- Haskell-Cafe - Haskell.org

[ANN] persistent-odbc
by grant weyburne 23 Mar '14

23 Mar '14

Hi, This package contains Persistent-compatible database backends using ODBC. It currently supports the following databases: MSSQL, MySql, Oracle, Sqlite, DB2 and Postgres. https://github.com/gbwey/persistent-odbc http://hackage.haskell.org/package/persistent-odbc-0.1.2.0/candidate Let me know if you have any problems or suggestions to improve it. Thanks, Grant

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No instance for (Eq v) trouble
by martin 22 Mar '14

22 Mar '14

Hello all, how can I make a type an instance of the Show class, if the functions used to implement show require e.g. Eq ? -------------------------------------------- I have a class: class Behavior b where at :: b a -> Day -> a and an instance data Change v = Change v [(Day,v)] deriving(Show) instance Behavior Change where ... and another instance (a Behavior of Behavior) data WeekdayPattern v = WeekdayPattern (Change v) instance Behavior WeekdayPattern where at (WeekdayPattern change) day = at change (day `mod` 7) And I wanted to write my own show function for WeekdayPattern. I started off like this: instance Show (WeekdayPattern v) where show wdp = show $ groupBy sameValue expanded where expanded = zip (map (at wdp) [0..6]) [0..6] sameValue x y = (fst x) == (fst y) ------------------------------------------ But I get: No instance for (Eq v) arising from a use of `sameValue' I believe the compiler wants to tell me, that it has no reason to assume that the values (the "v"s) can be compared for equality. Indeed the class itself and its "at" function do not require this (but it wouldn't hurt to add a constraint) I tried inserting an (Eq v) => in various places of the class or data declaration, but the compiler either didn't like that or it did not solve the problem. There is no problem just writing a show-like method as in xshow :: (Eq v, Show v) => (WeekdayPattern v) -> String but I cannot make this the "show" function of the Show class.

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Listing native package requirements based on cabal information
by David Thomas 22 Mar '14

22 Mar '14

Is there a way to extract this? I'm looking to make it easier for newcomers to my project to get things building across different linux distros.

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Re: [Haskell-cafe] Regular Expression Simplification
by Doug McIlroy 21 Mar '14

21 Mar '14

Apropos of regular expression simplification, > My first thought was: Maybe it would be possible to convert the regular expression to something like an NFA, > I'm slightly worried that the overhead of converting to an NFA is far too great for complicated regular expressions "Far too great" is only quadratic in the number of nonterminal symbols (with repetitions counted) in the regular expression. And the result is a particularly nice NFA, with one state for each of those symbols, and no epsilon moves. So I think there's merit in the proposal. See my Functional Pearl, JFP 14 (2004) 503-518, http://www.cs.dartmouth.edu/~doug/pubs.html Doug McIlroy ok(a)cs.otago.ac.nz

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SQL-like DSL for haskell List of Records
by martin 21 Mar '14

21 Mar '14

Hello all, I am currently prototyping some DB-related ideas in haskell. To make the next step I would have to write some joins. I am happy to work with just Lists of Records. Is there any DSL which relieves me from writing nested loops and all that caboodle? Doesn't have to provide SQL syntax either, just some expressiveness which comes close to basic SQL. Data volume will be miniscule.

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Re: [Haskell-cafe] Regular Expression Simplification
by Doug McIlroy 21 Mar '14

21 Mar '14

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How to obtain an expression type in a GHC compiler plugin?
by J. Stutterheim 21 Mar '14

21 Mar '14

Hi all, I'm trying to familiarise myself with writing GHC compiler plugins, but the GHC API is rather overwhelming, hence my question here. During compilation, I'm trying to determine whether a top-level function in my program has a certain type. Initially, if this is the case, I just want to print the function name. For me to do this check, I have to obtain the type for each top-level function. How can I achieve this? I already have a plugin with which I can print the name for a Bind, so I'm mainly interested in knowing how to obtain its type. Cheers, Jurriën

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PhD position at IT University of Copenhagen
by Iago Abal 21 Mar '14

21 Mar '14

The VARIETE project seeks an excellent PhD student to work on analysis methods for code and models found in highly configurable software systems (software product lines), especially in projects that use simple code generators and domain specific models. The objectives are to work on extensions of model checking and static analysis techniques for the verification of software systems implemented using model transformations. The ideal candidate has a solid background in semantics of programming languages and in algorithmic verification techniques (model checking, type checking, static analysis, satisfiability solving), combined with an appreciation for problem solving stemming from practice of software development. The project develops theories as well as tools. Functional programming skills are an advantage (but not essential). VARIETE is a highly prestigious research project awarded by the Danish Independent Research Council, within the Sapere Aude program. Project website: https://variete.wikit.itu.dk/ Positions are based in Copenhagen Denmark within the modern and lively research environment of IT University of Copenhagen (http://www.itu.dk <http://www.itu.dk/>). The position comes with entry level public servant salary (ca. 2200 EUR per month after taxes are deducted), social benefits (like paid vacation time, health care and pension saving scheme) along with a travel budget and all other support funds required for executing the research project. Prospective starting date is in fall 2014. Duration of the scholarship is 3 or 4 years. Personal contact via email prior to applying is strongly encouraged. Contact: Associate Professor Andrzej Wąsowski (wasowski(a)itu.dk) Postdoc Aleksandar Dimovski (adim(a)itu.dk) Research Group: Process and System Models < https://pure.itu.dk/portal/en/organisations/process-and-system-models%28780… >; Software and Systems Section <http://itu.dk/en/Om-IT-Universitetet/Organisation_tal_og_fakta> Deadline for application is April 23 at 23:59 CET. Please see the conditions of the call at https://delta.hr-manager.net/ApplicationInit.aspx?cid=119&ProjectId=168671&… Iago Abal Rivas

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RFC: GSoC proposal: Improve the Hackage login system
by Chris Wong 21 Mar '14

21 Mar '14

Hello Cafe, Like Matthew and Mateusz, I'm submitting a GSoC proposal as well! The initial draft is here: https://docs.google.com/document/d/1bcDiudULtaz3NFCqTHXD2WU6Ighsm199D7ojfS8… It's not a long read, so please do. Any comments, suggestions, &c would be appreciated greatly. If all is well, I will submit this in 48 hours. As an aside: would anyone like to mentor this project? Again, apologies for the late response. Thanks Chris

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Last call for Google Summer of Code 2014 applications
by Shachaf Ben-Kiki 21 Mar '14

21 Mar '14

Haskell.org is participating in Google Summer of Code again this year, and the student application deadline is nigh. Edward Kmett and I are the administrators this year. The deadline is 2014-03-21 at 19:00 UTC (noon PDT), or roughly 11 hours from now, so make sure to get your applications in before then. For a full timeline see <http://www.google-melange.com/gsoc/events/google/gsoc2014>. If you have any questions, feel free to ask on IRC (#haskell-gsoc on Freenode), or to contact me or Edward directly! Shachaf

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Re: [Haskell-cafe] Regular Expression Simplification
by Niklas Haas 21 Mar '14

21 Mar '14

On Thu, 20 Mar 2014 12:17:58 +0000, "Berg, Matias Juho" <m.berg.10(a)aberdeen.ac.uk> wrote: > ?Hi all, > > I am a final year undergraduate student at a university and I am doing my final honours project on natural language generation from regular expressions. For this to work efficiently I need to simplify the regular expressions before I translate them. It seems that there is some previous work done on this in Haskell but I have only been able to find this code (http://hackage.haskell.org/package/HaLeX-1.1/docs/src/Language-HaLex-RegExp…) which does some elementary simplification. > > Does anyone have any suggestions on where to look for more examples so I can see what kinds of attempts people have used to try and solve this problem? Also if someone has worked on this kind of problem was Kleene algebra a good starting point? > > Best regards, > > Matias My first thought was: Maybe it would be possible to convert the regular expression to something like an NFA, and then use one of the existing known algorithms to generate a minimal NFA from that, and then turn that back into a regular expression somehow? Although I'm slightly worried that the overhead of converting to an NFA is far too great for complicated regular expressions (finite doesn't mean reasonable), and that the conversion from NFA to regular expression will generate very suboptimal regular expressions despite the NFA being minimal. Maybe a similar analog could be used for another type of regular machine? Or a regular grammar, perhaps? Those are also fairly easy to bring into a normalized form and then convert back to a regex.

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GSOC and "scripting the inference process" .Was: Re: Haskell compilation errors break the complexity encapsulation on DSLs
by Alberto G. Corona 21 Mar '14

21 Mar '14

Now that GSOC comes again, I call you into attention one of the biggest problems that Haskell has to solve before being fully accepted in the IT industry: The (unnecessary) complexity of the DSL error messages. There is some work being done: http://dl.acm.org/citation.cfm?id=944707 The idea was to implement it in GHC after refining and testing it if I remember well. If some of this work can be carried out in a GSOC project, IMHO it should be given priority. In case that this work is not going to be available in the short-medium term, there is a ticket with some suggestions that can be carried out in a GSOC project, just in case any of you are interested into mentoring/carrying out this or any other solution to overcome this problem: http://hackage.haskell.org/trac/ghc/ticket/7870 2013-04-27 22:14 GMT+02:00 Ozgur Akgun <ozgurakgun(a)gmail.com>: > Hi, > > On 27 April 2013 10:07, Alberto G. Corona <agocorona(a)gmail.com> wrote: > >> I created a ticket for the feature request: >> >> Ticket #7870 >> >> Teachers, newbies and people working in Industry: Please push it! >> > > A link to the ticket may be helpful for the lazy. > > http://hackage.haskell.org/trac/ghc/ticket/7870 > > I quite like this idea, and I think this is one thing people use TH for > now instead. > (Using quasi-quotes, you can produce any compilation error you like...) > It would be great if we didn't have to pull in the full power of TH (or > QQ) for this. > > > Cheers, > Ozgur > > -- Alberto.

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Well-Typed Haskell courses now include a "Type System" course
by Andres Löh 20 Mar '14

20 Mar '14

Hi Haskell-Cafe. As you might already know, Well-Typed LLP are offering regular (commercial) Haskell courses in collaboration with Skills Matter in London for about 1.5 years now. The next courses will take place in the week from April 28 to May 2, 2014, and will for the first time include an all-new 1-day course on all the magic you can do with Haskell's (or more precisely GHC's) type system (taught by me, if that makes a difference). I hope this might be interesting to some on this list, and would certainly like to see many of you there. There's more info available here: http://www.well-typed.com/blog/88/ Here are direct links to the Skills Matter pages for each course. If you'd like to register, go to these: https://skillsmatter.com/courses/504-well-typed-s-guide-to-the-haskell-type… https://skillsmatter.com/courses/464-well-typed-fast-track-to-haskell https://skillsmatter.com/courses/465-well-typed-advanced-haskell Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

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ANN: timezone-olson V0.1.4
by Yitzchak Gale 19 Mar '14

19 Mar '14

Version 0.1.4 of timezone-olson has been released on Hackage. This version adds support for version 3 olson timezone files, which are now beginning to appear in the wild. Thanks to Renzo Carbonara and Oliver Charles for providing this update. The timezone-olson package provides a parser and renderer for binary Olson timezone files whose format is specified by the tzfile(5) man page on Unix-like systems. For more information about this format, see: http://www.twinsun.com/tz/tz-link.htm Functions are provided for converting the parsed data into 'TimeZoneSeries' objects from the timezone-series package [1]. On many platforms, binary Olson timezone files suitable for use with this package are available in the directory /usr/share/zoneinfo and its subdirectories on your computer. Hackage: http://hackage.haskell.org/package/timezone-olson Home page: http://projects.haskell.org/time-ng/ Note that support for tzfile versions 2 and 3 is not complete. We parse all of the explicit clock transitions, but we do not yet support the textual POSIX-style time zone specifications that can be included; currently, those only affect clock transitions that are predicted to occur far in the future. We also do not yet support parsing the leap second tables that are now provided in olson files; see the function Data.Time.Clock.TAI.parseTAIUTCDATFile in Ashely Yakeley's time library [2] for an alternative. Regards, Yitz [1] http://hackage.haskell.org/package/timezone-series [2] http://hackage.haskell.org/package/time

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Re: [Haskell-cafe] Listing native package requirements based on cabal information
by Tobias Dammers 19 Mar '14

19 Mar '14

On Wed, Mar 19, 2014 at 10:15:08AM +1300, Chris Wong wrote: > On Wed, Mar 19, 2014 at 9:47 AM, Tobias Dammers <tdammers(a)gmail.com> wrote: > > Hmm, but then, not all package managers/repositories use the same names > > for packages. So even if there were a way to extract the required > > libraries from cabal, feeding that information to the package manager > > isn't going to be trivial at all. > > It's not trivial, but I don't think it's difficult. > > For example, Debian Haskell packages all take the form > libghc-NAME-dev. Fedora uses haskell-NAME. Sure, there's a bit of hard > coding involved, but the set of prominent distributions is finite. I think installing *haskell* packages is not the problem; cabal already does a decent job at that. The problem we're facing is that cabal can *only* install haskell packages, but not any of the native libraries they depend on (usually through FFI, but other examples exist, such as the pg_config tool from PostgreSQL). Besides naming convention issues (which could be compensated for with lookup dictionaries; a brute-force solution, sure, but better than nothing), a bigger problem is that sometimes packages don't match 1:1 between distros, so one distro might just provide one monolithic foobar-dev package, while another might split it up into foobar-client-dev and foobar-server-dev, while yet another might provide more than one alternative. TL;DR: I think if we could just get a list of required native packages, we'd be a long way. > > One problem, though, would be versions. I know we often ask for the > latest and greatest packages, which may conflict with what the > distribution supplies. So the benefits may not be as high as they > seem. > > By the way -- has anyone looked into 0install? It hashes things, > similarly to Nix, but also tries to integrate with the existing > package manager. For example, if the user wishes to install GHC 7.6, > but the Debian repositories already supply it, it will invoke apt-get > instead of installing on its own. > > > (And frankly, looking at build systems > > for other languages, I've never seen anything that does this - even > > autotools doesn't really do a lot more than check whether a library is > > available). > > > > That said, even if we could just get a list of required libraries out of > > cabal, in a somewhat human and machine readable format, that would be a > > huge win in itself. > > > > On Tue, Mar 18, 2014 at 11:48:33AM -0700, David Thomas wrote: > >> One option that just occurred to me would be to allow passing a script to > >> cabal that could be passed the extra-libraries (if any), and could install > >> if it knew the relevant OS packages (or NIX packages), or abort with a > >> cleaner error message. > >> > >> Actually, wrapping ghc might be sufficient (though not ideal). > >> > >> > >> > >> On Tue, Mar 18, 2014 at 11:29 AM, Michal Antkiewicz < > >> mantkiew(a)gsd.uwaterloo.ca> wrote: > >> > >> > This is not an immediate solution but I can imagine listing NIX packages > >> > as dependencies inside a cabal file, then NIX would create a sandbox with > >> > these dependencies and cabal would build in that sandbox. The advantages > >> > are that you're not messing around with the underlying operating system's > >> > packages, NIX handles all dependencies transitively, and everything is > >> > specified declaratively. Sounds like a nice GSoC project :-) You could > >> > also do cross GHC version's builds as GHC itself can be sandboxed. Quite > >> > intriguing. > >> > > >> > Michal > >> > > >> > > >> > > >> > On Tue, Mar 18, 2014 at 2:16 PM, Michal Antkiewicz < > >> > mantkiew(a)gsd.uwaterloo.ca> wrote: > >> > > >> >> Certainly NIX is an interesting approach. It already comes with a large > >> >> base of dependencies, a format for specifying them. NIX can be installed in > >> >> any Linux distro and serve as an environment for building packages. That > >> >> might provide a cross-distribution solution to the native dependency > >> >> problem. > >> >> > >> >> See, a nice post by Oliver Charles > >> >> > >> >> http://ocharles.org.uk/blog/posts/2014-02-04-how-i-develop-with-nixos.html > >> >> > >> >> Michal > >> >> > >> >> > >> >> > >> >> On Tue, Mar 18, 2014 at 1:59 PM, David Thomas <davidleothomas(a)gmail.com>wrote: > >> >> > >> >>> Ok, well, if that's the case I'd like to see about remedying that. > >> >>> Anyone have any thoughts as to how to best go about this? I'm not clear on > >> >>> exactly what info lives where, especially across systems. Entirely manual > >> >>> population would be a (barely) acceptable fallback. > >> >>> > >> >>> > >> >>> On Tue, Mar 18, 2014 at 9:36 AM, Dan Burton <danburton.email(a)gmail.com>wrote: > >> >>> > >> >>>> I have wished for this on multiple occasions. I don't believe such a > >> >>>> thing exists as of yet. > >> >>>> > >> >>>> -- Dan Burton > >> >>>> > >> >>>> > >> >>>> On Tue, Mar 18, 2014 at 9:26 AM, David Thomas <davidleothomas(a)gmail.com > >> >>>> > wrote: > >> >>>> > >> >>>>> Is there a way to extract this? I'm looking to make it easier for > >> >>>>> newcomers to my project to get things building across different linux > >> >>>>> distros. > >> >>>>> > >> >>>>> _______________________________________________ > >> >>>>> Haskell-Cafe mailing list > >> >>>>> Haskell-Cafe(a)haskell.org > >> >>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe > >> >>>>> > >> >>>>> > >> >>>> > >> >>> > >> >>> _______________________________________________ > >> >>> Haskell-Cafe mailing list > >> >>> Haskell-Cafe(a)haskell.org > >> >>> http://www.haskell.org/mailman/listinfo/haskell-cafe > >> >>> > >> >>> > >> >> > >> >> <mantkiew(a)gsd.uwaterloo.ca> > >> >> > >> > > >> > > >> > > > > >> _______________________________________________ > >> Haskell-Cafe mailing list > >> Haskell-Cafe(a)haskell.org > >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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[ANN] cabal-cargs: A command line program for extracting compiler arguments from a cabal file.
by Daniel Trstenjak 18 Mar '14

18 Mar '14

Hi cafe, for further details please consult 'https://github.com/dan-t/cabal-cargs'. A little teaser, you can use cabal-cargs to make your default, non modified hdevtools aware of the settings in the cabal file and of a present cabal sandbox. Greetings, Daniel

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[GSoC] A proposal for GSoC regarding computational algebra
by Hiromi ISHII 18 Mar '14

18 Mar '14

Hello, cafe I am a currently an undergraduate student and will enter the graduate school on April. As a student, I have a project to propose for the GSoC: purely functional computer algebra library. I've been working on computational-algebra library [1], but this library needs more improvements. Especially, it needs much more performance improvements for Groebner bases computation. I want to implement efficient algorithms called F4 and F5 [2, 3] as the GSoC project. These algorithms require efficient linear algebra algorithms, so this project also involves the development of efficient symbolic linear algebra library. This project is not on any ideas list as far as I know, and I want to directly propose to the GSoC and I'm looking for a mentor. Is there anyone interested in my proposal? [1]: http://github.com/konn/computational-algebra [2]: http://www-polsys.lip6.fr/~jcf/Papers/F99a.pdf [3]: http://www.risc.jku.at/Groebner-Bases-Bibliography/gbbib_files/publication_… -- Hiromi ISHII konn.jinro(a)gmail.com

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Re: [Haskell-cafe] linking warnings when using regex-posix
by Rouan van Dalen 18 Mar '14

18 Mar '14

Thanks Kyra, I appreciate the feedback. Good to know it is in the process of being fixed. Regards --Rouan

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linking warnings when using regex-posix package in GHCi 7.8.1 RC2
by Rouan van Dalen 18 Mar '14

18 Mar '14

Hi Cafe, I have the following program: ========================= module Main where import Text.Regex.Posix main = do putStrLn "!!!" ========================= When I run the main function from within GHCi, I get the following output from GHCi: ========================= Loading package array-0.5.0.0 ... linking ... done. Loading package deepseq-1.3.0.2 ... linking ... done. Loading package containers-0.5.4.0 ... linking ... done. Loading package bytestring-0.10.4.0 ... linking ... done. Loading package transformers-0.3.0.0 ... linking ... done. Loading package mtl-2.1.2 ... linking ... done. Loading package regex-base-0.93.2 ... linking ... done. Loading package regex-posix-0.95.2 ... linking ... <interactive>: warning: isupper from msvcrt is linked instead of __imp_isupper <interactive>: warning: toupper from msvcrt is linked instead of __imp_toupper <interactive>: warning: tolower from msvcrt is linked instead of __imp_tolower <interactive>: warning: isalpha from msvcrt is linked instead of __imp_isalpha <interactive>: warning: isalpha from msvcrt is linked instead of __imp_isalpha <interactive>: warning: isalpha from msvcrt is linked instead of __imp_isalpha <interactive>: warning: iscntrl from msvcrt is linked instead of __imp_iscntrl <interactive>: warning: isupper from msvcrt is linked instead of __imp_isupper <interactive>: warning: isgraph from msvcrt is linked instead of __imp_isgraph <interactive>: warning: isprint from msvcrt is linked instead of __imp_isprint <interactive>: warning: ispunct from msvcrt is linked instead of __imp_ispunct <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalpha from msvcrt is linked instead of __imp_isalpha <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum <interactive>: warning: isalnum from msvcrt is linked instead of __imp_isalnum done. !!! *Main> ========================= Does anyone have an idea why GHCi is even linking against msvcrt? And is there a way to fix this? I am using GHC 7.8.1 RC2 x64 on Windows 8 x64 Regards --Rouan

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Bound library + recovering de Bruijn indices
by Benjamin Edwards 18 Mar '14

18 Mar '14

Hi Cafe I am currently experimenting with the excellent bound library. I want to implement the simply typed lambda calc from TAPL (no type inference yet). When moving under a binder I want to extend my context and then look up bound terms by their indices using list indexing. I would have thought I could use something like foldMapScope to recover the bound variables, then write some function to calculate the depth of the "listy" type value returned and thus recover the index. However! The type returned is non-regular and I cannot for the life of me figure out anything that doesn't involve GHC complaining about infinite types. Any pointers on how this can be accomplished? For the record, this is what I have that works at the moment. http://lpaste.net/101279

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FRP, Simulations and Time (Sodium et. al)
by martin 18 Mar '14

18 Mar '14

FPR is usually described as a way to model interactions with the real world. I believe the same ideas should be applicable to simulations. Instead of "real" events I would use fake events. This however only makes sense when the Time associated with the Events (and Behaviors) is not wallclock time, but some kind of virtual time. I looked briefly into Sodium and found no way to use "my own time". Steven Blackheath even said during a presentation, that Time in Sodium is just another Behavior. I am not sure if I understood this correctly, but it gives me the feeling, that the only Time available in Sodium is wallclock time, which would make it unsuitable for simulations. Could someone give me some insights whether or not FRP is suitable for simulations? Does my reasoning about Time make any sense? Do different libraries treat Time in different ways, such that some are suitable for simulations, while others are not? Martin

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GSoC Proposal: Pandoc improvements including EPUB 3.0 reader
by Matthew Pickering 18 Mar '14

18 Mar '14

I'm looking to submit a proposal to (mainly) add an EPUB reader to pandoc. I've spent the last few weeks getting to know the code base and wrote a proposal in the last few days. I would really appreciate any comments on the proposal and any further suggestions or things to look out for! Looking forward to doing some hacking on pandoc independent of this! Full proposal: https://www.dropbox.com/s/tdiimqa8mj22vq3/gsoc.pdf Has anyone looked into MathML -> Latex conversion? It would be nice to have this in the EPUB parser to deal with embedded equations. Below is a sketch outline of the suggested implementation. *Embedded Base64 images* - Replace Target in the Image constructor with a new constructor which can either be a Target as before or a base64 encoding. - Update HTML5 reader to read embedded images successfully. *EPUB 3.0 reader* - Utilise the HTML parser with rawTags enabled - Extract additional information about structure from walking over the AST

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Yi project proposal for GSOC 2014
by Mateusz Kowalczyk 18 Mar '14

18 Mar '14

Greetings, GSOC 2014 proposal period opens in ~4 hours and I'm hoping to participate this year as well. This time around I'd quite like to work on Yi. As we did last year, I think it's worthwhile to put up the proposals on café for people to comment on before they are submitted on Google's site. I paste it in full below so that it is easier to respond to parts of it (although I do ask that you don't quote the whole thing if it's not necessary). In case any changes happen, the most up-to-date version should be at https://gist.github.com/Fuuzetsu/9462709 Please feel free to nitpick on anything, throw in suggestions and ask for clarifications. I will give 5 days of discussion period on this after which point I'll submit it on Google's site. I appreciate all feedback. Thanks! Yi concurrency, usability and hackability ------------------------------------------ * What is the goal of the project you propose to do? There are two main goals of the project: the first is to implement concurrency in the Yi text editor. The second aim is to start bringing Yi into the territory of usable and hackable editors. Dmitry Ivanov who's currently in charge of Yi has agreed to mentor this project. * In what ways will this project benefit the wider Haskell community? While the project itself isn't one of the core ones (such as GHC, Haddock and Cabal), I feel that there are a couple of benefits to the community: 1. Work on Yi (now and in the future) will undoubtedly spawn new Haskell libraries usable in other projects. My personal experience with Yi shows that it's actually very comfortable to write a generic library which does what we need and then having a separate package which uses the library to actually interact with Yi. 2. Haskellers come closer to escaping the ELisp/vimscript hell. We can get a nicer programming environment, made and extensible in the language of our choice and get to use all the libraries that we're used to while we're at it. 3. We'll have more Real World™ Haskell applications. On a more serious note, it can serve as a good example of how to do certain things with Haskell: off the top of my head, it demonstrates the use of dyre and gtk2hs in a real-world scenario rather than a 5 line example on the Haskell wiki. If the project is successful, we can add concurrency to this. Other than the Haskell community in general, this project should benefit anyone with some interest in text editors. I think it's safe to say that happens to be a large majority of Haskellers: most of us want nicer integration with Haskell tools and libraries[citation needed] and now it'll be possible through direct, type-checked library access. * Can you give some more detailed design of what precisely you intend to achieve? The concurrency goal will involve careful study of Yi's inner workings in order to try and accommodate concurrency in Yi's editor state. There are various ways to do concurrency and the first part of the project will concentrate on settling for one. An example of two different ways is to extend the existing Yi engine with classical tools (MVars, channels) to accommodate for concurrency that way. An alternative way would be to modify the engine so that concurrency support is natural. Such experiment was started [here](https://github.com/ethercrow/y) using the sodium FRP package which would give us concurrency ‘for free’. The experiment is not complete and this is the kind of thing that will first be explored. Of course once we settle for a method, time will be spent implementing it. In the end, this should allow us to do things such as fire Yi events periodically or do network transfers without having to halt the whole editor. Editors such as emacs which are single-threaded effectively hop back-and-forth between tasks on a single thread. We aim to provide the ability to simply have tasks on different threads which allows us to take advantage of system resources much better. The second part of the project is to make Yi more usable and hackable. Usability here involves fixing bugs apparent to the user and hackability involves bugs apparent to developers. Further, as part of usability, I plan to implement as many editor modes as I find time for. Specifically, here are some open bugs that I hope to either fix or to make a considerate progress on: #445, #397, #517, #519, #515, #516, #513 (concurrency), #512, #507, #504, #502, #501, #499, #497, #493, #487, #478, #477, #468, #465, #399, #396, #391, #390, #382, #322, #295, #172, #160, #106, #145, #112, #82, #509. All the bug numbers can be viewed on [GitHub](https://github.com/yi-editor/yi/issues/). Please note that some of these are documentation bugs: Yi suffers from poor documentation and I believe that's what the main problems in gaining developers and users has been. When time or area I'm working on allows, missing documentation will be written. If I find any issue that have been fixed or are no longer applicable, the reports will simply be closed. The issues are very varied: unicode problems, keymap problems, highlighter problems, reloading problems, testing problems, mode problems… There is certainly enough work to entertain anyone for a longer amount of time while making Yi visibly better. The list of issues is simply an indicator of which problems the second goal of the project will concentrate on, rather than as a promise of which bugs are guaranteed to be fixed by the end of it. Alongside this goal, I'll write any modes for Yi as I find time for them. The completion of concurrency part of the project allows us to write many of the modes frequently requested by people wishing to use Yi which are currently impossible/unfeasible to write. * What deliverables do you think are reasonable targets? Can you outline an approximate schedule of milestones? The plan is based on the GSoC time line: 20 April - 19 May – while this is a bonding period, I'm already a part of the Yi community and have a fair grasp of it. I'd start to look into this project as early as this period (and in fact I plan to make steps towards it before this date which means some of the outlined issues might get fixed early ;) ). 19 May - 23 June – coding period; by this point I expect to have decided on which concurrency model we'll use and have a good idea of how it'll be implemented. By the end of this period, concurrency should either be completed or nearly done, depending on any unexpected problems that might come up. The deliverable would be Yi with (at least some) concurrency support. 24 June - 11 August – second part of the coding period; work on any of the listed (or unlisted bugs) and finish up concurrency if it is still not done. Write extra Yi modes, libraries and documentation as time allows. 11 August - 18 August – post-coding period; write any missing documentation, promote any cool new stuff we wrote ;) While I can not think of a specific deliverable, many bugs should now be fixed, Yi should have a lot more documentation, tests and modes. As a final note regarding the time line, it is not strictly necessary that the project implements concurrency first: while some bugs might need such support, many simply do not. If it's convenient to fix something that I had originally planned to for the second part of the project, I'll do so. * What relevant experience do you have? e.g. Have you coded anything in Haskell? Have you contributed to any other open source software? Been studying advanced courses in a related topic? Second year CS student. I program on regular basis using Haskell. I contribute to a bunch of FOSS projects as it seems necessary (see [my GitHub](https://github.com/Fuuzetsu)) I have successfully completed GSOC in 2013 which involved working on Haddock. To this day I help out with Haddock which often involves looking at the large GHC code base. * In what ways do you envisage interacting with the wider Haskell community during your project? e.g. How would you seek help on something your mentor wasn't able to deal with? How will you get others interested in what you are doing? I have a [blog](http://fuuzetsu.co.uk/blog) which gets propagated onto Haskell Planet. I'm active on IRC and many Haskell-related mailing lists. IRC, mailing lists and any relevant literature is where I'd seek help were I to get stuck on something my mentor can't help me with. I find that news about Yi are very popular and get propagated by the community itself very easily so I doubt there will be any problem getting people interested. I'm very easily reachable over e-mail and IRC and all the development is done in public. * Why do you think you would be the best person to tackle this project? I've been interested in Yi for a couple of months and have already wrote some commits, closed quite a few issues and filed even more issues on my own. I have access to the Yi repository and I help anyone looking to get started with Yi. I have about 2 years of Haskell experience and had my fair share of staring at library code. As mentioned before, I'm active as a member of the community and help out with one of the core Haskell projects (Haddock). -- Mateusz K.

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Hackage down?
by Iavor Diatchki 18 Mar '14

18 Mar '14

Hello, It looks like Hackage has stopped responding? (2:50pm PDT, 17 March 2014). -Iavor

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projects page
by Ketil Malde 18 Mar '14

18 Mar '14

I tried to follow the link from Google (Melange, http://www.google-melange.com/gsoc/org2/google/gsoc2014/haskell) to the ideas page - but it seems there are only outdated ideas there? Sorry if I wasn't paying attention in class, but how is this intended to work? Is there some other place I should look? -k -- If I haven't seen further, it is by standing in the footprints of giants

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FRP, Simulations and Time (Sodium et. al)
by Sacha Sokoloski 18 Mar '14

18 Mar '14

So I'm also doing a lot of simulations, and have experimented with FRP as a basic toolset for defining my simulations. I experimented with Netwire for a while, and while I still think it's a good library, I eventually threw it away to rely simply on Mealy Arrows (Netwire is Arrow based FRP). From what I've seen, the point of FRP libraries is to handle interactivity. That's what all the functions in the libraries are about. If you do have a virtual time stream that you can simply define at runtime and doesn't require side effects, you may find, as I did, that you'll create a simplified type synonym for the FRP structure that you're working with, and then not really using the libraries at all. The point is ultimately that, although pure simulations could nicely be part of any more complicated FRP program, if all you want is to do pure simulations, then you'll be introducing a fair bit of computational/structural overhead to fit it within the FRP framework, without any real payoff. Cheers, - Sacha On 17/03/14 10:50 AM, haskell-cafe-request(a)haskell.org wrote: > Message: 1 > Date: Sun, 16 Mar 2014 14:23:37 +0100 > From: martin<martin.drautzburg(a)web.de> > To:haskell-cafe@haskell.org > Subject: [Haskell-cafe] FRP, Simulations and Time (Sodium et. al) > Message-ID:<5325A5D9.8070408(a)web.de> > Content-Type: text/plain; charset=ISO-8859-15 > > FPR is usually described as a way to model interactions with the real world. I believe the same ideas should be > applicable to simulations. Instead of "real" events I would use fake events. This however only makes sense when the Time > associated with the Events (and Behaviors) is not wallclock time, but some kind of virtual time. > > I looked briefly into Sodium and found no way to use "my own time". Steven Blackheath even said during a presentation, > that Time in Sodium is just another Behavior. I am not sure if I understood this correctly, but it gives me the feeling, > that the only Time available in Sodium is wallclock time, which would make it unsuitable for simulations. > > Could someone give me some insights whether or not FRP is suitable for simulations? > > Does my reasoning about Time make any sense? Do different libraries treat Time in different ways, such that some are > suitable for simulations, while others are not? > > Martin

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Changes to cabal-install
by Amy de Buitléir 17 Mar '14

17 Mar '14

I have a couple of questions about the latest cabal-install release, and I haven't been able to find the release notes (checked GitHub, the Haskell.org wiki, googled, etc.) Here's what I'm running: amy@wombat$ cabal --version cabal-install version 1.18.0.3 using version 1.18.1.3 of the Cabal library Question 1 ---------- Until recently, the command cabal install --enable-tests used to build and run the tests. Now it seems I need to do cabal test which then compiles the tests and runs them. So is the --enable-tests flag doing anything, or is it now obsolete? Question 2 ---------- With my project, I do, in order: 1. cabal sandbox init --sandbox <directory> 2. cabal install --only-dependencies 3. cabal build --ghc-options=-Werror 4. cabal test But when I do step 3, I get a strange warning message: "The sandbox was created after the package was already configured." Huh? Creating the sandbox was the first thing I did! If instead, I replace step 3 with 3. cabal install --ghc-options=-Werror then I don't get the warning message until step 4.

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Two research positions in functional probabilistic programming
by Tom Nielsen 17 Mar '14

17 Mar '14

Dear Cafe, Two positions involving substantial Haskell work are available for a project that seeks to combine functional and probabilistic programming. We are looking for two postdocs to work on a practical system for large-scale inference in scientific and clinical datasets using bayesian statistical models, embedded in a typed functional programming language and based on stochastic dynamical systems. * A typed hierarchical database that uses a Hindley-Milner-like typesystem (with records) to organise large, complex and heterogeneous data from a hospital. * Probabilistic inference over these complex datasets * Parallelizing Bayesian inference * Modelling clinical datasets (for instance ECG) using dynamical systems. Some of these ideas have been explored in our Baysig Language ( http://tinyurl.com/Baysig) and BayesHive project (https://BayesHive.com) - both Baysig and BayesHive are implemented in Haskell. However, these are academic research posts and there is scope for exploring different designs to meet the same aims. Here are the official adverts: http://ig5.i-grasp.com/fe/tpl_UniversityOfLeicester01.asp?newms=jj&id=85616… http://ig5.i-grasp.com/fe/tpl_UniversityOfLeicester01.asp?newms=jj&id=85615… The application deadline is April 10. If you think you may be interested, you are welcome to ask me (tanielsen(a)gmail.com or tomn(a)openbrain.org) or Tom Matheson (tm75(a)le.ac.uk) any questions. Tom Nielsen OpenBrain Ltd http://openbrain.co.uk

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Want to get paid to hack on Haskell for the summer. GSoC deadline is 21 March
by Johan Tibell 17 Mar '14

17 Mar '14

Hi, If you're a student and want to get paid to write Haskell this summer, it's time to apply for this year's Google Summer of Code. The deadline is 21 March, 19:00 UTC. Here are some project ideas: http://blog.johantibell.com/2014/03/google-summer-of-code-projects.html -- Johan

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Google Summer of Code?
by Chris Wong 17 Mar '14

17 Mar '14

Hello Cafe, I'm currently considering applying for the Summer of Code. Johan Tibell's idea of improving the Hackage login system [1] looks compelling, since I have worked with Happstack before [2]. Is there enough time to start a proposal? I know it's a bit late now -- but I would very much like to participate. Thanks Chris [1] http://blog.johantibell.com/2014/03/google-summer-of-code-projects.html [2] https://github.com/lfairy/sparkle

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ffi call works fine in ghci but not as ghc compiledsegfault
by Miroslav Karpis 17 Mar '14

17 Mar '14

Hi, please can you help me with following? I have a call to an external dll (via ffi) which if executed from ghci works fine. If I compile my code with 'ghc -o fm Mycode.hs -L. -lextdll' and run it I get 'Segmentation fault/access violation in generated code'. main = do let param = "FilePath" let value = "C:/dev/misc/haskell/services/FM" result <- liftIO $ FM.setmodulestring param value return "done" setmodulestring :: String -> String -> IO CInt setmodulestring param value = do let cParamLength = fromIntegral $ length param ::CInt cValueLength = fromIntegral $ length value ::CInt setVarInArray = (-1)::CInt alloca $ \cParam -> do alloca $ \cValue -> do result <- c_setmodulestring cParam cParamLength cValue cValueLength setVarInArray return result If I try also with following, the behaviour is the same: setmodulestring2 :: String -> String -> IO CInt setmodulestring2 param value = do cParam <- newCWString param cValue <- newCWString value let cParamLength = fromIntegral $ length param ::CInt cValueLength = fromIntegral $ length value ::CInt setVarInArray = (-1)::CInt result <- c_setmodulestring cParam cParamLength cValue cValueLength setVarInArray free cParam free cValue return res Any comments/ideas more than appreciated. Cheers, Miro

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ANN: LogicGrowsOnTrees-1.1.0.2
by Gregory Crosswhite 17 Mar '14

17 Mar '14

Dear Haskellers, First, for those eager to skip directly to the more detailed documentation you can find an FAQ after the list of files at http://github.com/gcross/LogicGrowsOnTrees as well as a tutorial and users guide in respectively TUTORIAL.md<http://github.com/gcross/LogicGrowsOnTrees/blob/master/TUTORIAL.md>and USERS_GUIDE.md<http://github.com/gcross/LogicGrowsOnTrees/blob/master/USERS_GUIDE.md>in the list of files. The Hackage page with the reference documentation is at: http://hackage.haskell.org/package/LogicGrowsOnTrees Like many other packages on Hackage, LogicGrowsOnTrees provides an implementation of logic programming using MonadPlus; in this sense it is nothing new. What sets it apart is that it has been designed from the beginning to work in a distributed environment, allowing it to be parallelized over large numbers of processors with no shared memory. The benchmarks I have run (using the N-Queens problem with 17-19 queens) showed essentially perfect speed-up all the way up to 256 cores, and the only reason why this number is not larger is because I haven't had the opportunity to run tests on a larger cluster. Furthermore, I know from personal experience that complicated interfaces can take even the most powerful system and make it impotent, so I have tried hard to design interfaces that are easy to use and well-documented with a tutorial and several examples. There are three packages that are sibling to this: - LogicGrowsOnTrees-processes<http://hackage.haskell.org/package/LogicGrowsOnTrees-processes> - LogicGrowsOnTrees-network<http://hackage.haskell.org/package/LogicGrowsOnTrees-network> - LogicGrowsOnTrees-MPI<http://hackage.haskell.org/package/LogicGrowsOnTrees-MPI> The base package (LogicGrowsOnTrees<http://hackage.haskell.org/package/LogicGrowsOnTrees>) provides an "adapter" that allows one to use threads for parallelization; the other packages above provide adapters for, respectively: processes, a network, and MPI. Each one of these packages provides specialized functionality that can be used directly, but there is also an abstraction over all of them that can be used such that the only difference between the adapters for end-users is a single line of code. For more information, see the links at the top of the post. Thank you for your attention! :-) - Greg

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Re: [Haskell-cafe] ffi call works fine in ghci but not as ghc compiledsegfault
by Miro Karpis 16 Mar '14

16 Mar '14

still no luck ;-( Everything works well under ghci but if I compile it with ghc I get seg. fault ghc version 7.6.3 I found out that when I load my ghci without any .o and .hi files ghci function calls works fine. If I then compile my code with ghc and restart my ghci (.o and .hi files now exists) the ghci crashes. I have tried compiling with: ghc -O --make Service.hs -L. -lmodel ghc --make Service.hs -L. -lmodel ghc -o service Service.hs -L. -lmodel I have tried to call ffi function with: setmodulestring1 param value = do let param = cParamLength = fromIntegral $ length param ::CInt cValueLength = fromIntegral $ length value ::CInt setVarInArray = (-1)::CInt B.useAsCString (B.pack "FilePath") $ \cParam -> do B.useAsCString (B.pack "C:/dev/misc/haskell/services/FM") $ \cValue -> do result <- c_setmodulestring cParam cParamLength cValue cValueLength setVarInArray return result or with this: setmodulestring :: String -> String -> IO CInt setmodulestring param value = do let param = "FilePath" let value = "C:/dev/misc/haskell/services/FM" cParam <- newCString param cValue <- newCString value let cParamLength = fromIntegral $ length param ::CInt cValueLength = fromIntegral $ length value ::CInt setVarInArray = (-1)::CInt result <- c_setmodulestring cParam cParamLength cValue cValueLength setVarInArray free cParam free cValue return result my main is: main = do let param = "FilePath" let value = "C:/dev/misc/haskell/services/FM" result <- liftIO $ FM.setmodulestring param value return "done" and still no luck - works in ghci and crashes as exe interfacing the same library from a c++ code: extern "C" int __stdcall setmodulestring(char* param,unsigned int length, char* valuein,unsigned int valuelength,int index); interfacing it from Haskell: foreign import ccall "setmodulestring" c_setmodulestring :: CString -> CInt -> CString -> CInt -> CInt -> IO CInt please any ideas? On Sat, Mar 15, 2014 at 4:48 PM, Miro Karpis <miroslav.karpis(a)gmail.com>wrote: > thank you very much so far and sorry for my not 100% understanding ;-). > The dll is an external software - I have no control over it. > > This is how I interface it from c++ program: > > extern "C" int __stdcall setmodulestring(char* param,unsigned int length, > char* valuein,unsigned int valuelength,int index); > > > And this is how I'm trying to interface it from Haskell (with the seg-fault): > > foreign import ccall "setmodulestring" c_setmodulestring :: Ptr Char -> CInt -> Ptr Char -> CInt -> CInt -> IO CInt > > m. > > > On Sat, Mar 15, 2014 at 3:32 AM, Brandon Allbery <allbery.b(a)gmail.com>wrote: > >> On Fri, Mar 14, 2014 at 7:29 PM, Branimir Maksimovic < >> branimir.maksimovic(a)gmail.com> wrote: >> >>> You are passing wchar_t* and function expects char*. >>> >> >> FWIW they asked this before (possibly on a different list) and, absent >> the C information, I pointed out that Win32 API functions use CWString. >> This was apparently taken to mean that CWString was the only option.... >> >> Some awareness of one's platform conventions tends to be helpful when >> doing FFI. It seems to be absent here. :/ >> >> -- >> brandon s allbery kf8nh sine nomine >> associates >> allbery.b(a)gmail.com >> ballbery(a)sinenomine.net >> unix, openafs, kerberos, infrastructure, xmonad >> http://sinenomine.net >> > >

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Save the date: BayHac '14
by Mark Lentczner 15 Mar '14

15 Mar '14

The Silicon Valley / San Francisco Bay Area Haskell Hackaton's 4th year! Save the date for a weekend of Haskell hacking: *BayHac '14* *May 16th ~ 18th,* *Hacker Dojo* *Mountain View, California* Details and registration will follow in few weeks. Wiki page for event: BayHac2014 - HaskellWiki<http://www.haskell.org/haskellwiki/BayHac2014> — Jonathan Fischoff & Mark Lentczner

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overlapping/Incoherent closed type families
by Silvio Frischknecht 15 Mar '14

15 Mar '14

Hi I have been playing around a bit with closed type families. However, I somehow always bump my head at the fact that things usually doesn't work for Num without specifying the type. Here is an example. {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE IncoherentInstances #-} module Main where import Data.Typeable type family UnMaybed a where UnMaybed (Maybe a) = a UnMaybed a = a class UnMaybe x where unMaybe :: x -> UnMaybed x instance UnMaybe (Maybe a) where unMaybe (Just a) = a instance (UnMaybed a ~ a) => UnMaybe a where unMaybe a = a main = do print $ unMaybe 'c' print $ unMaybe (1::Int) print $ unMaybe (Just 1) print $ unMaybe 1 -- this line does not compile everything except the last line will compile. ../Example.hs:23:17: Occurs check: cannot construct the infinite type: s0 ~ UnMaybed s0 The type variable ‘s0’ is ambiguous In the second argument of ‘($)’, namely ‘unMaybe 1’ In a stmt of a 'do' block: print $ unMaybe 1 Now I know this is because numbers are polymorphic and (Maybe a) could be an instance of Num. I think for normal overlapping typeclasses this dilemma can be solved by using the IncoherentInstances PRAGMA. Anyway, I wanted to ask if there is a way to make this work in type families? I also thought about specifying Num explicitly in UnMaybed type family UnMaybed a where unMaybed (Num a => a) = a UnMaybed (Maybe a) = a UnMaybed a = a This compiles but i think the first case will never be matched this is probably a bug. Silvio

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HaL-9 - regional Haskell meeting in Halle/Saale, Germany, 2014-06-20 - Save the date
by Henning Thielemann 15 Mar '14

15 Mar '14

Save the date for our local Haskell meeting in Halle/Saale, Germany, presenting tutorials, talks, demonstrations ... everything you like. Workshop language is German (mainly), and English (by request). Switching to German: ------------------------------------------------- Was: Haskell-Treffen HaL-9 Wann: Freitag, 2014-06-20 Wo: Institut für Informatik an der Martin-Luther-Universität in Halle an der Saale Der offizielle Aufruf zum Einreichen von Beiträgen folgt demnächst. Mit besten Grüßen Henning Thielemann

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I seem to constantly abuse TypeFamilies; what do i really want?
by Brandon Simmons 15 Mar '14

15 Mar '14

I've used TypeFamilies numerous times in code and library APIs to get more powerful and creative type-checking, and I seem to always use it in the same way that ends up feeling like abuse. For instance here's an example I started to sketch up for a talk I'm giving; this is to be a type-checked RPN calculator, used e.g. like `eval () (1,(2,((+),())))`: ``` {-# LANGUAGE TypeFamilies , MultiParamTypeClasses , FlexibleInstances , UndecidableInstances #-} type family EvaledStack x stack type instance EvaledStack Int st = (Int,st) type instance EvaledStack (Int -> x) (Int,st) = EvaledStack x st type family FinalStack string initialStack type instance FinalStack () st = st type instance FinalStack (x,xs) st = FinalStack xs (EvaledStack x st) class EvalStep x stack where evalStep :: x -> stack -> EvaledStack x stack instance (EvalStep x st)=> EvalStep (Int -> x) (Int,st) where evalStep f (int,st) = evalStep (f int) st instance EvalStep Int st where evalStep int st = (int,st) class Eval string initialStack where eval :: initialStack -> string -> FinalStack string initialStack instance Eval () st where eval st () = st instance (EvalStep x st, Eval xs (EvaledStack x st))=> Eval (x,xs) st where eval st (x, xs) = eval (evalStep x st) xs ``` The code above is just a WIP, but notice several things: 1) It's intended to be "closed" but I can't express that 2) I need to use UndecidableInstances for the nested type family instances, even though the recursion I'm doing is simple 3) The classes are "ugly" with arbitrary instance heads 4) If I want to support polymorphic operators/operands I need to use OverlappingInstances, which is another layer of hack 5) Users get an unhelpful error from the type-checker if their RPN expression is ill-typed I think what I'm trying to do is fundamentally pretty simple, but I only have the tools to do it in the very ad-hoc way I've described. I think the new closed type families help me here, but I'm wondering: - have there been any proposals or discussions about this use case, or a name given to it? - do closed type families provide an elegant solution and I just don't realize it yet? - do other people find themselves using this pattern as well, or have I just gotten caught up in a strange way of abusing these extensions? Thanks a lot, Brandon

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π-day
by Henk-Jan van Tuyl 15 Mar '14

15 Mar '14

Happy Pi Day! [0] Henk-Jan van Tuyl [0] http://en.wikipedia.org/wiki/Pi_day -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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looking for alex+happy examples that uses location annotated tokens and location information in err msgs
by Ömer Sinan Ağacan 14 Mar '14

14 Mar '14

Does anyone have any example alex+happy programs that uses location location annotated tokens and location information of token while reporting parse errors? I already wrote lexer and my function type is something like: > runLexer :: String -> [(Token, AlexPosn)] where AlexPosn type contains location information. But I have no idea how can I use that in happy to parse and use location information in parse error messages. There are lots of examples in interwebs but as far as I can see none of them works on location-annotated tokens. Thanks.. --- Ömer Sinan Ağacan http://osa1.net

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Re: [Haskell-cafe] GSOC Project Ideas
by Michel Kuhlmann 14 Mar '14

14 Mar '14

I would like to see gitit2 [1] replacing gitit [2]. gitit2 uses yesod as a foundation; currently not all features of gitit are implemented yet; despite from that, it is not up-to-date with current yesod (not compiling). The old gitit has quite some bugs; and it is not easy to extent it. -Michel [1]: https://github.com/jgm/gitit2.git [2]: https://github.com/jgm/gitit.git

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Fwd: WSJ.com - Coding isn't just for those IT guys anymore
by Vasili I. Galchin 14 Mar '14

14 Mar '14

I wish Haskell could get this kind of media coverage .... ;<) Vasili ---------- Forwarded message ---------- From: vigalchin(a)gmail.com <vigalchin(a)gmail.com> Date: Thu, Mar 13, 2014 at 8:48 PM Subject: WSJ.com - Coding isn't just for those IT guys anymore To: vigalchin(a)gmail.com Cc: vigalchin(a)gmail.com [image: The Wall Street Journal] <http://www.wsj.com> * Please note, the sender's email address has not been verified You have received the following link from *vigalchin(a)gmail.com*<vigalchin(a)gmail.com> :. WSJ.com - Coding isn't just for those IT guys anymore<http://online.wsj.com/news/article_email/SB30001424052702304709904579411354…> Subscriber-only content will be available to non-subscribers for up to seven days after it is emailed. This article can also be accessed if you copy and paste the entire address below into your web browser. http://online.wsj.com/news/article_email/SB30001424052702304709904579411354…

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Track Exceptions
by Silvio Frischknecht 14 Mar '14

14 Mar '14

Hi everyone, It's always very annoying not knowing what could go wrong (what exceptions might be thrown) when calling a library function. In java doc, for instance, you can usually see what exceptions can be thrown by a specific function. I was wondering if this could be achieved for haskell by tracing "throw" and "catch" calls. Silvio

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Haskell Symposium 2014: Call for papers
by Wouter Swierstra 13 Mar '14

13 Mar '14

=================================================================== ACM SIGPLAN HASKELL SYMPOSIUM 2014 CALL FOR SUBMISSIONS Gothenburg, Sweden, 4-5 September 2014, directly after ICFP http://www.haskell.org/haskell-symposium/2014 haskell2014(a)easychair.org =================================================================== The ACM SIGPLAN Haskell Symposium 2014 will be colocated with the 2014 International Conference on Functional Programming (ICFP) in Gothenburg, Sweden. Like last year, the symposium will last 2 days. Thanks to broader participation from a growing community, we will be able to include more regular papers as well as system demonstrations, while upholding the scientific quality of the symposium. The Haskell Symposium seeks to present original research on Haskell, to discuss practical experience and future development of the language, as well as to promote other forms of denotative programming. Topics of interest include * Language Design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal semantics of the present language or future extensions, type systems, effects, metatheory, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management, as well as foreign function and component interfaces; * Tools, such as profilers, tracers, debuggers, preprocessors, and testing tools; * Applications, to scientific and symbolic computing, databases, multimedia, telecommunication, the web, and so forth; * Functional Pearls, being elegant and instructive programming examples; * Experience Reports, to document general practice and experience in education, industry, or other contexts. Such reports are shorter than regular papers; they are limited to six pages. Papers in the latter two categories need not necessarily report original research results. They may report instead, for example, reusable programming idioms, elegant ways to approach a problem, or practical experience that will be useful to other users, implementors, or researchers. The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a program! Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work (also for other languages where appropriate). In addition, we solicit proposals for * System Demonstrations (no longer than a regular paper talk), based on running (perhaps prototype) software rather than necessarily on novel research results. These proposals should summarize the system capabilities that would be demonstrated. The proposals should explain (and will be judged on) whether the ensuing session is likely to be important and interesting to the Haskell community at large, whether on grounds academic or industrial, theoretical or practical, technical or social. Please contact the program chair with any questions about the relevance of a proposal. Travel Support: =============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC programme, see its web page (http://www.sigplan.org/PAC.htm) Proceedings: ============ ACM Press will publish formal proceedings. Accepted papers will be included in the ACM Digital Library. Authors must grant ACM publication rights upon acceptance (http://authors.acm.org/main.html) but may retain copyright if they wish. Authors are encouraged to publish auxiliary material with their paper (source code, test data, and so forth). The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. Accepted proposals for system demonstrations will be posted on the symposium web page, but not formally published in the proceedings. Submission Details: =================== Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (http://www.acm.org/sigs/sigplan/authorInformation.htm) The text should be in a 9-point font in two columns. The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 pages. Papers need not fill the page limit. Each paper submission must adhere to SIGPLAN's republication policy, as explained on the web. Proposals for system demonstrations are limited to 2-page abstracts, in the same ACM format as papers. "Functional Pearls", "Experience Reports", and "Demo Proposals" should be marked as such with those words in the title at time of submission. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. Submission is via EasyChair: https://www.easychair.org/conferences/?conf=haskell14 * Abstract submission: Fri 09 May 2014 * Paper submission : Mon 12 May 2014 * Demo submission : Fri 30 May 2014 (prior abstract submission unnecessary) * Author notification: Wed 11 June 2014 * Final papers due : Sun 22 June 2014 All deadlines, except the final papers deadline, are in Standard Samoan Time. Programme Committee: ==================== George Giorgidze - Standard Chartered Bank Mauro Jaskelioff - Universidad Nacional de Rosario Mark Jones - Portland State University Lindsey Kuper - Indiana University José Pedro Magalhães - University of Oxford Geoffrey Mainland - Drexel University Simon Marlow - Facebook Shin Cheng Mu - Academia Sinica Keiko Nakata - Institute of Cybernetics, Tallinn University of Technology Bruno Oliveira - University of Hong Kong Lee Pike - Galois Josef Svenningsson - Chalmers University of Technology Wouter Swierstra - University of Utrecht (chair) Simon Thompson - University of Kent

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pure ghc
by joakim＠comex.se 12 Mar '14

12 Mar '14

Hello, If I compile my source code two time I would like to get the same binary. But currently when I run echo "main = return ()" > c.hs && ghc -fforce-recomp c.hs && xxd c > c1.hex && ghc -fforce-recomp c.hs && xxd c > c2.hex && diff c1.hex c2.hex my output is [1 of 1] Compiling Main ( c.hs, c.o ) Linking c ... [1 of 1] Compiling Main ( c.hs, c.o ) Linking c ... 55494c55494 < 00d8c50: 7400 6768 6333 3130 3830 5f30 2e63 0073 t.ghc31080_0.c.s --- > 00d8c50: 7400 6768 6333 3130 3935 5f30 2e63 0073 t.ghc31095_0.c.s So why does the binary differ? Is there a way to compile with ghc and get the same binary? Even though only one byte differ, the checksum (sha,md5,..) is completely changed... I'm running Linux and ghc 7.6.3. Regards, Joakim

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ANN: generic-church-0.1.0.1
by Danny Gratzer 12 Mar '14

12 Mar '14

Hi cafe! I'm happy to announce the first release of generic-church<http://hackage.haskell.org/package/generic-church-0.1.0.1>. A small library for converting types equipped with a [GHC.Generic] instance back and forth between their church representations automatically. For example > let x :: a -> (Int -> a) -> a; x = toChurch $ Just 1 > x False (const True) True And we can go the other way with [fromChurch] > fromChurch (\_ f -> f 1) :: Maybe Int 1 If you're interested in the typeclass/family hacking behind the scenes, I've blogged a <http://jozefg.bitbucket.org/posts/2014-03-06-church.html> few <http://jozefg.bitbucket.org/posts/2014-03-07-church-the-sequel.html> times<http://jozefg.bitbucket.org/posts/2014-03-10-revenge-of-churchrep.html>about it. Cheers, Danny Gratzer

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Read instance for constructors?
by Semen Trygubenko / Семен Тригубен ко 12 Mar '14

12 Mar '14

Dear Haskell-cafe, When deriving (Read), only values can be read in. If one wants to be able to read in constructors, too, is there an easy way out? E.g., the code below works, but the extra book-keeping f "A" = A ... is unpleasant — perhaps there's a simpler solution? {-# LANGUAGE FlexibleInstances #-} data D = A Int | B Int deriving (Show,Read) instance Read (Int -> D) where readsPrec = \_ s -> [(f s,"")] where f "A" = A f "B" = B f x = error $ "Invalid constructor " ++ x main = do let x = read "A 1" :: D print x let g s = read s :: (Int -> D) print $ g "B" 2 print $ g "C" 3 Many thanks in advance, Semen -- Семен Тригубенко http://trygub.com

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ANN: engineering-units-0.0.1
by Tom Hawkins 12 Mar '14

12 Mar '14

Hi, I just uploaded a handy library for managing engineering units. It provides a Num type that allows you to mix units into calculations. It also converts between units automatically and will error out if you try to mix values with inconsistent units. Here's an example that computes the horsepower of a hydraulic pump (power = pressure * flow): flow = 20 * gpm -- Gallons per minute. pressure = 3000 * psi -- Pounds per square inch. power = pressure * flow powerHP = value power hp -- Get the value in horsepower. powerKW = value power kw -- Get the value in Kilowatts. If you don't see your units in the library, it's easy to add new ones: mm :: Value mm = 0.001 * m Wish I had this back in college. -Tom http://hackage.haskell.org/package/engineering-units

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Library for service(windows) / daemon(linux)
by Miro Karpis 12 Mar '14

12 Mar '14

Hi Cafe, I need to write a service that should run on windows and on linux (after recompilation of course). One option is to use Qt Service Framework, but I was hoping that I could use Haskell for this task. Unfortunately I can not find anything similar. Please does anybody know about a library that could fulfil more or less the requirement? Cheers, Miro

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On Haskell IDEs
by Niklas Hambüchen 11 Mar '14

11 Mar '14

On 10/03/14 16:22, Michal Antkiewicz wrote:> > > I would say making ghci a full-blown IDE backend akin lisps > slime-swank or clojure nrepl would be the best approach. > > there's already hdevtools which plays that role - it provides all sorts > of information to an IDE, like types, location of declarations, etc. > It's a background process and it's quite responsive (tries to be > incremental). I don't know how it is implemented and whether it is a > wrapper around GHCi. > > But I agree that all IDEs should simply use GHCi as the official IDE > backend. Maybe some parts from scion/hdevtools/ghc-mod/etc. could be > pulled back into GHCi? > > Michal Mateusz just clarified that his proposal is more about improving Yi as an editor than making it more of a Haskell IDE, so I am replying to this in a new thread. Alexandr Ruchkin and me maintain SublimeHaskell, the most-used Haskell plugin for Sublime Text. It integrates ghc-mod, hdevtools, cabal etc. and can do things like showing compile errors, completing imports, displaying the type of an expression in the buffer or insert it over a function declaration. When we want to change a Haskell file's content, we get the relevant info from, say, ghc-mod and then do the actual modification logic in the editor scripting language (in our case Python). We recognise that this logic is duplicated in pretty much every editor with such Haskell features, and at the same think that Haskell is a much nicer language to program this logic. Technically, it is also less error prone to work with ghc-mod's API than with its command line interface. That's why we seem to move more and more to do the actual modifications in an external Haskell program, and have the part in the actual editor be as minimal as possible. Alexandr started a project called "hsdev" (https://github.com/mvoidex/hsdev) for that. It is very editor-agnostic and we think that sharing something like that across editors makes sense since it could save plugin writers a lot of time. The ideas is that you would tell this tool to perform an action, e.g. "import missing imports" or "go to declaration", and it would do it for you instead implementing the buffer changes in your editor plugin. There is one problem with that: Editors don't deal well with other programs editing currently opened files. In Sublime's case, the buffer will automatically refresh, but it is very well possible that your caret position and undo history get lost, which totally interrupts the work flow. So I was wondering what the most minimal interface would be that allows your editor to be aware of the changes that an external program (HaRE, hsdev, etc.) would like to make to your code base. At the moment we consider having hsdev outputting a diff format that describes the changes, and then each editor would have to implement a simple interpreter that goes through the files, applying the changes. That way we can avoid an external file change and the problems that come with it. The Sublime-related discussion about this is at https://github.com/SublimeHaskell/SublimeHaskell/pull/132#issuecomment-3388…. I think that IDE-style "smart language features" might have a better place in cross-editor tools than in the editors themselves. ghc-mod is a successful example. I also think that most Haskell-IDE maintainers would agree (please correct me otherwise) that doing program transformation features is much more pleasant to do in Haskell than in Elisp, Vimscript, Python or Java, and that specifying out a cross-editor "modification description" and implementing the smart parts in a shareable tool might save us all a lot of time. It looks like Scion (http://code.google.com/p/scion-lib/) followed similar goals, but doesn't seem to have found wide adoption and no real development for multiple years. I would welcome a discussion about why past attempts to this failed, whether you think that having a monolithic Haskell-IDE-tool output diffs for any editor to apply would lower the barriers for making editors Haskell-smart, and if you are an editor plugin maintainer, if you agree that sharing common functionality is worth it.

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GSOC Project Ideas
by Allan Gardner 11 Mar '14

11 Mar '14

I went through the GSOC trac, most of the Haskell IRC logs, and the Haskell idea reddit. So now I have a list of ideas (grouped by category): infrastructure: - C++ integration (e.g. finish https://github.com/ofan/hqt https://github.com/ofan/fficxx ) - Ada/Python/... integration - gobject bindings (gtk2hs stuff) - cabal improvements - Haddock flags ( https://github.com/haskell/cabal/issues/1585) custom setup improvements (https://github.com/haskell/cabal/issues/948) some sort of plugin system for tests/docs/other random things (BNFC), clean up and merge parallel build code, package management, ... - Hackage2 improvements - voting, rewrite auth system, speed up reverse-deps enough to enable on main site, package wiki/comments, deprecations w/ links, create haddock even if the package doesn't build, ... - scale up Hoogle so it can search all of hackage by default - scoutess work (integration with with hackage, ghc snapshots, ...) libraries: - email/PIM library (MIME, imap, ical, XMPP, WebDav, ... probably make use of a lot of old HaskellNet code) - comprehensive Date/Time API - there are lots of Hackage libs, but none do e.g. internationalization - aim to be backwards-compatible with time - highlight code using inferred types - HsColour + haskell-src/type-exts ? (maybe not enough for a GSOC?) - implement miscellaneous concurrent data structures - darcs improvements (performance, storage, ...) - fix pandoc conversion issues - ... (there were a lot of other libraries, but one of the blog posts said not do a library project because hackage already has too many and designing a good API from scratch is hard) GHC project ideas: - package improvements - finish multiple package stuff, sign and verify packages, ... - port GHC build system to shake (faster, progress indicators, written in Haskell, ...) - implement something like ccache or distcc for GHC - rewrite the GHC pattern overlap/exhaustiveness analysis ( https://ghc.haskell.org/trac/ghc/ticket/595, 10 years old!) - make GHC deterministic (refactor Unique + other nondeterminism sources, make a pure parser and/or typechecker API) - port Hat to the GHC api and integrate it into GHCi's :trace mode so omniscient debugging is available by default - implement typed core as another IL, either above or below old-core ("Types are Calling Conventions" paper) - other type-level programming library/project (units, extended arithmetic solver, true dependent types, linear types, ...) GHC backend-ish: - customizable RTS - remove unused RTS functions, write your own concurrency primitives, ... - resurrect Immix patches and try to get incremental GC - implement loop unrolling in GHC - allow LLVM phases to be written in Haskell (bindings already used, so it's just dealing with the FFI's and GHC's options handling...) I've tried putting these by two people (edwardk and carter) on #haskell-gsoc, and they both said that all the GHC ideas are too hard for a GSOC. Can I get a third opinion? Also, please tell me if I left anything important out (I see a Yi proposal on this mailing list; that would be in the ... in libraries). Finally, some sort of idea of which ideas are most likely to get accepted would be helpful. https://docs.google.com/forms/d/1rEobhHwFpjzPnra9L1TmrozWNFFyAVNPmdUMCcT--3… the top priorities are GHC, Cabal, and Hackage, so since GHC is (apparently) out I guess that means I should look more at my Cabal, Hackage, and Hoogle ideas... is this sensible? -- Allan Gardner

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