January 2020 - Haskell-Cafe

Problem using Persistent
by Debasish Ghosh 26 Jan '20

26 Jan '20

Hi - I have an entity declaration in persistent as follows: share [mkPersist sqlSettings { mpsGenerateLenses = True, mpsPrefixFields = False }, mkMigrate "migrateAll"] [persistLowerCase| Account accountNo Text accountType AccountType accountHolderName Text accountOpenDate UTCTime default=CURRENT_TIME accountCloseDate UTCTime Maybe default=NULL currentBalance MoneyUSD rateOfInterest Double Primary accountNo deriving Show |] I am getting this compilation error .. • No instance for (Generic (Key Account)) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (aeson-1.4.6.0:Data.Aeson.Types.ToJSON.ToJSON (Key Account)) | 40 | share [mkPersist sqlSettings { mpsGenerateLenses = True, mpsPrefixFields = False }, mkMigrate "migrateAll"] [persistLowerCase| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... and another exactly similar for FromJSON What exactly is going wrong here ? I am not using json in the above definition. regards. -- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg

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[ANN] bz2 library
by Vanessa McHale 25 Jan '20

25 Jan '20

Hi all, I wrote another high-level wrapper around libbz2, https://matrix.hackage.haskell.org/#/package/bz2. It includes low-level bindings (via c2hs) and a bundled copy of bzlib 1.0.8 as well. Cheers, Vanessa McHale

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[ANN] nix-freeze-tree-0.1.0.0 - Freeze a tree using fixed-output derivations
by Jack Kelly 24 Jan '20

24 Jan '20

I have just pushed an initial release of nix-freeze-tree[1] to Hackage. `nix-freeze-tree` is a utility that walks a directory tree, and writes out Nix expressions which rebuild that tree. The generated expressions have two important properties: 1. Each file in the tree is built by a separate fixed-output derivation, and 2. Directories are built by derivations that symlink their contents recursively. If you are using `nix copy` to ship a derivation between nix stores, copying the derivation built by evaluating the output of `nix-freeze-tree` can reuse existing files in the destination store, as fixed-output derivations can be checked against a hash before copying. I use this to speed up deploys to my personal website. Running `nix-freeze-tree` on the output from my site generator and importing the result means that `nix copy` only uploads new files, instead of copying a single giant derivation containing every image on the site. The code is on Sourcehut[2]; bug reports[3] and patches[4] are welcome. The repository also has a Nix helper function for running the tool on your own derivations. Best, -- Jack [1]: https://hackage.haskell.org/package/nix-freeze-tree-0.1.0.0 [2]: https://git.sr.ht/~jack/nix-freeze-tree/ [3]: https://todo.sr.ht/~jack/nix-freeze-tree [4]: https://lists.sr.ht/~jack/nix-freeze-tree-discuss

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[ghc-steering-committee] The GHC Committee welcomes its new members
by Joachim Breitner 24 Jan '20

24 Jan '20

Dear Haskell community, the GHC Steering committee welcomes three new members, Alejandro Serrano Mena, Cale Gibbard and Tom Harding. We are happy to see that there is continued interest in our work, and are looking forward to having fresh insights and energy on the committee. They take the seat of Sandy Maguire, who we thank for this productive work during his tenure. As you might notice, we picked more candidates than we filled slots. This is not because Sandy’s productivity need three people to fill. The idea is that there is no reason to turn away a motivated contributor just to stick to the arbitrary 10 members. Instead, we will aim at “roughly 10”, and probably not call for new members until we have dropped below that number again. On behalf of the committee, Joachim Breitner PS: Alejandro, Cale and Tom, please send me your github handles and subscribe to the mailing list at https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de https://www.joachim-breitner.de/

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Taking over double-conversion on Hackage
by Rinat Stryungis 21 Jan '20

21 Jan '20

Hi everyone, The double-conversion package seems unmaintained. I'd like to take over as maintainer. I have actually forked the repository, added new functionality, tests, and updated the original Google library: https://github.com/haskell-mouse/double-conversion I was advised to consider taking over the maintenance of double-conversion. I tried to contact Bryan O'Sullivan (the maintainer) a few times during the 2 years (!) using all available communication methods (e-mail, LinkedIn). He answered only once and asked me to send an e-mail. I did it but without any effect. I would still prefer not to have this responsibility, but I would also like people to be able to use the new functionality without creating redundant forks.

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a sort of chunk
by PICCA Frederic-Emmanuel 20 Jan '20

20 Jan '20

Hello, I need to write some code in order to do some computations with stack of images coming from hdf5 files. Each computation done per images is quite intensive, so I want to do this on multiple cores of my computer (24 in my case) A stack of images is composed of a list of files. each file contain n images. So I decided to proceed like this: create a data type which represent a chunk of this stack. data Chunk = Chunk { filename :: FilePath , from :: Int , to :: Int } And indeed the full stack is [Chunk] Now since I have n core, I need to split the full stack in equivalent chunk of images. Just for example. at the begining I have this image stack 30476 images [Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00077.nxs" 0 698,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00078.nxs" 0 1104,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00079.nxs" 0 1510,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00080.nxs" 0 1914,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00081.nxs" 0 2318,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00082.nxs" 0 2720,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00083.nxs" 0 2169,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00084.nxs" 0 2445,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00085.nxs" 0 2720,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00086.nxs" 0 490,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00087.nxs" 0 812,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00088.nxs" 0 1133,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00089.nxs" 0 1454,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00090.nxs" 0 1773,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00091.nxs" 0 2090,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00092.nxs" 0 2406,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00093.nxs" 0 2720] so i need to write a function which do something like chunksOf chunksOf :: Int -> [Chunk] -> [[Chunk]] In this case I need to create chunk of 1325 images, so the finla result should be [ [Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00077.nxs" 0 698,Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00078.nxs" 0 627] , [Chunk "/nfs/ruche-sixs/sixs-soleil/com-sixs/2019/Run3/FeSCO_Cu111/sample2_ascan_omega_00078.nxs" 627 1104, ...] ... ] I would like your help in order to give me advices or help me find information whcih can help me implement this chunksOf method. thanks for your help. Frederic

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10 PhD studentships in Nottingham for Home/EU applicants
by Graham Hutton 20 Jan '20

20 Jan '20

Dear all, The School of Computer Science at the University of Nottingham is seeking applications for 10 fully-funded PhD studentships for Home/EU students: https://tinyurl.com/ten-phds-2020 Applicants in the area of the Functional Programming Laboratory (https://tinyurl.com/fp-notts) are strongly encouraged! If you are interested in applying, please contact a potential supervisor as soon as possible (the application deadline is 6th March): Thorsten Altenkirch - constructive logic, proof assistants, homotopy type theory, category theory, lambda calculus. Venanzio Capretta - type theory, mathematical logic, corecursive structures, proof assistants, category theory, epistemic logic. Graham Hutton - not taking on any new students this year, but you may find these notes useful: https://tinyurl.com/scbkxkr Henrik Nilsson - functional reactive programming, domain- specific languages, generalised notions of computation. These positions are only open to Home/EU applicants. An advert for international students was posted earlier and is now closed. Best wishes, Graham +-----------------------------------------------------------+ 10 Fully-Funded International PhD Studentships School of Computer Science University of Nottingham, UK https://tinyurl.com/ten-phds-2020 Applications are invited for 10 fully-funded PhD studentships for Home/EU students in the School of Computer Science at the University of Nottingham, starting on 1st October 2020. The topics for the studentships are open, but should relate to the interests of one of the School’s research groups: Agents Lab; Computational Optimisation and Learning Lab; Computer Vision Lab; Functional Programming; Intelligent Modelling and Analysis; Mixed Reality Lab; Data Driven Algorithms, Systems and Design and Uncertainty in Data and Decision Making The studentships are for three and a half years and include a stipend of £15,009 per year and tuition fees. Applicants are normally expected to have a first-class Masters or Bachelors degree in Computer Science or a related discipline, and must obtain the support of a potential supervisor in the School prior to submitting their application. Initial contact with supervisors should be made at least two weeks prior to the closing date for applications. Eligible successful applicants are expected to apply for a EU VC Scholarship. Informal enquiries may be addressed to Kathleen.Fennemore(a)nottingham.ac.uk. To apply, please submit the following items by email to: Marc.Williams(a)nottingham.ac.uk: (1) a copy of your CV, including your actual or expected degree class(es), and results of all University examinations; (2) an example of your technical writing, such as a project report or dissertation; (3) contact details for two academic referees. (4) a research proposal – max 2 x sides A4 You may also include a covering letter but this is optional. Closing date for applications: Friday 6 March 2020. +-----------------------------------------------------------+ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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Haskell.org Committee Nomination Results
by Emily Pillmore 20 Jan '20

20 Jan '20

Hello Everyone, I'm pleased to announce that the results for the Haskell.org ( http://haskell.org/ ) committee nominations are in, and voting has completed. The following tally is the final result: Jaspe Van der Jeugt: 6 Alexandre Garcia de Oliveira: 5 April Gonçalves: 2 Rebecca Skinner: 5 With no votes for anyone else. This means that Jasper's renomination was successful, and we get to welcome 2 new faces to the Haskell.org ( http://haskell.org/ ) ; congratulations Rebecca and Alexandre! Thanks, Emily

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persistent and safe-money packages
by Debasish Ghosh 16 Jan '20

16 Jan '20

Hi - I am using "persistent" for database access in Haskell. However in my domain model I have used the Dense data type from safe-money ( https://hackage.haskell.org/package/safe-money) as the money datatype. It looks like this data type is not supported by default in persistent. Is there a way around ? Thanks in advance. -- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg

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[ANN] 2nd Call for Participation: BOB 2020 (February 28, Berlin, early-bird until Jan 20)
by Stefan Wehr 15 Jan '20

15 Jan '20

================================================================================ BOB 2020 Conference “What happens if we simply use what’s best?” February 28, 2020, Berlin http://bobkonf.de/2020/ Program: http://bobkonf.de/2020/en/program.html Registration: http://bobkonf.de/2020/en/registration.html ================================================================================ BOB is the conference for developers, architects and decision-makers to explore technologies beyond the mainstream in software development, and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experiences. The program features 14 talks and 8 tutorials on current topics: http://bobkonf.de/2020/en/program.html The subject range of talks includes functional programming, formal methods, architecture documentation, functional-reactive programming, and language design. The tutorials feature introductions to Idris, Haskell, F#, TLA+, ReasonML, and probabilistic programming. Heather Miller will give the keynote talk. Registration is open online: http://bobkonf.de/2020/en/registration.html NOTE: The early-bird rates expire on January 20, 2020! BOB cooperates with the Racketfest conference on the day before BOB: https://racketfest.com/ BOB cooperates with the :clojureD conference on the day after BOB: https://clojured.de/

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