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Replacing stdin from within Haskell
by Neil Davies 13 Jun '11

13 Jun '11

Hi Anyone out there got an elegant solution to being able to fork a haskell thread and replace its 'stdin' ? Why do I want this - well I'm using the pcap library and I want to uncompress data to feed into 'openOffile' (which will take "-" to designate read from stdin). Yes I can create a separate command and use the stuff in System.Process or uncompress it into a temporary file - but that all seems in-elegant. Suggestions? Cheers Neil

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Generating simple histograms in png format?
by Dmitri O.Kondratiev 12 Jun '11

12 Jun '11

I am looking for platform-independent library to generate simple histograms in png format. Does such thing exist? Thanks!

5 4

Instances and multiple inheritance
by Patrick Browne 12 Jun '11

12 Jun '11

How do I make an instance of the Vehicles class below? Thanks, Pat class Containers x y where insert :: y -> x y -> x y remove :: y -> x y -> x y whatsIn :: x y -> [y] instance Containers [] Char where insert y [] = y:[] insert y m = y:m remove _ [] = [] remove x (y:ys) | x == y = remove x ys | otherwise = y : remove x ys whatsIn = id instance Containers [] Integer where insert y [] = y:[] insert y m = y:m remove _ [] = [] remove x (y:ys) | x == y = remove x ys | otherwise = y : remove x ys whatsIn = id instance Containers [] [a] where insert x y = x:y -- lists of lists -- insert [1] [[3,6],[45]] class Surfaces a b where put :: b -> a b -> a b takeOff :: b -> a b -> a b whatsOn :: a b -> [b] instance Surfaces [] Integer where put y [] = y:[] put y m = y:m takeOff _ [] = [] takeOff x (y:ys) | x == y = takeOff x ys | otherwise = y : takeOff x ys whatsOn = id class Contacts a b where attach :: b -> a b -> a b detach :: b -> a b -> a b whatsAt :: a b -> [b] instance Contacts [] Integer where attach y [] = y:[] attach y m = y:m detach _ [] = [] detach x (y:ys) | x == y = detach x ys | otherwise = y : takeOff x ys whatsAt = id class Paths a b c where move :: c -> a b c -> a b c origin, destination :: a b c -> b whereIs :: a b c -> c -> b instance Paths (,) Integer Integer where -- `Integer -> (Integer, Integer) -> (Integer, Integer)' move c (a,b) = (a+1,b+1) origin (a,b) = a destination (a,b) = b whereIs (a, b) c = b -- Test -- move (1::Integer) (3::Integer,5::Integer) -- origin (3::Integer,5::Integer) class People p where class HeavyLoads l where class Surfaces w o => WaterBodies w o where instance WaterBodies [] Integer where class Containers h o => Houses h o where class (Surfaces v o, Paths a b (v o)) => Vehicles v o a b where -- I do not know how to make an instance of Vehicles -- What seems to be the *obvious* does not work -- instance Vehicles [] Integer (,) Integer Integer where -- Kind error: `Vehicles' is applied to too many type arguments -- In the instance declaration for -- `Vehicles [] Integer (,) Integer Integer' This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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pool: Why doesn't it block?
by Ertugrul Soeylemez 12 Jun '11

12 Jun '11

Hello Michael, hello fellow haskellers, there is something, which has bothered me for quite a while, but now it has become a serious problem for me, because I see it as a bug, and there is no elegant way to work around it. I wonder if it's the right semantics for Data.Pool to simply fail with an exception, if the pool is exhausted. It would be much more appropriate, if it would just block, until a resource becomes available. Otherwise it's just /safe/ for multi-threading, but not really /useful/ for it. I noticed this when I launched 512 worker threads, but my pool had only 16 database connections. I need the pool to block, until a resource is available. It's also common that my Yesod site just returns an internal server error, when the pool is exhausted, so you can only handle as many connections successfully as there are database connections. I would expect Yesod to wait for a connection to become available instead of simply blowing the request. Blocking should at least be an option and be somehow reachable from Yesod/persistent. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Yesod: How safe are forms?
by Ertugrul Soeylemez 12 Jun '11

12 Jun '11

Hello fellow haskellers, how far does Yesod.Form protect you from invalid input? I'm particularly interested in what happens, when you submit invalid data to select fields or radio groups. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Toy implementation of the STG machine
by Florian Weimer 12 Jun '11

12 Jun '11

I'm looking for a simple implementation of the STG machine to do some experiments, preferably implemented in something with memory safety. Performance is totally secondary. I'm also not interested in garbage collection details, but I do want to look at the contents of the various stacks.

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Fixed points
by Alexander Solla 11 Jun '11

11 Jun '11

Hi Everybody, I have a function which computes a fixed point in terms of iterate: equivalenceClosure :: (Ord a) => Relation a -> Relation a equivalenceClosure = fst . List.head -- "guaranteed" to exist . List.dropWhile (uncurry (/=)) -- removes pairs that are not equal . U.List.pairwise (,) -- applies (,) to adjacent list elements . iterate ( reflexivity . symmetry . transitivity ) Can this function be written in terms of fix? It seems like there should be a transformation from this scheme to something with fix in it, but I don't see it.

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How do you test a parser?
by Andrew Coppin 11 Jun '11

11 Jun '11

OK, so suppose you sit down and write a complicated string parser. Now how do you test that it works correctly? One way is to run the parser over a large corpus of "real world" samples. This has a number of problems: * If the parser is for a grammar you just invented yourself, presumably no "real world" corpus exists yet. * This tests that the most commonly used features work OK, but might not test whether more obscure features work right. * There's no really obvious way to check that what the parser returns is /correct/. You've got the input, but there's nothing to compare with. * This probably doesn't test whether the parser /fails/ for invalid input, since a real world corpus would presumably consist entirely of valid input. Most people's library of choice appears to be QuickCheck. But while it's not hard to have QuickCheck generate random strings and confirm that the returned parse tree (if any) doesn't violate any invariants, it's not so easy to check that the parse tree is actually what it should be. On top of that, depending on what the grammar is, the vast majority of random strings are probably just parse failures. Even if they aren't, there are probably specific constructs that are special-cased in the parser, which are very unlikely to appear by chance. (Things like keyword names, special combinations of punctuation, etc.) You can try to write your own text generator that constructs text more closely approximating what your parser is supposed to parse. But then how do you tell whether your generator is right? If you have a function that turns a parse tree back into text again, you can try verifying that a round-trip is the identity function. Except perhaps sometimes it isn't. Perhaps a given expression has more than one equivalent representation. A round-trip from string and back again is even less likely to be stable. So what's the best way to attack this problem?

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Announcing: hashtables-1.0.0.0
by Gregory Collins 11 Jun '11

11 Jun '11

Hi all, I’m very pleased to announce today the release of the first version of hashtables, a Haskell library for fast mutable hash tables. The hashtables library contains three different mutable hash table implementations in the ST monad, as well as a type class abstracting out the functions common to each and a set of wrapper functions to use the hash tables in the IO monad. It's also substantially faster than Data.HashTable. For more information, including performance benchmarks vs. some of the other common Haskell associative datatypes, please see my blog post: http://gregorycollins.net/posts/2011/06/11/announcing-hashtables Although I’ve made substantial efforts to test this code prior to release, it is a "version 1.0". Please send bug reports to the hashtables github issues page: https://github.com/gregorycollins/hashtables/issues. Thanks! G -- Gregory Collins <greg(a)gregorycollins.net>

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In ML books; functors only consume & produce structures; whereas, functions C&P values.
by KC 11 Jun '11

11 Jun '11

I've never seen such an easy description in Haskell books. Is there more going on with Haskell functors? -- -- Regards, KC

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Class and Instance
by Patrick Browne 11 Jun '11

11 Jun '11

Hi Below is a class that I wish to create some instances of. I do not wish to change the class definition. It is supposed to represent containers of type x that contain things of type y. My attempt at the insert function seems ok for Char and lists, but not ok for Integer. How do I instantiate this class for integers and lists? Does this class definition permit y to be a list and hence x to be a list of lists? Something like: instance Containers [] [] where .. Is this a constructor class? Thanks, Pat class Containers x y where insert :: y -> x y -> x y remove :: y -> x y -> x y whatsIn :: x y -> [y] instance Containers [] Char where insert y [] = y:[] insert y m = y:m instance Containers [] Integer where insert y [] = y:[] insert y m = y:m -- OK -- insert '1' ['u','u','l','i'] -- Not OK -- insert 2 [9,2] This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Vi mode in ghci?
by David McBride 11 Jun '11

11 Jun '11

Somehow in the distant past I managed to get vi editing mode working for ghci on ghc 6, but when I upgraded to ghc 7, I can't seem to figure out how to enable it anymore. I think I might have been using ghci-haskeline in the past. A post I read somewhere suggested that ghc 7 should have innate support for vi mode, but I can't figure out if it does, or how to enable it. Does anyone know? I'm willing to recompile ghc to use readline to get vi mode support if that is possible and the only way to do so.

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Cabal fail
by Andrew Coppin 11 Jun '11

11 Jun '11

How do you make Cabal fail? Let me clarify. I built a package that uses a post-configure hook to do some settings detection. Under certain circumstances, it can fail. When this happens, the hook prints "fatal error" and quits. Unfortunately, I didn't actually /test/ whether it works. On further investigation, I discovered that the hook considers itself to have failed, but it doesn't bother actually telling Cabal about this. (!) So Cabal still thinks everything went fine. OK, so, let's the correct way to notify Cabal that we need to retry the configure step? I can't figure it out from the minimal documentation in Distribution.Simple. (I'm guessing I need to use a pre-configure hook, since by the time post-configure runs, the configure stage is presumably over...?)

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best way to use ghc-7.0 under debian stable
by briand＠aracnet.com 11 Jun '11

11 Jun '11

Hi All, Currently debian stable installs ghc 6.12.1. I'd like to use the latest version of repa which is built on ghc 7.0, and my attempts to placate cabal's complaints about the installation of various packages isn't going anywhere. It's not obvious to me that there's an advantage to using debian for ghc if I want to stay on the latest ghc to support various packages. I'm wondering what the best way to maintain the latest ghc while using debian stable. Should I abandon the ghc debian package and install the haskell-platform ? Should I simply build ghc 7.0 independently ? The advantage of building ghc myself would of course be the ability to incorporate latest patches etc, and that seems like a good way to go. Any advice appreciated. Thanks, Brian

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Installing Gtk2Hs on Win32
by Dmitri O.Kondratiev 11 Jun '11

11 Jun '11

Please help to find pre-compiled binary libraries of Gtk+ for Win32. Gtk2Hs insatll instructions at: http://code.haskell.org/gtk2hs/INSTALL tells us: [quote] Building on Windows -------------------- Installation on Windows is nearly as easy as on Unix platforms. However, you need to download the pre-compiled binary libraries of Gtk+ and all it's dependent libraries. Point your browser to http://www.gtk.org/download-windows.html and download one of the "All-in-one bundles". Note that you do *not* need to install MinGW nor MSys (but it does not hurt if they are installed on your system). Install the binaries by unpacking them into a directory without spaces. [/quote] This link (http://www.gtk.org/download-windows.html) is dead. Where to get pre-compiled binary libraries of Gtk+ ? Thanks!

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Matplotlib analog for Haskell?
by Dmitri O.Kondratiev 11 Jun '11

11 Jun '11

Hello, Please advise on Haskell 2D plotting libraries to generate plots, histograms, power spectra, bar charts, errorcharts, scatterplots, etc, similar to what Matplotlib does: http://matplotlib.sourceforge.net/ Thanks! Dmitri

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Errors installing 'Haskell Charts'
by Dmitri O.Kondratiev 10 Jun '11

10 Jun '11

I am trying to install HaskellCharts at: http://dockerz.net/twd/HaskellCharts Running: cabal update cabal install gtk2hs-buildtools cabal install gtk cabal install chart goes well untill 'cabal install chart' which results in: C:\wks\RCA>cabal install chart Resolving dependencies... C:\Users\DKONDR~1\AppData\Local\Temp\cairo-0.12.03136\cairo-0.12.0\Gtk2HsSetup.h s:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilat ion of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when b uilding (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\Users\DKONDR~1\AppData\Local\Temp\cairo -0.12.03136\cairo-0.12.0\Gtk2HsSetup.hs, C:\Users\DKONDR~1\AppData\Local\Temp\ca iro-0.12.03136\cairo-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\Users\DKONDR~1\AppData\Local\Temp\cairo -0.12.03136\cairo-0.12.0\Setup.hs, C:\Users\DKONDR~1\AppData\Local\Temp\cairo-0. 12.03136\cairo-0.12.0\dist\setup\Main.o ) Linking C:\Users\DKONDR~1\AppData\Local\Temp\cairo-0.12.03136\cairo-0.12.0\dist\ setup\setup.exe ... Configuring cairo-0.12.0... setup.exe: The program pkg-config version >=0.9.0 is required but it could not be found. C:\Users\DKONDR~1\AppData\Local\Temp\glib-0.12.03136\glib-0.12.0\Gtk2HsSetup.hs: 25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilatio n of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when bui lding (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\Users\DKONDR~1\AppData\Local\Temp\glib- 0.12.03136\glib-0.12.0\Gtk2HsSetup.hs, C:\Users\DKONDR~1\AppData\Local\Temp\glib -0.12.03136\glib-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\Users\DKONDR~1\AppData\Local\Temp\glib- 0.12.03136\glib-0.12.0\Setup.hs, C:\Users\DKONDR~1\AppData\Local\Temp\glib-0.12. 03136\glib-0.12.0\dist\setup\Main.o ) Linking C:\Users\DKONDR~1\AppData\Local\Temp\glib-0.12.03136\glib-0.12.0\dist\se tup\setup.exe ... Configuring glib-0.12.0... setup.exe: The program pkg-config version >=0.9.0 is required but it could not be found. cabal: Error: some packages failed to install: Chart-0.14 depends on glib-0.12.0 which failed to install. cairo-0.12.0 failed during the configure step. The exception was: ExitFailure 1 gio-0.12.0 depends on glib-0.12.0 which failed to install. glib-0.12.0 failed during the configure step. The exception was: ExitFailure 1 gtk-0.12.0 depends on glib-0.12.0 which failed to install. pango-0.12.0 depends on glib-0.12.0 which failed to install. Where to get glib-0.12.0 ? Thanks!

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Building Haskell Platform natively for 64bit Windows
by John Sneer 10 Jun '11

10 Jun '11

Hello all, I know it is not probably good question to this list, but anyway, could anyone point me to some more detailed "how to" where is described building of Haskell Platform natively to 64bit Windows? I have no problem using 32bit version until I need more than 2GB of RAM. I went through some older threads, but description like: 1) install gcc 2) install ghc 3) build platform is something that didn't work well. Is there some more detailed description or recommended versions etc? If there is none then there is no need to respond. Thanks, regards J. -- John Sneer johnsneer(a)operamail.com -- http://www.fastmail.fm - Does exactly what it says on the tin

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Deadline June 17: ACM SIGPLAN Workshop on ML
by Chung-chieh Shan 10 Jun '11

10 Jun '11

ACM SIGPLAN Workshop on ML Sunday, 18 September 2011, Tokyo, Japan (co-located with ICFP) http://conway.rutgers.edu/ml2011/ CALL FOR CONTENT The ML family of programming languages includes dialects known as Standard ML, Objective Caml, and F#. These languages have inspired a large amount of computer-science research, both practical and theoretical. This workshop aims to provide a forum for discussion and research on ML and related technology (higher-order, typed, or strict languages). The format of ML 2011 will continue the return in 2010 to a more informal model: a workshop with presentations selected from submitted abstracts. Presenters will be invited to submit working notes, source code, and extended papers for distribution to the attendees, but the workshop will not publish proceedings, so any contributions may be submitted for publication elsewhere. We hope that this format will encourage the presentation of exciting (if unpolished) research and deliver a lively workshop atmosphere. SCOPE We seek research presentations on topics related to ML, including but not limited to * applications: case studies, experience reports, pearls, etc. * extensions: higher forms of polymorphism, generic programming, objects, concurrency, distribution and mobility, semi-structured data handling, etc. * type systems: inference, effects, overloading, modules, contracts, specifications and assertions, dynamic typing, error reporting, etc. * implementation: compilers, interpreters, type checkers, partial evaluators, runtime systems, garbage collectors, etc. * environments: libraries, tools, editors, debuggers, cross-language interoperability, functional data structures, etc. * semantics: operational, denotational, program equivalence, parametricity, mechanization, etc. Research presentations should describe new ideas, experimental results, significant advances in ML-related projects, or informed positions regarding proposals for next-generation ML-style languages. We especially encourage presentations that describe work in progress, that outline a future research agenda, or that encourage lively discussion. In addition to research presentations, we seek both Status Reports and Demos that emphasize the practical application of ML research and technology. Status Reports: Status reports are intended as a way of informing others in the ML community about the status of ML-related research or implementation projects, as well as communicating insights gained from such projects. Status reports need not present original research, but should deliver new information. In the abstract submission, describe the project and the specific technical content to be presented. Demos: Live demonstrations or tutorials should show new developments, interesting prototypes, or work in progress, in the form of tools, libraries, or applications built on or related to ML. In the abstract submission, describe the demo and its technical content, and be sure to include the demo's title, authors, collaborators, references, and acknowledgments. (Please note that you will need to provide all the hardware and software required for your demo; the workshop organizers are only able to provide a projector.) Each presentation should take 20-25 minutes, except demos, which should take 10-15 minutes. The exact time will be decided based on the number of accepted submissions. We plan to make videos of the presentations available on ACM Digital Library. SUBMISSION INSTRUCTIONS Email submissions to ccshan AT cs.rutgers.edu. Submissions should be at most two pages, in PDF format, and printable on US Letter or A4 sized paper. Persons for whom this poses a hardship should contact the program chair. Submissions longer than a half a page should include a one-paragraph synopsis suitable for inclusion in the workshop program. IMPORTANT DATES * 2011-06-17: Submission * 2011-07-22: Notification * 2011-09-18: Workshop PROGRAM COMMITTEE Amal Ahmed (Indiana University) Andrew Tolmach (Portland State University) Anil Madhavapeddy (University of Cambridge) Chung-chieh Shan (chair) (Rutgers University) Joshua Dunfield (Max Planck Institute for Software Systems) Julia Lawall (University of Copenhagen) Keisuke Nakano (University of Electro-Communications) Martin Elsman (SimCorp) Walid Taha (Halmstad University) STEERING COMMITTEE Eijiro Sumii (chair) (Tohoku University) Andreas Rossberg (Google) Jacques Garrigue (Nagoya University) Matthew Fluet (Rochester Institute of Technology) Robert Harper (Carnegie Mellon University) Yaron Minsky (Jane Street)

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Data.List / Map: simple serialization?
by Dmitri O.Kondratiev 10 Jun '11

10 Jun '11

Hello, Please advise on existing serialization libraries. I need a simple way to serialize Data.List and Data.Map to plain text files. Thanks, Dmitri

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Plug space leak with seq. How?
by Alexey Khudyakov 10 Jun '11

10 Jun '11

Hello café! This mail is literate haskell I have some difficulties with understanding how bang patterns and seq works. > {-# LANGUAGE BangPatterns #-} > import Control.Monad > import Control.Monad.Trans.State.Strict > > leak :: State Int () > leak = do > a <- get > put (a+1) > leak This function have obvious space leak. It builds huge chain of thunks so callling `runState leak 0' in ghci will eat all memory. Fix is trivial - add bang pattern. However I couldn't achieve same effect with seq. How could it be done? > noLeak :: State Int () > noLeak = do > a <- get > let !a' = (a + 1) > put a' > noLeak Thanks.

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SIGPLAN Programming Languages Software Award
by Isaac Potoczny-Jones 10 Jun '11

10 Jun '11

I'm pleased to be able to relay the following announcement from ACM SIGPLAN: The SIGPLAN Programming Languages Software Award is awarded to an institution or individual(s) to recognize the development a software system that has had a significant impact on programming language research, implementations, and tools. The impact may be reflected in the wide-spread adoption of the system or its underlying concepts by the wider programming language community either in research projects, in the open-source community, or commercially. The award includes a prize of $2,500. For 2011, the winners of the award are Simon Peyton Jones and Simon Marlow of Microsoft Research, Cambridge, for GHC The award winners are donating the entirety of the prize money to haskell.org. Citation: Simon Peyton Jones and Simon Marlow receive the SIGPLAN Software Award as the authors of the Glasgow Haskell Compiler (GHC), which is the preeminent lazy functional programming system for industry, teaching, and research. GHC has not only provided a language implementation, but also established the whole paradigm of lazy functional programming and formed the foundation of a large and enthusiastic user community. GHC's flexibility has supported experimental research on programming language design in areas as diverse as monads, generalized algebraic data types, rank-N polymorphism, and software transactional memory. Indeed, a large share of the research on lazy functional programming in the last 5–10 years has been carried out with GHC. Simultaneously, GHC's reliability and efficiency has encouraged commercial adoption, in the financial sector in institutions like Credit Suisse and Standard Chartered Bank, and for high assurance software in companies like Amgen, Eaton, and Galois. A measure of GHC's influence is the way that many of the ideas of purely functional, "typeful programming" have been carried into newer languages and language features. including C#, F#, Java Generics, LINQ, Perl 6, Python, and Visual Basic 9.0. Peyton Jones and Marlow have been visionary in the way that they have transitioned research into practice. They have been role models and leaders in creating the large and diverse Haskell community, and have made GHC an industrial-strength platform for commercial development as well as for research. Links: http://www.sigplan.org/award-software.htm http://corp.galois.com/blog/2011/6/7/sigplan-programming-languages-software…

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Delimited Continuations For Web Development?
by aditya siram 10 Jun '11

10 Jun '11

Hi all, Is anyone in the Haskell community doing web development using delimited continuations? Oleg had a talk on it [1] using Ocaml and CGI but I haven't heard of anyone taking it further. -deech [1]http://okmij.org/ftp/continuations/index.html#shift-cgi

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Re: [Haskell-cafe] Input and output of mathematical expressions
by Daniel Patterson 09 Jun '11

09 Jun '11

(I originally sent this reply from the wrong address, see below) On Jun 9, 2011, at 11:03 AM, Daniel Patterson wrote: > Also if you are doing this for web, I haven't used it personally (yet), but http://www.mathjax.org/ looks really good... would allow you to actually just write it in LaTeX (as writing MathML by hand is not something I'd like to subject anyone to). > > On Jun 9, 2011, at 10:59 AM, Chris Smith wrote: > >> Ae you looking to do this in a web application, or client-side? Since one of your requirements is to display a typeset equation, that makes a bit of difference. In a web-based setting, the best way to do that is probably MathML, whereas a GUI will be a bit harder. >> >> On Jun 9, 2011 8:24 AM, "Jacek Generowicz" <jacek.generowicz(a)cern.ch> wrote: >> > Greetings Cafe, >> > >> > What would you recommend as a Haskell-based means of interactively >> > reading and writing mathematical formulae? >> > >> > As a toy example, what might I use to write a program which presents >> > the user with >> > >> > Please simplify the expression: \pi x^2 + 3\pi x^2 >> > >> > (Where the TeX-style expression would be presented with a greek pi and >> > superscript twos on the xs.) >> > >> > The user should then have the ability to reply with something that >> > looks like the result of TeXing >> > >> > 5 \pi x^2 >> > >> > Whatever means the user uses to enter this expression, he should be >> > able to preview the "typeset" version of his input before submitting. >> > >> > Any ideas? >> > >> > Thanks. >> > >> > >> > _______________________________________________ >> > 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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Type Constraints on Data Constructors
by Guy 09 Jun '11

09 Jun '11

{- continuing discussion from beginners@ -} I have code such as class Foo f where foo :: a -> f a data Bar f a = Foo f => Bar {bar :: f a} instance Foo (Bar f) where foo a = Bar $ foo a GHC insists that I put Foo f => on the instance declaration, even though the constructor for Bar implies this. Is there any reason why GHC cannot infer this constraint from the Bar constructor? One issue raised in the beginners thread is that undefined :: Bar f a is not Foo f, but as undefined cannot be evaluated, this would not appear to be a problem.

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Re: [Haskell-cafe] Type Constraints on Data Constructors
by Guy 09 Jun '11

09 Jun '11

Can this be extended to records, without redundant repetition? data Baz f a = Baz {baz :: Foo f => f a, baz2 :: Foo f => f a} The type constraint for baz2 adds no information, as it's the same f as baz, but I can't leave it out. ----- Original Message ----- > From: Daniel Schüssler <danlex(a)gmx.de> > To: haskell-cafe(a)haskell.org > Cc: Guy <guytsalmaves-h(a)yahoo.com> > Sent: Thursday, 9 June 2011, 2:06 > Subject: Re: [Haskell-cafe] Type Constraints on Data Constructors > > Hello, > > you might be thinking of this type? > > {-# LANGUAGE Rank2Types #-} > > class Foo f where > foo :: a -> f a > > data Baz f a = Baz (forall f. Foo f => f a) > > instance Foo (Baz f) where > foo a = Baz (foo a) > > Maybe the difference between Bar and Baz ist best explained by writing it with > an explicit class dictionary for Foo: > > {-# LANGUAGE Rank2Types #-} > > data FooDict f = FooDict { > foo :: forall a. a -> f a > } > > data Bar f a = Bar (FooDict f) (f a) > > data Baz f a = Baz (FooDict f -> f a) > > fooDict_Baz :: FooDict (Baz f) > fooDict_Baz = FooDict (\a -> Baz (\d -> foo d a)) > > -- fooDict_Bar :: FooDict (Bar f) > -- fooDict_Bar = FooDict (\a -> Bar ? ?) > -- Doesn't work - you'd have to create a 'FooDict f' and a > 'f a' out of just > an 'a' > > > > Cheers, > Daniel

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Re: [Haskell-cafe] Type Constraints on Data Constructors
by Daniel Schüssler 09 Jun '11

09 Jun '11

Hello, you might be thinking of this type? {-# LANGUAGE Rank2Types #-} class Foo f where foo :: a -> f a data Baz f a = Baz (forall f. Foo f => f a) instance Foo (Baz f) where foo a = Baz (foo a) Maybe the difference between Bar and Baz ist best explained by writing it with an explicit class dictionary for Foo: {-# LANGUAGE Rank2Types #-} data FooDict f = FooDict { foo :: forall a. a -> f a } data Bar f a = Bar (FooDict f) (f a) data Baz f a = Baz (FooDict f -> f a) fooDict_Baz :: FooDict (Baz f) fooDict_Baz = FooDict (\a -> Baz (\d -> foo d a)) -- fooDict_Bar :: FooDict (Bar f) -- fooDict_Bar = FooDict (\a -> Bar ? ?) -- Doesn't work - you'd have to create a 'FooDict f' and a 'f a' out of just an 'a' Cheers, Daniel On 2011-June-08 Wednesday 20:45:56 Guy wrote: > {- continuing discussion from beginners@ -} > > I have code such as > > class Foo f where > foo :: a -> f a > > data Bar f a = Foo f => Bar {bar :: f a} > > instance Foo (Bar f) where > foo a = Bar $ foo a > > GHC insists that I put Foo f => on the instance declaration, even though > the constructor for Bar implies this. > > Is there any reason why GHC cannot infer this constraint from the Bar > constructor? One issue raised in the beginners thread is that > undefined :: Bar f a > is not Foo f, but as undefined cannot be evaluated, this would not appear > to be a problem. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

1 1

Computing the memory footprint of a HashMap ByteString Int (Was: How on Earth Do You Reason about Space?)
by Johan Tibell 09 Jun '11

09 Jun '11

Hi Aleksandar, I thought it'd be educational to do some back-of-the-envelope calculations to see how much memory we'd expect to use to store words in a HashMap ByteString Int. First, lets start by looking at how much memory one ByteString uses. Here's the definition of ByteString [1]: data ByteString = PS {-# UNPACK #-} !(ForeignPtr Word8) {-# UNPACK #-} !Int -- offset {-# UNPACK #-} !Int -- length The two Int fields are used to support O(1) slicing of ByteStrings. We also need the definitions of ForeignPtr [2] and Int. data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents data ForeignPtrContents = PlainForeignPtr !(IORef (Finalizers, [IO ()])) | MallocPtr (MutableByteArray# RealWorld) !(IORef (Finalizers, [IO ()])) | PlainPtr (MutableByteArray# RealWorld) data Int = I# Int# The UNPACK indicates to the compiler that it should unpack the contents of a constructor field into the constructor itself, removing a level of indirection. We'll end up with a structure that looks like this: data ByteString = PS Addr# ForeignPtrContents Int# -- offset Int# -- length To compute the size of a ByteString, count the number of constructor fields and add one (for the constructor itself). This is how many words the value is going to use. In this case it's 5 words. In addition we need to add the size of the ForeignPtrContents constructor, which happens to be a PlainPtr in this case, so we add two more words. Finally we need to look at the definition of MutableByteArray# [3], which is implemented by a C struct named StgArrWords: typedef struct { StgHeader header; StgWord bytes; StgWord payload[FLEXIBLE_ARRAY]; } StgArrWords; The StgHeader takes one word (when compiling with profiling turned off) so StgArrWords takes 2 words, plus the actual payload. If we add it all up we get 9 words, plus the size of the payload (i.e. the length of a word encoded using UTF-8 in your case). Now lets look at the definition of HashMap [4]: data HashMap k v = Bin {-# UNPACK #-} !SuffixMask !(HashMap k v) !(HashMap k v) | Tip {-# UNPACK #-} !Hash {-# UNPACK #-} !(FL.FullList k v) | Nil We also need the definition of FullList [5]: data FullList k v = FL !k v !(List k v) data List k v = Nil | Cons !k v !(List k v) For the sake of this discussion lets assume the tree is perfectly balanced. For a HashMap of size N this means that we have N leaves (Tip) and N-1 interior nodes (Bin). Each Bin takes 4 words. The size of the Tip depends on the number of hash collisions. These are quite rare so lets assume that the FullList only has one element. Also, the Nil constructor is free as it can be shared by all instances in the program. After applying the UNPACK pragmas the Tip constructor looks like: | Tip Int# !k v !(List k v) This takes another 5 words. Now when we know the overhead of both Bin and Tip we can compute the overhead per key/value pair as: (5N + 4(N-1)) / N = 9 - 4/N ~= 9 words. Given that an Int (not an Int#) takes 2 words, we can approximate the memory cost of a key/value pair in a HashMap ByteString Int as (9 + 9 + 2) * MachineWordSize + AvgByteStringLength bytes. For example, the average English word length is 5 characters, if you include stop words. We'd expect to use (9 + 9 + 2) * 8 + 5 = 165 bytes per unique word in our input corpus, on a 64-bit machine. Plus any GC overhead. This is probably more than one would expect. I'm working on switching HashMap to use another data structure, a Hash Array Mapped Trie, in its implementation. This will bring the overhead down from 9 words to about 4 words. You could try using the lower overhead SmallString type from the smallstring package [6]. It has an overhead of 4 words per string (instead of 9 like ByteString). There's some runtime overhead involved in converting a value (i.e. Text) to a SmallString. I don't know if this overhead will be noticeable in your program. 1. http://code.haskell.org/bytestring/Data/ByteString/Internal.hs 2. https://github.com/ghc/packages-base/blob/master/GHC/ForeignPtr.hs 3. https://github.com/ghc/ghc/blob/master/includes/rts/storage/Closures.h 4. https://github.com/tibbe/unordered-containers/blob/master/Data/HashMap/Comm… 5. https://github.com/tibbe/unordered-containers/blob/master/Data/FullList/Laz… 6. http://hackage.haskell.org/package/smallstring P.S. If your program indeed has a space leak this won't help you, but it's a good way to figure out if your program uses a reasonable amount of memory. Cheers, Johan

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Haskell Weekly News: Issue 185
by Daniel Santa Cruz 09 Jun '11

09 Jun '11

Welcome to issue 180 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of May 29 to Jun 4, 2011. You can find the HTML version at: http://goo.gl/Tm1Qu Announcements Sebastian Fischer wrote in to inform us of of a student internship for parallel Haskell at the National Institute of Informatics in Tokyo. The internship is for up to three months, between July and September 2011. http://goo.gl/xHjkn EIJIRO Sumii reminded us that the ICFP Programming Contest 2011 will be starting on June 17. http://goo.gl/eumwm Quotes of the Week * Tom Murphy: Great! New I really can say "Come on! It's fun! I can write foldr with foldl!" * Cale: C++, Y U SO UGLY? * xplat: nae, that'd be 'arr, my small example ain't half near as yellow as a lily-livered spaniard! scupper me wi' a cutlass, arr.' * Cale: rnf accidentally the entire data structure Top Reddit Stories * Haskell and parallelism in the Economist Domain: economist.com, Score: 66, Comments: 5 On Reddit: http://goo.gl/rBnc1 Original: http://goo.gl/vFtGY * I have the world's coolest hoodie (X-post from r/programming) Domain: imgur.com, Score: 28, Comments: 15 On Reddit: http://goo.gl/bk5wt Original: http://goo.gl/CT3fb * An OpenGL in Haskell anecdote which illustrates the obvious, HOpenGL is low level and imperative. Domain: haskell.org, Score: 27, Comments: 24 On Reddit: http://goo.gl/QtDNy Original: http://goo.gl/LUoL9 * Yesod for non-Haskellers (also, for Haskellers) Domain: yesodweb.com, Score: 27, Comments: 2 On Reddit: http://goo.gl/ekOQJ Original: http://goo.gl/UCyIv * Fast forwarding lrand48() Domain: blog.sigfpe.com, Score: 24, Comments: 0 On Reddit: http://goo.gl/Ki0in Original: http://goo.gl/5ICat * Silk (Haskell, semantic web start up) is hiring in Amsterdam Domain: news.ycombinator.com, Score: 24, Comments: 0 On Reddit: http://goo.gl/GUFVP Original: http://goo.gl/j8fRW * Nikki and the Robots 0.3 is out Domain: joyridelabs.de, Score: 23, Comments: 7 On Reddit: http://goo.gl/w3yrF Original: http://goo.gl/sJdwj * Introducing the Yesod Wiki Domain: yesodweb.com, Score: 19, Comments: 0 On Reddit: http://goo.gl/4TStP Original: http://goo.gl/noMVO * Communicating Haskell Processes: The Long And The Short Of It Domain: chplib.wordpress.com, Score: 19, Comments: 0 On Reddit: http://goo.gl/mC8cw Original: http://goo.gl/hHHoH * What libraries from scientific computing have yet to be written? Domain: self.haskell, Score: 17, Comments: 23 On Reddit: http://goo.gl/QXwyA Top StackOverflow Questions * "A proof is a program; the formula it proves is a type for the program" [migrated] votes: 17, answers: 0 Read on SO: http://goo.gl/LfzPM * Writing foldl using foldr votes: 16, answers: 2 Read on SO: http://goo.gl/O7I0Z * Binary Serialization for Lists of Undefined Length in Haskell votes: 13, answers: 1 Read on SO: http://goo.gl/pZkyI * From C ++ to Haskell Classes and States votes: 12, answers: 2 Read on SO: http://goo.gl/yYt9R * Why does Haskell force data constructor's first letter to be upper case? votes: 11, answers: 3 Read on SO: http://goo.gl/dnGIl * Why is such a function definition not allowed in haskell ? votes: 10, answers: 5 Read on SO: http://goo.gl/0RoHY About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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ANN: mecha-0.1.0
by Tom Hawkins 08 Jun '11

08 Jun '11

Mecha [1,2] is a constructive solid geometry (CSG) modeling language embedded in Haskell. This release adds OpenSCAD [3] as a backend target. OpenSCAD is a solid modeling DSL and a viewer. OpenSCAD uses OpenCSG [4] for rendering, which directly renders CSG objects with OpenGL without the need for complex mesh generation. (Thanks Corey, for pointing me to OpenCSG.) Here is a screenshot of OpenSCAD rendering a Mecha solid: http://tomahawkins.org/index.html#Mecha Here is the Mecha source: https://github.com/tomahawkins/mecha/blob/master/Language/Mecha/Examples/CS… -Tom [1] http://hackage.haskell.org/package/mecha [2] https://github.com/tomahawkins/mecha [2] http://www.openscad.org/ [3] http://opencsg.org/

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Parallel GHC project: new opportunity for an organisation to participate
by Duncan Coutts 08 Jun '11

08 Jun '11

GHC HQ and Well-Typed are pleased to announce a new opportunity for an organisation to take part in the Parallel GHC Project. The project started in November 2010 with four industrial partners, and consulting and engineering support from Well-Typed. Each organisation is working on its own particular project making use of parallel Haskell. The overall goal is to demonstrate successful use of parallel Haskell and along the way to apply engineering effort to any problems with the tools that the partner organisations might run into. We have capacity to support another partner organisation for the remaining duration of the project (at least another 12 months). Organisations do not need to contribute financially but should be prepared to make a significant commitment of their own time. Familiarity with Haskell would be helpful, but Haskell expertise is not needed. Partner organisations' choice of projects is similarly open-ended and could be based on anything from pre-existing code bases to green field endeavours. We would welcome organisations interested in pure parallelism, concurrency and/or distributed Haskell. Presently, two of our partner organisations are using mainly pure parallelism and two are using concurrency. What would be especially interesting for us is to diversify this mix further by working with an organisation interested in making use of of distributed Haskell, in particular the work highlighted in the recent paper "Haskell for the Cloud" [1]. To help give an idea what participating in the Parallel GHC Project is like, here is what some of what our current partner organisations have to say: The Parallel GHC Project has enabled us to make steady progress towards our goals. Well-typed has provided support in the form of best practice recommendations, general engagement with the project, and directly coding up key components. I have been getting lots of help from Well-Typed, and enjoy our weekly meetings. -- Finlay Thompson, Dragonfly My organization is now trying to implement highly concurrent Web servers. After GHC 7 was released we faced several critical bugs in the new IO manager and one guy at Well-Typed kindly fixed all the bugs. This has been a big benefit for our organization. Another benefit is feedback/suggestions from Well-Typed. Well-Typed and our organization have meetings every other week and we report progress to each other. During the discussions, we can set the right direction to go in. -- Kazu Yamamoto, IIJ Innovation Institute Inc. Well-Typed is coordinating the project, working directly with the participating organisations and the Simons at GHC HQ. If you think your organisation may be interested then get in touch with me via info(a)well-typed.com [1] http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remot… -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

1 0

haskellwiki slow/unresponsive
by Greg Weber 08 Jun '11

08 Jun '11

> > Those are definitely valid concerns. Has anyone made a wiki-like site with > Yesod? I hadn't heard of Yesod until I joined this mailing list, but I've > seen quite a bit of buzz around it since then. If a large enough chunk of > the community is backing a framework and focusing on making it secure and > reliable, then it should be possible to build applications with it (wikis, > blogs, etc.) that draw on the framework's strength and security. You may > still have security issues, but if they're continually addressed and > maintained at the framework level it benefits everyone building > applications > on top of that framework. I'm still relatively new to the Haskell > community > so I apologize if much of this has been addressed before! It is dead-simple to deploy a fast haskell web app, so haskell *might* have a chance at solving the problem, but likely it can be fixed much faster by doing a better job configuring the server or adding more resources. I think that security issues are a bit of a red herring. You can go through potential security issues one-by-one and basically make them impossible in a web/wiki framework (as we are doing in Yesod). Of course this is easier to do with a type system in Haskell. And of course some security is outside the scope of the software and needs to be established by procedure (admin server access should only be through ssh key, etc). Gitit uses darcs or git to store data, but through the command line interfaces. Unfortunately to my knowledge darcs does not expose a library interface. Gitit could be made faster and more secure by interfacing with libgit2. Even though it is may not be socially acceptable to state it, it is likely that gitit or any haskell library would be more secure in practice because it is more obscure. And the kind of security we really care about in the wiki- the wiki page content- can be easily replicated and restored. I definitely agree that the hard and boring part of switching to a haskell wiki is converting existing MediaWiki content and configuration to a new system- that would have to be investigated first. Unfortunately this is probably all bike-shedding and my original question of how can the community speed up the wiki hasn't been answered :) Greg Weber

3 2

Does this library already exist? template haskell + generate/compile C code + dlopen
by Ryan Newton 08 Jun '11

08 Jun '11

The Nikola GPU programming system has a very neat, flexible approach to how you compile the EDSL-generated code. You can do it dynamically, calling nvcc at runtime, OR it can play a trick where it calls nvcc at compile time (via template haskell) and caches the result in a string literal within the TH-generated Haskell code. That is a pretty cool trick IMHO. Moreover, we can do that with any tool that generates C code or native code, as follows: At compile time, via Template Haskell: 1. call tool which creates C code 2. create temp files and call C compiler 3. load resulting object file as a bytestring and store it as a string constant Then at runtime: 1. put bytestring back into a file 2. call dlopen 3. call dlsym, get FunPtr, voila! Anyway, I was going to put together a simple library that encapsulates the above steps. Such a library could be used, for example, in making this stencil compiler project <http://people.csail.mit.edu/yuantang/> available to Haskell users as well as C++ users. (The compiler is written in Haskell already anyway!) But first I thought I'd ask if this already exists. Also, is there a better way to do it? In particular, is there any way to get static linking to work, rather than the silliness with strings, tempfiles, and dlopen. Thanks, -Ryan

1 0

Proposal: remove "Stability" from haddock documentation on hackage
by Chris Smith 08 Jun '11

08 Jun '11

I got asked a question today about why Control.Applicative is labeled as "experimental" on Hackage. Perhaps that field is something of a failed experiment, and it remaining there is likely to confuse people. Just a thought... not sure of the best place to mention it. -- Chris Smith

9 11

Local Haskell Meeting (Stammtisch) Leipzig, Germany, June 27
by Johannes Waldmann 08 Jun '11

08 Jun '11

Informal Haskell Stammtisch on Monday, June 27, 8 p.m., at Cafe Grundmann, Leipzig, Germany. http://www.cafe-grundmann.de/ Pre-registration (by email to me) is appreciated. Please bring ideas for our local Haskell Workshop (planned for end of September). See you - J.W.

1 0

Twidge and identi.ca
by Mats Rauhala 08 Jun '11

08 Jun '11

Does twidge still support identi.ca? I was able to auth, but after that every single command returns 'twidge: user error (Bad response: 404)'. The commands work for twitter. -- Mats Rauhala MasseR

1 0

Comment Syntax
by Guy 08 Jun '11

08 Jun '11

-- followed by a symbol does not start a comment, thus for example, haddock declarations must begin with -- |, and not --|. What might --| mean, if not a comment? It doesn't seem possible to define it as an operator.

20 40

haskell platform question
by Tim Docker 08 Jun '11

08 Jun '11

I'd like to build the haskell platform against a recent GHC snapshot, for testing purposes. I see that I can download the source for the platform from: http://lambda.galois.com/hp-tmp/2011.2.0.1/haskell-platform-2011.2.0.1.tar.… with instructions: http://www.vex.net/~trebla/haskell/haskell-platform.xhtml However when I tried this some months ago, I found that some of the packages in the above source fail to build due to recent ghc changes (eg it's stricter handling of bang patterns). Is there a more recent "beta" snapshot of the platform? Or a darcs/git repo somewhere? I imagine wanting to build the platform against bleeding edge ghc would be a pretty common desire. Tim

4 6

HUnit false-positive stumper
by KQ 08 Jun '11

08 Jun '11

While working on a project of mine recently I realized that a particular HUnit test should have been failing. After paring things down, I came up with this shocker: test_perhaps.hs: > module Main where >import Test.HUnit >main = runTestTT $ TestList [ True ~=? True > , False ~=? True > , TestCase $ assertEqual "both true" True True > , TestCase $ assertEqual "false true" False True > ] $ cabal-dev install ... $ ./cabal-dev/bin/test_perhaps.hs ### Failure in: 3 false true expected: False but got: True Cases: 4 Tried: 4 Errors: 0 Failures: 1 The shock here is that there was only one failure, whereas the "False ~=? True" should have failed. Environment: MacOS 10.5.8 (Leopard) GHC 6.12.3 cabal-dev 0.7.4.1 Cabal 1.10.1.0 HUnit 1.2.2.3 I even tried removing the dist and cabal-dev directories and reattempting but got the same results. I tried the same test in a different environment (Linux PC w/GHC 7.0.2, all other elements the same) and got correct results: $ ./cabal-dev/bin/test_perhaps ### Failure in: 1 expected: False but got: True ### Failure in: 3 false true expected: False but got: True Cases: 4 Tried: 4 Errors: 0 Failures: 2 $ I realize it's very difficult to debug someone's setup remotely, but I have to confess I'm really stumped at this point. The input program is pretty straightforward, and I *have* gotten test failures in the past on the Mac. I did do a "cabal update" the other day after being informed that my hackage file was getting old, but HUnit hasn't changed in months. I even tried unpacking HUnit and reducing the (non-)failing test case by using the definitions of the elements: > module Main where > > import Control.Monad (unless) > import Test.HUnit > > main = runTestTT $ TestList [ True ~=? True > , False ~=? True > , TestCase $ assertEqual "both true" True True > , TestCase $ assertEqual "false true" False True > , TestCase $ assertEqual "fa" False True > , TestCase $ assertEqual "f" False True > , TestCase $ (False @?= True) > , TestCase $ unless (False == True) (assertFailure "f") > ] The results are very strange: $ ./cabal-dev/bin/test_perhaps ### Failure in: 3 false true expected: False but got: True ### Failure in: 4 fa expected: False but got: True ### Failure in: 7 f Cases: 8 Tried: 8 Errors: 0 Failures: 3 $ The assertEqual form doesn't fail as it should if the label is a single character (test 5), but in its fully expanded form (test 7) it will fail. Huh?! At this point I'm thoroughly confused. I'm using 6.12.3 on the Mac because I don't have the newer MacOS release for which there's a Haskell Platform release and I haven't wanted to build GHC by hand. I wouldn't expect 6.12.3 to have issues like this, but I wouldn't expect issues like this anywhere. If anyone has any suggestions (other than upgrading to GHC 7.x) I'll be most appreciative. -- -KQ

4 6

Non-advanced usage of Type classes
by Arnaud Bailly 08 Jun '11

08 Jun '11

Hello, In a recent thread, it has been asserted that defining type class is something you seldom need when programming in Haskell. There is one thing that as non-professional Haskell programmer I found type-classes useful for: Testing. This is probably very OO and is pretty much influenced by what I read in RWH but I find useful to define TC that abstract away from low-level interactions with other code, possibly IO related, in such a way that I can have one implementation for testing and one implementation for real work that is wired in caller context. This is what is called "mockist" TDD in some circles: Write code that expresses what it needs in its own terms, then implement "glue" to the code that provide the concrete behaviour. For example, while designing some program (a game...) I defined a type class thus: > class (Monad io) => CommandIO io where > readCommand :: io Command > writeResult :: CommandResult -> io () Then I defined in a module Commands.IO : > instance CommandIO IO where > readCommand = do input <- getLine > ... > writeResult r = putStrLn $ show r and for the purpose of testing I defined in some test module: > instance CommandIO (S.State ([Command],[CommandResult])) where > readCommand = do ((c:cs),rs) <- S.get > .... > writeResult r = do (cs,rs) <- S.get > ... Is this badly designed code that tries to mimic OO in a functional setting? If the answer is yes, how could I achieve same result (eg. testing the code that does command REPL) without defining type classes? Regards, Arnaud

4 6

Distributing Haskell GUI apps for Windows/MacOS?
by Pedro Vasconcelos 07 Jun '11

07 Jun '11

Hello, I've implemented a Haskell program to play a modern board game. I've put it on Hackage (http://hackage.haskell.org/package/hstzaar) but would also like to distribute to people without the Haskell development tools. Are there any easy ways of making installers for Windows/MacOS (preferably using free or open source tools)? Best regards, Pedro

4 3

Cons of -XUndecidableInstances
by Scott Lawrence 07 Jun '11

07 Jun '11

According to the haskell-prime wiki[1], -XUndecidableInstances removes checks on the form of instance declaration, and just impose a depth limit to ensure termination (of compilation, I assume?). The listed Con is that this removes the clear boundary between legal and illegal programs, and behaviour may be implementation-dependent as the edge of that boundary is reached. How can I tell when I'm nearing that boundary? (And where are the sorts of things GHC does with types documented? I can't seem to find any good explanation of these things.) More specifically, I have class Model m a | m -> a where ... class Entropy d where ... instance (Model m a) => Entropy m where ... The first line requires MultiParamTypeClasses and FunctionalDependencies (the two seem to go together) - the third requires UndecidableInstances (since the type variable 'a' appears on the left but not the right). Is this likely to cause a problem? My guess is it shouldn't, since it's equivalent to class Model m a | m -> a where ... class Entropy d a where ... instance (Model m a) => Entropy m a where ... without bothering to actually use 'a' in Entropy - but one never knows... (Actually, a third type variable has to be introduced to Entropy to remove the UndecidableInstances dependency - "Constraint is no smaller than the instance head". This only increases the illogic in my humble eyes. These examples seem simple enough for GHC to handle nicely...) [1] http://hackage.haskell.org/trac/haskell-prime/wiki/UndecidableInstances -- Scott Lawrence

8 18

Attoparsec concatenating combinator
by Yitzchak Gale 07 Jun '11

07 Jun '11

I often find while using attoparsec and attoparsec-text that I need to match a number of text parsers consecutively and concatenate the result. By "text parser" I mean "Parser ByteString" for attoparsec and "Parser Text" for attoparsec-text. It seems the best I can do is to collect them all in a list and then apply concat. But that still copies the text several times. Is there a combinator that does this without all that copying? If not, does the internal representation easily admit such a combinator? Thanks, Yitz

5 13

ANNOUNCE: time-recurrence-0.5.2
by Chris Heller 07 Jun '11

07 Jun '11

I'm happy to announce the first fully functional release of time-recurrence: http://hackage.haskell.org/package/time-recurrence As of version 0.5.2, the library is now powerful enough to express all recurring dates which can be specified via the iCalendar RFC. An example: Generate the 15th and the 30th of the month, but only during the work week: > jan2011 = ptime "Sat, 01 Jan 2011 00:00:00 -0400" > jan2012 = ptime "Sun, 01 Jan 2012 00:00:00 -0400" > takeWhile (<= jan2012) $ recur monthly `starting` jan2011 $ > enumDays [15,30] >=> > filterWeekDays [Monday .. Friday] [2011-02-15 04:00:00 UTC ,2011-03-15 04:00:00 UTC, 2011-03-30 04:00:00 UTC ,2011-04-15 04:00:00 UTC ,2011-05-30 04:00:00 UTC ,2011-06-15 04:00:00 UTC, 2011-06-30 04:00:00 UTC ,2011-07-15 04:00:00 UTC ,2011-08-15 04:00:00 UTC, 2011-08-30 04:00:00 UTC ,2011-09-15 04:00:00 UTC, 2011-09-30 04:00:00 UTC ,2011-11-15 04:00:00 UTC, 2011-11-30 04:00:00 UTC ,2011-12-15 04:00:00 UTC, 2011-12-30 04:00:00 UTC ] As always I encourage feedback, criticism, suggestions and pull requests. -Chris

1 0

Twidge and "lsfollowers --all"
by Keith Edmunds 07 Jun '11

07 Jun '11

The github Twidge page [https://github.com/jgoerzen/twidge/wiki] states that this is the right place for questions about Twidge, so here goes. When running twidge lsfollowers --all $USERNAME my expectation is that all followers of $USERNAME will be listed. However, what appears to happen is that the first 100 followers' names are displayed, then there is a pause, then the same 100 names are displayed again; this cycle repeats (I stopped it after four iterations). Is this the expected behaviour? Thanks, Keith -- "You can have everything in life you want if you help enough other people get what they want" - Zig Ziglar. Who did you help today?

2 1

cap 3: stopping thread 3 (stackoverflow)
by Johannes Waldmann 07 Jun '11

07 Jun '11

I am running a Haskell program with some par/pseq annotations. When I use threadscope to view the eventlog (not "timeline" but "events") I see a lot of stackoverflow messages. What is this? I don't get any RTS errors printed on the console. The program finishes normally (albeit with less speedup than expected) Should I file this as a possible GHC RTS bug? At least the output is confusing. (... and why can't I copy/paste the text from threadscope's output window)

5 7

Library Versioning
by Luis Cabellos 07 Jun '11

07 Jun '11

Hello, I have a question about cabal versioning. It's possible to export in a cabal library a version, so instead of getting version from: import Paths_my_package( version ) I want to get version from my library using: import MyPackage( version ) And then using this from programs. Thanks, Luis

2 4

Re: [Haskell-cafe] Can it be proven there are no intermediate "useful" type classes between Applicative Functors & Monads?
by David Barbour 07 Jun '11

07 Jun '11

On Tue, Jun 7, 2011 at 6:14 AM, Casey McCann <syntaxglitch(a)gmail.com> wrote: > On Mon, Jun 6, 2011 at 7:55 PM, David Barbour <dmbarbour(a)gmail.com> wrote: > > Earlier forms of my reactive demand programming model [1] - before I > > switched to arrows - would qualify. The model has limited side-effects > (e.g. > > power a camera on only when someone is observing it) so we cannot use > > branchApplicative. The reactivity requires continuously weaving the two > > branches over time and recombining the results, which is distinct from > > branchMonad. > > Oh, very nice, thank you. I'd actually suspected that models of > reactive behavior might be a case where the distinction is meaningful. > I do still wonder if there's something roughly equivalent to the > (grossly inefficient and unusable, but producing the same results > otherwise) monad instance for zipping infinite streams, but I don't > have time to work through it right now to be sure... > The main trouble with using monads directly is that they're simply too powerful. Monads allow ad-hoc joins and loops based on data. The number of reactive relationships during any given instant can vary widely and unpredictably based on data. This makes it difficult to maintain stable relationships over continuous time. Looping and Branching must be carefully managed in my reactive model in order to gain stability over time that Monads do not possess.

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Re: [Haskell-cafe] Can it be proven there are no intermediate "useful" type classes between Applicative Functors & Monads?
by David Barbour 07 Jun '11

07 Jun '11

On Mon, Jun 6, 2011 at 4:05 PM, Casey McCann <syntaxglitch(a)gmail.com> wrote: > ArrowChoice and ArrowApply are conceptually distinct and I expect > there are instances of the former that have no possible instance for > the latter. Branching vs. Monad I am much less certain of. > For a real-time or embedded DSL, or hardware modeling, you could easily desire 'Branching' and limited 'Loop' classes while rejecting the full power of Monads. > some type that's not obviously equivalent to one of these definitions: > branchMonad mb t f = do { b <- mb; if b then t else f } > branchApplicative = liftA3 (\b t f -> if b then t else f) > Earlier forms of my reactive demand programming model [1] - before I switched to arrows - would qualify. The model has limited side-effects (e.g. power a camera on only when someone is observing it) so we cannot use branchApplicative. The reactivity requires continuously weaving the two branches over time and recombining the results, which is distinct from branchMonad. [1] http://awelonblue.wordpress.com/2011/05/21/comparing-frp-to-rdp/

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Re: [Haskell-cafe] Can it be proven there are no intermediate "useful" type classes between Applicative Functors & Monads?
by David Barbour 07 Jun '11

07 Jun '11

On Sun, Jun 5, 2011 at 12:51 PM, KC <kc1956(a)gmail.com> wrote: > If new intermediate classes crop up then there would be no point in fixing > > class (Applicative m) => Monad m where > > since it would have to be changed if new intermediate classes are found. > You might check out a few articles regarding Kleisli arrows [1][2] for possibilities that live between applicative and monad. Applicative itself is also a little on the strong side. I had to reject Applicative for one model of signal transformers because 'pure' was not a legal constructor, even though 'fmap . const' and '<*>' were okay. And even Functor is too strong if you want effective linearity. I've found Adam Megacz's Generalized Arrows [3] to be a suitable chassis for weaker models. [1] http://www.haskell.org/haskellwiki/Arrow_tutorial#Kleisli_Arrows [2] http://lambda-the-ultimate.org/node/4273 [3] http://www.cs.berkeley.edu/~megacz/garrows/

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