May 2011 - Haskell-Cafe

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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hello:
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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How to make ghc 7 with llvm?
by Magicloud Magiclouds 02 May '11

02 May '11

Hi, As I recalled, ghc started to support llvm from version 7. But there is a problem: there is no option to make ghc with llvm. So Library within ghc source will be in gcc's binary format. Then when I install other packages, they may complain that the binary format is not llvm, so they install some libraries again. Any way I could make ghc 7 with llvm? -- 竹密岂妨流水过山高哪阻野云飞

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Posix IO & Unicode lookup exception
by Lee Pike 01 May '11

01 May '11

Hi, I wanted to see if anyone had an idea about why the following program results in an exception. The issue seems to be related to stdin to a POSIX file descriptors for a subprocess, and GHC's Unicode encoding lookup. I've tried to make the program as simple as possible to make the issue obvious. Consider: > module Pipe where > > import System.Process > import System.IO > import System.Posix.IO > > main = sequence_ $ replicate 100000 f > > f = do > (Just hin, _, _, prc) <- > createProcess (shell "./pipe_c.out") { std_in = CreatePipe > , std_out = UseHandle stdout > , std_err = UseHandle stderr} > > pin <- handleToFd hin > _ <- fdWrite pin ("h\n") > closeFd pin > waitForProcess prc and a trivial C program to read in the data (but doing nothing with it): --------------------------------------- #include <stdio.h> #define SIZE 10 int main(void) { char in[SIZE]; if(fgets(in, SIZE , stdin) != NULL); return 0; } ---------------------------------------- and executing > gcc -Wall -o pipe_c.out pipe_c.c > ghci Pipe.hs > main ... *** Exception: malloc: resource exhausted (out of memory) iconvOpen ("UTF-8","UTF-8"): resource exhausted (Cannot allocate memory) My best guess is that the GHC is calling iconv_open() <http://www.kernel.org/doc/man-pages/online/pages/man3/iconv_open.3.html>, but not making a subsequent call to iconv_close() <http://www.kernel.org/doc/man-pages/online/pages/man3/iconv_close.3.html>. (Actually, my best guess is I'm making a silly mistake, but I'm having trouble seeing it. :) ) I am running 2.6.35-28-generic (Ubuntu) and GHC 7.0.2, but I've observed the same issues with GHC 6.12.3. Lee

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Wai and http-enumerator not as lazy as I'd like
by Erik de Castro Lopo 01 May '11

01 May '11

Hi all, I'm using Wai and http-enumerator to build a http proxy. The core of the code looks like this: import qualified Network.HTTP.Enumerator as HE import qualified Network.Wai as Wai serveRequest :: forall (m :: * -> *). (MonadControlIO m, Failure HE.HttpException m) => HE.Request m -> m Wai.Response serveRequest request = do HE.Response sc rh bs <- HE.withManager $ HE.httpLbsRedirect request return $ Wai.responseLBS (mkStatus sc) rh bs This works but does not run in constant space as I would have hoped. The thing is, HE.httpLbsRedirect returns a lazy ByteString and Wai.responseLBS writes a lazy ByteString, so why isn't the whole thing lazy? I'd appreciate any clues. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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want to start a page of Haskell examples to go with Skiena's book "The Algorithm Design Manual" and also tie in Bird's equational reasoning for functional programming; ...
by caseyh＠istar.ca 01 May '11

01 May '11

I want to start a page of Haskell examples to go with Skiena's book "The Algorithm Design Manual" and also tie in Bird's equational reasoning for functional programming; where is a good place for such examples.

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