- Haskell-Cafe - Haskell.org

instance reification in TH
by Dmitry Olshansky 23 Apr '10

23 Apr '10

Hello cafe, Could you say can I determine in Template Haskell that datatype X has instance of class Y? Regards, Dmitry

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ANNOUNCE: c2hs 0.16.2
by Duncan Coutts 23 Apr '10

23 Apr '10

All, c2hs version 0.16.2 is out and is available from hackage http://hackage.haskell.org/package/c2hs The only significant change in this release is that it works with ghc-6.12. Note that it is not currently possible to bind to C functions that use "long double" as the corresponding Haskell type (CLDouble) is not provided by the version of the base library that comes with ghc-6.12. See ghc ticket http://hackage.haskell.org/trac/ghc/ticket/3353 What is c2hs ============ The c2hs tool assists in the development of Haskell bindings to C libraries. It extracts interface information from C header files and generates Haskell code with foreign imports and marshaling. The major advantages of using c2hs compared to writing foreign imports by hand (or using hsc2hs) are: * Cross-language type safety: C functions are imported with the correct Haskell types. * Saves time: boilerplate marshaling code is generated. Links ===== Home page: http://www.cse.unsw.edu.au/~chak/haskell/c2hs/ Bug tracker: http://hackage.haskell.org/trac/c2hs/ Duncan

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ANN: forkable-monad 0.1
by David Anderson 23 Apr '10

23 Apr '10

Dear Haskellers, I'm happy, and only slightly intimidated, to announce the initial release of forkable-monad. The short version is that forkable-monad exports a replacement forkIO that lets you do this: type MyMonad = ReaderT Config (StateT Ctx IO) startThread :: MyMonad ThreadId startThread = forkIO threadMain threadMain :: MyMonad () threadMain = forever $ liftIO $ putStrLn "Painless monad stack forking!" Note the lack of monad stack deconstruction and reconstruction to transport it over to the new thread. You'll find the details in the Haddock documentation for the module. forkable-monad is available: * On hackage: http://hackage.haskell.org/package/forkable-monad * Via cabal: cabal install forkable-monad * Source and issue tracker: http://code.google.com/p/forkable-monad/ Feedback is of course welcome. As this is my first published Haskell code and Hackage upload, I expect there will be quite a bit! - Dave

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Haskell (wiki) Logo license
by Jens Petersen 23 Apr '10

23 Apr '10

Hi, We use the Haskell Logo in Fedora OS installer for the Haskell packages group: http://git.fedorahosted.org/git/?p=comps-extras.git;a=tree I trying to get the logo updated to the nice "new" logo designed by Jeff Wheeler: http://haskell.org/sitewiki/images/a/a8/Haskell-logo-60.png To do that we need to update the license in http://git.fedorahosted.org/git/?p=comps-extras.git;a=blob;f=COPYING Can someone tell me what the correct license for the above image file is? Is it the wiki license or something else? Thank you, Jens -- on behalf of Fedora Haskell SIG https://fedoraproject.org/wiki/Haskell_SIG

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Does anyone know the reason for differeng measurements in profiling under -threaded?
by Jesper Louis Andersen 23 Apr '10

23 Apr '10

Hi, I am asking if anyone has seen the following behaviour from GHC under -threaded and heavy use of File I/O, network I/O and STM use. After having run Combinatorrent for a while and then terminating it, we get the following output from the RTS in GC statistics: INIT time 0.00s ( 0.00s elapsed) MUT time 0.22s ( 60.06s elapsed) GC time 0.05s ( 0.07s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.26s ( 60.12s elapsed) %GC time 18.2% (0.1% elapsed) Alloc rate 431,635,651 bytes per MUT second Productivity 80.3% of total user, 0.4% of total elapsed Note that I am running it for exactly 60 seconds of wall-clock time. Yet, the profile information reports: total time = 11.20 secs (560 ticks @ 20 ms) total alloc = 78,696,568 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc ticks bytes MAIN MAIN 80.4 0.2 450 20033 PeerControl Process.Peer 11.2 1.0 63 95272 Sender Process.Peer.Sender 6.2 0.0 35 387 sha1_digest Digest 1.8 1.4 10 136377 checkPiece FS 0.2 92.3 1 9083658 TorrentManager Process.TorrentManager 0.2 1.8 1 179587 which is clearly wrong. Total time is nowhere near 11.20 secs with 560 ticks here. I am almost certain that the 450 ticks attributed to MAIN is wrong as well. I have SCCs at each possible forkIO-point in the program. -prof -auto-all -caf-all doesn't make any difference either, I've tried. Has anyone seen this kind of problem and in that case, what was the fix? Barring nobody knows, the only thing I have left to try is to dive into the GHC RTS and figure out what is going on in there. As an aside: Before writing combinatorrent, I wrote etorrent, A BitTorrent client in Erlang. The two clients follow more or less the same basic structure and largely does the same ting. Combinatorrent has better memory usage right now, mostly due to representation choices: Haskell can do much better data representation than erlang. On the other hand, etorrent still beats out combinatorrent in the CPU department, using less CPU. And that somewhat perplexes me. This is why I am looking at performance profiles and trying to find the culprit. But with the above problem, I can't find it, since I don't trust the profiler anymore to give me the right results. All logic suggest that Haskell is ripping apart Erlang in all parts that has no communication, mostly due to being compiled and having a static type system (The erlang code is interpreted, even though they have a HiPE JIT). In practice however, my guess is that the binary handling in Erlang is quite effective, as is the communication between processes, so this might turn the tides in the favor of the erlang-RTS. As a fun thing, I tried downloading the same file in the test environment and running strace -cf on both programs. Here are the top winners. I don't think you can trust all the columns: Erlang: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 95.57 285.220815 2241 127300 10518 futex 2.16 6.453726 271 23828 9838 read 2.15 6.420512 279153 23 wait4 0.07 0.198851 3 64073 1 epoll_wait 0.02 0.048004 4000 12 poll 0.01 0.039131 2 16297 write 0.01 0.026917 0 88057 37842 recvfrom 0.01 0.018174 0 75803 36 epoll_ctl 0.00 0.008001 667 12 1 select 0.00 0.007133 1 5138 3 lseek 0.00 0.001931 64 30 6 execve 0.00 0.001912 0 5535 writev 0.00 0.001334 191 7 vfork 0.00 0.001209 0 2684 1995 open 0.00 0.001128 0 3094 2257 stat 0.00 0.000942 6 153 munmap Haskell/GHC: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 32.20 15834.075616 176163 89883 10065 futex 21.72 10682.355809 176009 60692 26672 recvfrom 12.22 6009.064699 176023 34138 18 select 10.25 5038.622913 176009 28627 1 write 8.45 4153.652812 176010 23599 1 read 4.84 2380.702238 176009 13526 1 writev 3.59 1765.107536 175965 10031 rt_sigprocmask 1.95 961.185526 176009 5461 fstat 1.94 956.257032 176009 5433 lseek 1.80 883.829682 175922 5024 getrusage 0.85 419.781465 176009 2385 107 rt_sigreturn 0.04 19.713008 176009 112 fcntl 0.03 12.320630 176009 70 mmap 0.02 9.680495 176009 55 socket 0.02 9.504619 176011 54 53 connect 0.02 8.976459 176009 51 getsockopt 0.01 5.456279 176009 31 close Apart from Erlang using epoll, there is not too much difference from the perspective of the kernel, which I think is rather good :) -- J.

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wxHaskell not able to link to libstdc++.so
by Ahn, Ki Yung 23 Apr '10

23 Apr '10

Dear Haskellers, I heard from a guy who was having problems with wxHaskell using GHC 6.10.x. I tried it myself and had the exact same problem occurring using GHC 6.12.x as well. I am using the debian unstable version of ghc6 package. kyagrd@kyagrd:~/tmp$ head bb.hs import Graphics.UI.WX import Graphics.UI.WXCore (bitmapGetSize) main = start viewer viewer :: IO () viewer = do bm <- bitmapCreateFromFile "~/sample.jpg" size <- bitmapGetSize bm f <- frame [text := "viewer", on paint := onPaint bm, clientSize :=size] kyagrd@kyagrd:~/tmp$ runghc bb.hs bb.hs: <command line>: can't load .so/.DLL for: stdc++ (libstdc++.so: cannot open shared object file: No such file or directory) kyagrd@kyagrd:~/tmp$ ghc bb.hs bb.o: In function `r43M_info': (.text+0x48): undefined reference to `wxzm0zi12zi1zi4_GraphicsziUIziWXziWindow_zdfDimensionsObject_closure' bb.o: In function `r43O_info': (.text+0xa8): undefined reference to `wxzm0zi12zi1zi4_GraphicsziUIziWXziWindow_zdfPaintObject_closure' ... The problem is there is no libstdc++.so in my Debian system as well as the guy who uses Ubunto. What exists is libstdc++.so.6. kyagrd@kyagrd:~/tmp$ ls /usr/lib/libstdc++.* /usr/lib/libstdc++.so.6@ /usr/lib/libstdc++.so.6.0.13 I wonder how wxHaskell was even able to successfully configure and compile at all. I didn't have wx package installed before, and I installed it just today and it successfully configured and compiled but unable to dynamically link to libstdc++.so. More serious problem is that wxHaskell doesn't work even making a symbolic link of libstdc++.so from libstdc++.so.6. I've also tried running ldconfig -v and ldconfig -p as a root, but still get the same problem. Is this a commom problem with the libraries linking to libstdc++.so or is this just the wxHaskell problem? Or, is this a ghc problem? -- Ahn, Ki Yung

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PhD student / postdoc positions in PLT+AD
by Barak A. Pearlmutter 22 Apr '10

22 Apr '10

Seeking PhD students and postdocs interested in an elegant combination of functional programming and big-iron style numeric computing. Functional Programming and Automatic Differentiation PhD Studentships Postdoctoral Positions We are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination into a production-quality optimising compiler. Our research prototype compiler generates object code competitive with the fastest current systems, which are based on FORTRAN. We are seeking PhD students and postdocs with interest and experience in relevant areas: programming language theory, numeric computing/numeric linear algebra, or differential geometry; and a burning drive to help lift big iron numeric computing out of the 1960s and into a newer higher order. Specific sub-projects include: compiler and numeric programming environment construction; writing, simplifying, and generalising numeric algorithms through the use of AD operators; and associated type/lambda calculus/PLT/real computation issues. The project headquarters will be in the Hamilton Institute, NUI Maynooth, Ireland, http://www.hamilton.ie/. Applications to: "Barak A. Pearlmutter" <barak+ad-fp-job(a)cs.nuim.ie> -- Prof Barak A. Pearlmutter Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland http://www.bcl.hamilton.ie/~barak/

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GHC lexer questions
by Phyx 22 Apr '10

22 Apr '10

Hi all, I have a few questions about the GHC lexer (currently using GHC Glasgow Haskell Compiler, Version 6.13.20100320, for Haskell 98, stage 2 booted by GHC version 6.12.1) And I notice that when invoking the lexer I get a few unexpected results that I was hoping someone could clarify. The first part has to do with braces. Opening braces are lexed fine, but the closing ones throw an error. I assume the lexer is keeping some kind of state and trying to do brace matching? (but isn't this supposed to be done by the parser?) ?parseLine("{")->tag cStatelessParseResultSOk ?parseLine("}")->tag cStatelessParseResultSFailed and ?parseLine("{-")->tag cStatelessParseResultSOk ?parseLine("-}")->tag cStatelessParseResultSFailed and in a statement "let foo = .." "foo" isn't lexed as an identifier, "f" is lexed as a ITvocurly and the "oo" as an identifier. I find this rather odd, and maybe it's me doing something wrong but doubt it. The original Haskell code I call from FFI is lexSourceString :: String -> IO (StatelessParseResult [Located Token]) lexSourceString source = do buffer <- stringToStringBuffer source let srcLoc = mkSrcLoc (mkFastString "internal:string") 1 1 let dynFlag = defaultDynFlags let result = lexTokenStream buffer srcLoc dynFlag return $ convert result -- | convert the build in ParseResult to out custom StateLessParseResult -- mainly because the State is difficult to marshal and we don't really seem to need it convert :: ParseResult a -> StatelessParseResult a convert (POk _ ty) = SOk ty convert (PFailed src msg) = SFailed src (showSDocDump msg)

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Hughes' parallel annotations for fixing a space leak
by Heinrich Apfelmus 22 Apr '10

22 Apr '10

Dear haskell-cafe, I've been thinking about space leaks lately. In particular, I've been studying the tricky example with pairs break [] = ([],[]) break (x:xs) = if x == '\n' then ([],xs) else (x:ys,zs) where (ys,zs) = break xs which was discussed in the papers Jan Sparud. Fixing some space leaks without a garbage collector. http://bit.ly/cOxcVJ Philip Wadler. Fixing some space leaks with a garbage collector. http://homepages.inf.ed.ac.uk/wadler/topics/garbage-collection.html As I understand it, GHC implements the technique from Sparud's paper, so this is a solved problem. (It will only kick in when compiled with optimization, though, so -O0 will make the above leak in GHC 6.10.4 if I checked that correctly.) Now, I'm wondering about alternative solutions. Wadler mentions some sort of parallel combinators break xs = (PAR before '\n' ys, PAR after '\n' zs) where (ys,zs) = SYNCLIST xs which were introduced by John Hughes in his Phd thesis from 1983. They are intriguing! Unfortunately, I haven't been able to procure a copy of Hughes' thesis, either electronic or in paper. :( Can anyone help? Are there any other resources about this parallel approach? Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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Adopting MissingPy?
by John Goerzen 22 Apr '10

22 Apr '10

Hi folks, MissingPy is a library I wrote a little while back that allows you to call Python code from Haskell. It's on Hackage and, as far as I know, still works. Trouble is, the need I used to have for it is gone. So I no longer use it myself for anything, and thus it is starting to bitrot. Would anyone like to take over maintenance of this project? Thanks, -- John

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Philadelphia Haskell hackathon -- reduced hotel rate ends Friday April 23!
by Brent Yorgey 21 Apr '10

21 Apr '10

There is still plenty of space to register [1] for Hac phi 2010, but time is running out to get a hotel room at a reduced rate --- if you want the special rate for the block of rooms we have reserved at the Club Quarters, you must contact the hotel by this Friday, April 23. Instructions are on the wiki [2]. If you have been thinking of coming but are not sure, let me emphasize that last year's Hac phi was great fun---it will be an excellent time to make new friends, expand your brain, eat good food, and of course write some Haskell. You are welcome and encouraged to attend whether you just want to learn Haskell or have been using it for 10 years! You also do not have to come for the whole time; feel free to come just for a day or even an afternoon. The original announcement is below. -Brent ---------- Hac phi 2010 is a Haskell hackathon/get-together to be held May 21-23 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac phi was a lot of fun, and many people have already expressed interest in coming back this year. I want to stress that everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac phi wiki [2]. Note that we're in a different space this year and may have to cap attendance, so register early. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! -The Hac φ team [1] http://haskell.org/haskellwiki/Hac_φ/Register [2] http://haskell.org/haskellwiki/Hac_φ [3] http://haskell.org/haskellwiki/Hac_φ/Talks

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Re: [Haskell-cafe] ANN: forkable-monad 0.1
by Limestraël 21 Apr '10

21 Apr '10

Nice initiative! By the way, since this is a monad, I think a better place than Control.Concurrent.Forkable would be Control.Monad.Forkable. It's just a suggestion. > 2010/4/21 David Anderson <dave(a)natulte.net> > > Dear Haskellers, >> >> I'm happy, and only slightly intimidated, to announce the initial >> release of forkable-monad. >> >> The short version is that forkable-monad exports a replacement forkIO >> that lets you do this: >> >> type MyMonad = ReaderT Config (StateT Ctx IO) >> >> startThread :: MyMonad ThreadId >> startThread = forkIO threadMain >> >> threadMain :: MyMonad () >> threadMain = forever $ liftIO $ putStrLn "Painless monad stack forking!" >> >> Note the lack of monad stack deconstruction and reconstruction to >> transport it over to the new thread. You'll find the details in the >> Haddock documentation for the module. >> >> forkable-monad is available: >> >> * On hackage: http://hackage.haskell.org/package/forkable-monad >> * Via cabal: cabal install forkable-monad >> * Source and issue tracker: http://code.google.com/p/forkable-monad/ >> >> Feedback is of course welcome. As this is my first published Haskell >> code and Hackage upload, I expect there will be quite a bit! >> >> - Dave >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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ANN: GPipe-Collada - Collada for Haskell!
by Tobias Bexelius 21 Apr '10

21 Apr '10

Hi, I've uploaded GPipe-Collada-0.1.0 to hackage (http://hackage.haskell.org/package/GPipe-Collada-0.1.0) that enables you to load Collada files to be used with GPipe. Check out Graphics.GPipe.Collada.Utils.viewScene for an example on how to use loaded Collada scenes (http://hackage.haskell.org/packages/archive/GPipe-Collada/0.1.0/doc/html/Gr…) For more information regarding Collada, including free sample files, visit http://www.collada.org. Cheers! Tobias _________________________________________________________________ Surfa tryggt med Internet Explorer 8 http://www.microsoft.com/sverige/windows/internet-explorer/default.aspx

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ANNOUNCE: Agata-0.2.0
by Jonas Almström Duregård 21 Apr '10

21 Apr '10

I'm pleased to announce Agata (Agata Generates Algebraic Types Automatically)! Avoiding excessive details, usage is best described by a small example: {-#LANGUAGE TemplateHaskell #-} import Test.QuickCheck import Test.AgataTH data X a b = X [Either a b] deriving Show data Y = Y deriving Show data Z = Z deriving Show $(agatath $ deriveall [''X,''Y,''Z]) main = sample (arbitrary :: Gen (X Y Z)) This code derives instances of Test.QuickCheck.Arbitrary for the data types X, Y and Z. http://hackage.haskell.org/package/Agata Regards Jonas

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Re: [Haskell-cafe] Bulk Synchronous Parallel
by Jason Dusek 21 Apr '10

21 Apr '10

2010/4/21 Aaron D. Ball <aarondball(a)gmail.com>: > I don't need a tool that automatically figures out how to distribute > any workload in an intelligent way and handles all the communication > for me. You are right in general. Only if you want to rely on purity and a few source code annotations to get you parallelism relatively cheaply do you care about these compiler approaches. This is something that Haskell can do that Ruby, C and friends really can not do -- thus I mention it. > If I have the basic building block, which is the ability to > serialize a Haskell expression with its dependencies and read them > into another Haskell instance where I can evaluate them, I can handle > the other pieces, which are > > - passing strings back and forth in whatever way is convenient > - deciding how to divide up my workload. Do add also, configuring servers and their connections. > In the Ruby universe, DRb combines the serialization and "passing > strings around" job and lets me figure out how to divide up the work, > and it would be delightful if there were something similarly simple in > the Haskell world. I think Holumbus has got some promising stuff for user-managed distributed workers: http://holumbus.fh-wedel.de/trac/browser/distribution What do you think? -- Jason Dusek

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instance Eq (a -> b)
by Ashley Yakeley 21 Apr '10

21 Apr '10

Why isn't there an instance Eq (a -> b) ? allValues :: (Bounded a,Enum a) => [a] allValues = enumFrom minBound instance (Bounded a,Enum a,Eq b) => Eq (a -> b) where p == q = fmap p allValues == fmap q allValues Of course, it's not perfect, since empty types are finite but not Bounded. One can nevertheless make them instances of Bounded with undefined bounds, and have enumFrom and friends always return the empty list. It seems one should also be able to write instance (Bounded a,Enum a) => Traversable (a -> b) where ??? But this turns out to be curiously hard. -- Ashley Yakeley

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hamming distance allocation
by Arnoldo Muller 21 Apr '10

21 Apr '10

Hello all: I want to generate some hamming distance statistics about a set of strings. As explained in another e-mail in this list, I used the following code to call the functions: (exampl holds the list of strings of size w) filter (\x -> x /= 0) $ map (uncurry hammingX) [(xs, ys) | xs <- exampl, ys <- exampl] I have two hamming functions: -- hamming distance for variable length strings hamming :: String -> String -> Int hamming x y = hamming' x y 0 where hamming' [] _ !c = c hamming' _ [] !c = c hamming' (x:xs) (y:ys) !c | x == y = hamming' xs ys c | otherwise = hamming' xs ys (c + 1) -- function posted in this mailing list hamming2 :: String -> String -> Int hamming2 xs ys = length (filter not (zipWith (==) xs ys)) I am executing these functions millions of times and the bottleneck of my program is in them as explained by running in profiling mode with +RTS -K400M -p -RTS The costlier function is the hamming distance COST CENTRE MODULE %time %alloc hamming Distances 66.6 41.9 It says that it is performing 41% of the allocations. In the case of hamming2 the allocations go as far as 52%. I could understand that there are allocations in "hamming2" because we are creating pairs, but in the case of "hamming" there should be no allocation. How can I execute my hamming functions without allocating memory? Best regards, Arnoldo Muller

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Re: [Haskell-cafe] Re: Move MonadIO to base
by Anders Kaseorg 21 Apr '10

21 Apr '10

On Tue, 20 Apr 2010, wren ng thornton wrote: > -- | The isomorphic Haskell98 version > class Monad m => MonadJoinIO m where > -- | Embed the IO into the monad m > joinIO :: IO (m a) -> m a > > -- | Extract the IO computation to the top level, > -- rendering the m pure from IO. > partIO :: m a -> IO (m a) > > -- | The isomorphisms > joinIO' m = morphIO (m >>=) > morphIO' f = joinIO (f partIO) To establish an isomorphism, you also need to define partIO from morphIO. For example, I don’t see how I could define partIO :: IO a -> IO (ReaderT r IO a) that extracts effects into the outer IO, because the effects depend on some unknown state of type r. By the way: > This bounced because I have different emails registered for cafe@ and > libraries@, so forwarding it along to the cafe. You can sign up both addresses for the list, then log in to the Mailman web interface ( http://www.haskell.org/mailman/listinfo/haskell-cafe → Unsubscribe or edit options) and disable mail delivery on one of them. Then you get one copy of each message but you can post from either address. Anders

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C variable access via FFI
by Tom Hawkins 20 Apr '10

20 Apr '10

I have a bunch of global variables in C I would like to directly read and write from Haskell. Is this possible with FFI, or must I write a bunch of C wrapper functions for the interface, i.e. a 'get' and a 'set' for each variable? I'm building a simulator for one of our embedded systems. The C is embedded code for an ECU -- the global variables are sensor and control IO. I'm using Haskell to integrate the C with vehicle models and to provide the user interface. With the help of the SDL hackage lib, I'm able to feed analog and discrete inputs from a gamepad into the simulator, which then runs the embedded code and streams VCD data into GTKWave, providing waveform visualization in realtime. Pretty fun stuff. -Tom

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what does the '~' mean ?
by zaxis 20 Apr '10

20 Apr '10

instance (BinaryDefer a, BinaryDefer b) => BinaryDefer (a,b) where put (a,b) = put2 a b get = get2 (,) size x = let ~(a,b) = x in size a + size b putFixed (a,b) = putFixed2 a b getFixed = getFixed2 (,) in `size` function, what does the `~` mean ? Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/what-does-the-%27%7E%27-mean---tp28263383p28263383.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Why does the transformers/mtl 'Error' class exist?
by John Millikin 20 Apr '10

20 Apr '10

Both the transformers[1] and mtl[2] define a class named 'Error', for use with MonadError and ErrorT. This class is required for the instance of Monad (and therefore MonadTrans, MonadIO, etc). However, I can't figure out why this class exists. Its presence means that instead of something like: ------------------------------------- data NameError = ErrorFoo | ErrorBar validateName :: Monad m => Text -> m (Either Error Text) validateName x = runErrorT $ do when (some condition) $ throwError ErrorFoo when (other condition) $ throwError ErrorBar return x ------------------------------------- I have to define this, which is more verbose, no more useful, and adds a "fake" class to the Haddock docs (with a warning not to use it): ------------------------------------- data Error = ErrorFoo | ErrorBar instance Error NameError where strMsg = error -- validateName ... ------------------------------------- Is there any good reason why the 'Error' class can't just be removed? [1] http://hackage.haskell.org/packages/archive/transformers/0.2.0.0/doc/html/C… [2] http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Mo…

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ANNOUNCE: hamlet 0.0.0
by Michael Snoyman 20 Apr '10

20 Apr '10

Hi all, I'd like to announce the first release of Hamlet[1], a templating system which is fully compile-time checked. Templates are parsed via quasi-quoting, giving you greater confidence in the validity of your templates. The syntax is inspired by Haml[2]; however, it is most definitely its own language, avoiding embedding of arbitrary code in favor of a more strict view-logic separation. Finally, it has a goal of allowing high performance uses; to facilitate this, it allows calling monadic code from inside the template, and can use enumerators for traversing elements. While these features could be abused to allow templates to perform unintended side-effects, the performance gains allowed by them are sometimes essential. I have started a documentation site for all Yesod-related packages[3], and in particular there is a Hamlet section[4]. I've also put up a small blog post[5] on this release, which explains how Hamlet fits into the Yesod ecosystem. (For those not in the know, Yesod is a web framework I'm working on.) Michael [1] http://hackage.haskell.org/package/hamlet-0.0.0 [2] http://haml-lang.com/ [3] http://docs.yesodweb.com/ [4] http://docs.yesodweb.com/hamlet/ [5] http://www.snoyman.com/blog/entry/hamlet-is-born/

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Profiling
by rodrigo.bonifacio 20 Apr '10

20 Apr '10

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DLL on Windows
by Serguey Zefirov 20 Apr '10

20 Apr '10

I tried the way described in http://www.haskell.org/ghc/docs/latest/html/users_guide/win32-dlls.html and i got this error message: -------------------------------------------------------------------------------------------- ghc -shared -o test.dll --make test.hs [1 of 1] Compiling Test ( test.hs, test.o ) ghc.exe: panic! (the 'impossible' happened) (GHC version 6.10.3 for i386-unknown-mingw32): link: GHC not built to link this way: LinkDynLib Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug -------------------------------------------------------------------------------------------- How can I fix it?

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ANN: Rika engine - alpha :)
by Jinjing Wang 20 Apr '10

20 Apr '10

Dear list: ANN: Rika Engine Source: http://github.com/nfjinjing/rika What is it: A 3D renderer, with some basic scripting ability, extremely hacky, not practical. Why: Example GLSL renderer? Deferred shading implementation ( though heavily cheated, you will see why .. ) ? This is one. Have fun folks :) -- jinjing

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newbie question how to pass data
by Mujtaba Boori 20 Apr '10

20 Apr '10

Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) ->Bool) -> Float -> ((Float,Float) -> Bool) I am trying to make calculation in this type ((Float,Float) ->Bool) with Float and then pass the information to ((Float,Float) -> Bool) Thank again appreciated.

4 6

memory needed for SAX parsing XML
by Daniil Elovkov 20 Apr '10

20 Apr '10

Hello haskellers! I'm trying to process an xml file with as little footprint as possible. SAX is alright for my case, and I think that's the lightest way possible. So, I'm looking at HaXml.SAX I'm surprised to see that it takes about 56-60 MB of ram. This seems constant relative to xml file size, which is expected. Only slightly depends on it as I recursively traverse the list of sax events. But it seems like too much. The size of the file is from 1MB to 20MB. The code is something like this main = do (fn:_) <- getArgs h <- openFile fn ReadMode c <- hGetContents h let out = proc $ fst $ saxParse fn c putStrLn out getChar -- 'f' is tail-recursive -- 'i' is used only to ensure that the list is completely traversed proc es = show $ f es 0 where f :: [SaxElement] -> Int -> Int f (SaxElementOpen name _ : rest) i = f rest i f (_ : rest) i = f rest i f [] i = i Thanks for you thoughts! -- Daniil Elovkov

2 2

Re: Move MonadIO to base
by Anders Kaseorg 20 Apr '10

20 Apr '10

On Sun, 18 Apr 2010, wren ng thornton wrote: > lift ma = morph (\k -> join (fmap (k . return) ma)) Monad laws simplify that to lift ma = morph (\k -> ma >>= k . return) > The type of morph requires us to Church-encode things needlessly; what > we mean to say is: morph (fmap return ma). Hmm. If I understand this (and your other emails) correctly, you’re saying my interface class Monad m => MonadMorphIO m where morphIO :: (forall b. (m a -> IO b) -> IO b) -> m a should be equivalent to a simpler interface class Monad m => MonadJoinIO m where joinIO :: IO (m a) -> m a via some isomorphism like morphIO f = joinIO (f return) joinIO m = morphIO (\w -> m >>= w) I would be very happy to get the simpler interface to work, because it’s Haskell 98. However, if I write joinIO m = morphIO (\w -> m >>= w) morphIO' f = joinIO (f return) and define catch using morphIO' instead of morphIO: m `catch` h = morphIO $ \w -> w m `Control.Exception.catch` \e -> w (h e) m `catch'` h = morphIO' $ \w -> w m `Control.Exception.catch` \e -> w (h e) then catch' fails to actually catch anything: *Main> throwIO NonTermination `catch` \NonTermination -> return "moo" "moo" *Main> throwIO NonTermination `catch'` \NonTermination -> return "moo" *** Exception: <<loop>> Am I doing something wrong? Anders

3 4

Continual delivery status notification failures
by Ivan Miljenovic 20 Apr '10

20 Apr '10

Can the owner of c10b66c97b5cd09384aa9f82ecd95c2b(a)orangeat.blackberry.com please fix their emails up (or the haskell-cafe admins remove that address), as I keep getting delivery status notification failure messages over a week after I send an email to the list (I would have emailed that address directly, except if they can't get -cafe they probably can't get any emails...). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Strange error with type classes + associated types
by Brent Yorgey 19 Apr '10

19 Apr '10

Hi all, Consider the following declarations. > -- from vector-space package: > (*.*) :: (HasBasis u, HasTrie (Basis u), > HasBasis v, HasTrie (Basis v), > VectorSpace w, > Scalar v ~ Scalar w) > => (v :-* w) -> (u :-* v) -> u :-* w > > -- my code: > data Affine v = Affine (v :-* v) v > > instance (HasBasis v, HasTrie (Basis v), VectorSpace v) => Monoid (Affine v) where > mempty = Affine idL zeroV > mappend (Affine a2 b2) (Affine a1 b1) = Affine (a2 *.* a1) (lapply a2 b1 ^+^ b2) When I try to compile this, I get the following error: No instance for (HasTrie (Basis u)) arising from a use of `*.*' at Diagrams.hs:107:50-58 Possible fix: add an instance declaration for (HasTrie (Basis u)) In the first argument of `Affine', namely `(a2 *.* a1)' In the expression: Affine (a2 *.* a1) (lapply a2 b1 ^+^ b2) In the definition of `mappend': mappend (Affine a2 b2) (Affine a1 b1) = Affine (a2 *.* a1) (lapply a2 b1 ^+^ b2) This seems bizarre to me; it seems like GHC ought to be able to infer that in my use of (*.*), u,v, and w are all instantiated to the v in the instance declaration, and hence all the required constraints are satisfied. I have no idea why it would be complaining about u --- there's nothing called u in my instance declaration. Can someone more well-versed in the intricacies of type checking with associated types explain this? Or is this a bug in GHC? -Brent

4 10

US Patent for the idea of using Haskell to implement UAX #9
by Brian Hulley 19 Apr '10

19 Apr '10

Hi everyone, It's been a long time since I last posted to this list since I'm currently working on something that is not directly Haskell-related, but it still relates to functional programming in general. Anyway imagine my surprise when an innocent search for some keywords (I can't remember the exact ones but the following gives similar results) haskell unicode bidirectional revealed a link to a US Patent (7120900) for the idea of implementing the Unicode Bidirectional Algorithm (UAX #9 http://www.unicode.org/reports/tr9) in Haskell, making use, as far as I can tell, of nothing more than the normal approach any functional programmer would use, namely separation of concerns etc. The link is http://www.freepatentsonline.com/7120900.html though I think it would be better if they had just called the website "free handcuffs online" because that is what it amounts to when people succeed in preventing others from using ideas, especially ideas everyone would easily think up by themselves. Before going further I would like to explicitly state that I would not wish to cast any aspersions upon the people or companies involved in this patent, since it is all too clear to me that these people have acted in a perfectly legal way and are just doing their various jobs: in other words it's an "I'm a lumberjack and I'm ok..." kind of scenario: everyone probably thinks of themsevles as being a perfectly nice upstanding person, and if I met any of them face to face I'm sure I would find that they are *indeed* really nice friendly people who believe that they are doing absolutely the right and logical thing. In fact many nice people I myself have talked to about my own ideas immediately suggest that I should "obviously" patent them: it takes a long time to explain to them why this would not actually further my goals at all, and it is difficult for them to understand my explanation because the problem is rather too subtle for people who have not already been thinking along these lines for a while. However all this does not affect the problem that this patent acts as a brick in the general structural evil in our world today: anyone who is trying to create a programming language environment that is accessible to people who use right-to-left languages is now between a rock and a hard place when it comes to implementing UAX #9, the standard Unicode algorithm for implementing the Unicode semantics of bidirectional text. In this particular case, I expect (but I'm not a lawyer so please don't take this as solid advice: it's just a hope) that a workaround for Haskell programmers would be to discard their natural desire to use a functional approach and instead just implement UAX #9 verbatim using the ST monad. (The patent also tries to extend itself to other functional languages so caution is needed all round.) But although the above workaround *might* be legal in this particular instance it seems to me that as programmers we must not just let our craft be dominated by the psychopathic tendencies of certain elements in society that work hard to try to squash the ``normals'' into a life of brutal misery while they rampage ruthlessly about with a biological inability to experience empathy for other human beings. (www.ponerology.com) The memetic virus of psychopathy is rapidly spreading throughout human civilization and most people are not conscious enough of the elements that are contributing towards their own behaviour, thus ignoring the fact that the rice they eat for supper might have been picked by a tiny little girl in China with bleeding fingers or the carpet they walk on may have been made by a little 8 year old boy slave chained all day to a loom in a shed in India. To this end I humbly appeal to everyone here to please help in the fight against software patents, so that we can begin the huge task of reclaiming our world for real people who understand that true meaning in life comes from extending our feeling of self into the world beyond our own body: http://petition.stopsoftwarepatents.eu/ http://ffii.org Thanks a lot for reading this, Brian. [expecting to be crucified, but if it helps just one little girl or boy it will be worth it!] Disclaimer: this email is entirely my responsibility and I am not acting on behalf of any of the above web sites.

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Re: [Haskell-cafe] hamming distance allocation
by John Lato 19 Apr '10

19 Apr '10

> Subject: Re: [Haskell-cafe] hamming distance allocation > > Am Montag 19 April 2010 01:03:14 schrieb Arnoldo Muller: >> Hello all: >> >> I want to generate some hamming distance statistics about a set of >> strings. As explained in another e-mail in this list, I used the >> following code to call the >> functions: >> (exampl holds the list of strings of size w) >> filter (\x -> x /= 0) $ map (uncurry hammingX) [(xs, ys) | xs <- exampl, >> ys <- exampl] >> >> I have two hamming functions: >> -- hamming distance for variable length strings >> hamming :: String -> String -> Int >> hamming x y = hamming' x y 0 >> where >> hamming' [] _ !c = c >> hamming' _ [] !c = c >> hamming' (x:xs) (y:ys) !c >> | x == y = hamming' xs ys c >> | otherwise = hamming' xs ys (c + 1) >> >> -- function posted in this mailing list >> hamming2 :: String -> String -> Int >> hamming2 xs ys = length (filter not (zipWith (==) xs ys)) >> >> I am executing these functions millions of times and the bottleneck of >> my program is in them as explained by running in profiling mode with >> +RTS -K400M -p -RTS >> >> The costlier function is the hamming distance >> COST CENTRE MODULE %time %alloc >> >> hamming Distances 66.6 41.9 >> >> It says that it is performing 41% of the allocations. In the case of >> hamming2 the allocations go as far as 52%. > > Allocations are cheap, so that's not necessarily a problem. More important > is, what's the maximum residency and how much is copied during GC? > Are you compiling with -O2 ? > >> I could understand that >> there are allocations in "hamming2" because we are creating pairs, but >> in the case of "hamming" there should be no allocation. > > Why not? I don't know how GHC counts allocations, but everytime you go from > (x:xs) to xs, you need a new pointer to the tail. If that counts as > allocation, hamming must allocate a lot, too. Is it really necessary to use Strings? I think a packed type, e.g. Vector or ByteString, would be much more efficient here. Of course this is only likely to be a benefit if you can move away from String entirely. I suspect that "hamming2" would perform better then. John

3 4

help with Haskell programming
by Mujtaba Boori 19 Apr '10

19 Apr '10

Hello I am kinda newbie in Haskell you can help help me with some programming I am trying to make function like for example func :: (a -> Bool) -> (a -> Bool) this function make calculation and return bool . I want to be able to make bool True when It is False and False when it is True while returning the a. Thank you -- Mujtaba Ali Alboori

5 8

Patch to add process group support
by Hamish Mackenzie 19 Apr '10

19 Apr '10

The attached patch for the process package adds support for creating and interrupting process groups on Unix and Win32 systems. Currently we have to use a nasty hack in Leksah that only works on Unix systems (and even then not very well). On Win32 Leksah's background build feature is dreadful as a results (because it has no way to interrupt the build and start over when the user makes more changes). I would love to get this onto Hackage somehow, if not as a new version of the process package, perhaps as an alternative package ("process-groups" using System.Process.Groups instead of System.Process). Please can you have a look and let me know what you think? Thanks, Hamish This patch introduces the following functions to System.Process * runCommandNewGroup, runProcessNewGroup, runInteractiveCommandNewGroup and runInteractiveProcessNewGroup __ These are variations on the existing functions for running processes. They create the child process as the lead process in a new process group. Unix - calls setpgid in both the parent and the child process after fork. Win32 - calls CreateProcess with the CREATE_NEW_PROCESS_GROUP set. I also had it unset CREATE_NO_WINDOW because this seems to prevent the child attaching to the parents console (and therefor stops interuptProcessGroup from working). * interruptProcessGroup This function can be used interrupt a running process group. Unix - calls signalProcessGroup to send sigINT Win32 - If the process ID is known it calls generateConsoleCtrlEvent to send cTRL_BREAK_EVENT __ Compatibility __ Backward Compatibility CreateProcess has a new field new_group which may need to be added to some existing code. If set to False the current behaviour is preserved. As far as I know this is the only change that could break existing code. Linker Errors I have renamed the C functions it used in order to prevent linker errors when using this new version with the existing process package. __ Win32 Only __ On Win32 the process handle is not the same as the process ID and there is no reliable way to convert from one to the other. I have added an interface that allows access to the process ID, but does not change the behaviour of the existing functions. * PINFO Replaces PHANDLE in the ProcessHandle type. type PINFO = (PHANDLE, Maybe Word32) -- Handle and Process ID * mkProcessHandleWithPID Like mkProcessHandle but allows you to specify the processes ID as well. This function is now used in all the run functions so the ProcessHandle they return will contain a process ID. * withProcessInfo and withProcessInfo_ Like withProcessHandle functions but gives you access to the PINFO (not just the PHANDLE it contains).

2 2

Re: [Haskell-cafe] hamming distance allocation
by John Lato 19 Apr '10

19 Apr '10

> From: Daniel Fischer <daniel.is.fischer(a)web.de> > > Am Montag 19 April 2010 14:37:33 schrieb John Lato: >> Is it really necessary to use Strings? I think a packed type, e.g. >> Vector or ByteString, would be much more efficient here. > > Not very much if the strings are fairly short (and the list isn't too long, > so there's not a big difference in cache-friendliness). > If eight-bit characters aren't enough, packing the strings into > UArray Int Char gives performance quite close to ByteStrings. I agree, provided that the Strings are used only once. I missed the first part of this thread, so there may be context I don't have, but if Strings are retained and the Hamming distance calculated for multiple pairings, I think it would be a noticeable difference. Although probably not as much as reducing the number of calls to Hamming as you and H. Apfelmus suggest. John

1 0

Functional Dependencies conflicts
by Limestraël 19 Apr '10

19 Apr '10

Hello, I'm trying to make two simple classe which would help me to transform unserializable datatypes to serializable ones. The classes are: class (Binary b) => Binarizable a b | a -> b where toBinary :: a -> b class (Binarizable a b, Monad m) => Unbinarizable a b m | a -> b where fromBinary :: b -> m a The idea is simple: if we have a type 'a' which cannot be serialized (for instance because it contains functions), we may turn it into a 'b' type which is instance of Binary, with of course a loss of information. And then, Unbinarizable enables us to get the original 'a' type back. fromBinary has to run inside a monad so that it can somehow recover the lost information. Now, for instance, in a simple role playing game, we would have the datatype : data GameObject = GameObject { objIdentifier :: String, objEffect :: Character -> Character } It can't obviously be declared instance of Binary, since the field objEffect is a function. So -- since an objIdentifier must be unique -- we can declare it instance of Binarizable/Unbinarizable: instance Binarizable GameObject String where toBinary = objIdentifier instance (MonadReader [GameObject] m) => Unbinarizable GameObject String m where fromBinary name = liftM getIt ask where getIt = maybe err id . find ((== name) . objIdentifier) err = error $ "Unbinarize: The object '" ++ name ++ "' doesn't exist!" To be unbinarized, we need to have a ReaderMonad which grants us access to the list of all the objects, so that we may find the object from its identifier. Well, here comes the trouble: GameStructs.hs:16:9: Functional dependencies conflict between instance declarations: instance (Binary a) => Binarizable a a -- Defined at MagBots/GameStructs.hs:16:9-37 instance Binarizable GameObject String -- Defined at MagBots/GameStructs.hs:38:9-37 GameStructs.hs:19:9: Functional dependencies conflict between instance declarations: instance (Binary a, Monad m) => Unbinarizable a a m -- Defined at MagBots/GameStructs.hs:19:9-50 instance (MonadReader [GameObject] m) => Unbinarizable GameObject String m -- Defined at MagBots/GameStructs.hs:41:9-73 I don't see why the functional dependencies conflict, since GameObject is not an instance of Binary...

6 13

Embedded funcional programming?
by Maurício CA 19 Apr '10

19 Apr '10

Hi, all, I've beeing working with some people who do programming for wireless devices. 100% of their code uses C, and I would like to show them nice things they could do with funcional programming (not necessarily Haskell. I believe, say, Standard ML could be also very nice.) I'm new to this, so the only problems I see are finding a compiler that targets the platform (ARM7, for instance, or others) and uploading the compiled firmware to the device. Do you think this can be a straightforward path? Thanks, Maurício

9 8

Re: Ocaml for Haskellers tutorial
by Pierre-Etienne Meunier 19 Apr '10

19 Apr '10

Hi, Strangely enough, I did the opposit some time ago. Ocaml is way simpler than haskell, in fact. There is no referential transparency, no laziness (though it can be simulated with "fun ()->x", but the type of a lazy expression is not the same as the reduced expressions), no parallelism (the GC is not parallel). There are no list comprehensions. The syntax for pattern matching is simpler but more restrictive, and there are loops. There are also objects, but even good caml programmers (i.e. at ICFP level) do not use it. Also, there are fewer available libraries, due to a quite immature FFI. I should also say that the top-level is of much greater help than ghci, and I strongly recommend using tuareg-mode with emacs, which make up a very powerful combination. Have a look at these examples : let fact n= if n=0 then 1 else n*(fact (n-1)) This function is of type Int->Int because the arithmetic operations are monomorphic (there are no typeclasses). If you wanted to do it with Doubles, you would have used *. +. and -. For other types, you would have been on your own ;-) let quickSort l=match l with []->[] | h::t -> let (a,b)=List.partition (fun x->x<=h) t in List.append (quickSort a) (h::(quickSort b)) As you can see, match is pretty close to case, except that it requires | between clauses, because the indentation is not significative. Also, lambda abstractions are called fun x-> instead of \x->. There is a syntactic sugar for combining lambdas and match : function []->M | h::t->M' which translates as fun x->match x with ... You cannot see it here, but camlists do not define variables with indentations as in : let a= b= in This would be "let a=... in let b=... in" with any number of lines and arbitrary indentation. Or "let a=... and b=... in", for instance if you want mutually recursive functions. This is because an ocaml file is evaluated in the order in which it is written, which allows for a cool top-level (by the way, the cool emacs command is C-x-e, "evaluate last expression" in tuareg mode). The module system seems sometimes obscure, as modules do not correspond always to files, and you can define several modules in a file. You can read http://caml.inria.fr/pub/docs/manual-ocaml/manual004.html to get an idea. You can define custom types. There is not newtype in haskell, and the keywords for type and data are the same : type tree a = Leaf of a | Node of (tree a)*(tree a) Then you can match on it : match t with Leaf x-> Node (x,y)-> How to do monads ? It is very simple, actually. Some expressions are of type (), with great similarity to IO (). >>= is expressed like in C, with ; print_string "This is a string"; print_int (a+b); a Would translate in haskell as : do putStr "This is a string" print $ a+b return a But the typesystem of ocaml does not require you to use monads. Also, you may have notice that there is no $ in ocaml. There is also a very fine exception system : exception MyException; exception MyException'; try if a=0 then raise MyException else raise MyException' with MyException -> print_string "MyException" | MyException' -> print_string "MyException'" Which allows you to match the value of what error you program did. The last thing you can use is references and loops : for i=0 to n do ... done; let a=ref 10 while (!a)=0 do print_int (!a); a := (!a)-1 done; Most of the time you use mapM_ in haskell, you can do it with loops and references in ocaml, and you save the memory allocation of the list (which is not a lot, I know). You can also notice that references are much simpler to use than in haskell, due to "implicit monads" and unsafePerformIO everywhere. What else ? Records and record updates (you may find this in the manual). And objects : maybe your future boss smokes objects, thus you can learn about this in the manual. A good point in ocaml is that the program does, to a large extent, exactly what you write, in the order you write it.Thus, it is often simpler to know where exceptions occur, and performance is less an issue than in haskell. In fact, haskell waited more than ten years before producing fast code, whereas ocaml did it almost from the beginning. Good luck ! Pierre-Etienne

2 3

Re: Move MonadIO to base
by David Menendez 19 Apr '10

19 Apr '10

On Sun, Apr 18, 2010 at 5:02 PM, wren ng thornton <wren(a)community.haskell.org> wrote: > Heinrich Apfelmus wrote: >> >> Anders Kaseorg wrote: >>> >>> This concept can also be generalized to monad transformers: >>> >>> class MonadTrans t => MonadTransMorph t where >>> morph :: Monad m => (forall b. (t m a -> m b) -> m b) -> t m a >> >> [...] >> However, not all control operators can be lifted this way. Essentially, >> while you may "downgrade" an arbitrary selection of t m a values you >> may only promote one m a in return and all have to share the same >> return type a . In particular, it's not possible to implement >> >> lift :: (Monad m, MonadTrans t) => m a -> t m a > > Why not? > * morph says m(t m a) is a subset of (t m a) > * Monad m says we can fmap :: (a->b) -> (m a->m b) > * Monad (t m) says we can return :: a -> t m a > > lift ma = morph (\k -> k (fmap return ma)) Maybe something like this? lift m = morph (\k -> m >>= k . return) -- n.b., return and >>= are from different monads > Again, having m(t m a)->(t m a) is strictly more expressive than only having > (m a)->(t m a) because the former may avail itself of operations/operators > of t. join . lift :: m (t m a) -> t m a morph is more powerful than lift, but it isn't because of the type. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

1 0

vector recycling
by Ben 19 Apr '10

19 Apr '10

hello -- this is mostly a question for roman, or don, i guess. suppose i have a list of similarly-sized vectors, and i want to add them up (possibly with coefficients), to yield a result vector. something like module Main where import qualified Data.Vector.Generic as V import qualified Data.Vector.Unboxed as UV type Vec = UV.Vector Double axpy :: Double -> Vec -> Vec -> Vec axpy a x y = V.zipWith (+) (V.map (* a) x) y sumVecs :: [(Double, Vec)] -> Vec sumVecs axs = let (a, x) = head axs in foldl adder (V.map (* a) x) (tail axs) where adder :: Vec -> (Double, Vec) -> Vec adder v1 (a, x) = axpy a x v1 how to i write this in a way which ensures recycling / fusion, e.g. in-place updates? best regards, ben

3 4

ANN: vcd-0.1.0
by Tom Hawkins 19 Apr '10

19 Apr '10

The release adds a simple VCD parser (via Parsec) and minor changes to the VCD generation API. http://hackage.haskell.org/package/vcd

1 0

cabal: other-modules
by Johannes Waldmann 18 Apr '10

18 Apr '10

What is the purpose of the "Other-Modules" section in a cabal file? (I.e., can we drop it?) If it's just the set of dependencies of the "Exported-Modules", then a reasonable build system should be able to figure this out on its own. The current situation is that I have to manage this list manually (or is there some tool?) and if I forget to name some "other-module", then "cabal install" happily builds and installs (!) the package anyway, but I'm getting linker errors when using it. This is a nuisance, to say the least. J.W.

8 9

Re: Move MonadIO to base
by wren ng thornton 18 Apr '10

18 Apr '10

This bounced because I have different emails registered for cafe@ and libraries@, so forwarding it along to the cafe. wren ng thornton wrote: > wren ng thornton wrote: >> Heinrich Apfelmus wrote: >>> Anders Kaseorg wrote: >>>> This concept can also be generalized to monad transformers: >>>> >>>> class MonadTrans t => MonadTransMorph t where >>>> morph :: Monad m => (forall b. (t m a -> m b) -> m b) -> t m a >>> >>> [...] >>> However, not all control operators can be lifted this way. Essentially, >>> while you may "downgrade" an arbitrary selection of t m a values you >>> may only promote one m a in return and all have to share the same >>> return type a . In particular, it's not possible to implement >>> >>> lift :: (Monad m, MonadTrans t) => m a -> t m a >> >> Why not? >> * morph says m(t m a) is a subset of (t m a) >> * Monad m says we can fmap :: (a->b) -> (m a->m b) >> * Monad (t m) says we can return :: a -> t m a >> >> lift ma = morph (\k -> k (fmap return ma)) > > Or rather, > > lift ma = morph (\k -> join (fmap (k . return) ma)) > > That's what I get for typing without checking. The type of morph > requires us to Church-encode things needlessly; what we mean to say is: > morph (fmap return ma). > >> Again, having m(t m a)->(t m a) is strictly more expressive than only >> having (m a)->(t m a) because the former may avail itself of >> operations/operators of t. -- Live well, ~wren

1 0

problem with cabal on snow leopard
by Carter Schonwald 18 Apr '10

18 Apr '10

Hello all, I can't seem to find it documented anywhere as to the default directories that cabal puts its information in (its certainly not in ~/.cabal ), as I'm finding that even when I try to do a "reinstall" of the haskell platform, cabal thinks that all the libraries i removed are still there. how can i fix this? thanks -Carter

3 5

Confusions about the Haskell Platform (for Mac)
by Sean Leather 18 Apr '10

18 Apr '10

1. Why can't the platform download site be hosted on www.haskell.org instead of hackage.haskell.org? I see that there's a redirect, but (imho) it would be ideal to have www.haskell.org/platform be the standard URL in my browser. It is easier to remember (for typing) and more obvious (for appearances). 2. What is the difference between "Haskell" and the "Haskell Platform"? I see one or the other in various places. To get from www.haskell.org to downloading the Mac software, I go through "Download Haskell," "Get the Haskell Platform > Mac," and "Download Haskell for Mac OS X (intel)." 3. By looking at http://hackage.haskell.org/platform/mac.html , I have no idea what I'm going to get when I click the "Download Haskell for Mac OS X (intel)" link. It would be nice to know what I'm getting myself into before I commit to waiting for this 137.9 MB file. Thanks to the wonder of deep links, one can also not be expected to traverse the path that I did in #2. Similarly, if I come to this link directly, I have no easy way of navigating to try to figure out what "Haskell" or "GHC" is. 4. The current link for the Mac image points to http://hackage.haskell.org/platform/2010.1.0.0/haskell-platform-2010.1.0.1-…. Note the inconsistency between the version in the directory and file names. 5. The directions on http://hackage.haskell.org/platform/mac.html say: After downloading: * Open the .dmg file * Follow the install instructions I see no "install instructions" in the .dmg file. I only see the following files: * GHC-6.12.1-i386.pkg * Haskell Platform 2010.1.0.1.pkg * Uninstall GHC 6. Since there are no "install instructions," I am not sure which .pkg to install first. Of course, fix #3 and you hopefully fix this one. 7. Why can't I have a single .pkg file? Why do I have an "Uninstall GHC" and not an "Uninstall Haskell Platform"? -- I don't expect all of the above to necessarily have a direct solution. They are just my observations to share. Some of them are from the point of view of a newcomer, and some are from a perfectionist. Regards, Sean

10 26

Re: [Haskell-cafe] Embedded funcional programming?
by Patai Gergely 18 Apr '10

18 Apr '10

Hello, > I'm new to this, so the only problems I see are finding a compiler > that targets the platform (ARM7, for instance, or others) and > uploading the compiled firmware to the device. I used Hume [1] to program Mindstorms NXT robots (ARM7) as well as Tmote Sky sensors (MSP430). In both cases I ported the HAM virtual machine and interpreted bytecode. In the case of NXT I used the hardware layer of leJOS [2], simply ripping out the JVM and replacing it with HAM. As for the Tmote Sky, the VM sat on top of Contiki [3], so it was possible to send HAM bytecode over the air, and the sensor would intercept it, stop the currently running program and execute the new one. Hume can also be compiled directly to C, and I got it to run on a home-brew LPC2106 based embedded platform (also with an ARM7 core) we use in education. The main problem was the size of the generated code. See [4] for details. My overall impression is that Hume could be a very nice language to program embedded systems with, given proper tool support (for starters, it's really crying for a visual editor) and a high-quality compiler. Gergely [1] http://www-fp.cs.st-andrews.ac.uk/hume/index.shtml [2] http://lejos.sourceforge.net/ [3] http://www.sics.se/contiki/ [4] http://www-fp.cs.st-andrews.ac.uk/hume/papers/pg_thesis/ -- http://www.fastmail.fm - Email service worth paying for. Try it for free

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GHC, odd concurrency space leak
by Jesper Louis Andersen 18 Apr '10

18 Apr '10

A problem with GHC? =================== This post describes some odd behaviour I have seen in GHC 6.12.1 when writing Combinatorrent. The post is literate Haskell so you can run it. The executive summary: A space leak occurs when a new process is spawned from inside another process - and I can't figure out why. I am asking for help on haskell-cafe. We begin by upgrading GHC from Haskell98 to something mature and modern: > {-# LANGUAGE GeneralizedNewtypeDeriving #-} And then we do the import-tango: > module Main where > > import Control.Monad.Reader > import Control.Monad.State > > In our system, a *Process* is an identifier for a thread of execution. Since these heavily makes use of the following imports, > import Control.Concurrent > import Control.Concurrent.STM it is beneficial to make processes live in the IO-monad. However, it turns out that having everything living in the IO monad is a clunky idea. A Process also has a current configuration: The channels and variables on which it can communicate for instance. The configuration is not expected to be changed over the course of the process running. For data which do change when the process runs, we want some state tracking. Luckily, the Xmonad X-monad comes to our rescue. In all its gory and glory details, we generalize it and rewrite it for our processes, which is how they are used in combinatorrent. Our model-kit for building new threads of execution is the following beast: > newtype Process a b c = Process (ReaderT a (StateT b IO) c) > deriving (Functor, Monad, MonadIO, MonadState b, MonadReader a) Note that the automatic derivations of *MonadState b* and *MonadReader a* makes GHC spit our some mkUsageInfo warnings in its generation of the .hi-files. They don't seem to be dangerous. Glueing instructions for our model kit is given by spawning off new threads: > run :: a -> b -> Process a b c -> IO (c, b) > run c st (Process p) = runStateT (runReaderT p c) st > > spawn :: a -> b -> Process a b () -> IO ThreadId > spawn c st p = forkIO $ run c st p >> return () Our first dummy --------------- Machinery for having fun is now in place. Here is a crash-test-dummy we would like to play with: > main1 = do > spawn () () (forever $ return ()) > threadDelay (3 * 1000000) > > -- main = main1 Note that the given units () and () are usually much more complicated, but for the sake of this minimal example units will do. And running that does what we expect it to do: ./Post +RTS -tstderr <<ghc: 1156600984 bytes, 2207 GCs, 2668/2668 avg/max bytes residency (1 samples), 1M in use, 0.00 INIT (0.00 elapsed), 2.72 MUT (2.67 elapsed), 0.28 GC (0.34 elapsed) :ghc>> At this point, we got our own little processor-heater installed in our room. But spring is coming to the Northern hemisphere, so we have little use for this until November or December. To actually make our system do something, we build this gem: > p1 :: Process () () () > p1 = forever $ return () > > startp1 :: IO ThreadId > startp1 = spawn () () p1 > > startp2 :: IO ThreadId > startp2 = spawn () () (forever $ > do liftIO startp1 > liftIO $ putStrLn "Delaying" > liftIO $ threadDelay (10 * 1000000)) > > main2 = do > putStrLn "Main thread starting" > startp2 > threadDelay (1 * 1000000) > > main = main2 Running *this* beast gives: ./Post +RTS -tstderr Main thread starting Delaying <<ghc: 2532328160 bytes, 4831 GCs, 63629171/319163472 avg/max bytes residency (10 samples), 629M in use, 0.00 INIT (0.00 elapsed), 1.02 MUT (1.09 elapsed), 1.57 GC (1.91 elapsed) :ghc>> Notice the improvement! Now we both get to burn the CPU and we have a space leak! Removing the "forever" in startp2 will make this code not leak space. startp2 only spawns a single process, which then runs and each time it takes a cycle in the forever-loop, it builds up live heap. This boggles my mind. Why does this code leak like mad? What could one do to remedy the leak? The leak is PAP and GHC.Base.sat_... (according to -hd heap profiling). In my combinatorrent code it is a ->StateT (does anyone know what PAP and ->StateT means and where to find more definitions like these?). In confusion, -- J.

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building Haskell Platform for CentOS 5.2 with a /tmp mounted noexec
by Chris Dornan 18 Apr '10

18 Apr '10

Hi Everyone, Just to report that I now have Haskell Platform built for CentOS 5.2 with a /tmp mounted noexec. Looking back, the first main hurdle was getting GHC built for CentOS 5.2 (which uses Linux 2.6.9/glibc 2.5), but the procedure was pretty straight forward: . Keep trying generic binary Linux ghc packages until one that works is found (in this case ghc-6.8.3); . With ghc-6.8.3 installed, download the src tar ball for the next ghc release (6.10.1) and build and install that; . With the newly built GHC on the PATH, rinse and repeat until ghc-6.12.1 installed. It took a fair bit of time but, as far as I can tell, every single build worked flawlessly. My next task was getting cabal-install to work, which nearly defeated me, but it turns out that because CentOS mounts /tmp noexec, so all I had to do was get cabal-install to use somewhere else. As I was pithily advised: alias cabal="TMPDIR=/foo cabal" With that in place, AFAIK, the Haskell Platform built from source with the various configure scripts giving me enough hints to 'yum install' all the required CentOS packages. Thanks to everyone for their help. Chris

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Vector to Monadic Stream and back, how?
by Xiao-Yong Jin 17 Apr '10

17 Apr '10

Dear list, I'm trying to change my code from using Data.Array to Data.Vector. I have some question regarding the monadic use of 'Vector'. Searching through the list couldn't give me enough hints, so I guess I'd try my luck asking on the list. I want to use 'mapM' on Data.Vector.Vector, but it looks like the only 'mapM' defined is in Data.Vector.Fusion.Stream.Monadic. I'm able to use 'stream' and 'liftStream' to convert a 'Vector' to a monadic stream, on which I can use 'mapM'. But I couldn't find a way to convert the monadic stream back to Vector without using an intermediate list. I don't think I understand the internal of monadic stream that much. But it looks not so fusion to me. Is it the only way back to Vector? Thanks, jxy -- J c/* __o/* X <\ * (__ Y */\ <

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redirecting cabal-install from /tmp
by Chris Dornan 17 Apr '10

17 Apr '10

Hi Everyone, I am trying to get cabal-install to work on a system in which /tmp is mounted noexec. Is there any way to configure it to use another directory? I would be happy to patch the source and rebuild if need be. Thanks in advance, Chris

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