- Haskell-Cafe - Haskell.org

Return value of a (windows) process
by Arnaud Bailly 21 Oct '10

21 Oct '10

Hello, I have the following code (fragment) I use to wrap execution of various processes in Haskell, in the spirit of Don Stewart's slides about "Scripting in Haskell". instance MonadExec IO where exec proc args = do (exit, out, err) <- liftIO $ readProcessWithExitCode proc args "" case exit of ExitFailure e -> throwError $ userError ("Fail execution of " ++ program proc args ++ ": " ++ (show e) ++", " ++err) ExitSuccess -> return out Here is the abridged declaration of MonadExec: class (Monad m) => MonadExec m where exec :: String -> [String] -> m String The issue I am facing is that readProcessWithExitCode does not seem to return the expected failures. When I run some failing process within windows command shell, I have: D:\projets\psug-dojo\lags>"d:/Program Files/scala-2.8.0/bin/scalac.bat" -d target\test-classes -classpath target\classes;D:\projets\ psug-dojo\lags\test-lib\specs_2.8.0-1.6.5.jar;D:\projets\psug-dojo\lags\test-lib\scalacheck_2.8.0-1.7.jar;D:\projets\psug-dojo\lags\ test-lib\junit-4.7.jar -d target\test-classes src\test\scala\oqube\lags\LagsTest.scala src\test\scala\oqube\lags\LagsTest.scala:86: error: not found: value beS_== bid.sublists(List(Request(1,2,3))) must beS_==(List(Nil,List(Request(1,2,3)))) ^ one error found D:\projets\psug-dojo\lags>echo %ERRORLEVEL% 1 D:\projets\psug-dojo\lags> but wrapping this same process in my "shell", I always got an ExitSuccess. What am I doing wrong ? Thanks Arnaud

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ANN: fountain-0.0.0
by Tom Hawkins 20 Oct '10

20 Oct '10

This library [1] implements a fountain code [2]. Fountain codes are forward error correction codes for erasure channels [3]. A fountain code encodes a message into an infinite stream of packets -- transmitters generate message packets at random, on-the-fly. To reconstruct the message, receivers simply need to capture enough packets for the decoding process. As a rateless code, fountain codes automatically adapt to varying channel conditions. Some of the more interesting applications of fountain codes include unsynchronized data broadcast and distributed download. For example, a multiple number of devices can transmitting content to multiple receivers without any coordination. Because packets are generated at random, receivers increase their bandwidth simply by listening to more transmitters. Note that receivers can also start generating packets and forwarding the message on even before they have decoded the complete message. This library provides a packet generator and a decoder for one of the first known fountain codes: LT codes [4]. It also includes a test function to experiment with message lengths, and encoding degrees -- it runs a simulation to determine the number of packets needed to decode a message. -Tom [1] http://hackage.haskell.org/package/fountain [2] http://en.wikipedia.org/wiki/Fountain_code [3] http://en.wikipedia.org/wiki/Binary_erasure_channel [4] http://en.wikipedia.org/wiki/LT_codes

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Getting results of of runSTUArray
by Henry Laxen 20 Oct '10

20 Oct '10

1{- Dear Group, I am trying to do something that I think should be very simple, but I just can't seem to figure out how. The idea is that I want to use runSTUArray to build up an array, but I want to look at the array after each iteration and possiblely accumulate some elements. -} import Debug.Trace import Control.Monad import Control.Monad.ST import Data.Array.ST incArray arr index = do x <- readArray arr index writeArray arr index (x+1) checkArray = filter (\(i,e) -> e == 1) run = runSTUArray ( do a <- newArray (0,9) 0 :: ST s (STUArray s Int Int) all <- forM [1..10] (\i -> do let j = i `mod` 4 incArray a j b <- getAssocs a -- now run something like checkArray and build up a list of "wanted" elements -- which I would like to return from run, but I don't see how return b) -- Or, if I cound run (lazily) on "all" (the result of the forM) -- I could do my accumulation let theAnswerIWant = concatMap checkArray all return (trace (show theAnswerIWant) a)) -- I can't seem to return anything but a here {- result is: array [(1,1),(1,1),(2,1),(1,1),(2,1),(3,1),(0,1), (1,1),(2,1),(3,1),(0,1),(2,1),(3,1),(0,1),(3,1), (0,1)] (0,9) [(0,2),(1,3),(2,3),(3,2),(4,0),(5,0),(6,0),(7,0),(8,0),(9,0)] is there any way to get theAnswerIWant out of run? Thanks in advance for any insights. -}

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Idea for a tool
by Michael Snoyman 20 Oct '10

20 Oct '10

Hi all, I saw a quote from Eric Kow via Neil Mitchell[1] that we don't spend enough time writing tools. Well, I've decided that the most annoying part of package maintainership right now is staying on top of new versions of dependencies. We essentially have two options right now: * Follow the PVP and put an upper bound on all dependencies, and people will be upset when your package only works with the old version of the dependency. * Skip the upper bound, and risk having your code break when there's a new version. I have an idea for a tool: you give it a list of packages you maintain, or even better yet, you give it your email address and it gets that list automatically. Then is looks through all your dependencies and sees if you have any upper bounds preventing newer versions from being used. Bonus points for making it a web service that just gives you an RSS feed. If anyone's interested in writing a tool like that, let me know. Otherwise, next time I'm twiddling my thumbs I'll try to throw it together. I've never dealt directly with the Cabal library, but there's a first time for everything. If someone else wants to write that tool and wants help sticking a web service on it, let me know. Michael [1] http://neilmitchell.blogspot.com/2010/10/enhanced-cabal-sdist.html

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Re: [Haskell-cafe] Haskellers.com skills list moderation?
by John Lato 20 Oct '10

20 Oct '10

My $.02 follows: > From: Michael Snoyman <michael(a)snoyman.com> > > There's only two skills which I think absolutely must go: > > Other languages I know: C# .NET, XSLT, Microsoft SQL Server, XML, SQL, > CSS, C, C++, Java, HTML, Visual Basic Script, Pascal, Rexx, Basic and > assembler > tool building > Agreed that both should go. > There are 11 skills I'm leaning towards dropping, all because they > fall in the too vague/too general category. Your input is requested on > these. They are: > > Attribute Grammar > Keep as "Attribute Grammars" > Cabal, packaging, build and distribution tools > This should be two categories: "Cabal internals" and "Software packaging/distribution tools". Keep Cabal internals, possibly keep the other > Categorical Programming Denotational design > Digital Forensics > keep > Fault Tolerant Server Software Mathematics > drop (possibly keep FTSS, maybe change the name) > Programming using Arrows > Possibly keep with a different name > Proving observational equivalence between Haskell programs drop > Transactional business applications development > I'm not entirely sure what this means, specifically if it's business transactions or db/software transactions. > UNIX Scripting and Tool Authoring > keep as "UNIX Scripting" > > Of the remaining 32 skills, some of them fall in the "too specific" > range just a bit (software transactional memory, property based > testing), but I'm inclined to let it slide. These 32 are: > > Advanced type-level programming (GADTs, TypeFamilies, proofs, etc.) > Algorithmic Problem Solving > Bioinformatics > Concurrent Haskell > DSL Design > Darcs internals > Foreign Function Interface (FFI) > Formal Verification > Functional graphics programming (2D, 3D, GPU) > GHC internals > Generic Programming > Graphical User Interfaces > Happstack Web Framework > Hardware Acceleration DSLs > Haskell on embedded devices > High Assurance Software Development > High-performance Haskell > Metaprogamming via Template Haskell > Natural Language Processing (tagging, parsing, translation,...) > Physics & Simulation > Programming language translation > Property based testing (QuickCheck) > Purely functional data structures — design and implementation > Reverse Engineering > Robotics and Automation > Signal Processing > Software Transactional Memory > Teaching Haskell > Web development (HTML, CSS and Javascript) > Yesod Web Framework > I would argue for keeping most of these. I do think that skills based on specific packages (Happstack, Yesod, STM?) perhaps should be dropped, except for packages that are integral to the Haskell universe (compilers, Cabal, Darcs?, QuickCheck?). For these packages, I think the "-internals" categories are the most useful. What about rolling Happstack, Yesod, etc. into "Haskell Web Frameworks", and possibly also keeping the "Web Development (HTML, CSS and Javascript)" category? John

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Why isn't there a cheaper "split-in-two" operation for Data.Set?
by Ryan Newton 20 Oct '10

20 Oct '10

Would there be anything wrong with a Data.Set simply chopping off half its (balanced) tree and returning two approximately balanced partitions -- i.e. Data.Set.split without the pivot argument? Even though it can't quite be constant-time (still need rebalancing) it could still be cheaper than Data.Set.split, right? In the context of Set.hs, I'm thinking along the lines of something like the following: cleave :: Set a -> (Set a, Set a) cleave Tip = (Tip, Tip) cleave (Bin _ x l r) | size l > size r = (l, insertMin x r) | otherwise = (insertMax x l, r) For one thing, this seems like the proposed function would be useful for consuming a set in parallel, similar to Guy Steele's arguments in recent talks (see Organizing Functional Code for Parallel Execution; or, foldl and foldr Considered Slightly Harmful<http://research.sun.com/projects/plrg/Publications/ICFPAugust2009Steele.pdf>). That is, one splits a tree and uses "par" to process the left and right half in parallel, enabling Alternatively, it would be simple to provide a parallel version of mapMonotonic that does this internally, i.e.: mapMonotonic _ Tip = Tip mapMonotonic f (Bin sz x l r) = Bin sz (f x) (mapMonotonic f l) (mapMonotonic f r) Becomes something like: parMapMonotonic f (Bin sz x l r) = par left $ seq right $ Bin sz (f x) left right where left = mapMonotonic f l right = mapMonotonic f r Cheers, -Ryan

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HDBC/ODBC/MySQL - working configuration?
by Neil Davies 20 Oct '10

20 Oct '10

Hi I'm having some difficulty getting the above combination to work, I've successfully used ODBC against MS but the MySQL stuff is creating some really interesting error conditions in the HDBC-ODBC module. My first thought is that it must be me, so does anyone out there have this combination working (with or without HaskellDB)? If so, can you tell me which versions of things you are using and which ODBC drivers (and configurations) you are using? Cheers Neil

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HaskellDB/ODBC/MySQL issue
by Neil Davies 19 Oct '10

19 Oct '10

Hi I can't seem to get the combination of HaskellDB/ODBC/MySQL to even get off the ground, example: import OmlqDBConnectData (connect'options) import Database.HaskellDB import Database.HaskellDB.HDBC.ODBC (odbcConnect) import Database.HaskellDB.Sql.MySQL (generator) main = odbcConnect generator connect'options $ \db -> tables db >>= print gives me Prelude.(!!): index too large Suggestions? Cheers Neil

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Happstack, RqData: (How) can I extract multiple data from form?
by Никитин Лев 19 Oct '10

19 Oct '10

-------- Пересылаемое сообщение -------- 20.10.10, 00:22, "Никитин Лев" <leon.v.nikitin(a)pravmail.ru>: >>Are formlets integrated with HSP? > hsp support is in a separate package: > http://hackage.haskell.org/package/formlets-hsp > There is a demo of using HSP+Formlets+Happstack here: > http://src.seereason.com/formlets-hsp/examples/happs-hsp/ > hope this helps! > - jeremy > Thanks! (Sorry for my stuped question) -------- Завершение пересылаемого сообщения --------

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Template Haskell: onward and upward
by Simon Peyton-Jones 19 Oct '10

19 Oct '10

Folks Following lots of feedback from users, especially at ICFP, I've evolved some proposals for Template Haskell, that should make it both more expressive, and more secure. http://hackage.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal Do let me know what you think. Discussion by email is fine (cc me if it's on Haskell-cafe), or comments direct on the Trac. (None of this will be in GHC 7.0; it's a proposed plan for post-release improvements.) Thanks Simon

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An interesting paper from Google
by Andrew Coppin 19 Oct '10

19 Oct '10

http://k1024.org/~iusty/papers/icfp10-haskell-reagent.pdf I'm sure some of you have seen this already. For those who lack the time or inclination to read through the (six) pages of this report, here's the summary... We [i.e., the report authors] took a production Python system and rewrote bits of it in Haskell, some of which is now in production use. We conclude the following: - Python lets you just do whatever the hell you want, but Haskell demands that you actually have a *plan* before you start churning out code and running it. The result is generally cleaner and more consistent when you get there. - Haskell's much-criticised immutable data is actually an *advantage* for backtracking search problems. - Haskell wins for thread-safety. - ADTs are nicer than exceptions. - The parts of Haskell stolen by Python aren't as nice in Python as they are in Haskell. [Well, duh.] - We like what GHC provides for profiling. - We are dissappointed by what GHC provides for debugging. - "String" is too slow. None of the alternatives seem to be widely supported. If your library consumes Strings and returns Strings, the fact that ByteString exists doesn't help you. - Recent changes to GHC broke our code. (Specifically, extensible exceptions.) We were quite surprised that such a "stable" and "mature" system as GHC would do this to us. - Haskell has quite a high barrier to entry. [Again, duh.] The paper also contains an interesting section that basically says "we tried porting the Python implementing of XYZ into Haskell, but there wasn't really any advantage because it's all I/O". In my humble opinion, "it's all I/O" is a common beginner's mistake. Reading between the lines, it sounds like they wrote the whole thing in the IO monad, and then decided it looked just like the existing Python code so there wasn't much point in continuing. I guess it's one of the bad habits that imperative programmers get into. With a little more experience, you eventually figure out that you can limit the stuff actually in the IO monad to a surprisingly small section, and so almost everything else in pure code, no matter how much the problem looks like it's "all I/O". But anyway, I'm only guessing from what I can actually see with my own eyes in the report itself. I'm surprised about the profiler. They seem really, really impressed with it. Which is interesting to me, since I can never seen to get anything sensible out of it. It always seems to claim that my program is spending 80% of its runtime executing zipWith or something equally absurd. I'm unsurprised which their dissappointment with debugging. I'm still quite surprised that there's no tool anywhere which will trivially print out the reduction sequence for executing an expression. You'd think this would be laughably easy, and yet nobody has done it yet. Their comments about String are sadly true.

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profiling cabal libraries
by Tom Hawkins 19 Oct '10

19 Oct '10

How do I profile cabal libraries? I "cabal install -p" a local package I am testing, and I compile a test of the library using "-prof -auto-all". But the profiling report only lists a CAF entry for the library, but does not detail any of the library's top level functions. What am I doing wrong? -Tom

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Re: [Haskell-cafe] Haskellers.com skills list moderation?
by Michael Snoyman 19 Oct '10

19 Oct '10

Hey JP, It's a tough question you're asking. I think areas directly applicable with Haskell, such as bioinformatics, games, physics simulations, are a pretty easy yes. Some more complicated things would be "related skills", such as knowing other programming languages, system administration, etc. I would like to hear the cafe's opinion on that; my gut feeling is a "yes is moderation." Having a web programming skill seems OK, but I wouldn't want to put in HTML5, Javascript, CSS 3 and so on as separate skills. Michael On Tue, Oct 19, 2010 at 3:45 PM, JP Moresmau <jpmoresmau(a)gmail.com> wrote: > Are skills only Haskell related? I mean, are they only subcategories of > "haskell programming". Because "bioinformatics" is there, and in that case > it shouldn't be. If skills include any application domain where people might > use Haskell, the list will be much bigger, and surely the Hackage categories > can be of use (for example, for me, I would request Games, Artificial > Intelligence...). > And, thanks for doing haskellers, great work! One day I want to really do a > web application in Haskell and I'll sure give a go to yesod. > JP > > On Tue, Oct 19, 2010 at 3:32 PM, Michael Snoyman <michael(a)snoyman.com> > wrote: >> >> Alright, adding skills is now only possible by an admin. In the place >> where we previously had "add a skill", we now have "request a new >> skill." That's the easy part. Now we need to determine which skills >> stay, and which ones go. I think the vast majority of them are fine, >> so I'll leave them at the end of this email. If anyone thinks I'm >> being to generous by allowing a specific still to say, just say so. >> >> There's only two skills which I think absolutely must go: >> >> Other languages I know: C# .NET, XSLT, Microsoft SQL Server, XML, SQL, >> CSS, C, C++, Java, HTML, Visual Basic Script, Pascal, Rexx, Basic and >> assembler >> tool building >> >> There are 11 skills I'm leaning towards dropping, all because they >> fall in the too vague/too general category. Your input is requested on >> these. They are: >> >> Attribute Grammar >> Cabal, packaging, build and distribution tools >> Categorical Programming >> Denotational design >> Digital Forensics >> Fault Tolerant Server Software >> Mathematics >> Programming using Arrows >> Proving observational equivalence between Haskell programs >> Transactional business applications development >> UNIX Scripting and Tool Authoring >> >> Of the remaining 32 skills, some of them fall in the "too specific" >> range just a bit (software transactional memory, property based >> testing), but I'm inclined to let it slide. These 32 are: >> >> Advanced type-level programming (GADTs, TypeFamilies, proofs, etc.) >> Algorithmic Problem Solving >> Bioinformatics >> Concurrent Haskell >> DSL Design >> Darcs internals >> Foreign Function Interface (FFI) >> Formal Verification >> Functional graphics programming (2D, 3D, GPU) >> GHC internals >> Generic Programming >> Graphical User Interfaces >> Happstack Web Framework >> Hardware Acceleration DSLs >> Haskell on embedded devices >> High Assurance Software Development >> High-performance Haskell >> Metaprogamming via Template Haskell >> Natural Language Processing (tagging, parsing, translation,...) >> Physics & Simulation >> Programming language translation >> Property based testing (QuickCheck) >> Purely functional data structures — design and implementation >> Reverse Engineering >> Robotics and Automation >> Signal Processing >> Software Transactional Memory >> Teaching Haskell >> Web development (HTML, CSS and Javascript) >> Yesod Web Framework >> >> Michael >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -- > JP Moresmau > http://jpmoresmau.blogspot.com/ >

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A State Monad Tutorial
by Artyom Kazak 19 Oct '10

19 Oct '10

Some time ago I have read A State Monad Tutorial (http://strabismicgobbledygook.wordpress.com/2010/03/06/a-state-monad-tutori…) While reading, I was fixing some minor mistakes (okay, a lot of mistakes). After all, I had an idea to create PDF with fixed version. So, here it is: http://dl.dropbox.com/u/8662438/A%20State%20Monad%20Tutorial.pdf

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Re: MonadCatchIO, finally and the error monad
by Paolo Losi 19 Oct '10

19 Oct '10

Thanks for you reply, Michael. See inline ... On Mon, Oct 18, 2010 at 11:21 AM, Michael Snoyman > > > > > As an haskell newbie, I've got some questions on this matter. > > > > Is it using ErrorT instead of extensible exeptions really necessary? > > I've read your comment stating that we cannot pass function arguments > > to Exceptions because of the Show class constraint. > > But is that really limiting? Wouldn't your use case (e.g. > > HTTP redirection in Yesod) be implementable with extensible exceptions? > > > > I would guess that the rule of thumb would be not to mix > > extensible exception and ErrorT whenever it is possibile. > > > > I've quickly read your post on inverting transformer stacks > > and, from my newbie point of view, I feel that the extra complexity > > isn't worth the gain. > > I'm not sure why you think inverting the ErrorT monad is more > complicated than MonadCatchIO: they're both doing basically the same > thing. The difference is that MonadInvertIO does this generically for > a whole class of functions, and therefore does it right. I could have written MonadCatchIO, I could not have written InvertIO :-) > Anyway, it *is* theoretically possible to use extensible exceptions > anywhere you would use ErrorT, but there's always a runtime hit in > using runtime exceptions. Also, it's a difference in philosophy: > coming from a language like Python or Ruby, it may feel natural to use > exceptions for these kinds of cases. In Haskell, we try to make > exceptions respect their name, and only use them in exceptional > circumstances. Is the performance penalty of using runtime exception so high? I would sacrifice some bits of performance for a more consistent error reporting / flow control. For the argument that Exceptions are for exceptional cases I'm totally with David MacIver [1]. I perfectly see the point of using ErrorT, but not when you need to handle runtime exceptions as well. As an haskell newbie I think that _THIS_, error handling strategies, is the single most troublesome point where some consistency (at least as a best practice) is really needed. I had hopes that extensible execptions could clarify the issue raised by [2] at least in "new" code... Thank again Paolo [1] http://www.drmaciver.com/2009/03/exceptions-for-control-flow-considered-per… [2] http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-err…

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Fwd: [Haskell-cafe] Happstack, RqData: (How) can I extract multiple data from form?
by Никитин Лев 18 Oct '10

18 Oct '10

-------- Пересылаемое сообщение -------- 18.10.10, 23:58, "Никитин Лев" <leon.v.nikitin(a)pravmail.ru>: -------- Пересылаемое сообщение -------- 18.10.10, 23:57, "Никитин Лев" : Well. Thanks. I've just have a look at formlets. I think is what i need. But I'm using now HSP. Are formlets integrated with HSP? (I saw olny using formlets with text.xhtml). And [after discussion of "Web" wiki page] what is props and cons of using HSP? For newbiers it will be good idia to include some comparison between web frameworks, templating systems etc. (their props and cons). > The following looks like something that you might want: > http://hackage.haskell.org/packages/archive/formlets/0.7.3/doc/html/Text-Fo… > See the source for more clues about its use: > http://hackage.haskell.org/packages/archive/formlets/0.7.3/doc/html/src/Tex… > More about the Formlets library here: > http://www.haskell.org/haskellwiki/Formlets > -------- Завершение пересылаемого сообщения -------- -------- Завершение пересылаемого сообщения --------

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An interesting paper on VM-friendly GC
by Andrew Coppin 18 Oct '10

18 Oct '10

Somebody showed me this the other day, and I thought it was interesting: http://www.cs.umass.edu/~emery/pubs/f034-hertz.pdf Basically, "we designed a garbage collector which tries to avoid touching memory pages that have been swapped out to disk just because we need to do a GC sweep". Which is a pretty obvious thing to do, when you think about it, and has several obvious performance implications. Maybe we should think about how GHC handles this? On the other hand, their implementation uses a modified Linux kernel, and no sane person is going to recompile their OS kernel with a custom patch just to run Haskell applications, so we can't do quite as well as they did. But still, and interesting read...

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Happstack, RqData: (How) can I extract multiple data from form?
by Никитин Лев 18 Oct '10

18 Oct '10

Hello. (How) can I query multiple data from form? Foor form ala <form> <input name="foo"/> <input name="foo"/> /form> I can use 'looks "foo"' to get array of values. Ok. But Is there any way to extract values from such form: <form> <input name="key" val="key1"/><input name="tag" val="tag11/><input name="tag" val="tag12/>... <input name="key" val="key2"/><input name="tag" val="tag21/><input name="tag" val="tag22/>... /form> to get something like [("key1", ["tag11", "tag12"]), ("key2", ["tag21", "tag22"]) ] ? One of requirement is possibility to add additionally key and tags for this key (via button, using javascript). Can I groupe such pairs using html? Maybe I have to do like this: <form> <input name="key" val="key1"/><input name="tag-key1" val="tag11/><input name="tag-key1" val="tag12/> <input name="key" val="key2"/><input name="tag-key2" val="tag21/><input name="tag-key2" val="tag22/> /form>, but i dont see any (simple) way to add dinamicly add other list of keyed tags in such manner (by user, o client side).

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Make with Version
by Jonas Fager 18 Oct '10

18 Oct '10

Hi. If you use ghc --make the autodepend do all the heavy-lifting for you but what's the way to work when you have two versions of a package and you have code compatible with both version? Is it possible to combine --hide-package with --make to compile old code against old package version? //Jonke

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MonadCatchIO, finally and the error monad
by Michael Snoyman 18 Oct '10

18 Oct '10

Hey all, In case anyone noticed, Haskellers occassionally dies with a "Pool exhausted exception." I've traced this to a bug in Yesod, which in turn is a bug in the neither package, which I believe is a flawed design in the MonadCatchIO-transformers package. Here are my thoughts on this and what I think needs to be done to fix it. In Control.Exception, we define a number of different ways of dealing with exceptions. All of these can be expressed in terms of block, unblock and catch. For our purposes here, I'm going to ignore block and unblock: they deal with asynchronous exceptions, which is not my point here. Keep that in mind with the code samples. Anyway, with this caveat, we can define finally as: finally :: IO a -> IO b -> IO a a `finally` sequel = do r <- a `catch` \e -> sequel >> throwIO (e :: SomeException) _ <- sequel return r The idea is simple: try to perform the action. If any exceptions get thrown, call sequel and rethrow the exception. If we ever get to line 4, it's because no exceptions were thrown. Therefore, we know that sequel has not yet been called, so we call it. Said another way: there are precisely two cases: * An exception was thrown * An exception was not thrown A downside of this finally function (and catch, for that matter) is that it requires all of the actions to live in the IO monad, when in fact we all love to let things run in complicated monad transformer stacks. So along comes MonadCatchIO-(transformers, mtl) and gives us a new magical definition of catch: catch :: (MonadCatchIO m, Exception e) => m a -> (e -> m a) -> m a Using this new, extended definition of catch, we can define a finally function with the type signature finally :: MonadCatchIO m => m a -> m b -> m a (Note that we need to replace throwIO with liftIO . throwIO.) You can try this with writers, readers, etc, and everything works just fine. You can even use an Error/Either monad transformer, throw an exception, and the finally function will correctly run your sequel function. However, things don't work out so well when you use a throwError. Let's see the code: {-# LANGUAGE PackageImports #-} import Control.Monad.Trans.Error import "MonadCatchIO-transformers" Control.Monad.CatchIO (finally) import Control.Monad.IO.Class main = runErrorT $ finally go $ liftIO $ putStrLn "sequel called" go :: ErrorT String IO String --go = return "return" --go = error "error" --go = throwError "throwError" Try running the code with each version of go uncommented. In the first two, "sequel called" gets printed. However, in the third, it does not. The reason is short-circuiting: if we remember from the definition of finally, there are two cases we account for. If an exception is called, catch addresses it. If not, we assume that the next line will be called. However, in the presence of short-circuiting monads like ErrorT, that line of code will never get called! I have a recommendation of how to fix this: the MonadCatchIO typeclass should be extended to include finally, onException and everything else. We can provide default definitions which will work for most monads, and short-circuiting monads like ErrorT (and I imagine ContT as well) will need to override them. Michael

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Re: [Haskell-cafe] Bulletproof resource management
by John Lato 18 Oct '10

18 Oct '10

> > From: Florian Weimer <fw(a)deneb.enyo.de> > > * Henning Thielemann: > > > Some open/close pairs have corresponding 'with' functions, that are > > implemented using Exception.bracket. You can also use them within > > GHCi. I think using both manual resource deallocation and finalizers > > makes everything more complicated and more unreliable. > > It seems that Exception.bracket does not work in all cases, see the > recent "MonadCatchIO, finally and the error monad" thread. > > Anyway, the ability of closures (and threads) means that something > like Exception.bracket does not prevent access to closed handles, so > I still need an additional safety net. > Enumeration-based interfaces will prevent access to closed handles[1], [2], etc. You can use this style for more than just handles and sockets, e.g. Takusen uses it for database connections. I think somebody else already mentioned region-based management, which is also completely safe AFAIK. [1] http://hackage.haskell.org/package/iteratee [2] http://hackage.haskell.org/package/enumerator

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ANNOUNCE: Haskell XML Toolbox Version 9.0.0
by Uwe Schmidt 18 Oct '10

18 Oct '10

Haskell XML Toolbox 9.0.0 I would like to announce a new version of the Haskell XML Toolbox. HXT has grown over the years. Components for XPath, XSLT, validation with RelaxNG, picklers for conversion from/to native Haskell data, lazy parsing with tagsoup, input via curl and native Haskell HTTP and others have been added. This has led to a rather large package with a lot of dependencies. To make the toolbox more modular and to reduce the dependencies on other packages, hxt has been split into various smaller packages since this version. Information about this release, about the new packages, changes and incompatibilities to older versions can be found at HXT home: "http://www.fh-wedel.de/~si/HXmlToolbox/index.html" and on the Haskell wiki page about HXT "http://www.haskell.org/haskellwiki/HXT" The source repo has been moved to GitHub: "http://github.com/UweSchmidt/hxt" Downloads and installation is available from hackage. Please email comments, bugs, etc. to hxmltoolbox(a)fh-wedel.de or si(a)fh-wedel.de Uwe -- University of Applied Sciences, Wedel, Germany http://www.fh-wedel.de/~si/index.html

16 32

concurrency vs. I/O in GHC
by Donn Cave 18 Oct '10

18 Oct '10

As I have migrated more of my application into Haskell, I find that I/O in one thread effectively blocks other threads. That's rather the opposite of what I need - I don't care so much if Haskell threads manage to compute in parallel, but the application should continue to function, in other threads, while an I/O has one thread blocked. Is that supposed to happen, is there anything I can do about it? Specifically I'm talking about OpenSSL SSL_read(). In this case, while a file descriptor is involved, there is also (I assume) some buffering, so it isn't a simple matter of deferring SSL_read() until data arrives at the fd. I'm hoping there's something more generic, that unblocks other threads during any foreign call - as one would do in Python, for example, bracking a foreign call with C macros that release and acquire the interpreter global lock. You'd also use this with functions like open(), which may not return immediately but don't answer to select(). All the Haskell code in question is executing in callbacks, from threads started in C++. The thread that actually does the network I/O could if necessary be started in 1st generation Haskell as a special case, if that might make any difference. I disable the RTS clock itimer signals, because something in the libraries I'm using doesn't thrive under a barrage of signals. (Environment setting GHCRTS=-V0.) The consequence, according to the documentation, should be "Context switches will still happen, but deterministically and at a rate much faster than normal", and anyway I suppose that's talking about green threads, not the OS threads where context switches are an OS function. Thanks! Donn Cave, donn(a)avvanta.com

2 3

Proving stuff about IORefs
by Ben Franksen 18 Oct '10

18 Oct '10

I have a formal proof where I am stuck at a certain point. Suppose we have a function f :: IORef a -> IO b I want to prove that f r == do s1 <- readIORef r r' <- newIORef s1 x <- f r' s3 <- readIORef r' writeIORef r s3 return x What happens here is that the temporary IORef r' takes the place of the argument r, and after we apply f to it we take its content and store it in the original r. This should be the same as using r as argument to f in the first place. How can I prove this formally? Cheers Ben

7 11

Text: find
by Antoine Latter 18 Oct '10

18 Oct '10

On Sun, Oct 17, 2010 at 3:25 PM, Bryan O'Sullivan <bos(a)serpentine.com> wrote: > On Mon, Oct 11, 2010 at 1:12 PM, Malcolm Wallace <malcolm.wallace(a)me.com> > wrote: >> >> find find find (Text -> Bool) -> Text -> Maybe Text >> >> Proposal: rename Text.findBy to Text.find. > > I agree with the change of type, not so much with the naming. That's roughly > my stance in the other cases, too. > What would the definition of a function of the form: > find :: (Text -> Bool) -> Text -> Maybe Text look like? What sort of requirements would be placed on the input predicate? Maybe I have a failure of imagination, but that sounds like a tricky thing to get correct for all predicates. Antoine

3 2

Haskell Front Page Ideas
by caseyh＠istar.ca 18 Oct '10

18 Oct '10

How about a bullet list of Haskell's features (maybe pros & cons) might be better; followed by a list of a few other languages and their trade-offs. As Jon Bentley has said, "design" is usually trade-offs but when you can come up with something that is a mutual benefit; so much the better. :)

3 2

Strict Core?
by Gregory Crosswhite 18 Oct '10

18 Oct '10

Hey everyone, Out of curiosity, are there any plans for GHC to eventually use the Strict Core language described in http://www.cl.cam.ac.uk/~mb566/papers/tacc-hs09.pdf? Cheers, Greg

7 9

Heterogenous network stream
by Antoine Latter 17 Oct '10

17 Oct '10

Forwarding to list. ---------- Forwarded message ---------- From: Antoine Latter <aslatter(a)gmail.com> Date: Sun, Oct 17, 2010 at 10:48 AM Subject: Re: [Haskell-cafe] Heterogenous network stream To: Charles-Pierre Astolfi <cpa(a)crans.org> On Sun, Oct 17, 2010 at 7:31 AM, Charles-Pierre Astolfi <cpa(a)crans.org> wrote: > Hi cafe, > > I'd like to implement a toy-protocol and I'm looking for the sanest > way (for me) to do so. > > Here's what it looks like: > The server receives, in that order: > - Ascii string (4 bytes) > - 16 bits int (2 bytes) > - 8 bits int (1 byte) > - UTF8 string (128 bytes) > > I tried to use hGetBuf, but I don't know if it's the way to go and I > did not understood what to do with the buf::Ptr a argument. I'm not > very concerned about efficiency. > > I don't know how I should convert my 16 and 8 bits int (read as Chars, > I guess) to an Int. Is there something for that in the standard > library? > I recommend reading the data in as one bytestring[1] (since you have a fixed length) and then parsing it using one of the many nice bytestring parsing libraries: * binary[2] * cereal[3] Others may have other suggestions. For the UTF8 parsing, I think the text[4] package has support for decoding UTF8 from a bytestring. Antoine [1] http://hackage.haskell.org/packages/archive/bytestring/0.9.1.7/doc/html/Dat… [2] http://hackage.haskell.org/package/binary [3] http://hackage.haskell.org/package/cereal [4] http://hackage.haskell.org/package/text

1 0

Re: [Haskell-cafe] UTF-8 problems when decoding JSON data coming from Network.HTTP
by Ionut G. Stan 17 Oct '10

17 Oct '10

On 17/Oct/10 8:02 AM, Michael Snoyman wrote: > In the gist you sent, the problem is that you are reading the HTTP > response as a String. The HTTP library doesn't deal well with > non-Latin characters when doing String requests; you should be using > ByteString and then converting. It's a little tedious using the HTTP > library with ByteStrings, which is one of the reasons I wrote > http-enumerator. Here's some working code. The main point is to > convert the UTF8 octets to a String. > > You could also consider using one of the JSON libraries that support > bytestrings directly instead of strings, which will likely result in > much better performance. Contenders include JSONb[1] and > yajl-enumerator[2]. > > import Network.HTTP.Enumerator > import qualified Text.JSON as JSON > import qualified Data.ByteString.Lazy.UTF8 as BSLU > > data GithubUser = GithubUser { > name :: String, > location :: String > } deriving (Eq, Show) > > > instance JSON.JSON GithubUser where > readJSON (JSON.JSObject object) = > let (Just a) = lookupM "user" $ JSON.fromJSObject object > (JSON.JSObject b) = a > user = JSON.fromJSObject b > in do name<- lookupM "name" user>>= JSON.readJSON > location<- lookupM "location" user>>= JSON.readJSON > return $ GithubUser { > name = name, > location = location > } > > showJSON user = JSON.makeObj [ > ("name", JSON.showJSON $ name user), > ("location", JSON.showJSON $ location user) > ] > > > lookupM :: (Monad m) => String -> [(String, a)] -> m a > lookupM x xs = maybe (fail $ "No such element: " ++ x) return (lookup x xs) > > main = do jsonLbs<- simpleHttp "http://github.com/api/v2/json/user/show/igstan" > let jsonText = BSLU.toString jsonLbs > let result = JSON.decode jsonText :: JSON.Result GithubUser > showResult result > where showResult (JSON.Ok json) = putStrLn $ name json > showResult (JSON.Error e) = putStrLn e > > Michael > > [1] http://hackage.haskell.org/package/JSONb-1.0.2 > [2] http://hackage.haskell.org/package/yajl-enumerator Thanks Michael, now it works indeed. But I don't understand, is there any inherent problem with Haskell's built-in String? Should one choose ByteString when dealing with Unicode stuff? Or, is there any resource that describes in one place all the problems Haskell has with Unicode? -- Ionuț G. Stan | http://igstan.ro

3 3

Heterogenous network stream
by Charles-Pierre Astolfi 17 Oct '10

17 Oct '10

Hi cafe, I'd like to implement a toy-protocol and I'm looking for the sanest way (for me) to do so. Here's what it looks like: The server receives, in that order: - Ascii string (4 bytes) - 16 bits int (2 bytes) - 8 bits int (1 byte) - UTF8 string (128 bytes) I tried to use hGetBuf, but I don't know if it's the way to go and I did not understood what to do with the buf::Ptr a argument. I'm not very concerned about efficiency. I don't know how I should convert my 16 and 8 bits int (read as Chars, I guess) to an Int. Is there something for that in the standard library? As I also write the client, I'm looking for a conversion in both ways. Thanks, -- Charles-Pierre

1 0

Opening a gtk2hs link in default browser
by Dmitry V'yal 17 Oct '10

17 Oct '10

Hello list, I'm trying to make a clickable link in gtk2hs program. Clicking it should open an url in the default browser. Can I hope to find some portable solution in gtk2hs (it should work in linux and windows) or should I start searching platform specific ones, like /usr/bin/xdg-open or something? Can any examples be found in existing haskell software? Dmitry

2 1

Systematic treatment of static arguments
by Stephen Tetley 17 Oct '10

17 Oct '10

Hello list The Monad and Applicative instances for functions are "equivalent" to the respective Reader vesions (I use "equivalent" along the lines of - operationally the same but without the type distinction / newtype). There is also the Monoid instance for functions which is pretty slick. Has anyone looked at variations with two or more static arguments though? For example mappend with two static arguments needs to be defined either as a new function: > mappendR2 :: Monoid a => (r1 -> r2 -> a) -> (r1 -> r2 -> a) -> r1 -> r2 -> a > mappendR2 f g = \x y -> f x y `mappend` g x y or an overlapping instance: > instance Monoid a => OPlus (r1 -> r2 -> a) where > f `mappend` g = \x y -> f x y `mappend` g x y Working in graphics I've found two static arguments comes up quite often - preliminarily most of my functions are functions from the DrawingContext to something (drawing context is an environment that tracks line width, stroke colour, fill colour, etc.): fn1 :: DrawingCtx -> a Many of my functions statically use a 'start' point as the only coordinate reference, so they are in a "coordinate free" style: fn2 :: Point -> DrawingCtx -> a Some functions even have have a third static argument, for example drawing arrowheads for lines seems nice if the angle of the line is parametric and not used explicitly: fn3 :: Radian -> Point -> DrawingCtx -> a The help from Applicative, Monad and Monoid is "used up" by the one static argument version, so I find that I have to introduce points (in the pointed / point-free sense not in the graphic sense) in the fn2 and fn3 versions where a point-free version might still be nice. Is there any prior work looking at sets of combinators for these higher arity functions - papers or code? I'd prefer not to introduce a whole new lexicon of combinator names if I can help it. Thanks Stephen

3 3

Text.Regex Segfault
by Duane Johnson 17 Oct '10

17 Oct '10

I bumped into a segmentation fault in the regex library today and thought I'd warn others in case similar behavior is observed: Prelude> :m Text.Regex > Prelude Text.Regex> map read (splitRegex (mkRegex "\\|") "0|1|2|4") :: > [Int] > Loading package syb ... linking ... done. > Loading package array-0.2.0.0 ... linking ... done. > Loading package bytestring-0.9.1.4 ... linking ... done. > Loading package regex-base-0.72.0.2 ... linking ... done. > Loading package regex-posix-0.72.0.3 ... linking ... done. > Loading package regex-compat-0.71.0.1 ... linking ... done. > [0,1,2,4] > Prelude Text.Regex> Segmentation fault > Regards, Duane

2 1

ghci - always show types
by Ozgur Akgun 17 Oct '10

17 Oct '10

Cafe, Just a quick question. Either I am hallucinating or there was a way of saying ghci to always show types. It was working as if you typed ":t it" after every line of input. Sorry, I searched but couldn't find the option via google. Hope someone here knows/remembers what I am talking about. Best, Ozgur

3 3

UTF-8 problems when decoding JSON data coming from Network.HTTP
by Ionut G. Stan 17 Oct '10

17 Oct '10

Hi, I'm trying to decode this JSON response: http://github.com/api/v2/json/user/show/igstan As you can see, the "name" field contains a non-latin character: ț, and it appears that Text.JSON. can't decode this response when it comes from Network.HTTP. I've tried Network.HTTP.Enumerator too, but the problem persists. Here's a simple (hopefully) reproducible test case: http://gist.github.com/630319 If you load it in ghci and call "main", you'll see that it doesn't properly show the user name. Also, calling: request "http://github.com/api/v2/json/user/show/igstan" will display the respective character encoded in a way that I have no idea whether or not is correct (Unicode is not one of my strong points for the moment). Can anyone shed some light on this problem? Which package is the culprit? Thanks, -- Ionuț G. Stan | http://igstan.ro

1 0

Golfing parsec code
by Jacek Generowicz 17 Oct '10

17 Oct '10

I'm trying to write a utility which is able to read Ratios from two distinct formats "2 / 3" -> 2 % 3 "4" -> 4 % 1 I'm sure that a skilled Haskell programmer could vastly improve on my code (do I even need Parsec?), so I've come to solicit advice. import Text.ParserCombinators.Parsec import qualified Text.ParserCombinators.Parsec.Token as P import Text.Parsec.Language (haskellDef) import Data.Ratio readFraction :: String -> Either ParseError (Ratio Integer) readFraction = parse ratio "-" lexer = P.makeTokenParser haskellDef integer = P.integer lexer natural = P.natural lexer whiteSpace = P.whiteSpace lexer ratio = try frac <|> integer' integer' = do i <- integer return $ fromInteger i frac = do whiteSpace num <- natural char '/' den <- integer return (num % den) f = readFraction " 3 / 2 " i = readFraction " 6 " main = print [f,i]

2 1

downloading GHC
by Ketil Malde 16 Oct '10

16 Oct '10

I needed GHC on a new machine, and went to download a binary tarball. First, I go to http://haskell.org/ghc/download_ghc_6_12_3.html, which kindly suggests to get the Haskell Platform instead. Then, at http://hackage.haskell.org/platform/linux.html, I'm told that I first need GHC, and pointed back to the GHC download page. I'll manage, of course - but I think this is a bit more confusing than it need be. -k -- If I haven't seen further, it is by standing in the footprints of giants

4 7

ANNOUNCE: DocTest-0.1.0
by Simon Hengel 16 Oct '10

16 Oct '10

I'm very excited to announce a new version of DocTest[1]. DocTest now uses Haddock for parsing of comments. Examples are now marked-up with Haddocks newly introduced support for "examples of interaction"[2][3]. A very basic example of usage is at [4]. DocTest is still experimental. Suggestions and patches are gladly welcome! Cheers, Simon [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/DocTest [2] http://www.haskell.org/haddock/CHANGES.txt [3] http://www.haskell.org/haddock/doc/html/ch03s08.html#id566093 [4] http://haskell.org/haskellwiki/DocTest

1 0

Ambiguous type variable
by Jacek Generowicz 16 Oct '10

16 Oct '10

-- Given a definition of view which is essentially a synonym for show: class View a where view :: a -> String instance View Int where view = show -- why does "show 2" compile, while "view 2" gives an -- 'Ambiguous type variable' error fine = view (2::Int) noProblem = show 2 ambiguousTypeVariable = view 2

4 8

IORef memory leak
by Jim Snow 16 Oct '10

16 Oct '10

I'm having some trouble with excessive memory use in a program that uses a lot of IORefs. I was able to write a much simpler program which exhibits the same sort of behavior. It appears that "modifyIORef" and "writeIORef" leak memory; perhaps they keep a reference to the old value. I tried both ghc-6.8.3 and ghc-6.10.1. Is this a known limitation, or is this a ghc bug, or am I using IORefs in the wrong way? -jim module Main where import Data.IORef import Control.Monad -- Leaks memory leakcheck1 ior = do go 1000000000 where go 0 = return () go n = do modifyIORef ior (+1) go (n-1) -- Leaks memory leakcheck2 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior writeIORef ior (x+1) go (n-1) -- Runs in constant memory leakcheck3 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior go (n-1) main :: IO () main = do ior <- newIORef 0 leakcheck2 ior compiled with: ghc -O2 --make Leak.hs -o Leak

10 17

Yi on Windows
by Peter Marks 16 Oct '10

16 Oct '10

Does anyone use Yi on Windows? I've managed to get it to build and run, but it doesn't seem to pick up a config file. This seems to be an issue identified over a year ago: http://code.google.com/p/yi-editor/issues/detail?id=269. This would seem to render the editor almost completely useless on Windows. The whole point is to have an editor you can customize in Haskell. Any suggestions appreciated. Thanks Peter

2 8

ANNOUNCE: darcs 2.5 release candidate 1
by Reinier Lamers 15 Oct '10

15 Oct '10

The darcs team would like to announce the immediate availability of darcs 2.5 release candidate 1 (also known as darcs 2.4.99.1 due to Cabal restrictions). If no blocking issues are found in the coming week, we will finally releease darcs 2.5. Important changes since darcs 2.4.4 are: * trackdown can now do binary search with the --bisect option * darcs always stores patch metadata encoded with UTF-8 * diff now supports the --index option * amend-record now supports the --ask-deps option * apply now supports the --match option * amend-record has a new --keep-date option * inventory-changing commands (like record and pull) now operate in constant time with respect to the number of patches in the repository * the push, pull, send and fetch commands no longer set the default repository by default * the --edit-description option is now on by default for the send command Changes since the last beta release are: * Fix a bug that let users add files outside the working copy directory * Fix excessive network usage when pulling from an old-fashioned repository * Make sure darcs builds on Windows with Haskell Platform 2010.2.0.0 If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.99.1.tar.gz and build it by hand as explained in the README file. Kind Regards, the darcs release manager, Reinier Lamers

1 0

Client-extensible heterogeneous types
by Jacek Generowicz 15 Oct '10

15 Oct '10

Greetings, I'm looking for dynamic dispatch on extensible sets of types. Here is a more detailed explanation of what I mean: ######################################################################## # Consider the following Python code as representative of something # you might see in Object-Orineted programs in all sorts of languages. # Create an abstract type from abc import ABCMeta, abstractmethod class Abstract: __metaclass__ = ABCMeta @abstractmethod def method(self): pass # Provide some reifications of the abstract type class Variant1(Abstract): def method(self): return "Variant 1 stuff" class Variant2(Abstract): def method(self): return "Variant 2 stuff" # Provide some utilities to process these data def heterogeneousProcessor(data): return [datum.method() for datum in data] # If you wrap all the above up in a library, clients can easily extend # it with their own new types which will still work within the # framework class ClientDefined(Abstract): def method(self): return "Client-defined stuff" heterogeneousContainer = [Variant1(), Variant2(), ClientDefined()] result = heterogeneousProcessor(heterogeneousContainer) ######################################################################## ------------------------------------------------------------------------ -- In Haskell, on the one hand, the heterogeneity is easily provided -- by algebraic data types data AbstractHeterogeneous = VariantHeterogeneous1 | VariantHeterogeneous2 methodHeterogeneous VariantHeterogeneous1 = "Variant 1 stuff" methodHeterogeneous VariantHeterogeneous2 = "Variant 2 stuff" heterogeneousProcessor dataa = [methodHeterogeneous datum | datum <- dataa] heterogeneousContainer = [VariantHeterogeneous1, VariantHeterogeneous2] resultHeterogeneous = heterogeneousProcessor heterogeneousContainer -- But in order to extend the set of variants, the client would have -- to modify the source code, changing the definiton of -- AbstractHeterogeneous and methodHeterogeneous, both of which belong -- to the library. ------------------------------------------------------------------------ -- On the other hand, the extensibility is easily provided by type -- classes -- Library code: class AbstractExtensible a where methodExtensible :: a -> String instance AbstractExtensible () where methodExtensible _ = "Variant 1 stuff" instance AbstractExtensible Char where methodExtensible _ = "Variant 2 stuff" -- Client extension: instance AbstractExtensible Int where methodExtensible _ = "Client-defined stuff" -- but in this case, there is no heterogeneity: you cannot create the -- equivalent of heterogeneousContainer above -- heterogeneousExtensibleContainer = ???? resultExtensible :: [String] resultExtensible = [methodExtensible (), methodExtensible 'a', methodExtensible (1::Int)] ------------------------------------------------------------------------ I can't see a Haskell solution which combines both of these orthogonal features without losing the benefits of the type system. (For example, I could create my own, weak, type system with tags to identify the type and maps to do the dispatch.) So my question is, what would be the Haskell approach to combining heterogeneity (as provided by variant (algebraic) types) with type-extensibility (as provided by type classes), without losing the benefits of Haskell's type system? I haven't looked into Template Haskell, but i guess that it is likely to provide a solution. But is there a solution in plain Haskell? Thanks.

9 26

dph question
by Warren Harris 15 Oct '10

15 Oct '10

I trying to learn a bit about data parallel haskell, and started from the wiki page here: http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell. Two questions: The examples express the dot product as: dotp_double xs ys = sumP [:x * <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> y | x <- xs | y <- ys:] Unless I'm missing something, shouldn't this actually be: dotp_double xs ys = sumP [:x * <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> y | x <- xs, y <- ys:] Second, when I run Main with the prescribed 10000 element array, everything seems to work quite nicely. The task takes about 2 seconds on my 4 processor x86_64, and threadscope shows all processors nicely utilized. However, when bumping this to 100000 elements, rather than taking 10x longer as I expected, the process never terminates. During one run I even lost control of my machine and needed to do a hard reset. Are there known limits to the array sizes that can be handled with dph, or can someone suggest what might be going wrong here? Thanks, Warren

3 3

All binary strings of a given length
by rgowka1 15 Oct '10

15 Oct '10

Hi - How can I generate all binary string of a given length? The type signature would something like - genbin :: Int -> [String] For example genbin 2 would give ["00","11","01","10"] and genbin 3 would give ["000","001","010","011","100","101","110","111"] etc.. thanks..

6 9

Fwd: [Haskell-cafe] Fuzzy time deltas
by Alberto G. Corona 15 Oct '10

15 Oct '10

Michael, The package Workflow has persistent timeouts (can wait for years and restart on system failure if embedded in the workflow monad, although it can run in the IO monad, with no recovery). They are composable with any action in the STM monad with orElse: flag <- getTimeoutFlag $ 5*24*60 * 60 -- wait exactly 5 days. even -- if the program restart ap <- step . atomically $ readSomewhere >> return False `orElse` waitUntilSTM flag >> return True case ap of False -> print "something received" True -> print "timeout" step" lift it from the IO to the workflow monad, and gives it persistence and recovery. without "step", it runs in the IO monad (No recovery on system failure): flag <- transientTimeout $ 5*24*60 * 60 -- wait 5 days, timeout -- restarts in case of failure ap <- atomically $ readSomewhere >> return False `orElse` waitUntilSTM flag >> return True case ap of False -> print "something received" True -> print "timeout" ------ transientTimeout t= do flag <- atomically $ newTVar False forkIO $ threadDelay(t * 1000000) >> atomically (writeTVar flag True) >> myThreadId >>= killThread return flag 2010/10/14 Michael Snoyman <michael(a)snoyman.com>: > Hey all, > > Is there a library that supports fuzzy time deltas? For example, given > two UTCTimes (or something like that) it could produce: > > 43 seconds > 13 minutes > 17 hours > 4 days > 8 months > > I want to use it for the news feature on Haskellers. It's not that > hard to write, just wondering if it's already been done. > > Michael > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

allocation for "pure" FFI functions
by Johannes Waldmann 15 Oct '10

15 Oct '10

Hi. I wonder how to do the following properly. I have one (large) C type, let's call it T, and I want to sell it as an abstract type in Haskell. I want to use C functions as if they were of type T -> T (pure function, returns a modified copy of the input) and the question is, how to do the memory allocation for that, in particular, how to avoid IO showing up in the (visible) types on the Haskell side: I don't want IO because I don't want to declare some artificial order of execution - instead I want lazy evaluation. E.g., I might have some Haskell record with a T component which may or may not be evaluated (accessed) at all. I came up with two straightforward approaches that don't work: Version 1: the C function does "malloc" on its own, and returns a Ptr. So far, this is pure (no IO). Problem: to make the "free" happen, I need to add a finalizer in Haskell, and this is IO. Version 2: allocation on the Haskell side with mallocForeignPtr. Problem: this is already in IO. Is there a solution? Workaround? Better approach? If it boils down to unsafePerformIO, then where do I put it, and what should I watch out for? J.W.

4 4

deploying applications that use plugins
by Stefan Kersten 15 Oct '10

15 Oct '10

hi all, i am working on an application that evaluates haskell code entered by the user using the plugins package [1]; now i want to deploy the application as a self-contained .app bundle on OSX, i.e. the end user should not have to install anything in addition. what do i need to bundle in order for plugins to work on a vanilla machine? i suppose i need ghc in PATH and have to ship modified versions of the package.conf files that point to the corresponding libraries within the application bundle? is there a parser for package.conf files? any pointers would be greatly appreciated! thanks, <sk> [1] http://hackage.haskell.org/package/plugins

1 0

Thunks
by Mihai Maruseac 15 Oct '10

15 Oct '10

Hi, Is there a way to determine the order in which thunks are created and expanded/evaluated in Haskell (GHC)? I'm looking mainly at some existing interface but if there is only something in the GHC source it will suffice. Thanks, -- Mihai

5 6

Re: MonadCatchIO, finally and the error monad
by oleg＠okmij.org 15 Oct '10

15 Oct '10

Michael Snoyman wrote: > I have a recommendation of how to fix this: the MonadCatchIO typeclass > should be extended to include finally, onException and everything > else. We can provide default definitions which will work for most > monads, and short-circuiting monads like ErrorT (and I imagine ContT > as well) will need to override them. It seems that `finally' can be fixed without all these proposed additions. The method catch is the only necessary method of the class. The subject of catching errors in non-IO monads has a long history, some of which is documented at http://okmij.org/ftp/Haskell/index.html#catch-MonadIO The page points out to an old CaughtMonadIO file. I have just updated it for new Exceptions, and added the final test: > test3c go = runErrorT $ go `gfinally` (liftIO $ putStrLn "sequel called") > test31 = test3c (return "return" :: ErrorT String IO String) *CaughtMonadIO> test31 sequel called Right "return" > test32 = test3c (error "error" :: ErrorT String IO String) *CaughtMonadIO> test32 sequel called *** Exception: error > test33 = test3c (throwError "throwError" :: ErrorT String IO String) *CaughtMonadIO> test33 sequel called *** Exception: ErrorException "\"throwError\"" As we can see, sequel is always called. Here is the updated file: http://okmij.org/ftp/Haskell/CaughtMonadIO.lhs Incidentally, one should be very careful of using `finally' with the continuation monad. The Cont monad lets us ``enter the room once, and exit many times''. So, finally may be called more than once. We need the ugly dynamic-wind -- or try to use less powerful monads if they suffice.

2 3