July 2017 - Haskell-Cafe

Safely reading RemoteRefs?
by amindfv＠gmail.com 14 Jul '17

14 Jul '17

I notice I'm able to retrieve haskell variables after a :r in ghci: :m + Data.Binary :m + GHCi.RemoteTypes > encodeFile "/tmp/foo" =<< mkRemoteRef "hello!" (:reload) > putStrLn =<< localRef =<< decodeFile "/tmp/foo" "hello!" Is this safe? It works correctly in the common case but are there things I need to be careful of? In the string case it's not too interesting but it would be great to e.g. retrieve the pid of a running process. Tom

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parallelism and state
by Dennis Raddle 14 Jul '17

14 Jul '17

I'm trying to parallelize a Monte Carlo backtracking search algorithm. The trick is that my algorithm is expressed in a State monad, because I need to hold a StdGen as well as keep several records of computations and metrics. So I know that if I'm going to run a State computation in several parallel lines of execution, I need to conceive of a way to split the state and later recombine it. Here is a simple example I came up with. I have no idea if I'm doing this in a good way, so any comments are welcome. http://lpaste.net/356879 D

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2nd CfP: IFL 2017 (29th Symposium on Implementation and Application of Functional Languages)
by publicityifl＠gmail.com 13 Jul '17

13 Jul '17

<div dir="ltr">Hello, Please, find below the second call for papers for IFL 2017. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL --- IFL 2017 - CALL FOR PAPERS ========================== 29th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES ======================================================================== University of Bristol, UK In cooperation with ACM SIGPLAN Wednesday 30 August - Friday 1 September, 2017 <a href="http://iflconference.org/" target="_blank">http://iflconference.org/</a> Scope ----- The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2017 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Peer-review ----------- Following the IFL tradition, IFL 2017 will use a post-symposium review process to produce the formal proceedings. All participants of IFL 2017 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: <a href="http://www.sigplan.org/Resources/Policies/Republication" target="_blank">http://www.sigplan.org/Resources/Policies/Republication</a> The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings are not subject to the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. The formal proceedings will appear in the International Conference Proceedings Series of the ACM Digital Library. Important dates --------------- Mon 31 July 2017 : Submission deadline draft papers Wed 2 August 2017 : Notification of acceptance for presentation Fri 4 August 2017 : Early registration deadline Fri 11 August 2017 : Late registration deadline Mon 21 August 2017 : Submission deadline for pre-symposium proceedings Wed 30 August 2017 - Fri 1 September 2017 : IFL Symposium Mon 4 December 2017 : Submission deadline for post-symposium proceedings Wed 31 January 2018 : Notification of acceptance for post-symposium proceedings Mon 12 March 2018 : Camera-ready version for post-symposium proceedings Submission details ------------------ Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must use the new ACM two columns conference format, which can be found at: <a href="http://www.acm.org/publications/proceedings-template" target="_blank">http://www.acm.org/publications/proceedings-template</a> For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. Authors submit through EasyChair: <a href="https://easychair.org/conferences/?conf=ifl2017" target="_blank">https://easychair.org/conferences/?conf=ifl2017</a> Topics ------ IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2017, please contact the PC chair at <a href="mailto:nicolas.wu@bristol.ac.uk" target="_blank">nicolas.wu(a)bristol.ac.uk</a>. Topics of interest include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee ------------------- Chair: Nicolas Wu, University of Bristol, UK - Kenichi Asai, Ochanomizu University, Japan - Sandrine Blazy, University of Rennes 1, France - Carlos Camarao, Universidade Federal de Minas Gerais, Brazil - Stephen Dolan, University of Cambridge, UK - Jurriaan Hage, Utrecht University, Netherlands - Yukiyoshi Kameyama, University of Tsukuba, Japan - Benjamin Lerner, Brown University, USA - Bas Lijnse, Radboud University, Netherlands - Garrett Morris, University of Kansas, USA - Miguel Pagano, Universidad Nacional de C√≥rdoba, Argentina - Tomas Petricek, Alan Turing Institute, UK - Maciej Pir√≥g, University of Wroclaw, Poland - Exequiel Rivas, Universidad Nacional de Rosario, Argentina - Neil Sculthorpe, Nottingham Trent University, UK - Melinda Toth, Eotvos Lorand University, Hungary - Phil Trinder, Glasgow University, UK - Kanae Tsushima, National Institute of Informatics, Japan - Marcos Viera, Universidad de la Republica, Uruguay - Meng Wang, University of Kent, UK Venue ----- The IFL 2017 will be held in association with the Department of Computer Science, University of Bristol, UK. Bristol is located in South West England, and can be easily reached from Bristol Airport. See the website for more information on the venue.</div>  <a style='display: block; margin: 32px 0 40px 0; padding: 10px; font-size: 1em; text-align: center; border: 0; border-top: 1px solid gray; ' href='https://goo.gl/2ksdRv'>powered by GSM. Free mail merge and email marketing software for Gmail.</a>

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Unicode silliness
by David Turner 13 Jul '17

13 Jul '17

Hi, One of my colleagues asked about using Unicode symbols in Haskell operators and I initially thought that this was supported. However, the code at the bottom of this message doesn't parse, reporting a lexical error on basically all of the funny characters of the form: Main.hs:1:5: error: lexical error at character '\65288' Character 65288 (0xff08) is punctuation [Ps] which I understand to mean it counts as a uniSymbol in the lexical structure described in the Haskell Report. https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-160002.2 Should this work? Not that I really want this specific example to work, but I'm interested to know why. Many thanks, David $ cat app/Main.hs foo （›’-’）› bar = False foo ‹（’-’‹） bar = False foo ∧（’-’）∧ bar = False foo ∨（’-’）∨ bar = False $ cat app/Main.hs | base64 # try and avoid mangling in email Zm9vIO+8iOKAuuKAmS3igJnvvInigLogYmFyID0gRmFsc2UKZm9vIOKAue+8iOKAmS3igJnigLnv vIkgYmFyID0gRmFsc2UKZm9vIOKIp++8iOKAmS3igJnvvIniiKcgYmFyID0gRmFsc2UKZm9vIOKI qO+8iOKAmS3igJnvvIniiKggYmFyID0gRmFsc2UK

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Issue 63 :: Haskell Weekly
by Taylor Fausak 13 Jul '17

13 Jul '17

\ Haskell Weekly \/\/ Issue 63 <https://haskellweekly.news/issues/63.html> Welcome to another issue of Haskell Weekly! Haskell is a purely functional programming language that focuses on robustness, concision, and correctness. This is a weekly summary of what's going on in its community. ## Featured ## - GHC 8.2.1 release candidate 3 available <https://mail.haskell.org/pipermail/ghc-devs/2017-July/014364.html> > This release candidate fixes a number of regressions from 8.0.2 found in release candidate 2, as well as a major correctness bug present in several previous GHC major releases. Users taking advantage of parallelism in their programs will be strongly encouraged to upgrade to 8.2.1 once it is released. Editor's note: Use this stack.yaml to test GHC 8.2.1-rc3 with Stack. <https://gist.github.com/tfausak/623d283cfe7a338bcbfca56ddfec0ecd/cc31f36373…> - Stack's new extensible snapshots <https://www.fpcomplete.com/blog/2017/07/stacks-new-extensible-snapshots> > There is a collection of features in Stack that have been added in bit by bit, as opposed to being designed into a cohesive whole from the start. The features work, but could be a bit better. We've known for a while that, instead of putting in place strategic fixes, a more general refactoring of the core dependency management logic was in order. - ZuriHac 2017 <https://www.youtube.com/playlist?list=PLOvRW_utVPVkoZ5GuodkejFU8MiH6_SB7> Videos of the Haskell keynotes from ZuriHac, the Zurich Haskell hackathon. Includes presentations from Edward Kmett, Stephen Diehl, Simon Thompson, Neil Mitchell, and Julie Moronuki. - Front Row is hiring a senior backend Haskell engineer <https://frontrow.workable.com/j/463B843754> > Come change how 6.5+ million K-12 US students learn Math, Language Arts, Social Studies and more. Use data, advanced type systems, great product design and deep pedagogy to change lives. - Iterators and streams in Rust and Haskell <https://www.fpcomplete.com/blog/2017/07/iterators-streams-rust-haskell> > Real streaming data is handled at the library level in Haskell, with many different options available. Rust does things differently: it bakes in a concept called iterators not only with the standard library, but the language itself: `for` loops are built-in syntax for iterators. - Introducing Vaultenv: Keeping your secrets secure with Vault and Haskell <https://tech.channable.com/posts/2017-07-06-introducing-vaultenv-keeping-yo…> > We're pleased to announce our first bit of open source code. It is a CLI utility that fetches secrets from the HashiCorp Vault secret store. It makes secrets available using environment variables to a process of your choosing. - Danger and HLint <http://allocinit.io/haskell/danger-and-hlint/> > Danger is a Ruby gem that you can use on your continuous integration process to automate code reviews. Danger can automatically go through the code in a an open pull request and check if it respects the rules your team and you have established. - Reader monad part 2 <https://medium.com/@jonathangfischoff/monad-reader-part-2-d812dda1d03e> > In part 1 I went over the basics of the reader monad. In this post I'll cover some more advanced topics and alternatives to the reader monad. - Encoding objects <https://www.schoolofhaskell.com/user/fumieval/encoding-objects> > Objects can be thought of as Mealy machines of effects, and their Haskell encoding is actually straightforward. This novel building block might be too strong to use everyday, but I'm pretty sure there are places where this abstraction fits well, like game programming. - Playing match maker <https://mmhaskell.com/blog/2017/7/10/playing-match-maker> > So at this point, are we condemned to choose between a fast inaccurate algorithm and a correct but slow one? In this case the answer is no! This problem is actually best solved by using a graph algorithm! ## Package of the week ## This week's package of the week is Solga, a library for easily specifying web APIs and implementing them in a type-safe way. <https://www.stackage.org/lts-8.22/package/solga-0.1.0.2> ## Call for participation ## Are you interested in contributing to open-source Haskell projects but not sure where to start? Here are some tasks from the Haskell community for you to pick and get started! - aeson: Hindent? <https://github.com/bos/aeson/issues/527> - cabal: Print out more information about the effective configuration when you build <https://github.com/haskell/cabal/issues/3945> - esqueleto: Make examples for the README buildable <https://github.com/bitemyapp/esqueleto/issues/4> - miso: phantomjs2 fails to build on OSX <https://github.com/dmjio/miso/issues/160> - servant: BasicAuthentication has no documentation for client-side use <https://github.com/haskell-servant/servant/issues/752> - stack: `stack new` can't be pointed at an intranet site <https://github.com/commercialhaskell/stack/issues/2804> - text: Add tshow function <https://github.com/bos/text/issues/183> Email <info(a)haskellweekly.news> with your beginner-friendly issues to get them included. ## In brief ## - A tale of two provers: Verifying monoidal string matching in Liquid Haskell and Coq <https://nikivazou.github.io/static/Haskell17/a-tale.pdf> - Named extensible effects <https://www.schoolofhaskell.com/user/fumieval/extensible/named-extensible-e…> - Profunctor optics: Modular data accessors <https://arxiv.org/abs/1703.10857> - Profunctor optics: The categorical view <https://bartoszmilewski.com/2017/07/07/profunctor-optics-the-categorical-vi…> - PureScript 0.11.6 <https://github.com/purescript/purescript/releases/tag/v0.11.6>

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problems with "stack ghc"
by Dennis Raddle 13 Jul '17

13 Jul '17

I'm using Haskell stack in the normal way (i.e. "stack build", "stack exec") for most of my application. However, I also need to build files that aren't in the stack source tree. My stack source tree is at ~/stack/cac/src/ Let's say I want to build the program /Users/Dennis/test.hs. So far I have been using stack ghc --stack-yaml ~/stack/cac/stack.yaml -- --make /Users/Dennis/test.hs I am confused. I thought this was working until today. I made some modifications in various places, including to my cac.cabal (but NOT my stack.yaml) and I'm having a problem. The libraries that test.hs imports are not getting rebuilt when they change. Are they *supposed* to be rebuilt? Any ideas how to debug this? D

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persistent in types
by Dmitry Olshansky 12 Jul '17

12 Jul '17

Hello cafe, I am trying to develop a library PersT <https://github.com/odr/perst/tree/a5c59833742f4cde35bbeb80394094fa1c0972a7> for working with database persistent in types (instead of e.g. template-haskell in persistent library). Now there are base DDL/DML functionality and additional DML functionality for tree-like structures. Small dirty example of using it is located here <https://github.com/odr/perst/blob/a5c59833742f4cde35bbeb80394094fa1c0972a7/…> . The main problem which I have is compile time. Compiling of my small example (five tables with five fields) needs 7 minutes and more than 6Gb RAM (!) I compile using stack and ghc 8.0.2. To check you need additionally only sqlite installed. Are there any ways to reduce resource utilization in compile time? Is it some problem in ghc which can be changed in future? What do you think? I also will be thankful for any feedback about library. Regards, Dima

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What are the essential parts of a binary GHC package
by Jan von Löwenstein 12 Jul '17

12 Jul '17

Hi, if I plan to compile Haskell code on the customers side, which parts of https://www.haskell.org/ghc/download_ghc_8_0_2.html#linux_x86_64 could I remove in order to reduce the size of my delivery? Some things are pretty clear: - GHC has to be in - LICENSE has to be in - libraries have to be in - docs can probably be left out But what about - compiler/stage1/** - compiler/stage2/** - libraries/Cabal/Cabal/** Any advice? Thanks and best regards Jan

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Encrypting streamed data
by Ivan Lazar Miljenovic 11 Jul '17

11 Jul '17

I have a use case for needing to use public key cryptography to encrypt a large amount of data in a streaming fashion (get it out of a DB, encrypt, put into an AWS S3 bucket). The command-line gpg tool seems to be able to encrypt/decrypt data from stdin to stdout in a streaming fashion, but in my attempts to use it it seems very file-based for things like the keys to use (whereas I would prefer to be able to pass the public key as an actual value rather than a file; if nothing else because this is for tools that don't have email addresses to use and base their keys on for addressing). Is there an existing library that can achieve this using conduit/pipes/whatever? cryptonite-conduit only covers hashing, hOpenPGP is poorly documented and I can't work out how to use it ("just follow the types" is difficult when Haddock docs don't link to the required types (seems to be because it uses the "import Module as X" trick for re-exporting everything but then everything from those modules isn't available). Can anyone recommend a solution? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Higher order functor package?
by Clinton Mead 11 Jul '17

11 Jul '17

Consider the illustrative code below: {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE UndecidableInstances #-} data Param = Param1 | Param2 data T (p :: Param) where TInt :: Int -> T Param1 TInteger :: Integer -> T Param1 TBool :: Bool -> T Param2 data U (p :: Param) where UDouble :: Double -> U Param1 UString :: String -> U Param2 data F (t :: Param -> *) where F :: t Param1 -> t Param2 -> F t f :: T a -> U a f (TInt x) = UDouble (fromIntegral x) f (TInteger x) = UDouble (fromIntegral x) f (TBool x) = UString (show x) class MyFunctor f where myFmap :: (forall a. t a -> u a) -> f t -> f u instance MyFunctor F where myFmap f (F x1 x2) = F (f x1) (f x2) deriving instance Show (U a) deriving instance (Show (t Param1), Show (t Param2)) => Show (F t) main = print $ myFmap f (F (TInt 42) (TBool False)) Basically this is a sort of "higher order" functor, but I can't seem to fit it into an ordinary functor. But it seems like I'm reinventing the wheel, as my code is suspiciously like `Functor` but only slightly different. Has this sort of class already been created and if so what package is it in?

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