December 2016 - Haskell-Cafe

Re: [Haskell-cafe] Work on mail.haskell.org beginning, please report any problems
by Viktor Dukhovni 23 Dec '16

23 Dec '16

> On Dec 22, 2016, at 11:09 PM, John Wiegley <jwiegley(a)gmail.com> wrote: > >> If available as a package for your O/S, might as well use 3.1 > > I'll consider that a second migration, after the dust settles this time > around. I'd need to gain experience with 3.x first on my own server, before > being comfortable enough to do that. The 2.11-based setup I've been running > for a month now on another server. Wietse maintains a strong commitment to backwards-compatibility. While Postfix 3.0 (could equally have been 2.12) brings in new features for distribution maintainers, with dynamically-loadable table drivers on multiple platforms it remains compatible with 2.x releases for users. Your 2.11 configuration will work with 3.1, you may just need to install additional table driver packages, if your O/S maintainer has just started delivering Postfix as multiple components. > >>> - [X] Enable postscreen for pre-queue RBL filtering > >> This makes it possible to combine multiple lower-weight RBLs, that >> individually are not sufficient to reject mail, but you should still use the >> SpamHaus zen RBL in smtpd(8). > > Why is it helpful to run it in smtpd as well? I'm already using: > > postscreen_dnsbl_sites = zen.spamhaus.org*2 ... This is because the TTLs on postscreen whitelists are rather longer than RBL TTLs. If an IP address is added to an RBL, you'll want to refuse access in a timely manner. -- Viktor.

1 0

RebindableSyntax and fail in GHC 8
by Chris Smith 21 Dec '16

21 Dec '16

Hey everyone, Can someone explain to me by GHC 8 causes list comprehension bindings to desugar to use 'fail', when RebindableSyntax is enabled in GHC 8? Note that I do not have monad comprehensions enabled. If I define fail, everything works (apparently regardless of what I define fail to do?!?), but I'd like to understand why it's necessary. Thanks, Chris

3 4

emit custom signal from gtk3 haskell side
by Ruben Astudillo 21 Dec '16

21 Dec '16

Hi all I implement the following custom signal in my code import qualified System.Glib.Signals as Sig updateSignal :: ButtonClass self => Sig.Signal self (IO ()) updateSignal = Sig.Signal $ Sig.connectGeneric "updateSignal" Such, will be called inside of "model" phase of the app. But looking on the gtk3 documentation (and grepping also) I can't find a equivalent of `gtk_signal_emit_by_name`. I don't believe this omission is accidental. Can Anybody guide where to look at? -- -- Ruben

1 0

We're recruiting … why not join us?
by Simon Thompson 21 Dec '16

21 Dec '16

We’re recruiting, and if you’re interested in programming languages, types, logics, or more practical aspects like refactoring and other tools, we’d really welcome an application from you. Simon T. Lecturer in Computing (2 posts) University of Kent - School of Computing We are seeking to appoint two Lecturers in Computing (the equivalent of tenure track, assistant professors), based at our Canterbury campus. We are particularly interested in appointments in Security, or Programming Languages but are happy to consider excellent candidates in other areas that would complement or enhance the School’s existing research strengths. Job description https://www11.i-grasp.com/fe/wrapdev_SendFile.asp?key=49170934&c=3362126282… Additional Information https://www11.i-grasp.com/fe/wrapdev_SendFile.asp?key=49170934&c=3362126282… Please apply at https://www11.i-grasp.com/fe/tpl_kent01.asp?newms=jj&id=40606&aid=14243 Closing date for applications: 6 February 2017 The School of Computing [1] is a welcoming and diverse environment whose commitment to gender equality has been recognised with a Bronze Athena SWAN [2] award. We are keen to enhance the balanced, inclusive and diverse nature of the community within our School and would particularly encourage female candidates to apply for these posts. We are committed to delivering high quality research and education. Our research is focused on five broad research areas: Programming Languages and Systems; Computer Security; Computational Intelligence; Data Science; and Computing Education. Full details can be found at: https://www.cs.kent.ac.uk/research. The 2014 Research Excellence Framework (REF) ranked our school 12th for research intensity [3], and we are well placed to improve on this in the next REF. The University of Kent has campuses in Canterbury and Medway, and specialist postgraduate centres in Athens, Brussels, Paris and Rome. It was ranked 16th in the UK in The Guardian University Guide 2016. It ranked 4th for overall student satisfaction (NSS, 2016) and 17th for the intensity of its excellent research (REF 2014). It has 20,000 students and 1,872 academic, teaching and research staff. Overlooking the city centre, and with 125 nationalities represented, the Canterbury campus has a very cosmopolitan feel. Canterbury is a small city that retains parts of its medieval walls. Famous for its heritage (Canterbury Cathedral; Chaucer’s Tales; etc), Canterbury is a vibrant community whose culture and leisure facilities are enhanced by hosting three universities. The city and surrounding region combines an attractive and affordable environment, good schools, and fast transport links to London and mainland Europe. Links: [1] https://cs.kent.ac.uk/ [2] http://www.ecu.ac.uk/equality-charters/athena-swan/ [3] https://www.timeshighereducation.com/sites/default/files/Attachments/2014/1… 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

1 1

ANNOUNCE: pdf-slave-1.0 tool for PDF generation from haskintex templates
by NCrashed . 21 Dec '16

21 Dec '16

I am glad to announce first release of pdf-slave, a tool for generation of PDF documents from LaTeX templates with embedded Haskell. Hackage: http://hackage.haskell.org/package/pdf-slave Github: https://github.com/NCrashed/pdf-slave Description ======= In many real-world systems we need to produce reports, cheque and other documents to users. One possible solution is rendering LaTeX files, but it is not flexible enough as you need to solve burden with generation of LaTeX file from varying inputs, manage static files like images and handling dependencies between pieces of document. The new tool takes the burden away. Now all static files and generation code is located within a single template project, that describes how to build your PDF document in a declarative way. Common work flow with pdf-slave: * Develop template from several LaTeX files with embedded Haskell (thanks to haskintex tool) * Pack the template project into bundle all-in file. * Distribute the bundle (for instance, to your service server). * Call pdf-slave with bundle and inputs in JSON file to generate new PDF document. Status ==== pdf-slave is designed as part of commercial Haskell system. The tool is very young and need time approval, although it already has all needed features one may need from such tool. Special thanks to hint authors and Daniel Díaz, developer of awesome HaTex and haskintex, without these packages pdf-slave wouldn't be possible! Reports, feature request, PR are always welcome! Anton Gushcha aka NCrashed

1 0

stack on Windows and C Preprocessor
by Olaf Klinke 20 Dec '16

20 Dec '16

Dear Cafe, not sure who the culprit is, thus trying on this list: My project builds fine with stack on Linux. I installed the latest (1.3.0 x86_64) stack on a Windows box and ran 'stack.exe setup' inside my project's directory. The build fails at the depencency HaXml-1.25.3 with a lexer error on line 34 of this file: http://hackage.haskell.org/package/HaXml-1.25.3/docs/src/Text-XML-HaXml-Wra… lexical error in string/character literal at character '\r' It seems the line (VERSION) is processed by the C preprocessor which somehow messes it up. Unfortunately, stack deletes its temporary files after running into the error, which deprives me of handles to further debug this. Does anyone have an idea how to continue from here? Thanks, Olaf

3 2

Re: [Haskell-cafe] Foo.Bar.hs filenames poll
by Dimitri DeFigueiredo 20 Dec '16

20 Dec '16

I believe it is a mistake to tie the module structure of a software system to a file structure on a hierarchical file system. I see those too structures as different entities. Why can't I coalesce two tiny modules into a single file? I also don't like having to capitalize my file names (Foo.hs instead of foo.hs) in linux, that seems rather ugly. It see this "ugliness" as a symptom of the "arbitrarily forced link" between the module structure and the file structure. Obviously, some conventions should apply, but a one-to-one correspondence seems too strict. Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

6 5

memoizing with a default
by Michael George 20 Dec '16

20 Dec '16

I've come across a slight variant of memoization and I'm curious if either there is a better way to think about (I have yet to reach Haskell Enlightement), or if it's been thought about before. I'm writing a program that does DFA minimization, as described here: https://www.tutorialspoint.com/automata_theory/dfa_minimization.htm. For me, the simplest way to understand the algorithm is we form an equivalence relation, where two states are different if either one is final and the other isn't, or if there are transitions from them to states that are different. different a b = isFinal a && not (isFinal b) || isFinal b && not (isFinal a) || exists chars (\c -> different (step a c) (step b c)) This definition diverges; it never decides that two states are the same. The standard algorithm builds a table, starting with the assumption that all states are the same, and iteratively applying this definition until it converges. I'm thinking of that algorithm as a memoized version of the implementation of different above. But it's not quite: different = memoFix2 different' where different' different'' a b = isFinal a && not (isFinal b) || isFinal b && not (isFinal a) || exists chars (\c -> different'' (step a c) (step b c)) for the same reason; it diverges. But with memoization I can detect the diversion by putting a marker in the memo table. If the computation makes a recursive call on an input that is started but not computed, then (in this case) I want to return false. In general, I think what I want is memoFixWithDefault :: (a -> b) -> ((a->b) -> (a -> b)) -> (a -> b) Where (memoFixWithDefault default f) is like (memoFix f) except that if (memoFix f a) would loop, then (memoFixWithDefault default f a) = default a. I suspect there's a nice denotational way of describing this, but my denotational semantics is rusty (maybe it's the least fixed point of f that is greater than g or something, but that's not it). I suspect that there needs to be a nice relationship between f and g for memoFixWithDefault f g to be sensible. I also suspect that there's a cute 3-line implementation like all the other implementations of memoize out there, but I don't see it. I also suspect that depth-first search has a cute implementation using this combinator. So I was wondering if other people had thoughts or references before I went off and thought about it. Thanks! -M

6 5

Foo.Bar.hs filenames poll
by Christopher Done 19 Dec '16

19 Dec '16

Short version: here is the poll < https://docs.google.com/forms/d/e/1FAIpQLSdniOfoaX7xflgdWRnjVQ6_VLtk1oxA00S… > I noticed recently that Foo.Bar.hs is supported by GHC. I had always assumed it wasn't because people always use directories. I've never liked having separate directories for each level of hierarchy. It's easier to just list a list of files and script (e.g. even copying a file X.hs to Y.hs is a bummer). When opening them on GitHub you have to click through to get a complete picture of a project. Other languages do and don't do this. Lispers, for example, don't. How do other Haskellers feel about it? Would it mess with anybody's tooling or mojo if I switched to that style in my packages? For one I know that Stack (my own implementation), actually assumes hierarchical filenames. So I'd have to patch that to implement this. E.g. > Unable to find a known candidate for the Cabal entry "HIndent.Types", but did find: HIndent.Types.hs. If you are using a custom preprocessor for this module with its own file extension, consider adding the file(s) to your .cabal under extra-source-files. I suppose the real question is, as a language standard and a community preference, should this be considered a bug? Should people be free to use X.Y.hs or X/Y.hs styles? Ciao!

9 9

ANN: New Haskell.org Committee Members
by Gershom B 19 Dec '16

19 Dec '16

Following the self-nomination period and discussion, the Haskell.org committee has selected new members: Full term: * Mitsutoshi Aoe * Jasper Van der Jeugt * Niki Vazou Short term: * Alan Zimmerman As per the rules of the committee, this discussion was held among the current members of the committee, and the outgoing members of the committee who were not seeking reappointment. Thank you to all the many candidates who submitted a self-nomination. We recieved many strong nominations, from a broad range of geographies and backgrounds. The choice amongst them was not easy, and we would encourage all those who nominated themselves to consider self-nominating again in the future. The outgoing members are: Adam Foltzer, Edward Kmett, Andres Loeh, and Nicolas Wu. Thanks all for your service! These are tough shoes to fill, but I have confidence that our incoming members will more than rise to the occasion. Cheers, Gershom

1 0