digital marketing agency](https://digitology.co). Contact us today to explore how our tailored solutions can revolutionize your online presence and drive tangible business growth. Explore [Digitology.co](https://digitology.co)'s services and witness firsthand how we transform your digital marketing endeavors. At [Digitology.co](https://digitology.co), your success is our priority! For more info, visit the best digital marketing agency in Egypt [here](https://digitology.co)

3 2

Elevate Safety with ATISystems.com: Your Destination for Cutting-Edge Giant Voice and Outdoor Warning Systems
by Ihab Mohsen 05 Dec '23

05 Dec '23

Ensuring safety and security in outdoor spaces stands as a paramount concern, and [ATISystems.com](http://atisystems.com/) leads the way with innovative solutions crafted to safeguard communities and businesses. Specializing in state-of-the-art giant voice systems and outdoor warning sirens, [ATISystems.com](http://atisystems.com/) offers comprehensive solutions tailored to meet your safety requirements. What's Available at [ATISystems.com](http://atisystems.com/)? 1. Giant Voice Systems [ATISystems.com](http://atisystems.com/) stands out in the industry by providing robust giant voice systems designed to effectively broadcast emergency messages across expansive outdoor areas. These systems play a crucial role in emergency preparedness, facilitating clear and immediate communication during critical situations. 2. Outdoor Warning Sirens Offering a range of outdoor warning sirens, [ATISystems.com](http://atisystems.com/) ensures that communities and facilities have access to resilient alerting mechanisms. These sirens are engineered to emit high-decibel warnings, alerting individuals outdoors to potential threats or emergencies, thus enhancing overall safety protocols. Why Opt for [ATISystems.com](http://atisystems.com/) for Your Safety Solutions? 1. Cutting-Edge Technology [ATISystems.com](http://atisystems.com/) leverages cutting-edge technology in the development and deployment of their giant voice systems and outdoor warning sirens. The integration of advanced features ensures reliability and effectiveness precisely when it matters most. 2. Tailored Solutions Recognizing the uniqueness of each location and scenario, [ATISystems.com](http://atisystems.com/) offers customized solutions to match specific safety requirements. Whether for municipalities, industrial sites, or educational campuses, their systems can be tailored for optimal performance. 3. Dedication to Safety At the heart of [ATISystems.com](http://atisystems.com/) lies an unwavering commitment to safety. Their solutions are engineered to offer peace of mind, enabling swift and efficient communication during emergencies, thereby minimizing potential risks. Discover Unmatched Safety Solutions at [ATISystems.com](http://atisystems.com/) The dedication of [ATISystems.com](http://atisystems.com/) in delivering top-tier giant voice systems and outdoor warning sirens positions them as the go-to resource for enhancing outdoor safety measures. Their comprehensive range of products and services equips you with the necessary tools to mitigate risks and safeguard lives. Secure Your Environment Today Explore the cutting-edge solutions offered by [ATISystems.com](http://atisystems.com/) and take essential steps to fortify safety in your outdoor spaces. Delve into their giant voice systems and outdoor warning sirens to enhance your emergency preparedness. For more information, please visit: [Giant Voice System]([https://atisystems.com](https://atisystems.com/)), [Outdoor Warning System]([https://atisystems.com](https://atisystems.com/)), [Outdoor Warning Siren]([https://atisystems.com](https://atisystems.com/)) Visit [[ATISystems.com](http://atisystems.com/)]([https://atisystems.com](https://a… today and empower your organization or community with reliable and effective safety solutions. With [ATISystems.com](http://atisystems.com/), safety is priority.

1 0

garn: a Nix/Cabal/Stack alternative
by Julian Arni 03 Dec '23

03 Dec '23

Dear list, garn is a new take on Nix - configured in Typescript rather than the Nix language, and with a nicer and simpler CLI (thanks optparse-applicative!). The repository is https://github.com/garnix-io/garn, and the website is https://garn.io/. I'm mentioning it here because we just added better Haskell support, which makes it now a plausible alternative to managing your Haskell projects with Nix, Stack or Cabal. An example Haskell project: import * as garn from "https://garn.io/ts/v0.0.18/mod.ts"; import * as pkgs from "https://garn.io/ts/v0.0.18/nixpkgs.ts"; export const project = garn.haskell.mkHaskellProject({ description: "My project", src: ".", executables: ["server", "openapi-generation"] overrideDependencies: { "servant": "0.19.1" }, ghcVersion: "ghc94" }).addExecutable("format")`${pkgs.ormolu}/bin/ormolu --mode inplace $(find . -name '*.hs')` Which allows for `garn build project` to build the project, and `garn enter project` to be in a devshell with cabal, ghc94 and all the dependencies installed, and `garn run project.format` for formatting all files with ormolu. (`garn init` is capable of generating most of that file for you.) # Comparisons First, compared to all of the options below, garn is much less mature - there'll be rougher edges or features not yet supported. It's also still changing very quickly, and backwards compatibility is not, for now, a priority. - Cabal: Like Cabal, garn still uses a cabal file, and in fact expects you to develop with cabal. garn brings in the dependencies itself and makes them known to ghc/cabal, and they're snapshot-based rather than resolver-based. In addition, garn makes it easy to include system (non-Haskell) dependencies, it sandboxes builds and tests for better reproduciblity (making CI in particular quite easy); it allows for shared caches; it supports scripts for project janitoring and management; and it supports devshells. - Stack: Like Stack, garn uses a snapshot-based approach to dependencies. In fact, it's based on the stackage releases. It also supports overriding versions from Hackage, but not yet from git. Unlike Stack, it allows specifying system dependencies (non-Haskell dependencies); it sandboxes builds and tests for better reproduciblity (making CI in particular quite easy); it allows for shared caches; it supports scripts for project janitoring and management; and it supports devshells. - Nix: the focus of garn has been a simplified and more productive user experience when compared to Nix. A typed language means you can get autocompletion, documentation, and type errors in your editor, for example. And, even if Typescript is by no means perfect, it's much easier to program in than Nix, so it becomes more fun to abstract away functionality (e.g.: an addGhcidToProject function). garn "transpiles" to Nix, and Nix can be embedded in garn, so the choice is not binary. # Feedback If you try it out, please let us know what you think, or if you run into any roadblocks. You can email me at this address, or open issues on GitHub, or join the Discord channel at https://discord.com/invite/XtDrPsqpVx. It also happens to be partly written in Haskell; hopefully that makes it more easy and fun to get involved! Cheers, Julian

1 0

ANN: hledger 1.32
by Simon Michael 02 Dec '23

02 Dec '23

G'day all, I'm very pleased to announce hledger 1.32, including work from contributors Jonathan Dowland, S. Zeid, Charlie Ambrose, Jacob Weisz, Peter Sagerson, Philipp Klocke, Stephen Morgan, and bobobo1618. Highlights: More precision control, beancount output, TSV output, --summary-only, strict/idempotent import, CSV rule enhancements, timedot letters, fixes - there's a lot in this one, set aside some time to read the notes. - https://github.com/simonmichael/hledger/releases/1.32 - https://hledger.org/release-notes.html#2023-12-01-hledger-132 - https://hledger.org/install hledger is free, fast, reliable, multicurrency, double-entry, plain text accounting software for unix, mac, windows, and the web; built around human-readable, version-controllable plain text files; inspired by and largely compatible with Ledger CLI; convertible to and from Beancount; written in Haskell for reliability and longevity. For help getting started or more info, see https://hledger.org and join our Matrix/IRC chat or mail list: https://hledger.org/support . Newcomers, experts, contributors, sponsors, feedback are welcome! For more about plain text accounting, see https://plaintextaccounting.org . May you be free from danger; may you have happy, peaceful, prosperous holidays. -Simon

1 0