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Timing pure functions?
by Magnus Therning 27 May '09

27 May '09

Yesterday I spent about 5 minutes trying to time a single function in haskell (after having spent about 30 minutes on the timeit module in Python). I found timeit[1] on Hackage but it only times an IO computation once, what I'd like to do is time a pure function several times. Timing it once was no problem, passing `return $! myPureFunc` to `timeIt` did that[2]. My feeble attempt at collecting several timings failed though. import System.CPUTime import qualified Codec.Binary.Base64 as B64 import System.IO import qualified Data.ByteString as BS import Control.Monad timeIt times ioa = let timeOnce = do t1 <- getCPUTime a <- ioa t2 <- getCPUTime let t = fromIntegral (t2-t1) * 1e-12 return t in sequence $ take times $ repeat timeOnce main = do fh <- openBinaryFile "/dev/urandom" ReadMode d <- liftM BS.unpack $ BS.hGet fh 100000 t <- timeIt 10 $ return $! B64.encode d print t Running this on my machine produces the output [2.3331e-2,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0]. I.e. the first time the data is encoded, but the following 9 times it's not. I suspect that it all comes from `B64.encode d` being pure, hence the encoding happens only once. Now I _really_ want the encoding to happen 10 times, is there some easy way to achieve this? /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/timeit [2]: -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Bidirectional programming in Haskell
by Shin-Cheng Mu 27 May '09

27 May '09

Hi, On Tue May 26 01:21:28 EDT 2009, Artyom Shalkhakov wrote: > Are there any libraries for "bidirectional" [1] programming > in Haskell? Any related work? Some of the early work in the PSD project (closely related to the "lenses") were developed in Haskell, http://www.iis.sinica.edu.tw/~scm/2007/inv/ It is not in active maintenance now, but if you are interested in doing something with it, I'd be happy to help. :) Ref: [1] S-C. Mu, Z. Hu and M. Takeichi, An injective language for reversible computation. In Mathematics of Program Construction 2004, LNCS 3125, pp. 289-313, July 2004. [2] Z. Hu, S-C. Mu and M. Takeichi, A programmable editor for developing structured documents based on bidirectional transformations. In Partial Evaluation and Semantics-Based Program Manipulation, pp. 178-189. August 2004

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The essence of my monad confusion
by Paul Keir 27 May '09

27 May '09

On the wiki page for Applicative Functors (http://www.haskell.org/haskellwiki/Applicative_functor) a familiar characteristic of monads is quoted; that they "allow you to run actions depending on the outcomes of earlier actions". I feel comfortable with Functors and Applicative Functors, but I don't yet get that "extra power" that monads provide. An example immediately follows that quotation on the wiki: do text <- getLine if null text then putStrLn "You refuse to enter something?" else putStrLn ("You entered " ++ text) For simplicity's sake, I modified it to avoid using the IO monad; the "text" binding is now provided by the first parameter, and (=<<) is used due to its similarity to fmap: bar :: Monad m => m String -> m String bar as = (=<<) (\a -> if null a then return "nothing" else return "something") as This works fine, so bar ["Blah"] gives ["something"], and bar (Just "") gives ["nothing"]. But, I can get the same effect using a Functor (replacing (=<<) with fmap): bar2 :: Functor f => f String -> f String bar2 as = fmap (\a -> if null a then "nothing" else "something") as Can anyone help me out of the swamp? Cheers, Paul

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Purely logical programming language
by Matthias Görgens 27 May '09

27 May '09

There are a number of ways to marry purely functional programming languages with IO. To name just two possibilities: Clean uses linear types, threading exactly one "World" through functions, Haskell uses Monads. The model in Prolog, however, looks more like the model used in most strict functional languages. It uses impure predicates to affect the outside world. Do you know of any attempt to do for logic programming what Monads did for functional programming?

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May all logos be freely used?
by Hans van Thiel 27 May '09

27 May '09

Hello, What's the status of the logos on http://www.haskell.org/logos/results.html I'd like to use one or more in an about message in a Haskell program. Is this allowed, and if so, are there any conditions? I've just looked at them again, and it's a really fine collection, I think. Best Regards, Hans van Thiel

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Bidirectional programming in Haskell
by Artyom Shalkhakov 27 May '09

27 May '09

Hello, Are there any libraries for "bidirectional" [1] programming in Haskell? Any related work? All I've found is a paper "There and back again: arrows for invertible programming", which I couldn't find the full text of. Cheers, Artyom Shalkhakov. [1] something along the lines of Boomerang <www.seas.upenn.edu/~harmony/>

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How does GHC's implementation of par work?
by Ryan Ingram 27 May '09

27 May '09

This is related to the long thread on a bug in cross-module strictness analysis for the "lazy" function in GHC. Simon posted a response to that bug ( http://hackage.haskell.org/trac/ghc/ticket/3259 ), which shows the Haskell source for GHC's "par" function: > {-# INLINE par #-} > par :: a -> b -> b > par x y = case (par# x) of { _ -> lazy y } While I understand that in GHC Core, case always reduces its argument to WHNF, this isn't true of Haskell in general. What causes par# to get executed? Does GHC overload case for unboxed return values to force the evaluation of the scrutinee even if its return value isn't used? -- ryan On Tue, May 26, 2009 at 10:29 AM, Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> wrote: > On Mon, 2009-05-25 at 09:40 -0400, Mario Blazevic wrote: >> >> $diff main.simpl imported.simpl >> >> ... >> >> 223c232 >> >> < a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer >> >> --- >> >>> a_s1sV [ALWAYS Just S] :: GHC.Integer.Internals.Integer >> >> ... >> > >> > Good find! >> > >> >> Does this S vs. L difference have anything to do with strictness and laziness? >> > >> > Yes. >> > >> > So, I think we should open a ghc but report with all the details, >> > particularly the example's source, the ghc version and that highlight of >> > that strictness difference. > >> I have logged the ticket at >> >> http://hackage.haskell.org/trac/ghc/ticket/3259 > > Thanks for being so persistent Mario! > > Simon has diagnosed the bug. It's really rather subtle. > > Duncan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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[wxHaskell] ghc-pkg: /usr/local/lib/imports doesn't exist ...
by jose.benjaminps 27 May '09

27 May '09

Hello, I'm installing wxhaskell from darcs, almost all found good, but after make "# runghc Setup.lhs install" the last lines this is: cat config/wxcore.pkg | sed -e "s|\${wxhlibdir}|/usr/local/lib|" | ghc-pkg update - Reading package info from stdin ... done. ghc-pkg: /usr/local/lib/imports doesn't exist or isn't a directory (use --force to override) make: *** [wxcore-register] Error 1 I don't know solve this, I'm using linux ubuntu-jaunty with ghc-6.10.1 and wxGTK-2.8.10 thanks, --benjo

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Haskell programmers in São Carlos - SP - Brazil?
by Maurício 27 May '09

27 May '09

Anybody else around here? Best, Maurício

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Re: Re: Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Duncan Coutts 27 May '09

27 May '09

On Sun, 2009-05-24 at 12:48 -0400, Mario Blažević wrote: > > How about diffing the whole core output (and using -ddump-simpl). If > > there's a performance difference then there must be a difference in the > > core code too. > I can't exactly use diff because the generated identifier names are not the same, > but after poring over with Emacs ediff I have found only one difference that's > not attributable to identifiers: > > $diff main.simpl imported.simpl > ... > 223c232 > < a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer > --- > > a_s1sV [ALWAYS Just S] :: GHC.Integer.Internals.Integer > ... Good find! > Does this S vs. L difference have anything to do with strictness and laziness? Yes. So, I think we should open a ghc but report with all the details, particularly the example's source, the ghc version and that highlight of that strictness difference. Duncan

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The Computer Language Benchmarks Game: pidigits
by Isaac Gouy 27 May '09

27 May '09

On Mon May 25 16:18:29 EDT 2009, Arnaud Payement wrote: > ... I thought it is better to show Haskell as one would naturally write it. One would naturally first write it in C ? :-)

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ANN: New repository and trac for haskell-src-exts
by Niklas Broberg 26 May '09

26 May '09

Hi all, In preparation for my GSoC project, I've set up some new infrastructure for the haskell-src-exts package. Instead of residing with the HSP packages, it's now old enough to be allowed to live on its own. The new repository is darcs get http://code.haskell.org/haskell-src-exts I've also finally set up a bug tracker for all things big and small, found at http://trac.haskell.org/haskell-src-exts I've added all issues from my old TODO list, but I'm sure there's more stuff that I've missed. Please help me by reporting anything you come across, or by requesting new and cool features. Cheers, /Niklas

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Anglohaskell?
by Philippa Cowderoy 26 May '09

26 May '09

Is anyone up for Anglohaskell this year? Perhaps more importantly, is anyone willing to step forward to run it? I can lend a hand, having organised and to some extent meta-organised in previous years, but I'm not sure I have spare time to do the same kind of job as I did in 2007. For those who're new or don't remember, http://www.haskell.org/haskellwiki/AngloHaskell contains links to info from previous years - the idea's a get-together with lots of talks from hobbyist to academic, and plenty of chat. -- Philippa Cowderoy <flippa(a)flippac.org>

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No VerboseCheck in QuickCheck 2?
by Shin-Cheng Mu 26 May '09

26 May '09

Hi, I believe this must have been raised before but I did some searching to no avail, so please allow me.. It seems that verboseCheck is gone in QuickCheck 2. In QuickCheck 2, how do we print the test cases, specify the number of tests, or the sizes of test cases, etc.? Thank you very much.. sincerely, Shin

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ANNOUNCE: Haskell Communities and Activities Report (16th ed., May 2009)
by Janis Voigtlaender 26 May '09

26 May '09

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (16th edition, May 2009) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of October 2009: target deadline for contributions to the November 2009 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in October (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Parsing command lines
by Patai Gergely 26 May '09

26 May '09

Hello all, is there a function that can safely split a command line into a FilePath to the executable and its parameters? I couldn't manage to find one. If not, what are the pitfalls in creating a cross-platform solution to this problem? Can I assume that the first space not within double quotes and not preceded by an unescaped backslash marks the end of the executable path, or is that too naive (or completely wrong)? Gergely -- http://www.fastmail.fm - A fast, anti-spam email service.

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Haskell type system and the lambda cube
by Petr Pudlak 26 May '09

26 May '09

Hi, I'm trying to get some better understanding of the theoretical foundations behind Haskell. I wonder, where exactly does Haskell type system fit within the lambda cube? <http://en.wikipedia.org/wiki/Lambda_cube> I guess it could also vary depending on what extensions are turned on. Thanks, Petr

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Haskell furniture?
by Steve Schafer 26 May '09

26 May '09

Here is some furniture that ought to appeal to the Haskell afficionado: http://karl-andersson.se/view_product.asp?rangeId=39&catId=2&picture=2 Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

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Any interest in forming a Haskell Meetup group in Edinburgh, Scotland
by A Smith 26 May '09

26 May '09

Anyone interested in just getting together, from complete beginner, to those with experience, to share ideas and help each other other out. The Filmhouse on Lothian Road, is quite handy for Edinburgh folk, and for anyone was coming through from Glasgow, and alighting(using train speak) at Haymarket. Its got plenty space, serves reasonable food, as well as having lots of good food places nearby. I'll suggest meeting on a Saturday to try and avoid conflict with the many other things people do in the evenings. Its also off-peak return from Glasgow all day. I'm a Haskell beginner, so I'm no good for answering more difficult questions, need someone else for that. -- Andrew

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The Computer Language Benchmarks Game: pidigits
by Arnaud Payement 26 May '09

26 May '09

Hi all, I recently decided to rewrite the pidigits benchmark of the debian shootout (shootout.alioth.debian.org) as toy project. However, it seems that on my machine, the code seems to be more performant than both the current entry and the proposed replacement (see http://www.haskell.org/haskellwiki/Shootout/Pidigits) for the same number of lines. Do you think it might be worth submitting my entry? Here is my code,: {-# OPTIONS -O2 -optc-O3 #-} -- -- The Great Computer Language Shootout -- http://shootout.alioth.debian.org/ -- by Arnaud Payement -- import System data F = F Integer Integer Integer Integer extract s@(F k n a d) = ((n*3+a) `div` d, (n*4+a) `div` d, s) update (F k n a d) = F (k+1) (n*k) ((a+n*2)*y) (d*y) where y = 2*k+1 next state = let (u, v, s'@(F k n a d)) = extract (update state) in if (n > a || (u /= v)) then next s' else (show u, F k (n*10) ((a-d*u)*10) d) digits = ("", (F 1 1 0 1)):[next state | state <- map snd digits] pr (d:t) k n | k > n = putStr "" | k `mod` 10 /= 0 = putStr d >> pr t (k+1) n | otherwise = putStrLn (d ++ "\t:" ++ show k) >> pr t (k+1) n main = pr (map fst (tail digits)) 1 . read . head =<< getArgs Best, Arnaud

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[ANN] multirec-binary
by Sebastiaan Visser 26 May '09

26 May '09

Hello all, MultiRec[1,2] is a great library for generic programming for families of possibly mutual recursive data types. Creating new generic functions might take some effort but the use of these functions is very straightforward. The multirec-binary[3] packages allows generic derivation of Data.Binary instances using the MultiRec library. Just try something like this: > import Data.Binary > import Generics.MultiRec.Base > import Generics.MultiRec.Binary > > instance Binary MyType where > put = gput MyFamily > get = gget MyFamily Have fun, -- Sebastiaan Visser [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/multirec-0.3 [2] http://www.cs.uu.nl/research/techreps/repo/CS-2008/2008-019.pdf [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/multirec-binary

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ANN: Haskell Hackathon in Philadelphia
by Brent Yorgey 25 May '09

25 May '09

Hi all! We are in the early stages of planning a Haskell hackathon/get together, Hac φ, to be held this summer at the University of Pennsylvania, in Philadelphia. Right now we're looking at two possible dates: June 19-21 or July 24-26 If you might be interested in attending, please add your name on the wiki page: http://www.haskell.org/haskellwiki/Hac_%CF%86 You can also note whether either of those dates absolutely doesn't work for you. (If you don't have an account on the wiki, you can email Ashley Yakeley for an account [1], or feel free to just respond to this email.) Expect more details (such as a nailed-down date) soon, once we have gauged the level of interest. Hope to see you in Philadelphia! Brent (byorgey) Daniel Wagner (dmwit) [1] http://haskell.org/haskellwiki/HaskellWiki:New_accounts

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FGL Question
by Hans van Thiel 25 May '09

25 May '09

Hello, I want to get the top or the bottom elements of a graph, but the following code appears to give the wrong answer in most cases, and the right answer in a few cases. Any ideas? -- get the most general or the least general elements graphMLGen :: Bool -> Gr [Rule] () -> Gr [Rule] () graphMLGen pm gr = buildGr unctxls where unctxls = map remadj ctxls remadj (_,n,r,_) = ([],n,r,[]) ctxls | pm = gsel (\x -> outdeg' x == 0) gr | otherwise = gsel (\x -> indeg' x == 0) gr Many thanks, Hans van Thiel

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Re: [Haskell-cafe] Data.Binary suboptimal instance
by Henning Thielemann 25 May '09

25 May '09

Khudyakov Alexey schrieb: > On Saturday 23 May 2009 02:55:17 Antoine Latter wrote: >> Or you could go for the compromise position, where the list can be >> part of a complex data structure so you're not relying on EOF to find >> the end. >> > Interesting solution however it does not perform very nice. I wrote > microbenchmark > >> xs :: [Word32] >> xs = [1..(10^6)] > > Writing chunked list of Word32 >> B.writeFile "chunked" . toLazyByteString . putList putWord32be $ xs > real 0m4.311s > user 0m3.272s > sys 0m0.096s > > Reading chunked list of Word32 >> print . last . runGet (getList getWord32be) =<< B.readFile "chunked" > real 0m0.634s > user 0m0.496s > sys 0m0.012s > > > Writing stream of Word32 >> B.writeFile "stream" . encodeStream $ xs > real 0m0.391s > user 0m0.252s > sys 0m0.020s > > Reading stream of Word32 >> print . (last :: [Word32] -> Word32) . decodeStream =<< B.readFile "stream" > real 0m0.376s > user 0m0.248s > sys 0m0.020s > > > I didn'd do any profiling so I have no idea why writing is so slow. If you use top-level definition 'xs' the program might cache the list and second write is faster. You may change the order of tests in order check, whether that is the cause.

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ANN: bsd-sysctl 1.0.3
by Maxime Henrion 25 May '09

25 May '09

I'm pleased to announce the release of bsd-sysctl 1.0.3, a package that provides a System.BSD.Sysctl module allowing access to the C sysctl(3) API. It should fully work on FreeBSD, NetBSD and Mac OS X platforms; it should also work on OpenBSD and Linux, although looking up sysctl's by name isn't supported there, and I believe the sysctl API doesn't return anything useful under Linux. The package is available here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bsd-sysctl-1.0.3 As this is my first package on hackage, I'm eager to hear about any comments on this code. Cheers, Maxime

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Defining a CPP macro when building documentation
by Maxime Henrion 25 May '09

25 May '09

Hello all, I have been exposed to a problem that have happened to others too, and since I have found a (scary) solution with the help of Duncan Coutts, I'm now sharing it with you. The reason I wanted to pass specific CPP flags to haddock was to allow the documentation of the full module to get built, even when only parts of the module would end up getting compiled because of restrictions on the build platform. In this mail, Duncan Coutts gives a very helpful bit of code to override the haddockHook and pass additional options to haddock : http://www.haskell.org/pipermail/haskell-cafe/2008-December/051785.html In my case, I needed something very similar, except that I needed to pass those additional options to hsc2hs which my code uses. This worked fine as long as the cabal commands sequence was ["configure","haddock"] and not ["configure","build","haddock"]. The reason for this is that in the latter case, the library wasn't being processed again, but output of previous runs of hsc2hs were reused. So I worked around this with an ugly hack that removes the .hs files before running haddock, in order to force re-processing! Here's the final code I ended up with : import Distribution.Simple import Distribution.Simple.LocalBuildInfo (LocalBuildInfo(withPrograms), buildDir) import Distribution.Simple.Program (userSpecifyArgs) import System.Directory import System.FilePath -- Define __HADDOCK__ when building documentation. main = defaultMainWithHooks simpleUserHooks { haddockHook = \pkg lbi h f -> do let progs = userSpecifyArgs "hsc2hs" ["-D__HADDOCK__"] (withPrograms lbi) removePreProcessedFiles (buildDir lbi) haddockHook simpleUserHooks pkg lbi { withPrograms = progs } h f } -- Horrible hack to force re-processing of the .hsc file. Otherwise -- the __HADDOCK__ macro doesn't end up being defined. removePreProcessedFiles :: FilePath -> IO () removePreProcessedFiles dir = removeFile (dir </> "System/BSD/Sysctl.hs") `catch` \_ -> return () Cheers, Maxime

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`seq` and categories
by Jason Dusek 25 May '09

25 May '09

On the IRC channel a few days ago, it was said that, as long as we allow `seq`, Hask is not a valid category. Doesn't this basically mean that a very large amount of Haskell -- anything with strictness annotations -- can not be described in a category Hask? -- Jason Dusek

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Re: Re: Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Mario Blažević 25 May '09

25 May '09

> I recommend using -ddump-simpl, as it produces more readable output. > >> Actually, I can't see any effect of that pragma in the >> core files whatsoever, but it certainly has effect on >> run time. > > How about diffing the whole core output (and using -ddump-simpl). If > there's a performance difference then there must be a difference in the > core code too. Ok. $ ghc-6.11.20090421 --make primes-test.hs -threaded -O2 -ddump-simpl &> main.simpl $ time ./primes-test +RTS -N2 4001 real 0m9.636s user 0m18.201s sys 0m0.088s $ ghc-6.11.20090421 --make primes-test.hs -threaded -O2 -ddump-simpl &> imported.simpl $ time ./primes-test +RTS -N24001 real 0m17.547s user 0m17.453s sys 0m0.052s I can't exactly use diff because the generated identifier names are not the same, but after poring over with Emacs ediff I have found only one difference that's not attributable to identifiers: $diff main.simpl imported.simpl ... 223c232 < a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer --- > a_s1sV [ALWAYS Just S] :: GHC.Integer.Internals.Integer ... Does this S vs. L difference have anything to do with strictness and laziness? That line is a part of the `Main.test' definition: $ diff -C 3 main.simpl imported.simpl *** 217,244 **** [Arity 2 Str: DmdType LL] Main.test = ! \ (n1_ahR :: GHC.Integer.Internals.Integer) ! (n2_ahT :: GHC.Integer.Internals.Integer) -> let { ! a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer LclId [Str: DmdType] ! a_s1rs = Data.List.prod1 (Main.factors2 (Data.List.prod1 ! (GHC.Num.up_list Main.lvl Main.lvl n1_ahR) Data.List.lvl1)) Data.List.lvl1 } in ! case GHC.Prim.par# @ GHC.Integer.Internals.Integer a_s1rs of _ { __DEFAULT -> case Data.List.prod1 (Main.factors2 (Data.List.prod1 ! (GHC.Num.up_list Main.lvl Main.lvl n2_ahT) Data.List.lvl1)) Data.List.lvl1 ! of x1_aUS { __DEFAULT -> ! case GHC.Real.$wdivMod x1_aUS a_s1rs of _ { (# ww1_aUo, _ #) -> ! ww1_aUo } } } >> Or do you mean to say that *your* installation of GHC >> behaves the same when the function `parallelize' is defined in >> the same module and when it's imported? > > Yes, exactly. With both ghc-6.10.1 and a very recent build of ghc-6.11 As you can see, I'm using the 6.11.20090421 build. I looked at recent ones, but the Linux builds seem to be less stable in May. I have the same results (though I didn't use -ddump-simpl) with 6.8.2. Can you recommend a different version to try?

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HPC Website & FAQ
by Dominic Steinitz 25 May '09

25 May '09

There's a nice website for HPC but it looks a bit out of date. http://projects.unsafeperformio.com/hpc/ I wanted to send a patch to the FAQ for using HPC with .lhs files (you have to run ghc -E to generate .hs files and strip some of the the lines ghc generates: "{-# LINE 1 "ASNTYPE.lhs" #-}" & "#line 1 "ASNTYPE.lhs""). Would it be possible for the author of HPC to make the website into a darcs repo so that we send in patches? Apparently the Cabal website is managed like that and it sounds like a good way of doing things in general to me. Thanks, Dominic.

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Re: Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Duncan Coutts 24 May '09

24 May '09

On Sat, 2009-05-23 at 20:42 -0400, Mario Blažević wrote: > On Sat 23/05/09 2:51 PM , Duncan Coutts duncan.coutts(a)worc.ox.ac.uk sent: > > On Sat, 2009-05-23 at 13:31 -0400, Mario Blažević wrote: > > ... > >> So the function is not strict, and I don't understand > >> why GHC should evaluate the arguments before the call. > > > > Right, it's lazy in the first and strict in the second argument. As far > > as I can see we have no evidence that is is evaluating anything before > > the call. > > > When I look at the Core definition of `test', it begins with > > > \ (n1axl::integer:GHCziIntegerziInternals.Integer) > (n2axn::integer:GHCziIntegerziInternals.Integer) -> > %let as1sU :: integer:GHCziIntegerziInternals.Integer = > base:DataziList.prod1 > (main:Main.factors2 > (base:DataziList.prod1 > (base:GHCziNum.upzulist main:Main.lvl main:Main.lvl n1axl) > base:DataziList.lvl1)) > base:DataziList.lvl1 > %in %case integer:GHCziIntegerziInternals.Integer (ghczmprim:GHCziPrim.parzh > @ > integer:GHCziIntegerziInternals.Integer > as1sU) > %of (dsapq::ghczmprim:GHCziPrim.Intzh) I recommend using -ddump-simpl, as it produces more readable output. > To my untrained eyes, this looks like it's evaluating > > > product $ factors $ product [1..n1]) > > which is the first argument to `parallelize'. That core code is doing: let blah = product $ factors $ product thelist in case par# blah of _ -> ... So it's calling the primitive par# but nothing is forced to WHNF (except the unused dummy return value of par#). > I assume that %case in Core evaluates the argument to WHNF, just like > case in Haskell. Yes. (In fact case in core always reduces to WHNF, where as in Haskell case (...) of bar -> (...) does not force anything.) > Then again, I could be completely misinterpreting what Core is, because > I can't find any call to `parallelize' before or after that. It appears > to be inlined in Core, regardless of whether the pragma > > > {-# INLINE parallelize #-} > > is there or not. Yes, because even without the pragma, ghc decides it's profitable to inline. > Actually, I can't see any effect of that pragma in the > core files whatsoever, but it certainly has effect on run time. How about diffing the whole core output (and using -ddump-simpl). If there's a performance difference then there must be a difference in the core code too. > Or do you mean to say that *your* installation of GHC behaves > the same when the function `parallelize' is defined in the same module and when > it's imported? Yes, exactly. With both ghc-6.10.1 and a very recent build of ghc-6.11 Duncan

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hackage version scheme survey
by brian＠lorf.org 24 May '09

24 May '09

It's not good to specify open version ranges in cabal build-depends like foo >= 1.1 because the foo maintainer will eventually release 2.0, containing API-breaking changes, and the build will fail. If you depend on foo 1.1, you can specify == 1.*, no problem. But if you depend on a package that uses a version scheme like 1.2.3 or 1.2.3.4, it's not clear (at least to me) where the major/minor division is. I did a survey of version schemes in 'cabal list': 484 packages use a.b. 630 packages use a.b.c. 180 packages use a.b.c.d. The rest: two packages use just one field for their versions; one uses five; one uses six. Maybe most of the a.b people are thinking major.minor, and most of the a.b.c people are thinking breaking.feature.implementation like the "rational" RubyGems scheme described in http://rubygems.org/read/chapter/7#page24 , but I don't know. It makes it hard to describe dependencies. Will there be a standard versioning scheme sometime?

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Cabal, Time & GHC 6.10.2
by Dominic Steinitz 24 May '09

24 May '09

I get > dom@linux-6ofq:~/asn1> runghc Setup.hs configure > Configuring PER-0.0.20... > Setup.hs: At least the following dependencies are missing: > time -any && -any but I have time > dom@linux-6ofq:~/asn1> ghc-pkg list | grep time > old-locale-1.0.0.1, old-time-1.0.0.2, packedstring-0.1.0.1, > time-1.1.2.4 I think I can see why cabal isn't finding it: > ghc-pkg dump --global | grep time-1.1.2.4 finds nothing and I believe that is what cabal uses to find things. What's not clear is what I do to fix things. Go back to 6.10.1? Thanks, Dominic. > dom@linux-6ofq:~/asn1> ghc --version > The Glorious Glasgow Haskell Compilation System, version 6.10.2 > dom@linux-6ofq:~/asn1> ghc-pkg --version > GHC package manager version 6.10.2 Here's my .cabal file. > Name: PER > Version: 0.0.20 > License: BSD3 > Author: Dominic Steinitz > Maintainer: dominic.steinitz(a)blueyonder.co.uk > Copyright: Dominic Steinitz 2003 - 2009 > Stability: Alpha > Category: Language > Homepage: http://www.haskell.org/asn1 > Synopsis: ASN.1 PER support for Haskell > Description: A formal and executable specification of the Packed Encoding Rules (PER) > for ASN.1 > build-depends: binary-strict == 0.4.2, > bytestring, > mtl, > containers, > time, > pretty, > base > build-type: custom > > Exposed-Modules: Language.ASN1.PER.Integer > Language.ASN1.PER.GenerateC > > Executable: PERTest > Main-Is: PERTest.hs

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Re: Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Mario Blažević 24 May '09

24 May '09

On Sat 23/05/09 2:51 PM , Duncan Coutts duncan.coutts(a)worc.ox.ac.uk sent: > On Sat, 2009-05-23 at 13:31 -0400, Mario Blažević wrote: > ... >> So the function is not strict, and I don't understand >> why GHC should evaluate the arguments before the call. > > Right, it's lazy in the first and strict in the second argument. As far > as I can see we have no evidence that is is evaluating anything before > the call. When I look at the Core definition of `test', it begins with \ (n1axl::integer:GHCziIntegerziInternals.Integer) (n2axn::integer:GHCziIntegerziInternals.Integer) -> %let as1sU :: integer:GHCziIntegerziInternals.Integer = base:DataziList.prod1 (main:Main.factors2 (base:DataziList.prod1 (base:GHCziNum.upzulist main:Main.lvl main:Main.lvl n1axl) base:DataziList.lvl1)) base:DataziList.lvl1 %in %case integer:GHCziIntegerziInternals.Integer (ghczmprim:GHCziPrim.parzh @ integer:GHCziIntegerziInternals.Integer as1sU) %of (dsapq::ghczmprim:GHCziPrim.Intzh) To my untrained eyes, this looks like it's evaluating > product $ factors $ product [1..n1]) which is the first argument to `parallelize'. I assume that %case in Core evaluates the argument to WHNF, just like case in Haskell. Then again, I could be completely misinterpreting what Core is, because I can't find any call to `parallelize' before or after that. It appears to be inlined in Core, regardless of whether the pragma > {-# INLINE parallelize #-} is there or not. Actually, I can't see any effect of that pragma in the core files whatsoever, but it certainly has effect on run time. >> Does anybody know of a pragma or another way to make a >> function *non-strict* even if it does always evaluate its argument? >> In other words, is there a way to selectively disable the strictness >> optimization? > > Yes, which is what pseq and par already do. > > If there's a bug, we need to reproduce it and report it. I cannot > reproduce it. If you mean that you can't reproduce anything that's contrary to the specification, that's not saying much: there are practically no guarantees on what `par' is supposed to accomplish. If you mean you can't reproduce anything you wouldn't expect, pray explain what is going on, because everybody else seems to be surprised. Or do you mean to say that *your* installation of GHC behaves the same when the function `parallelize' is defined in the same module and when it's imported?

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ANN: hledger 0.5 released
by Simon Michael 24 May '09

24 May '09

I'm pleased to announce the hledger 0.5 release on hackage. hledger is a (mostly) text-mode double-entry accounting tool that generates precise activity and balance reports from a plain text journal file. It is a partial clone, in haskell, of John Wiegley's excellent ledger. hledger implements a subset of ledger's commands and options, and also provides some new ones. For basic use you can use hledger and ledger pretty much interchangeably on the same data files. For screenshots, live demo, docs etc. see http://hledger.org . Release notes are at http://hledger.org/NEWS , and below. To install: cabal install hledger [-fvty] [-fhapps]. I'd like to hear feedback, especially if you are having trouble getting started. Happy tracking! - Simon (sm on #ledger) 2009/05/23 hledger 0.5 released ------------------------------- Changes: * the vty flag is disabled by default again, to ease installation on windows * use ledger 3 terminology: a ledger contains transactions which contain postings * new "add" command prompts for transactions interactively and adds them to the ledger * new "convert" command transforms bank CSV exports to ledger format, with rule-based cleanup * new "histogram" command shows transaction counts per day or other reporting interval * most commands now work properly with UTF8-encoded text (Sergey Astanin) * invoking as "hours" is now less different: it just uses your timelog, not your ledger * --quarterly/-Q option summarises by quarter * --uncleared/-U option looks only at uncleared transactions * be more accurate about checking balanced amounts, don't rely on display precision * enforce balancing for bracketed virtual postings * fix bug in eliding of posting amounts * don't show trailing spaces on amountless postings * parse null input as an empty ledger * don't treat comments as part of transaction descriptions * require some postings in ledger transactions * require a non-empty description in ledger transactions * don't fail when matching an empty pattern, as in "not:" * make the web server handle the null path * code, api and documentation updates * add a contributor agreement/list Release contributors: * Simon Michael * Sergey Astanin Release stats: * Days since last release: 51 * Committers: 2 * Commits: 101 * Lines of non-test code: 2795 * Known errors: 0 * Tests: 76 * Performance: || hledger-0.4 | hledger-0.5 | ledger =========================++=============+=============+======= -f sample.ledger balance || 0.01 | 0.01 | 0.06 -f 1000.ledger balance || 1.33 | 1.46 | 0.53 -f 10000.ledger balance || 15.28 | 16.35 | 4.67

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haddock could be a pretty-printer?
by Maurício 24 May '09

24 May '09

Hi, The new version of haddock makes use of GHC parser. How much of effort would take to make haddock generate pretty-print of the source code itself, including haddock documentation (although probably loosing other comments)? Maybe even an html version that documentation could point to? Thanks, Maurício

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Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Mario Blažević 24 May '09

24 May '09

On Sat 23/05/09 4:15 PM , Alexander Dunlap alexander.dunlap(a)gmail.com sent: >>> Does anybody know of a pragma or another way to make a >>> function *non-strict* even if it does always evaluate its >>> argument? In other words, is there a way to >>> selectively disable the strictness optimization? >> >> parallelize a b | False = (undefined, undefined) >> | otherwise = a `par` (b `pseq` (a, b)) >> >> might do, unless strictness analysis is smart enough to >> know that the False guard is always, well, False. >> _______________________________________________ > > GHC.Prim.lazy? It's GHC.Exts.lazy nowadays, and it doesn't have any effect. Neither does the `| False' guard. The only way I found to disable the eager argument evaluation is to pass them in as functions: > parallelize :: Num t => (t -> a) -> (t -> b) -> (a, b) > parallelize a b = let a' = a 1 > b' = b 1 > in (a' `par` (b' `pseq` (a', b'))) Then it can be imported and called like this: > test n1 n2 = let (p1, p2) = parallelize > (\n0-> product $ factors $ product [n0..n1]) > (\n0-> product $ factors $ product [n0..n2]) This solution is incredibly fragile. If the declared type of parallelize is modified by replacing t by Integer, the evaluation becomes eager again. Also, the argument functions can't simply take () for argument which would make this solution reasonable. If this is all the strictness optimizer's fault, I'm awed by how difficult it is to trick it into not doing its job.

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Pattern match question in HAXML code
by Max Cantor 24 May '09

24 May '09

Going through the instances for HTypeable (http://www.haskell.org/HaXml/HaXml/src/Text/XML/HaXml/TypeMapping.html#toHT… ) I saw the following instance for Either a b. My question is, why doesn't the pattern match in the where clause always fail? If (Left x) = m does not fail, doesn't that imply that m is a Left x and therefore the (Right y) = m should fail? thanks, max instance (HTypeable a, HTypeable b) => HTypeable (Either a b) where toHType m = Defined "Either" [hx, hy] [ Constr "Left" [hx] [hx] {-Nothing-} , Constr "Right" [hy] [hy] {-Nothing-}] where (Left x) = m (Right y) = m hx = toHType x hy = toHType y

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Haskell Weekly News: Issue 119 - May 23, 2009
by Brent Yorgey 24 May '09

24 May '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090523 Issue 119 - May 23, 2009 --------------------------------------------------------------------------- Welcome to issue 119 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements GHC porting works again. Ian Lynagh [2]announced that the [3]instructions for porting GHC to a new architecture now work again with the HEAD. If you get stuck when trying to do a port, feel free to ask on cvs-ghc at haskell.org or in #ghc on freenode. 6.10.4 plans. Ian Lynagh [4]announced plans for a 6.10.4 bugfix release of GHC. If you know of any bugs that you think should be looked into for 6.10.4, please let the development team know. The Timber compiler 1.0.3. Johan Nordlander [5]announced the release of version 1.0.3 of the Timber compiler. Timber is a modern language for building event-driven systems, based around the notion of reactive objects. It is also a purely functional language derived from Haskell, although with a strict evaluation semantics. 1.0.3 is a bug fix release, paving the way for future feature releases. mathlink-2.0.0.3. Tracy Wadleigh [6]announced the release of [7]mathlink, a library for writing Mathematica packages in Haskell. One simply writes some functions of type (MLGet a, MLPut b) => a -> IO b and provides a package specification in a simple DSL; the result is a program that exposes functions that can be called from Mathematica. text 0.2, fast and comprehensive Unicode support using stream fusion. Bryan O'Sullivan [8]announced the availability of [9]text 0.2, an efficient Unicode text library that uses stream fusion. New and notable in this release is support for lazy, chunked text, so you can process text files far larger than memory using a small footprint. Haskell Hackathon in Philadelphia. Brent Yorgey [10]announced Hac phi, a Haskell hackathon to be held in Philadelphia in July. Check out the [11]wiki page and add your name if you are interested in attending! More details to follow soon. feed2twitter 0.2 & hackage2twitter 0.2.1. Tom Lokhorst [12]announced the first release of [13]feed2twitter, a library for sending posts from a news feed to Twitter. EsotericBot 0.0.1. spoon [14]announced the release of [15]Esotericbot, a sophisticated, lightweight IRC bot, written in Haskell. atom 0.0.4. Tom Hawkins [16]announced a new release of [17]atom; this version adds an array datatype (A a). Hieroglyph-2.21 and buster, buster-gtk, and buster-network-2.0. Jeff Heard [18]announced new releases of [19]Hieroglyph, [20]buster, [21]buster-gtk, and [22]buster-network, with tons of changes; read Jeff's original announcement for details. TxtSushi 0.1. Keith Sheppard [23]announced the first version of [24]TxtSushi, a collection of command line utilities for processing tab-delimited and CSV files. It includes a utility for doing SQL SELECTs on flat files. Discussion Should exhaustiveness testing be on by default? Don Stewart started a [25]discussion, prompted by a [26]recent blog post, on whether coverage checking should be on by default, and other issues relating to compiler warnings and coding style. Proposal on the platform API policy question. Duncan Coutts [27]proposed a general policy for Haskell Platform release cycles and versioning, based on input from previous discussions. the problem of design by negation. Michael Mossey began a [28]discussion on software design philosophies. "Design by negation" considered harmful? Haskell in 3 Slides. John Van Enk [29]asked for ideas on a 3 to 4 slide introduction to Haskell. What do YOU think should be on those slides? Blog noise [30]Haskell news from the [31]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Bryan O'Sullivan: [32]Streaming Unicode support for Haskell: text 0.2. * Alex McLean: [33]Haskell hack. Music generation in Haskell. * Well-Typed.Com: [34]Building plugins as Haskell shared libs. A sneak preview of building Haskell shared libraries on Linux. * LHC Team: [35]New release: LHC 0.8. * Mark Wassell: [36]Grapefruit And Glade. * Conal Elliott: [37]The C language is purely functional. * >>> Will Donnelly: [38]Haskell: A Pretty Nice Language. * FP-Syd: [39]Sydney FP Group: FP-Syd #14.. * Dan Piponi (sigfpe): [40]Trace Diagrams with Monads. Quotes of the Week * roconnor: Damn it, I don't know how to make this as slow as python. * koeien: Let's register it [monomorphismrestriction.com] to prevent it from being used ;) * Elly: Rule 1 of malloc is the same as rule 1 of air travel: "Attempt at all costs to keep your number of landings equal to your number of takeoffs." * monochrom: I was trying to design a sensible language... then I downloaded ghc. * conal: The C ADT is implemented simply as String (or char *, for you type theorists, using a notation from Kleene) * Will Donnelly: monads are okay after a bit (though I'm still a little suspicious of them) About the Haskell Weekly News New editions are posted to [41]the Haskell mailing list as well as to [42]the Haskell Sequence and [43]Planet Haskell. [44]RSS is also available, and headlines appear on [45]haskell.org. To help create new editions of this newsletter, please see the information on [46]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [47]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/16984 3. http://hackage.haskell.org/trac/ghc/wiki/Building/Porting 4. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/16972 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17200 6. http://article.gmane.org/gmane.comp.lang.haskell.general/17197 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mathlink 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58862 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/text 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58817 11. http://www.haskell.org/haskellwiki/Hac_%CF%86 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58801 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/feed2twitter 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58796 15. http://www.killersmurf.com/projects/esotericbot 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58752 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/atom 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58683 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hieroglyph 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/buster 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/buster-gtk 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/buster%2Dnetwork 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/58635 24. http://www.keithsheppard.name/txt-sushi 25. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/16926 26. http://ocaml.janestreet.com/?q=node/64 27. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/11145 28. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/58784 29. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/58664 30. http://planet.haskell.org/ 31. http://haskell.org/haskellwiki/Blog_articles 32. http://www.serpentine.com/blog/2009/05/22/streaming-unicode-support-for-has… 33. http://yaxu.org/haskell-hack/ 34. http://blog.well-typed.com/2009/05/buildings-plugins-as-haskell-shared-libs/ 35. http://lhc-compiler.blogspot.com/2009/05/new-release-lhc-08.html 36. http://www.poundstone.org/blog/?p=231 37. http://conal.net/blog/posts/the-c-language-is-purely-functional/ 38. http://willdonnelly.wordpress.com/2009/05/19/haskell-a-pretty-nice-language/ 39. http://www.mega-nerd.com/erikd/Blog/FP-Syd/fp-syd-14.html 40. http://blog.sigfpe.com/2009/05/trace-diagrams-with-monads.html 41. http://www.haskell.org/mailman/listinfo/haskell 42. http://sequence.complete.org/ 43. http://planet.haskell.org/ 44. http://sequence.complete.org/node/feed 45. http://haskell.org/ 46. http://haskell.org/haskellwiki/HWN 47. http://code.haskell.org/~byorgey/code/hwn/

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ICFP Participation
by Matthias Görgens 24 May '09

24 May '09

Hello, Please pardon my naive question: Is there a way to sign on for ICFP 09? The homepage (http://www.cs.nott.ac.uk/~gmh/icfp09.html) only seems to mention how to submit papers. Is there a way to attend as a mere participant? Thanks, Matthias.

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Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Mario Blažević 24 May '09

24 May '09

>>> You could probably see exactly what's happening in >>> more detail by going through the Core output. >> >> Thank you, this advice helped. The Core output indicates >> that function `test' evaluates the arguments to >> `parallelize' before it calls it. In other words, the >> call to `parallelize' is optimized as a strict function >> call -- which it is. The problem is that this >> optimization evaluates the arguments sequentially. >> Compiling with optimizations turned off regains the >> parallel execution. >> >> I guess I will report this as a GHC bug. Or is it a >> feature request? > > As Duncan suggessted, try with GHC head (grab a snapshot). `par` et al > are much improved. I already have, with the snapshot from 21st of April. It behaves the same as 6.8.2, except it runs for twice as long. I'd like to take back a part of what I said before, though: `parallelize' should be strict only in its second argument. Its strictness in the first argument should be the same as with `par`. Even though `parallelize x y' always evaluates both x and y, the following test works fine with optimizations even if `parallelize' is imported: main = putStrLn (snd $ parallelize undefined "Hello, World!") So the function is not strict, and I don't understand why GHC should evaluate the arguments before the call. Does anybody know of a pragma or another way to make a function *non-strict* even if it does always evaluate its argument? In other words, is there a way to selectively disable the strictness optimization?

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Re: Extensible Exceptions and IO
by Alexander Dunlap 24 May '09

24 May '09

On Sat, May 23, 2009 at 9:09 AM, Ian Lynagh <igloo(a)earth.li> wrote: > On Sat, Apr 25, 2009 at 11:18:43AM -0700, Alexander Dunlap wrote: >> In the extensible exceptions paper[1], which I believe is the guide >> behind the current Control.Exception in GHC 6.10, a SomeIOException >> type is discussed so that it would be possible to use the nice >> interface for IOExceptions. Is this implemented anywhere? In the GHC >> libraries, all I see is the old interface just plugged into a simple >> wrapper, so you have to use guard . isDoesNotExistError (for example) >> with catchJust to catch a certain type of exception. If not, are there >> any plans to implement it? > > I'm not aware of anyone currently working on putting the various core > libs' exceptions into a sensible hierarchy, but it would be great if > someone was to do so! > > I'd suggest that the best way to do this would be initially a > combination of discussions on the mailing list (if anything is > non-obvious) and building a design (and, if appropriate, rationale) on a > wiki page, and ending with a library submissions proposal once you have > a design worked out: > > http://www.haskell.org/haskellwiki/Library_submissions > > > Thanks > Ian > > I'm moving this from the GHC users' mailing list to the haskell-cafe mailing list because I think the discussion probably involves the entire Haskell community. Hi all, We have this new exception hierarchy in GHC base-4 and a lot of new machinery for dealing with it. Other libraries also seem to be springing up to provide alternative methods of dealing with these new exceptions. However, I have found that in most of my programming, the new exception functionality cannot be used to its fullest extent because the exceptions in the core libraries are not part of any sensible "tree" of exceptions, even though having this giant exception tree seems to be one of the premises of the new exception library. For example, there is an ArithException type in Control.Exception which records various arithmetic exceptions. However, there are two problems: (1) in order to fit in with the extensible exceptions theme, the type ought to be called SomeArithException and all of the types of ArithExceptions ought to be sub-exceptions of it, and (2) to my knowledge, this exception type doesn't seem to be thrown by anything in the base libraries. I think the biggest culprit is the IOException type, which also happens to be probably the most commonly-used exception type, since exceptions from IOException cannot really be anticipated and prevented. The current design of IOException is as follows. There is an abstract type called IOException. This can be queried with functions like isDoesNotExistError (which determines if the exception is because a file did not exist). Furthermore, to make an IOException, one has to use the mkIOError function, which takes a value of type IOErrorType, which is again an abstract type. I believe the reason for the abstract types is to allow implementations to extend the range of exceptions. Unfortunately, this has even more problems. For example, the error returned by using readFile on a directory cannot be determined using any Haskell98 function, meaning that code that wants to detect this error cannot be portable to compilers that don't have the new InappropriateType IO exception. The biggest problem with the current design of IOException, however, is that it doesn't play very nicely with the main hierarchy. Like with ArithExceptions, there should be a SomeIOException type. http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf, which was the original paper on extensible exceptions, recommends having a class for IOExceptions that would allow us to get standard information from any specific IOException. I asked the GHC-users mailing list about this and Ian Lynagh recommended starting a discussion about how to put all of the core library exceptions into a sensible hierarchy. I think the goals here ought to be to make our new hierarchy as backward-compatible as possible. In particular, since many programs rely on the isWhateverError functionality, it would be nice to keep these in place. For IOExceptions, I think it would be nice to have a more multi-layered hierarchy than just SomeIOException and its subtypes. For example, I could see a hierarchy for IOExceptions looking something like this: SomeIOException - SomeCouldNotOpenException - AlreadyExists, NoSuchThing, PermissionDenied, InappropriateType - SomeResourceException - ResourceBusy, ResourceExhausted, ResourceVanished etc. I'm not all that familiar with the various exceptions and how they are used, so I'm not really able to propose a complete hierarchy for all the exceptions. I guess the purpose of this email was just to start off a discussion about what our exception hierarchy ought to look like. Thanks for reading this (admittedly long) message. Alex

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Data.Binary suboptimal instance
by Andrew Coppin 24 May '09

24 May '09

So lately I've been working on a little program to generate trippy graphics. (Indeed, some of you may remember it from a few years back...) Anyway, getting to the point, I just restructured my program. As part of the restructuring, I got rid of all the jiggery-pokery with Data.Array.Storable and so forth and decided to use Data.Binary. My first problem was quite simple. I want to write a program that reads an item from the input file, processes it, writes it to the output file, and repeats until the input file is empty. Data.Binary doesn't appear to provide any obvious way to do this. You can't be in the Get and Put monads simultaneously, and I can't figure out how to interleave them. In the end, I ended up with something like xs <- decodeFile f1 encodeFile f2 (map f xs) Unfortunately, it looks like this doesn't do what I want. Serialised lists have their length prepended to them, which means that encodeFile is going to have to generate the *entire* list in memory so it can count how big it is, before it can output a single byte to disk. (!) This is particularly annoying since the output list will be exactly the same size as the input one, and the very first thing decodeFile is doing is reading in that size figure. Maybe there's a way around this glitch, and I just haven't thought of it yet. But you'd think that wanting to lazily process data from an input file and write it into an output file would be an extremely common use-case. Given that, I'm not seeing an obvious way to handle it. Anyway, that's one small problem, but I can live with it. There's another, much bigger problem though, and that's really what I wanted to talk about... As we all know, Data.Binary is *built* for speed. So imagine my shock when my newly adjusted program now based around this library turned out to be massively slower, and the files it produced were drastically larger. It didn't take me that long to dig up the problem. The problem seems to boil down to this: The Binary instance for Double (and Float, by the way) is... well I guess you could argue it's very portable, but efficient it isn't. As we all know, an IEEE-754 double-precision floating-point number occupies 64 bits; 1 sign bit, 11 exponent bits, and 52 mantissa bits (implicitly 53). I had assumed that the Binary instance for Double would simply write these bits to disk, requiring approximately 0 computational power, and exactly 64 bits of disk space. I was wrong. ...what it *actually* does is convert the 64-bit Double into a 32-bit exponent and an arbitrary-precision integer part. (!) It appears to do this by floating-point arithmetic rather than just low-level bit shuffling. Looking up how arbitrary-precision integers are serialised, I see that there's an 8-bit "flag" indicating whether the integer fits into 32 bits, and if it doesn't, there's an 8-bit sign value, followed by a serialised list of bytes. So, in other words, a 32-bit length value, followed by the bytes themselves. To summarise, when I ask for a 64-bit Double to be serialised, I get this: 32-bit exponent. 8-bit flag (probably always "1") 8-bit sign (either "0" or "1") 32-bit size (probably always "8") 64-bit mantissa That's 144 bits in total, i.e., 225% larger than the original Double. (Let's not even go into how much computer power it takes to construct all this data before it's written to disk...) So, that's why my files balooned in size, and why my program slowed to a crawl. But how do I fix this? Well, my solution was simple, brutal, and probably *highly* non-portable. It begins with Unsafe.Coerce (that's never a good thing!) Put simply, if you take your 64-bit Double and unsafeCoerce it into a Word64 and then ask Data.Binary to serialise that, it writes it straight to disk without screwing around with it first. The result ought to be portable to any architecture that uses IEEE-754 arithmetic natively. (Anybody know of an arch this *doesn't* apply to?) But sure, if you wanted to do this on some other architecture with a different native float format, you'd have to write some trixy code to handle it. (And then add conditional compilation for it.) Is there any danger that there might be some kind of improvement to the Double instance in the next version of Data.Binary? END.

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Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Duncan Coutts 24 May '09

24 May '09

On Sat, 2009-05-23 at 13:31 -0400, Mario Blažević wrote: > >>> You could probably see exactly what's happening in > >>> more detail by going through the Core output. > >> > >> Thank you, this advice helped. The Core output indicates > >> that function `test' evaluates the arguments to > >> `parallelize' before it calls it. In other words, the > >> call to `parallelize' is optimized as a strict function > >> call -- which it is. The problem is that this > >> optimization evaluates the arguments sequentially. > >> Compiling with optimizations turned off regains the > >> parallel execution. > >> > >> I guess I will report this as a GHC bug. Or is it a > >> feature request? > > > > As Duncan suggessted, try with GHC head (grab a snapshot). `par` et al > > are much improved. > > I already have, with the snapshot from 21st of April. It behaves the same > as 6.8.2, except it runs for twice as long. > > I'd like to take back a part of what I said before, though: `parallelize' should > be strict only in its second argument. parallelize a b = a `par` (b `pseq` (a, b)) GHC infers that strictness of parallelize. It thinks that it is lazy in both args (you can check this yourself using ghc --show-iface). That's because the definitions of par and pseq use the function 'lazy': -- The reason for the strange "lazy" call is that it fools -- the compiler into thinking that pseq and par are non-strict in -- their second argument (even if it inlines pseq at the call site). -- If it thinks pseq is strict in "y", then it often evaluates -- "y" before "x", which is totally wrong. pseq x y = x `seq` lazy y par x y = case (par# x) of { _ -> lazy y } So GHC thinks that par is lazy in both arguments, while it thinks pseq is strict in the first and lazy in the second. > Its strictness in the first argument should be the same as with `par`. Yes, which it is. > Even though `parallelize x y' always evaluates both x and y, Be careful about what you mean. Yes, it starts the evaluation of x in parallel with y, but that does not mean it is strict in x, as you notice with your example of undefined below. > the following test works fine with optimizations even if `parallelize' > is imported: > > main = putStrLn (snd $ parallelize undefined "Hello, World!") > > So the function is not strict, and I don't understand why GHC should evaluate the > arguments before the call. Right, it's lazy in the first and strict in the second argument. As far as I can see we have no evidence that is is evaluating anything before the call. > Does anybody know of a pragma or another way to make a function *non-strict* even > if it does always evaluate its argument? In other words, is there a way to > selectively disable the strictness optimization? Yes, which is what pseq and par already do. If there's a bug, we need to reproduce it and report it. I cannot reproduce it. Duncan

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Fwd: [Haskell-cafe] Data.Binary suboptimal instance
by Antoine Latter 24 May '09

24 May '09

I suppose I should send my reply to the list ... ---------- Forwarded message ---------- From: Antoine Latter <aslatter(a)gmail.com> Date: Fri, May 22, 2009 at 5:54 PM Subject: Re: [Haskell-cafe] Data.Binary suboptimal instance To: Khudyakov Alexey <alexey.skladnoy(a)gmail.com> On Fri, May 22, 2009 at 4:36 PM, Khudyakov Alexey <alexey.skladnoy(a)gmail.com> wrote: >> You can! - It's again time to point out that Put shouldn't be a monad, but >> a monoid. But as it is, Put is a Writer monad on top of the Builder >> monoid. Better use that Builder monoid directly. > > Could you elaborate? I didn't quite understand. > > > Anyway I had similar problem and simply wrote few functions. They > encode/decode values of same type element by element. It's lazy enough so code > could be written in following style: > Or you could go for the compromise position, where the list can be part of a complex data structure so you're not relying on EOF to find the end. (warning, I don't have my compiler handy so this may not even typecheck) import Control.Monad import Data.Monoid import Data.Binary.Builder import Data.Binary.Get data ChunkedList a = Cons [a] (ChunkedList a) -- Non-null list | Nil chunkSize = 50 fromList :: [a] -> ChunkedList a fromList [] = Nil fromList xs = let (front,back) = splitAt chunkSize xs in Cons front (fromList back) toList :: ChunkedList a -> [a] toList Nil = [] toList (Cons front back) = front ++ toList back putList :: (a -> Builder) -> [a] -> Builder putList f xs = putChunkedList (fromList xs) where putChunkedList Nil = singleton 0 putChunkedList (Cons front back) = mconcat [ singleton (genericLength front) , mconcat $ map f front , putChunkedList back ] getList :: Get a -> Get [a] getList m = toList `liftM` getChunkedList where getChunkedList = do cLen <- getWord8 case cLen of 0 -> return Nil _ -> Cons `liftM` replicateM (fromIntegral cLen) m `ap` getChunkedList

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Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
by Mario Blažević 24 May '09

24 May '09

On Fri 22/05/09 10:51 AM , John Lato jwlato(a)gmail.com sent: > Hi Mario, > > It looks like the parallelize function is getting inlined when it's in > the same file, but not when it's in a separate file. > > Adding a {-# INLINE parallelize #-} pragma to the module with > parallelize recovers all the performance for me. > > You could probably see exactly what's happening in more detail by > going through the Core output. Thank you, this advice helped. The Core output indicates that function `test' evaluates the arguments to `parallelize' before it calls it. In other words, the call to `parallelize' is optimized as a strict function call -- which it is. The problem is that this optimization evaluates the arguments sequentially. Compiling with optimizations turned off regains the parallel execution. I guess I will report this as a GHC bug. Or is it a feature request?

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A problem with par and modules boundaries...
by Mario Blažević 23 May '09

23 May '09

I'll cut to the chase. The short program below works perfectly: when I compile it with -O2 -threaded and run with +RTS -N2 command-line options, I get a nearly 50% real-time improvement: $ time ./primes-test +RTS -N2 5001 real 0m9.307s user 0m16.581s sys 0m0.200s However, if I move the `parallelize' definition into another module and import that module, the performance is completely lost: $ time ./primes-test +RTS -N2 5001 real 0m15.282s user 0m15.165s sys 0m0.080s I'm confused. I know that `par` must be able work across modules boundaries, because Control.Parallel.Strategies is a module and presumably it works. What am I doing wrong? > module Main where > > import Control.Parallel > import Data.List (find) > import Data.Maybe (maybe) > > --import Parallelizable > parallelize a b = a `par` (b `pseq` (a, b)) > > test :: Integer -> Integer -> Integer > test n1 n2 = let (p1, p2) = parallelize > (product $ factors $ product [1..n1]) > (product $ factors $ product [1..n2]) > in p2 `div` p1 > > factors n = maybe [n] (\k-> (k : factors (n `div` k))) > (find (\k-> n `mod` k == 0) [2 .. n - 1]) > > main = print (test 5000 5001)

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import qualified?
by Vasili I. Galchin 23 May '09

23 May '09

Hello, I am working with some somewhat legacy code. I understand what "import qualified Blah as B" means but what does "import qualified Blah" mean? Is this a deprecated feature? I saw with user defined module as well as with "import qualified System" for example. REgards, Vasili

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MPI ... Globus APIs
by Vasili I. Galchin 23 May '09

23 May '09

Hello, 1) I think I read before that there some kind of MPI API (bindings) in Haskell. Yes? If so, current state? 2) http://www.globus.org/ Globus bindings? Kind regards, Vasili

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ANNOUNCE: text 0.2, fast and comprehensive Unicode support using stream fusion
by Bryan O'Sullivan 23 May '09

23 May '09

I'm pleased to announce the availability of text 0.2, an efficient Unicode text library that uses stream fusion. New and notable in this release is support for lazy, chunked text, so you can process text files far larger than memory using a small footprint. Hackage page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/text Darcs repo: darcs get http://code.haskell.org/text Report bugs: http://trac.haskell.org/text/

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community.haskell.org and code.haskell.org problems
by Neil Mitchell 22 May '09

22 May '09

Hi, I regularly (almost daily) have problems reaching both community and code.haskell.org, getting 500 server error messages. I've decided to make all my Haskell code available on community, which means that when it goes down, I can't access my repos - which is not great. Is there a reason for the poor reliability of these servers? Can anything be done to improve them? I know of servers running on Windows laptops which have far higher uptime :-) Thanks Neil

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