- Haskell-Cafe - Haskell.org

"cabal quine"
by Albert Y. C. Lai 19 Jul '17

19 Jul '17

Today a few of us on IRC (freenode #haskell) discovered: The following cabal-install command cabal manpage outputs cabal-install's man page (and is mainly how cabal-install can come with a man page without carrying an extra source file). This inspires the following feature request in jest: Could "cabal quine" output its source code please? :) kthxbye

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Call for Participation: ICFP 2017
by Lindsey Kuper 18 Jul '17

18 Jul '17

[ Early registration ends 4 August. ] ===================================================================== Call for Participation ICFP 2017 22nd ACM SIGPLAN International Conference on Functional Programming and affiliated events September 3 - September 9, 2017 Oxford, UK http://icfp17.sigplan.org/ ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. A full week dedicated to functional programming: ICFP, 2 co-hosted conferences, 1 co-hosted symposium, workshops, tutorials, programming contest results, student research competition, and mentoring workshop * Overview and affiliated events: http://icfp17.sigplan.org/home * Program: http://icfp17.sigplan.org/program/program-icfp-2017 * Accepted papers: http://icfp17.sigplan.org/track/icfp-2017-papers * Registration is available via: https://regmaster4.com/2017conf/ICFP17/register.php Early registration is due 4 August, 2016. * Programming contest, 4-7 August, 2016: http://2017.icfpcontest.org * Student Research Competition: http://icfp17.sigplan.org/track/icfp-2017-Student-Research-Competition * Follow @icfp_conference on twitter for the latest news: http://twitter.com/icfp_conference There are several events affiliated with ICFP: Sunday, September 3 Workshop on Higher-order Programming with Effects Workshop on Type-Driven Development Scheme and Functional Programming Workshop Programming Languages Mentoring Workshop ICFP Tutorials Monday, September 4 – Wednesday, September 6 ICFP FSCD - Days 1-3 Thursday, September 7 Haskell Symposium – Day 1 ML Family Workshop Workshop on Functional High-Performance Computing Commercial Users of Functional Programming – Day 1 FSCD - Day 4 Friday, September 8 Haskell Symposium – Day 2 OCaml Workshop Erlang Workshop Commercial Users of Functional Programming – Day 2 Saturday, September 9 Commercial Users of Functional Programming – Day 3 Haskell Implementors Workshop Functional Art, Music, Modeling and Design Conference Organizers: General Chair: Jeremy Gibbons (University of Oxford, UK) Program Chair: Mark Jones (Portland State University, USA) Artifact Evaluation Co-Chair: Matthew Flatt (University of Utah, USA) Artifact Evaluation Co-Chair: Ryan R. Newton (Indiana University, USA) Industrial Relations Chair: Ryan Trinkle (Obsidian Systems LLC, USA) PLMW Co-Chair: Neelakantan R. Krishnawami (University of Cambridge, UK) PLMW Co-Chair: Dan Licata (Wesleyan University, USA) PLMW Co-Chair: Brigitte Pientka (McGill University, Canada) Programming Contest Organiser: Sam Lindley (University of Edinburgh, UK) Publicity and Web Chair: Lindsey Kuper (Intel Labs, USA) Student Research Competition Chair: Ilya Sergey (University College London, UK) Student Volunteer Co-Captain: Yosuke Fukuda (Kyoto University, Japan) Student Volunteer Co-Captain: Yuki Nishida (Kyoto University, Japan) Student Volunteer Co-Captain: Jakub Zalewski (University of Edinburgh, UK) Video Chair: Jose Calderon (Galois, Inc., USA) Workshops Co-Chair: Andres Löh (Well-Typed LLP, UK) Workshops Co-Chair: David Christiansen (Indiana University, USA) Sponsors and industrial partners: Platinum partners Ahrefs Jane Street Capital Gold partners Bloomberg X Silver partners Galois Oracle Bronze partners Obsidian Systems Portland State University Well-Typed

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Is there something new in GHCi?
by Francisco Vieira de Souza 18 Jul '17

18 Jul '17

I´m using GHCi for Windows and I found sometrhing new. Mainly, there is no way for using GHC tooI for making a executable program. I had been doing all that is written in the page of Haskell and the same thing. I think that is something wrong in this new way of installation of GHC. Someone can help me in this case? Thanks in advance! Vieira -- "Embora ninguém possa voltar atrás e fazer um novo começo, qualquer um pode começar agora e fazer um novo fim". (Chico Xavier)

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Fwd: Memoization in Type Calculation
by Dmitry Olshansky 17 Jul '17

17 Jul '17

Rahul, thanks! It is really simple! 2017-07-17 21:33 GMT+03:00 Rahul Muttineni <rahulmutt(a)gmail.com>: > Hi Dmitry, > > You can effectively think of the type-level language of Haskell as a > strict functional language with almost no syntactic sugar, and recursion is > the *only* way of getting the behaviour of a loop. The best you can do to > improve the performance of that type family is to write it in > tail-recursive form using a helper function and which will make the type > family reduction look more linear vs a tree like it is now and make it > easier for the type checker to quickly spit out the result. > > type family Fib (n::Nat) :: Nat where > Fib n = GoFib 0 1 n > > type family GoFib (a :: Nat) (b :: Nat) (n :: Nat) where > GoFib a _ 0 = a > GoFib a b n = GoFib b (a + b) (n - 1) > > Hope that helps, > Rahul > > On Mon, Jul 17, 2017 at 11:19 AM, Dmitry Olshansky <olshanskydr(a)gmail.com> > wrote: > >> Hello, cafe! >> >> I wonder is there some possibility to get Memoization in Type Calculation >> (e.g. in closed Type Families)? >> >> Can we make something more efficient for famous Fib function then >> >> type family Fib (n::Nat) :: Nat where >> Fib 1 = 1 >> Fib 2 = 1 >> Fib n = Fib (n-1) + Fib (n-2) >> >> This Fib has obviously exponential calculation time and we need some >> memoization. >> >> If it is impossible right now, maybe there is a ticket for this? It seems >> to me very important things, no? >> >> Probably in this case we can write TypeChecker Plugin, but it looks like >> the common problem. >> >> Best regards, >> Dmitry >> >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > > > > -- > Rahul Muttineni >

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Memoization in Type Calculation
by Dmitry Olshansky 17 Jul '17

17 Jul '17

Hello, cafe! I wonder is there some possibility to get Memoization in Type Calculation (e.g. in closed Type Families)? Can we make something more efficient for famous Fib function then type family Fib (n::Nat) :: Nat where Fib 1 = 1 Fib 2 = 1 Fib n = Fib (n-1) + Fib (n-2) This Fib has obviously exponential calculation time and we need some memoization. If it is impossible right now, maybe there is a ticket for this? It seems to me very important things, no? Probably in this case we can write TypeChecker Plugin, but it looks like the common problem. Best regards, Dmitry

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[Hiring] NStack is hiring Haskellers in London (onsite / remote)
by Mandeep Gill 17 Jul '17

17 Jul '17

NStack (www.nstack.com / www.github.com/nstack/nstack) lets data scientists and analysts to do sophisticated data work in the cloud without a team of engineers. To accomplish this, we’re a building a platform for composable, data-driven microservices, using a mixture of Haskell and Linux systems tech (including Kubernetes, systemd, and local IPC) with some typed DSLs, systems code, and distributed systems thrown in - think Bash-piping for distributed, containerised microservices. Our aim is to bring many of the ideas from functional programming, such as typed APIs and composition to the analytics space. We have an early demo of the product available, see www.github.com/nstack/nstack to get started and https://demo.nstack.com (very early demo, built using GHCJS - see [1]), and are currently in trials with several enterprises. We’re looking for talented programmers experienced using typed functional languages with preferably some Linux/Unix systems knowledge. It’s a challenging role, working on hard problems, and offers the chance to work with a top technical team and shape a company and product from an early stage. NStack is funded by top-tier investors from the West Coast, the founders are both technical and ex-YC / academia, and our team is lucky enough to include world-class talent for the problem we're solving. Salaries are competitive and include generous stock options. We're generally hiring onsite, however several of us work across Europe and we welcome remote applications within suitable timezones. Unfortunately we can not sponsor work visas. We're a team of 10, including 5 full-time Haskellers, and are looking at a range of positions and experience levels - whether you’ve just left uni or been hacking for 20 years, so if you’re interested please get in touch. Any questions please email direct or take a look at https://angellist.com/nstack/jobs Cheers, Mandeep [1] log in using `user-id` = `be56de22d8444786ae3231953b2cf4b1` and `secret-key` = `8d245132f0a9402d9284cf2e0a280391`

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intero-neovim: Bring the joy of Intero to your Neovim!
by Matt 17 Jul '17

17 Jul '17

Hi everyone, I would like to invite you to try out my intero-neovim[1] plugin for giving Neovim IDE-like features. It's a port of the excellent intero plugin for emacs, and does many of the same things: type at point, go to definition, loading specific stack targets, etc. It has recently been significantly refactored and a ton of bugs have been fixed thanks to contributions by Reuben D'Netto and Jake Zimmerman. It's been working great for me on my work projects (stack-based with about a dozen packages) as well as one-off files and smaller projects. I would appreciate any and all feedback on features or bugs :D https://github.com/parsonsmatt/intero-neovim Thanks, Matt Parsons

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Haskell IDE Engine progress report for July 2017
by Alan & Kim Zimmerman 17 Jul '17

17 Jul '17

This report is also available online at [1] Haskell IDE Engine progress report for July 2017. <https://github.com/haskell/haskell-ide-engine/blob/master/docs/Report-2017-…>What is Haskell IDE Engine? Not an IDE. It is a common point for people in the Haskell community to pool their efforts with respect to tooling. For tool writers, provide tools as a HIE plugin, so it can be available on supported IDEs For IDE developers, integrate to HIE, and all the Haskell tools supported as plugins become available For users, it means the overall Haskell experience should improve. Currently, we are targeting the Language Server Protocol as the channel via which we talk to IDEs and editors. This would allow much broader IDE support. <https://github.com/haskell/haskell-ide-engine/blob/master/docs/Report-2017-…>Important developments Zubin Duggal started work on a Summer of Haskell project to improve the LSP support in HIE and implement more of the protocol. Large portions of the HIE codebase have been refactored and rearchitectured with LSP support in mind. Plugins for brittany and hoogle have been implemented. A new plugin request and dispatcher system has been implemented, along with a new architecture for the loading and caching of TypecheckedModules from source, which are used to drive many queries and features like type information, document symbols, document highlights and go to definition. Technical details regarding these changes are described in Architecture <https://github.com/haskell/haskell-ide-engine/blob/master/docs/Architecture…> . As of July 2017, HIE supports a large portion of the LSP. For the complete feature list, check the README <https://github.com/haskell/haskell-ide-engine/blob/master/README.md>. <https://github.com/haskell/haskell-ide-engine/blob/master/docs/Report-2017-…>Current project focus HIE (with vscode support via a plugin) is ready for an alpha release. Currently, the focus is on bug fixing and hooking up already existing features in HIE to LSP, like Hoogle queries and HaRe refactorings, and also providing additional features via LSP Code Actions and Code Lenses If you have experience writing Typescript or developing Visual Studio Code extensions, your help is needed to improve the vscode hie extension. <https://github.com/haskell/haskell-ide-engine/blob/master/docs/Report-2017-…>Contributors active in June and July Alan Zimmerman, Zubin Duggal, Lennart Spitzner <https://github.com/haskell/haskell-ide-engine/blob/master/docs/Report-2017-…> Contributing Haskell IDE Engine needs volunteers like any other open source project. For more information see: https://github.com/haskell/haskell-ide-engine Also drop by our IRC channel: #haskell-ide-engine at irc.freenode.net. Thanks! Zubin and Alan [1] https://github.com/haskell/haskell-ide-engine/blob/master/docs/Report-2017-…

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Wrapping Turtle ls function to work with 0 or 1 arguments (polyvariadic)
by Cody Goodman 16 Jul '17

16 Jul '17

I'm trying to get ls to work in ghci like it does in bash using variadiac arguments in Haskell. I'm stuck at the moment so I thought I'd send this to #haskell-cafe for some help. I'm not quite sure how to proceed. See the main function for what I want the end result to look like. Here is the code: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverloadedStrings #-} -- | An example module. module Example (main) where import qualified Turtle as Turtle import Control.Foldl import Control.Monad.IO.Class lsDirEx :: MonadIO io => io [Turtle.FilePath] lsDirEx = do Turtle.fold (Turtle.ls "/home") list class PrintAllType t where -- use PrintAllType from https://rosettacode.org/wiki/Variadic_function#Haskell process :: [Turtle.FilePath] -> t -- instance MonadIO io => PrintAllType (io [Turtle.FilePath]) where instance PrintAllType (IO [Turtle.FilePath]) where process [] = do -- ls received no args print current directory Turtle.pwd >>= \fp -> Turtle.fold (Turtle.ls fp) list process (filePath:[]) = do -- ls recieved one filePath liftIO $ Turtle.fold (Turtle.ls filePath) list process _ = error "multiple arguments not currently supported" -- instance (Show a, PrintAllType r) => PrintAllType (a -> r) where -- process args = \a -> process (args ++ [fmt a]) -- where fmt thing = _ $ Turtle.format Turtle.w thing ls :: (PrintAllType t) => t ls = process [] -- | An example function. main :: IO () main = do ls -- lists current directory ls ("/home" :: String) -- lists /home directory Thanks, Cody

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Re: [Haskell-cafe] parallelism and state
by dominic＠steinitz.org 16 Jul '17

16 Jul '17

Excellent news! How did you test it? One way would be to make it callable by C and then use testu01 but perhaps you have already done this or similar? See here for a bit more detail: https://idontgetoutmuch.wordpress.com/2017/01/14/calling-haskell-from-c/ <https://idontgetoutmuch.wordpress.com/2017/01/14/calling-haskell-from-c/>. Testing the split function might need thinking about though. > I recently made a SplitMix implentation: > http://hackage.haskell.org/package/splitmix <http://hackage.haskell.org/package/splitmix> > > When generating `Word64`s it's noticeable faster than `tf-random` (and > `mwc-random` and obviously `random). I didn't benchmark `Double` > generation, as `tf-random` own `Random` class doesn't support it [1], > and using `random`'s class would be very unfair. > > I haven't announced it (well, mentioned on Twitter), but please give it > a try! > > - Oleg Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Feedback on use of State monad
by Atrudyjane 15 Jul '17

15 Jul '17

Hello Everyone, In the HPFFP book, there's a homework problem where you're supposed to refactor a recursive function that rolls dice to return the number of rolls it takes to reach or exceed an input sum and the list of die that occurred. As practice, decided to take the solution and implement it using the State monad: -- Refactor rollsCountLoggged to use State -- limit sum count StdGen [Die] type RollsState = (Int, Int, Int, StdGen, [Die]) -- count [Die] type RollsValue = (Int, [Die]) runRolls :: State RollsState RollsValue runRolls = do (l, s, c, sg, xs) <- get if (s >= l) then return (c, reverse xs) else do let (d, ng) = randomR (1, 6) sg put (l, s + d, c + 1, ng, intToDie d : xs) runRolls rollsCountState :: Int -> IO () rollsCountState n = print $ evalState (runRolls) (n, 0, 0, mkStdGen 0, []) I'm wondering if that 'do' block under the else is a not so great stylistic choice. If the 'let' and 'put' is moved above the 'if', then that's just an unnecessary call at the end. Or is it just the nature of running inside a monad? Thank You, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.

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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>

1 0

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

2 5

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

1 0

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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I'm afraid of OverloadedLabels.
by Nikita Churaev 11 Jul '17

11 Jul '17

So gi-gtk is already using OverloadedLabels to fight the namespacing problem. What I'm afraid is that eventually people will pick up on this and stop using normal functions altogether. My problem with OverloadedLabels is that it is a class: if people start using it widely, the original modules of types are going to become even more privileged than they already are: only it will be canonically able to define functions which can be conveniently called, and everybody else will have to only define "second-class" normal functions or risk instance collisions. I mean yes, it's the same as in most OOP languages, but isn't it also bad? There must be a better way... Is there any alternative approach to name collisions coming?

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Fwd: constraint inference
by Dmitry Olshansky 10 Jul '17

10 Jul '17

Sorry, I didn't copy to cafe... ---------- Forwarded message ---------- From: Dmitry Olshansky <olshanskydr(a)gmail.com> Date: 2017-07-07 9:24 GMT+03:00 Subject: Re: [Haskell-cafe] constraint inference To: Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> I am not sure why but when I didn't set RankNTypes I got an appropriate error. So I specified it. But if I write let { f :: (forall n. KnownSymbol n => Proxy n) -> String; f p = symbolVal p } then nothing is changed. 2017-07-07 9:11 GMT+03:00 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com>: > You've specified RankNTypes but aren't using it; did you mean "(forall > n. KnownSymbol n => Proxy n)" ? > > Typically in these situations you also can't partially apply it, so > you might need to make it "f p = symbolVal p". > > On 7 July 2017 at 15:50, Dmitry Olshansky <olshanskydr(a)gmail.com> wrote: > > Hello, cafe! > > > > If we have many constraints then compiling is sometimes too slow. > Especially > > if there are some type calculations. > > And I don't know how to divide this time among different source files > > because all constraints are checked only when we call a function. > > > > I tried to add constraint info into argument definition instead of the > > function signature but this trick is not working (see below). > > > > Is it (at least theoretically) possible to infer the constraint inside a > > function from the type of function's argument? > > > > ------------------------------------ > > $ stack ghci > > ---- GHCi, version 8.0.2: > > Prelude> import Data.Proxy > > Prelude Data.Proxy> import GHC.TypeLits > > Prelude Data.Proxy GHC.TypeLits> :set -XRankNTypes > > Prelude Data.Proxy GHC.TypeLits> let { f :: (KnownSymbol n => Proxy n) -> > > String; f = symbolVal } > > > > <interactive>:4:54: error: > > • No instance for (KnownSymbol n) arising from a use of ‘symbolVal’ > > Possible fix: > > add (KnownSymbol n) to the context of > > the type signature for: > > f :: (KnownSymbol n => Proxy n) -> String > > • In the expression: symbolVal > > In an equation for ‘f’: f = symbolVal > > ------------------------------------ > > > > Best regards, > > Dmitry > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > > > > -- > Ivan Lazar Miljenovic > Ivan.Miljenovic(a)gmail.com > http://IvanMiljenovic.wordpress.com >

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[ANN] [Reminder] Budapest Haskell Hackathon 2017, 29-30th July 2017
by Daniel Berecz 10 Jul '17

10 Jul '17

Hi everybody, Just a last reminder about our hackathon in Budapest at the end of July: Our local user group will hold a Haskell Hackathon in Budapest this year (this will be the second)! We will hold it from the 29th to the 30th of July (Saturday & Sunday). You can find out more on the following link: https://wiki.haskell.org/Budapest_Hackathon_2017 We will reguraly update the site with new information. If you decide to come, please fill out the following Google Form: https://goo.gl/forms/QjTNdXTD1aEOMEQU2 If you have a project, demo, or talk that you want to bring to the event please contact us, and we can talk about the details. You can find our contact info the events wiki site, or you can just contact me directly. The event will be open to people of all experience levels, from beginners to gurus. The only requisite is that you’re interested in the Haskell language, and want to hang out with us, and have a good time! Greetings from Daniel, and the other BP-HUG organizers.

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Re: [Haskell-cafe] I'm afraid of OverloadedLabels.
by Anthony Clayden 10 Jul '17

10 Jul '17

> On Mon Jul 10 02:57:02 UTC 2017, Nikita Churaev wrote: > > You can maybe fix that up with FunDeps > > or TypeFamilies, but then you lose most of TDNR's > > benefits (esp not coping with existential quanted fields > > , which is typically a requirement to cope with > > OOP-style). > > What benefits exactly are lost? Hi Nikita, I'm not sure I (or anybody) has the energy for another round of the "records debate". The TDNR (ex-)proposal seems so attractive because it just doesn't consider in detail all the complexities. They're magically swept under the carpet with the special dot syntax. So you might expect TDNR would support all of the functionality with H98 field names, plus having multiple fields of the same name. But how? Note the comment on TDNR section 'The Proposal' "TDNR is not "overloading" in the sense of type classes." And there were plenty of complaints that it's confusingly similar-but-different to type classes. You could look at SPJ's re-working of TDNR [2010] as SORF [2013]: https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/SORF SORF took advantage of the new Symbol Kinds (not available in 2010), and tried to be more type-class-like, with the `Has` class. One of the motivations with SORF is "Records with polymorphic fields are very important in practice, so it would be a major wart not to support them." TDNR just didn't discuss higher-ranked types. It turned out that was just too hard, even for SORF. (Or rather for OverloadedRecordFields, which is what's being delivered from SORF.) SORF tries to tackle record update, for which TDNR said "However I propose not to apply it [TDNR] to record update. " I don't think our cousins in OOP would be impressed by a records system in which you can't write polymorphic record updates. (I.e. a method to update the `firstName` in any record that has that field.) Actually delivering improvements in GHC around records is being painfully slow. `DuplicateRecordFields` has eased the pain a little. Ironically `OverloadedRecordFields` doesn't tackle higher-ranked types, nor record updates. And needs special syntax `#firstName`. So perhaps TDNR's "benefits" were never achievable? ANtC

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Re: [Haskell-cafe] I'm afraid of OverloadedLabels.
by Anthony Clayden 10 Jul '17

10 Jul '17

> On Mon Jul 10 00:16:02 UTC 2017, Richard A. O'Keefe wrote: > ... > As a native speaker of an SVO language (English) > in a country where one of the official languages is VSO (Māori), > I sometimes find the OVS structure of > "mainstream OO languages" a little uncomfortable... > "Backward ran sentences until reeled the mind." lol. That's a cute comparison. I don't think "mainstream OO" is OVS. In: > ... button.colour <- red Gloss: 'button's colour goes red'. `button's colour` is the grammatical Subject. (Yes it represents an OOP 'Object.method', but that's because everything in OOP is an 'Object'; and the only way to access is by methods.) > ... button.reset Again `button` is the grammatical Subject. (It must be, because there's no grammatical Object.) Then `reset` must be the verb, used intransitively. (In French you'd use a reflexive: `the button resets itself`.) This dot-suffixing is already in Haskell: Data.ByteString.Lazy.Builder.Extras 'Data's ByteString's Lazy's Builder's Extras'. To me it seemed natural to carry on ....Extras.untrimmedStrategy.buffSize That is, a field is just a component that might have (sub-)sub-component(s). The field 'belongs to' the structure, so dot represents English possessive. And that's just how lenses look, deliberately. (But that's a mystifying pun on use of the dot: the dot changes from compound name separator to function composition.) But the outcry against TDNR's proposal for dot really was voiciferous. (And nobody paid so much attention to the semantics. So the flaw in TDNR wasn't discovered for a long time.) AntC

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Re: [Haskell-cafe] I'm afraid of OverloadedLabels.
by Anthony Clayden 10 Jul '17

10 Jul '17

> On Sun Jul 9 20:37:23 UTC 2017, Alexis King wrote: > > On Jul 9, 2017, at 10:36, Ertugrul Söylemez <esz at posteo.de> wrote: > > > AND PLEASE not the way an [existing proposal] suggests to do it! > > Please let's just do ad-hoc overloading. There is no reason to > > introduce new syntax, because syntax is completely orthogonal > > to this problem. > > > > [existing proposal]: > > https://prime.haskell.org/wiki/TypeDirectedNameResolution Folks, that is not an existing proposal. SPJ's write-up is seductive, but fatally flawed, as Edward Kmett pointed out during the OverLoadedRecordFields development work. Quite apart from its relying on special syntax, TDNR can't cope with nested records. In: > ... student.course.instructor ... the `.course.instructor` is irresolvably ambiguous, in exactly the same way as `(show . read)`. You can maybe fix that up with FunDeps or TypeFamilies, but then you lose most of TDNR's benefits (esp not coping with existential quanted fields, which is typically a requirement to cope with OOP-style). > If I understand correctly, what youre proposing is the syntax-free > variant of TDNR, which is already outlined here: > https://ghc.haskell.org/trac/ghc/wiki/SyntaxFreeTypeDirectedNameResolution I think everyone should read the accompanying discussion. That turned out to be not an 'outline' but just a piece of wishful thinking. It has all the flaws of the original TDNR, plus a few of its own. > I agree that would be a very nice feature to have, and I think doing it > without new syntax would be strongly preferable to adding new syntax. > The proposal outlines the basics of how it would work, but I have no > idea how difficult it would be to implement in practice. Quite. AntC

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Re: [Haskell-cafe] I'm afraid of OverloadedLabels
by Anthony Clayden 10 Jul '17

10 Jul '17

> On Sun Jul 9 21:04:33 UTC 2017, MarLinn wrote: > ... > Isn't it about time to allow explicit imports, and more importantly, > explicit non-imports for instances? > Some time ago I fantasised about some special syntax for this, > calling the (very rough) idea "aspects" ... Sorry, I didn't realise that "aspects" thread was about controlling instance scope. I did try to start a thread on instance scope a few days ago https://mail.haskell.org/pipermail/haskell-cafe/2017-July/127435.html I am interested in use cases for scope-controlling instances. Specifically I'm not seeing how scoping instances (instances of what, exactly?) helps with the "record problem". Typically if you have several record types with the same-named field, you want all of those to be in scope, where the field-access gets resolved according to the record's type. The `DuplicateRecordFields` extension now provides reasonable support for that, doesn't it? AntC

1 0

Collision Detection
by Yotam Ohad 10 Jul '17

10 Jul '17

Dear Cafe, I am thinking about writing a small physics engine with collision detection and I wanted to go over my ideas with you to help me refine them. I want to express objects as a group of inequalities and their domain. For example, a sphere would be only one inequality: x^2+y^2+z^2 -1 <= 0 on the whole domain. That way, to check a collision between two objects, one needs to check if at least one pair of inequalities with overlapping domains has a solution. I am unsure about how to express the inequalities in a way that could still allow me to compare between two of them. To add forces in, I think I can express them by adding a time dimension to the inequalities. For example, a constant force in the x-dimension on the previous sphere could be represented as (C+dx/dt*t+0.5*a*t^2)^2+y^2+z^2 <= 0. But then I am not sure about how to treat non-integrable forces. Another approach is to calculate the displacement of the object after each time interval. I don't like this approach as I want to integrate it with FRP in the end, and FRP continues time is something I would like to preserve. After I'll have everything above sorted, the next things would be to run simulations. I think at the start I'll check every pair of objects and try to calculate whether or not the will collide in the future. I'll save all the results in ascending time order. and every iteration, update the movement after the closest collision and update the collision pair order. Thanks, Yotam

3 2

Speed of simulation in Aivika
by David Sorokin 09 Jul '17

09 Jul '17

Hi Cafe, I’m glad to inform that the speed of simulation in the aivika [1] simulation library of version >= 5.2 has been dramatically increased after I started using the mwc-random [2] package for generating random numbers. It was possible to use custom random number generators before, but now the random number generator from the mentioned package is used by default. To improve the speed of simulation, in most cases it will be sufficient just to recompile your simulation models. In some my tests the speed of simulation has been increased up to 7 times when using default settings. The mwc-random package is just fantastic! Another good news is that I recently improved the distributed simulation module [3], which has recovering capabilities now in case of temporary connection errors. It should be quite unusual for the optimistic Time Warp method. In short, it means that we can build discrete event distributed simulation models using the ordinary Internet and cloud services. Best regards, David Sorokin [1] http://hackage.haskell.org/package/aivika [2] http://hackage.haskell.org/package/mwc-random [3] http://hackage.haskell.org/package/aivika-distributed

2 1

Haskell in Leipzig 2017: 2nd call for papers
by Wolfgang Jeltsch 07 Jul '17

07 Jul '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. Contributions ============= Everything related to Haskell is on topic, whether it is about current research, practical applications, interesting ideas off the beaten track, education, or art, and topics may extend to functional programming in general and its connections to other programming paradigms. Contributions can take the form of * talks (about 30 minutes), * tutorials (about 90 minutes), * demonstrations, artistic performances, or other extraordinary things. Please submit an abstract that describes the content and form of your presentation, the intended audience, and required previous knowledge. We recommend a length of 2 pages, so that the program committee and the audience get a good idea of your contribution, but this is not a hard requirement. Please submit your abstract as a PDF document at https://easychair.org/conferences/?conf=hal2017 until Friday, August 4, 2017. You will be notified by Friday, August 25, 2017. Hacking Projects ================ Projects for the hackathon can be presented during the workshop. A prior submission is not needed for this. Invited Speaker =============== * Ivan Perez, University of Nottingham, UK 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.

1 0

"stack script" and extra-deps
by Patrick Pelletier 07 Jul '17

07 Jul '17

When I use "stack script", how can I specify a package which is not in Stackage? When using a stack.yaml, I can just specify such a package in extra-deps, but I'm not clear how to translate this to "stack script". Thanks, --Patrick

2 1

Prelude's documentation on Hackage incomplete?
by Benjamin Rickels 07 Jul '17

07 Jul '17

I was recently looking at the various functions the RealFrac <http://web.archive.org/web/20170702220651/https://hackage.haskell.org/packa…> type class provides and noticed, that Double <http://web.archive.org/web/20170702220651/https://hackage.haskell.org/packa…> isn't listed as an instance of it; and neither are Num, Real and Fractional mentioned as implemented instances in the documentation for Double. Now, somebody who is familiar with the hierarchy of the numeric type classes could of course rather easily deduct, that, since Double is an instance of RealFloat (this is mentioned in the documentation on Hackage), it is also required to implement all of the above. And (surely enough) the Haskell 98 Report <https://webcache.googleusercontent.com/search?q=cache:GKdvwWYGFLoJ:https://…> lists all the relevant implementations and so does a quick *:info Double* in GHCi. However I'd (assume and thus) argue, that most beginners who google (or especially hoogle) anything about Haskell, are often directed to the Hackage documentation itself - and any missing information there could lead to quite a bit of a headache; even more so, if this concerns something as important as the Prelude. Therefore I'd like to ask, whether this is a known issue and independently propose, that (in the meantime) an official recommendation somewhere to do a quick *:info <type class>* or :info *<type> *in GHCi yourself, as to find out, which type class instances are currently available, might help to alleviate some confusion. Benjamin

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parallel Haskell
by Dennis Raddle 07 Jul '17

07 Jul '17

I have a program which does backtracking search in a recursive function. I followed the chapter in "Real World Haskell" to parallelize it. Without Control.Parallel, it uses about 25% of the CPU, namely one core out of four. With the suggestions in RWH it runs faster but still uses only 35% of the CPU max. So I set about creating a MWE in order to experiment and reach out for help. For some reason, my stripped down program isn't working with parallelism at all! There's no effect from the R.W.H. ideas. Can I get some suggestions as to why? Here's the program: -------------------------------------------- import qualified Data.List as L import Text.Printf import System.CPUTime import Data.Function import Control.Parallel -- Run backtracking search for a list of doubles, optimizing a -- fitness function on the list, called 'evalFunc'. At each step, use 'stepFunc' -- to generate a list of possible next Doubles that can be added to the list. -- We are done when the list has 'finalSize' elements. search1_par :: Int -> ([Double] -> Double) -> ([Double] -> [Double]) -> [Double] -> (Double,[Double]) search1_par finalSize evalFunc stepFunc listIn | length listIn == finalSize = (evalFunc listIn,listIn) | otherwise = let steps = stepFunc listIn (steps1,steps2) = divideListInTwo steps f s = search1_par finalSize evalFunc stepFunc $ s:listIn results1 = map f steps1 results2 = map f steps2 results = force results1 `par` (force results2 `pseq` (results1++results2)) in L.maximumBy (compare `on` fst) results force :: [a] -> () force xs = go xs `pseq` () where go (_:xs) = go xs go [] = 1 divideListInTwo :: [a] -> ([a],[a]) divideListInTwo [] = ([],[]) divideListInTwo xs = (take l xs,drop l xs) where l = length xs `div` 2 --------------------------------------------- -- some sample evaluation (fitness) functions and step generation functions. eval1 :: [Double] -> Double eval1 xs = v1 - v2 + v3 where v1 = sum $ zipWith (*) (cycle [1]) xs v2 = sum . map (*2) $ zipWith (*) (cycle [1,0]) xs v3 = sum . map (*3) $ zipWith (*) (cycle [1,0,0]) xs step1 :: [Double] -> [Double] step1 xs | l == 0 = take 8 $ xs | l == 1 = take 8 $ map (/2) xs | l == 2 = take 8 $ map (*3) xs where l = length xs `mod` 3 -------------------------------------------------------------------------------- -- main main = do t1 <- getCPUTime let f :: Double -> String f x = printf "%5.1f" x (_,result) = search1_par 13 eval1 step1 [1,2,3] putStrLn $ concatMap f result t2 <- getCPUTime putStrLn $ printf "CPU time: %.3f" ((fromIntegral $ t2-t1) / 1000000000000 :: Double)

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Issue 61 :: Haskell Weekly
by Taylor Fausak 07 Jul '17

07 Jul '17

\ Haskell Weekly \/\/ Issue 62 <https://haskellweekly.news/issues/62.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 ## - YOW! Lambda Jam 2017 <https://www.youtube.com/playlist?list=PLIpl4GKFQR6dqCNb2J0eQFR21Lwnk4I86> Videos of 23 talks by Edward Kmett, John Hughes, Conal Elliott, Brian McKenna, Bodil Stokke, and others. - Perhaps not the answer you were expecting but you asked for it <https://personal.cis.strath.ac.uk/conor.mcbride/so-pigworker.pdf> A collection of Conor McBride's Stack Overflow answers. - Unit testing effectful Haskell with monad-mock <https://lexi-lambda.github.io/blog/2017/06/29/unit-testing-effectful-haskel…> > While monad-mock might not be anything drastically different from > existing testing techniques, my hope is that it can provide an > opinionated mechanism to make testing easy and accessible, even for > complex interactions with other services and systems. - Front Row is hiring a senior backend Haskell engineer (ad) <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. - A major upgrade to Megaparsec: more speed, more power <https://markkarpov.com/post/megaparsec-more-speed-more-power.html> > The version 6 thus will aim to be not just a parser for human- > readable texts and source code, but "one size fits all" general > solution to parsing in Haskell, including low-level binary parsing. - Derive instances of representationally equal types <https://gist.github.com/Icelandjack/d258b88a0e0b3be2c0b3711fdd833045/fe806c…> > I made a way to get more free stuff and free stuff is good. - New version of FLTKHS with much better error messages and OpenGL integration <https://np.reddit.com/r/haskell/comments/6l85uj/announcement_new_version_of…> > Happy to announce a new major version of FLTKHS, the Haskell > binding to the FLTK for easy native cross-platform GUI apps in > Haskell. - Notes on fusion <http://teh.id.au/posts/2017/06/30/notes-on-fusion/index.html> > Fusion is one of those words that made me recoil for quite a few > years, since it sounds extremely complicated and arcane. A few > years into building applications in Haskell, I still hadn't forced > the thunk. - SuperRecord: Anonymous records for Haskell <https://www.athiemann.net/2017/07/02/superrecord.html> > A practical library for anonymous records that is both fast and has > an ergonomic interface for both using and extending it. - Typesafe modular arithmetic in Haskell <https://rkrishnan.org/posts/2017-06-20-typesafe-modulus-in-haskell.html> > We would like to get a type error (at compile time) if we do a > operation involving two numbers with a different modulus. The role > of a type system is to represent the invariants and so it will be > great to encode as many invariants of a particular value into the > type. - Lessons learned building a toy compiler <https://jaseemabid.github.io/2017/07/04/compiler.html> > We tend to think of compilers as big black boxes which transform > some high level language, let's say C into a binary in one big > step. I'd like to present it as a pipeline of languages and > transformations, each a bit simpler and slightly lower level than > the one before it. ## Package of the week ## This week's package of the week is Miso, a small isomorphic front-end framework. <https://hackage.haskell.org/package/miso-0.1.2.0> ## In brief ## - Diskhash: Disk-based, persistent hash tables <https://metarabbit.wordpress.com/2017/06/30/ann-diskhash-disk-based-persist…> - Graphing it out <https://mmhaskell.com/blog/2017/6/21/graphing-it-out> - highlight CLI application <https://functor.tokyo/blog/2017-07-01-highlight> - Idris dependent typing challenge: Bowling kata <https://deque.blog/2017/07/01/idris-bowling-kata/> - The Spiderman principle <https://www.snoyman.com/blog/2017/07/the-spiderman-principle> - What should a PostgreSQL migrator do? <https://medium.com/@jonathangfischoff/what-should-a-postgresql-migrator-do-…>

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constraint inference
by Dmitry Olshansky 07 Jul '17

07 Jul '17

Hello, cafe! If we have many constraints then compiling is sometimes too slow. Especially if there are some type calculations. And I don't know how to divide this time among different source files because all constraints are checked only when we call a function. I tried to add constraint info into argument definition instead of the function signature but this trick is not working (see below). Is it (at least theoretically) possible to infer the constraint inside a function from the type of function's argument? ------------------------------------ $ stack ghci ---- GHCi, version 8.0.2: Prelude> import Data.Proxy Prelude Data.Proxy> import GHC.TypeLits Prelude Data.Proxy GHC.TypeLits> :set -XRankNTypes Prelude Data.Proxy GHC.TypeLits> let { f :: (KnownSymbol n => Proxy n) -> String; f = symbolVal } <interactive>:4:54: error: • No instance for (KnownSymbol n) arising from a use of ‘symbolVal’ Possible fix: add (KnownSymbol n) to the context of the type signature for: f :: (KnownSymbol n => Proxy n) -> String • In the expression: symbolVal In an equation for ‘f’: f = symbolVal ------------------------------------ Best regards, Dmitry

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Re: [Haskell-cafe] representations of probability distributions
by David Banas 06 Jul '17

06 Jul '17

Hi Olaf, I can’t quite tell whether this is apropos to your request, but thought I’d mention it: https://wiki.haskell.org/Random_Processes <https://wiki.haskell.org/Random_Processes> -db

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Re: [Haskell-cafe] representations of probability distributions
by Olaf Klinke 05 Jul '17

05 Jul '17

Thanks for the link, Benjamin. Can one give a Hasekll type to a particle filter? In the arxiv paper you linked, I found this quite unnerving sentence: "The divergence between the true conditioned distribution on execution traces and the distribution encoded by the program may depend strongly on what inference instructions are chosen and how they are interleaved with the incorporation of data." Does this mean: Here is a programming language that may or may not compute what you meant it to do? --Olaf

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Instantiating a typeclass based on a runtime value?
by Saurabh Nanda 05 Jul '17

05 Jul '17

Overall context: I'm trying to port the DelayedJob library from Ruby/Rails world to Haskell. The basic idea is to serialise a job and write it to the DB, deserialise it, and run the job using a job-runner function, which is determined by the job-type. I'm using a type-class to define the custom job-runner, something on the lines of: class (FromJSON j, ToJSON j) => DelayedJob j where runJob :: j -> AppM () data SyncContactsJob = SyncContactsJob { userId :: UserId } deriving (Eq, Show, Generic, FromJSON, ToJSON) instance DelayedJob SyncContactsJob where runJob job = do -- entire job execution logic comes here Is there **any** type-system hackery, that will let me take a runtime value, eg. "SyncContactsJob", "DoBackupJob", and use that to run the correct version of the `runJob` function? That is, how do I write the following function: invokeJob :: JobId -> AppM () invokeJob jid = do jobRow <- fetchJob jid let jtype = jobRow ^. jobtype -- this will have "SyncContactsJob" jvalue = jobRow ^. jobdata -- this will have a Data.Aeson.Value runJob (......) -- QUESTION: What do I write here to make this compile?

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local instances / scoped instances
by Anthony Clayden 05 Jul '17

05 Jul '17

Can anybody explain the idea of typeclass instances being declared with a scope? Im not sure if these two terms mean the same. Perhaps the difference is: * local instances are declared in the familiar let-style: `let <instance> in <expr>` So the scope is the body of the `in` expr. * scoped instances are declared as usual at the top level in a module. Scope is controlled by the export/import naming. (That is, theres a way to name/identify instances for export/import. Note that currently you cant control scope of instances; Theyre imported to any module that (transitively) imports the module where an instance is declared.) (I'm doubtful that controlling scope of the name-for-instance would actually have the desired effect. Compare if I declare: > data T = MkT Int > f :: T -> String and export `f`, but not `T` nor `MkT`; nevertheless the type of `f` in some distant module is still `T -> String` (or `M.T -> String`). So detail of the type itself is exported. What are the use cases for instances being anything other than top-level/global scope? One seems to be so that a method can use a value/function declared in a surrounding scope, without needing an explicit parameter passed around the whole program. This is explored in [Kis04] (and linked references, esp. Implicit configurations), particularly for passing run-time parameters into methods. (What imperative programmers might do via global variables. In Haskell: nowadays Implicit Parameters.) [Kis04] (and especially the link to The pioneering work of Kaes) laments that Kaes original description of typeclasses/instances has never been delivered. Im not convinced Kaes is actually describing local instances. Hes giving the formal semantics for instances using let-binding; because hes using let as the standard way in Lambda Calculus to introduce any binding. Usual let-binding in LC provides a fresh name for each binding. That is, if that same name is in an outer scope, the outer binding is shadowed/unreachable within the scope of the fresh name. But instance binding is different: * Theres no fresh name: the class name is exactly the one already in scope. * Theres no shadowing: this instance is _in addition to_ those already in scope. thats what overloading needs. Overloading a class method with two or more instances in the same scope means theres no principal type for the method [W&B88 Appendix A.7]. Thats why in Haskell classes/instances/methods are at top level only. The methods then do have a most-general principal type `show :: (Show a) => a -> String`. This early work was of course before MPTCs, FunDeps, Overlaps, Flexible Instances/Contexts, constrained existentials, constrained GADTs. AFAICT it was expected there would be only one instance per type. So making instances global probably wasnt too limiting. Ive seen some later ideas (especially scoped instances blocking the export of an instance) where the intent seems to be to have _different_ instances in different scopes for the same class/same type. For example (apparently) a pretty print format for `show`. Or an alternative `Ord` for case-insensitive sorting of Chars/Strings. What are the use cases here? In particular, whats wrong with either: * using a different class/method name for different behaviour (for example supplying a different comparison via `sortBy`); or * wrapping in a newtype and declaring an instance of `Show/Ord` on the different nominal type? Im worried: classes come with laws. For example wrt case-insensitive `Ord`: `Eq` is a superclass. So if we get case-insensitive `A <= a` and `a <= A`, the laws expect ` A == a `. Specifically the base library `Data.Set` relies on those laws for `Ord` to build balanced trees. (Theres also a whole load of puzzlement to do with how the instance scoping rules work, as explored in [Kis04]. But I dont want to worry about the implementation yet.) AntC [Kis04] Local instances: frequently still discussed and still not implemented, Kiselyov O. ??originally 2004, current version 2014 http://okmij.org/ftp/Haskell/TypeClass.html#local See also papers linked/referenced from that site. (Thank you Oleg for bringing out the issues.) [Kaes88] Parametric Overloading in Polymorphic Programming Languages, Kaes S. 1988 https://link.springer.com/content/pdf/10.1007%2F3-540-19027-9_9.pdf [W&B88] How to make ad-hoc Polymorphism less ad-hoc, Wadler P. & Blott S. 1988 https://people.csail.mit.edu/dnj/teaching/6898/papers/wadler88.pdf

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on the number of ghc_worker threads on executables
by Ruben Astudillo 04 Jul '17

04 Jul '17

Dear list I noticed that on -threaded programs, even with just 1 capability (+RTS -N1) the number of worker tasks is usually more than 3. Examples of this are glirc and taffybar. What factors determine the number of Tasks/worker Tasks (OS threads) on a program? -- -- Ruben Astudillo -- PGP: 4EE9 28F7 932E F4AD

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representations of probability distributions
by Olaf Klinke 04 Jul '17

04 Jul '17

Dear cafe, following up the announcement of bali-phy and the discussion that ensued, I'd like ask the cafe for an overview of representations of probability distributions as data types. It seems that all packages mentioned to far use some flavour of Markov chain. Why? What else is there? Can someone recommend a survey publication/book/talk? Explicitly I am interested in: - the data type, as in Haskell's type system - how an element of the data type relates to the distribution it describes - how the monad instance works (if there is one) - how to compute conditionals (if computable) - how to compute integrals (if computable) - what base types are supported (finite types, reals, products ...) Below is an incomplete, and possibly oversimplified (or even wrong), list of my own. Additions and corrections welcome. -- Olaf 1. Markov chains - uses a random number generator - type is something like Markov a = a -> IO a - repeated execution produces an infinite trace xs :: [a] where for each predicate p :: a -> Bool, the limit of (length (filter p (take n xs))) % n as n tends to infinity approaches the probability of p under the distribution modeled. - monad instance? - conditionals? - integrals? - supported base types a in Markov a? 2. Riesz functionals: possibly the most functional representation - representation as linear functionals Riesz a = (a -> R) -> R for a suitable real number type R - embed predicates in the type (a -> R) via characteristic functions characteristic p = \a -> if p a then 1 else 0 Then for a functional phi :: Riesz a and p :: a -> Bool, phi (characteristic p) is the probability of p. - Monad instance is the same as for the continuation monad - conditionals? - integration is trivial: evaluation - arbitrary base types - but how would you even declare a uniform distribution in this type? 3. Density functions - represent a distribution on the reals as density function w.r.t. the Lebesgue measure - limited to reals (and products thereof) - data type possibly holds only parameters of parametric families (gaussian, beta, gamma, ...) - computing conditionals only for matching pairs of prior and sampling function (conjugate priors), analytically - integration analytically - no monad instance because no arbitrary base type 4. Finite distributions - type Dist a = [(a,R)] for suitable real number type R - optimizations use search trees instead of flat lists - basically composition of list and writer monads, where R is regarded as monoid under multiplication - represents a distribution of finite support. If phi :: Dist a, then probability p = sum $ map snd $ filter (p.fst) phi - conditioning possible if base type is of Eq class and R supports division - gets unwieldy without Ord instance of base type to keep the list small 5. Valuations - type Val a = (a -> Bool) -> R for suitable real number type R - models mathematical definition closely - monad bind is integration: requires exact reals R due to limit process - conditioning possible if R supports division

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Haskell Implementors' Workshop Call for Talks
by Richard Eisenberg 04 Jul '17

04 Jul '17

Call for Contributions ACM SIGPLAN Haskell Implementors' Workshop http://icfp17.sigplan.org/track/hiw-2017 Oxford, UK, 9 September, 2017 Co-located with ICFP 2017 http://www.icfpconference.org/icfp2017/ Important dates --------------- Proposal Deadline: Thursday, 6 July, 2017 Notification: Thursday, 20 July, 2017 Workshop: Saturday, 9 September, 2017 The 9th Haskell Implementors' Workshop is to be held alongside ICFP 2017 this year in Oxford. It is a forum for people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure, to share their work and discuss future directions and collaborations with others. Talks and/or demos are proposed by submitting an abstract, and selected by a small program committee. There will be no published proceedings. The workshop will be informal and interactive, with open spaces in the timetable and room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2017. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. The Implementors' Workshop is an ideal place to describe a Haskell extension, describe works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. Members of the wider Haskell community encouraged to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. Students working with Haskell are specially encouraged to share their work. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- We invite proposals from potential speakers for talks and demonstrations. We are aiming for 20-minute talks with 5 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 300 words. Submissions should be made via HotCRP. The website is: https://icfp-hiw17.hotcrp.com/ We will also have a lightning talks session which will be organised on the day. These talks will be 5-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Program Committee ----------------- * Richard A. Eisenberg -- chair (Bryn Mawr College) * Adam Gundry (Well-Typed) * Bartosz Nitka (Facebook) * Wren Romano (X, formerly Google[x]) * Alejandro Serrano Mena (Utrecht University) * Jan Stolarek (University of Edinburgh) Contact ------- * Richard A. Eisenberg <rae(a)cs.brynmawr.edu>

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speed of hashing
by Dennis Raddle 04 Jul '17

04 Jul '17

Hello, I have an application which could benefit from caching certain computations. For instance, I might have a function: listToWord :: [Int] -> Word16 which takes an integer X in the input as referring to bit X in the output word, and sets the bits. I have to run this computation many times within the inner loop. So I thought I could cache it. The only trouble is that it may not be any less expensive to convert the [Int] into a hash value, or use it as a key to look up a binary Map. What do you think? Does Haskell hashing use some kind of optimized computation that's faster than me writing a loop by hand? D

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Re: [Haskell-cafe] type (++) = (<>)
by Doug McIlroy 03 Jul '17

03 Jul '17

> What do you think of making (++) the same as (<>) Before Haskell 98, ++ was an overloadable operator associated with class MonadPlus. Why was that arrangement abolished? Doug

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Re: [Haskell-cafe] Master programme with Haskell prospects
by Olaf Klinke 02 Jul '17

02 Jul '17

Please follow Abhiroop's advice. The English midlands is a great region for functional minds. There is not only Nottingham: There is Leeds, Birmingham, Leicester and Swansea nearby, all of which have CS departments with very experienced scientists in that area. The midlands have the midlands graduate school [1]! I did my degree at the University of Birmingham, whose CS department is, as I dare say, run by a bunch of mathematicians. You can find any level of abstractness there, ranging from compiling ML to hardware up to topos theory. It was there I decided to learn Haskell. Recently they had a graduate whose entire dissertation [2] has an Agda version [3] so you can not only read it, but also execute. Olaf [1] http://www.cs.nott.ac.uk/MGS/ [2] http://etheses.bham.ac.uk/5967/ [3] http://www.cs.bham.ac.uk/~mhe/continuity-false/

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Re: [Haskell-cafe] Fast JSON validation - reducing allocations
by Ben Gamari 01 Jul '17

01 Jul '17

David Turner <dave.c.turner(a)gmail.com> writes: > On 11 May 2017 at 19:40, Ben Gamari <ben(a)smart-cactus.org> wrote: > >> Ccing Luke Maurer under the assumption that he will appreciate seeing >> the fruits of his labor. >> >> >> David Turner <dct25-561bs(a)mythic-beasts.com> writes: >> >> > Dear Café, >> > >> (snip) >> > >> > There's a copy of the relevant code for option 4 at >> > https://github.com/DaveCTurner/json-validator. I've hacked around with >> it a >> > bit since producing the numbers above, so it's now apparently a bit >> slower >> > than Aeson but allocates less too (~65kB). >> > >> > Could anyone help, e.g. by pointing me at the bit in the Core that is >> > allocating within the main loop? >> > >> While turning this over in my head I realized that this is the sort of >> program which may be helped significantly by GHC 8.2's improved join >> point handling. Indeed the timings seem to support this hypothesis: >> >> GHC 8.0.2 >> >> benchmarking json-validator/Automaton/testEvent >> time 22.84 μs (22.76 μs .. 22.94 μs) >> 1.000 R² (1.000 R² .. 1.000 R²) >> mean 22.84 μs (22.76 μs .. 22.94 μs) >> std dev 297.4 ns (221.8 ns .. 378.8 ns) >> >> Alloc rate 4,015,256,078,038 bytes per MUT second >> >> GHC 8.2 >> >> benchmarking json-validator/Automaton/testEvent >> time 9.221 μs (9.141 μs .. 9.318 μs) >> 0.998 R² (0.996 R² .. 1.000 R²) >> mean 9.163 μs (9.084 μs .. 9.356 μs) >> std dev 399.8 ns (193.0 ns .. 745.4 ns) >> variance introduced by outliers: 54% (severely inflated) >> >> Alloc rate 123,141,635 bytes per MUT second >> >> >> Wow! I suspect your allocations have now vanished. >> > > > Ooo, that's more like it. > > Could you run again using the following to get Criterion's estimate of the > allocations-per-call? > > json-validator-exe --regress allocated:iters +RTS -T >benchmarking json-validator/Automaton/testEvent Here it is, GHC 8.0.2 benchmarking json-validator/Automaton/testEvent time 34.81 μs (33.48 μs .. 36.10 μs) 0.991 R² (0.988 R² .. 0.994 R²) mean 34.04 μs (33.28 μs .. 34.99 μs) std dev 2.828 μs (2.423 μs .. 3.247 μs) allocated: 1.000 R² (1.000 R² .. 1.000 R²) iters 65924.651 (65895.364 .. 65951.510) y -4175.284 (-46736.295 .. 39814.235) variance introduced by outliers: 78% (severely inflated) GHC 8.2 benchmarking json-validator/Automaton/testEvent time 9.021 μs (8.923 μs .. 9.162 μs) 0.998 R² (0.994 R² .. 1.000 R²) mean 8.970 μs (8.905 μs .. 9.232 μs) std dev 341.4 ns (94.90 ns .. 711.7 ns) allocated: 0.973 R² (0.956 R² .. 0.984 R²) iters 343.033 (332.692 .. 354.606) y -7326.339 (-57269.638 .. 47222.231) variance introduced by outliers: 47% (moderately inflated) Cheers, - Ben

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ANN: hledger 1.3
by Simon Michael 01 Jul '17

01 Jul '17

I'm pleased to announce hledger 1.3, released yesterday. Thank you release contributors: Mykola Orliuk, Christian G. Warden, Dmitry Astapov, Justin Le, Joe Horsnell, Nicolas Wavrant, afarrow, Carel Fellinger, flip111, David Reaver, Felix Yan, Nissar Chababy, and Jan Zerebecki. Notable user-visible changes in this release: terminology/UI improvements for the status field, selection/scrolling/movement improvements in hledger-ui, negative amounts shown in red, bugfixes. Full release notes are at http://hledger.org/release-notes#hledger-1.3 The next major release is scheduled for September 30. hledger (http://hledger.org) is a dependable, precise, cross-platform program for tracking money, time, or any other commodity, using double-entry accounting and a simple plain text file format. It is a haskell reimplementation of Ledger, provides command-line, curses and web interfaces, and aims to be a robust, practical tool for daily use. To get started, install the stack tool, eg by downloading from http://haskell-lang.org/get-started. Then install hledger (and optionally the alternate UIs): $ stack install hledger-lib-1.3 hledger-1.3 [hledger-ui-1.3] [hledger-web-1.3] [hledger-api-1.3] (cabal experts can use that instead. See http://hledger.org/download for more install options.) Ensure $HOME/.local/bin is in your $PATH, and try some commands: $ hledger # list commands $ hledger -h # quick help $ hledger --man # man page for the hledger CLI tool $ hledger info journal # info manual for the journal format etc. Our IRC channel is #hledger on Freenode, and you can find out more at http://hledger.org. If you have benefitted from hledger, please give back and make it stronger! Donate using one of the funding links on the home page, give feedback, report bugs, send pull requests, write about it, or help in some other way. Thanks for your support; I hope you enjoy hledger and that it helps you achieve your goals. -Simon

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install haddock only with cabal?
by Evan Laforge 01 Jul '17

01 Jul '17

It's pretty common to wind up with a package installed, but not its haddock. Pretty much any time you install the traditional way via runhaskell Setup.hs you can forget to run haddock, since it doesn't use ~/.cabal/config. And even if you do, it doesn't update the haddock html index. But it seems like cabal install wants to install all targets in the cabal file, which means it wants to reinstall, which is no good for e.g. 'stm'. In fact I can't tell any way to address individual targets (e.g. if I just want to install the executable), and haddock doesn't seem to be a proper target at all, and I can't find any documentation about it. I can run 'haddock' and manually copy dist/doc/html to ~/.cabal/share/doc/$arch/$package/, but shouldn't cabal be able to do that? Also, it looks like regenerateHaddockIndex is hardcoded into the postInstallActions, and there's no way to run it independently, so it seems if you missed documentation on some package, you have to reinstall everything from scratch to fix it. Is there a way to install just haddock without going through all that? And shouldn't cabal-install/bootstrap.sh make the haddock index? Otherwise, you're guaranteed to be missing links for a bunch of fundamental packages. Or maybe it could default to using its (undocumented?) --sandbox flag to install just the binary, and not the various libraries (which are also likely to be missing desired flags, like --enable-profiling, since ~/.cabal/config is not available yet).

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Nice clean code example?
by Han Joosten 01 Jul '17

01 Jul '17

I'd like to showoff some nice Haskell code to a CTO with no experience with FP. I promised to send him a small piece of Haskell so he could get an idea. The code should be easy to read, without obscure looking operators (so sorry, no lens, arrows stuff like that), optionally even do something interesting. Who has the nicest example laying around? (and yes, I need it by yesterday ;-) ) Thanks!

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