June 2017 - Haskell-Cafe

[ANN] Budapest Haskell Hackathon 2017, 29-30th July 2017
by Daniel Berecz 11 Jun '17

11 Jun '17

Hi everybody, I'm happy to announce that 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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Last call for presentations: CUFP 2017, September 7-9, Oxford, UK
by Jasper Van der Jeugt 10 Jun '17

10 Jun '17

The deadline for CUFP presentations is this friday, the 9th of June. This CFP and the form for submitting presentations proposals can be found at: http://cufp.org/2017/call-for-presentations.html ------------------------------------------------------------------------ 2017 Call for Presentations Workshop for Commercial Users of Functional Programming 2017 Sponsored by SIGPLAN CUFP 2017 Co-located with ICFP 2017 Oxford, UK September 7-9 Talk Proposal Submission Deadline: 9 June 2017 The annual CUFP event is a place where people can see how others are using functional programming to solve real world problems; where practitioners meet and collaborate; where language designers and users can share ideas about the future of their favorite language; and where one can learn practical techniques and approaches for putting functional programming to work. ------------------------------------------------------------------------ Giving a CUFP Talk If you have experience using functional languages in a practical setting, we invite you to submit a proposal to give a talk at the event. We're looking for two kinds of talks: Retrospective reports are typically 25 minutes long. Now that CUFP has run for more than a decade, we intend to invite past speakers to share what they’ve learned after a decade spent as commercial users of functional programming. We will favour experience reports that include technical content. Technical talks are also 25 minutes long, and should focus on teaching the audience something about a particular technique or methodology, from the point of view of someone who has seen it play out in practice. These talks could cover anything from techniques for building functional concurrent applications, to managing dynamic reconfigurations, to design recipes for using types effectively in large-scale applications. While these talks will often be based on a particular language, they should be accessible to a broad range of programmers. We strongly encourage submissions from people in communities that are underrepresented in functional programming, including but not limited to women; people of color; people in gender, sexual and romantic minorities; people with disabilities; people residing in Asia, Africa, or Latin America; and people who have never presented at a conference before. We recognize that inclusion is an important part of our ission to promote functional programming. So that CUFP can be a safe environment in which participants openly exchange ideas, we abide by the SIGPLAN Conference Anti-Harassment Policy: http://www.sigplan.org/Resources/Policies/Anti-harassment If you are interested in offering a talk, or nominating someone to do so, please submit your presentation before 09 June 2017 via the CUFP 2017 Presentation Submission Form: https://goo.gl/forms/KPloANxHHwdiaoVj2 You do not need to submit a paper, just a short proposal for your talk. There will be a short scribe's report of the presentations and discussions but not of the details of individual talks, as the meeting is intended to be more of a discussion forum than a technical interchange. Nevertheless, presentations will be recorded and presenters will be expected to sign an ACM copyright release form. Note that we will need presenters to register for the CUFP workshop and travel to Oxford at their own expense. There are some funds available to would-be presenters who require assistance in this respect. ------------------------------------------------------------------------ Program Committee Alex Lang (Tsuru Capital), co-chair Rachel Reese (Mulberry Labs), co-chair Garrett Smith (Guild AI) Danielle Sucher (Jane Street) Jasper Van der Jeugt (Fugue) Yukitoshi Suzuki (Ziosoft) Evelina Gabasova (University of Cambridge) Brian Mitchell (Jet.com) ------------------------------------------------------------------------ More information For more information on CUFP, including videos of presentations from previous years, take a look at the CUFP website at http://cufp.org. Note that presenters, like other attendees, will need to register for the event. Acceptance and rejection letters will be sent out by July 15th. Guidance on giving a great CUFP talk Focus on the interesting bits: Think about what will distinguish your talk, and what will engage the audience, and focus there. There are a number of places to look for those interesting bits. Setting: FP is pretty well-established in some areas, including formal verification, financial processing, and server-side web services. An unusual setting can be a source of interest. If you're deploying FP-based mobile UIs or building servers on oil rigs, then the challenges of that scenario are worth focusing on. Did FP help or hinder in adapting to the setting? Technology: The CUFP audience is hungry to learn about how FP techniques work in practice. What design patterns have you applied, and to what areas? Did you use functional reactive programming for user interfaces, or DSLs for playing chess, or fault-tolerant actors for large-scale geological data processing? Teach us something about the techniques you used, and why we should consider using them ourselves. Getting things done: How did you deal with large-scale software development in the absence of pre-existing support tools that are often expected in larger commercial environments (IDEs, coverage tools, debuggers, profilers) and without larger, proven bodies of libraries? Did you hit any brick walls that required support from the community? Don't just be a cheerleader: It's easy to write a rah-rah talk about how well FP worked for you, but CUFP is more interesting when the talks also cover what doesn't work. Even when the results were all great, you should spend more time on the challenges along the way than on the parts that went smoothly.

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rts nursery and stopping the world minor gc
by Ruben Astudillo 09 Jun '17

09 Jun '17

I've been reading about garbage collection. I understand that is stop-the-world, parallel and generational. Per capability also exists "the nursery", which is a block of memory where new objects are allocated. When the nursery of a capability gets filled, do we have to stop-the-world just for that local nursery gc?. To me this is different than gen0 or gen1 gc because isn't really a shared resource. -- Ruben Astudillo.

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Haskell in Leipzig 2017: 1st call for papers
by Wolfgang Jeltsch 08 Jun '17

08 Jun '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. Program Committee ================= * Edward Amsden, Plow Technologies, USA * Heinrich Apfelmus, Germany * Jurriaan Hage, Utrecht University, The Netherlands * Petra Hofstedt, BTU Cottbus-Senftenberg, Germany * Wolfgang Jeltsch, Tallinn University of Technology, Estonia (chair) * Andres Löh, Well-Typed LLP, Germany * Keiko Nakata, SAP SE, Germany * Henrik Nilsson, University of Nottingham, UK * Ertuğrul Söylemez, Intelego GmbH, Germany * Henning Thielemann, Germany * Niki Vazou, University of Maryland, USA * Johannes Waldmann, HTWK Leipzig, Germany Questions ========= If you have any questions, please do not hesitate to contact Wolfgang Jeltsch at wolfgang-it(a)jeltsch.info.

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Arithmetic expressions with random operators
by Mahmoud Murad 08 Jun '17

08 Jun '17

I have the following problem, I want to randomly generate arithmetic expressions based on a number, for example: if I have n = 3, then the expression must have 3 operators like this (4)*(((3+5)-2)). Any advice or hint? I am relatively new to Haskell. Thanks.

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Issue 58 :: Haskell Weekly
by Taylor Fausak 08 Jun '17

08 Jun '17

\ Haskell Weekly \/\/ Issue 58 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. - A Haskell cross compiler for iOS <https://medium.com/@zw3rk/a-haskell-cross-compiler-for-ios-7cc009abe208> So far we have built a Haskell cross compiler for Raspberry Pi, as well as a Haskell cross compiler for Android. To round this off, we will build a cross compiler for iOS as well. With the WWDC signaling the end of 32-bit devices, we will only build the 64-bit cross compiler. - Event sourced aggregates in Haskell <http://blog.akii.de/posts/2017-06-04-eventsourcing-in-haskell.html> In this first post I'll be digging into event sourced aggregates in Haskell. It basically means that instead of persisting some state like "Johns account balance is $100", you keep track of the changes to John's account: "First he deposited $125, then withdrew $25". - The round-trip property <http://teh.id.au/posts/2017/06/07/round-trip-property/index.html> We know that parsers and printers are supposed to be dual. We can simply treat this as a law, and write a property test to enforce it. This is both the simplest useful property test I can think of for a working engineer and the most likely to reliably identify bugs of consequence. - How well do you know your programming tools? Take the survey and test your skills! (ad) <http://vmob.me/DE3Q17Haskellweek> "It is a good survey which, in addition to remembering what I already knew, gave me new tools to learn about and work with." This is how developers feel about the Developer Economics survey. The survey also shows you how you compare to other developers in your country. Plus you may win an iPhone 7 or a Pixel 32GB phone. - Assume it worked and fix it later <https://hackernoon.com/assume-it-worked-and-fix-later-8436d18b7ed3> If your email service is down, it can be beneficial to have the signup succeed regardless. By decoupling the success of an email request from the success of account signup, you can improve the reliability of your application. - Flexible data with Aeson <https://mmhaskell.com/blog/2017/6/5/flexible-data-with-aeson> At a certain point, our Haskell programs have to be compatible with other programs running on the web. This is especially useful given the growing usage of micro-services as an architecture. Regardless, it's very common to be transferring data between applications on different stacks. - Tagless final encoding of a test language <https://wickstrom.tech/programming/2017/06/05/tagless-final-encoding-of-a-t…> I have experimented with a test language encoded in tagless final style, instead of algebraic data types, to support the typed combinators `beforeEach` and `beforeAll`. I want to share the Haskell prototype I ended up with, and explain how I got there. - Can't see the four-est for the trees <http://www.gigamonkeys.com/trees/> It's a fun puzzle so you may want to take a crack at it yourself before reading on. But this isn't really about that puzzle. This is about how sometimes when the mathematical insights aren't flowing it's good to be a programmer. - The Typeclassopedia is now up to date <https://byorgey.wordpress.com/2017/06/07/the-typeclassopedia-is-now-up-to-d…> The title pretty much says it all: I have finally finished (I hope) updating the Typeclassopedia to reflect various recent changes to the language and standard libraries (such as the AMP and BBP/FTP). Along the way I also added more links to related reading as well as more exercises. - Continuations from the ground up <http://blog.ielliott.io/continuations-from-the-ground-up/> It's difficult to learn functional programming without hearing about continuations. Often they're mentioned while talking about boosting the performance of pure functional code, sometimes there's talk of control flow, and occasionally with "time-travel" thrown in there to make it all seem more obscure. - How to send me a pull request <https://www.snoyman.com/blog/2017/06/how-to-send-me-a-pull-request> I find myself repeating a lot of the same comments in pull requests, so I decided to put together a list of what I consider the most important features of a good pull request. Other people will have different feelings on some of these, but the points below are what apply to my projects. Package of the week ------------------- This week's package of the week is Yesod, a web framework for productive development of type-safe, RESTful, high performance web applications. <https://www.stackage.org/package/yesod>

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Re: [Haskell-cafe] [ghc-proposals/cafe] Partially applied type families
by Anthony Clayden 08 Jun '17

08 Jun '17

> On Tue Jun 6 19:22:45 UTC 2017, Richard Eisenberg wrote: > I still don't see why TypeError is problematic once we > have instance guards. Ah, "once we have instance guards", TypeError is never needed. With instance guards, you might sometimes want to put a TypeError instance, to give a more helpful message. (HList nowadays exploits TypeError that way.) OTOH, sometimes currently you want to say: "I don't want to provide an instance for that pattern here, but by all means somebody else can elsewhere." Example code to illustrate the above: > module A > class C a b ... > instance C Int b ... > > module B > instance C a Bool ... Those instances overlap irresolvably. The only way you can catch (not really improve) is: > instance (TypeError <help message>) => C Int Bool ... But a) which module do you put that in? b) what if somebody wants a `instance C Int Bool`? with guards > module A > class {-# INSTANCEGUARDS #-} C a b ... > instance C Int b | b /~ Bool ... > > module B > instance C a Bool | a /~ Int ... > > module C -- if 'somebody else' wants > instance C Int Bool ... AntC

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ANNOUNCE: streaming-conduit
by Ivan Lazar Miljenovic 08 Jun '17

08 Jun '17

I've recently found myself really enjoying using Michael Thompson's [streaming] library for, well, streaming data. However, a lot of packages that I want to use already use conduit for all their streaming needs. As such, I've just written the [streaming-conduit] library to convert between the two (rather than just switching this project to conduit entirely as I couldn't find a simple way to just stream data from a PostgreSQL database). I make no guarantees at this stage of performance, and it's quite possible that - especially for the asStream and asConduit functions - that they may not interoperate cleanly with monadic operations. [streaming]: https://hackage.haskell.org/package/streaming [streaming-conduit]: https://hackage.haskell.org/package/streaming-conduit -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Turtle library : grepping on Text vs FilePath
by Ruben Astudillo 07 Jun '17

07 Jun '17

Dear list On the Turtle library, the grep function got the following signature grep :: Pattern a -> Shell Line -> Shell Line For it to filter output of the `ls :: FilePath -> Shell FilePath` I got to fmap and transform via de Turtle.Format or Turtle.Line modules. Is this an indicator I shouldn't be doing this? Is the library telling me to use `find`? or is just accidental? -- -- Ruben

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Encoding superclass relationships as a constraint?
by Alexis King 07 Jun '17

07 Jun '17

Using ExistentialQuantification, it’s possible to create a rather generic type that packages some arbitrary dictionary up at runtime: data Some ctx = forall a. ctx a => Some a This is nice, but it’s not very interesting. It would be nice if we could define a Show instance for our type, and a naïve implementation is trivial: instance Show (Some Show) where show (Some x) = show x However, this isn’t very useful in practice, since it means we can only show values of precisely the type (Some Show), which are better represented as values of type String. However, we might reasonably have some class with Show as a superclass: class Show a => Foo a instance Foo () ...and we might want to show values of type (Some Foo). However, this won’t work with the naïve implementation, since Foo is not the same as Show. Clearly, what we want is some instance like this: instance (ctx ≈> Show) => Show (Some ctx) ...where (a ≈> b) can be read “a implies b”. Unfortunately, as far as I can tell, this isn’t actually possible in any general way. The closest thing I was able to come up with was a less general solution using Edward Kmett’s constraints package. It’s possible to write (≈>) with some manual boilerplate: import Data.Constraint import Data.Constraint.Forall class Class (ctx a) (ctx' a) => Class1' ctx ctx' a instance Class (ctx a) (ctx' a) => Class1' ctx ctx' a type Class1 ctx ctx' = Forall (Class1' ctx ctx') type a ≈> b = Class1 b a ...and then it’s possible to write that Show instance: instance (ctx ≈> Show) => Show (Some ctx) where show (Some (x :: a)) = case inst @(Class1' Show ctx) @a of Sub Dict -> case (cls :: ctx a :- Show a) of Sub Dict -> show x However, this means users have to manually implement Class instances for their classes. Using the above example of Foo, a user would have to write the following instance: instance Class (Show a) (Foo a) where cls = Sub Dict With that instance, we can properly show values of type (Some Foo): ghci> Some () :: Some Foo () But this instance is both trivial and annoying, since it means the machinery leaks out of the implementation code and into the code that uses it. I’m wondering if there’s any way to do this more generally, and if not, if there’s any reason it could not or should not be supported by GHC itself. Alexis

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