March 2017 - Haskell-Cafe

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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