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How to write a Monad instance for this type
by PICCA Frederic-Emmanuel 06 Sep '19

06 Sep '19

Hello, I try to write a monad instance for this type which represent the content of an hdf5 file. This file format is similar to a filesystem, where directories are Group and files Dataset. I end up with this type. data Hdf5M a = H5Root (Hdf5M a) | H5Group ByteString [Hdf5M a] -- A group can contain other groups and/or datasets | forall sh b. (NativeType b, Shape sh) => H5Dataset ByteString (Array F sh b) type Hdf5 = Hdf5M () hdf5 :: Hdf5 -> Hdf5 hdf5 = H5Root group :: ByteString -> [Hdf5] -> Hdf5 group g = H5Group g dataset :: (NativeType b, Shape sh) => ByteString -> (Array F sh b) -> Hdf5 dataset = H5Dataset I tryed to inspire myself from the blaze-html Markup type. And I would like to be able to describe a contain like this hdf5 $ do group "name" $ do dataset "name1" array1 dataset "name2" array2 group "other-name" $ do etc... instead of H5Root ( H5Group "name" [ dataset ]...) the final idea, is to be able to serialize object via a class ToHdf5 where toHdf5 :: a -> Hdf5 So my question is someone can help me design this ? thanks for your help Frederic

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Stack & Test Coverage
by Hilco Wijbenga 06 Sep '19

06 Sep '19

Hi all, I have a very simple newtype Index = Index Int deriving (Eq, Ord, Enum) for which I have written tests with both HSpec and QuickCheck. Specifically, I have written tests for "==", "/=", "compare" (for all of EQ, LT, and GT), and "toEnum" & "fromEnum". So I thought I had _very_ thoroughly covered that one line. :-) Unfortunately, after running "stack clean ; stack test --coverage" I see in the HTML report that only Eq has been tested and that Ord and Enum have _no_ coverage (i.e. "never executed"). (The module also has some regular functions and an explicit Show instance [all with tests] and those are apparently fine. So coverage does seem to work but not the way I expect?) What am I doing wrong? Cheers, Hilco

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c2hs syntax highlighting in vim
by Vanessa McHale 05 Sep '19

05 Sep '19

Hi all, I wrote a vim plugin to highlight c2hs files, viz. https://github.com/vmchale/c2hs-vim Cheers, Vanessa McHale

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ANN: optics-0.1
by Adam Gundry 02 Sep '19

02 Sep '19

We are delighted to announce the first Hackage release of optics, a Haskell library for defining and using lenses, traversals, prisms and more. For further information: https://www.well-typed.com/blog/2019/09/announcing-the-optics-library/ https://hackage.haskell.org/package/optics The optics library is broadly similar in functionality to the well-established lens library, but uses an abstract interface rather than exposing the underlying implementation of each optic kind. It aims to be easier to understand than lens, with clearer interfaces, simpler types and better error messages, while retaining as much functionality as possible. It is being developed by Andrzej Rybczak and Adam Gundry, with significant contributions from Oleg Grenrus and Andres Löh, and much copy-pasting of code and selective copying of ideas from lens. -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 118 Wymering Mansions, Wymering Road, London W9 2NF, England

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ghc and OpenMP
by PICCA Frederic-Emmanuel 01 Sep '19

01 Sep '19

Hello, I would like your help, in order to understand how to modify a cabal file in order to compile an Haskell modules with .c files (containing OpenMP pragma). I use FFI and a bunch of .c and .h files for the computation part. thanks for your help. I am runnng hc on Debian stable. Cheers Frederic

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ANN: hledger 1.15
by Simon Michael 01 Sep '19

01 Sep '19

I'm pleased to announce hledger 1.15 ! Thanks to release contributors Caleb Maclennan, Jakob Schöttl, Henning Thielemann, Dmitry Astapov, Ben Creasy, zieone, Boyd Kelly, Gabriel Ebner, Hans-Peter Deifel, and Andreas Pauley. hledger is a robust, cross-platform, multicurrency, plain text accounting tool, with command-line, console and web UIs. It is one of the leading implementations of plain text accounting, and is a modern and largely compatible reimplementation of Ledger. Use it for tracking money, time, stocks, cryptocurrencies or any other commodity, Learn more at http://hledger.org and http://plaintextaccounting.org . WHAT'S NEW: A new website, faster and more flexible valuation, more accurate close command, tags --values, new descriptions/payees/notes/diff commands, misc. fixes. For details, see https://hledger.org/release-notes#hledger-1-15 . GETTING STARTED: Many install methods are described at http://hledger.org/download . Some of these might take a few days to become up to date. You can build the latest by doing: stack install --resolver=lts-14.4 hledger-lib-1.15 hledger-1.15 hledger-web-1.15 hledger-ui-1.15 --verbosity=error or: cabal v2-update && cabal v2-install hledger-1.15 hledger-web-1.15 hledger-ui-1.15 To start recording transactions: "hledger add". To list more commands: "hledger". See http://hledger.org for tutorials and support, or join #hledger on Freenode (http://irc.hledger.org) or join via Matrix (http://riot.hledger.org) New and old users, contributors, sponsors, and all feedback are most welcome! Best, -Simon

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Book reviewers wanted
by John Whitington 31 Aug '19

31 Aug '19

Hello, I've prepared a book "Haskell from the Very Beginning" in the same manner as my earlier "OCaml from the Very Beginning". It would be useful to find some people to look it over before release. I did this with the OCaml book, and it improved it markedly. I'm particularly looking for people who have taught or learned Haskell as a first programming language (either in an academic setting or not). I should point out that the book has already benefited from a full, paid, technical review, so you would not be being used as a substitute for that. Do drop me an email if you're interested. There would be, of course, no obligation. Thanks, John -- John Whitington Director, Coherent Graphics Ltd

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[CFP] Final Day for submissions to Functional Conf 2019 in Bengaluru!
by John Sinclair 31 Aug '19

31 Aug '19

Are you using Haskell in innovative ways? Do you have expertise the community would benefit from? Functional Conf 2019 is India's premier Functional programming conference, and we would love to hear from you! Submit your session proposal here: https://confng.in/zmRYXh7N <https://llink.to/?u=https:%2F%2Fconfng.in%2FzmRYXh7N&e=b3dafcbd9791228f91b8…> Hurry! Submissions close today (Aug 31)! Functional Conf is the premiere event for Functional programmers in India. Our goal is to support the community with access to high quality speakers, who are innovating in this field. https://twitter.com/FnConf <https://llink.to/?u=https:%2F%2Ftwitter.com%2FFnConf&e=df52b49527f43fa29bda…> https://www.linkedin.com/showcase/functional-conf <https://llink.to/?u=https:%2F%2Fwww.linkedin.com%2Fshowcase%2Ffunctional-co…>

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A faithful strictly-accumulating writer
by David Feuer 30 Aug '19

30 Aug '19

It's widely known that the classic WriterT tends to leak space, and that in some cases this leak can be resolved by using StateT instead. The implementations I've seen of this idea both use the module system to prevent the computation from gaining unauthorized access to the state. I believe I've found a way to avoid this. Does this look right? {-# language RankNTypes, FlexibleInstances, MultiParamTypeClasses #-} import qualified Control.Monad.Writer.Class as W import Control.Monad.State.Strict import Control.Monad.Reader import Control.Applicative -- The key idea is that the computation -- can't inspect the state because it doesn't know the type newtype WriterT w m a = WriterT { unWriterT :: forall s. ReaderT ((w -> w) -> s -> s) (StateT s m) a } runWriterT :: Monoid w => WriterT w m a -> m (a, w) runWriterT m = runStateT (runReaderT (unWriterT m) id) mempty instance Functor m => Functor (WriterT w m) where fmap f m = WriterT $ fmap f (unWriterT m) instance Monad m => Applicative (WriterT w m) where pure a = WriterT (pure a) liftA2 f m n = WriterT $ liftA2 f (unWriterT m) (unWriterT n) instance Monad m => Monad (WriterT w m) where m >>= f = WriterT $ unWriterT m >>= unWriterT . f instance MonadTrans (WriterT w) where lift m = WriterT $ lift . lift $ m tell :: (Monad m, Semigroup w) => w -> WriterT w m () tell w = WriterT $ do p <- ask modify' $ p (<> w) pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a pass m = WriterT $ do p <- ask (a, ww) <- unWriterT m modify' (p ww) pure a instance (Monoid w, Monad m) => W.MonadWriter w (WriterT w m) where tell = tell listen m = do aw@(_a, w) <- lift $ runWriterT m tell w pure aw pass = pass

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Call for scholarship applications: PLMW @ POPL 2020
by Robbert Krebbers 30 Aug '19

30 Aug '19

CALL FOR SCHOLARSHIP APPLICATIONS (** DEADLINE: OCTOBER 7 **) ACM SIGPLAN Programming Languages Mentoring Workshop, New Orleans, Louisiana Tuesday, January 21, 2020 Co-located with POPL 2020 Web page: https://popl20.sigplan.org/home/PLMW-POPL-2020 After the success of the first eight Programming Languages Mentoring Workshops at POPL 2012-2019 we are announcing the 9th SIGPLAN Programming Languages Mentoring Workshop (PLMW), co-located with POPL 2020 and organized by Stephanie Balzer, Justin Hsu, Robbert Krebbers, and Nobuko Yoshida. The purpose of this mentoring workshop is to encourage graduate students and senior undergraduate students to pursue careers in programming language research. This workshop will bring together world leaders in programming languages research and teaching from academia and industry to provide (a) technical sessions on cutting-edge PL research and (b) mentoring sessions on how to prepare for a research career. The workshop will help students imagine how they might contribute to our research community. We especially encourage women and underrepresented minority students, and people with disabilities to attend PLMW. This workshop is part of the activities surrounding POPL, the Symposium on Principles of Programming Languages, and takes place the day before the main conference. One goal of the workshop is to make the POPL conference more accessible to newcomers. We hope that participants will stay through the entire conference. A number of sponsors (listed below) have generously donated scholarship funds for qualified students to attend PLMW. These scholarships should cover reasonable expenses (airfare, hotel, and registration fees) for attendance at both the workshop and the POPL conference. Students attending this year will get one year free student membership of SIGPLAN, unless they prefer to opt out during their application. The workshop registration is open to all. Students with alternative sources of funding are welcome as well. APPLICATION for PLMW scholarship: The scholarship application can be accessed at the following URL: https://forms.gle/dzV8ELgQUx2xfEjm9 The deadline for full consideration of funding is Monday, October 7. Selected participants will be notified by October 21. Confirmed speakers (so far): * Adriana Compagnoni, Stevens Institute of Technology * Isil Dillig, UT Austin * Derek Dreyer, MPI-SWS * Marco Gaboardi, SUNY Buffalo * Lindsey Kuper, UC Santa Cruz * Xavier Leroy, Inria Confirmed sponsors (so far): * An Anonymous Donor * NSF * ACM SIGPLAN * Amazon

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