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Threads and critical sections (threadWaitRead)
by Lars Petersen 30 May '15

30 May '15

Hello cafe, I'm currently trying to understand Haskell's threading model and GHC's implementation of it. Specifically I wondered about the safety of file IO. Consider this piece of code: > newtype Socket = Socket (MVar Fd) > > read :: Socket -> IO ByteString > read (Socket mfd) = do > fd <- readMVar mfd > -- (1) > threadWaitRead fd > -- (2) > withMVar mfd $ \fd'-> do > -- (3) the actual read... Context: - The socket is configured non-blocking. - No other operation will hold the MVar over a blocking call (because this would make it impossible to close it while it's blocking). - My close operation will fill the MVar with -1 after it's closed and will hold the MVar during the close operation. Questions: - Is it theoretically possible that my thread gets paused at (1) or within threadWaitRead, another thread closes the socket (or even worse opens another file with the same fd) _and_ my thread is resumed afterwards (I'm not asking about async exceptions)? - What constructs contain safepoints and which are guaranteed to be executed without interruption? Considerations (so far): - It is unlikely that (1) is a safepoint as no memory allocations occur, but is it guaranteed? - If the socket were closed during (1), threadWaitRead throws an exception complaining about a bad file descriptor which is unexpected but not harmful. - I found this thread (https://mail.haskell.org/pipermail/haskell-cafe/2014-September/115841.html) which is somewhat related, but I couldn't extract a clear answer from it. Best, Lars

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Disambiguating a Num/RealFrac instance
by amindfv＠gmail.com 30 May '15

30 May '15

Given: data GPA = F Float | Excuse String class ToGPA a where giveGPA :: a -> GPA Is there any way (without IncoherentInstances or Rebindablesyntax) that I can let the user write e.g. "giveGPA 4.0" (and "giveGPA 4") and get back "F 4" without getting type errors that "4.0"'s type is ambiguous? I can guarantee there won't be any additional instances to "ToGPA" (For my uses, I really (actually!) don't want to make the user write e.g. "F 4.0") I've gotten this working, once with IncoherentInstances and once with RebindableSyntax, but both of those flags feel pretty dirty. Thanks, Tom

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GHC + musl = easier static linking
by Marios Titas 29 May '15

29 May '15

TL;DR: I made some GHC binaries for linux that rely on musl instead of glibc for easier static linking. Scroll down for the link. Dear *, As most of you know, musl is a linux only C standard library that is optimized for static linking. Statically linked binaries can simplify distribution and deployment. If you have tried to statically link a program with glibc, you will probably know that this does not always work as expected: for example, in many cases the resulting binary will require to be executed in a system that has the exact same version of glibc as the one that was used to compiled it. In essence, static binaries linked with glibc are oftentimes *less* portable than dynamic ones. I decided to see if I could use GHC with musl. The problem was that while I could find some references online, I could not find a precompiled version of GHC that uses musl. So I decided to try and a build one myself. I was able to successfully bootstrap GHC under musl and I am posting it today in case someone else finds it usefull: https://drive.google.com/folderview?id=0B0zktggRQYRkbGJkbmU0UFFSYUE#list I posting this on google drive, I have no better place to host this, hopefully google will not disable my account :-) This is a fully bootstrapped (i.e. stage 2) GHC and *not* a cross compiler. So this will *not* work in a typical glibc based linux distribution. You need a complete musl based environment to use it (see bellow for that). Also, the binaries produced by this GHC will all depend on musl and not work on most glibc based distros. On the other hand, statically compiled binaries should be very portable and will not depend on any particular C standard library. I have done some minimal testing and it seems to work rather well; everything I tried to compile from hackage just worked. Additionally it can compile GHC itself which is always a good test. The size of the resulting static binaries is acceptable. In my 64 bit system, a simple hello world has the following sizes (stripped): 800K glibc dynanic, 1648K glibc static, and 1012K musl static. Why not use a cross-compiler? I have found that in practice it is easier to have a separete environment for producing static binaries. A cross-compiler requires that you also cross-compile all the C libraries that you use. Additionally, cabal has still problems (e.g. [1]) with cross-compiling and will not work out of the box. Note that musl is not 100% compatible with glibc, so some things might work slightly differently. If you do not like using a GHC precompiled by someone else, you can use my binaries to compile GHC under musl yourself; it would be easier than cross-compiling GHC from scratch. So how do you use this? You need a complete musl based environment. There a few musl based distros [2], albeit most of them are geared towards embedded applications. I personally use the experimental musl based gentoo image [3] which feels more like a regular distro. You can use it with chroot, lxc, or systemd-nspawn. For the sake of completeness, I have included at the end of this message some quick and dirty instructions on how to do this. [1] https://github.com/haskell/cabal/issues/1493 [2] http://wiki.musl-libc.org/wiki/Projects_using_musl [3] http://distfiles.gentoo.org/experimental/amd64/musl/ 8<------------------------------------- # gentoo stage3 image instructions CHROOT_PATH=/var/tmp/gentoo-musl cd /var/tmp wget http://distfiles.gentoo.org/experimental/amd64/musl/stage3-amd64-musl-vanil… mkdir "$CHROOT_PATH" tar xjf stage3-amd64-musl-vanilla-20150405.tar.bz2 -C "$CHROOT_PATH" for x in dev proc sys; do mount --bind /$x "$CHROOT_PATH/$x"; done cp -L /etc/resolv.conf "$CHROOT_PATH/etc" chroot "$CHROOT_PATH" /bin/bash -l PS1="(chroot) $PS1" # fetch the gentoo package db for the first time (slow): emerge-webrsync # enable static libraries (*.a), which are disabled by default: sed -ie 's/USE="/USE="static-libs /' /etc/portage/make.conf # reinstall/recompile gmp so that we have its static libs: emerge --oneshot dev-libs/gmp # install ghc: tar xJf /path/to/ghc-7.10.1-x86_64-unknown-linux-musl.tar.xz -C /tmp cd /tmp/ghc-7.10.1; ./configure --prefix=/opt/ghc-7.10.1; make install # compile a hello world: cd /tmp echo 'main = putStrLn "Hello world"' > test.hs /opt/ghc-7.10.1/bin/ghc --make -O -optl-static test.hs ./test file ./test # Of course, using chroot this way is insecure, don't use to install packages from hackage.

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Annoucing Eddie 1.0.0
by Mike Meyer 29 May '15

29 May '15

Eddie is a tool to let you use Haskell code in shell scripts, in a manner similar to the way awk and other scripting languages can be used. The jump to 1.0.0 isn't because it's finished, but because this is a rewrite from the ground up to incorporate more modern Haskell features (the old version was my first non-trivial Haskell program), resulting in changes to various behaviors as well as some option tweaks. Most notably, it no longer uses [Char] for everything. In text mode, it uses Text, with options to specify both input and output encodings. In binary mode, it uses ByteString, with an option to set the output back to Text so you can generate summaries as opposed to simply filter. To make using Text and ByteString sane, it also uses ClassyPrelude instead of Prelude + Data.Char & Data.List so your Haskell list habits work - at least mostly. See CHANGES.md in the source or ChangeLog in the wiki for detailed changes. Otherwise, the manual page is available on the web site.

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ANNOUNCE: Haskell Communities and Activities Report (28th ed., May 2015)
by Mihai Maruseac 29 May '15

29 May '15

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (28th edition, May 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2015/report.pdf http://haskell.org/communities/05-2015/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of September 2015: target deadline for contributions to the November 2015 edition of the HC&A Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac and Alejandro Serrano Mena <hcar at haskell.org> -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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Status of Acid-State and IxSet
by JP Moresmau 29 May '15

29 May '15

Hello, for a project I decided to use Acid-State and IxSet, and was pretty happy with them. I enjoyed working with Haskell structures for storing and querying data, and my data could fit in memory. However, now I've moved to GHC 7.10, and for example syb-with-class, a dependency of ixset, does not build with template-haskell 2.10. So I'm wondering, are these projects still alive and kicking? Are if I want some memory based storage and indexing, should I look at some other projects (maybe VCache http://hackage.haskell.org/package/vcache) Or is it just bad timing and GHC 7.10 compatible versions of these libraries are in the works? Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

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recursive datakinds
by Nicholls, Mark 29 May '15

29 May '15

Hello, If I were trying to do something like creating types like.... > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE ScopedTypeVariables #-} > data MyList a where > MyEnd :: MyList '[] > MyCons :: a -> MyList b -> MyList (a ': b) > mysimplelist :: MyList '[Integer] > mysimplelist = MyCons 1 (MyEnd) what about > myrecursivelist = MyCons 1 myrecursivelist Cafe2.lhs:23:30: Occurs check: cannot construct the infinite type: b ~ a : b Expected type: MyList b Actual type: MyList (a : b) Relevant bindings include myrecursivelist :: MyList (a : b) (bound at Cafe2.lhs:23:3) In the second argument of 'MyCons', namely 'myrecursivelist' In the expression: MyCons 1 myrecursivelist can I "fix" this somehow? I obviously want the type of myrecursivelist to be MyList '[Integer, Integer, Integer, Integer, Integer, Integer, Integer, Integer,.....] Something like a completely nonsensical... MyList (Fix (Integer ': )) P.S. Yes, I barely understand "fix" as a function, let alone a data type. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Haskell Weekly News
by Kim-Ee Yeoh 29 May '15

29 May '15

*Top Picks:* - Tony Day, Brisbane-based investment strategist and high-frequency-trading hacker, rides 100% idiomatic Haskell <http://tonyday567.github.io/blog/mvc-todo/> into the Single Page (web)-App <http://en.wikipedia.org/wiki/Single-page_application> space. How? He spins the GHCJS transpiler on Gabriel Gonzalez's Model-View-Controller library to obtain his own TodoMVC benchmark demo <http://tonyday567.github.io/static/index-auto.html>. Per GHCJS, the production spans multi-megabytes of javascript. Not to be missed: auto-run, i.e. click the QuickCheck-powered checkbox labeled "Let haskell do the work." Github repo <https://github.com/tonyday567/mvc-todo>. Along the way, he discovers how crippled Javascript is without sum types. As noted on HN <https://news.ycombinator.com/item?id=9591868#up_9601842>, "Typos and missing cases represent a very large set of trivial bugs." He believes superior FP features such as sum types makes it "much harder for haskell to avoid success." Woe is us. /r/haskell <http://www.reddit.com/r/haskell/comments/36y9jc/haskell_as_an_mvc_framework/> - Michael Walker, a Ph.D. student at York, reveals <http://www.reddit.com/r/haskell/comments/37dv8y/what_side_projects_have_you…> his personal book-collection management web-app <http://www.barrucadu.co.uk/bookdb/> that runs on top of persistent <https://hackage.haskell.org/package/persistent>, WAI <https://hackage.haskell.org/package/wai>, and web-routes <https://hackage.haskell.org/package/web-routes>. Public domain. <https://github.com/barrucadu/bookdb> - David Christiansen announces on /r/haskell <http://www.reddit.com/r/haskell/comments/36az3n/idris_0918_released_with_fa…> Idris 0.9.18 <http://www.idris-lang.org/idris-0-9-18-released/> with fancier records. Top comment says Idris is more type-friendly than Haskell despite the "esoteric academics behind dependent type theory." Why? Because Idris, helpfully offers suggestions that turn ill- into well-typed code. Haskell doesn't. - Remember JP Moresmau dropping EclipseFP <http://haskell.1045720.n5.nabble.com/Haskell-Weekly-News-tp5809360.html>? Can Leksah take its place? Hamish Mackenzie announces a 7.10-ready Leksah 0.15.0 <https://github.com/leksah/leksah/wiki/Leksah-0.15.0> on /r/haskell <http://www.reddit.com/r/haskell/comments/37emp2/leksah_0150/>. Top new feature? Support for GHCJS, which excites Phil Freeman of PureScript fame. Here's a 2min video clip on How to make a ghcjs-dom application in Leksah <https://www.youtube.com/watch?v=zQnExdDL63c>. - Roman Cheplyaka writes a short 'n sweet tutorial <https://ro-che.info/articles/2015-05-28-force-list> on how a list may be variously forced. He summarizes the similarities and differences in a table that goes from the shallowestly evaluated seq () to forceSpine to forceElements to the deepest rnf. /r/haskell <http://www.reddit.com/r/haskell/comments/37ky62/how_to_force_a_list/> - Justin Leitgeb, Rails developer and Co-Founder / CTO of Stack Builders, a Haskell-enabled software consultancy, deprioritizes learning Clojure, Go, Erlang, and Scala in favor of Agda, Coq, Idris, Elm, and Liquid Haskell <http://www.stackbuilders.com/news/another-personal-programming-language-roa…>. He will invest a couple of weeks at the Oregon PL Summer School starting June 15 <https://www.cs.uoregon.edu/research/summerschool/summer15/>. - Tony Morris announces a three-day Haskell-based Intro to FP course in Melbourne, July 21-23 this year. Pitched at beginners, it features learning by coding. Free; application deadline: July 10 <https://groups.google.com/forum/#!topic/nicta-fp/Dnq-oh4nJbg>. - By popular request <http://haskell.1045720.n5.nabble.com/Workshop-on-Type-Inference-and-Automat…>, František Farka and his team recorded all of a workshop on type inference in Dundee, Scotland <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/> held a couple of weeks ago. Highly-viewed talks <https://www.youtube.com/user/qmbevents/videos> include Edwin Brady <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/#edwin> on implementing dependent types in Idris and Conor McBride <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/#conor> on "Type Inference Needs Revolution." /r/haskell <http://www.reddit.com/r/haskell/comments/36dfcx/workshop_on_type_inference_…> - Michael Hicks at UMD, PC chair of POPL 2012, writes a PL research apologia <http://www.pl-enthusiast.net/2015/05/27/what-is-pl-research-and-how-is-it-u…> cum pitch for new grad students. An informal poll he did shows PL Ph.D.s get good jobs. He explains that "The ethos of PL research is to not just find solutions to important problems, but to find the *best expression of those solutions.*" As ethos specimens, he gives three: probabilistic programming, incremental computation a.k.a. self-adaptive computation, and authenticated data structures (see LambdaAuth <http://amiller.github.io/lambda-auth/>). HN-worthy <https://news.ycombinator.com/item?id=9614178>. - Jan Stolarek gives a glimpse <http://lambda.jstolarek.com/2015/05/injective-type-families-for-haskell/> of GHC's new injective type families feature, which is joint research with SPJ and Richard Eisenberg. It brings Haskell one step closer to having a notion of type functions as opposed to mere 'constructors.' The feature is useful in type-level hacking. It allows the arguments of a type family to be inferred solely by result type. But Lennart Augustsson on /r/haskell <http://www.reddit.com/r/haskell/comments/37bvh0/injective_type_families_for…> gives a trivial example where even when a type family isn't injective, its argument can still be inferred knowing a particular result type. He regrets that GHC doesn't do this. - Joe Nelson uploads the video and a summary <http://begriffs.com/posts/2015-05-24-safe-haskell.html> of Kristen Kozak introducing the Safe Haskell extension to the SF Bay Area Haskell Users Group <http://www.meetup.com/Bay-Area-Haskell-Users-Group/events/220312001/> on May 12. Slides here <http://wordroute.com/grayjay/safe-haskell.pdf>. Redditor beerdude26 <http://www.reddit.com/r/haskell/comments/37a1cq/using_the_safe_haskell_lang…> adds the link to and tidies up the Safe Haskell wiki <https://wiki.haskell.org/Safe_Haskell> entry. - Out of frustration with existing Windows GUI FFIs, Luka Horvat <http://lukahorvat.github.io/programming/2015/05/24/haskellforms/> creates bindings for WinForms <https://github.com/LukaHorvat/HaskellForms>, Microsoft's .NET menus-and-widgets library. He binds to F# instead of lower-level C++/C# because it's easier. /r/haskell <http://www.reddit.com/r/haskell/comments/37464x/haskellforms_winforms_bindi…> - Gabriel Gonzalez <http://www.haskellforall.com/2015/05/the-internet-of-code.html> demoes another piece of Morte, what he describes as his "pandoc for programming languages." He prototypes "distributing typed code over the internet where the unit of compilation is individual expressions." /r/haskell <http://www.reddit.com/r/haskell/comments/36d12v/haskell_for_all_the_interne…> - Taylor Fausak forsakes TagSoup <https://hackage.haskell.org/package/tagsoup> and scrapes websites <http://taylor.fausak.me/2015/05/21/scraping-websites-with-haskell/> the hard way using xml-conduit. He concludes that it's "tougher than doing the same thing in scripting languages, but hopefully easier than [his blog readers] expected." - Stuart Popejoy writes a monad tutorial <http://slpopejoy.github.io/posts/Effectful01.html>. A redditor finds it an "entertaining explanation of monads." Another asks the OOP-ish question: are monads "basically wrappers that contain 'impure' data and actions with some common functions?" See /r/haskell <http://www.reddit.com/r/haskell/comments/374c3o/effectful_haskell_io_monads…> *Announcement:* Semen Trygubenko has stepped down from publishing his edition of HWN. Losing him means there will be no issue next week. *Quotes of the Week:* - UnoOuzo <https://twitter.com/UnoOuzo/status/601658244201848832>: "The type of my love is parametrically polymorphic. It is unbounded." Looking into ways to cite this in my thesis. - Conor McBride <http://www.reddit.com/r/haskell/comments/36y9jc/haskell_as_an_mvc_framework…>: Algebra is a posh way of saying "construction kit". - Michael Hicks <http://www.pl-enthusiast.net/2015/05/27/what-is-pl-research-and-how-is-it-u…>: The ethos of PL research is to not just find solutions to important problems, but to find the *best expression of those solutions. * - There is a fountain of youth: it is your mind, your talents, the creativity you bring to your life and the lives of people you love. When you learn to tap this source, you will truly have defeated age. -- Sophia Loren -- Kim-Ee

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-fdefer-more-errors
by Tom Ellis 29 May '15

29 May '15

Deferring type errors until runtime is a feature which probably helps a lot with the adoption of Haskell. Newcomers can get the encouragement of a mostly-working program even if there are type errors in a few places. I can write foo = 1 + 1 bar = head True main = print foo and still run the program or load it in GHCi. Does anyone think it would also be beneficial to have "-fdefer-name-errors"? If I write foo = 1 + 1 bar = baz main = print foo but baz doesn't exist it would still be nice to let the program run or be loaded in GHCi. This could be achieved by replacing all missing variables by "error 'baz was not defined'" or similar. If a suitable parser could be written it might even be possible to defer syntax errors! This kind of thing would make the experience with Haskell gentler for newcomers, but also more pleasant for veterans! Tom

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Re: [Haskell-cafe] Any recommendations on how to interact with the running state of a program?
by Anatoly Yakovenko 29 May '15

29 May '15

How would the program get updated values? via an IORef? This thing is running until the user decides the output looks good Anatoly On Thu, May 28, 2015 at 12:16 PM Carlos López-Camey <c.lopez(a)kmels.net> wrote: > Cloud Haskell + Event streaming via some Protocol (like json on > websockets) ? > > > > 2015-05-25 15:14 GMT-06:00 Anatoly Yakovenko <aeyakovenko(a)gmail.com>: > >> I am playing around with some deep learning algorithms and I need a way >> to periodically read the intermediate result and change some of the system >> variables. >> >> Any ideas on how to best approach this problem? I was thinking of >> something basic, like having the program periodically read and write from >> some files. >> >> Thanks, >> Anatoly >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> >> >

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