April 2014 - Haskell-Cafe

What should I use if I want both deriving SQL 'create table' from haskell types automatically and relational queries ?
by ducis 04 Apr '14

04 Apr '14

Persistent and acid-state are both 'No SQL' ? I guess there is only groundhog?

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Fun and Profit with Stongly-Typed Data Schemas
by Chris Dornan 04 Apr '14

04 Apr '14

Folks, I will be giving a talk in London on Wednesday evening at Skills Matter on _api-tools_, a cool new Haskell package Iris Connect have been developing with Well Typed to help us with the new web platform. See the Skills Matter web site for details: https://skillsmatter.com/meetups/6201-fun-and-profit-with-stongly-typed-data -schemas Cheers, Chris

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Inline makes program slow?
by Kai Zhang 03 Apr '14

03 Apr '14

Hi cafe, Inline sometimes can cause problems, at least in following case: import qualified Data.Vector as V import Data.List f ∷ String → (String → Int → Char) → [Int] → String f str g idx = map (g str) idx h ∷ String → Int → Char *{-# INLINE h #-}* h s i = (V.fromList $ sort s) V.! i slow ∷ String → [Int] → String slow str = f str h fast ∷ String → [Int] → String fast str = map ((V.fromList $ sort str) V.!) main = do let testString = replicate 100000 'a' iterations = replicate 1000 100 putStrLn $ fast testString iterations putStrLn $ slow testString iterations Without inline (remove the inline pragma), "slow" would be much faster. I suspect this is because ghc can build a "persistent structure" for the partial applied function. After inline, each call of "g" will try to build a new vector. How can I tell ghc not to inline some specific functions? Or are there other ways to resolve this issue?

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Haskell Weekly News: Issue 289
by Daniel Santa Cruz 03 Apr '14

03 Apr '14

Welcome to issue 289 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from March 1 to 31, 2014 So... where have I been? My wife and I recently welcomed into the world our new baby, and as some of you have already experienced, I was doing good getting *some* sleep at night. Life has slowly started to get back to a more predictable rhythm, and I am sure glad to be back doing this. For a while there I thought that any project of mine before bambina was going to have to be put into permament hold, but things are getting very manageble now. Sorry for the abscense! I was somewhat pleased to see that some in -cafe wondered if this was coming back... this one is for you! Some have mentioned that they wish to see more of the "reviews" that HWN used to have of blog entries, and/or email chains in the different mailing lists. I understand that dessire. I'm probably going to need some extra hands to write those, if there are to be back. Maybe we can start with something "lighter", a la tl;dr. I'm stil trying to work out how we'd do that exacly. I'll keep you all posted. Anyway, enough rambling. Quotes of the Week * acowley: I will push machines-concurrent to hackage one fine day edwardk: not concurrent-machines ? acowley: Man, I walked right into that bike shed * malc: I have a coworker who constantly reads haskell stuff at work, nudged him towards the lens talk, that should push him towards actually doing work at work. * Fuuzetsu: I know someone who pulls in Lens just for & and ?? * Eduard_Munteanu: Don't put your money in monads, you can never get it back. :P Top Reddit Stories * Haskell for all: Introductions to advanced Haskell topics Domain: haskellforall.com, Score: 121, Comments: 23 On Reddit: [1] http://goo.gl/M6ZL9f Original: [2] http://goo.gl/YEKyCc * GHC 7.8.1 RC2 released Domain: well-typed.com, Score: 99, Comments: 36 On Reddit: [3] http://goo.gl/jxlLQl Original: [4] http://goo.gl/05xnqX * Book review: Parallel and Concurrent Programming in Haskell Domain: serpentine.com, Score: 87, Comments: 17 On Reddit: [5] http://goo.gl/BfWSWC Original: [6] http://goo.gl/nDe2UG * The Haskell Cast #6 - Gabriel Gonzalez and Michael Snoyman on Pipes and Conduit Domain: haskellcast.com, Score: 83, Comments: 13 On Reddit: [7] http://goo.gl/PwL2aV Original: [8] http://goo.gl/hwemr6 * Performance in Haskell from a noob's perspective Domain: self.haskell, Score: 74, Comments: 111 On Reddit: [9] http://goo.gl/IuT0uJ Original: [10] http://goo.gl/IuT0uJ * New Haddock -- a visual guide to changes Domain: fuuzetsu.co.uk, Score: 72, Comments: 13 On Reddit: [11] http://goo.gl/KWJSEh Original: [12] http://goo.gl/IXzESN * Full-time position available: computer scientist to create data types for scientific computing, in Haskell. Location: anywhere (telecommute). Domain: self.haskell, Score: 67, Comments: 17 On Reddit: [13] http://goo.gl/Z8y9py Original: [14] http://goo.gl/Z8y9py * Haskell Type Classes Cheatsheet Domain: fundeps.com, Score: 62, Comments: 21 On Reddit: [15] http://goo.gl/oAWk4C Original: [16] http://goo.gl/SqHbro * Malicious link in haskell.org docs for 6 years, 6 months, 6 days Domain: self.haskell, Score: 61, Comments: 6 On Reddit: [17] http://goo.gl/V5y3hk Original: [18] http://goo.gl/V5y3hk * What's your "killer app" for your scientific/statistical programming environment? Domain: self.haskell, Score: 57, Comments: 92 On Reddit: [19] http://goo.gl/jfbwdj Original: [20] http://goo.gl/jfbwdj * Find out the type of an expression/function with typed holes Domain: ro-che.info, Score: 56, Comments: 5 On Reddit: [21] http://goo.gl/hcjX1v Original: [22] http://goo.gl/TWLT5R * Elm 0.12 - making interactive UI elements easy and pure Domain: elm-lang.org, Score: 56, Comments: 5 On Reddit: [23] http://goo.gl/SRoog7 Original: [24] http://goo.gl/7FoFAz * QuickCheck 2.7 released Domain: hackage.haskell.org, Score: 55, Comments: 7 On Reddit: [25] http://goo.gl/RN66bR Original: [26] http://goo.gl/3S38Uz Top StackOverflow Questions * Why is Haskell unable to read "7e7" but able to read "7a7"? votes: 42, answers: 3 Read on SO: [27] http://goo.gl/ZI8qm8 * Strange GHCi lazy evaluation votes: 17, answers: 1 Read on SO: [28] http://goo.gl/aoHvVp * Fast, branchless unsigned int absolute difference votes: 16, answers: 3 Read on SO: [29] http://goo.gl/TT3Oxi * How can I write human-language units as postfixes in Haskell, like `3 seconds`? votes: 15, answers: 2 Read on SO: [30] http://goo.gl/6MJk5C * Running Haskell on Xeon-Phi votes: 14, answers: 1 Read on SO: [31] http://goo.gl/QEIayp Until next time, [32]+Daniel Santa Cruz References 1. http://www.haskellforall.com/2014/03/introductions-to-advanced-haskell-topi… 2. http://www.reddit.com/r/haskell/comments/21bcb5/haskell_for_all_introductio… 3. http://www.well-typed.com/blog/87 4. http://www.reddit.com/r/haskell/comments/1zgaln/ghc_781_rc2_released/ 5. http://www.serpentine.com/blog/2014/03/18/book-review-parallel-and-concurre… 6. http://www.reddit.com/r/haskell/comments/20sdt4/book_review_parallel_and_co… 7. http://www.haskellcast.com/episode/006-gabriel-gonzalez-and-michael-snoyman… 8. http://www.reddit.com/r/haskell/comments/1zfo9g/the_haskell_cast_6_gabriel_… 9. http://www.reddit.com/r/haskell/comments/21g6uo/performance_in_haskell_from… 10. http://www.reddit.com/r/haskell/comments/21g6uo/performance_in_haskell_from… 11. http://fuuzetsu.co.uk/blog/posts/2014-03-24-New-Haddock-released%21-A-visua… 12. http://www.reddit.com/r/haskell/comments/218rov/new_haddock_a_visual_guide_… 13. http://www.reddit.com/r/haskell/comments/202pvl/fulltime_position_available… 14. http://www.reddit.com/r/haskell/comments/202pvl/fulltime_position_available… 15. http://fundeps.com/posts/cheatsheets/2014-03-04-cheat-sheets/ 16. http://www.reddit.com/r/haskell/comments/1zn3h9/haskell_type_classes_cheats… 17. http://www.reddit.com/r/haskell/comments/21959m/malicious_link_in_haskellor… 18. http://www.reddit.com/r/haskell/comments/21959m/malicious_link_in_haskellor… 19. http://www.reddit.com/r/haskell/comments/1zowv1/whats_your_killer_app_for_y… 20. http://www.reddit.com/r/haskell/comments/1zowv1/whats_your_killer_app_for_y… 21. http://ro-che.info/articles/2014-03-13-type-of-local-function.html 22. http://www.reddit.com/r/haskell/comments/20bixd/find_out_the_type_of_an_exp… 23. http://elm-lang.org/blog/announce/0.12.elm 24. http://www.reddit.com/r/haskell/comments/218isr/elm_012_making_interactive_… 25. http://hackage.haskell.org/package/QuickCheck-2.7.1/changelog 26. http://www.reddit.com/r/haskell/comments/213da9/quickcheck_27_released/ 27. http://stackoverflow.com/questions/22689122/why-is-haskell-unable-to-read-7… 28. http://stackoverflow.com/questions/22491700/strange-ghci-lazy-evaluation 29. http://stackoverflow.com/questions/22445019/fast-branchless-unsigned-int-ab… 30. http://stackoverflow.com/questions/22109333/how-can-i-write-human-language-… 31. http://stackoverflow.com/questions/22253311/running-haskell-on-xeon-phi 32. https://plus.google.com/105107667630152149014/about

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I guess this is "Play it again Sam"?? If so, sorry ...
by Vasili I. Galchin 03 Apr '14

03 Apr '14

http://www.infoq.com/presentations/haskell-newsroom-nyt

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Eta Reduction
by Dan Doel 03 Apr '14

03 Apr '14

In the past year or two, there have been multiple performance problems in various areas related to the fact that lambda abstraction is not free, though we tend to think of it as so. A major example of this was deriving of Functor. If we were to derive Functor for lists, we would end up with something like: instance Functor [] where fmap _ [] = [] fmap f (x:xs) = f x : fmap (\y -> f y) xs This definition is O(n^2) when fully evaluated,, because it causes O(n) eta expansions of f, so we spend time following indirections proportional to the depth of the element in the list. This has been fixed in 7.8, but there are other examples. I believe lens, [1] for instance, has some stuff in it that works very hard to avoid this sort of cost; and it's not always as easy to avoid as the above example. Composing with a newtype wrapper, for instance, causes an eta expansion that can only be seen as such at the core level. The obvious solution is: do eta reduction. However, this is not operationally sound currently. The problem is that seq is capable of telling the difference between the following two expressions: undefined \x -> undefined x The former causes seq to throw an exception, while the latter is considered defined enough to not do so. So, if we eta reduce, we can cause terminating programs to diverge if they make use of this feature. Luckily, there is a solution. Semantically one would usually identify the above two expressions. While I do believe one could construct a semantics that does distinguish them, it is not the usual practice. This suggests that there is a way to not distinguish them, perhaps even including seq. After all, the specification of seq is monotone and continuous regardless of whether we unify ⊥ with \x -> ⊥ x or insert an extra element for the latter. The currently problematic case is function spaces, so I'll focus on it. How should: seq : (a -> b) -> c -> c act? Well, other than an obvious bottom, we need to emit bottom whenever our given function is itself bottom at every input. This may first seem like a problem, but it is actually quite simple. Without loss of generality, let us assume that we can enumerate the type a. Then, we can feed these values to the function, and check their results for bottom. Conal Elliot has prior art for this sort of thing with his unamb [2] package. For each value x :: a, simply compute 'f x `seq` ()' in parallel, and look for any successes. If we ever find one, we know the function is non-bottom, and we can return our value of c. If we never finish searching, then the function must be bottom, and seq should not terminate, so we have satisfied the specification. Now, some may complain about the enumeration above. But this, too, is a simple matter. It is well known that Haskell programs are denumerable. So it is quite easy to enumerate all Haskell programs that produce a value, check whether that value has the type we're interested in, and compute said value. All of this can be done in Haskell. Thus, every Haskell type is programatically enumerable in Haskell, and we can use said enumeration in our implementation of seq for function types. I have discussed this with Russell O'Connor [3], and he assures me that this argument should be sufficient even if we consider semantic models of Haskell that contain values not denoted by any Haskell program, so we should be safe there. The one possible caveat is that this implementation of seq is not operationally uniform across all types, so the current fully polymorphic type of seq may not make sense. But moving back to a type class based approach isn't so bad, and this time we will have a semantically sound backing, instead of just having a class with the equivalent of the current magic function in it. Once this machinery is in place, we can eta reduce to our hearts' content, and not have to worry about breaking semantics. We'd no longer put the burden on programmers to use potentially unsafe hacks to avoid eta expansions. I apologize for not sketching an implementation of the above algorithm, but I'm sure it should be elementary enough to make it into GHC in the next couple versions. Everyone learns about this type of thing in university computer science programs, no? Thoughts? Comments? Questions? Cheers, -- Dan [1] http://hackage.haskell.org/package/lens [2] http://hackage.haskell.org/package/unamb [3] http://r6.ca/

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HGamer3D - 0.3.x - Building Fixed
by Peter Althainz 02 Apr '14

02 Apr '14

Dear All, I recognized that there were some serious bugs in build code for HGamer3D, which basically prevented building it successfully. I'm very sorry for this! As always there are some explanations how this could happen, but anyhow, I could imagine how frustrating it is to try, spend time and then: no success. This has been fixed now. The detailed build instructions on http://www.hgamer3d.org/Install.html do work, they have been tested on 4 different linux distros and Windows exactly the way described there. regards Peter

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New GHC Features for 7.10.1 and Beyond
by Gershom Bazerman 01 Apr '14

01 Apr '14

There's been lots of exciting work going into the forthcoming GHC 7.8.1 release. But even with all these new features, our language is far from complete and I wouldn't want the GHC team to rest on their laurels. Especially with so much renewed community involvement in GHC development, it seems appropriate to share some ideas some of us have been discussing for future releases, and take a poll of community consensus regarding which ones people might be excited to jump in and help out with, or might find particularly helpful. One important area that needs work is exceptions. These are famously difficult, and many libraries have been written to provide checked exceptions, or unify error and exceptions in various ways. It seems to me that the problem, all along, has been that we've decided to give Haskell _imprecise exceptions_ with special semantics. It would alleviate a great deal of confusion if we implemented a -fprecise-exceptions flag that remedied this. Additionally, while we have long known that Haskell is not a "lazy" language but a "non-strict" one, our leading compiler only rarely takes advantage of this. It is well known that for every program there is a reduction strategy that preserves non-strict semantics while using as few reductions as possible. This is known as the "optimal reduction" strategy, and it is calculable and well understood. We should put some work into a -foptimal-reduction flag. Another important area of research is quantum computation. The D-Wave Two, with 512 qbits, is a commercially available quantum computer now deployed in a few leading institutions. As more and more people begin to purchase D-Wave systems for home hacking, every language worth its salt will need to be able to target this system. Now that cross compilation is more fully supported, it would be good to start putting work into a dwave backend. We've also seen a lot of interest in distribution and cloud computing. From the articles I've read, efficient concurrent programming involves using node.js, so I think we should put some work into writing a new-new-new-IO Manager built on top of this technology. Furthermore, it's ridiculous that while Haskell allows simple concurrent and distributed programming, it offers no simple way to set up a distributed environment. Work should be put into a compiler mode that provisions cloud resources automatically and distributes the target binary among them, with a flag such as: -fvia-s3 _credit_card_number_ There are a few cases where a good idea in GHC can be taken further, and I don't know why we haven't tried already. For example, we've found that despite introducing some occasional problems, the state hack also has drastically improved performance. Since it works so well, we should provide a dual to -fno-state-hack, -fmore-state-hack, for those who really need every last drop of performance. Similarly, now we have type holes that let us see what types should be inserted in various spots. -fvalue-holes is the next logical step, to tell us what terms we should be writing. Also, while we've put so much emphasis on correctness, we've also loosened the bolts optionally the other way, with flags such as -XIncoherentInstances. Along the same lines, I would like to introduce -XImpossibleInstances to let us express such useful things as "instance Int String," "instance IO Comonad," and "instance data if". -fdefer-type-errors allows us to defer type errors to runtime. Having spent some time with JavaScript lately, I think this doesn't go nearly far enough. It would be good to run programs with other sorts of problems as well, for educational purposes and for quick hacking. So work should be put into a -fdefer-scope-errors flag, as well as -fon-error-resume-next, which has done wonders for the resilience of Visual Basic code (see also the wildly useful ability to set the top level error handler in PHP). With all these features put together, we have a powerful new way to approach Haskell programming, and it would be good to enable them by default on .ilhs files (illiterate haskell). It also was pointed out to me recently by Jason Dagit that while we have had debates and confusion over closed and open type families and functions for years, topologists have solved this problem elegantly. With some basic point set topology under one's belt, a -XClopenTypeFamilies extension seems almost trivial to implement. Finally, I'm very sorry to see that my proposal for youtube syntax has languished, and I hope it can be revived: http://www.haskell.org/pipermail/haskell-cafe/2012-April/100527.html HTH HAND, Gershom

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[TFPIE2014] final call for papers
by Peter Achten 01 Apr '14

01 Apr '14

3rd International Workshop on Trends in Functional Programming in Education (TFPIE 2014) May 25, 2014 Utrecht University Soesterberg, The Netherlands (http://www.cs.uwyo.edu/~jlc/tfpie14/) The 3rd International Workshop on Trends in Functional Programming in Education, TFPIE 2014, will be co-located with the Symposium on Trends in Functional Programming (TFP 2014) at Soesterberg, at the “Kontakt der Kontinenten” hotel in the Netherlands on Sunday, May 25th. TFP will follow from May 26-28. The goal of TFPIE is to gather researchers, teachers and professionals that use, or are interested in the use of, functional programming in education. TFPIE aims to be a venue where novel ideas, classroom-tested ideas and work-in-progress on the use of functional programming in education are discussed. The one-day workshop will foster a spirit of open discussion by having a review process for publication after the workshop. The program chair of TFPIE 2014 will screen submissions to ensure that all presentations are within scope and are of interest to participants. Potential presenters are invited to submit an extended abstract (4-6 pages) or a draft paper (up to 16 pages) in EPTCS style. The authors of accepted presentations will have their preprints and their slides made available on the workshop's website/wiki. Visitors to the TFPIE 2014 website/wiki will be able to add comments. This includes presenters who may respond to comments and questions as well as provide pointers to improvements and follow-up work. After the workshop, presenters will be invited to submit (a revised version of) their article for review. The PC will select the best articles for publication in the journal Electronic Proceedings in Theoretical Computer Science (EPTCS). Articles not selected for presentation and extended abstracts will not be formally reviewed by the PC. TFPIE workshops have previously been held in St Andrews, Scotland (2012) and in Provo Utah, USA (2013). ** Invited Speaker ** TFPIE is pleased to announce that professor Johan Jeuring of Utrecht University and Open University, both in The Netherlands is giving an invited talk entitled: "Automatic tutoring and assessing functional programs". ** Program Committee ** James Caldwell, (Program Chair) University of Wyoming Peter Achten, Radboud University, Nijmgen Edwin Brady, University of St Andrews, St Andrews Jurriaan Hage, Universiteit Utrecht Philip Holzenspies, University of Twente Daniel R. Licata, Wesleyan University Marco T Morazan, Seton Hall University Christian Skalka, University of Vermont David Van Horn, Northeastern University ** Submission Guidelines ** There will be two types of presentations at TFPIE 2014. Regular papers and “best lecture” presentations. The best lecture talks are intended to allow for presentations or short lectures of purely pedagogical material. Papers and abstracts can be submitted via easychair at the following link: https://www.easychair.org/conferences/?conf=tfpie2014 ** Papers ** TFPIE 2014 welcomes submissions describing techniques used in the classroom, tools used in and/or developed for the classroom and any creative use of functional programming (FP) to aid education in or outside Computer Science. Topics of interest include, but are not limited to: * FP and beginning CS students * FP and Computational Thinking * FP and Artificial Intelligence * FP in Robotics * FP and Music * Advanced FP for undergraduates * FP in graduate education * Engaging students in research using FP * FP in Programming Languages * FP in the high school curriculum * FP as a stepping stone to other CS topics * FP and Philosophy ** Best Lectures ** In addition to papers, this year we are requesting “best lecture” presentations. What’s your best lecture topic in an FP related course? Do you have a fun way to present FP concepts to novices or perhaps an especially interesting presentation of a difficult topic? In either case, please consider sharing it. Best lecture topics will be selected for presentation based on a short abstract describing the lecture and its interest to TFPIE attendees. ** Important Dates ** * 1 February 2014: TFPIE submissions open on easychair. * 14 April 2014: Early registration for TFP closes * 21 April 2014: Submission deadline for draft TFPIE papers and abstracts * 27 April 2014: Notification of acceptance for presentation * 15 May 2014: Registration for TFPIE closes - as does late registration for TFP * 25 May 2014: Presentations in Soesterberg, Netherlands * 29 June 2014: Full papers for EPTCS proceedings due * 16 August 2014: Notification of acceptance for proceedings * 8 September 2014: Camera ready copy due for EPTCS Submission of an abstract implies no obligation to submit a full paper; abstracts with no corresponding full versions by the full paper deadline will be considered as withdrawn. At least one author from each accepted presentation must attend the workshop.

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Problems with Network/MVar
by Pierre-Étienne Meunier 01 Apr '14

01 Apr '14

Hello cafe, I’m trying to run a server to synchronize a bunch of machines, and one of the threads keeps crashing every ~50 hours. One of the threads is an HTTP server and stays alive forever, so it’s not a Unix signal problem. I don’t have much information on its stdout/stderr, and the bug is not very reproducible, since it can happen even though very few of its maximum ~200 clients are connected. Is there a reason why the following can stop after some time? server::Config -> MVar State -> IO () server config state=withSocketsDo $ do installHandler sigPIPE Ignore Nothing threads<-Sem.new $ maxThreads config forever $ do E.catch (bracket (listenOn $ port config)) sClose $ \sock->forever $ do (s,a,_)<-accept sock wait threads forkIO $ (reply state s a)`finally`(signal threads)`E.catch`(\e->let _=e::IOException in return ()) (\e->let _=e::IOException in return ()) threadDelay 100000 This was compiled with GHC 7.6.3, -threaded and run on Debian Jessie (I have not tested other settings), and Config and State are both record types. Thanks, Pierre

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