- Haskell-Cafe - Haskell.org

Binary and Serialize classes
by Evan Laforge 03 May '11

03 May '11

It's been remarked to me that relying on the Binary and Serialize classes is dangerous, because there's no guarantee they won't maintain a consistent format. So if my app uses the Serialize instances that come with cereal, it could suddenly fail to read old saves after an upgrade to cereal. However, neither binary nor cereal expose the underlying serialization algorithms for various types except through the instances, so I would have to copy and paste the code over if I want control over it. If I don't trust 'instance Serialize String' to not change behind my back, maybe I could at least trust 'Data.Serialize.stringToUtf8' to not change since if it did the name would now be wrong. Are these fears justified? I imagine if the Int instance for Serialize changed there would be an uproar and it would probably have to be changed back. I sent a bug to the maintainers of data-binary a long time ago about the Double instance not serializing -0, and they ignored it, probably because it would be bad to change the instance. So can I use the included instances without fear of them changing between releases? Of course I still run the risk of an instance from some other package changing, but I'm less likely to be using those. Speaking of that Double instance... both data-binary and cereal use decodeFloat and encodeFloat which mean they suffer from the same problems as realToFrac, namely that -0 becomes 0 and NaN becomes -Infinity (I'm not sure why the latter happens since the decoded values differ... must be a problem with encodeFloat). It's tempting to just get the ieee754 bitmap out and serialize that. I know I've seen this question around before, but is it possible to somehow cast a D# directly to bytes? I know I can write a C function and FFI that in, but it would be tidier to do it all in haskell. I guess I can probably use castPtr and memCpy, but I don't see the required addressOf. I.e. how would I write 'memcpy(buf, &d, sizeof(double));'?

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Re: [Haskell-cafe] Interfacing C++ iterators does not work as expected
by Huynh Huu Long 03 May '11

03 May '11

Big thanks, that works for me as well. Doesn't GHC check whether a type is allowed (like CInt,...) or not (like Bool,...)? 2011/5/2 Gábor Lehel <illissius(a)gmail.com>: > It works if I replace the 'bool' return type of 'finished' with int > and CInt on the C++ and Haskell side, respectively, and test for (/= > 0).

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Reformatter for Show
by Alexey Khudyakov 02 May '11

02 May '11

Hello everyone! Haskell have nice automatically derivable Show type class. It's quite useful for debugging with one exception. String representation of even moderately large value is completely unreadable (example below). My question is there any tool for reformatting result of show so it could be read by human beings? Real life example of world salad produced by show > Module (SrcLoc {srcFilename = "rec.hs", srcLine = 1, srcColumn = 1}) > (ModuleName "Main") [OptionsPragma (SrcLoc {srcFilename = "rec.hs", > srcLine = 1, srcColumn = 1}) (Just GHC) "-F -pgmF ./preprocessor "] > Nothing (Just [EVar (UnQual (Ident "main"))]) [] [DataDecl (SrcLoc > {srcFilename = "rec.hs", srcLine = 3, srcColumn = 1}) DataType [] > (Ident "Foo") [] [QualConDecl (SrcLoc {srcFilename = "rec.hs", > srcLine = 3, srcColumn = 12}) [] [] (RecDecl (Ident "Foo") [([Ident > "asd"],UnBangedTy (TyCon (UnQual (Ident "Int")))),([Ident > "fgh"],UnBangedTy (TyCon (UnQual (Ident "String"))))])] [(UnQual > ...

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versioning of Haskell packages
by Daniil Elovkov 02 May '11

02 May '11

Hello list I have an idea on versioning of Haskell packages and a small question about release model of Haskell Platform. Since the latter is shorter let's start with it. So, what is the release model of Haskell Platform? Is it released every N months and frozen at that point? Or some intermediate package/compiler updates can make the new version to appear out of schedule? Now, about package versioning. Haskell packages are flourishing and evolving at a high speed. Packages depend on other package and they evolve asynchronously. In result it is easy to end up in need of multiple versions of the same package in one program or maybe stumble upon other conflicts/problems. I'm expressing myself a little vaguely because I don't have a good example in my head right now. However, the idea is to maintain not just multi-digit version names that don't carry any semantics except that 1.1.3 is later than 1.1.2, but also somehow describe compatibility between versions. This way, even if the package A has jumped from 1.0 to 1.5 and the dependent package B has been stagnant all this time (and B depends on A 1.0), if we have information that 1.5 is compatible with 1.0, then we can safely use A 1.5 while building B. Or we could use whatever version of A is found in the system as long as its compatible with A 1.0 that B depends on. The compatibility relation can be made more fine grained and be applied not at the level of packages but that of modules, for example. This way, if we only use modules that are compatible with the version we depend on, it's fine to go with the newer version. Even if the package as a whole is not compatible. Also, when doing this at the level of modules, package authors could deliberately maintain compatibility by leaving the API of old modules intact and making them sort of wrappers to the new API which itself is not compatible with the older version. Of course, if the case when a newer version is backwards compatible is rare for most Haskell packages, then the idea doesn't make a lot of sense. But if it's common, then this could simplify building packages/programs and maintaining installed packages. What do you think? -- Daniil Elovkov

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Interfacing C++ iterators does not work as expected
by Huynh Huu Long 02 May '11

02 May '11

Hi, I tried to access C++ iterators from Haskell via the FFI. There are two example programs attached, in Haskell as well as in C++. Both utilise the same iterator wrapper example.cpp, but when running the Haskell program it prints an empty list, where it should print the contents of some vector. The C++ program meanwhile runs fine though. What does not seem to work is this part: bool finished (std::vector<int>::iterator *begin, std::vector<int>::iterator *end) { return *begin == *end; } In Haskellland, "finished :: Ptr Iterator -> Ptr Iterator -> IO Bool" always returns IO True. Is this a bug lurking in my program or is it a GHC issue? Long

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GLUT error while using Gloss
by Artyom Kazak 02 May '11

02 May '11

Hi Cafe! When I'm running examles from Gloss graphics library, all examples except gloss-hello are crashing with the following error: "GLUT: Warning in gloss-easy: GL error: gloss-easy: <stderr>: hPutChar: invalid argument (character is not in the code page)". A window with produced picture is showed for one second and then it crashes. gloss-hello, strangely, is working perfectly. I'm using Windows 7 (but on XP there is same behaviour), Haskell Platform 2011.2, OpenGL 2.4.0.1, GLUT 2.2.2.0, Gloss 1.3.1.2. At the first time, exampes didn't want to run at all, but I placed glut32.dll from http://www.xmission.com/~nate/glut.html in my .dlls folder. I tried another glut32.dll (from the Raincat game), but it didn't change anything. How can it be fixed?

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by Eran L. 02 May '11

02 May '11

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Odd "Ambiguous type variable `a0' in the constraint:" error (with Yesod)
by Mathew de Detrich 02 May '11

02 May '11

Ok so I have a problem that was described here ( http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431) in regards to returning a "Ambiguous type variable `a0' in the constraint error" when compiling. Originally I thought it was due to the way I was coding that part of the code (or to be more accurate the code specifically for creating the so called widgets with addHTML/addWidget,addHamlet). So I copied the example code given here exactly (http://www.yesodweb.com/book/example-widgets) Compiling this worked fine, so at the next point I changed the definition of getRootR to getRootR = defaultLayout $ wrapper $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|I was added with addHtml|] addHamlet [$hamlet|I was added with addHamlet|] addWidget [$hamlet|I was added with addWidget|] and then to getRootR = defaultLayout $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|I was added with addHtml|] addHamlet [$hamlet|I was added with addHamlet|] addWidget [$hamlet|I was added with addWidget|] Both times compiled fine, so the issue wasn't what I originally thought that it was (as described in http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431) The problem is, that when I use the above example code in my WebServer, I get this Ambigious type error when compiling (even though I have set up the route the exact same way). Unfortunatley I can't provide the code for my webserver, since its commercially owned (and it would be pointless because its just renamed variables, but otherwise its the same), so does anyone have any ideas what could possibly cause such an error (such as a missing/extra import or some package or something), or possibly some missing instances? Also sorry for creating another mailing list, but its a different issue then what I thought it was originally (and I also wanted to put it on haskell-cafe since its a more general issue)

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Links into list
by John Sneer 02 May '11

02 May '11

Hello all, I have rather strange question. I wonder whether there is an obvious solution or not in Haskell. I tried but without defining quite ugly tree-like structures and incorporating IORefs it seems to me like impossible. Note: I use ASCII art to explain. let l be a Haskell list +-+-+-------------------------------------+ l = | | | .... | +-+-+-------------------------------------+ let li be another Haskell list +-+-+-+ li = |a|b|c| +-+-+-+ let hi be a hook/pointer list such, that it links some/all members of the list li +--+--+--+ hi = |pa|pb|pc| +--+--+--+ | | | V V V +--+--+--+ li = |a |b |c | +--+--+--+ When I insert li into l, I would like to get something like: +--+--+--+ hi = |pa|pb|pc| +--+--+--+ | | | V V V +-+-+----------+--+--+--+---------------------------+ l' = | | | .... |a |b |c | | +-+-+----------+--+--+--+---------------------------+ where l' is still a Haskell list. Now, I would like to modify, directly via pb the list l' to have: +--+--+--+ hi = |pa|pb|pc| +--+--+--+ | | +-------+ V V V +-+-+----------+--+--+-+-+-+-+--+---------------------------+ le = | | | .... |a |b |q|w|e|r|c | | +-+-+----------+--+--+-+-+-+-+--+---------------------------+ where le is a Haskell list, but, in fact accessible through the same ID/reference all the time. Simply: I would like to have direct access into several places in a very long list. Moreover, the efficiency and features of Haskell list and all the functions around it are to be used as well (not all, but many, nevertheless, I could make them new, if I can achieve good performance, not a big trouble). Is it possible, or not? Could you possibly point me to some resource I can get more information about such "tricks". Thanks John -- John Sneer johnsneer(a)operamail.com -- http://www.fastmail.fm - A no graphics, no pop-ups email service

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Re: [Haskell-cafe] Links into list
by Ivan Lazar Miljenovic 02 May '11

02 May '11

Re-CCing the list. On 2 May 2011 11:33, John Sneer <johnsneer(a)operamail.com> wrote: > I took a look at > http://cvs.haskell.org/Hugs/pages/libraries/base/Data-Sequence.html > but it doesn't seem to be helpful - cannot insert sequence into the > sequence, just > modification of a single element is supported. Moreover, indexing via > Ints would > require extra recalculation when something is somewhere inserted. > > So, either it is not the solution or I have not understood the Sequence > well. :-( Well, you can split the list (probably using splitAt) and then insert another one in between (using ><). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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