September 2017 - Haskell-Cafe

Haskell job opportunity at RELEX Solutions in Helsinki, Finland
by samuli.thomasson＠paivola.fi 19 Sep '17

19 Sep '17

Would you like to get paid for writing Haskell? And do you live in Helsinki, Finland area (or could re-locate)? If yes, please read on. RELEX Solutions is Europe’s fastest growing provider of integrated retail and supply chain planning solutions. We are now hiring one or two Haskell programmers in Helsinki, Finland to join our small but impactful group of Haskellers to develop our internal tools. You would be working pretty much exclusively in Haskell and Elm. Exact role is somewhat flexible, depending on what other skills you might bring to the table. Read more and apply now here: https://www.relexsolutions.com/open-positions/#op-209192-haskell-and-elm-en… A good working knowledge of general Haskell programming is preferred, although eager learners can and should apply as well - if it's a good match, you just pick up the skills on the job. Unfortunately, we can't accept full remote for this position, so you should be able to be be present at the Helsinki office at least some days every week. Other details are largely negotiable. Note that the deadline for applications is quite soon, by October 2nd. Regards, Samuli Thomasson

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Eindhoven Haskell Meetup session - Oct 3, 2017
by Damian Nadales 19 Sep '17

19 Sep '17

Dear All, A new edition of our Haskell meetup will be held in Eindhoven: https://www.meetup.com/preview/Eindhoven-Haskell-Meetup/events/243342181 I would like to invite all the people that might be around this area. Also if you don't live close by, but you're visiting and want to give a talk please let me know, as we can always use a speaker. And at the same time I want to thank to the speakers that came (and will come) from far away to support his meetup (the Haskell community is -in my experience- great!). Kind regards, Damian.

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Haskell Programmers Wanted
by Brian Hurt 19 Sep '17

19 Sep '17

Code Haskell. Build great things. We’re Wrinkl, a well-funded startup working closely with Obsidian Systems to build a powerful new communications platform. We use Haskell on both the backend and the frontend (using GHCJS and Reflex). And we’re looking to hire Haskell developers, preferably in the Philly area or willing to relocate to Philly. If this sounds interesting, visit https://www.wrinkl.com/jobs/ for more info. Women and minorities strongly encouraged to apply. Brian

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ghc-devs@haskell.org
by Robin Palotai 19 Sep '17

19 Sep '17

Hello GHC devs, Before inventing the wheel, want to check if there is a GHC API way to look up the (fully) resolved instance method from a class method. For example, given a code data Foo Int deriving Show bar = show (Foo 3) when inspecting the Typechecked AST for bar's show call, I would like to get to the Name / Id of 'show' of the 'Show' typeclass. I believe I could use splitHsSigmaTy on the HsType of the function call to get the context, and then evaluate the HsWrapper somehow to find out what instance dictionary is applied to the class restriction in the context, and then look up the instance method from the dictionary.. Two questions: 1) Is there maybe functionality for this? 2) If not, is there any guarantee about the constraint order in the context, at the method call? So I could more easily determine which constraint's application to look for.. Any hints welcome && Thank you! Robin

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Haskell in Leipzig 2017: 1st call for participation
by Wolfgang Jeltsch 18 Sep '17

18 Sep '17

Event: Haskell in Leipzig 2017 Time: October 26–28, 2017 Place: HTWK Leipzig, Germany Homepage: https://hal2017.softbase.org/ About ===== Haskell is a modern functional programming language that allows rapid development of robust and correct software. It is renowned for its expressive type system, its unique approaches to concurrency and parallelism, and its excellent refactoring capabilities. Haskell is both the playing field of cutting-edge programming language research and a reliable base for commercial software development. The workshop series Haskell in Leipzig (HaL), now in its 12th year, brings together Haskell developers, Haskell researchers, Haskell enthusiasts, and Haskell beginners to listen to talks, take part in tutorials, join in interesting conversations, and hack together. To support the latter, HaL will include a one-day hackathon this year. The workshop will have a focus on functional reactive programming (FRP) this time, while continuing to be open to all aspects of Haskell. As in the previous year, the workshop will be in English. Invited Speaker =============== * Ivan Perez, University of Nottingham, UK Invited Performer ================= * Lennart Melzer, Robert-Schumann-Hochschule Düsseldorf, Germany Registration ============ Registration information is available on the web page of the local organizers at http://nfa.imn.htwk-leipzig.de/HAL2017/. Program Committee ================= * Edward Amsden, Plow Technologies, USA * Heinrich Apfelmus, Germany * Jurriaan Hage, Utrecht University, The Netherlands * Petra Hofstedt, BTU Cottbus-Senftenberg, Germany * Wolfgang Jeltsch, Tallinn University of Technology, Estonia (chair) * Andres Löh, Well-Typed LLP, Germany * Keiko Nakata, SAP SE, Germany * Henrik Nilsson, University of Nottingham, UK * Ertuğrul Söylemez, Intelego GmbH, Germany * Henning Thielemann, Germany * Niki Vazou, University of Maryland, USA * Johannes Waldmann, HTWK Leipzig, Germany Questions ========= If you have any questions, please do not hesitate to contact Wolfgang Jeltsch at wolfgang-it(a)jeltsch.info.

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Cabal: updating environment variable in a dependency
by aditya siram 16 Sep '17

16 Sep '17

Hi all, I have two separate packages, a library and an app. In the library's Setup.hs I set the LIBRARY_PATH environment variable to a place where I install a custom C shared library, but when the app is built LIBRARY_PATH is empty. I realize that GHC can build and link C code but for the purposes of this project that wasn't workable. So is there a way to make this environment variable hack work or, even better, a good way a dependency can pass custom information to its dependents? Thanks! -deech

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Maintenance of streaming and streaming-bytestring
by Andrew Martin 16 Sep '17

16 Sep '17

David Feuer brought this up two weeks ago [0]. The maintainer of streaming and streaming-bytestring has been unreachable, and provided his continued absence, I would like to become a maintainer of these two libraries. I have put up PRs fixing issues [1] and I've offered on the issue tracker to become a comaintainer [2]. I use this library a lot at work, so I have an interest in keeping it in good shape. [0] https://mail.haskell.org/pipermail/libraries/2017-August/028141.html [1] https://github.com/michaelt/streaming-bytestring/pull/11 [2] https://github.com/michaelt/streaming-bytestring/issues/19 -- -Andrew Thaddeus Martin

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How to write a polymorphic serializer?
by Viktor Dukhovni 16 Sep '17

16 Sep '17

I tried to implement a polymorphic serializer that uses a enclosed MVar () to serialize an action in an arbitrary IO monad, without a need for its user to resort to explicit liftIO calls. So, as a first step I create a typeclass that has a natural default implementation, and some instances for the Monads of interest that just use the natural default: type Serially m = (MonadIO m) => forall a. IO a -> m a class (MonadIO m) => Serializable m where serialize :: MVar () -> Serially m serialize lock = liftIO . withMVar lock . const instance Serializable IO instance Serializable (StateT Int IO) ... With this, given: foo :: Serially IO -> ... -> IO () foo serially ... = do ... serially $ ... ... bar :: Serially (StateT Int IO) -> ... -> StateT Int IO () bar serially ... = do ... serially $ ... ... I can write: lock <- newMVar () foo (serialize lock) ... bar (serialize lock) and the type system figures out the correct version of serialize for foo's and bar's actual monad. Is it possible to create a single "serialize lock" closure that works for both "foo" and "bar"? Something that works along the lines of: let x = liftIO . withMVar lock . const :: ??? foo x ... bar x ... If I leave out the "liftIO", then I can of course use: x :: forall a. IO a -> IO a and the "liftIO" can be put explicitly into "foo" and "bar". foo x ... = liftIO $ x $ ... bar x ... = liftIO $ x $ ... but is it possible for "x" to both be polymorphic with respect to its user's monad and at the same time to be a *closure* around some MVar, and thus not directly a method of a type class. Of course needing to add an extra "liftI0" here and there is not onerous, I'm mostly just curious whether I'm missing something that can hide that boilerplate call in the serializer implementation. -- Viktor.

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Executing conduit streams in parallel leads to memory leaks
by Simon Hafner 13 Sep '17

13 Sep '17

When I run my conduit without any additions, it works as expected, with low constant memory usage, as advertised. It's a bit slow, so I tried to speed it up with worker pools (via parallel-io) and staged folding (via stm-conduit). However, then the memory usage indicates all the ByteString from the file readings are being fully allocated and kept in memory, even though they're not being used after a step of conduit. [1] I thought maybe because of the closing IO, the release of the file handle somehow keeps the read string in memory, so I wanted to make absolutely sure that's not the problem. [2] Switch out the `Lib.readFile` with `B.readFile` to undo that specific part. I was not using a worker pool in the beginning, so maybe the `mapConcurrently_` somehow allocated all the threads, but with the pooled solution, that should be solved as well. What else could cause all the ByteStrings to be kept in memory in the parallel version? The example is available on: https://github.com/reactormonk/non-constant-memory [1] https://github.com/reactormonk/non-constant-memory/blob/master/src/Lib.hs#L… [2] https://github.com/reactormonk/non-constant-memory/blob/master/src/Lib.hs#L…

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HPC and coverage report
by Baa 13 Sep '17

13 Sep '17

Hello Everyone. To generate coverage, Haskellers use HPC. HPC has command report [--xml-output] ... which allows to get XML output of coverage. Jenkins CI "understands" Cobertura Coverage XML format, which is shown as code source marked with red/green/etc background lines - uncovered/covered code lines. Also it's possible to mark branches (if/else/etc). It's true for Python, Java, etc. Can HPC (or other Haskell tool) does the same, i.e. generate Cobertura-compatible XML (to be understandable by Jenkins CI) ? What peoples use for coverage reporting under Jenkins CI w/ Haskell ? Is this `hpc report --xml-output` compatible w/ Jenkins CI ? === Best regards, Paul

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