March 2014 - Haskell-Cafe

Type checking string literal in Haskell?
by KwangYul Seo 03 Mar '14

03 Mar '14

In Java, the Checker Framework ( http://types.cs.washington.edu/checker-framework/) provides a way to type check string literals. For example, Java signatures type system differentiates strings literals in different forms: 1. Unqualified strings : "Hello, world!" -> @Unqualified String 2. Fully qualified names: "package.Outer.Inner" -> @FullyQualifiedString String 3. Binary names: "package.Outer$Inner" -> @BinaryName String 4. Field descriptors: "Lpackage/Outer$Inner;" -> @FieldDescriptor String It can do the similar checks with regular expressions or SQL statements. Is it possible to type check string literals in Haskell? I think it would be nice if we can check if a given string literal is a valid URL or an email address at compile time. Regards, Kwang Yul Seo

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 03 Mar '14

03 Mar '14

============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 30th April 2014 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year, but there is currently no common location in which to promote and advertise the resulting work. The Journal of Functional Programming would like to change that! As a service to the community, JFP is launching a new feature, in the form of a yearly publication of abstracts from PhD dissertations that were completed during the previous year. To start this new feature off, we would like to reach back three years for the first round of abstracts. The abstracts will be freely available on the JFP website, i.e. not behind any paywall, and will not require any transfer for copyright, merely a license from the author. Please submit dissertation abstracts according to the instructions below. A dissertation is eligible if parts of it have or could appear in JFP, that is, if it is in the general area of functional programming. JFP will not have these abstracts reviewed. We welcome submissions from both the PhD student and PhD advisor/supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 30th April 2014. o Student: (full name) o Advisor/supervisor: (full names) o Dissertation title: (including any subtitle) o Dissertation abstract: (plain text, maximum 1000 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting in the abstract, but do get in touch if this causes significant problems) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Awarding institution: (full name and location) o Date of PhD award: (depending on the institution, this may be the date of the viva, date when corrections were approved, date of graduation ceremony, or otherwise) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kindgdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: [Haskell-cafe] [TYPES] OutsideIn(X) question
by Dimitrios Vytiniotis 03 Mar '14

03 Mar '14

Dear Alejandro, thanks for your notice! You are right, the entailment relation in the JFP paper is too weak for type classes. As we formulated it, the simplifier can take a type class reduction step but there is no way to show that the new set of constraints is equivalent to the original class constraint. There is nothing very deep going on, it's an oversight in the definition of entailment. The fix -- at least in the case of non-overlapping instances and vanilla multi-parameter type classes -- is to ensure that all axioms for type class instances form bi-implications. In fact in the constraint handling rules formulation of type classes, class instances always gave rise to such bi-implications. I will post the correction to the article on my web page in the next couple of weeks and let you know, but I thought I should send an update. Best regards, Dimitrios -----Original Message----- From: Types-list [mailto:types-list-bounces@lists.seas.upenn.edu] On Behalf Of Alejandro Serrano Mena Sent: Thursday, February 27, 2014 10:18 AM To: types-list(a)lists.seas.upenn.edu Subject: [TYPES] OutsideIn(X) question [ The Types Forum, http://lists.seas.upenn.edu/mailman/listinfo/types-list ] Dear Types List, I have a small question related to the "OutsideIn(X): Modular type inference with local assumptions" and was hoping that you could help me a bit. My question related about the proof of soundness and principality, specifically Lemma 7.2 (to be found in page 67). In that lemma, it's stated that QQ and \phi' Q_q ||- \phi Q_w <-> \phi' Q_w'. I'm trying to recover the proof (which is omitted in the text), but I stumble upon a wall when trying to work out what happens in the case an axiom is applied. In particular, I'm playing with an example where QQ (the set of axioms) = { forall. C a => D a } (where C and D are one-parameter type classes) Q_q = { } Q_w = { D Int } Thus, if I apply the rule DINSTW (to be found in page 65), I get a new Q_w' = { C Int } Now, if the lemma 7.2 is true, it should be the case that (1) QQ ||- C Int <-> D Int which in particular means that I have the two implications (2) { forall. C a => D a, C Int } ||- D Int (3) { forall. C a => D a, D Int } ||- C Int (2) follows easily by applying the AXIOM rule of ||- (as shown in page 54). However, I don't see how to make (3) work :( I think that understanding this example will be key for my understanding of the whole system. Anybody could point to the error in my reasoning or to places where I could find more information? Thanks in advance.

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I'm trying to design a GUI library -- a design question
by Ömer Sinan Ağacan 03 Mar '14

03 Mar '14

Hi all, I'm designing a ncurses based GUI library. I'm calling smallest GUI elements "widgets". A widget can have internal state of any type. Let's call this type `s`. I want to be able to have something like this: > data Widget s = Widget > { handleKey :: Key -> s -> s > , draw :: Int -> Int -> Int -> Int -> IO () > , getFocus :: s -> s > , loseFocus :: s -> s > -- more methods may be added in the future > } I also want to be able to have container types like this: > data Container = Container [Widget s] but obviously this does not work because widgets may have different types of states. So problem I'm trying to solve here is that I should somehow abstract internal state used in a Widget but still be able to keep track of that state so that I can pass it to methods in sequential invocations of that methods. Also, once I have a container like this, updating widget states would become a problem. I'd have to somehow keep all those states like: > data Container = Container [Widget] [WidgetState] or > data Container = Container [(Widget, WidgetState)] and then manually pass state to widgets when calling methods, and update the list using return values of methods. In a sense I'm like trying to emulate something like closures with mutable closed-over data in impure languages. I think one way to have a similar effect is to use closures with closed-over IORefs. Then I could modify that state but then I'd need to have methods with types `IO ()`. I want to have more "precise" types. i.e. IO is a lot more general than what I'd like to have as my widget methods. (side effect of a widget should be limited with changes in it's internal state) Sorry for badly organized post, I'm a bit tired right now and I hope my points are clear. I'm trying to figure out Haskell way of doing something like this, without going into IO world. Ideas/suggestions would be appreciated. Thanks, --- Ömer Sinan Ağacan http://osa1.net

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New Functional Programming Job Opportunities
by Functional Jobs 03 Mar '14

03 Mar '14

Here are some functional programming job opportunities that were posted recently: NixOS/Haskell DevOps Engineer at Zalora http://functionaljobs.com/jobs/8687-nixos-haskell-devops-engineer-at-zalora functional software developer at OpinionLab http://functionaljobs.com/jobs/8685-functional-software-developer-at-opinio… Platform Engineer at Signal Vine LLC http://functionaljobs.com/jobs/8684-platform-engineer-at-signal-vine-llc Cheers, Sean Murphy FunctionalJobs.com

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ANN: tasty-0.8
by Roman Cheplyaka 03 Mar '14

03 Mar '14

I'm glad to announce the 0.8 release of tasty, a modern Haskell testing framework. http://hackage.haskell.org/package/tasty-0.8 Among the important user-visible changes are: - New running modes `--hide-successes` and `--quiet` - Short flags for some existing options (`-p` for `--pattern`, `-j` for `--num-threads`) - Timeout support - Possibility to pass options via environment variables - Fix of a resources-related bug For details, see the CHANGELOG and README: https://github.com/feuerbach/tasty/blob/master/CHANGES.md http://documentup.com/feuerbach/tasty tasty now has a mailing list[ml] and an IRC channel #tasty at FreeNode. The IRC channel is logged at ircbrowse.net[logs] (thanks to Chris Done). [ml]: https://groups.google.com/forum/#!forum/haskell-tasty [logs]: http://ircbrowse.net/tasty Read more: http://ro-che.info/articles/2014-03-03-tasty-0.8.html Roman

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ANN: poppler-0.12.3 is released.
by Ian-Woo Kim 02 Mar '14

02 Mar '14

Hi, Haskellers! A new version of haskell poppler library is now released. Poppler is a pdf rendering library, based on xpdf code base. It's being developed and has quite complete features. Haskell poppler library is a binding to the native poppler library. Recently, gtk2hs-0.12.5 was released and we now have support to gtk3. We also have a great advance of Cabal-1.18, but the previous version of poppler was not buildable with the new Cabal. So this version (0.12.3) of poppler is to support gtk2hs-0.12.5 and Cabal-1.18. By default, poppler is compiled against gtk version 2, but with -fgtk3 option, one can build poppler with gtk3. The new version of poppler is available on hackage. Enjoy! Ian-Woo Kim

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Scaling Haskell/Yesod on OpenShift
by Joey Eremondi 02 Mar '14

02 Mar '14

I was wondering, does anyone have experience with using Haskell/Yesod on the OpenShift PaaS? I've seen the Haskell and Yesod cartridges, but I'm wondering if there's any way to get Haskell playing nice with their load-balancing and automatic scaling capabilities? Would CloudHaskell be useful at all in this? I'm reasonably experienced with Haskell but very new to Cloud computing and Web stuff, so any advice, even if basic, is helpful. Thanks!

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ICFP 2014 deadline discrepancy: UTC -11 /= anywhere in the world
by Philip Dexter 02 Mar '14

02 Mar '14

ICFP 2014's website [1] (and all notices) list the deadline as Saturday, 1 March 2014, 23:59 UTC-11 (anywhere in the world) Anywhere in the world time, also known as Anywhere on Earth (AoE), is UTC-12, not UTC-11. Is there anybody that can confirm that this is intentional? EasyChair does not list a deadline. -- [1] http://www.icfpconference.org/icfp2014/index.html

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questions about ghc-clang-wrapper
by Richard Cobbe 02 Mar '14

02 Mar '14

I've just run the ghc-clang-wrapper script (from https://gist.github.com/tibbe/8448715) to try to get GHC 7.6.3 (part of Haskell Platform 2013-2.0.0) working with XCode 5.0.2 on a MacOS 10.8.5 machine. Things look good, but I have two questions: Question 1: How do I know this worked? I tested it by modifying a package that I'm writing so that it requires CPP to build correctly, and successfully built it with "cabal clean; cabal configure; cabal build". (More details available on request.) Does that mean I'm in good shape? (I'm asking because I was never entirely clear on what the incompatibility was; all I know is that it has something to do with cpp.) Question 2: Is there any harm in adding -Wno-return-type to extraClangArgs in ghc-clang-wrapper? This silences an annoying but apparently harmless warning I get from "cabal configure". Thanks, Richard

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