- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 15 Mar '17

15 Mar '17

Well, Joachim, it certainly can not get easier than that. Thanks a lot! Olaf

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Lazy vs Strict ByteStrings
by Dimitri DeFigueiredo 15 Mar '17

15 Mar '17

I just noticed that I avoid using ByteStrings because I tend to spend a huge amount of time reading GHC output and trying to figure out if I am/should be using the strict or the lazy version. I have to use both lazy/strict because different libraries use different versions. Maybe having different names would have helped. Am I doing something wrong or is the interface to ByteStrings a poor design? Cheers, Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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Register for BayHac 2017
by Dan Burton 15 Mar '17

15 Mar '17

Registration form: https://goo.gl/forms/GWShzl5m2icmhf8L2 [image: Inline image 1] <https://goo.gl/forms/GWShzl5m2icmhf8L2> All attendees are requested to register for the event via the registration form. Registration form: https://goo.gl/forms/GWShzl5m2icmhf8L2 More info: https://wiki.haskell.org/BayHac2017 -- Dan Burton

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PLDI ACM Student Research Competition (SRC)
by Tobias Grosser 14 Mar '17

14 Mar '17

The ACM Student Research Competition (SRC), sponsored by Microsoft Research, offers a unique forum for undergraduate and graduate students to present their original research on programming language design, implementation, theory, applications, and performance at PLDI 2017. The goal is to give students a place to discuss their research with experts in their field and to help them sharpen their research and communication skills. Please find more information at http://pldi17.sigplan.org/track/pldi-2017-student-research-competition Deadline for submission of the extended abstracts: Friday April 7th, 2017. ## Rounds Three rounds of competition will take place before and during PLDI 2017. All rounds are held in two categories: Undergraduate Students and Graduate Students (Masters and PhD level). Winners of the third round will be invited to participate in the SRC Grand Finals competition hosted by the ACM. The PLDI rounds are: * Extended abstract round. All students are encouraged to submit an extended abstract outlining their research. * Poster session. Based on the abstracts, a panel of judges will select the most promising authors to participate in the poster session which will take place at PLDI. In the poster session, students will have the opportunity to present their work to the judges, who will select a group of semi-finalists in each category to advance to the next round. * PLDI presentation. The last PLDI round will consist of an oral presentation at the conference to compete for the three top places. ## Prizes Winners of the three top places in each category receive prizes of $500 for the first place winner, $300 for the second place winner and $200 for the third place winner, respectively. The top three undergraduate and graduate winners receive an award medal and a one-year complimentary ACM student membership with a subscription to ACM’s Digital Library. ## ACM SRC Grand Finals First place winners in each category will be invited to participate in the ACM SRC Grand Finals, an on-line round of competition between first-place SRC winners from different ACM conferences held in 2017. Grand Finals will be judged by a different, ACM-appointed panel of judges. Winners of the three top Grand Finals places in each category will receive additional prizes of $500 for the first place winner, $300 for the second place winner and $200 for the third place winner, respectively. They will be also invited to the annual ACM Award Banquet along with prestigious ACM award winners, including the winner of the Turing Award. ## Eligibility criteria Current student status, either graduate or undergraduate, at the time of submission deadline. In order to advance to the poster session round, participants of the SRC must be current ACM (student) members. ## Extended Abstract Submission Details Each submission should include the student author’s name, institutional affiliation, e-mail address, and postal address; research advisor’s name; ACM student member number; category (undergraduate or graduate); research title; and an extended abstract addressing the following: The extended abstract must not exceed 1000 words and must not be longer than 2 pages. Reference lists do not count towards these limits. ## SRC Panel Xavier Martorell, BSC and UPC (chair). Tobias Grosser, ETH Zurich Sandrine Blazy, IRISA Martin Kong, RICE University Timothy Bourke, INRIA Cole Schlesinger, Barefoot Networks, Inc.

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3 Lectureships in Computer Science at University of Bristol
by Nicolas Wu 13 Mar '17

13 Mar '17

Dear all, The University of Bristol is currently advertising three lectureships that you may be interested in: 2 x Lecturer or Senior Lecturer in the Department of Computer Science [1] 1 x Lecturer or Senior Lecturer in Computer Science – Software Engineering [2] These roles would be a good fit for candidates interested in Software Engineering and Programming Languages. The University of Bristol was ranked 5th on research intensity by the Research Excellence Framework (REF 2014) [3], and in Computer Science and Informatics was ranked 4th on research intensity [4]. Bristol has had a long tradition of teaching Haskell to its first year undergraduates in their first term, and attracts strong students. If you're interested, then do consider applying and feel free to get in touch! Best wishes, Nick Links: [1] http://www.bristol.ac.uk/jobs/find/details.html?nPostingID=5674&nPostingTar… [2] http://www.bristol.ac.uk/jobs/find/details.html?nPostingID=5737&nPostingTar… [3] https://www.timeshighereducation.com/sites/default/files/Attachments/2014/1… [4] https://www.timeshighereducation.com/sites/default/files/Attachments/2014/1… Dr Nicolas Wu Department of Computer Science University of Bristol

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What is a good way to test library on several ghc's?
by Anton Kholomiov 13 Mar '17

13 Mar '17

Someone asked to add support in my lib for older version of GHC. I know that there are GHC-version specific pragmas to activate the code on specific version. I'm not quite sure how to setup the environment to use several GHCs. How do you do that kind of thing? Cheers! Anton

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Authenticated APIs in Servant
by Ivan Lazar Miljenovic 13 Mar '17

13 Mar '17

I'm needing to write and deploy a web API with authenticated endpoints. I was planning on using Servant, but it looks like there is still no in-built support for OAuth-style token-based APIs. Is that correct, or did I miss something? (That said, the actual API is of a "submit one POST and get a response back" style rather than continuing to do more requests, so a username/password basic authentication approach may also work if all else fails.) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] To [] Or Not To []
by Doug McIlroy 13 Mar '17

13 Mar '17

This stirred ancient memories in praise of wrong programs: > > * If your program sorts a list, then your program is wrong. > > This seems a very strange claim. > > The whole thing is an abuse of the word "wrong". > A program can be all of ugly, inefficient, unidiomatic, > &c &c without being WRONG. Fifty-plus years ago, when computing was 1000 times slower and cost $600/hour, it was typical for professional programmers to mediate between scientists and computers so that those expensive machines would be used efficiently. At Bell Labs, though, Dick Hamming insisted on open-shop computing because "It is more important to have the right problem done the wrong way, than to have the wrong problem done the right way." Doug

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Re: [Haskell-cafe] reading and writing Data.Ratio.Rational
by Mike Ledger 12 Mar '17

12 Mar '17

I can never seem to remember to reply-all ... On Wed, Mar 8, 2017 at 3:24 PM, Mike Ledger <eleventynine(a)gmail.com> wrote: > There is also the exact-real and numbers packages which provide exact > reals with their respective CReal types. > > On Wed, Mar 8, 2017 at 6:22 AM, Olaf Klinke <olf(a)aatal-apotheke.de> wrote: > >> Before asking on this list, I wrote my own version which did just what >> David suggested: Parse the integer before the decimal point, parse the part >> behind the decimal point and divide the latter by 10^length, then sum both >> parts. This function has type >> >> Fractional t => String -> t >> >> I thought that this was maybe very inefficient and therefore looked for >> prior art. If someone feels it is worth adding to some package, I'm happy >> to provide the code. >> >> Olaf >> >> >> > Am 06.03.2017 um 21:46 schrieb David Feuer <david.feuer(a)gmail.com>: >> > >> > I think your best bet is probably to get your hands dirty and parse it >> yourself: first grab an integer, then optionally a decimal point, etc. >> > >> > On Mar 6, 2017 3:43 PM, "Olaf Klinke" <olf(a)aatal-apotheke.de> wrote: >> > Ah, thanks! That is something to build on. >> > >> > Olaf >> > > Am 24.02.2017 um 15:35 schrieb Patrick Chilton <chpatrick(a)gmail.com>: >> > > >> > > Prelude> import Numeric >> > > Prelude Numeric> fst $ head $ readFloat "0.1234" :: Rational >> > > 617 % 5000 >> > > >> > > On Fri, Feb 24, 2017 at 1:17 PM, Olaf Klinke <olf(a)aatal-apotheke.de> >> wrote: >> > > Dear cafe, >> > > >> > > when processing text files containing numbers of the form "xxxx.yyyy" >> I used to parse them into Double using that type's Read instance. >> Obviously, even with arithmetic no more complicated than the field >> operations the result might have ugly rounding errors like 12.000000000002 >> due to the fact that numbers like 0.7 are not dyadic rationals. The math in >> my program is not complicated and the numbers are not large, so I don't >> care about Rationals having potentially huge memory footprints. >> > > >> > > So here's my question. A literal like 0.7 has type Fractional a => a, >> but the Read instance of Rational rejects the string "0.7". Must it be this >> way? Do I have to go via toRational.(read :: String -> >> Data.Scientific.Scientific)? >> > > >> > > Note that the documentation of Data.Scientific explicitly states that >> using (/) is unsafe, so I'd rather stay with the field Rational. >> > > >> > > For the output as decimal expansion, there is of course long division >> as described here [1], but I wonder whether either this exists in some >> library or there is even a more efficient solution. >> > > >> > > Any pointers are appreciated. >> > > Thanks, >> > > Olaf >> > > >> > > [1] http://stackoverflow.com/questions/30931369/how-to-convert- >> a-rational-into-a-pretty-string >> > > _______________________________________________ >> > > Haskell-Cafe mailing list >> > > To (un)subscribe, modify options or view archives go to: >> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > > Only members subscribed via the mailman list are allowed to post. >> > > >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > To (un)subscribe, modify options or view archives go to: >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > Only members subscribed via the mailman list are allowed to post. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > >

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Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 11 Mar '17

11 Mar '17

And I was merely pointing that out: That there are recommended use cases in the article, while the abstract given by the author on this list was quite negative. (Possibly it had to be, in order to gain the necessary attention.) I was not arguing against the syntactic preference Haskell gives to singly linked lists over other data structures. If Haskell was designed from scratch, what would take the place of []? Possibly Data.Sequence? Mildly complex data processing programs are somewhat special: You read in and parse one or more input files, do some transformation on the parsed data and write some results back to disk. As the article explained, the Haskell compiler will be able to optimise away the intermediate list structure (at least in simple cases). If data dependency is local, you'll be fine with data input to String rather than Text or ByteString, with map rather than fmap or foldl', and you'll be fine to use a mapM_ on a list for output. Olaf > I think you are missing the point of the article, as that is pretty much > exactly the type of situation where it recommends lists be used. > > On Mar 10, 2017 7:45 PM, "JK" <jerzy.karczmarczuk(a)unicaen.fr> wrote: > >> Hi. >> Le 11/03/2017 à 00:28, Olaf Klinke a écrit : >> >> Actually, your negative abstract turns out to be for a quite positive article. List are like iterators, that's the essence, and as long as you are iterating, it's fine to use []. The prominence of singly linked lists in Haskell has tought me to write my data processing programs in a streaming style. >> >> *Yes.* I've been teaching not just "data processing" - after all almost >> everything we program is "data processing", no?... but such concrete stuff >> as physics simulation (diff. eqs.), some other numerics (asymptotic >> expansions, etc.) signal processing (including sound generation), and I >> liked to present several examples in a *dataflow style* with plenty of >> co-recursive contraptions. Haskell lazy lists were natural, concise, and >> easy to manipulate. We enjoyed it, wrong or not. >> >> Nothing is perfect, not only mister Nobody; calling ANY approach to >> programming "wrong" is sectarian. A professional coder working on a >> concrete project may say bad words about anything he wishes, but for a >> teacher this is a pedagogical sin, and inefficient programs can be more >> inspiring than some "correct" doctrines. >> >> Jerzy Karczmarczuk

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Is this a "monad" for contravariant functors?
by Zemyla 10 Mar '17

10 Mar '17

There originally seems like there shouldn't be one, because it's impossible to, for any permutation of the arguments of (>>=), get the covariances/contravariances right on the arguments. However, based on the fact that there is a mapping: a -> ((a -> r) -> r) and what seems like an isomorphism: (a -> r) -> (((a -> r) -> r) -> r) (((a -> r) -> r) -> r) -> (a -> r) then this looks like it might be some kind of monad-like structure: class Contravariant f => Contramonad f where contrareturn :: a -> f (f a) contrabind :: (a -> f (f b)) -> f b -> f a I have no idea what the laws should be for it, though. So is this worth pursuing, or have I just stumbled down a blind alley?

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regex and Regular Expressions Libraries
by Chris Dornan 10 Mar '17

10 Mar '17

Dear Haskell Cafe, With the regex announcement, I wanted to try get other folks' perspective on what has been happening with the Haskell regular expression libraries. I have been in turn impressed at how good the engineering of the Regex packages while gobsmacked by how difficult the traditional Text.Regex API is to use. In this blog post, http://engineers.irisconnect.net/posts/2017-03-07-regex.html I rather cheekily speculate that the Haskellers perhaps have been a bit disdainful of regular expressions (not important in a language capable of doing proper parsing, etc.). What do you think? Chris

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CEF3 bindings and game dev in Haskell
by Maksymilian Owsianny 10 Mar '17

10 Mar '17

Hey I'm interested in making gamedev in haskell more viable and one thing that I believe is missing in that part of the environment is a good GUI story. So I decided to create proper bindings to cef3, you can check out what I've done so far [here][1]. In this current version I basically took the work done by Daniel Austin [here][2] split it into parts to prevent my machine from dying from lack of memory and packed it up with some nix glue for convenient usage. Now all this doesn't look like much but it sill took me much more time then I'm willing to admit in public. That's one of the eternal truths of software development, that everything always takes an order of magnitude more time then you suspect it would. Anyway, what I would like to do here is add support for all platforms. Create some nice scripts that help with automatizing building, developing and deployment of code with this library for users. Possibly write a script that automatically generates bindings from c headers to bring this lib up to speed with the current version of cef. Write some proper guides on using this lib for creating game GUI or standalone desktop application. All of that is nice and well to wish for but it is quite a lot of frankly not very fun work. If I had to choose I would rather spend my time working on either something fun or something that pays the rent, so usually at this point I stop and walk away. But this time I decided to go about it differently, and ask for community support. I have set up a BountySource campaign [here][3] so I could focus on making this happen. Alternatively, if that's your thing, I have set up a bitcoin address [4]. Depending on how well this campaign will go I'm also planing on creating a full 2D game engine... but yeah, that's in the future. So yeah, if you'd like to help me build this please support me. Thanks, Max. [1]: https://github.com/haskell-ui/cef3-raw [2]: https://github.com/fluffynukeit/bindings-cef3 [3]: https://salt.bountysource.com/teams/haskell-ui [4]: 1LskxSUyLDLpMCBfUUnqcRT8mEFMW511Eq PS. I have also created a reddit thread for this proposal here <https://www.reddit.com/r/haskell/comments/5yi1b7/cef3_bindings_and_game_dev…>.

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To [] Or Not To []
by Johannes Waldmann 09 Mar '17

09 Mar '17

Dear Cafe, I wrote a text (well, you might call it a rant) on the tragic over-use of lists in Haskell, based on my experience in programming and teaching. http://www.imn.htwk-leipzig.de/~waldmann/etc/untutorial/list-or-not-list/ * If your program accesses a list by index (with the !! operator), then your program is wrong. * If your program uses the length function, then your program is wrong. * If your program sorts a list, then your program is wrong. * If you wrote this sort function yourself, then it is doubly wrong. * The ideal use of a list is such that will be removed by the compiler. * The enlightened programmer writes list-free code with Foldable. Have fun reading, and avoiding lists from here on! - J.W.

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Parsing a bunch of strings without backtracking
by Olaf Klinke 09 Mar '17

09 Mar '17

The tree with internal nodes of type Char and leaves of type v is a representation of a finite state machine, a common tool in parsing certain grammars. I'm not sure your type Compact v captures exactly this, since a value v may be followed by more Chars down in the tree. You'd want type Compact v = [Either (More v) v] -- akin to Data.Tree.Forest data More v = More Char (Compact v) -- akin to Data.Tree.Tree What you may want to look at is a so-called trie [1,2]. In textbooks, it is common to assume that there is a charcter '$' not element of your alphabet. Then '$' is appended to each string before compressing all into a tree structure. This way, one can put both a string and a prefix of it into the same tree. A special kind of trie is the suffix trie. It holds all suffixes of a string and facilitates fast full-text search. More efficient variants are extensively used in bioinformatics [3]. Olaf [1] http://hackage.haskell.org/package/word-trie-0.3.0/docs/Data-Trie.html [2] https://en.wikipedia.org/wiki/Trie [3] http://mummer.sourceforge.net

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Haskell-related PhD studentship
by Meng Wang 09 Mar '17

09 Mar '17

Dear Haskellers, I am seeking a PhD student on Haskell-related projects, particularly in the area of bidirectional programming (lenses), property-based testing (QuickCheck) and refactoring. This would be a good opportunity for someone who is interested in applying Haskell ideas to real problems. The student will be part of the Programming Languages and Systems (PLAS) group in the School of Computing at the University of Kent. A major focus within the group is functional programming (FP), where it has had an international reputation for more than twenty five years. The group currently has 13 FTE academic staff, 4 RAs and 12 doctoral students. . The funding covers maintenance, EU student fees and research related expenses. But non-EU students are welcome to apply too. For Sep 2017 starting, the application deadline is 7 April 2017. If you are interested, please contact me at m.w.wang(a)kent.ac.uk<mailto:m.w.wang@kent.ac.uk>. More information can be found at https://www.cs.kent.ac.uk/research/studyingforaphd/phd-wang-2017.html. Best wishes, Meng Dr. Meng Wang | Lecturer in Computer Science School of Computing, University of Kent Canterbury, Kent, CT2 7NF, UK m.w.wang(a)kent.ac.uk<mailto:m.w.wang@kent.ac.uk> | ☎ +44 (0)1227 827699 www.cs.kent.ac.uk/~mw516<http://www.cs.kent.ac.uk/~mw516>

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[ANN] DataHaskell mailing list
by Marco Zocca 09 Mar '17

09 Mar '17

Dear all, DataHaskell now has a mailing list hosted on haskell.org : https://mail.haskell.org/cgi-bin/mailman/listinfo/data-haskell This is an addition to our current Gitter chatroom ; the purpose is to host longer-form archived discussions on libraries, documentation and more. With kind regards, Marco Zocca (https://github.com/ocramz)

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Re: [Haskell-cafe] formal treatment of module dependencies
by Johannes Waldmann 08 Mar '17

08 Mar '17

> specifying and solving module dependencies related: http://www.mancoosi.org/papers/ (not for Haskell modules, but for Debian-like package specs) PS: keep in mind that even the most elaborated versioned dependency mechanism is just a work-around - as long as we don't have *all* software properties in the types. Only then could we have a formal proof that a module A can use module B2 instead of module B1, etc. And, we do not "just" want correctness w.r.t. a specification, but also resource consumption. - J.W.

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reading and writing Data.Ratio.Rational
by Olaf Klinke 07 Mar '17

07 Mar '17

Dear cafe, when processing text files containing numbers of the form "xxxx.yyyy" I used to parse them into Double using that type's Read instance. Obviously, even with arithmetic no more complicated than the field operations the result might have ugly rounding errors like 12.000000000002 due to the fact that numbers like 0.7 are not dyadic rationals. The math in my program is not complicated and the numbers are not large, so I don't care about Rationals having potentially huge memory footprints. So here's my question. A literal like 0.7 has type Fractional a => a, but the Read instance of Rational rejects the string "0.7". Must it be this way? Do I have to go via toRational.(read :: String -> Data.Scientific.Scientific)? Note that the documentation of Data.Scientific explicitly states that using (/) is unsafe, so I'd rather stay with the field Rational. For the output as decimal expansion, there is of course long division as described here [1], but I wonder whether either this exists in some library or there is even a more efficient solution. Any pointers are appreciated. Thanks, Olaf [1] http://stackoverflow.com/questions/30931369/how-to-convert-a-rational-into-…

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Professor of Cyber Security at Kent
by Simon Thompson 07 Mar '17

07 Mar '17

We’re looking to appoint a professor of Cyber Security at Kent, to head up our existing group, with its Masters course and research activities. The Cyber Security group also sits alongside our Programming Languages and Systems group, and someone with strengths in PL aspects of security would fit very well. The advert for a Professor of Cyber Security is live at https://jobs.kent.ac.uk/fe/tpl_kent01.asp?s=4A515F4E5A565B1A&jobid=40739,21… <https://jobs.kent.ac.uk/fe/tpl_kent01.asp?s=4A515F4E5A565B1A&jobid=40739,21…> and post has been advertised with an associated lectureship in security, so the successful candidate will have a chance to shape that appointment too. If you’re interested, do think about applying, or get in touch with us. I’d be happy to answer emails, as would Richard Jones, head of school: cs-hos AT kent DOT ac DOT uk Simon Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk <mailto:s.j.thompson@kent.ac.uk> | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt <http://www.cs.kent.ac.uk/~sjt>

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Yesod app uses wrong port when connecting to PostgreSQL
by Geraldus 06 Mar '17

06 Mar '17

Hi folks! I'm struggling with following error: libpq: failed (could not connect to server: Connection refused Is the server running on host "localhost" (127.0.0.1) and accepting TCP/IP connections on port 5432? I see such messages in Keter's log (in app folder). The VPS where the app deployed running PostgreSQL 9.6 on port 5433. I have changed default port in: - config/settings - config/keter.yaml - /opt/keter/etc/postgres.yaml on server side None of these steps helped. I wasn't able to find other port configuration places. What else can I do?

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Maintaining fb, yesod-fb, yesod-auth-fb
by Sibi 06 Mar '17

06 Mar '17

Posting here to share my intent on taking the maintainership of fb, yesod-fb and yesod-auth-fb packages. The packages has not been updated for quite some time and are currently not building in GHC 8.0.2. Also, I use this package at my day job and we are planning to switch to GHC 8 soon. Regards, Sibi

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Blast: a simple distributed computing library
by jean-christophe mincke 06 Mar '17

06 Mar '17

Hi everyone. Over the past few months, I have written Blast, a distributed computing library, inspired by Apache Spark. It is now open source and can be found on github: https://github.com/jcmincke/Blast Regards J-C

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Mailing list request for DataHaskell
by Marco Zocca 06 Mar '17

06 Mar '17

Dear all, I write on behalf of DataHaskell [0, 1], an open source community devoted to data science, scientific computing and machine learning. We would like haskell.org to host a mailing list for our community, to archive discussions that might otherwise get lost on the Gitter chatroom [2]. With this email I'd also like to extend the invitation to join to all interested people and companies. Thank you and kind regards, Marco Zocca (https://github.com/ocramz) [0] datahaskell.org [1] github.com/DataHaskell [2] https://gitter.im/dataHaskell/Lobby

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Inlinable (>>>)
by David Sorokin 05 Mar '17

05 Mar '17

Hi Cafe, I offer to define the functions of the Control.Category module inlinable: -- | Right-to-left composition (<<<) :: Category cat => cat b c -> cat a b -> cat a c {-# INLINABLE (<<<) #-} (<<<) = (.) -- | Left-to-right composition (>>>) :: Category cat => cat a b -> cat b c -> cat a c {-# INLINABLE (>>>) #-} f >>> g = g . f Perhaps all functions from this module should be marked by this pragma as possible. I suppose that the current definition without the pragma is the cause why the execution slows down in my applications, when using monad parameters in the constraints. I noticed it yet two or three years ago, while playing with the monad transformers, but decided to write now. Best regards, David Sorokin

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Declaring Functors for Type constrained data types
by Guru Devanla 01 Mar '17

01 Mar '17

Hello All, I am working on a program that will define a bunch of tasks. Each task will have to implement certain methods as part of a type class. -- task 1 data UpdateAcctsTask = UpdateAccts -- task 2 data EmailConfig = EmaiConfig {someattrs::String} data SendEmailTask = SendEmailsTask EmailConfig -- task 3 data GeneralWriterTask a = GeneralWriterTask a Each of these tasks implement a class, Taskable. The return values are simplified for this example. class Taskable a where process :: a -> Bool can_run :: a -> Bool This works fine. I can expand on these tasks and execute them. Now, I wanted to be able to defined dependencies between these (Taskable's). I decided I could create a data type for this dependency and may be also get a FreeMonad around this structure for further processing using a graph of Tasks. But, before that I wanted to create an wrapper for these Taskables and create a functor for it as follows The first thing I did was, define a Task, which generalizes over all the above defined (and future Taskables) data Task a where Task :: (Taskable a) => a -> Task a instance Functor Task where fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b --- THIS DOES NOT WORK fmap f (Task a) = Task $ f a But, I realized that I cannot define an fmap over a type constraint. My questions are: 1. Is there any way to do this. I see there is an answer of SO. I wanted to make sure if there were any improvements to this since that answer' was posted. http://stackoverflow.com/questions/17157579/functor-instance-for-a-gadt-wit… 2. Secondly, I would like to know why this is not possible. Is it a current limitation of GHC or if there is some fundamental category theory concepts that dis-allows such declarations that I need to grok! Appreciate any help on this. Thank you!

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Re: [Haskell-cafe] Looking for maintainers or comaintainers on my Haskell projects
by sighingnow＠163.com 01 Mar '17

01 Mar '17

Hi! I’m interested in co-maintaining the Hindent project. I use Hindent extensively and I have submitted several patches to Hindent on github during the last year. I would like to contribute to improving Hindent and making it better. Thanks. Tao He

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block comments in cabal files?
by Johannes Waldmann 01 Mar '17

01 Mar '17

I totally could not understand why "stack install" suddenly failed with "ghc: unrecognised flag: -" Then it dawned on me: you cannot use {- .. -} for block comments in cabal files. (the syntax has no block comments at all?) yes, Wadler's law and all, but still. - J.

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Summer of Haskell 2017
by Niki Vazou 28 Feb '17

28 Feb '17

Dear Haskellers, We are sorry to announce that this year Haskell.org <http://haskell.org/> was not accepted for the 2017 Google Summer of Code. But, as last year, we plan to organize our own Summer of Haskell! We ask for your help to plan a successful Summer of Haskell 2017. As last year, one student will be funded directly out of Haskell.org <http://haskell.org/> general funds. The key factor in how many further students we can fund is the generosity and support of donors. Funding a single student plus overhead costs 6000 USD. We are looking for donors willing to cover half, one or more students. If you (or your organization) are interested in becoming a Summer of Haskell donor please reach out to committee(a)haskell.org. We also encourage you to make a smaller donation, as if we receive enough of small donations in total, we will combine them to fund more students. Donation information is at: https://wiki.haskell.org/Donate_to_Haskell.org More information will be forthcoming as we work out the details. Feel free to contact us at (nvazou(a)cs.umd.edu and m(a)jaspervdj.be) if want to help or if you have any ideas, questions, or concerns. Best, Niki Vazou & Jasper Van der Jeugt for the Haskell.org Committee

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[Announce] ZuriHac 2017
by Jasper Van der Jeugt 28 Feb '17

28 Feb '17

Hello all, It is our pleasure to announce ZuriHac 2017, which will take place on 9th to 11th of June, 2017. The Haskell Hackathon is an international, grassroots collaborative coding festival whose goal is to expand the community and to build and improve Haskell libraries, tools, and infrastructure. This is the already the 6th Haskell Hackathon in Zurich! This year, we are privileged to enjoy keynotes from: - Edward Kmett - Julie Moronuki - Neil Mitchell - Stephen Diehl - Simon Thompson This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! This event is open to any experience level, from beginners to gurus. We will have a dedicated beginners' track, and there will be mentors on site whom you can directly approach during the whole event with any Haskell-related question you have. We will be hosting the Hackathon in a new location [1] near the lake which provides space for up to 300 people! Experience from the previous years tells us these places might be gone quickly, so don't wait too long to register. You can find more information and register for the Hackathon here: http://zurihac.info We would also like to thank our sponsors Digital Asset [2], Google Open Source Programs Office [3] and HSR [4] for supporting this great event! Looking forward to see you there, the Zurich HaskellerZ meetup group [1]: https://zurihac.info/#venue [2]: https://digitalasset.com/careers.html [3]: https://developers.google.com/open-source/ [4]: https://www.hsr.ch/

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Re: [Haskell-cafe] Declaring Functors for Type constrained data types
by Jan Bracker 28 Feb '17

28 Feb '17

Hi, As mentioned before there are several ways of doing this. 1. Constrain the functions working with your types keeping the type itself unconstrained. 2. Write/Use a replacement for the Functor class that allows you to do this. This will usually involve associated type synonyms or functional dependencies. In my own work [1] I use associated type synonyms to do this. But there are other libraries that wrote similar replacements for the Functor class [2,3 and probably some more]. So if you want to use these you fill have to swap out the Functor (Applicative and Monad) hierarchy with another one that supports constraints. So this is definitly possible and no limitation of GHC. The standard library just does not allow it. Categorically a standard functor goes from the category Hask to Hask. A restricted or constrained functor can be seen as a functor that goes from a category C (Hask with the constraints) to Hask together with an injective functor that embeds C in a subcategory of Hask (which allows you to use it in Haskell). Best, Jan [1]: http://hackage.haskell.org/package/supermonad-0.1/docs/Control-Supermonad-C… [2]: https://hackage.haskell.org/package/rmonad-0.8.0.2/docs/Control-RMonad.html… [3]: http://hackage.haskell.org/package/constrained-monads-0.1.0.0/docs/Control-… 2017-02-28 12:00 GMT+00:00 <haskell-cafe-request(a)haskell.org>: > I've done a bit of work on a library that allows you to `fmap` say over > sets, (i.e. fmap :: Ord a, Ord b => (a -> b) -> Set a -> Set b) which is > more powerful than Monofunctor as it doesn't require the types to be the > same. > > And it will involve importing a new version of "Functor" and "fmap", > however it should be largely backwards compatible. > > However, it still needs a little bit of work to be in a state ready to be > uploaded to Hackage (and even then, it probably needs even more work) but > I'll give you a buzz when its up, hopefully in the next week or so. > > On Tue, Feb 28, 2017 at 2:01 PM, Guru Devanla <gurudev.devanla(a)gmail.com> > wrote: > > > Thanks, Will. I think adding the constraint to the functions is a good > > idea. Plus, it makes the intent clearer at the function level. > > > > > > > > On Mon, Feb 27, 2017 at 6:02 PM, Will Yager <will.yager(a)gmail.com> > wrote: > > > >> 1. You could use MonoFunctor (complicated and probably not a good idea > >> here) or just put the Taskable constraint on functions instead of on the > >> Task definition (good and easy). > >> > >> So it would be > >> > >> data Task a = Task a deriving Functor > >> > >> And then put "Taskable a" on functions that require it. > >> > >> 2. You can't do it because it doesn't really make sense. A big part of a > >> functor is that it has to be totally agnostic of what it's parametrized > >> over. Otherwise you could easily violate the functor laws. > >> > >> Good question though, I used to wonder the same thing. > >> > >> cheers, > >> Will > >> > >> On Feb 27, 2017, at 6:48 PM, Guru Devanla <gurudev.devanla(a)gmail.com> > >> wrote: > >> > >> Hello All, > >> > >> I am working on a program that will define a bunch of tasks. Each task > >> will have to implement certain methods as part of a type class. > >> > >> -- task 1 > >> data UpdateAcctsTask = UpdateAccts > >> > >> -- task 2 > >> data EmailConfig = EmaiConfig {someattrs::String} > >> data SendEmailTask = SendEmailsTask EmailConfig > >> > >> -- task 3 > >> data GeneralWriterTask a = GeneralWriterTask a > >> > >> Each of these tasks implement a class, Taskable. The return > >> values are simplified for this example. > >> > >> class Taskable a where > >> process :: a -> Bool > >> can_run :: a -> Bool > >> > >> > >> This works fine. I can expand on these tasks and execute them. > >> > >> Now, I wanted to be able to defined dependencies between these > >> (Taskable's). I decided > >> I could create a data type for this dependency and may be also get a > >> FreeMonad > >> around this structure for further processing using a graph of Tasks. > But, > >> before that I wanted > >> to create an wrapper for these Taskables and create a functor for it as > >> follows > >> > >> The first thing I did was, define a Task, which generalizes over all > >> the above defined (and future Taskables) > >> > >> data Task a where > >> Task :: (Taskable a) => a -> Task a > >> > >> > >> instance Functor Task where > >> fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b > --- > >> THIS DOES NOT WORK > >> fmap f (Task a) = Task $ f a > >> > >> > >> But, I realized that I cannot define an fmap over a type constraint. > >> > >> My questions are: > >> > >> 1. Is there any way to do this. I see there is an answer of SO. I wanted > >> to make sure if there were any improvements to this since that > answer' > >> was posted. > >> http://stackoverflow.com/questions/17157579/functor-instance > >> -for-a-gadt-with-type-constraint > >> > >> 2. Secondly, I would like to know why this is not possible. Is it a > >> current > >> limitation of GHC or if there is some fundamental category theory > >> concepts > >> that dis-allows such declarations that I need to grok! > >> > >> Appreciate any help on this. Thank you! > >> > >> _______________________________________________ > >> Haskell-Cafe mailing list > >> To (un)subscribe, modify options or view archives go to: > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >> Only members subscribed via the mailman list are allowed to post. > >> > >> > > > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > > >

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Call For Presentations: Compose 2017, New York, May 18-19
by Gershom B 27 Feb '17

27 Feb '17

Compose is a conference for typed functional programmers, focused specifically on Haskell, OCaml, F#, SML, and related technologies. It will be held in New York on Thursday and Friday, May 18-19, 2017. Registration will be open shortly. http://www.composeconference.org/2017 To get a sense of Compose, you can check out the great talks from past conferences: https://www.youtube.com/channel/UC0pEknZxL7Q1j0Ok8qImWdQ Below is our call for presentations. We recognize the deadline is tight, so feel free to submit proposals and ideas on the less-polished side. http://www.composeconference.org/2016/cfp/ *** The audience for Compose is Haskell, OCaml, F#, or SML developers who are looking to increase their skills or learn new technologies and libraries. Presentations should be aimed at teaching or introducing new ideas or tools. We are also interested in presentations aiming at taking complex concepts, such as program derivation, and putting them into productive use. However proposals on anything that you suspect our audience may find interesting are welcome. The following are some of the types of talks we would welcome: Library/Tool Talks — Exploring the uses of a powerful toolkit or library, be it for parsing, testing, data access and analysis, or anything else. Production Systems — Experience reports on deploying functional techniques in real systems; insights revealed, mistakes made, lessons learned. Theory made Practical — Just because it’s locked away in papers doesn’t mean it’s hard! Accessible lectures on classic results and why they matter to us today. Such talks can include simply introducing the principles of a field of research so as to help the audience read up on it in the future; from abstract machines to program derivation to branch-and-bound algorithms, the sky’s the limit. We also welcome proposals for more formal tutorials. Tutorials should be aimed at a smaller audience of beginner-to-novice understanding, and ideally include hands-on exercises. The due date for submissions is March 23, 2017. We will send out notice of acceptance by March 30th. We prefer that submissions be via the EasyChair website (https://easychair.org/conferences/?conf=compose2017) Please suggest a title, and describe the topic you intend to speak on. Talks can be either 30 or 45 minutes, please indicate how much time you would prefer to take. Additional information may be included on both your expertise and the interesting elements of your topic, going on what might be included in a public abstract. Furthermore, if your abstract doesn't feel "final"—don't worry! We'll work with you to polish it up. If you want to discuss your proposal(s) before submitting, or to further nail down what you intend to speak on, please feel free to contact us at info(a)composeconference.org. We're happy to work with you, even if you are a new or inexperienced speaker, to help your talk be great. *** Diversity We would like to put an emphasis on soliciting a diverse set of speakers - anything you can do to distribute information about this CFP and encourage submissions from under-represented groups would be greatly appreciated. We welcome your contributions and encourage you to apply! Best, Gershom

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ECOOP/PLDI 2017 in Barcelona / Call for Student Participation
by Tobias Grosser 27 Feb '17

27 Feb '17

ECOOP/PLDI 2017 in Barcelona / Call for Student Participation http://conf.researchr.org/home/pldi-ecoop-2017 Interested in hearing the latest about Scala, Perl, Clojure, Rust, Swift, JavaScript, and many many other languages and systems? Curious about new ideas such as gradual types, JIT compilers for dynamic languages, differential privacy or deep learning? Always wanted to talk about computer science face to face with senior researchers and practitioners? Let’s come to share, challenge and complement your views! Join us in Barcelona from Sunday 18 to Friday 23 June 2017 The ECOOP/PLDI conferences offers support to students and professionals interested in attending. To apply: application form # Bachelor, Master, and early Doctoral Students The ECOOP Summer School provides an easy introduction to the world of research in programming languages. No background is required other than an interest in languages technologies. Info: http://2017.ecoop.org/track/ecoop-2017-Summer-School # Doctoral Students The Doctoral Symposium is designed to provide a forum for PhD students at any stage in their research to present their topic and get detailed feedback and advice. The main objectives of this event are: to allow PhD students to practise writing clearly and to communicate and present their research effectively to receive constructive feedback from other researchers and peers to offer opportunities to form research collaborations to contribute to the conference goals through interaction with other researchers at the main conference Info: http://2017.ecoop.org/track/ecoop-2017-Doctoral-Symposium # All Students Register to be Student Volunteers, help with the organization of the conference, get a free registration and a ticket for one of the banquets. This year PLDI and ECOOP are co-located. Student Volunteers can attend events from both conferences and might be working for either. Preference can be stated in the application form. Info: http://2017.ecoop.org/track/ecoop-2017-Student-Volunteers#About

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Re: [Haskell-cafe] What does "1 = 2" mean in Haskell?
by Harendra Kumar 27 Feb '17

27 Feb '17

CCing the list. I guess you intended to cc but forgot. On 24 February 2017 at 09:27, <ok(a)cs.otago.ac.nz> wrote: > In Erlang, the equivalent of a let fails. > 1> 1=2. > ** exception error: no match of right hand side value 2 > > In SML, the equivalent of a let fails. > - val 1 = 1; > - val 1 = 2; > > uncaught exception Bind [nonexhaustive binding failure] > raised at: stdIn:2.5-2.10 > > The problem is not that let 1 = 2 ... is *legal* but that > - the compiler is *silent* about it > - the runtime is *silent* about it. > Compiling the little program > > main = let 1 = 2 in print "hi" > > I expected that the compiler would be silent but that > there would be some sort of "matched failed" error at > run time. Silly me. > > > The thing is, it is not just bindings that bind no variables > that act as if they were not there. > > main = let [x] = [1,2] in print "hi" > > also compiles silently and runs without error. Change it to > > main = let [x] = [1,2] in print ("hi" ++ show x) > > and you get a runtime error > > <object>: <source>: Irrefutable pattern failed for pattern [x]. > > I wish the compiler would report an error something like > > "<location>: possibly failing match deleted > because it binds no live variables" > > > >

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Munich Haskell Meeting, 2017-02-28 @ 19:30
by Christian Neukirchen 26 Feb '17

26 Feb '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, Feburary 28 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html The location is carnival-free. If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-feb-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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What does "1 = 2" mean in Haskell?
by Harendra Kumar 25 Feb '17

25 Feb '17

Kids have this amazing ability to break any toy in minutes. I gave my seven year old daughter ghci to play with and in a little while she said it is broken: >> let 1 = 2 >> 1 1 >> Earlier, I had explained to her about symbols and assigning values to symbols, and I said numbers are not symbols. But when she came up with this I could not explain what's going on. How can "1 = 2" be a valid equation? Am I missing something fundamental here, or it is just broken? -harendra

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Postdoc at Inria/Irisa on translation validation from LiquidHaskell
by Ranjit Jhala 25 Feb '17

25 Feb '17

Hi all, Inria Project-team TEA is seeking a talented PhD with demonstrated experience in theory and implementation of refinement types in programming and automated verification and proof of programs. The aim of our post-doctoral project is to design a certified code generator from Liquid Haskell to verified, imperative and functional, compiler suites (CompCert, CakeML). We will start by initially focusing on first-order functions in the absence of dynamic memory management. In the spirit of Pnueli's translation validation principle, we will prove code generated from the code generator to obey program properties inferred from source programs, using refinement reflection. We will study extensions to hardware synthesis as well as dynamic memory management, higher-order functions, etc. For context, details, and application, please visit: https://www.inria.fr/ institut/recrutement-metiers/offres/post-doctorat/sejours- post-doctoraux/(view)/details.html?id=PGTFK026203F3VBQB6G68LONZ&LOV5 =4508&nPostingID=11123&nPostingTargetID=17708

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[Haskell-Cafe] Different memory profile in cabal project.
by Thomas Vanhelden 24 Feb '17

24 Feb '17

I have a (dummy) program that reads the contents of a file into a ByteString and then converts the ByteString to a Vector of Floats. To test the laziness of this program, I’ve taken a slice of the the Vector and printed it to the standard output. The program looks like this: import qualified Data.Vector.Unboxed as V import qualified Data.ByteString as BS import Data.Word import System.Environment import GHC.Int main = do [file] <- getArgs samples <- getSamplesFromFile file let slice = V.slice 0 50000 samples print slice getSamplesFromFile = fmap toVector . BS.readFile toVector :: BS.ByteString -> V.Vector Float toVector bs = vgenerate (fromIntegral (BS.length bs `div` 3)) $ \i -> myToFloat [BS.index bs (3*i), BS.index bs (3*i+1), BS.index bs (3*i+2)] where myToFloat :: [Word8] -> Float myToFloat = sum . map fromIntegral vgenerate n f = V.generate n (f . fromIntegral) So I compile this program and generate a memory profile via: ghc Main.hs -O2 -rtsopts -prof ./Main debug48.wav +RTS -hy hp2ps -e8in -c Main.hp ps2pdf Main.ps The file debug48.wav is a 12.9MB file. This is the result: If I now create a cabal project and add the exact same program as the main file by: mkdir testing cd testing cabal init add the program’s code to src/Main.hs add bytestring and vector to the build dependencies (in testing.cabal) add -O2 -rtsopts -prof to the ghc-options (in testing.cabal) cabal install testing debug48.wav +RTS -hy hp2ps -e8in -c testing.hp ps2pdf testing.ps This is the result: How can there be such a big difference in memory usage, just by the code being part of a cabal project?

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Working at Chordify? Working with Music. Coding in Haskell.
by Bas de Haas 24 Feb '17

24 Feb '17

Dear Haskellers, Chordify is hiring! Chordify is a young and fast growing music e-learning platform that helps musicians to play their favorite music. We automatically analyse the chords of a piece of music and display them in an intuitive player. Try it yourself at: https://chordify.net/ . The cool thing is: our backend has been written almost exclusively in Haskell. We hope to broaden our team with a researcher and a developer. We are looking for people who are pro-active, independent, and creative to improve Chordify. * We offer a digital signal processing researcher position in which you can prototype, evaluate, and deploy new algorithms that form the core of Chordify. * We also offer a (front-end) developer position in which you can improve the Chordify user experience, launch new ideas, write Haskell, and have thousands of users use your code every day. If you are interested in working at Chordify, you can find the details here: https://chordify.homerun.co/ All the best, Bas de Haas

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Extensions to Haskell 98 in the GHC Prelude?
by Dylan Thurston 24 Feb '17

24 Feb '17

Pardon me if this is a stupid/frequently asked question... The GHC Prelude has some extensions to the Prelude in Haskell 98 or Haskell 2010. For instance, it has the Applicative type class, whose bindings overlap with some old code of mine. Where are these non-standard extensions documented? The Applicative class is documented at https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.9.0.0/… but it's not picked out as something non-standard. I didn't find it documented where I would have expected in the GHC docs at https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#h… nor did I see a way to get a standards-compliant prelude? In this particular case I'll probably rework my code, as Applicative seems like a reasonable core addition. (I am a little annoyed that it uses so many short infix operators; I was using (*>).) I'm just surprised not to see it documented anywhere. Thanks, Dylan Thurston

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Read my thesis draft on Backpack!
by Edward Z. Yang 24 Feb '17

24 Feb '17

Hello all, I have finished a draft of my thesis on Backpack, a new module system for Haskell (building on top of the work of Scott Kilpatrick et al). This thesis also doubles as the definitive manual on Backpack, which will be released with the upcoming GHC 8.2. I've posted a copy of the draft here: https://github.com/ezyang/thesis/releases and I would love it if you took a look at it, gave it a read perhaps, and told me about anything that was unclear or not well explained. You can file an issue on the GitHub, or shoot me an email or tweet. Looking forward to your comments. Thanks, Edward

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ghc on xeon phi?
by Johannes Waldmann 23 Feb '17

23 Feb '17

Dear Cafe - would a program compiled by ghc, or ghc itself, run as-is on Intel Xeon Phi (KNL)? I found this reference http://stackoverflow.com/questions/22253311/running-haskell-on-xeon-phi but it seems to be about the pre-KNL version. Thanks - J.

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Trends in Functional Programming in Education ITFPIE) 2017, call for papers
by Simon Thompson 23 Feb '17

23 Feb '17

TFPIE 2017 Trends in Functional Programming in Education, 2017 The sixth workshop on Trends in Functional Programming in Education, 2017, which is to be held on the Canterbury campus of the University of Kent on Thursday, 22 June, following the 2017 TFP meeting on 19–21 June. TFPIE workshops have previously been held in St Andrews, Scotland (2012), Provo Utah, USA (2013), Soesterberg, The Netherlands (2014), and Sophia-Antipolis, France (2015), College Park, USA (2016). 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 2017 will screen submissions to ensure that all presentations are within scope and are of interest to participants. After the workshop, presenters will be invited to submit revised versions of their articles for publication in the journal Electronic Proceedings in Theoretical Computer Science (EPTCS). Call for papers TFPIE 2017 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 - The pedagogy of teaching FP - FP and e-learning: MOOCs, automated assessment etc. - Best Lectures – more details below In addition to papers, 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. Submission 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. Papers and abstracts can be submitted via easychair at the following link: https://easychair.org/conferences/?conf=tfpie2017 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 rejected for presentation and extended abstracts will not be formally reviewed by the PC. Programme committee Dr Laura Castro, University of A Coruña Prof Ralf Lämmel, University of Koblenz-Landau Dr Elena Machkasova, University of Minnesota, Morris Prof Michel Mauny, Inria, Paris Dr Jeremy Singer, University of Glasgow Prof Simon Thompson, University of Kent (chair) Important dates Submissions of draft papers: 10 May, 2017 Notification: 17 May, 2017 Registration: 11 June, 2017 Workshop: 22 June 2017 Submission for formal review: 18 August, 2017 Notification of acceptance: 6 October, 2017 Camera ready paper: 3 November, 2017 Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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-Werror -Woverlapping-patterns -fglasgow-exts ?
by Johannes Waldmann 23 Feb '17

23 Feb '17

Dear Cafe, I want to chase overlapping patterns in a larger code base. So I use -Werror -Woverlapping-patterns . But the code also needs -fglasgow-exts and this makes compilation stop right at the beginning, with "warning: -fglasgow-exts is deprecated: ..." which will be turned into an error because of -Werror -Wno-warnings-deprecations does not help, apparently that warning is not a deprecation. Is there a way to get -Werror for specific warnings? - J.

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Size-indexed monoids
by Kosyrev Serge 23 Feb '17

23 Feb '17

Good day! What is the proper type class for stronger-typed (size-indexed) monoids: - that is, monoids carrying their "size" in the type - preferably as GHC.TypeLits.Nat - preferably on Hackage ? I'm quite prepared to the idea that a monoid is an entirely wrong abstraction, from a category-theoretic standpoint, so I would gladly learn of a better one : -) Use case: > {-# LANGUAGE DataKinds, GADTs, KindSignatures, TypeOperators, UnicodeSyntax, StandaloneDeriving #-} > > module Understanding.Types where > > import GHC.TypeLits > > data T (depth ∷ Nat) p where > TZ ∷ T 0 p > TS ∷ (Show p, CmpNat (m + 1) n ~ EQ) ⇒ > { payload ∷ p > , next ∷ T m a } → T n a > deriving instance Show p ⇒ Show (T d p) > > instance Monoid (T d p) where > mempty = TZ > mappend TZ TZ = TZ > mappend TZ t@(TS _ _) = t > mappend t@(TS _ _) TZ = t > mappend tl@(TS pl nl) tr = TS pl $ mappend nl tr As it is, even the mempty case rejects such a blatant violation of polymorphism, since `T 0 p` cannot unify with `T n p`. So, ideally (I think), I would like something like this: > class TypedMonoid a where > tmempty ∷ a 0 > tmappend ∷ a n → a m → a (n + m) -- с уважениeм / respectfully, Косырев Сергей -- “Most deadly errors arise from obsolete assumptions.” -- Frank Herbert, Children of Dune

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CFP: International Workshop on OCL and Textual Modeling (OCL 2017)
by Achim D. Brucker 22 Feb '17

22 Feb '17

(Apologies for duplicates) CALL FOR PAPERS 17th International Workshop on OCL and Textual Modeling Co-located with STAF 2017 SOFTWARE TECHNOLOGIES: APPLICATIONS AND FOUNDATIONS July 20, 2017, Marburg, Germany http://oclworkshop.github.io Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but they have weaknesses: for example, detailed visual representations bear the risk of becoming overcrowded faster than textual models and some of the visual features lack the level of precision required to create complete and unambiguous specifications. These weaknesses of graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is to create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. ## Topics of interest Topics of interest include (but are not limited to): - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for: - validation, verification, and testing, - model transformation and code generation, - meta-modeling and DSLs, and - query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports: - usage of textual modeling languages and tools in complex domains, - usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing applications and case studies of textual modeling as well as test suites and benchmark collections for evaluating textual modeling tools. ## Venue This workshop will be organized as a part of STAF 2017 Conferenze in Marburg, Germany. It was previously organized as part of the MODELS conference. Similar to its predecessors , the workshop addresses both people from academia and industry . The aim is to provide a forum for addressing integration of OCL and other textual modeling languages , as well as tools for textual modeling , and for disseminating good practice and discussing the new requirements for textual modeling . ## Workshop Format The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. ## Submissions Two types of papers will be considered: * Short contributions (between 6 and 8 pages) describing new ideas, innovative tools or position papers. * Full papers (between 12 and 16 pages). in LNCS format. Submissions should be uploaded to [EasyChair](https://easychair.org/conferences/?conf=ocl17). The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a post-conference edition of [CEUR](http://www.ceur-ws.org). ## Important Dates - Submission of papers: April 28, 2017 - Notification: May 25, 2017 - Workshop date: July 20, 2017 -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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Takeover of http-client-lens
by Simon Hafner 22 Feb '17

22 Feb '17

Hello I wanted to take over http-client-lens. The corresponding email address is out of service: > 550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. https://github.com/reinh/http-client-lens/issues/2 Posting here in another attempt to reach the original author. Cheers, Simon

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ghc on xeon phi?
by Dominic Steinitz 22 Feb '17

22 Feb '17

> Hi Levent, > >> The whole point of the Xeon-phi is the availability of >> large-vector sized floating-point support and many-many cores. > > Sure, that's what I'm contemplating - use the many options > of writing parallel and concurrent Haskell programs. > > So, GHC's RTS should "just work"? > > I was hoping someone already had actually seen this > on their machine. > > - J. I recall we thought about using a Xeon phi for our application but abandoned the attempt for reasons I cannot recall. I’ll see if I can find out. But maybe you would better using something like accelerate-llvm? I think you wrote your application was numerical. BTW I am currently adding a small improvement to ghc floating point code generation but I am beginning to think it’s going to be difficult to get ghc to generate really good numerical / floating point code and I am experimenting with accelerate-llvm. Dominic

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A question about GHC test coverage
by Olaf Klinke 22 Feb '17

22 Feb '17

You should not have to write tests for functions you did not define. Correct me if I'm wrong, but any property of max can be derived from the properties of <=. Sadly, the laws for Ord are not stated in the documentation of Data.Ord. (Because there is no consensus?) They are: Reflexive: if x == y then x <= y Transitive: if x <= y and y <= z then x <= z Antisymmetric: if x <= y and y <= x then x == y Olaf

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What is this applicative functor?
by Joachim Breitner 20 Feb '17

20 Feb '17

Hi, I recently wrote this applicative functor: data OneStep a = OneStep a [a] instance Functor OneStep where fmap f (OneStep o s) = OneStep (f o) (map f s) instance Applicative OneStep where pure x = OneStep x [] OneStep f fs <*> OneStep x xs = OneStep (f x) (map ($x) fs ++ map f xs) takeOneStep :: OneStep t -> [t] takeOneStep (OneStep _ xs) = xs This was useful in the context of writing a shrink for QuickCheck, as discussed at http://stackoverflow.com/a/41944525/946226. Now I wonder: Does this functor have a proper name? Does it already exist in the libraries somewhere? Should it? Greetings, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata(a)joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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