- Haskell-Cafe - Haskell.org

runhaskell flags: What is going on?
by Niklas Hambüchen 05 May '13

05 May '13

> runhaskell -fno-warn-unused-matches Myfile.hs [some compile error] > runhaskell -fno-warn-unused-matches Myfile.hs [no output whatsoever but exit code 127] > runhaskell -asdf Myfile.hs ghc: unrecognised flags: -asdf > runhaskell -fasdf Myfile.hs [no output whatsoever but exit code 127] Not sure if that's how it should work or missing error reporting? Thanks Niklas

3 4

ANNOUNCE: grid 5.0
by Amy de Buitléir 05 May '13

05 May '13

I'm happy to announce a new major release of the grid package: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) WHAT'S NEW: * Octagonal grids (just the thing for tiling that hyperbolic bathroom floor!) * Want to use the square grid, but need diagonal movement too? Try the octagonal grid. See the userguide for more information. * Updated the userguide to reflect the changes. ABOUT GRID: Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, hexagonal, and octagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome.

1 0

Backward compatibility
by Adrian May 04 May '13

04 May '13

Hi All, Please don't interpret this as a rant: I'm just feeling a bit disappointed about probably having to give up on Haskell. Let's face it: this decision to change the default syntax in GHC7 means that right now Haskell looks about as stable as Ruby on Rails. I just tried to use Flippi. It broke because of the syntax change so I tried WASH. I couldn't even install it because of the syntax change. I persisted for a while but gave up because getPackageId doesn't exist in any form at all anymore. This was only the install script: what would WASH itself have in store for me to get my brain around? What are my choices here: 1) Revert to GHC6 or put pragmas and compiler switches everywhere, switch 2010 off globally with cabal or even make an alias of ghc: That means I'll gradually clash with people who decide ... 2) Convert all my code and a lot of other peoples' to the new syntax, thereby exacerbating the problem that ruled out 1. Either way, we're looking at a long period during which a large portion of the libraries will be incompatible with the other portion, and nobody will know which style to write. I don't know if or when WASH or any other library will convert, or even if I'd prefer that to happen sooner or later, because that would depend on when other libraries do and how I'd worked around it in the meantime. Altogether that means I can't sensibly decide to rely on any library, so I can't use Haskell. I'll just have to go back to fumbling around in XSL, PHP and the like. Is Haskell 2010 really so much better that it justifies this? I just saw that movie "The Words": the moral of the story is that you shouldn't try to change your mistakes. ... Apparently it's not only 2010. I now find that buildng the Haskell Platform wants GHC 7.4.2, not 7.4.1 because of the line "import Prelude" (if I remember rightly,) and even when I follow the rules precisely I still get several different deprecation warnings. The prelude is not exactly obscure. If you deprecate that you've broken everything. Is it really impossible to keep such a basic mantra meaningful from one minor version to the next? Java was fond of deprecating things in the early days, but when they said "deprecated" they didn't mean "switched off", let alone that it would lead to a syntax error. They just meant "not trendy anymore." It's a very common illusion to believe that the central thing in your life is also the central thing in everybody else's. That's why things like Norton take over your whole machine: those guys believe that the only reason you bought the computer was to fight viruses, because that's what most of the machines in their office were bought for. There seems to be something similar going on in the way Haskellers are expected to update all their code whenever GHC decide to issue an update. But in reality we have jobs of our own. I'd like to choose Haskell over XSL because I think it'll enable me to write web sites more efficiently, not because I want to forget all about my job and savour the brilliance of the latest Haskell version. But in reality I'm just sitting here waiting for the Platform to compile just in case it's the Ubuntu package's fault, but I know it won't help. I'll just get other problems instead. Reality is that the whole ecosytem is in disarray because of this lack of respect for backward compatibility. At least Rails can plead that it's relatively new, but Haskell has been around for over 20 years. I understand that progress has to be made, and it would be nice if people did just update all their code quickly so you could switch off old stuff and move on. But it's not hard to survey the code that's out there and see how much stuff you'd be breaking if you did. If it's not a lot, then switching it off to wake them up would be an acceptable compromise. But it looks to me as if a lot of very important stuff is still failing on the GHC from November 2010, so clearly things are going too fast. Adding new stuff is great, and sometimes the new stuff clashes with the old stuff. But how much of that deprecated stuff really *needed* to be switched off, and couldn't the new stuff have been designed so as not to force that? In principle this is the best language on the planet, but with all these version gotchas I don't know that I can use it anymore. What a tragedy. I can't even think of a suggestion as to how Haskell should try to get out of this mess now. Adrian.

30 80

BayHac '13
by Mark Lentczner 04 May '13

04 May '13

Please join us for a weekend of Haskell hacking: *BayHac '13* *May 17th ~ 19th, 2013* *Hacker Dojo* *Mountain View, CA* Full details on the Haskell Wiki: BayHac '13<http://www.haskell.org/haskellwiki/BayHac2013> - Mark

2 2

Lambda Calculus question on equivalence
by Ian Price 04 May '13

04 May '13

Hi, I know this isn't perhaps the best forum for this, but maybe you can give me some pointers. Earlier today I was thinking about De Bruijn Indices, and they have the property that two lambda terms that are alpha-equivalent, are expressed in the same way, and I got to wondering if it was possible to find a useful notion of function equality, such that it would be equivalent to structural equality (aside from just defining it this way), though obviously we cannot do this in general. So the question I came up with was: Can two normalised (i.e. no subterm can be beta or eta reduced) lambda terms be "observationally equivalent", but not alpha equivalent? By observationally equivalent, I mean A and B are observationally equivalent if for all lambda terms L: (L A) is equivalent to (L B) and (A L) is equivalent to (B L). The definition is admittedly circular, but I hope it conveys enough to understand what I'm after. My intuition is no, but I am not sure how to prove it, and it seems to me this sort of question has likely been answered before. Cheers -- Ian Price -- shift-reset.com "Programming is like pinball. The reward for doing it well is the opportunity to do it again" - from "The Wizardy Compiled"

7 13

A three page web application in a tweet (in Haskell)
by Alberto G. Corona 03 May '13

03 May '13

Hi, Just to share my excitement with you: https://twitter.com/AGoCorona/status/329648864082677760 See the link in the tweet for an explanation Best Regards, -- Alberto.

1 0

Cabal / cabal-install: special installation
by Christopher Howard 03 May '13

03 May '13

Hi. This question dovetails off my previous thread. I described how I got ghc-7.6.3 installed from source onto an old RHEL5 machine. Naturally, I want to get cabal-install installed and start building great Hackage software. However, I have this quirk: I installed GHC to a special directory using ./configure --prefix=/specialdirectory/local. It is in my path, and works just fine. But when I try to build cabal-install, it claims it can't find Base, Cabal, and a few other things I know are installed. I know they are installed because I can see where they are installed and I can import stuff from them within GHCI. The result is the same whether I use the boot script, or whether I use runhaskell Setup configure (with or without a --prefix option). Is there another environment variable I need to set or something like that? (I need to emphasize that I do not want to install anything to $HOME, because my $HOME directory is shared across multiple systems.) -- frigidcode.com

1 0

Markdown extension for Haddock as a GSoC project
by Mateusz Kowalczyk 03 May '13

03 May '13

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Greetings, In light of fairly recent discussion, on this mailing list, I decided to investigate the topic of Markdown support for Haddock. The archives of the recent discussion can be seen at [1]. This post aims to summarise the current state of discussion. I do aim to file a proposal for a GSoC project on this issue but it'd be foolish to file a proposal for a project aiming to benefit the community without consulting the community itself. Here are some main points and ideas gathered: * reSt seems to have a small following - quite a bit smaller than the Markdown community. In fact, there seems to be a significant amount of people pushing for Markdown which contrasts what we can read in a topic from 2008 at [2]. I guess it just shows how much Markdown has gained in popularity in recent years. * There are issues with using Markdown even before we attempt to use it for Haskell documentation: * There exists no formal specification or semantics. It would seem that a significant number of Markdown parsers are creating by reverse engineering an already existing parser. This is bad because we end up propagating the bugs and workarounds around ambiguity that the original parser has. * As a follow-up to the previous point, the (vanilla) Markdown is ambiguous and there is nothing to resolve it. As Richard A. O'Keefe pointed out, there exist situations where it's not possible to infer the semantics of Markdown from its official implementation and the result is parser/writer-specific [6]. * John MacFarlane has already written a Markdown parser in Haskell. It can be read at [3]. This means that the new extension would not need to rely on Pandoc. He says ``I have an experimental thing here that could be used as a basis (it's 7x faster than pandoc and uses 1/5 the memory, BSD licensed)''. This is great! The post can be seen in full at [4]. * An alternative idea was to simply write a writer module for Pandoc for Haddock. * A reader module is already present. * According to John MacFarlane, Haddock is not expressive enough to take advantage of this. Furthermore, Pandoc doesn't have some constructions that Haddock does [4]. * Comes back to the problem on relying on such a large package as Pandoc. * Yet another proposal was rather than introducing Markdown to concentrate on fixing current issues and adding features to Haddock as it stands [8]. This is one of the options listed at the short blog post at [14] by Johan Tibell. * David Waern, one of Haddock maintainers admits that Haddock lacks active development and it has been that way for a longer time. Having said that, he seems to believe that Markdown integration is a project that can realistically be completed over summer. Such project would be able to use the existing HTML backend in Haddock. [9]. * Math expressions were requested and MathJAX was suggested as a solution at [5]. math.stackexchange.com uses MathJAX and it works quite well. Personally, I believe that Haskell documentation would benefit from this simply due to the academic nature of the language. * Support for Literate Haskell would be a welcome addition as suggested by Andrew Butterfield at [7]. * There are issues with CPP and LHS in regards to using Markdown in documentation. They are pointed out at [10] by John MacFarlane and others that he's replying to. * As pointed out 5 years ago at [11], we'd have to do some preprocessing on current, fairly critical sections of comments used by Haddock. I believe these are fairly useful and it would be a shame to see them go. I hope I haven't missed anything of high importance in a list above. When researching the topic, issues with Markdown quickly become apparent. Today, there are tens of different Markdown flavours: each company has different needs and each company interprets the vague original documentation in a way that's convenient to them. In these situations, topics, e-mails and blog posts like this one happen. There is a call to action from October 2012 at [12] but there seems to be absolutely no progress on any of the miraculous things mentioned in the post. As a result of that post, a W3C community was formed at [13]. It's clear that the community is inactive and no progress towards a solution was made. Having considered all information gathered here, I believe this would make a good GSoC project. There has been interest in this for Haskell since 2008 judging by the size of the last thread, it's clear that the community interest is high. In fact, it surprises me that this hasn't been done in previous years. The main issue that I see with using Markdown is the fact that we can't simply add a {-# HADDOCK Markdown #-} and let some parser rip through it. There are many issues that need to be resolved that I outlined above. This will no doubt result in a Markdown flavour suited to documenting Haskell rather than the wonderful, standardised Markdown that [13] is talking about. Is this different in any way from what GitHub or StackOverflow is doing? Introducing yet another flavour? In essence, no. For all Haskellers however, there would be a difference: this is a chance to use the pleasant-to-eye Markdown that many seem to like and use it for documentation. We have the chance to formalise it and make it work properly. It's not a perfect solution for every single person writing documentation out there but it's a good solution for Haskellers. Obviously, the ideal would be to have the wonderful, standardised, formalised Markdown that [13] seems to try and conjure and then extend that. Alas, we have no such luxury. As a closing paragraph, I would like to ask some questions. - - Is creating yet another Markdown flavour acceptable? I see no better solution: we'll have to take Markdown and fix it up to our needs. I'm sure most people are aware of the fairly famous Internet web-comic relating to standards [15]. We're however not trying to push a new standard - writing a proper standard, implementing it and then extending it for Haskell documentation is far too much for a single summer. - - Is there a mentor that would be willing to oversee the project? There's a fairly fresh ticket at [16] but there seems to be no indication of mentor-ship. I'm very interested in the project - it seems to hold high value for the community while not being completely out of scope for a student with a couple of months of Haskell experience, especially considering the large ``Getting up to scratch'' period. Thank you for your time. Links - ----- [1] - http://comments.gmane.org/gmane.comp.lang.haskell.cafe/104398 [2] - http://www.haskell.org/pipermail/haskell-cafe/2008-February/039846.html [3] - https://github.com/jgm/Markdown [4] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104421 [5] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104443 [6] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104422 [7] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104428 [8] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104523 [9] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104545 [10] - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/104444 [11] - http://www.haskell.org/pipermail/haskell-cafe/2008-February/039939.html [12] - http://www.codinghorror.com/blog/2012/10/the-future-of-markdown.html [13] - http://www.w3.org/community/markdown/ [14] - http://blog.johantibell.com/2013/04/haskellorg-gsoc-ideas.html [15] - http://xkcd.com/927/ [16] - http://trac.haskell.org/haddock/ticket/244 - -- Mateusz K. -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (GNU/Linux) iQIcBAEBAgAGBQJRdt+WAAoJEM1mucMq2pqXspkP/j+SCyfho65/ji5OvNCEBFTb YLGC1YY/dk/zbb67gDDxmFtxw/UBGnUifliGUKhCy2Zzm6mwF7BI1U4NWhbR2nkQ VWkysqHMn1JM8uQNtkdXEe/KUi2KxcM+VtOkuCUQeF0mlHB7Hthlz58lGZHD+cMK EvhN0BGrL1/xnKwSexTSNexXIDCmvCW19jTxuR9uwPqVNVPMjWxO0wRVplnzcqyf S8jeG0+Q28S0Su/CxNUdUQ+pE2+FWksNY4a/d/8Tce9sFoTTXUEGAp6Nbgyi+uKd yS+AtBqvcsw7KYw03kQAB1qCD05dtc7ptp9ohjgwYY6jzgRMBrIRdDUvmqT6nPh1 MHgG9rkg9yj+gKhSlNiTZ7MCl/o/CkCtPG0rNtd6//QXDZlH3Xx6CpYKm/GnyAWh a2vX5ZX0urVIapFPPEpClMkleLbHfi2UWGVbC43jP0lvsZTMlr4uvwHzJ8N4e9AI 2MdK4yZes6b/DSH47FPdoAeYksUvwI0xpI5rpkLuoOh+O094gQVhm4yb+ceqF6CE VddqnHPPF4gn7NTeYh6fnsAvzCm5PYgyhEUySbkmlGHjAODtvs6uC9MW4zwYPeCp BvGTSsGx7DbtjHwDYQIzMI/oyjFXMVsQ/+bE+pzIo/tQJevXkILaZKYhBvUyAbE5 ypdJkLdQb8EcWGT5Cx0v =sTE0 -----END PGP SIGNATURE-----

7 12

a bug of 32bit ghc on mac ?
by Sray 02 May '13

02 May '13

hi all it seems i have met some bug on a MAC 32bit GHC version 7.4.2 to make long stroy short, my code is about parse a json file using aeson here is my code http://pastebin.com/0VcVhdvX and here is test data http://pastebin.com/PvtSvst5 and test steps save the code , and name it a.hs (or what you want) save the test data ,name it a.json (do not change its name) $ ghc a.hs -o a $ ./a what i get from the output is "fail" and when i run the command below for a few times $ runghc a.hs i even got some ok and some fail mixed together but i copy the code and compile it on my linux, everything goes fine, the output is "OK" i have run "uninstall-hs" and install a 64bit ghc, also goes fine i delete one line randomly chosen from my test data , got an ok roll back ,delete another line, also an ok... I just want to make sure is this my bug or a bug of ghc p.s. the reason using 32bit ghc is yesod, which has met another bug on 64bit mac ghc :( thanks sray

2 2

Summer school on Applied Functional Programming at Utrecht University; deadline for registration May 15
by Doaitse Swierstra 02 May '13

02 May '13

Again we will teach an "Applied Functional Programming Summer in Haskell" school this year at Utrecht University. In the previous four occasions students were all very happy with the school and we plan to repeat this success this year. Among the intended audience are prospective master students who gained an interest in Functional Programming, e.g. by taking a general course on programming languages, and want to learn more about Haskell and its typical programming patterns. In previous years we have taught an introductory part (advanced bachelor level), an advanced part (beginning master level) and a shared part for both groups. Topics covered are, besides some examples of domain specific languages, also monads, monad transformers, arrows, parser combinators and self-analysing programs, underlying principles, type inferencing, etc. Half of the course time is spent on a larger programming exercise; you can also come with a problem of your own if you want, and get help from the Utrecht University Software Technology group in finding the proper Haskell idioms, tools and libraries, for solving it. Important links: -- our own page where we supply information based on questions asked http://www.cs.uu.nl/wiki/USCS/ -- on this page you can find the poster you can print and hang somewhere (why not your office door): Furthermore we ask for your cooperation to bring this announcement under the attention of potential participants. Best, Doaitse Swierstra, Atze Dijkstra and Johan Jeuring

2 1

remote-build-reporting: cabal phoning home?!
by Ertugrul Söylemez 02 May '13

02 May '13

Hello there, could somebody please shed some light on the following line that appeared in my ~/.cabal/config? remote-build-reporting: anonymous The option doesn't seem to be documented anywhere, and I'm very nervous about undocumented "remote reporting" features. I expect cabal-install to communicate only to fetch the latest package index and to upload and download packages, the former only when i actually use the "upload" command. If it performs any other communication, please tell me how to disable it. In that case I'd also be interested in an explanation for why this was enabled in the first place. I certainly didn't enable it myself, because it would be a serious security breach in my case. Thank you. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.

2 2

Haskell job / adaptive learning
by Carl Baatz 02 May '13

02 May '13

Dear Haskellers, Erudify (http://erudify.com/) is looking for software engineers to join our team. We are developing an autonomous interactive online tutor. We sell our product to enterprise and provide it for free to schools and consumers. We are looking for software engineers who are smart and friendly, have several years of programming experience, and a passion for building practical and high quality software. Haskell is our main language and we are using JavaScript on the client side. The field of automated learning and teaching is still in its infancy and we are thrilled about the work we are doing to improve how people learn. So what are we looking for? We expect you to prefer design principles compatible with Unix and the Web. You make an effort to learn new technologies that could be helpful and you work to master those you already use. You have worked with a broad range of technologies in your career. You get things done and realise that trade-offs are necessary, but compromising the long-term goal is not. You have experience with Haskell, but might not be an expert. Erudify consists of eleven people -- six of us are Engineers, of which four are specialised in Haskell (with another Haskeller joining soon). We are based in Zürich and our working language is English. Please email us at jobs(a)erudify.com if you are curious. Also, two of our engineers, Bas van Dijk and Simon Meier, will be at the Odessa hackathon from 2 May to 6 May -- they'd be delighted to chat with you. Best wishes, Carl Baatz

1 0

Re: [Haskell-cafe] Is it time to start deprecating FunDeps?
by oleg＠okmij.org 02 May '13

02 May '13

> In your class Sum example, > > class Sum x y z | x y -> z, x z -> y > > your own solution has a bunch of helper classes First of all, on the top of other issues, I haven't actually shown an implementation in the message on Haskell'. I posed this as a general issue. In special cases like below > class Sum2 a b c | a b -> c, a c -> b > instance Sum2 Z b b > instance (Sum2 a' b c') => Sum2 (S a') b (S c') > -- note that in the FunDeps, var a is not a target > -- so the instances discriminate on var a I didn't doubt the translation would go through because there is a case analysis on a. But the general case can be more complex. For example, class Sum2 a b c | a b -> c, a c -> b instance Sum2 Z Z Z instance Sum2 O Z O instance Sum2 Z O O instance Sum2 O O Z > In your overlap example you introduce a definition that won't compile! > > > > {- -- correctly prohibited! > > instance x ~ Bool => C1 [Char] x where > > foo = const True > > -} > You expect too much if you think a mechanical translation will 'magic' a > non-compiling program into something that will compile. > I do expect equality constraints to not play well with overlaps. Combining > FunDeps with overlaps is also hazardous. I'm only claiming that EC's will > be at least as good. I don't understand the remark. The code marked `correctly prohibited' is in the comments. There are no claims were made about that code. If you found that comment disturbing, please delete it. It won't affect the the example: the types were improved in t11 but were not improved in t21. Therefore, EC's are not just as good. Functional dependencies seem to do better.

1 0

Haskell Weekly News: Issue 266
by Daniel Santa Cruz 02 May '13

02 May '13

Welcome to issue 266 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 7 to 27, 2013. Quotes of the Week * hpc: the dirty secret of haskell programmers is that their power does not come from their PhD, but their bowtie * tibbe: I think I have implemented a reverse state monad by mistake. The numbers suggest I'm getting the values tunneled back from the future. :/ * SaulGorn: A formalist is one who cannot understand a theory unless it is meaningless. * cmccann: laziness by default is a shortcoming. eagerness by default is a bigger shortcoming. * monochrom: #haskell-blah has brilliant people too. for example me. * limo: OK, guys was nice meeting you and now I'm a proud owner of a GHCi icon on my desktop * otters: lens law #1: the implementation must be shorter than the type * MartinDeMello: Any sufficiently well-commented Lisp program contains an ML program in its comments. * tgeeky: whatever man. Jesus only had twelve or so deciples, lens has 80! * SatnamSingh: I always thought that category theory was something that happened to other people. (#ICFP) * cmccann: see the thing is tabs are good for indentation. haskell doesn't use indentation, it uses alignment. it is not indenting. it is ASCII art. * JohnVonNeumann: In mathematics you don't understand things. You just get used to them. * edwardk: you can view Yoneda f as enforced fmap fusion and Codensity f as enforced bind fusion. Top Reddit Stories * Functors, applicatives, and monads in pictures Domain: adit.io, Score: 136, Comments: 67 On Reddit: [1] http://goo.gl/xRC6K Original: [2] http://goo.gl/QcfqM * What I Wish I Knew When Learning Haskell, Stephen Diehl [39 slides, Left/Rgt-arrow key to back/advance] Domain: dev.stephendiehl.com, Score: 119, Comments: 37 On Reddit: [3] http://goo.gl/T8uzi Original: [4] http://goo.gl/eZTA0 * git clone in Haskell from the bottom up Domain: stefan.saasen.me, Score: 86, Comments: 6 On Reddit: [5] http://goo.gl/Al86J Original: [6] http://goo.gl/nWSXV * Vinyl: Modern Records for Haskell Domain: jonmsterling.com, Score: 56, Comments: 27 On Reddit: [7] http://goo.gl/pSUph Original: [8] http://goo.gl/ra0Jb * Follow-up: Web Woes for Haskell Domain: self.haskell, Score: 49, Comments: 9 On Reddit: [9] http://goo.gl/PJZEA Original: [10] http://goo.gl/PJZEA * What does “coalgebra” mean in the context of programming? (StackOverflow) Domain: stackoverflow.com, Score: 45, Comments: 23 On Reddit: [11] http://goo.gl/nMqBF Original: [12] http://goo.gl/UH2Wl * The Mother of all Monads - School of Haskell Domain: self.haskell, Score: 42, Comments: 27 On Reddit: [13] http://goo.gl/XO7mQ Original: [14] http://goo.gl/XO7mQ * Why do all the Haskell Web Frameworks rely on Template Haskell? Domain: self.haskell, Score: 40, Comments: 59 On Reddit: [15] http://goo.gl/v5p6F Original: [16] http://goo.gl/v5p6F * Job opening for Haskell developer in Nijmegen (NL) Domain: self.haskell, Score: 39, Comments: 6 On Reddit: [17] http://goo.gl/Uzqs8 Original: [18] http://goo.gl/Uzqs8 * Tutorial: Building a Sample Application with Haskell Snap, PostgreSQL, and the PostgreSQL Simple Snaplet Domain: janrain.com, Score: 39, Comments: 17 On Reddit: [19] http://goo.gl/TAM6J Original: [20] http://goo.gl/37Nry * Haskell for all: Defaults Domain: haskellforall.com, Score: 38, Comments: 20 On Reddit: [21] http://goo.gl/7NYex Original: [22] http://goo.gl/FqiuW * Haskell for all: pipes-concurrency-1.0.0: Reactive programming Domain: haskellforall.com, Score: 38, Comments: 23 On Reddit: [23] http://goo.gl/DaSaZ Original: [24] http://goo.gl/eGw3F * Haskell in Ipython Domain: github.com, Score: 38, Comments: 7 On Reddit: [25] http://goo.gl/Y7DEP Original: [26] http://goo.gl/324JX * A taste of Curry (Wolfgang Jeltsch) Domain: jeltsch.wordpress.com, Score: 38, Comments: 0 On Reddit: [27] http://goo.gl/Ijh6Y Original: [28] http://goo.gl/17Ry1 * Complete and Easy Bidirectional Typechecking for Higher-Rank Polymorphism Domain: semantic-domain.blogspot.co.uk, Score: 37, Comments: 21 On Reddit: [29] http://goo.gl/Yv9RY Original: [30] http://goo.gl/qW1Qq * Template Haskell on impossible architectures (x-post from r/programming) Domain: joeyh.name, Score: 37, Comments: 14 On Reddit: [31] http://goo.gl/m1Jvp Original: [32] http://goo.gl/GqiAR * inefficient Ackermann implementation not obvious how to improve Domain: stackoverflow.com, Score: 37, Comments: 20 On Reddit: [33] http://goo.gl/CEaOT Original: [34] http://goo.gl/JploA * Haskell for all: pipes and io-streams Domain: haskellforall.com, Score: 37, Comments: 20 On Reddit: [35] http://goo.gl/fmLhr Original: [36] http://goo.gl/8BoLg * Build applications with Shake build system Domain: lethalman.blogspot.it, Score: 35, Comments: 5 On Reddit: [37] http://goo.gl/MZBLj Original: [38] http://goo.gl/bP7lK * What’s in a number? criterion edition Domain: serpentine.com, Score: 34, Comments: 6 On Reddit: [39] http://goo.gl/cfVMa Original: [40] http://goo.gl/hqBfW * Announcing postgresql-simple 0.3.1 Domain: blog.melding-monads.com, Score: 34, Comments: 5 On Reddit: [41] http://goo.gl/LEuEv Original: [42] http://goo.gl/jrn5v * LGtk: Lens-based Gtk Interface Domain: people.inf.elte.hu, Score: 32, Comments: 64 On Reddit: [43] http://goo.gl/SC7kn Original: [44] http://goo.gl/FfYeE * Introduction to QuickCheck testing (new FP complete SoH tutorial) Domain: fpcomplete.com, Score: 32, Comments: 7 On Reddit: [45] http://goo.gl/2BnS6 Original: [46] http://goo.gl/6DKRG * Introduction to Haskell, Lecture 12 is Live (Category Theory). This is the last lecture in this series. Thank you for your support! Domain: shuklan.com, Score: 31, Comments: 4 On Reddit: [47] http://goo.gl/to9jd Original: [48] http://goo.gl/JZYSv Top StackOverflow Questions * What does “coalgebra” mean in the context of programming? votes: 134, answers: 3 Read on SO: [49] http://goo.gl/UH2Wl * Ackermann very inefficient with Haskell/GHC votes: 39, answers: 7 Read on SO: [50] http://goo.gl/JploA * How to concisely express function iteration? votes: 21, answers: 3 Read on SO: [51] http://goo.gl/OknK3 * Does the GHC garbage collector have any special optimisations for large objects? votes: 16, answers: 1 Read on SO: [52] http://goo.gl/p1zvM * Parallel computations with fast randomness and purity? votes: 16, answers: 4 Read on SO: [53] http://goo.gl/O3FzX * Idiomatic Scala translation of Kiselyov's zippers? votes: 15, answers: 1 Read on SO: [54] http://goo.gl/TCBaL * Singleton types in Haskell votes: 14, answers: 1 Read on SO: [55] http://goo.gl/u3867 * Encoding correctness constraints directly into the Haskell type system? votes: 14, answers: 2 Read on SO: [56] http://goo.gl/in0xv * Monad transformers with identity monad votes: 12, answers: 2 Read on SO: [57] http://goo.gl/hx2Ic * Extensible Haskell Type Classes votes: 12, answers: 4 Read on SO: [58] http://goo.gl/zdYpL * GADT's failed exhaustiveness checking votes: 12, answers: 2 Read on SO: [59] http://goo.gl/GIFDu * QuickCheck: Arbitrary instances of nested data structures that generate balanced specimens votes: 11, answers: 1 Read on SO: [60] http://goo.gl/WH3FV * What is this special functor structure called? votes: 11, answers: 1 Read on SO: [61] http://goo.gl/cKt7o Until next time, [62]+Daniel Santa Cruz References 1. http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictu… 2. http://www.reddit.com/r/haskell/comments/1co0s5/functors_applicatives_and_m… 3. http://dev.stephendiehl.com/hask/ 4. http://www.reddit.com/r/haskell/comments/1cbjqm/what_i_wish_i_knew_when_lea… 5. http://stefan.saasen.me/articles/git-clone-in-haskell-from-the-bottom-up/ 6. http://www.reddit.com/r/haskell/comments/1ch3l2/git_clone_in_haskell_from_t… 7. http://www.jonmsterling.com/posts/2013-04-06-vinyl-modern-records-for-haske… 8. http://www.reddit.com/r/haskell/comments/1buh5y/vinyl_modern_records_for_ha… 9. http://www.reddit.com/r/haskell/comments/1cy58c/followup_web_woes_for_haske… 10. http://www.reddit.com/r/haskell/comments/1cy58c/followup_web_woes_for_haske… 11. http://stackoverflow.com/questions/16015020/what-does-coalgebra-mean-in-the… 12. http://www.reddit.com/r/haskell/comments/1cht4v/what_does_coalgebra_mean_in… 13. http://www.reddit.com/r/haskell/comments/1cyajx/the_mother_of_all_monads_sc… 14. http://www.reddit.com/r/haskell/comments/1cyajx/the_mother_of_all_monads_sc… 15. http://www.reddit.com/r/haskell/comments/1czxvj/why_do_all_the_haskell_web_… 16. http://www.reddit.com/r/haskell/comments/1czxvj/why_do_all_the_haskell_web_… 17. http://www.reddit.com/r/haskell/comments/1clkjb/job_opening_for_haskell_dev… 18. http://www.reddit.com/r/haskell/comments/1clkjb/job_opening_for_haskell_dev… 19. http://janrain.com/blog/tutorial-building-a-sample-application-with-haskell… 20. http://www.reddit.com/r/haskell/comments/1d5c38/tutorial_building_a_sample_… 21. http://www.haskellforall.com/2013/04/defaults.html 22. http://www.reddit.com/r/haskell/comments/1c2g1r/haskell_for_all_defaults/ 23. http://www.haskellforall.com/2013/04/pipes-concurrency-100-reactive.html?m=1 24. http://www.reddit.com/r/haskell/comments/1cc6fe/haskell_for_all_pipesconcur… 25. https://github.com/creswick/ihaskell-notebook 26. http://www.reddit.com/r/haskell/comments/1d0151/haskell_in_ipython/ 27. http://jeltsch.wordpress.com/2013/04/27/a-taste-of-curry/ 28. http://www.reddit.com/r/haskell/comments/1d8n51/a_taste_of_curry_wolfgang_j… 29. http://semantic-domain.blogspot.co.uk/2013/04/thanks-to-olle-fredriksson.ht… 30. http://www.reddit.com/r/haskell/comments/1c50t3/complete_and_easy_bidirecti… 31. http://joeyh.name/blog/entry/Template_Haskell_on_impossible_architectures/ 32. http://www.reddit.com/r/haskell/comments/1cm4n1/template_haskell_on_impossi… 33. http://stackoverflow.com/questions/16115815/ackermann-very-inefficient-with… 34. http://www.reddit.com/r/haskell/comments/1cq3r4/inefficient_ackermann_imple… 35. http://www.haskellforall.com/2013/04/pipes-and-io-streams.html 36. http://www.reddit.com/r/haskell/comments/1ct65e/haskell_for_all_pipes_and_i… 37. http://lethalman.blogspot.it/2013/04/build-vala-applications-with-shake.html 38. http://www.reddit.com/r/haskell/comments/1bv703/build_applications_with_sha… 39. http://www.serpentine.com/blog/2013/04/13/whats-in-a-number-criterion-editi… 40. http://www.reddit.com/r/haskell/comments/1cauc5/whats_in_a_number_criterion… 41. http://blog.melding-monads.com/2013/04/27/announcing-postgresql-simple-0-3-… 42. http://www.reddit.com/r/haskell/comments/1d8mf1/announcing_postgresqlsimple… 43. http://people.inf.elte.hu/divip/LGtk.html 44. http://www.reddit.com/r/haskell/comments/1cemr2/lgtk_lensbased_gtk_interfac… 45. https://www.fpcomplete.com/user/pbv/an-introduction-to-quickcheck-testing 46. http://www.reddit.com/r/haskell/comments/1cus3k/introduction_to_quickcheck_… 47. http://shuklan.com/haskell/lec12.html 48. http://www.reddit.com/r/haskell/comments/1c00tj/introduction_to_haskell_lec… 49. http://stackoverflow.com/questions/16015020/what-does-coalgebra-mean-in-the… 50. http://stackoverflow.com/questions/16115815/ackermann-very-inefficient-with… 51. http://stackoverflow.com/questions/16010778/how-to-concisely-express-functi… 52. http://stackoverflow.com/questions/16186488/does-the-ghc-garbage-collector-… 53. http://stackoverflow.com/questions/16248600/parallel-computations-with-fast… 54. http://stackoverflow.com/questions/15940473/idiomatic-scala-translation-of-… 55. http://stackoverflow.com/questions/16017294/singleton-types-in-haskell 56. http://stackoverflow.com/questions/16207008/encoding-correctness-constraint… 57. http://stackoverflow.com/questions/15946633/monad-transformers-with-identit… 58. http://stackoverflow.com/questions/15967167/extensible-haskell-type-classes 59. http://stackoverflow.com/questions/16225281/gadts-failed-exhaustiveness-che… 60. http://stackoverflow.com/questions/15959357/quickcheck-arbitrary-instances-… 61. http://stackoverflow.com/questions/16123588/what-is-this-special-functor-st… 62. https://plus.google.com/105107667630152149014/about

1 0

Re: [Haskell-cafe] Looking for portable Haskell or Haskell like language
by Christopher Howard 02 May '13

02 May '13

On 05/01/2013 12:21 AM, Gabor Greif wrote: > Am 27. April 2013 um 07:21 schrieb Christopher Howard > <christopher.howard(a)frigidcode.com>: > > > I can feel your pain... Here is a blog post I have written some time ago > http://heisenbug.blogspot.de/2011/09/ghc-704-on-centos.html > about how to bridge the gap. This was actually a RHEL5 system, > but did not want to admit it :-) > > My writeup may be useful to you. > > Cheers, > > Gabor > > For posterity's sake: I actually got this figured out already. First, I downloaded the binary version of GHC 6.8.3 and installed it without issues. Then I used it to build the sources of GHC 6.10.4 and install them. Then I used 6.10.4 to build and install 6.12.3. Then 7.0.4, then 7.2.2, then 7.4.2, and finally 7.6.3. Had no problems doing it that way, except for the optimization flags issue in the 7.* series which I mentioned in my other thread. That was easily fixed by setting the build.mk to the "quick" build profile. -- frigidcode.com

1 0

ANN: hledger 0.20
by Simon Michael 02 May '13

02 May '13

I'm pleased to announce hledger and hledger-web 0.20! hledger is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. In addition to reporting, it can also help you record new transactions, or convert CSV data from your bank. Add-on packages include hledger-web (providing a web interface), hledger-irr and hledger-interest. hledger is inspired by and compatible with John Wiegley's Ledger. For more, see http://hledger.org . Install it: # cabal update; cabal install hledger-web For more installation help, see http://hledger.org/MANUAL.html#installing . Or, sponsor a ready-to-run binary for your platform: http://hledger.org/DOWNLOAD.html . Release notes (http://hledger.org/NEWS.html#hledger-0.20) **Bugs fixed:** * balance: a 0.19 regression which showed wrong total balance with `--flat` has been fixed (#94) * register: when `--date2` is used, the register is now sorted by the secondary date * web: some missing static & template files have been added to the package, fixing cabal-dev and hackage builds (#97, #98) * web: some hardcoded static urls have been fixed * Dependencies and code have been updated to support the latest libraries and GHC versions. For now, hledger requires GHC 7.2+ and hledger-web requires GHC 7.4+. **Journal reading:** - DOS-style line-endings are now also supported in journal and rules files. - `!` is now accepted in the status field as well as `*`, like ledger - The *actual date* and *effective date* terminology has changed to *primary date* and *secondary date*. Use `--date2` to select the secondary date for reports. (`--aux-date` or `--effective` are also accepted for ledger and backwards compatibility). - Per-posting dates are supported, using hledger tags or ledger's posting date syntax - Comment and tag handling has been improved **CSV reading:** - CSV conversion rules have a simpler, more flexible [syntax](MANUAL.html#csv-files). Existing rules files will need to be updated manually: - the filename is now `FILE.csv.rules` instead of `FILE.rules` - `FIELD-field N` is now `FIELD %N+1` (or set them all at once with a `fields` rule) - `base-currency` is now `currency` - `base-account` is now `account1` - account-assigning rules: add `if` before the list of regexps, add indented `account2 ` before the account name - parenthesised amounts are parsed as negative **Querying:** - Use `code:` to match the transaction code (check number) field - Use `amt:` followed by `<`, `=` or `>` and a number N to match amounts by magnitude. Eg `amt:<0` or `amt:=100`. This works only with single-commodity amounts (multi-commodity amounts are always matched). - `tag:` can now match (exact, case sensitive) tag values. Eg `tag:TAG=REGEXP`. **add comand:** - Transaction codes and comments (which may contain tags) can now be entered, following a date or amount respectively. (#45) - The current entry may be restarted by entering `<` at any prompt. (#47) - Entries are displayed and confirmed before they are written to the journal. - Default values may be specified for the first entry by providing them as command line arguments. - Miscellaneous UI cleanups **register command:** - The `--related`/`-r` flag shows the other postings in each transaction, like ledger. - The `--width`/`-w` option increases or sets the output width. **web command:** - The web command now also starts a browser, and auto-exits when unused, by default ("local ui mode"). With `--server`, it keeps running and logs requests to the console ("server mode"). - Bootstrap is now used for styling and layout - A favicon is served - The search field is wider - yesod devel is now supported; it uses `$LEDGER_FILE` or `~/.hledger.journal` - the `blaze_html_0_5` build flag has been reversed and renamed to `blaze_html_0_4` **Add-ons:** - The hledger-interest and hledger-irr commands have been released/updated. - hledger-chart and hledger-vty remain unmaintained and deprecated. **Documentation and infrastructure:** - The hledger docs and website have been reorganised and updated - Manuals for past releases are provided as well as the latest dev version - hledger has moved from darcs and darcs hub to git and github (!) - The bug tracker has moved from google code to github - Feature requests and project planning are now managed on trello - A build bot builds against multiple GHC versions on each commit Release contributors: - Sascha Welter commissioned register enhancements (--related and --width) - David Patrick contributed a bounty for add enhancements - Joachim Breitner added support for ! in status field - Xinruo Sun provided hledger-web build fixes - Peter Simons provided hledger-web build fixes, and a build bot - Marko Kocić provided hledger-web fixes

1 0

What is the process for GSOC?
by Dominic Steinitz 01 May '13

01 May '13

How is it decided which Haskell projects get chosen? Do we discuss them here and take a collective view? Thanks, Dominic. PS I should point out I have an interest in the proposal to port charts to use diagrams (http://www.google-melange.com/gsoc/proposal/review/google/gsoc2013/jbracker…) i.e. I'd really like this as a user.

2 1

Re: [Haskell-cafe] Space leak in hexpat-0.20.3/List-0.5.1
by oleg＠okmij.org 01 May '13

01 May '13

Wren Thornton wrote: > So I'm processing a large XML file which is a database of about 170k > entries, each of which is a reasonable enough size on its own, and I only > need streaming access to the database (basically printing out summary data > for each entry). Excellent, sounds like a job for SAX. Indeed a good job for a SAX-like parser. XMLIter is exactly such parser, and it generates event stream quite like that of Expat. Also you application is somewhat similar to the following http://okmij.org/ftp/Haskell/Iteratee/XMLookup.hs So, it superficially seems XMLIter should be up for the task. Can you explain which elements your are counting? BTW, xml_enum already checks for the well-formedness of XML (including the start-end tag balance, and many more criteria). One can assume that the XMLStream corresponds to the well-formed document and only count the desired start tags (or end tags, for that matter).

1 0

GSoC - A Cabal Project
by Martin Ruderer 30 Apr '13

30 Apr '13

Hi, I am proposing a GSoC project on Cabal. It aims to open up the dependency solver for debugging purposes. The details are here: https://gist.github.com/mr-/7995081f89cff38e9443 I would really like to hear what you think about it. Best regards, Martin

3 2

ghc-7 -fPIC error
by Christopher Howard 30 Apr '13

30 Apr '13

Hey guys, this probably isn't the official GHC mailing list, but I've been trying to build and install a new GHC on an old RHEL5 system, as mentioned in my previous Cafe thread. I was able to make some good headway: I installed a ghc-6.8 binary, and then used that to build ghc-6.10 source, and then used that to build ghc-6.12 source. The idea is to keep going until I get to ghc-7.6. However, when I get to 7.0, my builds all eventually die with this error: quote: -------- /usr/bin/ld: rts/dist/build/RtsStartup.dyn_o: relocation R_X86_64_PC32 against `StgRun' can not be used when making a shared object; recompile with -fPIC /usr/bin/ld: final link failed: Bad value collect2: ld returned 1 exit status -------- After this failed the first time, I tried adding -fPIC to SRC_HC_OPTS, GhcStage1HcOpts, and so forth in build.mk. But ultimately I get the same error. I tried skipping a version and building ghc-7.2 instead, but the same error pops up. -- frigidcode.com

3 2

Haskell compilation errors break the complexity encapsulation on DSLs
by Alberto G. Corona 30 Apr '13

30 Apr '13

Hi I ever was worried about the barrier that the complexity of the Haskell errors impose to users of DSLs. Many DSLs look so simple that even someone without knowledge of Haskell can make use of them for some domains. However when the program is compiled then al the monsters of the deep appear in the surface: polymorphisms, undefined instances, type errors reported in a line produced by a type assumption in another, etc. This is a problem for an industrial use of Haskell in the large scale. For obvious reasons. I would like to know 1) which techniques are used to minimize the problem 2) To suggest some kind of extension that would allow for example to add extra information to the error messages by the library developer.. For example, in http://hackage.haskell.org/packages/archive/control-monad-exception/0.10.3.… There is a nice documentation about two kind of errors that are frequent when using that library: *A type error of the form: * * No instance for (UncaughtException MyException) arising from a use of `g' at examples/docatch.hs:21:32-35 Possible fix: add an instance declaration for (UncaughtException MyException) In the expression: g ()* *is the type checker saying: * *"hey, you are trying to run a computation which throws a MyExceptionwithout handling it, and I won't let you" * *Either handle it or declare MyException as an UncaughtException<http://hackage.haskell.org/packages/archive/control-monad-exception/0.10.3.…>. * The question: Is it possible to develop a GHC extension that attach (or prepend) such an explanation to the ghc error? Or any other alternative that carry out the same functionality. -- Alberto.

3 5

Packages in distro mentioned on hackage?
by Magnus Therning 30 Apr '13

30 Apr '13

I've noticed that some packages have a mention of distributions that include it, and which version. How does a distro get to be added like that? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

2 1

Re: [Haskell-cafe] Markdown extension for Haddock as a GSoC project
by Mateusz Kowalczyk 30 Apr '13

30 Apr '13

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 28/04/13 00:08, Joe Nash wrote: > Managed not to send to all: > > I think the reason markdown was the original suggestion was due to > the fact it is a very widespread and popular syntax, and as Johan > commented in the original thread, has to an extent "won". Having a > consistent, standard documentation syntax for Haskell is of course > important, but this project also seeks to standardise the syntax > beyond Haskell. I don't think standardising it beyond Haskell is the goal at all. I think that coming up with Markdown that doesn't repeat the mistakes of all the flavours out there is quite difficult in itself, but coming up with such Markdown AND making it making it flexible enough to be of any use with Haddock as is? I think it might be a little out of scope. Extending Haddock itself and providing a writer (and a new reader) for Pandoc will let you write Markdown if you please and then convert it to (and from) many of the formats supported by Pandoc. > It may be hedging our bets, as a lot could change and markdown > could suddenly fall from fame, but I believe the large variety of > opposing markup languages are failing to gain traction in > comparison and it is indeed a safe bet. While I don't think it will fall from fame overnight, it would be a shame to restrict people to just using Markdown because that's how all the extensions were implemented. > > If the flexibility of having it pandoc compatible is a desired > feature, can this not be achieved through implementing markdown for > haddock as well? Depending on what haddock specific features were > required to be added to the superset, it may require only minimal > changes to an existing markdown reader/writer in pandoc already. It can, but the restriction comment applies here as well. - -- Mateusz K. -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (GNU/Linux) iQIcBAEBAgAGBQJRfPllAAoJEM1mucMq2pqXaKAP/20x2549NvfJgib9oitm1Qn8 CCpGRHdSw4xlIh0pHUE1MKyiJrGGgchuUErN2CJBOKXPhn1fgCWFKJyfOhcPbyr7 2Tpb9t++9G9u0JtIb6/UQpgy6RDXQAqOUs78KxRx3DCb3rYuT5TNpr5ynS58T1+9 7eGQXeUQwhCkKZH6EuIF0qTwxPTsMOAT66oRVnaZJFl2+rOBcw8ZmxMsHnk3bX9t VAR+ySwN4FWce6QZwGU43tMwFHoIxsKrmZ3ZydQPfwdQjJjFjCdTaZhtEYS1Hcvu gg0yU5mjO/1frauqjgC/qo4ez/JrJr5aYHrV91o5S2w0Wl7jfUADAIRDLfhAnTe6 shbiPPBPJeY7EAt6JEytHBmlbwZLyE3zrB0GhlhyaKY344Z5k5Woq2u6zInPx5gi rtiWOUbuerBl8autaKwnPa9i+DSmRwMD8esFIyUMyCbPOMj89YirZtxWGD+ol71i cnBXV8DbX9aYfR8c8U3WNeR8FTOvxKYEC8cc3xErsustrwFlDKzlJD41VGHVZvjB qayuRIR/qMy+fy+8UrxzCFQA32gtZ/ZmjfyHWXHhe2DujThmTi95PtgpLHe1Y2Nh hYuGhj6fqzFvSvSJkE2uWlNPVIu7viBcF1bQEZz07OdYuvU+GbA8OK8q3EzCq+l3 l5uWSedwC4cdyNRxigFS =cgRe -----END PGP SIGNATURE-----

10 14

Fwd: [darcs-users] Google Summer of Code
by Guillaume Hoffmann 30 Apr '13

30 Apr '13

---------- Forwarded message ---------- From: Ganesh Sittampalam <ganesh(a)earth.li> Date: 2013/4/25 Subject: [darcs-users] Google Summer of Code To: darcs-users <darcs-users(a)darcs.net> Hi, darcs.net will participating in Google Summer of Code this year, under the umbrella of haskell.org - Google will be allocating 1 or 2 extra slots specifically for Darcs. Time is a little short, as we'd originally thought this arrangement was coming to an end: the student application deadline is 1900UTC on next Friday (May 3rd). Please see http://darcs.net/GSoC for ideas and what to do next if you are interested in participating. The most important thing is to get in touch with us informally asap. Many thanks to Guillaume Hoffmann both for making our initial application to be a separate organisation (which was unfortunately rejected), and then for talking to Google and establishing that our existing arrangement of having a dedicated slot under the haskell.org umbrella could continue. Cheers, Ganesh _______________________________________________ darcs-users mailing list darcs-users(a)darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-users

1 0

Re: [Haskell-cafe] GSoC Push Notifications project - communicating with mobile devices
by Kristopher Micinski 30 Apr '13

30 Apr '13

On Mon, Apr 29, 2013 at 4:46 PM, Marcos Pividori <marcospividori(a)gmail.com> wrote: > Hi, thanks for your response! this really help me. > > * About the code in Spanish: I will replace it for an English version in the > next weeks. As Michael said, I had to write it in Spanish because it was a > project for my university. Sorry, I know it is not clear now. > No worries, I don't think this makes it bad, just harder to read for English speakers > * Kristopher, thanks for your comments and availability. Yes, I am looking > to provide a Haskell API to interface with these different push notification > services. I know that maybe it isn't enough to fill up a summer, so I will > get into account this ideas you gave me. I am thinking about some useful > tools I could provide to people programming for mobile devices. I would also be interested in hearing about what you come up with. I have a lot of experience with mobile hacking, and I'd be glad to help you validate ideas as they come to you, or point you at related material. >>"I would suggest also adding to your schedule writing up a >> Yesod app which has "back and forth" communication with a device, >> seeing what common problems pop up, and then writing an API for >> handling *that* as well. (That may broaden the scope of your project >> enough.)" > > I created this "back and forth" communication for the project I mentioned > before. But, a lot of things could be improved. For example, about > mantaining a state of the connection and being able to manage with a lot of > devices at the same time. (As I implemented this, I think it wont work > really well with a lot of devices). And I could develop a demo app for each > OS (Android, iOS, Windows Phone, etc) to manage this communication. > I 'll continue investigating, every contribution is welcome! The key aspects are to use Haskell to write as little as boilerplate code as possible. Then identify the common elements of the API for each and write a common API. That strikes me as the simple part. I'm not sure if it's sensible or not, but maybe it would be possible to formalize this "back and forth" computation using something similar to session types. Maybe next steps would be to automatically take the types in the program and generate a contract for the JSON sent between the device and server, which might give you some fun experience in type level programming. This is very hand wavy, so I'm not sure if that's realistic or not... Kris

1 0

GSoC proposal: Haskell AST-based refactoring and API upgrading tool
by Niklas Hambüchen 29 Apr '13

29 Apr '13

I would like to propose the development of source code refactoring tool that operates on Haskell source code ASTs and lets you formulate rewrite rules written in Haskell. Objective --------- The goal is to make refactorings easier and allow global code changes that might be incredibly tedious to do in a non-automated way. By making these transformations convenient, we can make it easier to maintain clean code, add new features or clean up leftovers faster, and reduce the fear and effort to upgrade to newer versions of packages and APIs. Transformations --------------- First, here are a few operations you would use this tool for. Some of them are common operations you would also do in other programming languages, some are more specific to Haskell. * Changing all occurrences of "import Prelude hiding (catch)" to "import qualified Control.Exception as E" * Replacing all uses of a function with that function being imported qualified or the other way around * Adding a field to data constructor a record, setting user-supplied defaults for construction and destruction: -- Suppose you want to change one of these data User = User { name :: String, age :: Int } data User = User String Int -- into one of these data User = User { name :: String, age :: Int, active :: Bool } data User = User String Int Bool -- the refactoring tool could perform, in all relevant locations: show (User name age) = ... show (User name age _) = ... -- and also this transformation: ... u { name = "deleted" } ... ... u { name = "deleted", active = False } ... -- or equivalently with records. -- Special cases could be taken care of as specified, such as -- "whenever an object of [this User type] has -- of its records passed into some function 'email', do this -- now only if the user is active, so modify all relevant code -- email (name u) -- to -- if (active u) then email (name u) else return () -- Other examples include adding a position counter to attoparsec. * Adding a type parameter to a type -- This happens a lot on monad transformer stacks, e.g. newtype MyMonad a b c = MyMonad (ReaderT a (WriterT b ... -- and as you would probably agree on, this is not the most -- comfortable change to make; in big project this can mean -- hour-long grinding. -- It has also recently happened in the basic underlying types -- of packages like conduit and pipes. * Adding a new transformer around a monad * Addressing problems like mentioned in http://blog.ezyang.com/2012/01/modelling-io/: "There is one last problem with this approach: once the primitives have been selected, huge swaths of the standard library have to be redefined by “copy pasting” their definitions ..." * Extracting a value into a let or where clause * Renaming a variable, and all its occurrences that are semantically same variable (based on its scope) * Changing the way things are done, such as: * Replacing uses of fmap with <$>, also taking care of the corresponding import, and such cases were partial application is involved * Replacing uses of "when (isJust)" to "forM_" * Making imports clearer by adding all functions used to the file to the import list of the module that gets them in scope * Finding all places where an exported function does not have all its arguments haddock-documented. * Performing whole-project refactorings instead of operating on single files only, allowing operations like "Find me all functions of this type, e.g. Maybe a -> (a -> m a) -> m a in the project and extract them into this new module, with the name 'onJust'." Some of the problems above can be tried to address using regex-based search and replace, but this already fails in the simplest case of "import Prelude hiding (catch)" in case there is more than that imported from Prelude or newlines involved in the import list. Transformation on the AST are much more powerful, and can guarantee that the result is, at least syntactically, valid. No text base tool can do that. Other uses ---------- In addition to being able to perform transformations as mentioned above, the refactoring tool as a library can be leveraged to: * Support or be the base of code formatting tools such as haskell-stylish, linters, style/convention checkers, static analyzers, test coverage tools etc. * Implement automatic API upgrades. Imagine the author of a library you use deprecates some functions, introduces replacements, adds type parameters. In these cases, it is very clear and often well-documented which code has to be replaced by what. The library author could, along with the new release, publish "upgrade transformations" that you can apply to your code base to save most of the manual work. These upgrade transformations could be either parts of the packages themselves or be separately maintained and refined on the feedback of users applying them to their code bases. This could allow the Haskell community to keep up their fast pace while making API breakage a non-problem. Automation is what makes us deal well with test suites, source control, error checking. Taking the pain out of this will increase people's incentive to upgrade their code an to keep it well-maintained by easy refactorings. This concept of automatic code upgrades would, to my knowledge, be quite unique in the programming language world, and the possibility to have this is, as many things, the result of Haskell's excellent type system. In comparison to dynamic and "unsafe" languages, we actually have a lot of information around that we can use to write powerful and expressive tools. Splitting into GSoC tasks ------------------------- I think that the project is too large for one summer and should be split into two parts: 1. Implementation of a "full-source" transformation-enabling AST 2. The transformation engine and API / DSL About 1: Creating a full-source AST Existing parsers for Haskell tend to throw away information that is not necessary for compiling the code. Of course this is not good for a code-to-code transformation tool, stripping comments or whitespace the programmer cares about is not an option. haskell-src-exts has gained support for comments around version 1.1 (as another GSoC project?), but still comments are treated as foreigners, given to you in a list detached from the AST; their positions are specified with integers, which means that when you modify the AST, you have to take care to adjust the comments as well. The problem seems to be that haskell-source-exts is made for parsing, its AST is made for transformations. I have yet to see a tool that actually *modifies* haskell-src-exts's AST - most of them use it for parsing, and then apply some form of pretty-printing to get back to code. We need an AST that contains *all* information about the original source code, which means render . parse = id; this is why I called it "full source" AST. Also, this AST should have all elements available in a way that encourages modification and does not discriminate against operations like re-indentation or alignment. I like to think of this as something that could eventually be part of ghc, in a pipeline like GHC's parser -> full-source AST -> reduced AST that strips parts irrelevant for compilation -> usual compilation pipeline I am thinking of this being in a GHC environment because it would guarantee that it is up-to-date with the currently supported language extensions etc. However, I also see that this might slow down development and make things harder, so at least for the beginning it might be better to do it as as a normal library; I would love to get feedback on this. It might be a good idea to start with GHC's or haskell-source-exts parser and modify them to obtain the full-source AST. I believe though that it would be beneficial if the AST would be to a certain extent be decoupled from the parser, such that other people could write other parsers (e.g. using other parsing libraries like Parsec, attoparsec, uu-parsinglib, trifecta) that produce the same compatible AST type. I believe that creating this AST makes a GSoC project for itself, and that it would be beneficial for all efforts processing Haskell as a source language. About 2: Transformation engine and how to write transformations Transformations should be Haskell code that operates on the AST. They would be similar to how you write TemplateHaskell in a way, yet much easier and more intuitive to use in most cases. I suggest the creation of a monadic DSL that allows you to select and match those parts of the code you are interested in, perform some case analysis and computation to determine how you want to transform it, and then give you convenient ways to express your changes in that DSL. Functions in this DSL would roughly belong to one of the kinds * finding/matching * transforming/rewriting. I have not thought about how this DSL would exactly look like, but I could imagine myself writing a high-level transformation like this: is <- getImports let matching = filter (importsFunction "catch") . filter (hasModuleName "Prelude") mapM_ (rewrite . removeImportFunction "catch") matching One of the priority goals of this GSoC project is making this API convenient; transformations should read more naturally and be much shorter than equivalent TemplateHaskell. This extensive, convenience oriented monadic DSL should aim to have a reasonably large set of functions, and "do things for you" even if you could build them yourself with three or four combinators. It should base itself on a minimal set of core transformations. I think that the Haskell community has a lot of expertise, especially from parsing libraries, in creating this core API + convenience combinators duo, that could aid the GSoC student to get this right. Along with this way to specify transformations, a transformation engine has to be constructed which can perform them somewhat efficiently over large code-bases. While speed should should not be a main target for this project and functionality and expressiveness are the main goals, the transformation engine should be designed at least with speed in mind, which can be worked on at a later stage outside of the GSoC project. This second part of the proposal should result in a library to perform transformations and an executable that can apply them to a folder of source files. Possible extensions and follow-up projects ------------------------------------------ * Interactive refactoring tool The refactoring tool could be driven by an interactive application that lets you write some transformations, apply them to your code base, view the diffs and build the project; on build failures or otherwise not-complete transformations it would allow to refine your transformations, or write custom special cases for selected files, line ranges or functions. This way you could quickly upgrade your code base in a fast write-check-repeat cycle. * An API upgrade infrastructure As mentioned above, transformations could be published along with new software packages that allow their dependants to easily upgrade their code to the latest API version with minimal manual effort. An infrastructure or standard way of way of doing this could be established. For now, I think that package maintainers would publish their upgrade transformations in their projects' or separate code repositories, and users would apply them with the refactoring tool as needed. I can also imagine a more dedicated infrastructure though, where API upgrades are stored on Hackage or a similar database. Summary ------- This proposal contains two GSoC projects that I believe to be reasonably sized for one summer each. I think that they make good projects according to GSoC standards as they are focused, feasible, and aimed at creating real-world code that bring a clearly visible benefit. The entry barrier is not too high since knowledge of compiler or runtime internals is not required. However, applicants should probably have a fairly good understanding of parsers and how they are dealt with in Haskell, and be somewhat familiar with the Haskell community in order to find sources of good feedback and for being able to judge whether the tools being created would be convenient for the community to use. Discussion ---------- Now I would be glad to get some responses on this proposal; I have written a bit of text, but it is still a very rough idea and I would love to hear your thoughts about it.

6 6

Fwd: Google Summer of Code, news
by Marcos Pividori 29 Apr '13

29 Apr '13

Greetings, I am a Computer Science student from Argentina. I am interested in working this summer in a project related to Haskell for the Google Summer of Code. I have been discussing my idea with Michael Snoyman in order to have a clearer idea. Now, I would like to know the community interest in this project. I want to develop a server-side library in Haskell for sending push notifications to devices running different OS, such as Android, iOS, Windows Phone, BlackBerry, and so on. To pass a subject, I have recently worked with Yesod (a Web Framework based in Haskell) developing a server to comunicate with Android-powered devices through Google Cloud Messaging. (It is available: https://github.com/MarcosPividori/Yesod-server-for-GCM ) To develop this project, I have read a lot about this service and Yesod libraries, and I developed two programs, a server written in Haskell and an Android application for mobile phones. Also, I developed an EDSL to write programs which exchange information with the devices. I would be grateful if you could give me your opinion about this project and the proposal I am starting to write. *Proposal GSoC 2013:* *Abstract* The aim of this project is to develop a server-side library in Haskell for sending push notifications to devices running different OS, such as Android, iOS, Windows Phone, BlackBerry, and so on. The fact is that every company is developing Push Notification services, and these are very similar. Then, I want to find the fundamental concepts to construct a library which enable to configure the options for the different services and send messages easily. When I say they are very similar, I refer to the fact that they all are asynchronous, best-effort services that offers third-party developers a channel to send data to apps from a cloud service in a power-efficient manner. The most popular are: - Google Cloud Messaging (Android) - Apple Push Notification Service (iPhone / iPad) - Microsoft Push Notification Service (Windows Phone) - BlackBerry Push Service (BlackBerry) - Windows Push Notification Services (Windows 8) - etc. *Motivation and expected benefits* I think my idea would be very useful because it will allow all Haskell developers to open to a new world of mobile devices and to build useful programs/services that interact with them. Pushing data to smartphones provides users with instant access to desired updates as they happen, such as news and weather, sports scores, stock prices and other time-sensitive content. The push services provide an efficient way to quickly push timely information updates to many smartphones at once, in a centrally managed and controlled manner. Generally, you can also be very selective in who you send information to, including individual customers or many customers (multicast). This services minimizes the impact on the smartphones battery life. Instead of actively checking for new data, the applications can remain closed. Once the data is delivered, the application can be launched in the background to process it as needed. This processes offer an alternative to other less efficient methods, such as polling, where a device regularly polls an application server to see if new content is available. The main differences between the services, refer to details as: the maxim payload length, the quality of service, queueing the messages or not, and the time limit for this, the way the messages are handled in the devices, etc. As all the libraries to access to these services are developed in Java, I thought that it would be a good idea to offer an option to Haskell programmers. Taking advantage of the similarity of these services, I could develop a very adaptable library which fits the necessities for each one and at the same time offer an abstraction to the user. *Deliverables.* * An API library to build and send messages including: - GCM and a demo Android app. - APN and a demo iOS app. - Microsoft Push Notification Service (Windows Phone) and a demo app. - Documentation for all the code developed. Including the explantation on how to use the server library and how to try the demo apps. * A demo server taking advantage of this libraries to communicate with the demo apps through push notifications. * Optionally: (Only in the case that I finalize the rest of the objectives before the deadline) - API for communication through BlackBerry Push Service (BlackBerry). - API for communication through Windows Push Notification Services (Windows 8). *Technical Considerations* I have to complete this.... *Timeline* May 27: (Accepted students announced) - 'Community Bonding Period' (~1 month) + Get to know mentor(s). + Refine this proposal with mentor(s). + Set up svn accounts. + Set up a wiki page or blog for this project. + Make sure that everything is ready for coding. + Try to involve the community as much as possible, ask for new ideas/suggestions/etc. June 17: (Start of the program) - 'Library to use the GCM Service' (Total: ~4 weeks) + Familiarize with the GCM service and understand the details related. (~1 week) + Develope the server library to send GCM messages and a demo Android app. (~2 weeks) + Document all the code developed, and check details. (~1 week) - 'Library to use the APNS Service' Part 1 (Total: ~2 weeks) + Familiarize with the APNS service and understand the details related. (~1 week) + Start to develope the server library to send APNS Push Notifications and a demo iOS app. (~1 week) July 29: (Midterm evaluation period begins) - 'Midterm evaluation period' (~1 week) + Submit the midterm review. August 2: (Midterm evaluation deadline) - 'Library to use the APNS Service' Part 2 (Total: ~2 weeks) + Continue to develope the server library to send APNS Push Notifications and a demo iOS app. (~1 week) + Document all the code developed, and check details. (~1 week) - 'Library to use the MPNS Service' (Total: ~4 weeks) + Familiarize with the MPNS service and understand the details related. (~1 week) + Develope the server library to send MPNS Push Notifications and a demo Windows Phone app. (~2 weeks) + Document all the code developed, and check details. (~1 week) September 16: (Suggested pencils down date) - 'Pencils down' (~1 week) + Final code clean up. + Present library to the community and analyze feedback. September 23: (Firm pencils down date) - 'Summing up' (~1 week) + Submit Final Code to Google. + Present final evaluation. + Analyze possible future work and document it. September 23: (Final evaluation deadline) *About myself and my interest in this project* -------

4 3

GSoC Push Notifications project - communicating with mobile devices
by Marcos Pividori 29 Apr '13

29 Apr '13

Sorry, I am resending this email because I didn't write a correct title before. ------- Greetings, I am a Computer Science student from Argentina. I am interested in working this summer in a project related to Haskell for the Google Summer of Code. I have been discussing my idea with Michael Snoyman in order to have a clearer idea. Now, I would like to know the community interest in this project. I want to develop a server-side library in Haskell for sending push notifications to devices running different OS, such as Android, iOS, Windows Phone, BlackBerry, and so on. To pass a subject, I have recently worked with Yesod (a Web Framework based in Haskell) developing a server to comunicate with Android-powered devices through Google Cloud Messaging. (It is available: https://github.com/MarcosPividori/Yesod-server-for-GCM ) To develop this project, I have read a lot about this service and Yesod libraries, and I developed two programs, a server written in Haskell and an Android application for mobile phones. Also, I developed an EDSL to write programs which exchange information with the devices. I would be grateful if you could give me your opinion about this project and the proposal I am starting to write. *Proposal GSoC 2013:* *Abstract* The aim of this project is to develop a server-side library in Haskell for sending push notifications to devices running different OS, such as Android, iOS, Windows Phone, BlackBerry, and so on. The fact is that every company is developing Push Notification services, and these are very similar. Then, I want to find the fundamental concepts to construct a library which enable to configure the options for the different services and send messages easily. When I say they are very similar, I refer to the fact that they all are asynchronous, best-effort services that offers third-party developers a channel to send data to apps from a cloud service in a power-efficient manner. The most popular are: - Google Cloud Messaging (Android) - Apple Push Notification Service (iPhone / iPad) - Microsoft Push Notification Service (Windows Phone) - BlackBerry Push Service (BlackBerry) - Windows Push Notification Services (Windows 8) - etc. *Motivation and expected benefits* I think my idea would be very useful because it will allow all Haskell developers to open to a new world of mobile devices and to build useful programs/services that interact with them. Pushing data to smartphones provides users with instant access to desired updates as they happen, such as news and weather, sports scores, stock prices and other time-sensitive content. The push services provide an efficient way to quickly push timely information updates to many smartphones at once, in a centrally managed and controlled manner. Generally, you can also be very selective in who you send information to, including individual customers or many customers (multicast). This services minimizes the impact on the smartphones battery life. Instead of actively checking for new data, the applications can remain closed. Once the data is delivered, the application can be launched in the background to process it as needed. This processes offer an alternative to other less efficient methods, such as polling, where a device regularly polls an application server to see if new content is available. The main differences between the services, refer to details as: the maxim payload length, the quality of service, queueing the messages or not, and the time limit for this, the way the messages are handled in the devices, etc. As all the libraries to access to these services are developed in Java, I thought that it would be a good idea to offer an option to Haskell programmers. Taking advantage of the similarity of these services, I could develop a very adaptable library which fits the necessities for each one and at the same time offer an abstraction to the user. *Deliverables.* * An API library to build and send messages including: - GCM and a demo Android app. - APN and a demo iOS app. - Microsoft Push Notification Service (Windows Phone) and a demo app. - Documentation for all the code developed. Including the explantation on how to use the server library and how to try the demo apps. * A demo server taking advantage of this libraries to communicate with the demo apps through push notifications. * Optionally: (Only in the case that I finalize the rest of the objectives before the deadline) - API for communication through BlackBerry Push Service (BlackBerry). - API for communication through Windows Push Notification Services (Windows 8). *Technical Considerations* I have to complete this.... *Timeline* May 27: (Accepted students announced) - 'Community Bonding Period' (~1 month) + Get to know mentor(s). + Refine this proposal with mentor(s). + Set up svn accounts. + Set up a wiki page or blog for this project. + Make sure that everything is ready for coding. + Try to involve the community as much as possible, ask for new ideas/suggestions/etc. June 17: (Start of the program) - 'Library to use the GCM Service' (Total: ~4 weeks) + Familiarize with the GCM service and understand the details related. (~1 week) + Develope the server library to send GCM messages and a demo Android app. (~2 weeks) + Document all the code developed, and check details. (~1 week) - 'Library to use the APNS Service' Part 1 (Total: ~2 weeks) + Familiarize with the APNS service and understand the details related. (~1 week) + Start to develope the server library to send APNS Push Notifications and a demo iOS app. (~1 week) July 29: (Midterm evaluation period begins) - 'Midterm evaluation period' (~1 week) + Submit the midterm review. August 2: (Midterm evaluation deadline) - 'Library to use the APNS Service' Part 2 (Total: ~2 weeks) + Continue to develope the server library to send APNS Push Notifications and a demo iOS app. (~1 week) + Document all the code developed, and check details. (~1 week) - 'Library to use the MPNS Service' (Total: ~4 weeks) + Familiarize with the MPNS service and understand the details related. (~1 week) + Develope the server library to send MPNS Push Notifications and a demo Windows Phone app. (~2 weeks) + Document all the code developed, and check details. (~1 week) September 16: (Suggested pencils down date) - 'Pencils down' (~1 week) + Final code clean up. + Present library to the community and analyze feedback. September 23: (Firm pencils down date) - 'Summing up' (~1 week) + Submit Final Code to Google. + Present final evaluation. + Analyze possible future work and document it. September 23: (Final evaluation deadline) *About myself and my interest in this project* -------

2 2

Looking for portable Haskell or Haskell like language
by Christopher Howard 29 Apr '13

29 Apr '13

Hi. I've got this work situation where I've got to do all my work on /ancient/ RHEL5 systems, with funky software configurations, and no root privileges. I wanted to install GHC in my local account, but the gnu libc version is so old (2.5!) that I can't even get the binary packages to install. I've had success installing some other simple functional languages (like CLISP) on these same systems, so I was wondering if there was perhaps another language very similar to Haskell (but presumably simpler) with a super portable compiler easily built from source, which I could try. I'll admit -- I haven't tried the HUGS compiler for Haskell. The quick description didn't make it sound much more portable than GHC, but I guess I could try it if I heard some good reasons to think it would be more portable. -- frigidcode.com

13 13

Space leak in hexpat-0.20.3/List-0.5.1
by wren ng thornton 29 Apr '13

29 Apr '13

Hello all, So I'm processing a large XML file which is a database of about 170k entries, each of which is a reasonable enough size on its own, and I only need streaming access to the database (basically printing out summary data for each entry). Excellent, sounds like a job for SAX. However, after whipping up a simplified version of the program using hexpat, there's a space leak. Near as I can tell, it's not a problem with my code, it's a problem with Data.List.Class (or hexpat's use thereof). The simplified code follows, just compile it for profiling and use hp2ps to see what I mean. The file I'm running it on can be found at: ftp://ftp.monash.edu.au/pub/nihongo/JMdict.gz Any ideas on what the problem really is, or how to fix it? ---------------------------------------------------------------- ---------------------------------------------------------------- module JMdict (main) where import Text.XML.Expat.SAX (SAXEvent(..)) import qualified Text.XML.Expat.SAX as SAX import Text.XML.Expat.Tree (NodeG(..)) import qualified Text.XML.Expat.Tree as DOM import qualified Text.XML.Expat.Proc as Proc import qualified Text.XML.Expat.Internal.NodeClass as Node import qualified Data.ByteString.Lazy as BL import Data.Char (isSpace) import Data.Text.IO as TIO import qualified Data.Text as T import Control.Applicative ((<$>)) import Control.Monad (forM_) import qualified System.IO as IO import qualified System.Environment as Sys (getArgs) import qualified System.Exit as Sys (exitFailure) import qualified System.Directory as Sys (doesFileExist, getPermissions, readable) ---------------------------------------------------------------- -- | A variant of 'Control.Monad.unless' for when the boolean is -- also monadic. unlessM :: Monad m => m Bool -> m () -> m () unlessM mb handle = do b <- mb if b then return () else handle -- | If the file does not exist or is not readable, then crash the -- program. assertFileExistsReadable :: FilePath -> IO () assertFileExistsReadable file = do unlessM (Sys.doesFileExist file) $ do IO.hPutStrLn IO.stderr $ "No such file: "++file Sys.exitFailure unlessM (Sys.readable <$> Sys.getPermissions file) $ do IO.hPutStrLn IO.stderr $ "File not readable: "++file Sys.exitFailure main :: IO () main = do files <- Sys.getArgs forM_ files $ \file -> do assertFileExistsReadable file countElements 0 . filter (not . isWhitespace) . dropPreamble . SAX.parse SAX.defaultParseOptions =<< BL.readFile file where dropPreamble (StartElement t _ : xs) | t == T.pack "JMdict" = xs dropPreamble (_:xs) = dropPreamble xs dropPreamble [] = [] isWhitespace (CharacterData c) | T.all isSpace c = True isWhitespace _ = False countElements :: Int -> [SAXEvent T.Text T.Text] -> IO () countElements n [] = print n countElements n xs = case anyElement xs of (Left err, xs') -> fail $ show err ++": "++ show (take 3 xs') (Right ell, xs') -> do print (n+1) (countElements $! n+1) xs' ---------------------------------------------------------------- data ElementError = EmptyStream | NoStartElement | EndOfStream | InvalidXML deriving (Read, Show, Eq) -- | Split an event stream into an initial element and the remainder -- of the stream. Use 'DOM.saxToTree' to convert the element to a -- tree. anyElement :: (Eq tag) => [SAXEvent tag text] -> (Either ElementError [SAXEvent tag text], [SAXEvent tag text]) anyElement = start where start [] = (Left EmptyStream, []) start xxs@(x:xs) = case x of StartElement t _ -> loop [t] (x:) xs _ -> (Left NoStartElement, xxs) loop _ _ [] = (Left EndOfStream, []) loop [] k xs = (Right (k []), xs) loop tts@(t:ts) k xxs@(x:xs) = step (k . (x:)) xs where step = case x of StartElement t' _ -> loop (t':tts) EndElement t' | t' == t -> loop ts | otherwise -> \_ _ -> (Left InvalidXML, xxs) _ -> loop tts ---------------------------------------------------------------- ----------------------------------------------------------- fin. -- Live well, ~wren

1 0

Why were datatype contexts removed instead of "fixing them"?
by harry 28 Apr '13

28 Apr '13

If I understand correctly, the problem with datatype contexts is that if we have e.g. data Eq a => Foo a = Foo a the constraint Eq a is thrown away after a Foo is constructed, and any method using Foos must repeat Eq a in its type signature. Why were these contexts removed from the language, instead of "fixing" them? PS This is following up on a discussion on haskell-beginners, "How to avoid repeating a type restriction from a data constructor". I'm interested in knowing whether there's a good reason not to allow this, or if it's just a consequence of the way type classes are implemented by compilers.

10 17

Display an inferred type during compilation
by Corentin Dupont 28 Apr '13

28 Apr '13

Hi Cafe, can I ask the compiler to display the type of an inferred value during compile time? It would be great if I can output a string during compilation with the type. A little bit like running :type in GHCi, but without GHCi... Because running GHCi is sometime painful (I have to clean my code first). I'm thinking of something like: main :: IO () main = do a <- someCode displayTypeAtCompileTime a return () $ ghc -c test.hs test.hs:4:3: your type is: Foo Thanks, Corentin

3 3

Diving into the records swamp (possible GSoC project)
by Adam Gundry 28 Apr '13

28 Apr '13

Hi, I am hoping to do a GSoC project this year working on GHC, and have been pointed in the direction of the records issue (in particular, the desire to overload field names). This has been discussed on-and-off for years, and while there are lots of ideas [1], little has been implemented in GHC itself. The plan would be to implement a solution to the "narrow issue" of overloaded field names, along the lines of Simon PJ's SORF proposal (on the wiki). This would provide a basis for experimentation with first-class record types. While there are still design issues to resolve, the broad plan is clear and I'm confident it can be implemented in a summer and without overly restricting future record system designs. Does this sound like a reasonable strategy? I'd appreciate comments and criticism, although arguing about the details of the design should perhaps wait. (A little about me: I'm a PhD student working on type inference for Haskell and dependent types, with about four years' Haskell experience including work on big type-system related projects. I am familiar with the theory behind GHC, but I haven't worked on the code before.) Thanks, Adam Gundry [1] http://hackage.haskell.org/trac/ghc/wiki/Records

6 11

text-icu on Windows
by John MacFarlane 28 Apr '13

28 Apr '13

Hello café: I'd very much like to get text-icu working on Windows, as then I could ship pandoc binaries that do proper unicode collation in bibliographies. But I'm having a devil of a time. This may be due to my very limited Windows knowledge. Any help would be appreciated, especially from someone who actually has text-icu working on Windows. I was able to cabal install text-icu without errors. I used --extra-lib-dirs and --extra-include-dirs to point to the lib and include directories in the 32-bit binary distribution of icu4c. I'm using the latest Haskell Platform, 2012.4.0.0 with ghc 7.4.2. I was also able to build the following simple program that uses text-icu, by doing ghc --make icu.hs: ~~~ -- icu.hs import Data.Text.ICU main = print $ Locale "tr-TR" ~~~ No errors or warnings in either of these steps. But when I try to run the compiled program, icu.exe, I get no output at all. I expected to get a line with 'Locale "tr-TR"', but instead I get nothing -- not even an error or warning. This remains the case if I try ~~~ main = do print "Start" print $ Locale "tr-TR" print "Done" ~~~ 'echo $?' yields False. Any ideas? John (I've posted a similar question to StackOverflow [1], but it hasn't gotten an answer yet there, so I thought I'd try here.) [1]: http://stackoverflow.com/questions/16127710/how-do-i-get-text-icu-working-o…)

3 4

HList with DataKinds [Was: Diving into the records swamp (possible GSoC project)]
by oleg＠okmij.org 28 Apr '13

28 Apr '13

Aleksandar Dimitrov wrote: > I've been kicking around the idea of re-implementing HList on the basis of the > new DataKinds [1] extension. The current HList already uses DataKinds (and GADTs), to the extent possible with GHC 7.4 (GHC 7.6 supports the kind polymorphism better, but it had a critical for me bug, fixed since then.) See, for example http://code.haskell.org/HList/Data/HList/HListPrelude.hs (especially the top of the file). or, better http://code.haskell.org/HList/Data/HList/HList.hs HList now supports both type-level folds and unfolds.

1 0

Markdown extension for Haddock as a GSoC project
by Chris Smith 28 Apr '13

28 Apr '13

Oops, forgot to reply all. ---------- Forwarded message ---------- From: "Chris Smith" <cdsmith(a)gmail.com> Date: Apr 27, 2013 12:04 PM Subject: Re: [Haskell-cafe] Markdown extension for Haddock as a GSoC project To: "Bryan O'Sullivan" <bos(a)serpentine.com> Cc: I don't agree with this at all. Far more important than which convention gets chosen is that Haskell code can be read and written without learning many dialects of Haddock syntax. I see an API for pluggable haddock syntax as more of a liability than a benefit. Better to just stick to what we have than fragment into more islands. I do think that changing Haddock syntax to include common core pieces of Markdown could be a positive change... but not if it spawns a battle of fragmented documentation syntax that lasts a decade. On Apr 27, 2013 11:08 AM, "Bryan O'Sullivan" <bos(a)serpentine.com> wrote: > On Sat, Apr 27, 2013 at 2:23 AM, Alistair Bayley <alistair(a)abayley.org>wrote: > >> How's about Creole? >> http://wikicreole.org/ >> >> Found it via this: >> >> http://www.wilfred.me.uk/blog/2012/07/30/why-markdown-is-not-my-favourite-l… >> >> If you go with Markdown, I vote for one of the Pandoc implementations, >> probably Pandoc (strict): >> http://johnmacfarlane.net/babelmark2/ >> >> (at least then we're not creating yet another standard...) >> > > Probably the best way to deal with this is by sidestepping it: make the > support for alternative syntaxes as modular as possible, and choose two to > start out with in order to get a reasonable shot at constructing a suitable > API. > > I think it would be a shame to bikeshed on which specific syntaxes to > support, when a lot of productive energy could more usefully go into > actually getting the work done. Better to say "prefer a different markup > language? code to this API, then submit a patch!" > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

3 2

partially applied data constructor and corresponding type
by TP 28 Apr '13

28 Apr '13

Hello, I ask myself if there is a way to do the following. Imagine that I have a dummy type: data Tensor = TensorVar Int String where the integer is the order, and the string is the name of the tensor. I would like to make with the constructor TensorVar a type "Vector", such that, in "pseudo-language": data Vector = TensorVar 1 String Because a vector is a tensor of order 1. Is this possible? I have tried type synonyms and newtypes without any success. Thanks a lot, TP

2 2

Operations on functional graphs
by Francesco Mazzoli 28 Apr '13

28 Apr '13

Hi list, I’ve been lately thinking about how to implement an algorithm efficiently, and I need a directed graph that can perform the following tasks: 1. Finding the strongly connected components 2. Condensing strongly connected components 3. Contract single edges The condensing shouldn’t prevent successive operations to work with the condensed vertices (treating them all as the same), but should get rid of the edges. Point one is easy, for example as described in [1]. I’m wondering if a nice way to implement the other two with functional structures has been described. I’d guess it would be a mix of a graph and disjoint sets data structure... Thanks, Francesco [1]: Structuring Depth-First Search Algorithms in Haskell, by David King and John Launchbury.

1 1

Request for help: Recompile of ghc producing "...-ghc7.4.2.so"s, which break project builds.
by Captain Freako 27 Apr '13

27 Apr '13

Recently, I had to recompile ghc, in order to get the "-dyn" versions of the standard libraries installed. (The standard Ubuntu 12.10 64-bit Linux distribution doesn't include them in its "haskell-platform" package, and you can't upgrade "base" using the normal "cabal iinstall" approach, from what I understand.) When I did this, the "-dyn" versions of the standard libraries were, in fact, installed. However, they were given names with a "-ghc7.4.2.so" suffix. And this is causing the build of a previously working project to break, thusly: ghc -o libami.so -shared -dynamic -package parsec *-lHSrts* -lm -lffi -lrt AMIParse.o AMIModel.o ami_model.o ExmplUsrModel.o Filter.o /usr/bin/ld: /usr/lib/ghc-7.4.2/*libHSrts.a*(RtsAPI.o): relocation R_X86_64_32S against `ghczmprim_GHCziTypes_Czh_con_info' can not be used when making a shared object; recompile with -fPIC The problem is this: the linker is picking up the "*.a" version of the * HSrts* library, instead of the "*.so" version, which is what it really needs. And the reason it's doing this is the "*.so" version has been given a "-ghc7.4.2" suffix: dbanas@dbanas-lap:~/prj/AMI-Tool$ l /usr/lib/ghc-7.4.2/libHSrts* /usr/lib/ghc-7.4.2/libHSrts.a {snip} /usr/lib/ghc-7.4.2/libHSrts*-ghc7.4.2*.so* {snip} So, it seems to me that I need to either: 1. Tell the linker about the possibility of a "-ghc7.4.2" file name suffix, or 2. rebuild my "*.so"s without that suffix. *Can anyone tell me:* - *How to accomplish either #1 or #2, above?* - *Which one is the preferred solution to the problem?* - *Am I completely off in the weeds?* - *If so, what should I really be doing, in order to fix this issue?* Thanks! -db

3 2

Re: [Haskell-cafe] Why were datatype contexts removed instead of "fixing them"?
by wren ng thornton 26 Apr '13

26 Apr '13

On 4/25/13 9:49 PM, Dan Doel wrote: > I don't really think they're worth saving in general, though. I haven't > missed them, at least. The thing I've missed them for (and what I believe they were originally designed for) is adding constraints to derived instances. That is, if I have: data Bar a => Foo a = ... deriving Baz Then this is equivalent to: data Foo a = ... instance Bar a => Baz (Foo a) where ... where the second ellipsis is filled in by the compiler. Now that these constraints have been removed from the language, I've had to either (a) allow instances of derived classes which do not enforce sanity constraints, or (b) implement the instances by hand even though they're entirely boilerplate. The behavior of these constraints is certainly unintuitive for beginners, but the constraints themselves are very helpful when programming with phantom types and type-level functions for constraints. -- Live well, ~wren

2 1

Instances for continuation-based FRP
by Hans Höglund 26 Apr '13

26 Apr '13

Hi everyone, I am experimenting with various implementation styles for classical FRP. My current thoughts are on a continuation-style push implementation, which can be summarized as follows. > newtype EventT m r a = E { runE :: (a -> m r) -> m r -> m r } > newtype ReactiveT m r a = R { runR :: (m a -> m r) -> m r } > type Event = EventT IO () > type Reactive = ReactiveT IO () The idea is that events allow subscription of handlers, which are automatically unsubscribed after the continuation has finished, while reactives allow observation of a shared state until the continuation has finished. I managed to write the following Applicative instance > instance Applicative (ReactiveT m r) where > pure a = R $ \k -> k (pure a) > R f <*> R a = R $ \k -> f (\f' -> a (\a' -> k $ f' <*> a')) But I am stuck on finding a suitable Monad instance. I notice the similarity between my types and the ContT monad and have a feeling this similarity could be used to clean up my instance code, but am not sure how to proceed. Does anyone have an idea, or a pointer to suitable literature. Best regards, Hans

5 7

pattern matching on data families constructors
by Alexey Egorov 26 Apr '13

26 Apr '13

Hi, suppose that there is following data family: > data family D a > data instance D Int = DInt Int > data instance D Bool = DBool Bool it is not possible to match on constructors from different instances: > -- type error > a :: D a -> a > a (DInt x) = x > a (DBool x) = x however, following works: > data G :: * -> * where > GInt :: G Int > GBool :: G Bool > > b :: G a -> D a -> a > b GInt (DInt x) = x > b GBool (DBool x) = x The reason why second example works is equality constraints (a ~ Int) and (a ~ Bool) introduced by GADT's constructors, I suppose. I'm curious - why data families constructors (such as DInt and DBool) doesn't imply such constraints while typechecking pattern matching? Thanks.

3 6

Munich Haskell Meeting
by Heinrich Hördegen 25 Apr '13

25 Apr '13

Dear all, our next regular meeting will take place in Munich on the 29th of April at 19h30 at Cafe Puck. If you plan to join us, please go to http://www.haskell-munich.de/dates and click the button. Until then, have a nice time, Heinrich

1 0

Haskell training in San Francisco Bay Area and New York
by Duncan Coutts 25 Apr '13

25 Apr '13

Well-Typed are offering Haskell courses in the San Francisco Bay Area and New York in early June. They are for professional developers who want to learn Haskell or improve their skills. There is a 2-day introductory course and a 2-day advanced course. Full course and registration details: http://www.well-typed.com/services_training Well-Typed are running these courses in partnership with FP Complete and Skills Matter. Locations, dates ================ San Francisco Bay Area * Introductory Course: June 4-5th, 2013 * Advanced Course: June 6-7th, 2013 New York * Introductory Course: June 10-11th, 2013 * Advanced Course: June 12-13th, 2013 * Early bird discount before April 29th -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

1 0

Why the `-ghc7.4.2' suffix on *.SO base names?
by Captain Freako 25 Apr '13

25 Apr '13

In trying to compile a 2 year old, previously working project on a new machine with a fresh Haskell Platform installation, I bumped into this: ghc -o libami.so -shared -package parsec -lHSrts -lm -lffi -lrt AMIParse.o AMIModel.o ami_model.o ExmplUsrModel.o Filter.o /usr/bin/ld: /usr/lib/ghc-7.4.2/libHSrts.a(RtsAPI.o): relocation R_X86_64_32S against `ghczmprim_GHCziTypes_Czh_con_info' can not be used when making a shared object; recompile with -fPIC /usr/lib/ghc-7.4.2/libHSrts.a: could not read symbols: Bad value collect2: error: ld returned 1 exit status When I investigated why the *.A, instead of the *.SO, was being picked up, I found that the *.SO had a `-ghc7.4.2' appended to its base name. Is this new? What is its purpose? Do I need to add a `LIB_SUFFIX' variable to my makefile and append it to all of my `-l<...>'s, or is there a more elegant solution? Thanks! -db

1 0

Fwd: How to do automatic reinstall of all dependencies?
by capn.freako＠gmail.com 25 Apr '13

25 Apr '13

-db ----- Forwarded message ----- From: "Captain Freako" <capn.freako(a)gmail.com> Date: Tue, Apr 23, 2013 9:21 pm Subject: How to do automatic reinstall of all dependencies? To: <haskell-cafe(a)haskell.org>

4 3

Re: [Haskell-cafe] Instances for continuation-based FRP
by Conal Elliott 25 Apr '13

25 Apr '13

The intuition intrigues me. If, upon inspection, it survives morphs into something else, I'd like to hear about it. Good luck! -- Conal "The object of mathematical rigor is to sanction and legitimize the conquests of intuition, and there was never any other object for it." - Jacques Hadamard "I call intuition cosmic fishing. You feel a nibble, then you've got to hook the fish." -- Buckminster Fullero On Wed, Apr 24, 2013 at 7:26 PM, earl obscure <theanswertoproblems(a)gmail.com > wrote: > > His description of the different frp approaches starts at section 2.1 of > the thesis. > http://www.testblogpleaseignore.com/wp-content/uploads/2012/04/thesis.pdfTh… in 3.1 describes implementation of discrete signals. I don't think he > gives a denotational semantics. > I was thinking, the event, is the derivative of the specific continuous > signal it corresponds to, all other continuous signals of the system held > equal. Applying the partial derivative, is like sampling, or discrete time > stepping. But it is samplying the entire state, or multivariate structure > not just the specific symbol. This made more sense unarticulated. I'll > need to think a bit.

1 0

Re: [Haskell-cafe] Instances for continuation-based FRP
by Conal Elliott 25 Apr '13

25 Apr '13

Hi Jared, Oh -- does Elm have a denotational semantics? I haven't heard of one. I just now skimmed the informal description of the Signal type<http://elm-lang.org/docs/Signal/Signal.elm>, and from the reference to "updates" in the description of "merge", it sound like whatever semantics it might have, it couldn't be function-of-time. I'm intrigued with your interpretation. I wonder what it could mean for an event to be a derivative, especially partial one, and for arbitrary types. -- Conal On Wed, Apr 24, 2013 at 1:48 PM, earl obscure <theanswertoproblems(a)gmail.com > wrote: > Hi Conal, > > > Caveat pre-emptor I'm new to haskell, frp, etc.... .. anyway how I was > interpreting Elm's Eventbased strict FRP, was that each event was the > partial derivative of the continuous time variable, and then since it was > being strict, it would evaluate the tangent line or state of the system at > that point, only update when necessary. > > > Now related to Continuations, this is something I've been thinking about > as well,but haven't gotten very far; apparently cont monad, and comonad are > closely related. I was hoping to use the comonad rules, extend/duplicate > to encode different continuations paths, and then extract when, a > continuation path is chosen. Was hoping maybe the analog would be PDE's, > or something more general than my interpretation of Elm's FRP. > > These are just random thoughts that I wanted to get out. Thanks. > > Jared Nicholson. > >

1 0

Request for help with shared object link failure, involving relocation and -fPIC.
by Captain Freako 25 Apr '13

25 Apr '13

Can anyone help me solve this link failure? ghc -o libami.so -shared -package parsec -lHSrts -lm -lffi -lrt AMIParse.o AMIModel.o ami_model.o ExmplUsrModel.o Filter.o /usr/bin/ld: /usr/lib/ghc-7.4.2/libHSrts.a(RtsAPI.o): relocation R_X86_64_32S against `ghczmprim_GHCziTypes_Czh_con_info' can not be used when making a shared object; recompile with -fPIC /usr/lib/ghc-7.4.2/libHSrts.a: could not read symbols: Bad value collect2: error: ld returned 1 exit status Thanks! -db

1 0

Re: [Haskell-cafe] Implementing an embedded language that threads a global structure.
by Ian Bloom 24 Apr '13

24 Apr '13

Incidentally I did find a solution that fits into the type system and also produces the correct result. This was done by making the global parameter change type as well at each lambda so that it becomes a function with the same arrity as the term. Unfortunately to evaluate these terms the program needs fork and do redundant work at every application of lam (so it's a major hack), though it does give me some confidence that a real solution is possible perhaps by using dependent types?: ---- lam :: ((g->(g,a)) -> (g->(g',b))) -> (g->(a->g',a->b)) lam = (\ f -> \ g -> (\x -> fst $ (f (\ gv->(gv,x))) g, \y -> snd $ (f (\ gv->(gv,y))) g )) app :: (g->(a->g1,a->b)) -> (g->(g,a)) -> (g->(g1,b)) app = (\ eA eB -> \ gz -> let (gB, fB) = eB gz in let (lgA, lfA) = eA gB in (lgA fB, lfA fB) ) ext :: ((g1,a)->(g2,b))->(g0->(g1,a))->(g0->(g2,b)) ext = \f -> \a -> \g -> f (a g) idCounter :: Num g => (g,a) -> (g, a) idCounter = \(g, x) -> (g+1, x) -- Example terms trmA = lam (\x -> lam (\y -> app x y)) trmB = (lam (\y -> app trmC y)) trmC = (\g -> (\c->g,\c->c+3)) trmD = (\g -> (g,3)) trmE = app trmC trmD trmF = (ext idCounter) trmD trmG = app trmC (app trmC trmF) trmH = app trmC trmE --- Here's my original statement of the problem http://hastebin.com/raw/bewiqihiyo And here's all my code with new, http://hpaste.org/86273 Unfortunately, I don't know of a way to tell the compiler that essentially the variables x and y in the lam function will always be applied to the same value. Unfortunately, I've reached a limit of my understanding of how pairs integrate with the type system in Haskell, I've considered returning something other than a pair from my base function (such as a function) but I have yet to figure that out. Thanks for your insights, Ian Bloom

1 0