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Re: [Haskell-cafe] c2hs and cabal?
by Magnus Therning 29 Nov '06

29 Nov '06

On Tue, Nov 28, 2006 at 11:11:30 +0000, Duncan Coutts wrote: >On Tue, 2006-11-28 at 10:56 +0000, Magnus Therning wrote: >> On Tue, Nov 28, 2006 at 00:33:42 +0000, Duncan Coutts wrote: >> >On Mon, 2006-11-27 at 22:26 +0000, Magnus Therning wrote: >> >> Can I use cabal to build packages that incorporates C libraries and FFI >> >> Haskell created using c2hs? >> > >> >Yes you can. Cabal understands a bit about .chs files. I think all you >> >need to do is specify the module in the exposed-modules or >> >other-modules and Cabal will find the .chs file and try to build a .hs >> >file from it. > >> I'm still having some problems though :-( I thought I'd start with a >> minimal, rather silly, test to see if I can get Cabal to do what I want. >> I've stuck what I have at >> http://www.therning.org/magnus_files/cnh.tar.gz >> >> It fails to build for me with the following error: >> >> Preprocessing executables for cnh-0.1... >> Building cnh-0.1... >> [1 of 2] Compiling Foo ( src/Foo.hs, dist/build/cnh/cnh-tmp/Foo.o ) >> src/Foo.chs:6:8: parse error on input `import' >> >> I tried using '-v5' but it didn't really offer anymore insight for a >> novice like me :( > >Check the generated Foo.hs file to see if it looks obviously bogus. Can't really see anything obviously bogus about the following: % cat Foo.chs module Foo where #include "foo.h" fooOne i = {# call foo1 #} i % cat Foo.hs -- GENERATED by C->Haskell Compiler, version 0.14.5 Travelling Lightly, 12 Dec 2005 (Haskell) -- Edit the ORIGNAL .chs file instead! {-# LINE 1 "src/Foo.chs" #-}module Foo where fooOne i = foo1 i foreign import ccall safe "src/Foo.h foo1" foo1 :: (CInt -> (IO CInt)) Or am I missing something? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. "There are of course many problems connected with life, of which some of the most popular are `Why are people born?' `Why do they die?' `Why do they spend so much of the intervening time wearing digital watches?'" -- The Book.

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Compatibility between Data.ByteString.Base and Data.ByteString.Lazy
by Neil Bartlett 29 Nov '06

29 Nov '06

Hi, Firstly my apologies if this is an outrageously newbie question. I am trying to write a binary protocol parser using Data.ByteString. I have created a module "ByteParser" containing my parsing utilities, which imports ByteString as: import qualified Data.ByteString as B In my Main module, where all the IO happens, I want to use lazy ByteStrings so I ask for the following imports: import ByteParser import qualified Data.ByteString.Lazy as L The problem is that when I try to call a function in ByteParser with an L.ByteString, the compiler complains that: Couldn't match expected type `Data.ByteString.Base.ByteString' against inferred type `L.ByteString' Does this mean that the lazy version of ByteString is a completely separate, incompatible type from the base version? Obviously I can work around this problem by making ByteParser import Data.ByteString.Lazy instead of Data.ByteString, but then ByteParser would not be able to work with strict ByteStrings. Ideally I would like ByteParser to be agnostic about the use of lazy versus strict ByteStrings. Is this a sensible and/or possible thing to do? Many thanks, Neil

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haddock infix questions
by Nicolas Frisby 29 Nov '06

29 Nov '06

I found nothing in the Gmane archives, so I turn to you. I'm hoping to Haddock-a-chop my library but the ole infix type constructor (and there are many of them) is causing a parse error. It seems a common enough problem, so I was wondering... 1) What ended up happening with David Waern's SoC project? I tried the darcs repo at http://darcs.haskell.org/SoC/haddock.ghc/, but got some error messages that I think indicate I don't have an appropriate GHC As a Library package ("`HsDoc' not in scope"). I'm running GHC 6.6 on a Mac. I'll avoid a manual build of a snapshot GHC until it's my last option, so I haven't made strides to check if HsDoc is actually in the dev GHC repo. 2) Is there a good workaround out there for such a parse error? I'm thinking someone might have a handy pre-processor to help out Haddock? Thanks for your time, Nick

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RE: [Haskell] ANNOUNCE: Visual Haskell prerelease 0.2
by Bayley, Alistair 29 Nov '06

29 Nov '06

> From: haskell-bounces(a)haskell.org > [mailto:haskell-bounces@haskell.org] On Behalf Of Johannes Waldmann > > sounds great - but can I use this without buying Visual Studio first, > i. e. does it work with some free beta version or similar? > > and, does it then work under wine :-) > > seriously, how hard would it be to adapt VH to Eclipse? > the interfaces (for syntax highlighting, typechecking etc.) > should be similar - in theory. Johannes, Are you aware that there is a Haskell plugin for eclipse? http://eclipsefp.sourceforge.net/ It's probably not as sophisticated as VH (I haven't tried VH, so cannot comment with authority), but you get syntax highlighting/colouring, compile-on-save, and compile errors are highlighted in the source editor. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: [Haskell] ANNOUNCE: Visual Haskell prerelease 0.2
by Pepe Iborra 28 Nov '06

28 Nov '06

On 28/11/2006, at 12:25, Krasimir Angelov wrote: > On 11/28/06, Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de> wrote: >> seriously, how hard would it be to adapt VH to Eclipse? >> the interfaces (for syntax highlighting, typechecking etc.) >> should be similar - in theory. > Parsing and typechecking in VH is all done by GHC via the ghc-api. Thus strong interoperability support is needed. > Ah, and about Eclipse. Visual Studio and Eclipse are very different > platforms and you have to rewrite VSHaskell almost from scratch if you > want an Eclipse port. Perhaps the only common parts will be Cabal & > GHC. > Not only the interfaces are completely different, but an entirely new set of interoperability problems would need to be solved. The guys from EclipseFP have a document[1] about that topic. That said I would love being able to run an eclipsified VH in my Mac :) [1] - http://eclipsefp.sourceforge.net/haskell/ ExtendingEclipseInHaskell.pdf Cheers pepe

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c2hs and cabal?
by Magnus Therning 28 Nov '06

28 Nov '06

Can I use cabal to build packages that incorporates C libraries and FFI Haskell created using c2hs? Any pointers on how to do that? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. If voting could really change things it would be illegal.

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GHCi and HXT
by Alexis Hazell 28 Nov '06

28 Nov '06

Hi all, i'm having a bit of difficulty using GHCi to try out HXT. i brought the Text.XML.HXT.Arrow module into scope using :m. Then i entered: let contact = mkelem "stream:stream" [ sattr "xmlns:stream" "http://etherx.jabber.org/streams", sattr "xmlns" "jabber:client", sattr "to" "livejournal.com" ] [] and got the error message: <interactive>:1:14: Ambiguous type variable `a' in the constraint: `ArrowXml a' arising from use of `mkelem' at <interactive>:1:14-19 Probable fix: add a type signature that fixes these type variable(s) The type signature of mkelem is mkelem :: (ArrowXml a) => String -> [a n XmlTree] -> [a n XmlTree] -> a n XmlTree So then i modified it to: let contact = mkelem "stream:stream" [ sattr "xmlns:stream" "http://etherx.jabber.org/streams", sattr "xmlns" "jabber:client", sattr "to" "livejournal.com" ] [] :: ArrowXml a => a XmlTree XmlTree And got: <interactive>:1:14: Ambiguous type variable `a' in the constraint: `ArrowXml a' arising from instantiating a type signature at <interactive>:1:14-194 Probable fix: add a type signature that fixes these type variable(s) i don't understand why i need to specify type signatures when they should already be in scope? And if i /do/ need to specify the type signatures for some reason, how do i do so in a manner that works? Any help much appreciated! Alexis.

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Binary code
by escafia 28 Nov '06

28 Nov '06

Hi, i've one fuction receiving an int . The objective is return the respective binary code of that int. For example, if i receive 28 the fuction will return 011100. My question is that there is any fuction in libraries that do this? If not, anyone has any suggestion? Thank you. -- View this message in context: http://www.nabble.com/Binary-code-tf2704645.html#a7541561 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANN: NeHe Tutorials in Haskell
by Jason Dagit 27 Nov '06

27 Nov '06

Hello, I was recently reminded by http://haskell.org/haskellwiki/How_to_write_a_Haskell_program that you should announce your haskell projects! About a year ago I started converting the (somewhat) famous NeHe tutorials for OpenGL to HOpenGL. I have created a darcs repository for anyone interested (and I welcome darcs patches). You may find it here: http://codersbase.com/index.php/Nehe-tuts If you're interested in contributing, please observe one requirement, I want the examples to be as much in vanilla Haskell98 as possible to help make it accessible to beginners. Also, if you have feedback that is also very welcome. Thanks Jason

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Optimizing a hash function
by Ivan Tomac 27 Nov '06

27 Nov '06

Hi, Yesterday evening I had a go at porting Bob Jenkins' hash function (http://www.burtleburtle.net/bob/c/lookup3.c) to Haskell. The first version I came up with ran 20 times slower than C. Thanks to Don Stewart's suggestions on IRC, I managed to improve the performance by replacing tuples with a strict type so the current version is about 10 times slower than C code. Is there any way this can be further optimized? To keep things fair I've simplified the C function so it works on one byte at a time as opposed to an int at a time. The code was compiled with GHC-6.6 on a G5 iMac running OS X 10.4. Options used to compile the version using the hash function written in Haskell: ghc -O -funbox-strict-fields -fglasgow-exts -fbang-patterns -cpp -o test hashByteString.hs test.hs Options used to compile the version using the hash function in C: ghc -O -fglasgow-exts -ffi -fbang-patterns -cpp -DCHASH -o ctest ctest.c test.hs Ivan Tomac

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Glade Gtk2Hs Tutorial
by Hans van Thiel 27 Nov '06

27 Nov '06

Hello, The Haskell Gtk2Hs adaptation of the 'Developing Gnome Apps with Glade' tutorial for beginners is now also on http://eddy.writelinux.com/ Note there are translations there of the original version for C in French, Dutch, Spanish, Turkish and Korean. Native speakers of those languages who are interested could use these and check out the Haskell Gtk2Hs listings later. Thanks, Hans van Thiel

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Priority Queue?
by Ken Takusagawa 27 Nov '06

27 Nov '06

Is there a Haskell implementation of an efficient priority queue (probably heap-based) out there that I can use? I do not see it in the GHC libraries.

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Implementations of bit vectors
by Dominic Steinitz 27 Nov '06

27 Nov '06

You could also look at http://darcs.haskell.org/crypto/Data/LargeWord.hs but it would need modifying to produce a 33 bit word. Dominic.

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re: Improving library documentation
by brad clawsie 26 Nov '06

26 Nov '06

don, i am glad you raised this point, i was going to write a note to this list soon with a similar request i would suggest comparing perldoc to haddock and other haskell documentation tools. generally speaking, i find that documentation for perl libraries is written as if the author is enthusiastic about users understanding it. there are typically detailed function descriptions, and often inlined examples. on the other hand i have found most haddock documentation to consist merely of function signatures. as a result i have to employ spotty supplemental references like the haskell ref at zvon.org (which i am sure is a copy of a well-known reference set floating around out there), which includes some examples, or google's code search with haskell support. furthermore, perldoc has querying capabilities (-f etc) which preclude having to surf around references, another great feature. regardless of format, well-written documentation will go a long way to forwarding haskell adoption. brad

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Haddock question
by Luis Cabellos 26 Nov '06

26 Nov '06

Hello, I follow the How to Write a Haskell program from http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program, but I have a problem with Haddock. I use Windows XP, I when I install GHC + Haddock I have this warning output C:\code\haskell\test> runhaskell Setup.hs haddock Preprocessing executables for Test-0.0... Running Haddock for Test-0.0... Warning: cannot use package Test-0.0: ghc-pkg failed Warning: cannot use package base-2.0: does not exist.y $topdir\html\libraries\base There is something generated in dist\doc\html\test path but only the main function page but it don't has the comments from *.hs file. I use --prefix=c:\programs instead of --prefix=$HOME in configure. Because I am working on Windows. I don't use darcs, but I think that it's not the problem. -- Thanks a lot, Luis Cabellos

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Improving library documentation
by dons＠cse.unsw.edu.au 26 Nov '06

26 Nov '06

People sometimes complain that the standard libraries need more documentation. Since we have so many eyes reading the haddock source, a low barrier way for people to improve the docs is needed. I've created a new wiki page, linked from the libraries page: http://haskell.org/haskellwiki/Improving_library_documentation This page is to track documentation improvments suggested by *you*. So if you're reading the haddocks, and notice something's not documented, or missing details, please record it on this page, so it can be improved and committed. -- Don

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Implementations of bit vectors
by Maurício 26 Nov '06

26 Nov '06

Hi, Does anyone knows of a nice implementation of bit vectors? I like the functions I see in Data.Bits (rotate, shift etc.), but that only works with predefined sizes and I need a bit vector of exactly 33 bits. Thanks, Maurício

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ghc 6.6 crash
by Tim Newsham 25 Nov '06

25 Nov '06

I just built GHC 6.6 on my linux Fedora Core box (athlon 700mhz). When I run it, it crashes. The 6.4 version works fine (which I also built). # ghc-6.6 Segmentation fault # gdb /usr/local/lib/ghc-6.6/ghc-6.6 GNU gdb Red Hat Linux (6.0post-0.20040223.19rh) Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i386-redhat-linux-gnu"...(no debugging symbols found)...Using host libthread_db library "/lib/libthread_db.so.1". (gdb) run -B/usr/local/lib/ghc-6.6 Starting program: /usr/local/lib/ghc-6.6/ghc-6.6 -B/usr/local/lib/ghc-6.6 Error while mapping shared library sections: : Success. Error while reading shared library symbols: : No such file or directory. (no debugging symbols found)...(no debugging symbols found)...[Thread debugging using libthread_db enabled] [New Thread 16384 (LWP 20880)] Error while reading shared library symbols: : No such file or directory. Error while reading shared library symbols: : No such file or directory. Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 16384 (LWP 20880)] 0x00421d3b in __pthread_clock_gettime () from /lib/i686/libpthread.so.0 (gdb) where #0 0x00421d3b in __pthread_clock_gettime () from /lib/i686/libpthread.so.0 #1 0x00dc0331 in clock_gettime () from /lib/i686/librt.so.1 #2 0x0888fb4d in ?? () #3 0x00000003 in ?? () #4 0xfef22228 in ?? () #5 0xfef222e8 in ?? () #6 0x0888fe26 in ?? () #7 0x09462dd4 in ?? () #8 0x00000000 in ?? () (gdb) x/64 $esp 0xfef221a4: 0x13ea7d62 0x00000000 0x00000003 0x29a79300 0xfef221b4: 0x006ec840 0x00421cf0 0x00dc3ff4 0x00000003 0xfef221c4: 0x00000000 0xfef22208 0x00dc0331 0x00000003 0xfef221d4: 0x29a79300 0x00000000 0xfef22228 0x00dbc7ec 0xfef221e4: 0x09462d88 0x29a79300 0x00000000 0xffffffff 0xfef221f4: 0x006ec840 0x00000000 0x09462d88 0xfef222d0 0xfef22204: 0x00000002 0xfef222e8 0x0888fb4d 0x00000003 0xfef22214: 0xfef22228 0xfef222e8 0x0888fe26 0x09462dd4 0xfef22224: 0x00000000 0x0945c280 0x00000000 0x00000000 0xfef22234: 0x09462d88 0xfef222d0 0x0888f127 0x09462dd4 0xfef22244: 0x08962469 0xfef22268 0x0041d525 0xfef222d4 0xfef22254: 0xfef222d0 0x00000002 0x0888f23d 0xfef222d0 0xfef22264: 0x00000002 0xfef222d4 0x08888c73 0x089e2aa0 0xfef22274: 0x00000000 0x00000000 0x00000000 0x00000000 0xfef22284: 0x00000000 0xfef222d4 0x08890019 0xfef222d4 0xfef22294: 0xfef222d0 0xfef222d4 0x08889bde 0xfef222d0 # uname -a Linux box 2.6.5-1.358 #1 Sat May 8 09:04:50 EDT 2004 i686 athlon i386 GNU/Linux # ls -ld /lib/*pthread* -rwxr-xr-x 1 root root 95012 May 11 2004 /lib/libpthread-0.10.so* lrwxrwxrwx 1 root root 18 Jun 21 15:00 /lib/libpthread.so.0 -> libpthread-0.10.so* Tim Newsham http://www.thenewsh.com/~newsham/

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Starting your own Haskell project: part 2
by dons＠cse.unsw.edu.au 25 Nov '06

25 Nov '06

We've expanded the wiki page on 'How to write a Haskell project', to include a complete walkthrough creating: * Darcs * Cabal * QuickCheck infrastructure. http://haskell.org/haskellwiki/How_to_write_a_Haskell_program Feedback welcome! -- Don P.S. It might even be useful to have a tool, haskell-project, which sets up all these files automatically.

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Foldl
by Marco Comini 25 Nov '06

25 Nov '06

Donald Bruce Stewart: > zacara: > > i'd like to write a function that given a list like [1,2,3,4...] > > returns a list of couple where the first element is the corresponding > > element of the string, and the second is the sum of the previous > > elements. > Looks like a scanl (a prefix fold) -- the "of the previous > elements" is a clue that you're describing a scan: > > Prelude> :t scanl > scanl :: (a -> b -> a) -> a -> [b] -> [a] > > Prelude> scanl (+) 0 [1..4] > [0,1,3,6,10] > > You should be able to work out the rest from here (or rewrite it as a > fold). I agree Don, certainly with scanl one can solve the problem elegantly. [For ClareZako: it could be something like f (x:xs) = scanl acc (x,0) xs where acc (x,s) y = (y, s+x) ] On the contrary a solution in terms of a fold is indeed more tricky. Probably ClareZako was actually looking for a solution in term of a scan instead of a fold... otherwise would be interesting to know why he's willing to solve this problem in such a twisted way... don't you agree? Cheers, -- Marco.

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list monad and controlling the backtracking
by isto 25 Nov '06

25 Nov '06

Hi all, Weekly news had a link to article "Local and global side effects with monad transformers" and the following is from there (minor modification done): import Control.Monad.List import Control.Monad.State import Control.Monad.Writer test5 :: StateT Integer (ListT (Writer [Char])) Integer test5 = do a <- lift $ mlist aList b <- lift $ mlist bList lift $ lift $ tell ("trying "++show a++" "++show b++"\n") modify (+1) guard $ a+b<5 return $ a+b go5 = runWriter $ runListT $ runStateT test5 0 If the aList = [1..5] as well as bList, then there will be 25 tryings. If aList and bList are [1..10000000], there will be a lot of tryings... However, guard is saying that we are interested in only values whose sum is less than 5. Is it possible to control easily in the above example that when we have tried out pairs (1,1), (1,2), (1,3), (1,4), that now we are ready to stop trying from the bList? And then similarly when we finally arrive to a pair (4,1), that now we are ready to finish also with aList? This would give a nice way to build branch & bounding algorithms, even though it does not take much more lines to do it in some other way. br, Isto

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strange type mismatch when trying to use takusen
by Seth Gordon 25 Nov '06

25 Nov '06

I have a simple test program for takusen and PostgreSQL: > import Database.Enumerator > import Database.PostgreSQL.Enumerator > import Control.Monad.Trans > > gazdbSession dbname = connect [CAdbname dbname] > > resultCollector :: (Monad m) => String -> IterAct m [String] > resultCollector str accum = result' (str:accum) > > main = do withSession (connect [CAdbname "template1"]) $ main' > > main' = do r <- doQuery (sql "select 'fred'") resultCollector [] > liftIO $ putStrLn $ show r But it won't compile: [1 of 1] Compiling Main ( takusen-test.hs, takusen-test.o ) takusen-test.hs:11:57: Couldn't match expected type `forall mark. DBM mark Session a' against inferred type `DBM mark sess ()' In the second argument of `($)', namely `main'' In the expression: (withSession (connect [CAdbname "template1"])) $ main' In the expression: do (withSession (connect [CAdbname "template1"])) $ main' Adding an explicit "main' :: DBM mark Session ()" type signature doesn't change the error message. The takusen source that I'm using was updated from darcs on November 14 and compiled with ghc 6.6.

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Re: [Haskell] Re: [Haskell-cafe] SimonPJ and Tim Harris explain STM - video
by C.M.Brown 25 Nov '06

25 Nov '06

Hi, I got this working on Mac OS X. I had to download media player 9: http://www.microsoft.com/windows/windowsmedia/software/Macintosh/osx/defaul… This contains the WMV3 codec. Cheers, Chris. On Fri, 24 Nov 2006, James William Pye wrote: > On Fri, Nov 24, 2006 at 10:26:38AM +0100, Tomasz Zielonka wrote: > > Does anybody know how to watch this on Linux? I would prefer to simply > > download the movie file and use MPlayer on that, but I failed. > > > > . or on Mac OS X (haven't tried yet) > > The latest mplayer works for me on FreeBSD/amd64 (1.0rc1, iirc). > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Implicit params and typeclasses
by Grzegorz Chrupała 24 Nov '06

24 Nov '06

Hi all, When I try to compile the following in GHC (v 6.6) (with -fno-monomorphism-restriction -fimplicit-params) class Configuration a where thestring:: a -> String foo c = let { ?c = c } in bar bar = thestring ?c I get: Ambiguous type variable `a' in the constraint: `Configuration a' arising from use of `thestring' at /home/grzegorz/Foo.hs:9:6-17 Possible cause: the monomorphism restriction applied to the following: bar :: (?c::a) => String (bound at /home/grzegorz/Foo.hs:9:0) foo :: a -> String (bound at /home/grzegorz/Foo.hs:8:0) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction Adding the type signature bar :: (Configuration a,?c :: a) => String the compiler says: Ambiguous constraint `Configuration a' At least one of the forall'd type variables mentioned by the constraint must be reachable from the type after the '=>' In the type signature for `bar': bar :: (Configuration a, ?c :: a) => String Similar code without use of implicit params compiles fine: class Configuration a where thestring:: a -> String foo c = thestring c Why do implicit params cause this error and is there a way to make it work? Thanks, -- Grzegorz

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Foldl
by ClareZako 24 Nov '06

24 Nov '06

Hello, i'd like to write a function that given a list like [1,2,3,4...] returns a list of couple where the first element is the corresponding element of the string, and the second is the sum of the previous elements. An example: input: [1,2,3,4] output: [(1,0)(2,1)(3,3)(4,6)] The problem is that I'd like to use foldl and a solution like: buildCouples = snd . foldl op (0,[]) where op (v,xs) x = (v+x,xs++[(x,v)]) is not good since the operator ++ let the function take 2*n time and not n. Can anyone help me please? Clare -- View this message in context: http://www.nabble.com/Foldl-tf2698202.html#a7524410 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Command line prompt templates
by Dougal Stanton 24 Nov '06

24 Nov '06

I was trying to write my own equivalent to Don Stewart's mkcabal but ended up getting sidetracked. I made some generalised prompts for use at the command line and wanted to get some feedback on them. The full code can be found at [1] but the basic summary is like this: > prompt :: String -> IO String Simple prompt - supply a question and receive the user's answer. > promptYesNo :: String -> Maybe String -> IO Bool A yes/no question, with an optional value to ask about. I suppose this could be simplified to just one string. > promptList :: String -> [String] -> Maybe Integer -> IO Integer Ask the user to choose from a list, where the result is the number chosen. The optional value is a default if the user doesn't pick a number. > promptListEdit :: String -> [String] -> Maybe Integer -> IO String As above, but the last option in the list is an invitation to provide your own answer. Again the optional value is the default choice if nothing else is chosen. The result here is necessarily the string rather than the integer cos I didn't want to complicate matters further with stuff like Either types. If you can see any flaws or have any suggestions please let me know! Cheers, D. [1]: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/CommandPrompts.hs>

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Equivalent of if/then/else for IO Bool?
by Dougal Stanton 24 Nov '06

24 Nov '06

Is there some sort of equivalent of the if/then/else construct for use in the IO monad? For instance the following can get quite tedious: > do bool <- doesFileExist filename > if bool > then sth > else sth' Is there a more compact way of writing that? Something akin to: > condM (doesFileExist filename) (sth) (sth') Or maybe there's a more sensible way of doing the above that I've missed. I seem to use endless alternate conditions sometimes and there's bound to be a better way.

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Re: [Haskell-cafe] working with lists of couples
by Clare 24 Nov '06

24 Nov '06

Yes the code you are suggesting is certainly linear and it takes without doubt time n. But I was looking for a solution using foldl that of course takes time n. The problem of the following solution is that it gives a reversed result, and of course i can't just reverse the result. couples = snd . foldl f (0,[]) where f (s,[]) x = (s+x, [(x,0)]) f (s,xs) x = (s+x, (:) (x,s) xs) Clare 2006/11/17, Valentin Gjorgjioski <valentin.gjorgjioski(a)ijs.si>: > > On 17.11.2006 21:04 Clare wrote: > > I'm not sure it takes time n couse of the operator ++ and the lazy > > stuffs in haskell. > Ok, you can use > buildCouples (x:xs) s = (x,s) : (buildCouples xs (x+s)) > instead of ++ > > this algorithm is linear, I don't know why(?) you think it is not. > > Valentin > > -- > Valentin Gjorgjioski > Bachelor of Computer Science > Department of Knowledge Technologies, Jozef Stefan Institute > Jamova 39, SI-1000 Ljubljana, Slovenia > Phone: +386 1 477 3343 > Fax: +386 1 477 3315 > Web: http://kt.ijs.si/ValentinGjorgjioski/ > Email: Valentin.Gjorgjioski(a)ijs.si > >

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type keeping rounding, typeable (and a difficulty)
by isto 24 Nov '06

24 Nov '06

Hi, I've been trying to compile the following function (rounding to a desired degree): roundDec :: (Num a, Typeable a) => Int -> a -> a roundDec d a = let t = show (typeOf a) in case t of "Double" -> roundDDec d a "Complex Double" -> roundCDec d a otherwise -> a -- or something The two other functions are roundCDec :: (RealFloat a) => Int -> Complex a -> Complex a roundCDec d (c :+ b) = (roundDDec d c :+ roundDDec d b) and roundDDec :: (RealFloat a) => Int -> a -> a roundDDec d a = a -- or somegthing Compiler gives the following error message: Couldn't match expected type `Complex a' against inferred type `a1' (a rigid variable) `a1' is bound by the type signature for `roundDec' at FFT.hs:57:17 In the second argument of `roundCDec', namely `a' In the expression: roundCDec d a In a case alternative: "Complex Double" -> roundCDec d a If in the roundDDec a's are replaced with Double, there will be similar error message from the "Double"-line. The functionality can be written differently, but I wanted to try write rounding having in a signature at least "(Num a) => Int -> a -> a". Again, any help would be appreciated a lot! Thanks in advance! br, Isto

3 5

Type Error. Why!?
by Tim Newsham 24 Nov '06

24 Nov '06

Can anyone tell me why I get a type error in the following code? I have to define liftA2 twice to avoid the type error. Both times its defined identically but with different type signature. If I use the original liftA2 in place of liftA2' I get: whyerror.lhs:36:25: Ambiguous type variable `a' in the constraint: `Arrow a' arising from use of `>>>' at whyerror.lhs:36:25-27 Possible cause: the monomorphism restriction applied to the following: liftA2' :: forall b a1 b1 c. (a1 -> b1 -> c) -> a b a1 -> a b b1 -> a b c (bound at whyerror.lhs:36:1) unsplit' :: forall a1 b c. (a1 -> b -> c) -> a (a1, b) c (bound at whyerror.lhs:34:1) split' :: forall b. a b (b, b) (bound at whyerror.lhs:33:1) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction ----- whyerror.lhs ---- > {-# OPTIONS_GHC -farrows #-} > module WhyError where > import Control.Arrow > newtype SimpleFunc a b = SimpleFunc { runF :: (a -> b) } > instance Arrow SimpleFunc where > arr f = SimpleFunc f > first (SimpleFunc f) = SimpleFunc (mapFst f) > where mapFst g (a,b) = (g a, b) > (SimpleFunc f) >>> (SimpleFunc g) = SimpleFunc (g . f) > split :: (Arrow a) => a b (b, b) > split = arr (\x -> (x,x)) > unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d > unsplit = arr . uncurry > -- arr (\op (x,y) -> x `op` y) > liftA2 :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d > liftA2 op f g = split >>> first f >>> second g >>> unsplit op > -- f &&& g >>> unsplit op > f, g :: SimpleFunc Int Int > f = arr (`div` 2) > g = arr (\x -> x*3 + 1) > h = liftA2 (+) f g > hOutput = runF h 8 > -- XXX I am getting type problems with split, unsplit and liftA2! why? > split' = arr (\x -> (x,x)) > unsplit' = arr . uncurry > -- liftA2' :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d > liftA2' op f g = split' >>> first f >>> second g >>> unsplit' op > plusminus, double, h2 :: Kleisli [] Int Int > plusminus = Kleisli (\x -> [x, -x]) > double = arr (* 2) > h2 = liftA2 (+) plusminus double > h2Output = runKleisli h2 8 > main = do > print hOutput > print h2Output Tim Newsham http://www.thenewsh.com/~newsham/

5 6

LLVM back end
by Michael T. Richter 23 Nov '06

23 Nov '06

I've been eyeing LLVM[1] as interesting technology -- brief executive summary: a virtual machine suited as the back end of compiler output with optimised native code then coming from it as either JIT-based execution of the LLVM bytecode or as a further compilation step -- and couldn't help but immediately think of the possibility of one of the Haskell compiler projects providing an LLVM code generator. I think this would help in several areas: * it could make porting the compiler to other architectures -- including oddball ones that would be too small to otherwise support -- easier; * it could help remove the nigh-ubiquitous reliance upon GCC as a back-end (while I think that GCC is a pretty good piece of software, I'm not sure it's really suited to its current role as the "do-everything" back end); * it could leverage some of the really interesting work that's going on in optimisation technology by letting one VM's optimiser do the work for any number of languages; * it could improve interaction between source code written in multiple languages. Is this me opening up a Pandora's Box of ignorance here? Or is LLVM potentially interesting? (And were someone motivated into perhaps trying to make an LLVM back-end, where would one start to poke around in, say, the GHC codebase to even begin to implement this? And how insane would they be driven by the process?) [1]http://llvm.org -- Michael T. Richter Email: ttmrichter(a)gmail.com, mtr1966(a)hotpop.com MSN: ttmrichter(a)hotmail.com, mtr1966(a)hotmail.com; YIM: michael_richter_1966; AIM: YanJiahua1966; ICQ: 241960658; Jabber: mtr1966(a)jabber.cn "I have never seen the poor people being so familiar with their heads of state as they were with [Michele Duvalier]. It was a beautiful lesson for me. I've learned something from it." --Mother Theresa

2 2

Re: help with happy/alex
by Simon Marlow 23 Nov '06

23 Nov '06

robert bauer wrote: > Hi, > > I have a .y and .x file. The alex -g _.x gives me a .hs file and happy -g -a -c _.y gives me a .hs file. > I then use ghc -c alex.hs to get a a .o and a .hi file. This works great. > Unfortunately > ghc -c happy.y doesn't work -- it says that it the module name for the lexer doesn't match. I've attached the > .y and .x files. > > Also, I cannot figure out what I need to do in the parser to call the lexer and I don't know how to kick start the > parser -- I have no clue what to put in third, main.hs, module. > > Any help would be appreciated. You have a typo in the module name in TVScriptLexer.x. As for tying it together, I suggest taking a look at some of the examples, eg. http://darcs.haskell.org/alex/examples/tiny.y from the Alex distribution. Cheers, Simon

1 0

YCR2JS Programmers Guide Draft available
by Dimitry Golubovsky 23 Nov '06

23 Nov '06

Hi, I have finished typing in the draft of low-level programming guide for Yhc Core to Javascript converter. Everyone interested in future use of this tool is encouraged to read and review the Guide. Its purpose is to give some ideas about interaction of Haskell programs converted into Javascript with a web browser on the lowest possible level, without application frameworks and support libraries (just because these haven't been developed). Any feedback and ideas are appreciated. The document is located at: http://haskell.org/haskellwiki/Yhc/Javascript/Programmers_guide Thanks. -- Dimitry Golubovsky Anywhere on the Web

1 0

trivial ghc problem, help needed
by brad clawsie 23 Nov '06

23 Nov '06

i have a program tb.hs: --- module Main where import Network.URI (URI(..), URIAuth(..), parseURI) myFunc :: String -> Maybe URI myFunc u = parseURI u main = do { return () } --- when i attempt to build it with ghc, i get the following output: tb.o: In function `Main_myFunc_info': (.text+0x11): undefined reference to `networkzm2zi0_NetworkziURI_parseURI_closure' tb.o: In function `Main_myFunc_srt': (.rodata+0x0): undefined reference to `networkzm2zi0_NetworkziURI_parseURI_closure' collect2: ld returned 1 exit status any clue why? i would send this to the ghc list but i presume my issue is due to a trivial misunderstanding of the language thanks in advance

2 1

Articles on programming with (IO) exceptions in Haskell?
by Magnus Therning 22 Nov '06

22 Nov '06

I'm having problems finding a good introduction to exceptions. Yet Another Haskell Tutorial leaves it for chapter 10, which is empty. I'd prefer something that suits a noob. :-) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. `Time is an illusion. Lunchtime doubly so.' `Very deep,' said Arthur, `you should send that in to the "Reader's Digest". They've got a page for people like you.' -- Ford convincing Arthur to drink three pints in ten minutes at lunchtime.

4 3

Why are new releases of libraries required for every GHC release?
by DavidA 22 Nov '06

22 Nov '06

Some recent comments suggested that every time there is a new GHC release, there needs to be a new wxHaskell release (or insert name of other library here) to work with it. This seems to be true even for minor upgrades, like 6.4.1 -> 6.4.2. Why is this? Does GHC break binary compatibility at every release? Is this avoidable? This makes using Haskell unnecessarily hard compared to other platforms.

4 3

help with threadDelay
by Neil Davies 22 Nov '06

22 Nov '06

Hi, I'm using GHC 6.6 (debian/etch) - and having some fun with threadDelay. When compiled without the -threaded compiler argument it "behaves" as documented - waits at least the interval - for example: Tgt/Actual = 0.000000/0.036174s, diff = 0.036174s Tgt/Actual = 0.000001/0.049385s, diff = 0.049384s Tgt/Actual = 0.000005/0.049492s, diff = 0.049487s Tgt/Actual = 0.000025/0.049596s, diff = 0.049571s Tgt/Actual = 0.000125/0.049655s, diff = 0.04953s Tgt/Actual = 0.000625/0.04969s, diff = 0.049065s Tgt/Actual = 0.003125/0.049684s, diff = 0.046559s Tgt/Actual = 0.015625/0.04962s, diff = 0.033995s Tgt/Actual = 0.078125/0.099668s, diff = 0.021543s Tgt/Actual = 0.390625/0.399637s, diff = 0.009012s Tgt/Actual = 1.953125/1.999515s, diff = 0.04639s Tgt/Actual = 9.765625/9.799505s, diff = 0.03388s however when -threaded is used you get some interesting effects, including returning too early: Tgt/Actual = 0.000000/0.000093s, diff = 0.000093s Tgt/Actual = 0.000001/0.000031s, diff = 0.00003s Tgt/Actual = 0.000005/0.000029s, diff = 0.000024s Tgt/Actual = 0.000025/0.000028s, diff = 0.000003s Tgt/Actual = 0.000125/0.000034s, diff = -0.000091s Tgt/Actual = 0.000625/0.000035s, diff = -0.00059s Tgt/Actual = 0.003125/0.000029s, diff = -0.003096s Tgt/Actual = 0.015625/0.000028s, diff = -0.015597s Tgt/Actual = 0.078125/0.058525s, diff = -0.0196s Tgt/Actual = 0.390625/0.389669s, diff = -0.000956s Tgt/Actual = 1.953125/1.939513s, diff = -0.013612s Tgt/Actual = 9.765625/9.749573s, diff = -0.016052s The program I used to generate this is :- import Control.Concurrent import Data.Time import Text.Printf main = mapM_ delay (0 : take 11 (iterate (*5) 1)) delay n = do tS <- getCurrentTime threadDelay n tE <- getCurrentTime let n' = toRational n / 10^6 n'' = fromRational (n') :: Double obs = diffUTCTime tE tS printf "Tgt/Actual = %0.6f/%s, diff = %s\n" n'' (show obs) (show $ obs - fromRational n') return () Any suggestions? Neil

3 2

why are implicit types different?
by S. Alexander Jacobson 22 Nov '06

22 Nov '06

Why do g and g' have different types? g x y = let ?f = \x-> x in ?f x ++ (show (?f y)) g' x y = let f = \x-> x in f x ++ (show (f y)) Is there a way I can use implicit types and let g be as general as g'? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

1 0

Friday Afternoon Play: The Burrows Wheeler Transformation
by Jón Fairbairn 22 Nov '06

22 Nov '06

So, it's Friday afternoon (well, here anyway), and presumably everyone is thinking of leaving for the weekend. Is anyone interested in playing a little game? I'm going to make the first move, and then just wait and see if anyone makes the next. I'm certainly not promising to make any further moves myself, but I'm not promising I won't, either. Below you'll see a naïve implementation of the Burrows Wheeler transformation and its inverse. That's my move. (And don't complain too much about style and such -- I'm half asleep.) Further moves are made by transforming the programme so that it becomes more efficient. The first target is not raw speed, but to adjust it until the computational complexity gets to be what it should be. I'm hoping for elegant moves here. After that, folk might start replacing the lists with more efficient datastructures and so on, but still /elegantly/. The object of the game isn't ultimate speed -- I've no particular wish to see a Haskell version that's as fast as a good C version, especially not if it ends up /looking/ like a C version. Really the objective is to think about Haskell programming -- and probably come up with some observations such as "there really ought to be a class of indexable structures, so that we don't have to change (!!) to (!) all over the place". And to have fun, anyhow... * * * Demonstration of the principle of the Burrows Wheeler transform. The implementations are simplistic (and have worse complexity than is possible). > module Burrows_Wheeler_Transform where > import List This is surely the wrong module to have to find (><) from: > import Data.Graph.Inductive.Query.Monad((><)) The forward transformation. Input a list, output the transformed list and possibly an int (Nothing if there were no elements in the input -- perhaps the result type should be Maybe (NonEmptyList t,Int)) > slow_bwt:: Ord t => [t] -> ([t], Maybe Int) This version works by computing a list of all the rotations of the input and sorting it. The transformation is then the last element of each line of the sorted rotations. Tag the data with an index so that it's possible to find where the original form of the unput ends up after sorting. This is essentially the specification of the transform coded up directly. > slow_bwt l > = (map fst tagged_bwt, index_of_original) > where tagged_bwt = map (last><id) > $ sort > $ rotations l`zip`[0..] > index_of_original = findIndex ((==0).snd) tagged_bwt that looks like worst case n²×log n to me (where n is the length of the string): if every element is the same, the n×log n comparisons the sort does will each look at every element, and that's really more than is necessary. The inverse transform. We should have that @ slow_inv_bwt . slow_bwt == id @ Observe that sorting the input gives the first column of the sorted rotations (and the input is the last). So after one sort we have X ... c Y ... b Z ... a . . . . . . . . . Since they are rotations, rotating each row of this gives pairs that belong to the original list: cX ... by ... aZ ... . . . . . . sorting these gives us the first two columns of the sorted rotations, and again we already know the last column, which can be rotated into the first and so on. > slow_inv_bwt ([],_) = [] > slow_inv_bwt (l,Just index_of_original) > = iterate catsort (replicate len []) After (length input) iterations we have the original sorted rotations, > !! len and we have the index of where the untransformed string is, so we can just pick it back out. > !! index_of_original > where catsort s = sort $ map (uncurry (:)) $ l `zip`s > len = length l This version does rather fewer sorts: > mark2_inv_bwt ([],_) = [] > mark2_inv_bwt (l,Just n) > = (transpose $ take (length l) $ iterate (permuteWith perm) l)!!(perm!!n) > where perm = map snd $ sort $ l `zip` [0..] Should I simply have left it out and hoped that it would turn up after a few moves? I'm not going to explain it, anyway. > rotations l = init $ zipWith (++) (tails l) (inits l) > permuteWith ns l = map (l!!) ns -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

6 6

Newbie Haskell optimization question
by Slavomir Kaslev 21 Nov '06

21 Nov '06

Hello, I am new to Haskell and I am having fun with it writing L-system generator. I was very pleased with concise syntax. It's an order of magnitude better than any C++, Lisp, etc program I've ever written or read. Doing algorithmic optimizations on my generator I ended up with 4 different versions (you can see the other 3 in the file attached). The most optimal of them is: import qualified Data.Map type LSystemElement = (Char, [Int]) type LSystem = [LSystemElement] type LSystemRules = Data.Map.Map Char (LSystemElement -> LSystem) generate'' :: LSystemRules -> LSystem -> Int -> LSystem generate'' rules axiom steps = concatMap (iterate f axiom !!) (ind !! steps) where ind = [0] : [g x | x <- ind] where g [] = [] g [x] = [x, x + 1] g xs = xs ++ g (drop (length xs `div` 2) xs) f = concatMap $ \elem -> Data.Map.findWithDefault (\x -> [x]) (fst elem) rules elem -- Tests a (_, [x, y]) | y <= 2 = [('A', [x + 2, y + 2])] | otherwise = [('B', [2]), ('A', [x - 1, y - 1])] b (_, [x]) | x <= 2 = [('C', [])] | otherwise = [('B', [x - 1])] rules = Data.Map.fromList [('A', a), ('B', b)] axiom = [('A', [2, 2])] Now I am thinking of doing some language level optimizations. Can you give me any (random) directions? Any style recommendations are welcome as well. Cheers. -- Slavomir Kaslev

3 2

Re: topEq for types
by oleg＠pobox.com 21 Nov '06

21 Nov '06

Nicolas Frisby described the problem and the solution of the type equality comparison that takes into account the top-most type constructor only. Thus `Maybe Bool' should be considered equal to `Maybe Char' because both types have the same top constructor Maybe. The following complete code shows another solution. The code includes several tests, test1 through test6, along with the expected answers. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} -- type equality for the top-level type constructor -- That is, decide two types to be equal if the top-most type constructor -- matches. Thus `Maybe Bool' should be considered equal to `Maybe Char' module TEQ where -- Our approach is simple. Given a type, we normalize it: -- replace all arguments of the type constructor with unit. -- After that, we use the regular HList's TypeEq. class Normalize t r | t -> r instance TypeCast (c ()) r => Normalize (c x) r instance TypeCast (c () ()) r => Normalize (c x y) r instance TypeCast (c () () ()) r => Normalize (c x y z) r -- add instances for type constructors of higher arities instance TypeCast y x => Normalize x y top_eq :: (Normalize a a', Normalize b b', TypeEq a' b' r) => a -> b -> r top_eq = undefined test1 = top_eq (undefined::Int) (undefined::Char) -- :t test1 -- test1 :: HFalse test2 = top_eq (undefined::Bool) (undefined::Bool) -- test2 :: HTrue test3 = top_eq (undefined::Maybe Bool) (undefined::Maybe Char) -- test3 :: HTrue test4 = top_eq (undefined::Maybe Bool) (undefined::[Char]) -- test4 :: HFalse test5 = top_eq (undefined::Either Int Char) (undefined::Either Char Int) -- test5 :: HTrue test6 = top_eq (undefined::Either Int Char) (undefined::[Char]) -- test6 :: HFalse -- The following is borrowed from HList and included here for completeness data HTrue data HFalse class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b

1 0

Squash ladder software in Haskell
by Robin Bate Boerop 21 Nov '06

21 Nov '06

I've recently been looking for some open source software that will enable me to run a squash/tennis/badminton ladder on the Web. I haven't found any. So, it's time to write my own. Haskell is my language of choice. Before I start this project, is there anyone out there who has written some code that might be of use to me? Want to share? -- Robin Bate Boerop

3 2

RFC and Announcement: HLADSPA, LADSPA for Haskell
by Alfonso Acosta 21 Nov '06

21 Nov '06

Hi all, Let me introduce myself. I'm a computer science engineering student, writing his masters thesis about a VHDL translator for ForSyDe (http://www.imit.kth.se/info/FOFU/ForSyDe/ , a Hardware Description Language embedded in Haskell) In order to show a practical application of ForSyDe I decided to try with its audio processing design capabilities. For that purpouse I decided to port LADSPA (http://www.ladspa.org/ ) to Haskell. The result so far is HLADSPA 0.1, which can be downloaded from http://www.student.nada.kth.se/~alfonsoa/HLADSPA-0.1.tgz and contains a testing plugin (Null.hs). It requires GHC>=6.6 due to the use of template haskell and existentially quantified records (read on for more on this). Even for people not interested in LADSPA itself, the project can be regarded as an example of how to create plugins and shared libraries in Haskell without making use hs-plugins. I'll be happy to read any comments and/or questions regarding the project. Here are the design questions on my side: The most important part for creating the HLADSPA, was modelling the LADSPA header file (http://www.ladspa.org/ladspa_sdk/ladspa.h.txt ) in Haskell (http://www.student.nada.kth.se/~alfonsoa/HLADSPA-0.1/HLADSPA.hs ) The essential code snippet is, ===================== -- Existentially quantified records allow the plugin developer -- to chose hd and id types at will and, allowing to declare "heterogeneous" -- Descriptor lists. Drawback: it only works in ghc 6.6 and -- makes the design tricky. The problem comes from modelling (void*) in Haskell -- id is the implementation data data Descriptor = forall id. Descriptor {uniqueID :: LadspaIndex, label :: String, properties :: LadspaProperties, name, maker, copyright :: String, portCount :: LadspaIndex, portDescriptors :: [PortDescriptor], portNames :: [String], portRangeHints :: [PortRangeHint], _implementationData :: id, _instantiate :: Descriptor -> LadspaIndex -> Maybe Instance} -- hd is the handle data Instance = forall hd. Instance { _this :: hd, -- initial handle -- In this case we are using lists to represent the port I/O buffers, so the -- port connections (buffer pointers of ports) is handled by the marshaller -- connectPort :: (hd -> LadspaIndex -> Ptr LadspaData -> IO hd) _activate :: Maybe(hd -> IO ()), -- (LadspaIndex,PortData) indicates the portnumber and its data _run :: hd -> LadspaIndex -> [(LadspaIndex,PortData)] -> ([(LadspaIndex,PortData)], hd), -- Not yet implemented (is not mandatory for a plugin to provide them) -- _runAdding :: -- _setAddingGain :: _deactivate :: Maybe (hd -> IO ()), _cleanup :: hd -> IO () } ===================== This is the best solution I could come up with and I'm not still happy with it. The trickiest part, was modelling (void*) in Haskell. I know that using lists for I/O buffers is inefficient but I coded it having ForSyDe in mind. If people show interest I can code a faster StorableArray-version. Here are my questions. * I'm using GHC's existentially quantified records extension to hide the hd and id parameters because the plugin programmer has to be able to provide a collection of Descriptor (a Descriptor list in current release). I would love to find a solution in plain Haskell98. Any ideas? * This approach requires splitting the original C LADSPA_Descriptor struct in the Descriptor and Instance Haskell types, which leads to a design error: there are functions (e.g. _run, _activate ... ) in Instance which really belong to Descriptor (those functions shouldn't change with the plugin instance). For example, with this approach, it is not possible to tell the plugin host if activate() or deactivate() will be used because this is "asked" any instances is created. * Real Time support in HLADSPA. From LADSPA's documentation: "Property LADSPA_PROPERTY_HARD_RT_CAPABLE indicates that the plugin is capable of running not only in a conventional host but also in a `hard real-time' environment. To qualify for this the plugin must satisfy all of the following: (1) The plugin must not use malloc(), free() or other heap memory management within its run() or run_adding() functions. All new memory used in run() must be managed via the stack. These restrictions only apply to the run() function." Should I forget about HARD_RT_CAPABLE with Haskell? Is there a way to control the use of the heap by a function? * I'm not so sure about the types of _activate, _deactivate and _cleanup. I don't even know if I should include them because they only seem to be required by a language with side effects. Furthermore, _activate() and _deactivate() don't seem to have sense with current implementation cause they are "separated from instantiate() to aid real-time support" and using lists as I/O buffers discards RT support (see above) Thanks in advance for your comments/answers, Alfonso Acosta PS1: Big thanks and claps for the people at #haskell@Freenode . They helped a lot to make this initial release possible. PS2: I would like to get the project hosted at the darcs repository at haskell.org. Do you consider it interesting enough for it?

3 5

importing Distribution.Compat.FilePath fails
by Magnus Therning 21 Nov '06

21 Nov '06

Putting import Distribution.Compat.FilePath in my source results in Could not find module `Distribution.Compat.FilePath': it is hidden (in package Cabal-1.1.6) I want to use 'joinFileName'. I found this thread from the cabal-devel list http://www.mail-archive.com/cabal-devel@haskell.org/msg00162.html and I was hoping that my cabal version would have an unhidden FilePath module... apparently not. Can I unhide it somehow? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. It's important for a corporate leader to know the difference between what is actually illegal, and what people assume should be illegal. -- Bill Gates, in interview for The Register, 19th June 2006

5 8

ghci stack overflow
by jim burton 20 Nov '06

20 Nov '06

This code produces a stack overflow in ghci when I call `makeSpiral' with large values, e.g. big enough to produce a 1001x1001 spiral. (makeSpiral produces a list of lists which form a clockwise 'spiral', it's a puzzle from mathschallenge.net.) I'm sure there is a way to increase the stack space in ghc which I will look into, but is there a way I could avoid the problem in the first place by attacking the problem differently? Does stack space run out because the list is an argument being passed around (1001x1001 versions of it)? If so would the state monad help me? data Dir = R | D | L | U deriving (Show, Eq, Enum) type Spiral = ([[Int]], Int, Dir) -- (rows, current row, next direction) rows :: Spiral -> [[Int]] rows (rs, i, d) = rs currentrow :: Spiral -> Int currentrow (rs, i, d) = i nextdir :: Spiral -> Dir nextdir (rs, i, d) = d getrow :: Int -> [[Int]] -> Maybe [Int] getrow i sp = if i < 0 || i >= length sp then Nothing else Just (sp!!i) ndir :: Dir -> Dir ndir d = if d == U then R else succ d newsp :: Spiral newsp = ([[1]], 0, R) makeSpiral :: Int -> Spiral makeSpiral i = makeSpiral' 2 newsp where makeSpiral' j sp = if j > i then sp else makeSpiral' (j+1) (update j sp) update :: Int -> Spiral -> Spiral update i (sp, cr, d) = (sp', cr', d') where oldrow = if (d == U && cr' == cr && cr == 0) || (d == D && cr' == length sp) then [] else fromJust $ getrow cr' sp cr' | d == L || d == R = cr | d == U = if cr == 0 then 0 else cr-1 | otherwise = cr+1 cr'' = if d == U && cr == 0 then -1 else cr' sp' = insertrow cr'' newrow sp newrow = case d of R -> oldrow++[i] D -> oldrow++[i] L -> i:oldrow U -> i:oldrow d' | d == R || d == L = if length oldrow == maximum (map length sp) then ndir d else d | d == U = if cr'' == -1 then ndir d else d | otherwise = if cr' == length sp then ndir d else d insertrow :: Int -> [Int] -> [[Int]] -> [[Int]] insertrow i r rs = if i == -1 then r:rs else front++[r]++back where (front, rest) = splitAt i rs back = if null rest then [] else tail rest printSpiral :: Spiral -> IO () printSpiral (sp, i, d) = putStrLn (concat $ intersperse "\n" (map show sp)) sumdiags :: Spiral -> Int sumdiags (sp, i, d) = (sumdiags' 0 0 (+1)) + (sumdiags' 0 end (subtract 1)) - centre where row1 = sp!!0 end = length row1 - 1 halfx = (length row1 `div` 2) halfy = (length sp `div` 2) centre = (sp!!halfy)!!halfx sumdiags' row col f = if row == length sp then 0 else (sp!!row)!!col + sumdiags' (row+1) (f col) f -- View this message in context: http://www.nabble.com/ghci-stack-overflow-tf2666036.html#a7435185 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Collection of objects?
by Valentin Gjorgjioski 20 Nov '06

20 Nov '06

Is some kind of collection of object with different types in Haskell exist? Except the tuples, which have fixed length. I find this * Tuples heterogeneous, lists homogeneous. * Tuples have a fixed length, or at least their length is encoded in their type. That is, two tuples with different lengths will have different types. * Tuples always finite. But I need something which is heterogeneous and non-fixed length. I'm used do Java, and this switch to functional languages is very strange to me. So, to be clear, I need something like LinkedList<Object> in java. Can you please help me or suggest me, what can I use in this case? Valentin

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Yet more type confusion
by chris moline 20 Nov '06

20 Nov '06

Hey all, I'm thoroughly confused by the type error produced by the following: applies a fun returning a tuple of lists to a list of values and then repeatedly applies it to the snd of the result, returning a list of the fst's of successive applications > iterT :: ([a] -> ([a], [a])) -> [a] -> [[a]] > iterT f a = let (b, c) = f a in b : iterT f c like break, but drops the separator > breakDrop :: (a -> Bool) -> [a] -> ([a], [a]) > breakDrop p l = (takeWhile (not . p) l, drop 1 $ dropWhile (not . p) l) like break, but returns a list of lists of values separated by the separator > sep :: (a -> Bool) -> [a] -> [[a]] > sep p = takeWhile (/= "") . iterT (breakDrop p) > main = print $ sep (== 1) "" compling with ghc --make returns this: test.hs:14:45: Couldn't match the rigid variable `a' against `Char' `a' is bound by the type signature for `sep' Expected type: Char -> Bool Inferred type: a -> Bool In the first argument of `breakDrop', namely `p' In the first argument of `iterT', namely `(breakDrop p)' Setting the type signature of sep to (Char -> Bool) -> [Char] -> [[Char]] fixes the problem but I don't get why the signature isn't as general as I think it should be. Something to do with defaulting perhaps? __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Wiki page for rewrite rules tips, types and tricks
by dons＠cse.unsw.edu.au 20 Nov '06

20 Nov '06

After suggesting solutions based on rewrite rules to three different questions this week, I though maybe we better have a wiki page on using rewrite rules for non-traditional ad-hoc tricks. http://haskell.org/haskellwiki/Playing_by_the_rules If you've used rewrite rules for something fun (other than implementing a major deforestation system), please add it to the page. -- Don

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Arrows tutorial
by Tim Newsham 20 Nov '06

20 Nov '06

Small arrows tutorial (sorta, I'm learning myself and trying to doc as I go): http://www.thenewsh.com/~newsham/x/arrow.txt comments welcome. I ran into a type issue I couldnt resolve. I'd love any comments on that (documented near the bottom, liftA2'). Also, I'd like to put some info about ghci -farrows syntax when I figure that stuff out. If anyone feels like translating some examples into do-notation or coming up with new examples that showcase do-notation, please post em or send them to me. Tim Newsham http://www.thenewsh.com/~newsham/

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jhc, whole program optimizing compiler.
by szefirov＠ot.ru 20 Nov '06

20 Nov '06

Do anyone had any experience with JHC? I tried to install it second time and again get an error during library build. It's a pity, we need a speed in our very lazy code. ;)

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