March 2016 - Haskell-Cafe

Odd list comprehension behaviour
by Krisztian Pinter 21 Mar '16

21 Mar '16

Hello, I noticed some odd behaviour with list comprehensions. [1..1] == [1] BUT [1,1..1] == [1..] I noticed this while writing a Clean program, but it seems Haskell inherited this as well. In the case of integer lists with step size >= 0 the up_list function[1] is used: up_list :: Integer -> Integer -> Integer -> [Integer] up_list x0 delta lim = go (x0 :: Integer) where go x | x > lim = [] | otherwise = x : go (x+delta) In the case of [1,1..1] x0 == lim, so go will recurse infinitely, producing an infinite list. I think the reasonable behaviour would be [1,1..1] == [1]. Is there a reason it doesn't work like this? [1] http://hackage.haskell.org/package/base-4.8.2.0/docs/src/GHC.Enum.html#up_l… Thanks, Krisztián

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Ambiguous type variable ‘f0’ arising from a use of ‘f’
by Tom Ellis 20 Mar '16

20 Mar '16

Can anyone explain what happened in GHC 8 such that it forced this change: -data PackMap a b s t = PackMap (Applicative f => (a -> f b) -> s -> f t) +data PackMap a b s t = PackMap (forall f. Applicative f => (a -> f b) -> s -> f t) ... instance PP.SumProfunctor (PackMap a b) where - f +++! g = (PackMap (\x -> eitherFunction (f' x) (g' x))) - where PackMap f' = f - PackMap g' = g + f +++! g = + PackMap (\x -> + case f of + PackMap f' -> + case g of + PackMap g' -> + eitherFunction (f' x) + (g' x)) https://github.com/tomjaguarpaw/haskell-opaleye/pull/140/files The errors are Ambiguous type variable ‘f0’ arising from a use of ‘f’ etc., as seen here https://github.com/tomjaguarpaw/haskell-opaleye/pull/140#issuecomment-19829… Thanks, Tom

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Gaussian Elimination over GF2
by Mick Francis 20 Mar '16

20 Mar '16

Does anyone know of any implementation in Haskell of Gaussian Elimination over GF2? I'm experimenting with congruence-based factoring algorithms, and need an implementation for the Linear Algebra stage. Mick.

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[ANN] servant 0.5
by Julian Arni 19 Mar '16

19 Mar '16

Dear all, We have just released a new major version (0.5) of the official servant packages: - servant - servant-server - servant-client - servant-docs - servant-foreign - servant-js - servant-cassava - servant-blaze - servant-lucid - servant-mock (servant-js and servant-foreign are being released for the first time. servant-jquery will be deprecated in favour of servant-js) This new version brings quite a number of significant changes. Servant is now faster, more correct, more replete with features, and more consistent. We have a blog post going into more details about the changes: http://haskell-servant.github.io/posts/2016-03-19-servant-0.5-release.html Note also that we have revamped our tutorial, and moved it to readthedocs: http://haskell-servant.readthedocs.org/en/master/ Enjoy the weekend, Julian

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 151, Issue 22
by Barak A. Pearlmutter 19 Mar '16

19 Mar '16

> If I were to propose this, which I'm not, I would discuss what the > [a,b..c] notation is meant to represent. To me, personally and for no > good reason, it looks like an iterator that yields numbers starting at > a, adding (b-a) at each step and yielding the next number as long as > it's less than or equal to c, and continuing as long as c is neither > reached nor exceeded. Yes, the current situation is somewhat odd. $ ghci GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> [0,5..9] [0,5] Prelude> [0,0.5..0.9] [0.0,0.5,1.0] Prelude> [0.0..0.49999999999999991] [0.0] Prelude> [0.0..0.49999999999999992] [0.0,1.0] Prelude> :m Data.Ratio Prelude Data.Ratio> [0..1%2] [0 % 1,1 % 1] This makes the construct into a serious bug generator for anything numeric. For instance, consider a naive numeric integration of the sort one might wish to exhibit while teaching Haskell this past pi day, Monday the fourteenth of March. Prelude> let h=1e-1 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] 3.304518326248318 Prelude> let h=1e-2 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] NaN Prelude> let h=1e-3 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] NaN Prelude> let h=1e-4 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] 3.141791477784753 Prelude> let h=1e-5 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] 3.1416126164824103 Prelude> let h=1e-6 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] NaN One invariant that people seem to find natural regarding [x,y..z] is that its elements are all between x and z (inclusive), regardless of the value of y. This is, in part, because the mathematical notation [x,z] often denotes the closed interval whose endpoints are x and z. I myself cannot think of any advantage of the behaviour exhibited above. To take a slightly deeper violated invariant, it seems like map f [x,y..z] and [f x,f y..f z] should be the same when f is a linear (in the numeric sense) function. The current behaviour violates this.

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Re: [Haskell-cafe] Trying desperately to make a c library binding on Windows
by Nicu Ionita 18 Mar '16

18 Mar '16

Am 17.03.2016 um 23:24 schrieb Theodore Lief Gannon: > > I am on Win 7, using stack (configured for lts-4.2) under MinGW. > > Stack itself provides MSYS2/MinGW during builds on Windows. Do you > mean you have an independent MinGW shell that you're opening, and then > running stack from there? Yes, I work under the git bash (git for Windows comes with an own msys). > If so, assuming you're not already using this flag, try: > > stack build --skip-msys Just tried that now, same result. > > This tells stack that it's already running inside a unix-style shell, > and shouldn't assert its related path overrides. > > Beyond that, it would help to have more information about exactly > where everything is and how it's being built/invoked/etc. Project is under: J:\Projects\BaseOh Library is under: J:\BO\lib and is called: libbayesopt.a (under bash they are called /j/Projects/BaseOh and /j/BO/lib, needless to say I tried those too, but they are not found, as the programs know only Windows style paths). So beeing under the git bash in the project directory, I type: > stack build or even: > stack build --extra-lib-dirs=j:\\BO\\lib But in both cases, if the line "extra-libraries: bayesopt" is present in the cabal file, then configure does not work (message below). Inbetween I upgraded stack (now is version 1.0.4.2 x86_64). When I try to build, I get the message: Continuing despite missing tool: msys2 after which the build continues and brings exactly the same failures. > > On Thu, Mar 17, 2016 at 2:19 PM, Nicu Ionita <nicu.ionita(a)acons.at > <mailto:nicu.ionita@acons.at>> wrote: > > Hi all, > > I am really stuck with my try to make a Haskell binding for the > library bayesopt on Windows. > In theory it looks all simple, just put some lines in the cabal > configuration file: > > ... > extra-lib-dirs: /path/to/the/lib -- contains the file > libbayesopt.a > extra-libraries: bayesopt > ... > > But whatever I tried, I can't step over the configuration phase, > coz I get (I'm compiling with stack): > > Configuring BaseOh-0.1.0.0... > setup-Simple-Cabal-1.22.5.0-x86_64-windows-ghc-7.10.3.exe: Missing > dependency > on a foreign library: > * Missing C library: /j/BO/lib/libbayesopt.a > This problem can usually be solved by installing the system > package that > provides this library (you may need the "-dev" version). If the > library is > already installed but in a non-standard location then you can use > the flags > --extra-include-dirs= and --extra-lib-dirs= to specify where it is. > > If I comment those lines out, then the configure phase succeeds, > the library is compiled (i.e. my binding library), but a test > executable in the same project does of course not compile, as > symbols are missing: > > ... > In-place registering BaseOh-0.1.0.0... > Preprocessing executable 'test' for BaseOh-0.1.0.0... > [1 of 2] Compiling Optimisation.Stochastic.BayesOpt ( > src\Optimisation\Stochasti > c\BayesOpt.hs, > .stack-work\dist\x86_64-windows\Cabal-1.22.5.0\build\test\test-tm > p\Optimisation\Stochastic\BayesOpt.o ) > ghc.exe: unable to load package `BaseOh-0.1.0.0' > ghc.exe: > J:\Projects\BaseOh\.stack-work\dist\x86_64-windows\Cabal-1.22.5.0\build > \HSBaseOh-0.1.0.0-GdNObdePATgDIW1I2OVwEc.o: unknown symbol > `initialize_parameter > s_to_default' > > -- While building package BaseOh-0.1.0.0 using: > ... > > Does somebody have some idea, what is going on? > > I am on Win 7, using stack (configured for lts-4.2) under MinGW. > (I'm using inline-c to make the bindings, and the same trick for > include - include-dirs: ... - worked fine.) > > Thanks, and sorry for the longer email. > Nicu > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >

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CUFP 2016 Call for Presentations
by Alex Lang 18 Mar '16

18 Mar '16

Apologies for any duplicates you may receive. http://cufp.org/2016/call-for-presentations.html ====================================================================== CUFP 2016 Call for Presentations Commercial Users of Functional Programming 2016 Sponsored by SIGPLAN Co-located with ICFP 2016 Nara, Japan September 22-24 Talk Proposal Submission Deadline: 24 June 2016 CUFP 2016 Presentation Submission Form: http://goo.gl/forms/gWDSoKfizW 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 talks 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 mission 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 24 June 2016 via the CUFP 2016 Presentation Submission Form (http://goo.gl/forms/gWDSoKfizW) 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 all presenters to register for the CUFP workshop and travel to Japan at their own expense. There are some funds available to would-be presenters who require assistance in this respect. ====================================================================== Program Committee Katie Ots (Facebook), co-chair Alex Lang (Tsuru Capital), co-chair Rúnar Óli Bjarnason (Verizon Labs) Mark Hibberd (Ambiata) Mirai Ikebuchi (Nagoya University) Paul Khuong (AppNexus) Carin Meier (Cognitect) Kenji Rikitake (Kenji Rikitake Professional Engineer's Office) ====================================================================== 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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Calling Haskellers to Bryn Mawr & Haverford
by Richard Eisenberg 18 Mar '16

18 Mar '16

Anyone who has probed deeply (or at all!) into the delay behind the release of GHC 8.0 may have come across protestations from me that I'm on a job search and have been struggling to find the time to finish off the last few show-stopper bugs. That search has now come to a very happy conclusion, and so I'm writing to share the news: I will be starting as an Assistant Professor at Bryn Mawr College this fall. In my role there, I'm expecting to introduce a new course (or two) on functional programming using Haskell (though the emphasis would be more on pure, strongly typed functional programming than on Haskell, per se), as well as to continue to pursue my research interests in the design and implementation of functional languages along with the requisite type theory. Undergrads will feature prominently in my research efforts, joining my research lab and collaborating on research papers. That's where you come in. Are you, reading this list, a high school student? (Or perhaps an undergrad looking for a change?) Do you know a Haskeller who is? Then come to Bryn Mawr and work with me to design the functional language of the future. "But wait!" many of you might say, "isn't Bryn Mawr only for women?" Yes, indeed it is. However, right down the street is Haverford College, a co-ed school. Haverford and Bryn Mawr share a close relationship, with regular free transportation (< 10 mins) between the schools and very frequent cross-registration for courses. Haverford students even sometimes declare their major at Bryn Mawr and vice versa. The CS departments regularly coordinate in deciding which courses to offer and when, in order to allow students to access the resources of both colleges. So, for you young men out there, I'll rephrase: Are you, reading this list, a high school student? (Or perhaps an undergrad looking for a change?) Do you know a Haskeller who is? Then come to Haverford (or Swarthmore, or University of Pennsylvania) and work with me to design the functional language of the future. I've included Swarthmore and UPenn (where I'm finishing my dissertation) above as members of the Quaker Consortium, where students have cross-registration privileges. Swarthmore is also in the Tri-College Consortium with Bryn Mawr, which also allows easier inter-college research experiences. Swarthmore students can get to Bryn Mawr via free van service (~25 mins) and UPenn students can get to Bryn Mawr via 20 minutes on SEPTA regional rail (these links are admittedly less convenient than the Haverford bus). Of course, I'm happy to answer questions. Though, being new to Bryn Mawr, I may have to refer you to others to get the answers! Hope to see you on campus! Richard

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GHC Performance Regression
by Dominic Steinitz 18 Mar '16

18 Mar '16

Some time ago I wrote a little test program for random number generation. Under 7.8.3 I get > Total time 9.03s ( 9.15s elapsed) Under 7.10.3 I get > Total time 24.773s ( 25.288s elapsed) Now of course it could be the libraries that I am using rather than GHC itself so I have tried to make them as similar as possible. For 7.8.3 I have > build-depends: base ==4.7.0.1, > mtl ==2.1.3.1, > primitive == 0.6, > mwc-random == 0.13.3.2, > vector == 0.10.12.3, > random ==1.1, > random-fu == 0.2.6.2, > random-source == 0.3.0.6 For 7.10.3 I have > build-depends: base ==4.8.2.0, > mtl ==2.2, > primitive == 0.6, > mwc-random == 0.13.3.2, > vector == 0.10.12.3, > random ==1.1, > random-fu == 0.2.6.2, > random-source == 0.3.0.6 So the only differences are in mtl and base. I don’t seem to be able to coax cabal into using mtl-2.2 for 7.8.3 with all the other required libraries. > dominic@ghcPerformance:~$ cabal install 'random-source ==0.3.0.6' 'random-fu ==0.2.6.2' 'random ==1.1' 'primitive ==0.6' 'mwc-random ==0.13.3.2' 'mtl ==2.2' 'vector ==0.10.12.3' --with-ghc=ghc-7.10.3 > Resolving dependencies... > All the requested packages are already installed: > mtl-2.2 > mwc-random-0.13.3.2 > primitive-0.6 > random-1.1 > random-fu-0.2.6.2 > random-source-0.3.0.6 > vector-0.10.12.3 > Use --reinstall if you want to reinstall anyway. > dominic@ghcPerformance:~$ cabal install 'random-source ==0.3.0.6' 'random-fu ==0.2.6.2' 'random ==1.1' 'primitive ==0.6' 'mwc-random ==0.13.3.2' 'mtl ==2.2' 'vector ==0.10.12.3' --with-ghc=ghc-7.8.3 > Resolving dependencies... > cabal: Could not resolve dependencies: > trying: random-source-0.3.0.6/installed-70e... (user goal) > next goal: mtl (user goal) > rejecting: mtl-2.2.1, 2.2.0.1 (global constraint requires ==2.2) > rejecting: mtl-2.2/installed-cc5..., 2.2 (conflict: random-source => > mtl==2.1.3.1/installed-8bc...) > rejecting: mtl-2.1.3.1/installed-8bc..., 2.1.3.1, 2.1.2, 2.1.1, 2.1, 2.0.1.1, > 2.0.1.0, 2.0.0.0, 1.1.1.1, 1.1.1.0, 1.1.0.2, 1.1.0.1, 1.1.0.0, 1.0 (global > constraint requires ==2.2) > Backjump limit reached (change with --max-backjumps). Cabal seems to be telling me that random-source-0.3.0.6 is the problem but if I look at the constraints for that package here https://hackage.haskell.org/package/random-source <https://hackage.haskell.org/package/random-source> then I see > mtl <https://hackage.haskell.org/package/mtl> (>=1 && <3) I am not sure how to proceed from here. Should I raise an issue on the GHC bug tracker? I’d like to solve this myself but I don’t want to start building versions of ghc to see which change caused the regression without first eliminating mtl. Any ideas would be gratefully received. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com > {-# LANGUAGE TemplateHaskell #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE FlexibleInstances #-} > > import Data.Random > import Data.Random.Source > import qualified System.Random.MWC as MWC > import Control.Monad.Reader > import Control.Monad.Primitive > > $(monadRandom [d| > instance (PrimMonad m, s ~ PrimState m) => MonadRandom (ReaderT (MWC.Gen s) m) where > getRandomWord16 = ask >>= lift . MWC.uniform > getRandomWord32 = ask >>= lift . MWC.uniform > getRandomWord64 = ask >>= lift . MWC.uniform > |]) > > testUniform :: MonadRandom m => Int -> m [Double] > testUniform n = replicateM (fromIntegral n) (sample stdUniform) > > n :: Int > n = 10^7 > > main :: IO () > main = do > seed <- MWC.create > xs <- runReaderT (testUniform n) seed > print (sum xs / fromIntegral n) This cabal file will build this on 7.8.3 > name: PerfTest8 > version: 0.1.0.0 > homepage: TBD > license: MIT > author: Dominic Steinitz > maintainer: idontgetoutmuch(a)gmail.com > category: System > build-type: Simple > cabal-version: >=1.10 > > executable Random8 > main-is: TestMwcViaRandomSource.hs > build-depends: base ==4.7.0.1, > mtl ==2.1.3.1, > primitive == 0.6, > mwc-random == 0.13.3.2, > vector == 0.10.12.3, > random ==1.1, > random-fu == 0.2.6.2, > random-source == 0.3.0.6 > default-language: Haskell2010 This cabal file will build this on 7.10.3 > name: PerfTest10 > version: 0.1.0.0 > homepage: TBD > license: MIT > author: Dominic Steinitz > maintainer: idontgetoutmuch(a)gmail.com > category: System > build-type: Simple > cabal-version: >=1.10 > > executable Random10 > main-is: TestMwcViaRandomSource.hs > build-depends: base ==4.8.2.0, > mtl ==2.2, > primitive == 0.6, > mwc-random == 0.13.3.2, > vector == 0.10.12.3, > random ==1.1, > random-fu == 0.2.6.2, > random-source == 0.3.0.6 > default-language: Haskell2010

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Trying desperately to make a c library binding on Windows
by Nicu Ionita 17 Mar '16

17 Mar '16

Hi all, I am really stuck with my try to make a Haskell binding for the library bayesopt on Windows. In theory it looks all simple, just put some lines in the cabal configuration file: ... extra-lib-dirs: /path/to/the/lib -- contains the file libbayesopt.a extra-libraries: bayesopt ... But whatever I tried, I can't step over the configuration phase, coz I get (I'm compiling with stack): Configuring BaseOh-0.1.0.0... setup-Simple-Cabal-1.22.5.0-x86_64-windows-ghc-7.10.3.exe: Missing dependency on a foreign library: * Missing C library: /j/BO/lib/libbayesopt.a This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. If I comment those lines out, then the configure phase succeeds, the library is compiled (i.e. my binding library), but a test executable in the same project does of course not compile, as symbols are missing: ... In-place registering BaseOh-0.1.0.0... Preprocessing executable 'test' for BaseOh-0.1.0.0... [1 of 2] Compiling Optimisation.Stochastic.BayesOpt ( src\Optimisation\Stochasti c\BayesOpt.hs, .stack-work\dist\x86_64-windows\Cabal-1.22.5.0\build\test\test-tm p\Optimisation\Stochastic\BayesOpt.o ) ghc.exe: unable to load package `BaseOh-0.1.0.0' ghc.exe: J:\Projects\BaseOh\.stack-work\dist\x86_64-windows\Cabal-1.22.5.0\build \HSBaseOh-0.1.0.0-GdNObdePATgDIW1I2OVwEc.o: unknown symbol `initialize_parameter s_to_default' -- While building package BaseOh-0.1.0.0 using: ... Does somebody have some idea, what is going on? I am on Win 7, using stack (configured for lts-4.2) under MinGW. (I'm using inline-c to make the bindings, and the same trick for include - include-dirs: ... - worked fine.) Thanks, and sorry for the longer email. Nicu

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