- Haskell-Cafe - Haskell.org

example of monad from http://learnyouahaskell.com not working
by Damien Mattei 27 Feb '19

27 Feb '19

i'm trying this example (see code below) from : http://learnyouahaskell.com/for-a-few-monads-more#making-monads when trying to compile this: import Data.Ratio newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show instance Functor Prob where fmap f (Prob xs) = Prob $ map (\(x,p) -> (f x,p)) xs thisSituation :: Prob (Prob Char) thisSituation = Prob [( Prob [('a',1%2),('b',1%2)] , 1%4 ) ,( Prob [('c',1%2),('d',1%2)] , 3%4) ] flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob [] l1 = Prob [('a',2%3),('b',1%3)] multAllExt :: (Prob a, Rational) -> [(a, Rational)] multAllExt (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs --Main> :type multAllExt --multAllExt :: (Prob a, Rational) -> [(a, Rational)] --Main> multAllExt (l1,1 % 4) --[('a',1 % 6),('b',1 % 12)] i get this error: GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :load monade.hs [1 of 1] Compiling Main ( monade.hs, interpreted ) monade.hs:21:10: error: • No instance for (Applicative Prob) arising from the superclasses of an instance declaration • In the instance declaration for ‘Monad Prob’ | 21 | instance Monad Prob where | ^^^^^^^^^^ Failed, no modules loaded. it fails when i add the last part of the example: instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob [] seems the Monad needs an instance of the Applicative to be instanciated... what is wrong? regards, Damien

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Haskell.Org accepted for GSoC 2019
by Jasper Van der Jeugt 26 Feb '19

26 Feb '19

Hi all, We're very excited to announce that Haskell.Org has been accepted [1] into the Google Summer of Code 2019 program [2]. We hope that, like last year, it will lead to a whole range of improvements to the Haskell ecosystem, and to new faces joining our community! We would like to thank everyone who submitted ideas -- this is a key part of being accepted into GSoC. Now, here's the near term timeline: - Today - March 25: Potential student participants discuss application ideas with mentors - March 25 - April 9: Students can submit applications - May 6: Accepted student proposals announced At this point, we're looking for both students and extra mentors. We would like to assign at least two mentors to each project if possible, so the students get the support they deserve. Additional ideas for projects are still welcome! There's a lot of information on our Summer of Haskell page [3]. If there are any students who are not sure where to begin, feel free to reach out to us directly [4]! [1]: https://summerofcode.withgoogle.com/organizations/5556388114202624/ [2]: https://summerofcode.withgoogle.com/ [3]: https://summer.haskell.org/ [4]: https://summer.haskell.org/contact.html Warm regards Jasper Van der Jeugt on behalf of the Haskell.Org Committee

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generics-sop equivalent of everywhere/mkT?
by Scott Michel 26 Feb '19

26 Feb '19

Before I cut down my code to a test case, are there any examples of a generics-sop equivalent of syb's everywhere/mkT? What's the way to operate on a product, replace the "I" argument with a compatible argument and walk back through the isomorphism, i.e. "to $ <something> $ from". I'm hacking on a Z80 system emulator (TRS-80 Model I system, more specifically). There are a couple of places where it'd be smoother in the disassembler to transform the disassembled instruction sequence (converting addresses to labels) before output. Consequently, cutting down code to an example is a bit painful -- examples would help. -scooter

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Where should I report about https://www.haskell.org/ghc/
by Yuji Yamamoto 25 Feb '19

25 Feb '19

Hello Haskellers, I found a typo [in the download page of GHC][1]: the "Current Stable Release" section must be "8.6.3" instead of "8.6.2"! So I want to file an issue for the repository or sending a patch via pull request or merge request. But as long as I looking for https://gitlab.haskell.org/ghc/ghc , I can't get sure of where the download page is managed. Can you tell me where is the right place to report or how to send a patch? Thanks in advance! [1] https://www.haskell.org/ghc/download.html -- 山本悠滋 twitter: https://twitter.com/igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

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Munich Haskell Meeting, 2019-02-26 @ 19:30
by Leah Neukirchen 24 Feb '19

24 Feb '19

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, February 26 at Rumpler 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 can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-feb-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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Feedback on new warning for MINIMAL pragmas
by Levent Erkok 24 Feb '19

24 Feb '19

Dear Haskell enthusiasts: I recently came across a scenario where GHC's output confused me regarding MINIMAL pragmas, and filed a "feature request" for GHC: https://github.com/ghc-proposals/ghc-proposals/pull/210 Simon PJ asked me to turn it into a proposal to gather feedback: https://github.com/ghc-proposals/ghc-proposals/pull/210 Alas, it gathered no feedback so far, aside from Simon PJ and myself. It's a very simple change and hopefully one that is also very easy to implement. If you could take a moment to comment on the proposal (or better yet, volunteer to implement it!), it would be much appreciated. I'm always very pleasantly surprised on the thoughtfulness and the technical depth of the comments this community provides, and it would be nice to make sure there are no gotcha scenarios. To keep the traffic on the haskell-cafe low, please comment directly on the proposal itself. Thanks, -Levent.

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segfault depending on code position in source file
by Damien Mattei 22 Feb '19

22 Feb '19

my code is structured like this , in summary: -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getAlphaDelta :: Connection -> String -> IO (Double,Double) getAlphaDelta conn name = do let qry_head_AlphaDelta_AngularDistance = "select alpha,delta from AngularDistance where Nom = ?" :: Query (alpha_delta_rows :: [(Double,Double)]) <- query conn qry_head_AlphaDelta_AngularDistance (Only (name::String)) -- putStrLn $ show alpha_delta_rows return (head alpha_delta_rows) main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie" } -- check getAlphaDelta works: let name3 = "A 7" putStrLn $ show name3 ad3 <- getAlphaDelta conn name3 putStr "ad3 =" putStrLn $ show ad3 this works i got result such as: "A 7" ad3 =(11297.0,-619.0) if i move the 5 lines of code at the end of file it segfault between the begin and end i have some DB connection and queriung , filtering, mapping... any idea? should i really upgrade the version of GHCI? Damien

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Advert for: 12 fully-funded 3-year PhD scholarships at Kent
by Simon Thompson 22 Feb '19

22 Feb '19

The Programming Languages and Systems (PLAS) group at the University of Kent's School of Computing invites applications for 12 fully-funded 3-year PhD scholarships (for UK/EU students). Applications are due by the 26th April 2019. The PLAS group is one of the largest programming languages research groups in Europe. It is currently ranked 17th worldwide by the independent CSrankings website: http://csrankings.org/#/index?plan&world. We provide a supportive environment for research and we have a vibrant postgraduate population. We encourage our students to engage with the wider research community through attending conferences and taking internships with leading industrial companies. We are located in Canterbury, a lively and cosmopolitan historic town with convenient travel links to London and Europe. You can apply to study for a PhD in any topic that falls within our range of expertise. We have studentships up to a value of £19,945 per annum that are available by competition. Application process: Select a potential supervisor (see below) and send them an informal project proposal as well as a brief CV (preferably by the first week of April 2019). Staff contact details can be found on their web pages. Submit your formal applications through the university admission system by the 26th April 2019. Your application should include a completed online admission form; the name and contact details of two referees; an original document providing confirmation of your degree (or a transcript if the degree is not yet awarded). For non-native English speakers, a certificate of competence in English is required at IELTS 6.5 or higher, with no element less than 6.0 (or equivalent). Programming Languages and Systems Group: https://www.cs.kent.ac.uk/research/groups/plas/index.html Topics suggested by our group https://www.cs.kent.ac.uk/research/groups/plas/pgprojects.html Applications process: https://www.cs.kent.ac.uk/research/studyingforaphd/howtoapply.html For general inquiries about the process, please e-mail: cs-phd-plas(a)kent.ac.uk. PLAS is a large research group with potential supervisors who work across the breadth of programming languages and systems research. *Dr Mark Batty* (Additional scholarship: https://www.cs.kent.ac.uk/people/staff/mjb211/studentship/studentship.htm) Concurrency; software verification; systems; relaxed memory; programming language semantics; GPU concurrency. *Dr Laura Bocchi* Foundations and engineering of API with complex behaviour, verification of distributed concurrent systems; behavioural types; real-time systems; transactions and transaction protocols. *Dr Olaf Chitil* Tracing, semantics, algorithmic debugging, type error debugging, compilation and functional programming. *Dr Radu Grigore* Program analysis; runtime verification; probabilistic models of computation. *Dr Rogerio de Lemos* Software engineering for self-adaptive systems: assurances and resilience evaluation; architecting resilient systems. *Prof. Richard Jones* Language implementation; memory management; garbage collection; object demographics; program analysis for improved memory management; program visualisation, rigorous performance evaluation. *Dr Stefan Kahrs* Expressiveness of programming languages, type systems, term rewriting, infinitary rewriting. *Dr Stephen Kell* Language implementation, tools, interoperability, runtimes and operating systems. *Prof. Andy King* Abstract interpretation, logic programming, decompilation and reverse engineering *Dr Julien Lange* Process calculi, automata theory, behavioural types, model checking and their application to the implementation and verification of concurrent and distributed systems. *Dr Stefan Marr* Language implementation techniques, concurrency, parallel programming, optimizations, tooling, debugging, virtual machines, interpreters, compilation. *Dr Matteo Migliavacca* On-line data processing, distributed publish-subscribe, and high-performance event processing in large scale and cloud scenarios. *Dr Dominic Orchard* Mathematical structure of programs; logical foundations of programming; categorical semantics; behavioural type theories; programming language design; program verification for computational science. *Dr Scott Owens* Semantics of shared memory concurrency; design of programming languages; formal verification for software and interactive theorem proving, especially for CakeML (https://cakeml.org) *Dr Tomas Petricek* Programming languages and tools, especially for data science, studying interactions of programming, bridging the gap between data and types; foundations of programming in a broad sense, including design and human experience; philosophy and history of computing and programming. *Prof. Simon Thompson* Functional programming in Haskell, Erlang and OCaml; refactoring functional programs: tool building. theory and practice: dependently-typed functional programming; DLT: languages for smart contracts on blockchains, including Marlowe on Cardano. Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk <mailto:s.j.thompson@kent.ac.uk> | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt <http://www.cs.kent.ac.uk/~sjt>

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ANNOUNCE: waargonaut-0.6.0.0 (better mousetrap)
by Sean Chalmers 22 Feb '19

22 Feb '19

## Waargonaut 0.6.0.0 This release of Waargonaut sees several issues fixed and a generous helping of cleanups and refactoring. This is a breaking change as the API has been drastically altered for some parts. Other changes include more granular modules and generalisation. ### Bug fixes - Unicode was not being handled correctly, sometimes. The encoding/decoding to a `Text` or `String` value wasn't correct due to my own lack of understanding. - Upper-case hexadecimal input was being silently zeroed out during some conversion steps. Those issues should all be resolved now. In the process of fixing those issues, it tugged on a couple of threads for things I've been wanting to do. So I did them, see below: ### Changes: #### Decoding! Using `parseWith` and the requirement to DIY almost an entire parsing function is a thing of the past. Replaced with the following structure: Functions: - `decodeWith` - `decodeFromText` - `decodeFromString` - `decodeFromByteString` - `decodeWithInput` - as well as their pure variants These functions only require you provide a function from the respective input type and has an implementation of the `CharParsing` interface. The greatly simplifies the process of providing your own parsing function. #### Default Parsing Functions! Waargonaut now provides some default parsing functions using the `attoparsec` package. These are in the `Waargonaut.Attoparsec` module and should make it easier for people to be up and running with the package when requirements of a custom parser aren't important. #### New Optics! The `Waargonaut.Lens` module provides some Prisms for use with the `Json` type. Working in a similar way to the `lens-aeson` package. These optics are law abiding and they have tests to prove it. #### Better Builder To complement the more streamlined parsing functions, the encoder has had its builder internals reworked to support either `Text` or `ByteString`. In the style of "scrap your typeclasses" to keep things simple. Added tests to check their output matches. #### More modules! - Factored out components into more focused submodules: - UnescapedJChar - EscapedJChar - HexDigit4 - Elem - Elems - JAssoc - Decode.Runners - Builders for all `Json` types now live under `Waargonaut.Encode.Builder` #### Deprecate Traversal decoder The decoder in `Waargonaut.Decode.Traversal` has been marked as deprecated and will be removed in a future release. It is bit rotting somewhat compared to the succinct powered decoder and doesn't operate at anywhere near the same level of speed or memory efficiency. If people care they can open an issue to let me know. #### Test re-structure Tests have been reordered into more granular modules. Some tests have been removed or reworked to be more accurate. More tests have been added to try to protect from regressions. More tests have been added to confirm that various instances or functions are law abiding. As a consequence there have been a few instances and functions that have been removed.

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ghci segmentation fault
by Damien Mattei 21 Feb '19

21 Feb '19

when executing this code i have a segmentation fault: printNameAndData :: (String,Float,Float) -> IO () printNameAndData (name,bdSidonie,bdWDS) = do putStrLn $ name ++ " " ++ show bdSidonie ++ " " ++ show bdWDS return () main :: IO () main = do rm <- mapM printNameAndData fltrdNamesBDsApprox *Main> :load UpdateSidonie.hs [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) Ok, one module loaded. *Main> main Segmentation fault (core dumped) if i hat a print statement: putStrLn $ show rm it works: [output cut] ... STT 448 28.4162 28.4162 STT 453 6.493 6.493 STT 487 79.761 79.761 STT 518 7.952 7.952 STT 524 19.3533 19.3533 STT 531 37.878 37.878 STT 534 21.3296 21.3296 VDK 1 26.1293 26.1293 VDK 2 7.4946 7.4946 VOU 42 -16.461 -16.461 WOR 7 71.851 71.851 WOR 12 27.4445 27.4445 WOR 17 -9.956 -9.956 WOR 20 16.2316 16.2316 WOR 24 31.25 31.25 [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),()] strange? the version is: GHCi, version 8.2.2 Damien

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