- Haskell-Cafe - Haskell.org

Should circular deriving "via" be rejected?
by Viktor Dukhovni 12 Nov '21

12 Nov '21

GHC accepts: {-# LANGUAGE DerivingStrategies, DerivingVia #-} newtype Foo = Foo Int deriving (Show, Eq) via (Bar) newtype Bar = Bar Int deriving (Show, Eq) via (Foo) should there be any effort to avoid circular definitions? Of course one could always implement an explicit `show` method that diverges, so perhaps this is no worse, but on the other hand, statically deciding divergence looks potentially easier in this case... On the other hand actual definitions like this seem unlikely in practice, so perhaps not worth burning precious compiler cycles to detect them? -- Viktor.

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Want to join a Team and Looking for good first issues to contribute
by Hima Bindu 11 Nov '21

11 Nov '21

Hi There! I am Hima Bindu, a CS Undergrad in GVPCE(A), AP, India. I like to be a polyglot and started learning a few languages, and came across Haskell. I was fascinated by its laziness and learned it for a couple of months. I skimmed through the repos, GSoC projects and a few Twitter threads. I want to contribute to anything that is beginner-friendly. It's overwhelming but I want to give it a try. Looking for any kind of suggestions or a walkthrough. This is very new to me, so please feel free to point out if I'm doing something wrong or moving in the wrong direction. *My area of interests:* Compilers(Currently working on one by following Bob Nystrom's book), Operating Systems/Kernals(have no experience but interested), anything related specifically to Haskell language(Data Structures, error handling etc) *Also, I eagerly want to join monthly meetings or something of that sort if there are any. How can I do that?* Really looking forward to your replies. Thank you Have a good day :D -- *Hima Bindu Tenneti* *Reach me here! <https://portfolios.talentsprint.com/~hima_bindu/>* *Programs must be written for people to read, and only incidentally for machines to execute. --Hal Abelson, SICP*

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[meetup announcement] Haskell Utrecht Meetup Group
by Ida Bzowska 10 Nov '21

10 Nov '21

Hi Everyone, Just letting you know that the freshly formed Haskell meetup from the Netherlands will have the first meetup on the 22nd of Nov; if you are in the area - don't hesitate: come! https://www.meetup.com/haskell-utrecht/events/281978445/ λCheers Ida Bzowska

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[ANN] linear-generics-0.2
by David Feuer 09 Nov '21

09 Nov '21

This new major version improves the GHCGenerically1 DerivingVia target. Previously, it worked on strictly fewer types than GHC could derive Generic1 for. Now it works on strictly more types than that. It does this by using a derived Generic instance instead of a derived Generic1 instance, thanks to the magic of quantified constraints. For anyone already using the package, the upgrade path is trivial: just derive any additional Generic instances GHC asks for.

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Re: [Haskell-cafe] Locate errors in TH for Haskell code
by Michal J Gajda 07 Nov '21

07 Nov '21

Dear Anton, > I have a task to convert one Haskell expression to another. > And I'd like to report errors properly. > > [interpret| -- error reported here > valid haskell code > invalid code <- error > |] > > I can parse it with haskell-src-exts or haskell-src-meta. > but if I have error in the code (type-check) error is positioned on the > first line of the QQ-expression. > > Do you know is it possible to report error at the line where it has > happened or > to keep original source code location in the Exp. > > In the function I do transformation Exp -> Exp. > > Or maybe there is a better way to do it instead of QQ? QQ is a good way. The simplest way would be to use `Language.Haskell.TH.location :: Q Loc` [1] and update it with the `Language.Haskell.Exts.SrcLoc.Loc` you get from `haskell-src-exts`[2]. [1] https://hackage.haskell.org/package/template-haskell-2.18.0.0/docs/Language… [2] https://hackage.haskell.org/package/haskell-src-exts-1.23.1/docs/Language-H… Note that [1] is available since template-haskell version 2.8.0.0. -- Best regards Michał

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ghc-9.2.1 unboxed types compatibility
by Bruno Damour 06 Nov '21

06 Nov '21

Hello, A significant number of libraries are broken by the changes in unboxed types. A lot of them are in the process (or already have) of adding compatibility layers to allow them to support the new version while preserving compatibility with older ones, introducing code like : #if MIN_VERSION_base(4,16,0) word32ToWordCompat# :: Word32# -> Word# word32ToWordCompat# = word32ToWord# #else word32ToWordCompat# :: Word32# -> Word# word32ToWordCompat# x = x #endif for the functions they use (usually a small subset). Some of the solutions appear to differ. Is there a preferred way to make this consistently or/and avoid unnecessary code duplication ? Thanks Bruno

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printing unicode in Windows console: hPutChar error
by Julian Bradfield 06 Nov '21

06 Nov '21

Some of my poor misguided students are still using Windows, and when they run my (predecessor's) Haskell program which uses Unicode box drawing characters in its output, they get *** Exception <stdout>: hPutChar: invalid argument (invalid character) Another student told them to change the console encoding via magic: chcp.com 65001 but a bit of googling suggests that a "modern Windows console program" should be using the widechar API to print Unicode and things should Just Work. Does Haskell have a principled view about character encodings? Or does it just say "I do byte strings, it's the user's problem"? Decades ago I was all in favour of charset-agnosticism, but these days I tend to think "go Unicode, and leave the legacy Latin-1 etc. data as the user's problem" :) Julian. -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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Type inference for lambda function
by Matthias Güdemann 04 Nov '21

04 Nov '21

Hi cafe, today while explaining lambda functions I came across this case which I don't really understand. This is what I get using ghci λ> :t (\a b -> a + b) (\a b -> a + b) :: Num a => a -> a -> a but when loading a .hs file with just this function (without signature) add = \a b -> a + b I get λ> :t add add :: Integer -> Integer -> Integer but I can explicitly define add to have this type: add :: Num a => a -> a -> a add = \a b -> a + b Also ghc infers this polymorphic type for example for add0 a = \b -> a + b Why is the type of add without a signature inferred to Integer and not Num a? Best regards Matthias

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TOP is hiring Functional Programmers for developing multi-user web applications
by rinus plasmeijer 03 Nov '21

03 Nov '21

Dear Haskell Fans, TOP Software Technology (top-software.com <http://top-software.com>) is a recently founded (2018) spin-off company located in the Netherlands, building on decades of research on Functional Programming at the Radboud University of Nijmegen (clean.cs.ru.nl <http://clean.cs.ru.nl>). By using advanced Functional Programming techniques and tools, we develop collaborative, distributed, multi-user / multi-system / multi-platform, web-oriented software applications for industry. We focus on Command and Control type of applications such as VIIA <https://top-software.nl/VIIA.html> <https://top-software.nl/VIIA.html> where real-time information is used for monitoring vessels. We use Clean, a Haskell like pure and lazy functional language, and the iTask system offering Task Oriented Programming (TOP), a special flavour of Functional Programming to construct reliable software applications for our customers at a high-level of abstraction on an Agile manner. We are looking for Experienced Functional Programmers to join our team. See the attached pdf. More information on what we do can be found ontop-software.com <http://top-software.com>. Mail torinus(a)top-software.com <mailto:rinus@top-software.com> orrinus(a)cs.ru.nl <mailto:rinus@cs.ru.nl> if you are interested. Greetings, Rinus Plasmeijer

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[ANN] text-display 0.0.1.0: A typeclass for user-facing output
by Hécate 02 Nov '21

02 Nov '21

Hi everyone, I am proud to release the first version of text-display¹, a typeclass for user-facing output. `Display` is a typeclass that doesn't abide by the rules of `Show` & `Read`, and brings with it a nice and ergonomic way to derive instances through `DerivingVia` when they already have a `Show` instance: ```haskell {-# LANGUAGE DerivingVia #-} data AutomaticallyDerived = AD -- We derive 'Show' deriving Show -- We take advantage of the 'Show' instance to derive 'Display' from it deriving Display via (ShowInstance AutomaticallyDerived) ``` But let's say you want to redact an instance of `Display`? You can do it locally, through the `OpaqueInstance` helper. It is most useful to hide tokens or passwords: ```haskell data UserToken = UserToken UUID deriving Display via (OpaqueInstance "[REDACTED]" UserToken) display $ UserToken "7a01d2ce-31ff-11ec-8c10-5405db82c3cd" -- => "[REDACTED]" ``` I hope you will have fun with this library! Cheers! ¹ https://hackage.haskell.org/package/text-display-0.0.1.0 -- Hécate ✨ 🐦: @TechnoEmpress IRC: Hecate WWW: https://glitchbra.in RUN: BSD

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