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  • Need help manipulating WAV (RIFF) Files at a byte level

    - by Eric
    I'm writing an an application in C# that will record audio files (*.wav) and automatically tag and name them. Wave files are RIFF files (like AVI) which can contain meta data chunks in addition to the waveform data chunks. So now I'm trying to figure out how to read and write the RIFF meta data to and from recorded wave files. I'm using NAudio for recording the files, and asked on their forums as well on SO for way to read and write RIFF tags. While I received a number of good answers, none of the solutions allowed for reading and writing RIFF chunks as easily as I would like. But more importantly I have very little experience dealing with files at a byte level, and think this could be a good opportunity to learn. So now I want to try writing my own class(es) that can read in a RIFF file and allow meta data to be read, and written from the file. I've used streams in C#, but always with the entire stream at once. So now I'm little lost that I have to consider a file byte by byte. Specifically how would I go about removing or inserting bytes to and from the middle of a file? I've tried reading a file through a FileStream into a byte array (byte[]) as shown in the code below. System.IO.FileStream waveFileStream = System.IO.File.OpenRead(@"C:\sound.wav"); byte[] waveBytes = new byte[waveFileStream.Length]; waveFileStream.Read(waveBytes, 0, waveBytes.Length); And I could see through the Visual Studio debugger that the first four byte are the RIFF header of the file. But arrays are a pain to deal with when performing actions that change their size like inserting or removing values. So I was thinking I could then to the byte[] into a List like this. List<byte> list = waveBytes.ToList<byte>(); Which would make any manipulation of the file byte by byte a whole lot easier, but I'm worried I might be missing something like a class in the System.IO name-space that would make all this even easier. Am I on the right track, or is there a better way to do this? I should also mention that I'm not hugely concerned with performance, and would prefer not to deal with pointers or unsafe code blocks like this guy. If it helps at all here is a good article on the RIFF/WAV file format.

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  • What is node.js?

    - by Jeffrey
    I don't fully get what node.js is all about. Maybe it's because I am mainly a web based business app developer. Can someone please explain what it is and the use of it? Thanks. My understanding so far is that: The programming model is event driven, especially the way it handles IO. It uses javascript and the parser is V8. It can be easily used to create concurrent server apps. Are my understandings correct? If yes, then what are the benefits of evented IO, is it just more for the concurrency stuffs? Also is the direction of node.js to become a framework like, javascript based (v8 based) programming model?

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  • Package management fails in update-manager with gzip problems and compilation errors. U12.04LTS

    - by HarveyP
    Similar to but not the same as Package management system corrupted. Cannot install or remove packages. U12.04LTS (an earlier problem) with package management system. Followed all of L. D. James suggestions in his answer to no avail. This time as well as the gzip error I am also getting compilation errors. The difference may be due to a lack of compilation in my earlier problem so it may be the same error. The packages concerned are enumerated in the output from update-manager below. Also included below that is the output from apt-get -f install apt-get autoremove gives same output. Tried update without SSL updates - 9 to install and got "Unhandled Error in aptdaemon". Output number 3 below. One at a time - output 4 - is for firefox, first in the list of packages. Falls over at libssl1.0.0 despite deselection of it from update ... Tried apt-get install --reinstall dpkg which succeeded, apt-get install --reinstall tar and apt-get install --reinstall gzip both of which failed at libssl1.0.0 as ever. (as suggested by Subv3rsion elsewhere in this forum) Now cannot apt-get update with complete success even after changing server and apt-get clean - output number 5 below ... 1). Output from update-manager The following packages will be upgraded:<> firefox firefox-globalmenu firefox-locale-en libavcodec-extra-53 libavformat53 libavutil-extra-51 libjson0 libpostproc52 libssl1.0.0 libswscale2 openssl 11 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.<br> Need to get 0 B/46.5 MB of archives. After this operation, 1,416 kB of additional disk space will be used.<br> Do you want to continue [Y/n]? y debconf: Perl may be unconfigured (Bareword "gensym" not allowed while "strict subs" in use at /usr/lib/perl/5.14/IO/Handle.pm line 67. BEGIN not safe after errors--compilation aborted at /usr/lib/perl/5.14/IO/Handle.pm line 366. Compilation failed in require at /usr/lib/perl/5.14/IO/Seekable.pm line 9. BEGIN failed--compilation aborted at /usr/lib/perl/5.14/IO/Seekable.pm line 9. Compilation failed in require at /usr/lib/perl/5.14/IO/File.pm line 11. BEGIN failed--compilation aborted at /usr/lib/perl/5.14/IO/File.pm line 11. Compilation failed in require at /usr/share/perl/5.14/FileHandle.pm line 9. Compilation failed in require at (eval 1) line 3. BEGIN failed--compilation aborted at (eval 1) line 3. ) -- aborting (Reading database ... 160575 files and directories currently installed.) Preparing to replace libssl1.0.0 1.0.1-4ubuntu5.14 (using .../libssl1.0.0_1.0.1-4ubuntu5.15_i386.deb) ... Unpacking replacement libssl1.0.0 ... dpkg-deb (subprocess): data: internal gzip read error: '<fd:4>: data error' dpkg-deb: error: subprocess <decompress> returned error exit status 2 dpkg: error processing /var/cache/apt/archives/libssl1.0.0_1.0.1-4ubuntu5.15_i386.deb (--unpack):<br> subprocess dpkg-deb --fsys-tarfile returned error exit status 2 No apport report written because MaxReports has already been reached Bareword "gensym" not allowed while "strict subs" in use at /usr/lib/perl/5.14/IO/Handle.pm line 67. BEGIN not safe after errors--compilation aborted at /usr/lib/perl/5.14/IO/Handle.pm line 366. Compilation failed in require at /usr/lib/perl/5.14/IO/Seekable.pm line 9. BEGIN failed--compilation aborted at /usr/lib/perl/5.14/IO/Seekable.pm line 9. Compilation failed in require at /usr/lib/perl/5.14/IO/File.pm line 11. BEGIN failed--compilation aborted at /usr/lib/perl/5.14/IO/File.pm line 11. Compilation failed in require at /usr/share/perl/5.14/FileHandle.pm line 9. Compilation failed in require at /usr/share/perl5/Debconf/Template.pm line 8. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Template.pm line 8. Compilation failed in require at /usr/share/perl5/Debconf/Question.pm line 8. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Question.pm line 8. Compilation failed in require at /usr/share/perl5/Debconf/Config.pm line 7. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Config.pm line 7. Compilation failed in require at /usr/share/perl5/Debconf/Log.pm line 10. Compilation failed in require at /usr/share/perl5/Debconf/Db.pm line 7. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Db.pm line 7. Compilation failed in require at /usr/share/debconf/frontend line 6. BEGIN failed--compilation aborted at /usr/share/debconf/frontend line 6. dpkg: error whale cleanang up: subprgcess installed post-installation script returned error exit status 2 Errors were encountered while processing: /var/cache/apt/archives/libssl1.0.0_1.0.1-4ubuntu5.15_i386.deb E: Sub-process /usr/bin/dpkg returned an error code (1) 2). Output from install -f harveyp@harveyp:~$ sudo apt-get -f install [sudo] password for harveyp: Reading package lists... Done Building dependency tree Reading state information... Done 0 to upgrade, 0 to newly install, 0 to remove and 11 not to upgrade. 1 not fully installed or removed.<br> After this operation, 0 B of additional disk space will be used. E: Internal Error, No file name for libssl1.0.0 3). Unhandled error from aptdaemon Traceback (most recent call last): File "/usr/lib/python2.7/dist-packages/aptdaemon/worker.py", line 1045, in _simulate trans.unauthenticated = self.__simulate(trans) File "/usr/lib/python2.7/dist-packages/aptdaemon/worker.py", line 1160, in __simulate unauthenticated = self._get_unauthenticated() File "/usr/lib/python2.7/dist-packages/aptdaemon/worker.py", line 347, in _get_unauthenticated for pkg in self._iterate_packages(): File "/usr/lib/python2.7/dist-packages/aptdaemon/worker.py", line 1356, in _iterate_packages for enum, pkg in enumerate(self._cache): File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 216, in __iter__ yield self[pkgname] File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 201, in __getitem__ pkg = self._weakref[key] = Package(self, self._cache[key]) KeyError: 'librqrcode-rubq-doc 4). output from update of firefox installArchives() failed: Error in function: < Setting up libssl1.0.0 (1.0.1-4ubuntu5.14) ... Bareword "gensym" not allowed while "strict subs" in use at /usr/lib/perl/5.14/IO/Handle.pm line 67. BEGIN not safe after errors--compilation aborted at /usr/lib/perl/5.14/IO/Handle.pm line 366. Compilation failed in require at /usr/lib/perl/5.14/IO/Seekable.pm line 9. BEGIN failed--compilation aborted at /usr/lib/perl/5.14/IO/Seekable.pm line 9. Compilation failed in require at /usr/lib/perl/5.14/IO/File.pm line 11. BEGIN failed--compilation aborted at /usr/lib/perl/5.14/IO/File.pm line 11. Compilation failed in require at /usr/share/perl/5.14/FileHandle.pm line 9. Compilation failed in require at /usr/share/perl5/Debconf/Template.pm line 8. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Template.pm line 8. Compilation failed in require at /usr/share/perl5/Debconf/Question.pm line 8. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Question.pm line 8. Compilation failed in require at /usr/share/perl5/Debconf/Config.pm line 7. BEGIN failed--compilation aborted at /usr/share/perl5/Debconf/Config.pm line 7. Compilation failed in require at /usr/share/perl5/Debconf/Log.pm line 10. 5. output from apt-get update ...snip ... Hit http://ubuntu-archive.mirrors.free.org precise-security/multiverse Translation-en Hit http://ubuntu-archive.mirrors.free.org precise-security/restricted Translation-en Hit http://ubuntu-archive.mirrors.free.org precise-security/universe Translation-en Fetched 368 kB in 6s (59.5 kB/s) W: Failed to fetch gzip:/var/lib/apt/lists/partial/ubuntu-archive.mirrors.free.org_ubuntu_dists_precise_universe_source_Sources Hash Sum mismatch E: Some index files failed to download. They have been ignored, or old ones used instead.

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  • aio_write on linux with rtkaio is sometimes long

    - by Drakosha
    I'm using async io on linux with rtkaio library. In my tests everything works perfectly, but, in my real application i see that aio_write which is supposed to return very fast, is very slow. It can take more than 100 milis to write a 128KB to a O_DIRECT padded file. Both my test and the application use same I/O size, i check on the same file system (GFS). I added counting and i see that there are about 50% of async io operations that are short (shorter then 2 milis) and 50% that are long (longer than 2 milis). I also checked that the test and the application both use the same rtkaio library. I'm pretty lost, anyone any ideas where should i look? Another my related question: http://stackoverflow.com/questions/1799537/proc-sys-fs-aio-nr-is-never-higher-than-1024-aio-on-linux

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  • Drawbacks with reading and writing a big file to or from disk at once instead of small chunks?

    - by Johann Gerell
    I work mainly on Windows and Windows CE based systems, where CreateFile, ReadFile and WriteFile are the work horses, no matter if I'm in native Win32 land or in managed .Net land. I have so far never had any obvious problem writing or reading big files in one chunk, as opposed to looping until several smaller chunks are processed. I usually delegate the IO work to a background thread that notifies me when it's done. But looking at file IO tutorials or "textbook examples", I often find the "loop with small chunks" used without any explanation of why it's used instead of the more obvious (I dare to say!) "do it all at once". Are there any drawbacks to the way I do that I haven't understood?

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  • Java: opening and reading from a file without locking it.

    - by rogue780
    I need to be able to mimic 'tail -f' with Java. I'm trying to read a log file as it's being written by another process, but when I open the file to read it, it locks the file and the other process can't write to it anymore. Any help would be greatly appreciated! Here is the code that I'm using currently: public void read(){ Scanner fp = null; try{ fp = new Scanner(new FileReader(this.filename)); fp.useDelimiter("\n"); }catch(java.io.FileNotFoundException e){ System.out.println("java.io.FileNotFoundException e"); } while(true){ if(fp.hasNext()){ this.parse(fp.next()); } } }

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  • Ruby: Read large data from stdout and stderr of an external process on Windows

    - by BinaryMuse
    Greetings, all, I need to run a potentially long-running process from Ruby on Windows and subsequently capture and parse the data from the external process's standard output and error. A large amount of data can be sent to each, but I am only necessarily interested in one line at a time (not capturing and storing the whole of the output). After a bit of research, I found that the Open3 class would take care of executing the process and giving me IO objects connected to the process's standard output and error (via popen3). Open3.popen3("external-program.bat") do |stdin, out, err, thread| # Step3.profit() ? end However, I'm not sure how to continually read from both streams without blocking the program. Since calling IO#readlines on out or err when a lot of data has been sent results in a memory allocation error, I'm trying to continuously check both streams for available input, but not having much luck with any of my implementations. Thanks in advance for any advice!

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  • Hard drive write speed - finding a lighter antivirus?

    - by Shingetsu
    I recently have been getting a lot of system lag here (for example, the mouse and the display in general take about 15 seconds to react in the worst cases). After a lot of monitoring the resources, I found that the problem mainly happens when too much Disk I/O is being done. Three culprits have been identified: My browser had the highest write I/O with 35,000,000 I/O Write Bytes. Steam had the highest read I/O (when IDLE!!!) with 106,000,000 I/O Read Bytes. My antivirus (in both cases I will soon mention) was the runner up in both cases with: 30,000,000ish write and 80,000,000ish read. The first AV I had was Avast! which I had liked on my previous system. After noticing it taking so much I/O I switched to Panda (supposing it wouldn't use TOO much during idle phase). However it only used a bit less I/O. Just a lot less memory and cpu and somewhat more network. My browser at the moment is Maxthon 3 (which I like a lot). Before this I was running chrome which had similar data and much higher cpu when running in the background was enabled. I'm not going to be running steam all the time and there aren't many alternatives to it. I like my browser very much, but I AM willing to switch if there's an obvious problem (I'm in programming, however I'm not a very good sysadmin, especially not when it comes to windows). Finally, my system almost stops lagging when I turn off the antivirus (and preferably steam) (some remains but once in every 5-6 hours for a few seconds so it isn't a big problem). My question (has a few parts): Is it possible to configure steam to lower it's I/O usage? (and maybe network while we're at it?) Which antivirus (very preferably free) uses lowest I/O while idle (I leave PC alone during active scans so that isn't a problem). Is there an obvious problem with my current browser and, if so, is there a way to fix it or should I switch and, if so, to what? (P.S. I've been on FFox for some time too). Info on system: Windows 7 (32 bit T_T, I am getting a new one in a few months but I want to keep using system during that time though). Hard Drive (main) is a Raid0. (Also have an external 1TB one which contains steam (and steam alone). As such it doesn't get used by much anything other than steam and isn't a very large problem. However steam still uses some I/O of registry) CPU: Intel(R) Core(TM)2 CPU [email protected] RAM: 6GB (3.25GB usable) (this and CPU have little effect as shown in next section) Additional info: Memory usage during problematic times: 44% CPU usage during problematic times: 35% Page File: main drive: system managed. 1TB drive: none. The current system I'm using is about 6 years old and is mainly a place holder while I await the new one in a few months. Final words: this is my 1st post on Super User (this question wouldn't feel right on Stack Overflow where I usually stay). If it doesn't have it's place here please tell me. If anything is wrong with it, same. Edit Technically I'm looking for a live thread detection program with minimal IO usage. I already have good active scan capability: Kaspersky (the free scanner uses the paid database) and MalwareBytes. Edit 2 Noticed another one, it seems that windows media player has been using stuff even when off! Turning it off and restarting now. If the problem is fixed I'll tell you guys. The reason I didn't notice it before was because I didn't have resource manager in front of me at the MOMENT of the problem. Now I did and it was at the very top of the list!

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  • How to close IOs?

    - by blackdog
    when i managed IO, i found a problem. i used to close it like this: try { // my code } catch (Exception e) { // my code } finally{ if (is != null) { is.close(); } } but the close method also would throw exception. if i have more than one IO, i have to close all of them. so the code maybe like this: try { // my code } catch (Exception e) { // my code } finally{ if (is1 != null) { is1.close(); } if(is2 != null{ is2.close(); } // many IOs } if is1.close() throws an exception, is2, is3 would not close itself. So i have to type many try-catch-finally to control them. is there other way to solve the problem?

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  • many1 no longer works with Parsec 3.x

    - by Zak
    After updating to Parsec 3.1 from 2.x, code using many1, such as word = many1 letter fails with No instance for (Stream s m Char) arising from a use of `letter' I found a mailing list post claiming that adding {-#LANGUAGE NoMonomorphismRestriction #-} to the top of the source file would solve the problem, but it did not.

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  • Parsec: backtracking not working

    - by Nathan Sanders
    I am trying to parse F# type syntax. I started writing an [F]Parsec grammar and ran into problems, so I simplified the grammar down to this: type ::= identifier | type -> type identifier ::= [A-Za-z0-9.`]+ After running into problems with FParsec, I switched to Parsec, since I have a full chapter of a book dedicated to explaining it. My code for this grammar is typeP = choice [identP, arrowP] identP = do id <- many1 (digit <|> letter <|> char '.' <|> char '`') -- more complicated code here later return id arrowP = do domain <- typeP string "->" range <- typeP return $ "("++domain++" -> "++range++")" run = parse (do t <- typeP eof return t) "F# type syntax" The problem is that Parsec doesn't backtrack by default, so > run "int" Right "int" -- works! > run "int->int" Left "F# type syntax" unexpected "-" expecting digit, letter, ".", "`" or end of input -- doesn't work! The first thing I tried was to reorder typeP: typeP = choice [arrowP, identP] But this just stack overflows because the grammar is left-recursive--typeP never gets to trying identP because it keeps trying arrowP over and over. Next I tried try in various places, for example: typeP = choice [try identP, arrowP] But nothing I do seems to change the basic behaviours of (1) stack overflow or (2) non-recognition of "-" following an identifier. My mistake is probably obvious to anybody who has successfully written a Parsec grammar. Can somebody point it out?

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  • Type error while trying to implement the (>>=) function in order to create a custom monad transforme

    - by CharlieP
    Hello, I'm trying to create a monad transformer for a future project, but unfortunately, my implementation of the Monad typeclasse's (=) function doesn't work. First of all, here is the underlying monad's implementation : newtype Runtime a = R { unR :: State EInfo a } deriving (Monad) Here, the implementation of the Monad typeclasse is done automatically by GHC (using the GeneralizedNewtypeDeriving language pragma). The monad transformer is defined as so : newtype RuntimeT m a = RuntimeT { runRuntimeT :: m (Runtime a) } The problem comes from the way I instanciate the (=) function of the Monad typeclasse : instance (Monad m) => Monad (RuntimeT m) where return a = RuntimeT $ (return . return) a x >>= f = runRuntimeT x >>= id >>= f The way I see it, the first >>= runs in the underlying m monad. Thus, runRuntimeT x >>= returns a value of type Runtime a (right ?). Then, the following code, id >>=, should return a value of type a. This value is the passed on to the function f of type f :: (Monad m) => a -> RuntimeT m b. And here comes the type problem : the f function's type doesn't match the type required by the (=) function. Jow can I make this coherent ? I can see why this doesn't work, but I can't manage to turn it into something functionnal. Thank you for you help, and do not hesitate to correct any flaws in my message, Charlie P.

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  • Removing the contents of a Chan or MVar in a single discrete step

    - by Bill
    I'm writing a discrete simulation where request values from multiple threads accumulate in a centralized queue. Every n milliseconds, a manager wakes up to process requests. When the manager wakes up, it should retrieve all of the contents of the central queue in a single discrete step. While processing these, any client threads attempting to submit to the queue should block. When processing completes, the queue reopens and the manager goes back to sleep. What's the best way to do this? The retry behavior of STM isn't really what I want. If I use a Chan or MVar, there's no way to prevent clients from enqueuing additional requests during processing. One approach is to use an MVar as a mutex on a Chan holding the queue. Are there other ways to do this?

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  • I'm working on Peano Axioms in Agda and I've hit a bit of a sticking point

    - by Schroedinger
    PA6 : ?{m n} -> m = n -> n = m is the axiom I am trying to solve and support, I've tried using a cong (from the core library) but am having troubles with the cong constructor PA6 = cong gets me nowhere, I know for cong I am required to supply a refl for equality and a type, but I'm, not sure what type I'm supposed to supply. Ideas? This is for a small assignment at University, so I'd rather someone demonstrate what I've missed rather than write the acutual answer, but I'd appreciate any degree of support.

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  • How to Convert Type in Tuples

    - by Pradeep
    how to convert a String type to a Int i have a tuple and i want to convert it to a tuple which has different types tupletotuple :: (String,String,String) ->(String,Int,Int) tupletotuple (a,b,c) = (a,read(b),read(c)) i get this Error Msg Project tupletotuple ("cha",4,3) ERROR - Cannot infer instance * Instance : Num [Char] * Expression : tupletotuple ("cha",4,3)

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  • How does 'lazy' work?

    - by Matt Fenwick
    What is the difference between these two functions? I see that lazy is intended to be lazy, but I don't understand how that is accomplished. -- | Identity function. id :: a -> a id x = x -- | The call '(lazy e)' means the same as 'e', but 'lazy' has a -- magical strictness property: it is lazy in its first argument, -- even though its semantics is strict. lazy :: a -> a lazy x = x -- Implementation note: its strictness and unfolding are over-ridden -- by the definition in MkId.lhs; in both cases to nothing at all. -- That way, 'lazy' does not get inlined, and the strictness analyser -- sees it as lazy. Then the worker/wrapper phase inlines it. -- Result: happiness Tracking down the note in MkId.lhs (hopefully this is the right note and version, sorry if it's not): Note [lazyId magic] ~~~~~~~~~~~~~~~~~~~ lazy :: forall a?. a? -> a? (i.e. works for unboxed types too) Used to lazify pseq: pseq a b = a `seq` lazy b Also, no strictness: by being a built-in Id, all the info about lazyId comes from here, not from GHC.Base.hi. This is important, because the strictness analyser will spot it as strict! Also no unfolding in lazyId: it gets "inlined" by a HACK in CorePrep. It's very important to do this inlining after unfoldings are exposed in the interface file. Otherwise, the unfolding for (say) pseq in the interface file will not mention 'lazy', so if we inline 'pseq' we'll totally miss the very thing that 'lazy' was there for in the first place. See Trac #3259 for a real world example. lazyId is defined in GHC.Base, so we don't have to inline it. If it appears un-applied, we'll end up just calling it. I don't understand that because it refers to lazyId instead of lazy. How does lazy work?

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  • Getting Cabal to work with GHC 6.12.1

    - by Dan Dyer
    I've installed the latest GHC package (6.12.1) on OS X, but I can't get Cabal to work. I've removed the version I had previously that worked with GHC 6.10 and tried to re-install from scratch. The latest Cabal version available for download is 1.6.0.2. However, when I try to build this I get the following error: Configuring Cabal-1.6.0.2... Setup: failed to parse output of 'ghc-pkg dump' From what I've found searching, this seems to suggest that the version of Cabal is too old for the version of GHC. Is there any way to get Cabal to work with GHC 6.12.1 yet? EDIT: To be clear, I'm trying to set-up cabal-install.

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  • "Ambigous type variable" error when defining custom "read" function

    - by Tener
    While trying to compile the following code, which is enhanced version of read build on readMay from Safe package. readI :: (Typeable a, Read a) => String -> a readI str = case readMay str of Just x -> x Nothing -> error ("Prelude.read failed, expected type: " ++ (show (typeOf > (undefined :: a))) ++ "String was: " ++ str) I get an error from GHC: WavefrontSimple.hs:54:81: Ambiguous type variable `a' in the constraint: `Typeable a' arising from a use of `typeOf' at src/WavefrontSimple.hs:54:81-103 Probable fix: add a type signature that fixes these type variable(s)` I don't understand why. What should be fixed to get what I meant? EDIT: Ok, so the solution to use ScopedTypeVariables and forall a in type signature works. But why the following produces very similar error to the one above? The compiler should infer the right type since there is asTypeOf :: a -> a -> a used. readI :: (Typeable a, Read a) => String -> a readI str = let xx = undefined in case readMay str of Just x -> x `asTypeOf` xx Nothing -> error ("Prelude.read failed, expected type: " ++ (show (typeOf xx)) ++ "String was: " ++ str)

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  • Continuation monad "interface"

    - by sdcvvc
    The state monad "interface" class MonadState s m where get :: m s put :: s -> m () (+ return and bind) allows to construct any possible computation with State monad without using State constructor. For example, State $ \s -> (s+1, s-1) can be written as do s <- get put (s-1) return (s+1) Similarily, I never have to use Reader constructor, because I can create that computation using ask, return and (>>=). Precisely: Reader f == ask >>= return . f. Is it the same true for continuations - is it possible to write all instances of Cont r a using callCC (the only function in MonadCont), return and bind, and never type something like Cont (\c -> ...)?

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  • Confusion regarding laziness

    - by Abhinav Kaushik
    I have a function myLength = foldl (\ x _ -> x + 1) 0 which fails with stack overflow with input around 10^6 elements (myLength [1..1000000] fails). I believe that is due to the thunk build up since when I replace foldl with foldl', it works. So far so good. But now I have another function to reverse a list : myReverse = foldl (\ acc x -> x : acc) [] which uses the lazy version foldl (instead of foldl') When I do myLength . myReverse $ [1.1000000]. This time it works fine. I fail to understand why foldl works for the later case and not for former?

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  • "Pattern matching" of algebraic type data constructors

    - by jetxee
    Let's consider a data type with many constructors: data T = Alpha Int | Beta Int | Gamma Int Int | Delta Int I want to write a function to check if two values are produced with the same constructor: sameK (Alpha _) (Alpha _) = True sameK (Beta _) (Beta _) = True sameK (Gamma _ _) (Gamma _ _) = True sameK _ _ = False Maintaining sameK is not much fun, it is potentially buggy. For example, when new constructors are added to T, it's easy to forget to update sameK. I omitted one line to give an example: -- it’s easy to forget: -- sameK (Delta _) (Delta _) = True The question is how to avoid boilerplate in sameK? Or how to make sure it checks for all T constructors? The workaround I found is to use separate data types for each of the constructors, deriving Data.Typeable, and declaring a common type class, but I don't like this solution, because it is much less readable and otherwise just a simple algebraic type works for me: {-# LANGUAGE DeriveDataTypeable #-} import Data.Typeable class Tlike t where value :: t -> t value = id data Alpha = Alpha Int deriving Typeable data Beta = Beta Int deriving Typeable data Gamma = Gamma Int Int deriving Typeable data Delta = Delta Int deriving Typeable instance Tlike Alpha instance Tlike Beta instance Tlike Gamma instance Tlike Delta sameK :: (Tlike t, Typeable t, Tlike t', Typeable t') => t -> t' -> Bool sameK a b = typeOf a == typeOf b

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  • hackage package dependencies and future-proof libraries

    - by yairchu
    In the dependencies section of a cabal file: Build-Depends: base >= 3 && < 5, transformers >= 0.2.0 Should I be doing something like Build-Depends: base >= 3 && < 5, transformers >= 0.2.0 && < 0.3.0 (putting upper limits on versions of packages I depend on) or not? I'll use a real example: my "List" package on Hackage (List monad transformer and class) If I don't put the limit - my package could break by a change in "transformers" If I do put the limit - a user that uses "transformers" but is using a newer version of it will not be able to use lift and liftIO with ListT because it's only an instance of these classes of transformers-0.2.x I guess that applications should always put upper limits so that they never break, so this question is only about libraries: Shall I use the upper version limit on dependencies or not?

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  • Examples of attoparsec in parsing binary file formats?

    - by me2
    Previously attoparsec was suggested to me for parsing complex binary file formats. While I can find examples of attoparsec parsing HTTP, which is essentially text based, I cannot find an example parsing actual binary, for example, a TCP packet, or image file, or mp3. Can someone post some code or pointer to some code which does this using attoparsec?

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