- Haskell-Cafe - Haskell.org

Cabal-1.10.1.0 and bytestring-0.9.2.1 hackage problem.
by Alan Pogrebinschi 03 Feb '12

03 Feb '12

That Cabal-1.10.1.0 bug seems to be back, now with bytestring-0.9.2.1 just uploaded to hackage. Same workaround as last time works (or maybe its time to upgrade...). For more info see: http://www.haskell.org/pipermail/haskell-cafe/2011-August/094883.html Thanks, Alan

3 2

Adding Html formatting to formlets
by Alberto G. Corona 03 Feb '12

03 Feb '12

I came across the idea that is easy to define additional operators to Text.FormLets for adding custom HTML formatting. Had anyone tried that? For example to enclose the Prod formulary in a Table using Text.XHtml tags. I defined additional operators <<< and <++ for enclosing and prepending Html to a formLet, respectively: >data Prod= Prod{pname :: String, pprice :: Int} >getProd= table <<< ( > Prod <$> tr <<< (td << "enter the name" <++ td <<< getString (pname <$> mp)) > <*> tr <<< (td << "enter the price" <++ td <<< getInt ( pprice <$> mp))) even: >p << "paragraph" <++ getProd ++> (more Html stuff) is possible or even it is possible an operator <+> > getProd <+> someOtherStuff to return Either Prod OtherStuff I did it in my own version of FormLets. So It is too heavy to put here a running example. It is part of a package that I will upload soon to hackage. This also may work for embedding formLets in other haskell HTML formats besides Text.XHtml.

2 2

Fwd: Contributing to http-conduit
by Chris Wong 03 Feb '12

03 Feb '12

Sorry, accidentally clicked "Reply" rather than "Reply to all". Here's the message I sent: ---------- Forwarded message ---------- From: Chris Wong <chrisyco+haskell-cafe(a)gmail.com> Date: Fri, Feb 3, 2012 at 7:44 PM Subject: Re: [Haskell-cafe] Contributing to http-conduit To: "Myles C. Maxfield" <myles.maxfield(a)gmail.com> On Fri, Feb 3, 2012 at 5:32 PM, Myles C. Maxfield <myles.maxfield(a)gmail.com> wrote: > I propose making an entirely unrelated package, public-suffix-list, with a > module Network.PublicSuffixList, which will expose this function, as well as > functions about parsing the list itself. Thoughts? I'll volunteer to write this module, but I'm not sure whether to read it in at run time, or generate the module containing the data in advance. The latter seems less error-prone -- no messing around with file paths or IO. Chris

1 0

ANN: combinatorics
by wren ng thornton 03 Feb '12

03 Feb '12

-------------------------------------------- -- combinatorics 0.1.0 -------------------------------------------- The combinatorics package offers efficient *exact* computation of common combinatorial functions like the binomial coefficients and factorial. (For fast *approximations*, see the math-functions package instead.) -------------------------------------------- -- Long Description -------------------------------------------- * Math.Combinatorics.Primes: provides the prime numbers via Runciman's lazy wheel sieve algorithm. Provided here since efficient algorithms for combinatorial functions often require prime numbers. The current version memoizes the primes as an infinite list CAF, which could lead to memory leaks in long-running programs with irregular access to large primes. I'm looking into a GHC patch to allow resetting individual CAFs from within compiled programs so that you can explicitly decide when to un-memoize the primes. (In GHCi when you reload a module all the CAFs are reset. However, there's no way to access this feature from within compiled programs as yet.) * Math.Combinatorics.Binomial: offers a fast computation of the binomial coefficients based on the prime factorization of the result. As it turns out, it's easier to compute the prime factorization of the answer than it is to compute the answer directly! And you don't even need the factorial function to do so. Albeit, with a fast factorial function, the naive definition of binomial coefficients gives this algorithm a run for its money. * Math.Combinatorics.Factorial: offers a fast computation of factorial numbers. As Peter Luschny comments[1], the factorial function is often shown as a classic example of recursive functions, like addition of Peano numbers, however that naive way of computing factorials is extremely inefficient and does a disservice to those learning about recursion. The current implementation uses the split-recursive algorithm which is more than sufficient for casual use. I'm working on implementing the parallel prime-swing algorithm, which should be a bit faster still. [1] http://www.luschny.de/math/factorial/FastFactorialFunctions.htm -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/combinatorics Darcs: http://community.haskell.org/~wren/combinatorics Haddock (Darcs version): http://community.haskell.org/~wren/combinatorics/dist/doc/html/combinatorics -- Live well, ~wren

3 7

cabal-install package precedence with --extra-lib-dirs
by Scott Lawrence 03 Feb '12

03 Feb '12

When running cabal install with --extra-lib-dirs=./lib, if a package is found both in ~/.cabal/lib and ./lib, cabal seems to favor the ~/.cabal/lib one. Is there some way to specify the correct precedence to use? -- Scott Lawrence

2 1

Re: [Haskell-cafe] strict version of Haskell - does it exist?
by Marc Weber 02 Feb '12

02 Feb '12

> jsonLines :: C.Resource m => C.Conduit B.ByteString m Value > jsonLines = C.sequenceSink () $ do > val <- CA.sinkParser json' > CB.dropWhile isSpace_w8 > return $ C.Emit () [val] Adding a \state -> (the way Felipe Lessa told me) make is work and it runs in about 20sec and that although some conduit overhead is likely to take place. omitting my custom data type using bytestrings operating on Value of Aeson reduces running time to 16secs. PHP/C++ still wins: less than 12secs. Now I can imagine again that even a desktop multi core system is faster than a single threaded C application. Thanks for your help. Maybe I can setup profiling again to understand why its still taking little bit more time. Marc Weber

7 8

ANN: test-framework-doctest 0.2 (now works also with test-framework 0.5)
by Sakari Jokinen 02 Feb '12

02 Feb '12

test-framework-doctest provides integration between doctest-haskell and test-framework.. See http://hackage.haskell.org/package/test-framework-doctest for details. Big thanks to Simon Hengel who did all the real work in making the library work with test-framework 0.5 cheers Sakari

1 0

zlib 0.5.3.2 broken?
by Michael Snoyman 02 Feb '12

02 Feb '12

Hi all, I've received a number of reports of a broken zlib 0.5.3.2 (and experienced it myself). Is this a generally known issue? As a temporary workaround, I've released a new version of zlib-bindings that has an upper bound on zlib to avoid 0.5.3.2 (and later). Once a new patched version is released, I'll release an updated version of zlib-bindings to remove that overly restrictive upper bound. Michael

5 4

PhD studentship in Nottingham (closing date 10th February)
by Graham Hutton 02 Feb '12

02 Feb '12

Dear all, I am currently advertising a PhD Studentship in Functional Programming at the University of Nottingham in the UK: http://www.nottingham.ac.uk/jobs/currentvacancies/ref/SCI1088 The closing date is *** Friday 10th February 2012 ***. If you would like to apply, please follow the instructions in the advert, rather than clicking on "apply online". Many thanks, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

ANN: NubFinder : Mining Opinions on the Web
by dokondr 02 Feb '12

02 Feb '12

Hello, I am pleased to announce NubFinder research project. Goal: develop technology to search and analyze user opinions on the Web. NubFinder and NubTrend are research prototypes trying first to accomplish a more 'simple' task - classification of emotions in Twitter messages, and then approach opinion mining. NubFinder project site: https://sites.google.com/site/nubfinder NubFinder discussion group: http://groups.google.com/group/nubfinder Thanks for your interest in NubFinder Research! -- All the best, Dmitri O. Kondratiev "This is what keeps me going: discovery" dokondr(a)gmail.com http://sites.google.com/site/dokondr/welcome

2 2

ANN: hledger 0.17
by Simon Michael 02 Feb '12

02 Feb '12

Happy new year all. hledger and hledger-web 0.17 are released. Not quite as planned, this release mainly fixes bugs and updates dependencies. Also the hledger-vty and hledger-chart packages have been demoted to "unmaintained extras" for now to save time. hledger is a library, command-line tool, and web app for reporting on financial transactions stored in a text file. It is a haskell port and friendly fork of John Wiegley's Ledger. Home: http://hledger.org To install: cabal update, then cabal install hledger (or cabal-install hledger-web). If you have any trouble, please let me know, or watch the mail list/bug tracker for updates. (Ready to run binaries are provided on request to project donors, a great way to support the project - see http://hledger.org/DOWNLOAD.html.) Release notes for 0.17: * support HP 2011.4.0.0 * support and require cmdargs 0.9 * allow non-threaded builds, supporting more debian architectures * parsing: give a clearer error when journal file path contains ~ * parsing: -B/--cost now ignores P historical prices, like ledger * parsing: inferred amounts now use the cost commodity if known, like ledger (#69) * balance: report differently-priced lots in an account as a single amount, like ledger * web: support and require yesod >= 0.9.4 * web: use the main aeson package again * web: fix a regression with dollar signs in hamlet templates * web: add form allowed blank account names (#81) * chart, vty: hledger-chart and hledger-vty demoted to non-maintained extras for now Best, -Simon

1 0

Haskell Weekly News: Issue 213
by Daniel Santa Cruz 02 Feb '12

02 Feb '12

Welcome to issue 213 of the HWN, a newsletter covering quotes, stories, and questions seen around the net concerning Haskell during the week of January 22 to 28, 2012. You can read the HTML version at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-213.h… Quotes of the Week * monochrom: great way to answer a semantics question by commenting on the font ... "what is the semantics of IO?" "the I may be narrower than O in some fonts, and same width in some other fonts" ... "on very old typewriters, it also denotes the number 10 (ten), for those of you looking for a denotation" * elliott: |\/|/-\|-|-|=|\||} is my preferred mappend operator * merijn: rostayob: I used to be a functional programmer like you, but then I took an UML diagram to the knee >.> * mauke: monoids are simple elliott: And as we all know, monads are basically just monoids! monochrom: haskell is basically just ascii * O'Keefe: "Elegance is not optional." * O'Keefe: "If I don't write it, I won't wrong it." * kmc: "monads are like containers, as long as you forget everything you know about containers, and treat it as a synonym for 'monad'" * Enigmagic: this calls for mfix and some tequila Top Reddit Stories * SPJ : Escape From the Ivory Tower: The Haskell Journey, From 1990 to 2011 Domain: yow.eventer.com, Score: 67, Comments: 12 On Reddit: [1] http://goo.gl/JqRUW Original: [2] http://goo.gl/9ILW1 * Make Things Now! - Pragmatic FP With Haskell (slides) Domain: code.haskell.org, Score: 44, Comments: 22 On Reddit: [3] http://goo.gl/WJQnH Original: [4] http://goo.gl/UMd9y * ANNOUNCE: GHC 7.4.1 Release Candidate 2 Domain: haskell.org, Score: 37, Comments: On Reddit: [5] http://goo.gl/D9F37 Original: [6] http://goo.gl/sdD1L * Web application that uses UHC to compile Haskell to JavaScript Domain: alessandrovermeulen.me, Score: 36, Comments: 13 On Reddit: [7] http://goo.gl/A3S0k Original: [8] http://goo.gl/lyX2K * Web app that shows you the changes between releases of packages on hackage Domain: hdiff.luite.com, Score: 33, Comments: 4 On Reddit: [9] http://goo.gl/6ioFh Original: [10] http://goo.gl/aZAyc * Combining Regions and Iteratees Domain: haskell.org, Score: 30, Comments: 10 On Reddit: [11] http://goo.gl/LEPiQ Original: [12] http://goo.gl/k3wpd * MonadBaseControl is unsound : Inside 233 Domain: blog.ezyang.com, Score: 29, Comments: 19 On Reddit: [13] http://goo.gl/ptCF6 Original: [14] http://goo.gl/Y5bbh * Yesod example: wiki, markdown, chat subsite, event source Domain: yesodweb.com, Score: 29, Comments: 1 On Reddit: [15] http://goo.gl/GyfzS Original: [16] http://goo.gl/on96C * inner beauty of tree traversals: pre- and post-order Domain: blog.moertel.com, Score: 29, Comments: On Reddit: [17] http://goo.gl/6fKzq Original: [18] http://goo.gl/NpjrO * Seem like there's a boycott of Elsevier going on. Domain: thecostofknowledge.com, Score: 29, Comments: 1 On Reddit: [19] http://goo.gl/2B106 Original: [20] http://goo.gl/dvxML * Unifying Monoids and Monads with Polymorphic Kinds Domain: jonmsterling.com, Score: 26, Comments: 29 On Reddit: [21] http://goo.gl/kTafx Original: [22] http://goo.gl/A3IrU * Modelling IO: MonadIO and beyond Domain: blog.ezyang.com, Score: 26, Comments: 4 On Reddit: [23] http://goo.gl/gr2M1 Original: [24] http://goo.gl/0n1a9 Top StackOverflow Questions * Is it a good idea to compile a language to C? votes: 26, answers: 7 Read on SO: [25] http://goo.gl/V8sSx * Haskell GHC: what is the time complexity of a pattern match with N constructors? votes: 19, answers: 1 Read on SO: [26] http://goo.gl/SU0l8 * What to use instead of a main loop in Haskell? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/iYasz * Associated types and container elements votes: 12, answers: 2 Read on SO: [28] http://goo.gl/zBNWU * Undefined at the type level votes: 12, answers: 1 Read on SO: [29] http://goo.gl/m9Acn * Is there a good reason why `deleteBy` does not have its most general type? votes: 10, answers: 3 Read on SO: [30] http://goo.gl/dQDrf * Using low bitsize integral types like `Int8` and what they are for votes: 9, answers: 2 Read on SO: [31] http://goo.gl/YaE68 * How is the ghc runtime support for profiling implemented? votes: 9, answers: 2 Read on SO: [32] http://goo.gl/yz7eC * how to achieve “product of two monads” effect? votes: 9, answers: 1 Read on SO: [33] http://goo.gl/pLhVK * Combine two monads when neither has a transformer? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/yw8tr * Why don't type synonyms permit recursion in Haskell? votes: 8, answers: 2 Read on SO: [35] http://goo.gl/hPzVq Until next time, Daniel Santa Cruz References 1. http://yow.eventer.com/events/1004/talks/1054 2. http://www.reddit.com/r/haskell/comments/ovc9s/spj_escape_from_the_ivory_to… 3. http://code.haskell.org/~dons/talks/padl-keynote-2012-01-24.pdf 4. http://www.reddit.com/r/haskell/comments/oxg3c/make_things_now_pragmatic_fp… 5. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-January/021714.… 6. http://www.reddit.com/r/haskell/comments/p0x96/announce_ghc_741_release_can… 7. http://alessandrovermeulen.me/2012/01/26/getting-rid-of-javascript-with-has… 8. http://www.reddit.com/r/haskell/comments/oz27a/web_application_that_uses_uh… 9. http://hdiff.luite.com/ 10. http://www.reddit.com/r/haskell/comments/ozjys/web_app_that_shows_you_the_c… 11. http://www.haskell.org/pipermail/haskell-cafe/2012-January/098704.html 12. http://www.reddit.com/r/haskell/comments/orh4f/combining_regions_and_iterat… 13. http://blog.ezyang.com/2012/01/monadbasecontrol-is-unsound/ 14. http://www.reddit.com/r/haskell/comments/ot5f2/monadbasecontrol_is_unsound_… 15. http://www.yesodweb.com/blog/2012/01/wiki-chat-subsite 16. http://www.reddit.com/r/haskell/comments/oub3p/yesod_example_wiki_markdown_… 17. http://blog.moertel.com/articles/2012/01/26/the-inner-beauty-of-tree-traver… 18. http://www.reddit.com/r/haskell/comments/oyu7d/inner_beauty_of_tree_travers… 19. http://thecostofknowledge.com/ 20. http://www.reddit.com/r/haskell/comments/ozi7i/seem_like_theres_a_boycott_o… 21. http://www.jonmsterling.com/posts/2012-01-12-unifying-monoids-and-monads-wi… 22. http://www.reddit.com/r/haskell/comments/osn5d/unifying_monoids_and_monads_… 23. http://blog.ezyang.com/2012/01/modelling-io/ 24. http://www.reddit.com/r/haskell/comments/ourfu/modelling_io_monadio_and_bey… 25. http://stackoverflow.com/questions/8976423/is-it-a-good-idea-to-compile-a-l… 26. http://stackoverflow.com/questions/9027384/haskell-ghc-what-is-the-time-com… 27. http://stackoverflow.com/questions/9015318/what-to-use-instead-of-a-main-lo… 28. http://stackoverflow.com/questions/9016521/associated-types-and-container-e… 29. http://stackoverflow.com/questions/9036554/undefined-at-the-type-level 30. http://stackoverflow.com/questions/9004937/is-there-a-good-reason-why-delet… 31. http://stackoverflow.com/questions/8963942/using-low-bitsize-integral-types… 32. http://stackoverflow.com/questions/9010808/how-is-the-ghc-runtime-support-f… 33. http://stackoverflow.com/questions/9013329/how-to-achieve-product-of-two-mo… 34. http://stackoverflow.com/questions/9034060/combine-two-monads-when-neither-… 35. http://stackoverflow.com/questions/8963376/why-dont-type-synonyms-permit-re…

1 0

C++ Parser?
by Christopher Brown 01 Feb '12

01 Feb '12

Hi, I have stumbled across language-c on hackage and I was wondering if anyone is aware if there exists a full C++ parser written in Haskell? Many thanks, Chris.

8 13

network-conduit failing to close sockets
by Ertugrul Söylemez 01 Feb '12

01 Feb '12

Hello there, I have tried to implement a simple echo server using the network-conduit library version 0.2.1. This is the code: module Main where import Data.Conduit import Data.Conduit.Network main :: IO () main = runTCPServer (ServerSettings 4000 Nothing) ($$) It works, but at some point it dies with too many open files, even though I never open two connections simultaneously: % ./echo-server echo-server: accept: resource exhausted (Too many open files) Apparently it fails to close the sockets properly. This is the client side code I have used for testing in GHCi: > :m Network Control.Monad System.IO > replicateM_ 512 (connectTo "127.0.0.1" (PortNumber 4000) >>= hClose) I'm running the action once, then wait a few seconds to give the server a chance to close the handles. Then I run it again, causing the server program to die with the above mentioned error message. Am I doing something wrong or is this a bug in network-conduit? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

3 3

Memory usage with TVar?
by Johan Brinch 01 Feb '12

01 Feb '12

Here's the example program: https://gist.github.com/1cbe113d2c79e2fc9d2b When I run the program (which maintains a list inside an STM TVar), I get the following statistics: > ./Test +RTS -s 176,041,728 bytes allocated in the heap 386,794,976 bytes copied during GC 69,180,224 bytes maximum residency (7 sample(s)) 42,651,080 bytes maximum slop 179 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 336 colls, 0 par 0.44s 0.44s 0.0013s 0.0033s Gen 1 7 colls, 0 par 0.39s 0.40s 0.0570s 0.1968s INIT time 0.00s ( 0.00s elapsed) MUT time 0.23s ( 0.23s elapsed) GC time 0.83s ( 0.84s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.06s ( 1.07s elapsed) %GC time 77.9% (78.3% elapsed) Alloc rate 749,153,093 bytes per MUT second Productivity 22.1% of total user, 22.0% of total elapsed How come this program uses 179 MB of memory? I'm on a 64-bit machine where 2'000'000 integers uses 32 MB. Where does the overhead come from? -- Johan Brinch

2 2

Serialize instance for System.Posix.Files.FileStatus
by Johan Brinch 01 Feb '12

01 Feb '12

Are there plans on implementing an efficient Serialize instance of the FileStatus type? I'm aware that it's just a wrapper around a C stat structure, and I reckon it can be done more efficiently than copying from C to Haskell for every value. -- Johan Brinch

2 1

[ANN] Crypto-API 0.9 Release
by Thomas DuBuisson 01 Feb '12

01 Feb '12

Crypto-API is a generic interface for cryptographic operations, defining classes for hashes, ciphers, and random number generation while also providing convenience functions such as block cipher modes and padding. Maintainers of hash and cipher implementations are encouraged to add instances for the classes defined in Crypto.Classes. Crypto users are similarly encouraged to use the interfaces defined in the Classes module. Release 0.9 Changes: * Crypto.Classes now exports 'Data.Serialize.encode' * AsymCipher now has proper fundeps * cpolysArr is no longer one big line Cheers, Thomas

2 2

Stuck on HXT basics
by Pēteris Paikens 01 Feb '12

01 Feb '12

I'm quite stuck on getting HXT working to do my xml parsing, can you help me to get started? Even in http://www.haskell.org/haskellwiki/HXT#Getting_started:_Hello_world_examples the basic filter example that's given there in section 6: import Text.XML.HXT.Core import Text.XML.HXT.DOM.XmlTreeFilter selectAllText :: ArrowXml a => a XmlTree XmlTree selectAllText = deep isXText fails with the following error message. What could I be doing wrong? Couldn't match expected type `Data.Tree.NTree.TypeDefs.NTree XNode' with actual type `hxt-8.5.4:Data.Tree.NTree.TypeDefs.NTree hxt-8.5.4:Text.XML.HXT.DOM.TypeDefs.XNode' Expected type: a (Data.Tree.NTree.TypeDefs.NTree XNode) XmlTree Actual type: Text.XML.HXT.DOM.XmlTreeTypes.XmlFilter In the first argument of `deep', namely `isXText' In the expression: deep isXText On the other hand, perhaps you can suggest something other than HXT - my needs would be taking "heavy" XML's (a lot of little-neccessary structure, but correctly validated), extracting text tokens from it and outputting them with some light tagsoup annotation. Regards, Peteris

4 4

Haskell User Group Hamburg - Meeting on 9th Feb.
by Andreas Baldeau 31 Jan '12

31 Jan '12

Hello cafe, in Hamburg this month a new Haskell User Group has formed. We are trying to establish a monthly meetup with hopefully interesting talks. For the start we are having our first regular meetup on the 9th of February, 19:00 in the betahaus hamburg. On this day I will give a talk about datastructures in Haskell and give some introduction into amortized O(1) queues and Tries. So if you are interested to come have a look at http://www.doodle.com/da9tr6zeynmcq3z6 (german). Further announcements will be made via Twitter, so you may want to follow https://twitter.com/hug_hh. Andreas Baldeau

1 0

help with safecopy + acid-state
by Johannes Waldmann 31 Jan '12

31 Jan '12

Dear all, I can't quite get safecopy to work with acid-state: old version of code : data T = T Foo $(deriveSafeCopy 0 'base ''T) new version : data T_orig = T_orig Foo $(deriveSafeCopy 0 'base ''T_orig) data T = T Foo Bar $(deriveSafeCopy 0 'extension ''T) instance Migrate T where type MigrateFrom T = T_Orig ... but when my (new) application reads the state from disk (written by the old application) I get "Could not parse saved checkpoint due to the following error: Failed reading: Duplicate version tags: [0,0]" When I change the version number in deriveSafeCopy, I get "Failed reading: safecopy: Map: Cannot find getter associated with this version number: Version {unVersion = 2}" I don't even know where the "2" comes from. Any hints appreciated - J.W.

3 5

Re: [Haskell-cafe] [Haskell] ANNOUNCE multiarg - parser combinators for command line parsing
by Henning Thielemann 31 Jan '12

31 Jan '12

On Sun, 29 Jan 2012, Simon Meier wrote: > I'm currently using Neil Mitchell's cmdargs package [1]. How does your > package compare to that? Last time I checked cmdargs it was not referential transparent. Is multiarg better in this respect?

3 2

Monad-control rant
by Mikhail Vorozhtsov 31 Jan '12

31 Jan '12

On 11/12/2011 07:34 AM, Bas van Dijk wrote: >> Are you going to release a new version of monad-control right away > > Not just yet. I've split `monad-control` into two packages: > > * `monad-control`: just exports `Control.Monad.Trans.Control`. This part is finished. > * `lifted-base`: wraps all modules of the `base` package which export `IO` computations and provides > lifted version instead. For example we have `Control.Exception.Lifted`, `Control.Concurrent.Lifted`, etc. > > As you can imagine the latter is a lot of boring work. Fortunately it's easy to do so will probably > not take a lot of time. BTW if by any chance you want to help out, that will be much appreciated! > > The repos can be found [here](https://github.com/basvandijk/lifted-base) Maybe I should elaborate on why I stopped using monad-control and rolled out my own version of lifted Control.Exception in monad-abort-fd package. I'm CC-ing the Cafe just in case someone else might be interested in the matter of IO lifting. Imagine we have a monad for multiprogramming with shared state: -- ContT with a little twist. Both the constructor and runAIO -- are NOT exported. newtype AIO s α = AIO { runAIO ∷ ∀ β . (α → IO (Trace s β)) → IO (Trace s β) } runAIOs ∷ MonadBase IO μ ⇒ s -- The initial state → [AIO s α] -- The batch of programs to run. -- If one program exits normally (without using -- aioExit) or throws an exception, the whole batch -- is terminated. → μ (s, Maybe (Attempt α)) -- The final state and the result. -- "Nothing" means deadlock or that -- all the programs exited with -- aioExit. runAIOs = liftBase $ mask_ $ ... bloody evaluation ... data Trace s α where -- Finish the program (without finishing the batch). TraceExit ∷ Trace s α -- Lift a pure value. TraceDone ∷ α → Trace s α -- A primitive to execute and the continuation. TraceCont ∷ Prim s α → (α → IO (Trace s β)) → Trace s β -- Primitive operations data Prim s α where -- State retrieval/modification PrimGet ∷ Prim s s PrimSet ∷ s → Prim s () -- Scheduling point. The program is suspended until -- the specified event occurs. PrimEv ∷ Event e ⇒ e → Prim s (EventResult e) -- Scheduling point. The program is suspended until the state -- satisfies the predicate. PrimCond ∷ (s → Bool) → Prim s () -- Run computation guarded with finalizer. PrimFin ∷ IO (Trace s α) → (Maybe α → AIO s β) → Prim s (α, β) -- Run computation guarded with exception handler. PrimHand ∷ IO (Trace s α) → (SomeException → AIO s α) → Prim s α aioExit ∷ AIO s α aioExit = AIO $ const $ return TraceExit aioAfter ∷ (s → Bool) → AIO s () aioAfter cond = AIO $ return . TraceCont (PrimCond cond) aioAwait ∷ Event e ⇒ e → AIO s (EventResult e) aioAwait e = AIO $ return . TraceCont (PrimEv e) runAIOs slices the programs at scheduling points and enqueues the individual pieces for execution, taking care of saving/restoring the context (finalizers and exception handlers). The Functor/Applicative/Monad/MonadBase/etc instances are pretty trivial: instance Functor (AIO s) where fmap f (AIO g) = AIO $ \c → g (c . f) instance Applicative (AIO s) where pure a = AIO ($ a) (<*>) = ap instance Monad (AIO s) where return = pure AIO g >>= f = AIO $ \c → g (\a → runAIO (f a) c) instance MonadBase IO (AIO s) where liftBase io = AIO (io >>=) instance MonadState s (AIO s) where get = AIO $ return . TraceCont PrimGet put s = AIO $ return . TraceCont (PrimSet s) instance MonadAbort SomeException (AIO s) where abort = liftBase . throwIO trace ∷ AIO s α → IO (Trace s α) trace (AIO g) = g (return . TraceDone) instance MonadRecover SomeException (AIO s) where recover m h = AIO $ return . TraceCont (PrimHand (trace m) h) instance MonadFinally (AIO s) where finally' m f = AIO $ return . TraceCont (PrimFin (trace m) f) -- finally m = fmap fst . finally' m . const -- No async exceptions in AIO instance MonadMask () (AIO s) where getMaskingState = return () setMaskingState = const id Now we have a problem: we can throw/catch exceptions and install finalizers in AIO, but we can't use Control.Exception.Lifted because we can't declare a MonadBaseControl instance for our "ContT with limited interface". So now, instead of trying to wrap IO control operations uniformly, I just reimplement them in terms of the classes I mentioned above (MonadAbort+MonadRecover+MonadFinally+MonadMask), for example: bracket ∷ (MonadFinally μ, MonadMask m μ) ⇒ μ α → (α → μ β) → (α → μ γ) → μ γ bracket acq release m = mask $ \restore → do a ← acq finally (restore $ m a) (release a) It requires more typing (more classes => more instances), but it works for a broader class of monads and I get proper side affects in finalizers for a bonus. The code is on Hackage (monad-abort-fd) and on the GitHub (https://github.com/mvv/monad-abort-fd/blob/master/src/Control/Monad/Excepti…)

4 24

TCP Server
by Alexander V Vershilov 31 Jan '12

31 Jan '12

Hello, café. Recently I asked about tcp server libraries [1] and there was only one answer haskell-scallable-server [2], but in that package there was some dependencies and server logic that are not good for my task. So I decided to make a library with skeletons for different types of tcp servers and a basic library to make server [3]. Now there is only warp based (simplified) tcp server. Main logic is: * handle new connection and start iteratee green thread * enumerator (library side) reads socket and send data into enumeratee (application side) * enumeratee consumes input and produces server command (now simply output) and push it into iteratee (server side) * iteratee server side reads command and performs action (now simply output command) It gives application possibility to store state in enumerator. My questions to community: 1). can you help to make code better ;) maybe there are some stupid mistakes 2). is there any ideas what thing can be add to the server to make it reusable And another thing that I want to add other types of server e.g. single threaded, with workers pool, or other types. And I want to ask what types of servers will be effective in haskell. [1] http://www.haskell.org/pipermail/haskell-cafe/2012-January/098594.html [2] https://github.com/jamwt/haskell-scalable-server [3] https://github.com/qnikst/TcpServers -- Alexander V Vershilov.

7 12

[ANNOUNCE] biostockholm 0.2
by Felipe Almeida Lessa 31 Jan '12

31 Jan '12

Hello! I'm pleased to announce the second major release of the biostockholm library! This library allows you to parse and render files in the Stockholm 1.0 format, which is used by Pfam, Rfam, Infernal and others for holding information about families of proteins or non-coding RNAs. http://hackage.haskell.org/package/biostockholm Despite this low increase in number from 0.1 to 0.2, this is actually a big rewrite of the library. Now we have: - An streaming interface similar to what SAX parsers provide. This allows you to consume Stockholm files using constant memory (80k in a simple case). - More test cases. It's able to consume its own pretty printed version of Rfam through the document interface, and is also capable of reading the full Rfam stockholm file (which has some huge families) through the streaming interface. - QuickCheck properties. Now we have three different QuickCheck properties covering almost everything. These have helped uncover some tricky bugs that were never found before. However, two of these three properties still don't pass, but I consider the failing examples that I've investigated just corner cases. Unfortunately, Stockholm lacks a formal specification. - Conduit interface. Besides a lazy I/O version, now there's a conduit interface. - Code much easier to read and reason about. - Fast enough: the streaming interface achieves 12 MiB/s for parsing, which is pretty nice considering that there are some known overheads on its implementation. For the tasks that biostockholm 0.1 already handled, biostockholm 0.2 tends to be slightly slower. However, biostockholm 0.2 is able to handle some previously impossible cases where an streaming solution is required. Cheers! -- Felipe.

1 1

strict version of Haskell - does it exist?
by Marc Weber 31 Jan '12

31 Jan '12

A lot of work has been gone into GHC and its libraries. However for some use cases C is still preferred, for obvious speed reasons - because optimizing an Haskell application can take much time. Is there any document describing why there is no ghc --strict flag making all code strict by default? Wouldn't this make it easier to apply Haskell to some additional fields such as video processing etc? Wouldn't such a '--strict' flag turn Haskell/GHC into a better C/gcc compiler? Projects like this: https://github.com/thoughtpolice/strict-ghc-plugin show that the idea is not new. Eg some time ago I had to do some logfile analysis. I ended doing it in PHP because optimizing the Haskell code took too much time. Marc Weber

12 18

[Haskell Cafe] strict version of Haskell - does it exist?
by Doug McIlroy 31 Jan '12

31 Jan '12

> Is there any document describing why there is no ghc --strict flag > making all code strict by default? > Wouldn't such a '--strict' flag turn Haskell/GHC into a better C/gcc > compiler? I agree that a strict flag would turn Haskell into C--but that's a perversion of Haskell. Almost all Haskell code I write depends critically on laziness. Sure, there are little bits here and there that would run faster if I carefully decorated them with strict flags. But the genius of Haskell is architectural clarity, achieved in no small part by relegating nasty issues of sequencing to the implementation. If you even have to think once about what needs to be strict or non-strict, then non-strict is the only viable default. I can't imagine writing any interesting Haskell code that will successfully run all strict. Doug McIlroy

4 3

Invitation to connect on LinkedIn
by Sunny Basi 31 Jan '12

31 Jan '12

LinkedIn ------------ I'd like to add you to my professional network on LinkedIn. - Sunny Sunny Basi Software Development Engineer Intern at Amazon.com Greater Seattle Area Confirm that you know Sunny Basi: https://www.linkedin.com/e/uc6lxc-gy2lv1bv-z/isd/5727371010/HFPCz0eC/?hs=fa… -- You are receiving Invitation to Connect emails. Click to unsubscribe: http://www.linkedin.com/e/uc6lxc-gy2lv1bv-z/XvIdBwmueHfd6vFMPXXdLaqreCbl5oO… (c) 2012 LinkedIn Corporation. 2029 Stierlin Ct, Mountain View, CA 94043, USA.

1 0

Speculative Parallelism
by Burak Ekici 30 Jan '12

30 Jan '12

Dear List, my question is in which cases, I could understand the speculative parallelism in parallel Haskell? Does the case in which all and the same number of created sparks are converted by both original and second generation strategies, speak of the speculative parallelism effect? If it does not, how can I prove the argument that indicating speculative parallelism could be exploited in second generation strategies while it is not possible to exploit when usages of original ones are in question? Kind regards, Burak.

1 0

space leak when repeatedly calling Control.Monad.State.Strict.modify
by Joey Hess 30 Jan '12

30 Jan '12

The attached test case quickly chews up hundreds of MB of memory. If modified to call work' instead, it runs in constant space. Somehow the value repeatedly read in from the file and stored in the state is leaking. Can anyone help me understand why? (ghc 7.0.4) -- see shy jo

3 4

Re: [Haskell-cafe] Some thoughts on Type-Directed Name
by Anthony Clayden 30 Jan '12

30 Jan '12

> There is an effort underway to make Haskell's Records > better. The discussion is ongoing on the ghc-users mail > list, ... > in the direction of making the most minimal changes > possible to achieve some simple record name-spacing. > > Thanks, > Greg Weber Thank you Greg, Yes I know, and I have been trying to follow along (intermittently). Thank you for your attempts to marshall the discussion. What would really, really help me is for someone to have a look at the 'solution' I posted to the difficulties SPJ saw with the SORF approach. (I ref'd it in my reply to Steve.) It seemed from my testing to address the needs. Since I got it working in GHC 7.2.1, there's a good chance it will need only "minimal changes" to implement (I'm thinking mostly syntactic sugar) -- providing of course that it is workable and generalisable enough. It could possibly benefit from some of the new Kind-level stuff in 7.4.1 (that SPJ used, but wasn't available to me at the time). I keep trying to make the time to write up the full proposal on the Wiki. I see it as a 'tweak' to SORF. Given that I'm supposed to have a day job, I'm reluctant to make time until/unless someone else double-checks whether I'm barking up the wrong tree. Anthony

2 1

Need advice: Haskell in Web Client
by dokondr 30 Jan '12

30 Jan '12

**Bardur Arantsson wrote: >On 01/26/2012 11:16 AM, dokondr wrote: >> Ideally, I would be happy to be able to write in Haskell a complete >> front-end / GUI, so it could be compiled to different back-ends: Javascript >> to run in the Browser and also a standalone app. >> In Python world this is already done with Pyjamas (http://pyjs.org/) - "a >> Rich Internet Application (RIA) Development Platform for both Web and >> Desktop." >> Also from Pyjamas site: >> Pyjamas "... contains a Python-to-Javascript compiler, an AJAX framework >> and a Widget Set API. >> Pyjamas Desktop is the Desktop version of Pyjamas >> Pyjamas Desktop allows the exact same python web application source code to >> be executed as a standalone desktop application (running under Python) >> instead of being stuck in a Web browser." >> >> Architecture diagram >> http://pyjs.org/wiki/pyjamasandpyjamasdesktop/ >> >> I wonder if somebody works on similar Haskell "Rich Internet Application >> (RIA) Development Platform" ? >> Any ideas, comments on implementation of such system in Haskell? What >> existing Haskell GUI libraries can be used for a desktop GUI, etc.? >> > >Well, it's basically just proof-of-concept at the moment, and it's not >really usable for real applications at the moment, but there is > > http://hackage.haskell.org/package/dingo-core-0.1.0 > http://hackage.haskell.org/package/dingo-widgets-0.1.0 > http://hackage.haskell.org/package/dingo-example-0.1.0 > >The basic client<->server communication, server-side state handling, >etc. is there, but it's missing a couple of things before it could be >used for real apps: There's no real security, and there are *very* few >widgets. The few widgets that exist at the moment are also probably >lacking a few operations. On the plus side, it's should be pretty easy >to create new widgets. > >You can get a feel for how the thing looks from an application >programmer's perspective by looking at the source for the example. Looks neat, thanks! If I got this write, in dingo all web page content is created by running Haskell on the server side in run-time and then sending generated html + js to the client (browser). I am looking for the opposite - when client does more work running Javascript generated by Haskell in advance, the approach that Pyjamas use ( http://pyjs.org/) with Python to Javascript compilation.

1 0

ANN: bytestring-lexing 0.3.0
by wren ng thornton 29 Jan '12

29 Jan '12

-------------------------------------------- -- bytestring-lexing 0.3.0 -------------------------------------------- The bytestring-lexing package offers efficient reading and packing of common types like Double and Integral types. -------------------------------------------- -- Administrative Changes (since 0.2.1) -------------------------------------------- * Change of maintainer. Don Stewart handed maintainership of the package over to myself when I voiced interest. * Change of repo type. The old repo for the package used Darcs-1 style patches. I've converted the repository to Darcs-2 hashed. This means that the new repository cannot exchange patches with the old Darcs-1 repo (or any other Darcs-2 conversions that may be floating around out there). So anyone who's interested in contributing should scrap their local copies and get the new repo. -------------------------------------------- -- Code Changes (since 0.2.1) -------------------------------------------- * Added Data.ByteString.Lex.Integral which provides efficient implementations for reading and packing/showing integral types in ASCII-compatible formats including decimal, hexadecimal, and octal. * The readDecimal function in particular has been highly optimized. The new version is wicked fast[1] and perfectly suitable for hot code locations like parsing headers for HTTP servers like Warp. In addition, attention has been paid to ensuring that parsing is efficient for larger than native types like Int64 on 32-bit systems (including 64-bit OS X), as well as Integer. The optimization of this function was done in collaboration with Erik de Castro Lopo, Vincent Hanquez, and Christoph Breitkopf following a blog post by Erik[2] and ensuing discussion on Reddit[3] [1] A Criterion report is available for 64-bit Intel OS X running 32-bit GHC 6.12.1: http://code.haskell.org/~wren/bytestring-lexing/test/bench/readDecimal.html The benchmark is included in the repo and has also been run on 64-bit GHC 7 systems, which differ primarily in not showing slowdown for Int64 vs Int (naturally). If you're curious about the different implementations: * readIntBS / readIntegerBS --- are the readInt and readInteger functions in Data.ByteString * readDecimalOrig (correct) --- was my original implementation, prior to collaboration with Erik, Vincent, and Christoph. * readIntegralMH (buggy) --- or rather a non-buggy version very much like it, is the implementation currently used in Warp. * readDecimal (current) --- is the current implementation used in this package. [2] http://www.mega-nerd.com/erikd/Blog/CodeHacking/Haskell/read_int.html [3] http://www.reddit.com/r/haskell/comments/otwxe/benchmarking_and_quickchecki… -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/bytestring-lexing Darcs: http://community.haskell.org/~wren/bytestring-lexing Haddock (Darcs version): http://community.haskell.org/~wren/bytestring-lexing/dist/doc/html/bytestri… -- Live well, ~wren

2 3

ANN: pointless fun
by wren ng thornton 29 Jan '12

29 Jan '12

-------------------------------------------- -- pointless-fun 1.1.0 -------------------------------------------- The pointless-fun package offers some common point-free combinators (common for me at least). -------------------------------------------- -- Long Description -------------------------------------------- * Perhaps the most useful is that it packages up Matt Helige's classic multicomposition trick[1]. These combinators allow you to easily modify the types of a many-argument function with syntax that looks like giving type signatures. For example, > foo :: A -> B -> C > > albert :: A -> X > beth :: B -> Y > carol :: C -> Z > > bar :: X -> Y -> Z > bar = foo $:: albert ~> beth ~> carol I've found this to be especially helpful for defining non-derivable type class instances for newtypes since it both abstracts away the plumbing and also makes explicit what you mean. * Other prevalent combinators include, ** (.:) for binary composition: > (f .: g) x y = f (g x y) > -- or, > f .: g = curry (f . uncurry g) This is the same as the common idiom @(f .) . g@ but more easily extended to multiple uses, due to the fixity declaration. ** (.!) for function composition which calls the right-hand function eagerly; i.e., making the left-hand function strict in its first argument. > (f .! g) x = f $! g x This defines the composition for the sub-category of strict Haskell functions. If the 'Functor' class were parameterized by the domain and codomain categories (e.g., a regular @Functor f@ would be @CFunctor (->) (->) f@ instead) then this would allow us to define functors @CFunctor (->) (!->) f@ where @fmap f . fmap g = fmap (f .! g)@. [1] http://matt.immute.net/content/pointless-fun -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/pointless-fun Darcs: http://community.haskell.org/~wren/pointless-fun Haddock (Darcs version): http://community.haskell.org/~wren/pointless-fun/dist/doc/html/pointless-fun -- Live well, ~wren

2 3

Reminder Munich Haskell Meeting
by Heinrich Hördegen 29 Jan '12

29 Jan '12

Dear all, just to remind you: Tomorrow, Monday 30. Jan 2012, Haskellers meet in Munich. Check out the details and click the button, if you plan to join: http://www.haskell-munich.de/dates See you, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

1 0

ANN: data-or
by wren ng thornton 29 Jan '12

29 Jan '12

-------------------------------------------- -- data-or 1.0.0 -------------------------------------------- The data-or package offers a data type for non-exclusive disjunction. This is helpful for things like a generic merge function on sets/maps which could be union, mutual difference, etc. based on which 'Or' value a function argument returns. Also useful for non-truncating zips (cf. 'zipOr') and other cases where you sometimes want an 'Either' and sometimes want a pair. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/data-or Darcs: http://community.haskell.org/~wren/data-or Haddock (Darcs version): http://community.haskell.org/~wren/data-or/dist/doc/html/data-or -- Live well, ~wren

1 0

Named captures in regex-pcre?
by Ilya Portnov 28 Jan '12

28 Jan '12

Hi haskell-cafe. Is there a way to get named captures from regex using regex-pcre (or maybe other PCRE-package)? For example, I want to write something like let result = "ab 12 cd" =~ "ab (?P<number>\d+) cd" :: SomeCrypticType and then have namedCaptures result == [("number", "12")]. I do not see somewhat similar in regex-pcre documentation. It parses such regexs fine, and captures work, but i do not see way to get _named_ captures. WBR, Ilya Portnov.

3 2

Re: [Haskell-cafe] Natural Transformations and fmap
by Janis Voigtländer 28 Jan '12

28 Jan '12

Ryan Ingram wrote: > However, the type of natural transformations comes with a free theorem, for > example > > concat :: [[a]] -> [a] > > has the free theorem > > forall f :: a -> b, f strict and total, fmap f . concat = concat . fmap > (fmap f) > > The strictness condition is needed; consider > > broken_concat :: [[a]] -> [a] > broken_concat _ = [undefined] > f = const () > > fmap f (broken_concat []) = fmap f [undefined] = [()] > broken_concat (fmap (fmap f) []) = broken_concat [] = [undefined] > > The 'taming selective strictness' version of the free theorem generator[1] > allows removing the totality condition on f, but not the strictness > condition. > > But in the case of concat, you can prove a stronger theorem: > > forall f :: a -> b, fmap f . concat = concat . fmap (fmap f) > > My suspicion is that this stronger theorem holds for all strict and total > natural transformations, but I don't know how to go about proving that > suspicion. I'm a hobbyist mathematician and a professional programmer, not > the other way around:) > > ... > > [1]http://www-ps.iai.uni-bonn.de/cgi-bin/polyseq.cgi There is an analogous approach to the "taming selective strictness" version of the free theorem generator where it is general recursion that is tamed. In that setting, you then get free theorems like that for concat without either strictness or totality side conditions. It is really very similar, indeed the "taming selective strictness" work takes over and develops further the much earlier "taming general recursion" ideas. The original source for the latter is: http://dblp.uni-trier.de/rec/bibtex/conf/esop/LaunchburyP96 Just nobody ever bothered to implement it in a tool. (Well, actually, such an implementation is essentially hidden inside the counterexample generator http://www-ps.iai.uni-bonn.de/cgi-bin/exfind.cgi) Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

1 0

Something's weird about System.Directory.removeDirectoryRecursive
by John Millikin 28 Jan '12

28 Jan '12

http://hackage.haskell.org/packages/archive/directory/1.1.0.1/doc/html/Syst… The documentation says that removeDirectoryRecursive follows symlinks. However, the implementation doesn't (in most cases, see below). ------------------------------------------------------------------------------------------------------------ $ mkdir test-removeDirectoryRecursive $ cd test-removeDirectoryRecursive $ mkdir a b $ touch a/a.txt b/b.txt $ ln -s $PWD/b a/ $ ln -s $PWD/b/b.txt a/ $ ls -l a total 8.2k -rw-rw-r-- 1 john john 0 2012-01-27 23:33 a.txt lrwxrwxrwx 1 john john 65 2012-01-27 23:33 b -> /home/john/test-removeDirectoryRecursive/b lrwxrwxrwx 1 john john 71 2012-01-27 23:34 b.txt -> /home/john/test-removeDirectoryRecursive/b/b.txt # OK, a/ has a normal file and two symlinks in it. Let's recursively remove a/ and see what happens. $ ghci Prelude> import System.Directory Prelude System.Directory> removeDirectoryRecursive "a" Prelude System.Directory> Leaving GHCi. $ ls -l a b ls: cannot access a: No such file or directory b: total 0 -rw-rw-r-- 1 john john 0 2012-01-27 23:33 b.txt # a/ was removed -- good! # # b/ and its contents are untouched, good, but goes against the docs ------------------------------------------------------------------------------------------------------------ Now, there is one case where this function *will* follow symlinks. However, I believe it is a bug because it produces odd behavior: ------------------------------------------------------------------------------------------------------------ $ sudo mkdir a [sudo] password for john: $ sudo ln -s $PWD/b a/ $ ls -l a b a: total 4.1k lrwxrwxrwx 1 root root 65 2012-01-27 23:38 b -> /home/john/test-removeDirectoryRecursive/b b: total 0 -rw-rw-r-- 1 john john 0 2012-01-27 23:33 b.txt # Now a/ has a symlink, which cannot be deleted, because its containing directory is read-only to the current user. $ rm a/b rm: remove symbolic link `a/b'? y rm: cannot remove `a/b': Permission denied # What happens if removeDirectoryRecursive is called now? $ ghci Prelude> import System.Directory Prelude System.Directory> removeDirectoryRecursive "a" *** Exception: a/b: removeDirectory: permission denied (Permission denied) Prelude System.Directory> Leaving GHCi. $ ls -l a b a: total 4.1k lrwxrwxrwx 1 root root 65 2012-01-27 23:38 b -> /home/john/test-removeDirectoryRecursive/b b: total 0 # a/ is untouched, but b/ has been emptied! ------------------------------------------------------------------------------------------------------------ So what is the expected behavior of this function? What should it do in the presence of symlinks? IMO, the function should be documented as *not* following symlinks, and the directory check should be changed so that it returns False for symlink-to-directory.

1 0

Natural Transformations and fmap
by Ryan Ingram 28 Jan '12

28 Jan '12

I've been playing around with the relationship between monoids and monads (see http://www.jonmsterling.com/posts/2012-01-12-unifying-monoids-and-monads-wi… http://blog.sigfpe.com/2008/11/from-monoids-to-monads.html) and I put together my own implementation which I'm quite happy with, that you can see at http://hpaste.org/56903 ; relying only on the extensions RankNTypes, TypeOperators, NoImplicitPrelude, ScopedTypeVariables; At the end of that paste, I prove the three Haskell monad laws from the functor laws and "monoid"-ish versions of the monad laws, but my proofs all rely on a property of natural transformations that I'm not sure how to prove; given type m :-> n = (forall x. m x -> n x) class Functor f where fmap :: forall a b. (a -> b) -> f a -> f b -- Functor identity law: fmap id = id -- Functor composition law fmap (f . g) = fmap f . fmap g Given Functors m and n, natural transformation f :: m :-> n, and g :: a -> b, how can I prove (f . fmap_m g) = (fmap_n g . f)? Is there some more fundamental law of natural transformations that I'm not aware of that I need to use? Is it possible to write a natural transformation in Haskell that violates this law? -- ryan

5 8

Re: [Haskell-cafe] Some thoughts on Type-Directed Name
by Greg Weber 27 Jan '12

27 Jan '12

Hi guys, There is an effort underway to make Haskell's Records better. The discussion is ongoing on the ghc-users mail list, feel free to join, or at least read on gmane. The wiki pages are up to date. Please note that we are moving in the direction of making the most minimal changes possible to achieve some simple record name-spacing. Thanks, Greg Weber

1 0

Terminology: different levels of strictness
by Yves Parès 27 Jan '12

27 Jan '12

2 1

Call for Papers - Haskell Symposium 2012
by Janis Voigtländer 27 Jan '12

27 Jan '12

======================================================================== "Haskell 2012" ACM SIGPLAN Haskell Symposium 2012 Copenhagen, Denmark 13th September, 2012 CALL FOR PAPERS http://www.haskell.org/haskell-symposium/2012/ ======================================================================== The ACM SIGPLAN Haskell Symposium 2012 will be co-located with the 2012 International Conference on Functional Programming (ICFP), in Copenhagen, Denmark. The purpose of the Haskell Symposium is to discuss experiences with Haskell and future developments for the language. Topics of interest include, but are not limited to: * 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 treatments of the semantics of the present language or future extensions, type systems, 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, in the form of profilers, tracers, debuggers, pre-processors, testing tools, and suchlike; * Applications, using Haskell for scientific and symbolic computing, database, multimedia, telecom and web applications, and so forth; * Functional Pearls, being elegant, instructive examples of using Haskell; * Experience Reports, general practice and experience with Haskell, e.g., in an education or industry context. Papers in the latter three categories need not necessarily report original research results; they may instead, for example, report practical experience that will be useful to others, reusable programming idioms, or elegant new ways of approaching a problem. (Links with more advice appear on the symposium web page.) 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, based on running (perhaps prototype) software rather than necessarily on novel research results. Such short demo proposals should explain why a demonstration would be of interest to the Haskell community. 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: ============ There will be formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. Authors must transfer copyright to ACM upon acceptance (for government work, to the extent transferable), but retain various rights (http://www.acm.org/publications/policies/copyright_policy) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Accepted demo proposals, assessed for relevance by the PC, will be published on the symposium web page, but not formally published in the proceedings. Submission Details: =================== * Abstract Submission: 31st May 2012 * Submission Deadline: 3rd June 2012, 11:00 am, UTC * Author Notification: 27th June 2012 * Final Papers Due : 10th July 2012 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (9pt format, more details appear on the symposium web page). The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 pages. Each paper submission must adhere to SIGPLAN's republication policy, as explained on the web. Demo proposals are limited to 2-page abstracts, in the same format. "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. Programme Committee: ==================== * Amal Ahmed, Northeastern University * Jost Berthold, University of Copenhagen * Nils Anders Danielsson, University of Gothenburg * Iavor Diatchki, Galois Inc. * Jeremy Gibbons, University of Oxford * Jurriaan Hage, Utrecht University * Zhenjiang Hu, National Institute of Informatics Tokyo * Daan Leijen, Microsoft Research * Ben Lippmeier, University of New South Wales * Simon Peyton Jones, Microsoft Research * Colin Runciman, University of York * Eijiro Sumii, Tohoku University * Janis Voigtländer (chair), University of Bonn * Brent Yorgey, University of Pennsylvania Links: ====== * http://www.haskell.org/haskell-symposium (the permanent web page of the Haskell Symposium) * http://www.haskell.org/haskell-symposium/2012 (the 2012 Haskell Symposium web page) * http://www.icfpconference.org/icfp2012 (the ICFP 2012 web page) -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

1 0

Haskell Weekly News: Issue 212
by Daniel Santa Cruz 27 Jan '12

27 Jan '12

Welcome to issue 212 of the HWN, a newsletter covering developments in the Haskell community. This issue covers top Haskell stories from around the web found between January 11 to 21, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/01/haskell-weekly-news-issue-212.h… Quotes of the Week * shachaf: Haskell's type system is the perfect mix of useless and stupid. * roconnor: gez, you write one paper explaining how a lens is really a higher order monoidal natural transformation, and suddenly everyone thinks you are an expert on lenses. * roconnor: a lens is a monoidal natural transformation between higher-order coalgebra functors, what's the problem? * Enigmagic: we use the 'rsync' deploy model * cmccann: someone should implement a dependently-typed unsafeCoerce in Agda, you'd have to provide a proof that the coercion is invalid * ghc: Couldn't match type `No' with `Yes' Top Reddit Stories * Idris - A language with dependent types Domain: idris-lang.org, Score: 43, Comments: 30 On Reddit: [1] http://goo.gl/pZdKX Original: [2] http://goo.gl/MxXKx * Purely Functional, Declarative Game Logic Using Reactive Programming Domain: github.com, Score: 38, Comments: 17 On Reddit: [3] http://goo.gl/ByPJj Original: [4] http://goo.gl/1PYa4 * A guided tour through the bytestring library Domain: meiersi.github.com, Score: 32, Comments: 10 On Reddit: [5] http://goo.gl/0YUQ2 Original: [6] http://goo.gl/A2u2S * Comprehensions Domain: patternsinfp.wordpress.com, Score: 26, Comments: 7 On Reddit: [7] http://goo.gl/9Zv3F Original: [8] http://goo.gl/SNA14 * Thinking of writing a tutorial series on FP and Haskell. Here's a demo. Domain: codingtales.com, Score: 24, Comments: 9 On Reddit: [9] http://goo.gl/eU99Y Original: [10] http://goo.gl/1efUI * UHC compiles Haskell to JavaScript. Web game using Haskell for Ludum Dare? Domain: chrisdone.com, Score: 23, Comments: 4 On Reddit: [11] http://goo.gl/og088 Original: [12] http://goo.gl/8Op3M * Warp Conduits Released! Domain: yesodweb.com, Score: 22, Comments: 3 On Reddit: [13] http://goo.gl/zpHnZ Original: [14] http://goo.gl/5Vl3w * Why are guards in the list comprehension faster than in the do-notation? Domain: self.haskell, Score: 19, Comments: 22 On Reddit: [15] http://goo.gl/1T8Kk Original: [16] http://goo.gl/1T8Kk * Hac Boston Domain: self.haskell, Score: 19, Comments: 7 On Reddit: [17] http://goo.gl/xWWZF Original: [18] http://goo.gl/xWWZF * Easy steps to install gtk and use all its features on OS X lion Domain: gist.github.com, Score: 16, Comments: 10 On Reddit: [19] http://goo.gl/Teh6P Original: [20] http://goo.gl/Su6Y1 Top StackOverflow Questions * What is this haskell syntax? votes: 19, answers: 1 Read on SO: [21] http://goo.gl/JCYg0 * Why is the type of this function (a -> a) -> a? votes: 18, answers: 4 Read on SO: [22] http://goo.gl/BTr8e * `Integer` vs `Int64` vs `Word64` votes: 13, answers: 1 Read on SO: [23] http://goo.gl/Pc2CX * Did I just write a continuation? votes: 12, answers: 1 Read on SO: [24] http://goo.gl/aXnTw * How lazy is Haskell's `++`? votes: 12, answers: 3 Read on SO: [25] http://goo.gl/GxPxC * Can Haskell be used to write shell scripts? votes: 12, answers: 4 Read on SO: [26] http://goo.gl/u3Qke * Why does this Haskell code compile? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/yz6VB * Any nice tools for untying knots in Haskell? votes: 11, answers: 4 Read on SO: [28] http://goo.gl/YGNeq * Two-dimensional zipper votes: 10, answers: 2 Read on SO: [29] http://goo.gl/37E9J * the way merge-sort faster than insertion-sort puzzles me votes: 10, answers: 2 Read on SO: [30] http://goo.gl/mkmdc * Haskell function that takes a type and a value and checks if value has that type votes: 10, answers: 1 Read on SO: [31] http://goo.gl/tJO81 Until next time, Daniel Santa Cruz References 1. http://idris-lang.org/ 2. http://www.reddit.com/r/haskell/comments/ol90m/idris_a_language_with_depend… 3. https://github.com/leonidas/codeblog/blob/master/2012/2012-01-17-declarativ… 4. http://www.reddit.com/r/haskell/comments/ol7vm/purely_functional_declarativ… 5. http://meiersi.github.com/HaskellerZ/meetups/2012%2001%2019%20-%20The%20byt… 6. http://www.reddit.com/r/haskell/comments/oqjhy/a_guided_tour_through_the_by… 7. http://patternsinfp.wordpress.com/2012/01/19/comprehensions/ 8. http://www.reddit.com/r/haskell/comments/on8a6/comprehensions/ 9. http://codingtales.com/2012/01/16/understanding-foldl-using-foldr-from-real… 10. http://www.reddit.com/r/haskell/comments/oi4l2/thinking_of_writing_a_tutori… 11. http://chrisdone.com/posts/2012-01-06-uhc-javascript.html 12. http://www.reddit.com/r/haskell/comments/olthb/uhc_compiles_haskell_to_java… 13. http://www.yesodweb.com/blog/2012/01/warp-conduits 14. http://www.reddit.com/r/haskell/comments/on285/warp_conduits_released/ 15. http://www.reddit.com/r/haskell/comments/oolyt/why_are_guards_in_the_list_c… 16. http://www.reddit.com/r/haskell/comments/oolyt/why_are_guards_in_the_list_c… 17. http://www.reddit.com/r/haskell/comments/opgi1/hac_boston/ 18. http://www.reddit.com/r/haskell/comments/opgi1/hac_boston/ 19. https://gist.github.com/1655271 20. http://www.reddit.com/r/haskell/comments/or0ro/easy_steps_to_install_gtk_an… 21. http://stackoverflow.com/questions/8905178/what-is-this-haskell-syntax 22. http://stackoverflow.com/questions/8918990/why-is-the-type-of-this-function… 23. http://stackoverflow.com/questions/8873000/integer-vs-int64-vs-word64 24. http://stackoverflow.com/questions/8867837/did-i-just-write-a-continuation 25. http://stackoverflow.com/questions/8872714/how-lazy-is-haskells 26. http://stackoverflow.com/questions/8925780/can-haskell-be-used-to-write-she… 27. http://stackoverflow.com/questions/8935568/why-does-this-haskell-code-compi… 28. http://stackoverflow.com/questions/8945096/any-nice-tools-for-untying-knots… 29. http://stackoverflow.com/questions/8905030/two-dimensional-zipper 30. http://stackoverflow.com/questions/8919785/the-way-merge-sort-faster-than-i… 31. http://stackoverflow.com/questions/8957601/haskell-function-that-takes-a-ty…

2 1

Wow you have to check this out Haskell
by R J 27 Jan '12

27 Jan '12

hello Haskell the holidays are coming up soon and I think this can help http://www.news13open.com

3 2

Re: [Haskell-cafe] Need advice: Haskell in Web Client
by dokondr 26 Jan '12

26 Jan '12

On Tue, Jan 17, 2012 at 6:42 PM, John Lenz <lenz(a)math.uic.edu> wrote: > > HTML5 Canvas is great for charts. If you go this route you might as well > use a library which draws charts for you instead of writing all this code > yourself. > > Personally, I use extjs version 4 which has some amazing charts, but there > are other libraries out there. > > http://www.sencha.com/**products/extjs/examples/#**sample-3<http://www.sencha.com/products/extjs/examples/#sample-3> > > Essentially the server provides the data in JSON or XML some other format, > and the extjs code draws the charts on the client side. > > If you go with extjs, then the server side I would suggest a small, simple > yesod or snap server. You could probably get the server under a hundred > lines of code with yesod; see some of the examples in the yesod book. > yesod or snap would serve JSON of the statistics on request, and also > serve the javascript files which draw the charts. > Yes, I was thinking about using Haskell to generate everything that specific Javascript library needs to display charts in browser. Naturally charts are to be displayed by this library itself. I also would like to have Haskell tools to generate Web GUI in Javascript. As for yesod, I am not sure that I like approach which mixes HTML with code, or even worse - creates a new weird HTML-like language like 'whamlet quasi-quotation', for example:  I prefer using Turing complete PL to program web client, like the one used in GWT (Java) or Cappuccino (Objective-J). http://cappuccino.org/learn/ In this case you /almost/ don't need to know HTML, CSS, DOM, Ajax, etc. to develop WebUI and good PL lets you concentrate on problem domain instead of bothering about browser support. It is a real pity that Haskell still has no such tools to generate Web GUI in Javascript. (((

3 3

Why were unfailable patterns removed and "fail" added to Monad?
by Gregory Crosswhite 26 Jan '12

26 Jan '12

Today I learned (tldr; TIL) that the "fail" in the Monad class was added as a hack to deal with the consequences of the decision to remove "unfailable" patterns from the language. I will attempt to describe the story as I have picked it up from reading around, but please feel free to correct me on the details. :-) An "unfailable" pattern (which is a generalization of an "irrefutable" pattern) is a pattern which can never fail (excluding the possibility of _|_), such as let (x,y) = pair Before "fail" was a method of the Monad class, using refutable patterns in a monad required the type to be an instance of MonadZero (that is, MonadPlus without the plus), so that for example do Just x <- m required that the monad be an instance of MonadZero. If you avoided such patterns, your Monad did not have to have this instance, so that for example do (x,y) <- pair would not require MonadZero because the pattern is unfailable. To me this seems like a lovely way of handling the whole matter, and much improved over the incredibly ugly wart of having a "fail" method in the Monad class. In fact, I think I remember people on this list and in other forums occasionally bringing something like this approach up as a way of getting rid of the "fail" wart. So my question is, why did we go to all of the trouble to transition away from the MonadZero approach to the current system to begin with? What was so bad about "unfailable" patterns that it was decided to remove them and in doing so replace MonadZero with a mandatory "fail" method in Monad? I mean, this *is* Haskell, so my safest assumption is that smart people were involved in making this decision and therefore the reasons much have been really good (or at least, seemed good given the information at the time). :-) Cheers, Greg

12 20

LDTA 2012: call for participation
by Emilie Balland 26 Jan '12

26 Jan '12

*************************************************************************************** LDTA 2012 Call for Participation 12th International Workshop on Language Descriptions, Tools, and Applications http://ldta.info Tallinn, Estonia March 31 & April 1, 2012 an ETAPS workshop *************************************************************************************** You are cordially invited to LDTA'12. LDTA is an application and tool-oriented workshop focused on grammarware - software based on grammars in some form. Grammarware applications are typically language processing applications and traditional examples include parsers, program analyzers, optimizers and translators. -- REGISTRATION-- To register for LDTA'12, please follow the instructions at: http://www.etaps.org/2012/registration The early registration deadline is January 29, 2012. --HOTEL INFORMATION-- LDTA will be co-located with ETAPS. Please visit ETAPS's web site to get more information for the accomodation (http://www.etaps.org/2012/accommodation) Special rates and/or block bookings for ETAPS 2012 participants at a number of central hotels also expire Sunday, 29 January 2012. --PROGRAM-- The workshop will include an invited talk by Janis Voigtländer (University of Bonn, Germany) and the presentation of the following technical papers: - Sebastian Erdweg, Paolo G. Giarrusso and Tillmann Rendel. Language Composition Untangled - Johnni Winther. Improving Precision of Generated ASTs - Tobi Vollebregt, Lennart C. L. Kats and Eelco Visser. Declarative Specification of Template-Based Textual Editors - Marcos Viera, Doaitse Swierstra and Atze Dijkstra. Grammar Fragments Fly First-Class - Marcos Viera, Doaitse Swierstra and Arie Middelkoop. UUAG Meets AspectAG: How to make Attribute Grammars First-Class - Daniel Lincke and Sibylle Schupp. From HOT to COOL - Transforming Higher-Order Typed Languages to Concept-Constrained Object-Oriented Languages - Maartje De Jonge and Eelco Visser. A Language Generic Solution for Name Binding Preservation in Refactorings - Vadim Zaytsev. Notation-Parametric Grammar Recovery - Jean-Christophe Bach, Pierre-Etienne Moreau, Marc Pantel and Xavier Crégut. Models Transformations with Tom The workshop will also feature presentations devoted to a de-brief of the 2011 tool challenge, based on the paper currently being prepared by the challenge participants.

1 0

Hierarchical tracing for debugging laziness
by Eugene Kirpichov 26 Jan '12

26 Jan '12

Hi cafe, Look how one can watch the evaluation tree of a computation, to debug laziness-related problems. {-# LANGUAGE BangPatterns #-} module HTrace where import Data.List (foldl') import Data.IORef import System.IO.Unsafe level = unsafePerformIO $ newIORef 0 htrace str x = unsafePerformIO $ do lvl <- readIORef level putStrLn (replicate (4*lvl) ' ' ++ str) writeIORef level (lvl+1) let !vx = x writeIORef level lvl return vx xs = map (\x -> htrace (show x) x) [1..10] s = foldl (\a b -> htrace "+" (a+b)) 0 xs s2 = foldl' (\a b -> htrace "+" (a+b)) 0 xs b = htrace "b" 2 c = htrace "c" 3 a = htrace "a" $ b + c x = htrace "x" $ b + c *HTrace> a a b c 5 *HTrace> x x 5 *HTrace> s + + + + + + + + + + 1 2 3 4 5 6 7 8 9 10 55 (reload) *HTrace> s2 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 55 -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

5 7

where to put general-purpose utility functions
by Joey Hess 26 Jan '12

26 Jan '12

I'm finding a rather unusual problem as I write haskell.. Unlike every other language I've used, large portions of my haskell code are turning out to be general-purpose, reusable code. Fully 20% of the haskell code I've written for git-annex is general purpose. Now, I came out of a decade of perl with maybe 1% reusable code. So I'm sure this is a credit to haskell, and not to me. My problem now is that as I start new projects, I want to have my haskell utility functions available, and copying them around is not ideal. So, put them on hackage. But where, exactly? It already has several grab bag utility libraries. The only one with much traction is MissingH. Using the others makes a program have an unusual dependency, which while only a cabal install away, would make work for distributions that want to package the program. I've ruled out using a couple on that basis. Doesn't encourage me to add another one. My 2000+ lines of reusable code are a grab-bag of generic utility functions. Looking them over (see Appendix), I could try to get portions into existing libraries on hackage, but it's unlikely I'd find a home for most of them, so I'm still left with this problem of what to do. I wonder if the model used for xmonad-contrib, of a big library package, that is very open to additions from contributors, would be helpful here? John, any interest in moving MissingH in this direction? I get the impression it's not otherwise changing much lately, and parts of it are becoming naturally obsolete, maybe this could inject some life into it. Any other thoughts you have on grab-bag utility libraries on hackage also appreciated. ---- Appendix: A sample of a a few of the better functions from my utility library. Some quite generic monadic control functions, few of them truely unique: whenM :: Monad m => m Bool -> m () -> m () -- also >>? unlessM :: Monad m => m Bool -> m () -> m () -- also >>! firstM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a) A module that exports functions conflicting with partial functions in the Prelude, to avoid them being accidentially used. And provides some alternatives (which overlap somewhat with Safe): headMaybe :: [a] -> Maybe a readMaybe :: Read a => String -> Maybe a beginning :: [a] -> [a] Various path manipulation functions such as: dirContains :: FilePath -> FilePath -> Bool dotfile :: FilePath -> Bool absPath :: FilePath -> IO FilePath Other stuff: separate :: (a -> Bool) -> [a] -> ([a], [a]) catchMaybeIO :: IO a -> IO (Maybe a) readSize :: [Unit] -> String -> Maybe ByteSize -- parses "100 kb" etc format :: Format -> Variables -> String findPubKeys :: String -> IO GpgKeyIds boolSystem :: FilePath -> [CommandParam] -> IO Bool withTempFile :: Template -> (FilePath -> Handle -> IO a) -> IO a -- see shy jo

11 14

"Re: Wow you have to check this out Haskell" - appols accidental resend of this scam spam
by Steve Horne 26 Jan '12

26 Jan '12

Lesson = don't open e-mail client while borderline asleep.

1 0