- Haskell-Cafe - Haskell.org

how to send a string to a tuple
by Pradeep Wickramanayake 03 Mar '10

03 Mar '10

Hi I have taken a string output from a file But I want to send that string to another function which will convert in to a tuple Can someone teach me please

2 1

Monad laws
by David Sabel 03 Mar '10

03 Mar '10

Hi, when checking the first monad law (left unit) for the IO-monad (and also for the ST monad): return a >>= f ≡ f a I figured out that there is the "distinguishing" context (seq [] True) which falsifies the law for a and f defined below > let a = True > let f = \x -> (undefined::IO Bool) > seq (return a >>= f) True True > seq (f a) True *** Exception: Prelude.undefined Is there a side-condition of the law I missed? Regards, David

5 7

using haskell to serve with apache
by brad clawsie 03 Mar '10

03 Mar '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 hi i'm currently doing some work on a personal site and was considering giving a haskell web tool a spin. i have a fairly complex apache configuration, so i don't want to try to replace it with a native haskell server, but instead use haskell via an interface to apache (i.e. something like wsgi, plack, etc). some searching shows hyena and yesod in various states of development, but i can't figure out if they are appropriate or ready for use. anyone have experiences using haskell behind apache? should i just try out something based on fastcgi? any anecdotes welcome thanks brad -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (FreeBSD) iEYEARECAAYFAkuKoJEACgkQxRg3RkRK91PVJwCdFD4dS47KjJuMcNVZmPkYXKyU ItsAnRoYBE39XtXyXDluh+pHSxzP+Qcd =91Ql -----END PGP SIGNATURE-----

5 4

references for compiler optimizations for functional languages
by Michael Vanier 03 Mar '10

03 Mar '10

Hi everyone, I'm interested in collecting good references for compiler optimizations for functional languages (lazy, strict, statically-typed or not). Any suggestions? Thanks in advance, Mike

4 4

new syntax highlighting library (lexers wanted!)
by John MacFarlane 03 Mar '10

03 Mar '10

Cafe: I wanted to let you know about a new syntax highlighting library I've been working on, called 'illuminate'. The (BSD3 licensed) code is on github: http://github.com/jgm/illuminate illuminate is meant eventually to replace my other highlighting library, highlighting-kate, which is too big, slow, and memory-hungry. In my benchmarks, illuminate is around 70X faster than highlighting-kate and uses half the memory. It can produce output in ANSI, HTML, XHTML, and LaTeX. Highlighting styles can be defined independently of any output format. So far I've written lexers for haskell, literate haskell, cabal, alex, c, c++, c#, python, ruby, java, html, rhtml, xml, javascript, css, tex, and bibtex. Want to highlight a language not on this list? Why not help out by writing a lexer? (Study the lexer for a similar language to get started; if you aren't familiar with Alex, you may want to look at the Alex User Guide too. And let me know if you have any questions.) John

1 0

FFI and lists?
by Kimberly Wallmark 03 Mar '10

03 Mar '10

I'm working with FFI to make a Haskell DLL that's called by C# code. I understand how to share simple types. I've found reasonable documentation for struct-equivalents. Is there a clean way to share lists, or should I make a linked-list struct and do it manually? In other words, if I have adder :: CInt -> CInt -> CInt adder x y = (x+y) then I can use foreign export stdcall adder :: CInt -> CInt -> CInt and on the other side it behaves like int adder(int a, int b); I'd like to know what to do if I have squares :: [CInt] -> [CInt] squares = map (^2) Advice? --Kim

2 1

Evolving Faster Haskell Programs (now with LLVM!)
by Don Stewart 03 Mar '10

03 Mar '10

http://donsbot.wordpress.com/2010/03/01/evolving-faster-haskell-programs-no… … In which I use genetic algorithms to search for optimal LLVM optimizer passes to make Haskell programs faster … LLVM + GHC is a lot of fun. -- Don

2 2

Fwd: [Haskell-cafe] minimal interface
by Sean McLaughlin 02 Mar '10

02 Mar '10

Hi Don, This doesn't tell me what is extraneous in the exports. For instance If I have two modules A and Main module A (f, g) where f :: a -> a f = id g :: a -> a g = id module Main (main) where import qualified A main :: IO () main = do let a = A.f 7 putStrLn $ show a I'd like something to tell me that A.g is not used in the program. Thanks, Sean On Tue, Mar 2, 2010 at 4:28 PM, Don Stewart <dons(a)galois.com> wrote: > seanmcl: > > Hello, > > > > I have a midsize program, and would like to simplify the module > interfaces > > for my own sanity. I know there are tools to check for extraneous > imports, but > > is there something similar for exports? For instance, if module A > exports f, > > but A.f is never used in my compiled program, I would like to be warned > of this > > fact. > > ghc -ddump-minimal-imports > > I think that's the flag. > > -- Dump >

1 0

A few ideas about FRP and arbitrary access in time
by Phil Jones 02 Mar '10

02 Mar '10

...Are hereby presented at: http://www.ee.bgu.ac.il/~noamle/_downloads/gaccum.pdf Comments are more than welcome. (P.S Thanks to a whole bunch of people at #haskell for educating me about this, but most notably Conal Elliott)

1 0

lhs2TeX \eval{} problem
by Tom Lokhorst 02 Mar '10

02 Mar '10

Hello, I'm having a problem with lhs2Tex and \eval{}. It doesn't work. I have the following in a file test.lhs: -------------------------------- %include polycode.fmt One %option ghci Two \eval{4} Three -------------------------------- When I try to run this file through lhs2Tex, it crashes: > lhs2Tex test.lhs ... polycode junk removed ... One %option ghci Two lhs2TeX: fd:7: hGetLine: end of file -------------------------------- Has someone encountered this problem before, or knows how to solve this? - Tom Lokhorst

2 4

Where is HEAD?
by Louis Wasserman 02 Mar '10

02 Mar '10

Where is GHC 6.13 head? I can find sources of 6.13, and darcs for 6.12, but not darcs for 6.13...I'm trying to play with the LLVM backend, and this is the one question it seems to presuppose that you know the answer to. Heh. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

2 1

ANN: Progression-0.3 (supporting benchmarking in Haskell)
by Neil Brown 02 Mar '10

02 Mar '10

Hi all, I've just uploaded the new version of my Progression benchmarking library to Hackage (http://hackage.haskell.org/package/progression) Progression is a utility built on top of Criterion that helps you record benchmark times for several different versions of your code and then draw graphs to compare them. The latest version adds: support for normalisation of benchmark times, support for drawing bar charts, and allows for more detailed configuration of how the data is arranged before plotting. I've written in more detail about the changes in a blog post (http://chplib.wordpress.com/2010/03/02/progression-0-3-bar-charts-and-norma…) which also has an example of a normalised bar chart generated with the new version. Thanks, Neil.

1 0

Multiple Interpretations for a monad?
by Günther Schmidt 02 Mar '10

02 Mar '10

Hi everyone, in my attempts to remove boilerplate and thus to do more "abstraction" I come across a number of interesting things and suggestions. Especially blog posts from Dan Piponi and also Heinrich Apfelmus. I think what they both are saying is that you can construct / implement some sort of 2 layered monad which can then have more than one interpretation. In the responses to one of my posts on DSLs Dan Piponi also points out that he considers monads also to be DSLs. It didn't click with me when he said it, but reading more of his blog posts made me remember that. Now I know this is probably something obvious to most haskellers, but to me it's not. Did I understand this correctly so far? Günther

4 3

haskelldb problem
by Immanuel Normann 02 Mar '10

02 Mar '10

Hi, I have problems with the usage of the DBSpec module. The following used to work a year ago: testDB :: DBInfo testDB = makeDBSpec "testDB" (DBOptions False) [t1] t1 :: TInfo t1 = makeTInfo "t1" [c1,c2] c1 :: CInfo c1 = makeCInfo "c1" (IntT,False) c2 :: CInfo c2 = makeCInfo "c2" (StringT,False) Now the problem is type error in (DBOptions False): Couldn't match expected type `DBOptions' against inferred type `Database.HaskellDB.DBSpec.PPHelpers.MakeIdentifiers So my precise problem is the usage of that "MakeIdentifiers": MakeIdentifiers moduleName :: String -> String identifier :: String -> String toType :: String -> String Could someone please demonstrate how to use MakeIdentifiers reasonably e.g. for the above testDB? And more general: I cannot find uptodate documentation on haskelldb. Is there something around? Thanks Immanuel

2 2

Gtk2hs on Snow Leopard: "Failed to load interface for `System.Glib.UTFString'"
by Michael Goy 02 Mar '10

02 Mar '10

Hi, I am very close to suicide or similar acts of pure desperation... Since weeks, I am trying different approaches to get gtk2hs running on my Snow Leopard MacBook Pro... no success, you guess it... My current GHC is 6.10.4 (but I also tried 6.10.3 before) when I try to make GTK2HS from Source GTK2HS-0.10.1 I get the following error: Failed to load interface for `System.Glib.UTFString' It seems to be a glib problem; I attached a log of make with everything disabled except glib. BTW: I am interested in ANY hints how to get gtk2hs running on Snow Leopard: Is there any known solution for this problem that i wasn't able to find via google? Best regards Mitch

1 0

What are "free" Monads?
by Günther Schmidt 02 Mar '10

02 Mar '10

Hello, I see the term "free monad" quite a lot, but don't really see an explanation what a free monad is. What sets a monad free and why in this day and age are there unfree monads? Günther

6 7

Linear programming in Haskell
by Louis Wasserman 02 Mar '10

02 Mar '10

Is there a nice package out there somewhere with a linear programming implementation? Preferably with a nicely functional interface? Kthxbai, Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

10 20

Optimizing hash array mapped tries
by Edward Z. Yang 01 Mar '10

01 Mar '10

Hello all, I spent this weekend attempting to impement Phil Bagwell's Hash Array Mapped Tries in Haskell. You can view the fruits of this work at this repository: http://github.com/ezyang/hamt Unfortunately, I'm getting pretty abysmal performance out of the implementation I wrote, so I'm hoping to get some performance tuning insights here. My cost centers look about like this: time% alloc% i-Full-update HAMT 29.1 31.4 lookup' HAMT 13.7 4.3 main Main 12.8 19.7 subkey HAMT 11.5 10.0 i-Bitmap HAMT 9.4 12.0 i-Full HAMT 8.1 12.9 insert' HAMT 5.1 0.6 popCount PopCount 4.7 1.0 And my GC stats look like: ./HAMTTest 256000 +RTS -t 275576630962922 <<ghc: 410718260 bytes, 788 GCs, 19646424/46880276 avg/max bytes residency (9 samples), 138M in use, 0.00 INIT (0.00 elapsed), 1.14 MUT (1.13 elapsed), 1.88 GC (2.00 elapsed) :ghc>> + 0:03.16 ./IntMapTest 256000 +RTS -t -710684043813 <<ghc: 172126372 bytes, 329 GCs, 5552955/10483896 avg/max bytes residency (9 samples), 31M in use, 0.00 INIT (0.00 elapsed), 0.51 MUT (0.51 elapsed), 0.62 GC (0.67 elapsed) :ghc>> + 0:01.18 IntMap is included for comparison. The HAMT does about x3-5 worse than IntMap, and uses about x4 more memory (and gets correspondingly penalized when garbage collection occurs). My observations/suspicions are as such: * i-Full-update essentially copies a 32-size vector, with a change to one element. I think I am getting brutally punished for this, in terms of both memory usage as well as runtime. What I'm curious is whether or not this is intrinsic to the algorithm, or if it's something special that GHC is doing. * Bagwell suggests that each node in the Array-Mapped Trie should take only two words. I think I'm losing another two words to storing bounds information in vector, as well as the costs of boxing (though I can't tell if GHC is boxing or not). I've done some playing around with the data declaration, but it's current form seems to result in the fastest implementation. I wonder if this is what is causing the x4 ballooning of memory, or if it's something else. I haven't tried rewriting the code to use GHC's arrays. * Increasing the heap size seems to generally improve the performance the HAMT; a 1G heap on a 512K sample set reduces the difference to about x1.5-2 slower. It's hard to tell if the memory allocation system is working for or against me. What bothers me is even if I could magically wave away all GC time in HAMT, it would only barely win against IntMap. Maybe the algorithm is simply not as good as a good ole' Patricia Trie; but then again, maybe not. Comments and suggestions greatly appreciated. Thanks, Edward

4 8

Performance of Knight's Tour
by Artyom Kazak 01 Mar '10

01 Mar '10

Hi! I'm learning Haskell, and now I'm trying to make framework for solving searching problems, such as Knight's Tour. For small boards it answers instantly. For 7x8 board - 23 seconds. For 8x8 board - more than 30 minutes (it hasn't finished yet). Where is the root of the evil? --program module Main where import Data.List import Data.Array.Unboxed import qualified Data.Array.IArray as IArr import Data.Ix data SResult = Good | GoodProcess | Process | Bad data SDPSearch a p r = SDPSearch (a -> p -> [a]) --expand (p -> p) --update (a -> p -> SResult) --sort ([a] -> r) --result runSDPSearch :: SDPSearch a c b -> [a] -> c -> b runSDPSearch (SDPSearch e u s r) list p = r (rec list params) where params = iterate u p rec [] _ = [] rec (l:lp) pr@(n:np) = case s l n of Good -> l : rec lp pr GoodProcess -> l : (rec (e l n) np) ++ (rec lp pr) Process -> (rec (e l n) np) ++ (rec lp pr) Bad -> rec lp pr main = do (a, b) <- (break (== ' ')) `fmap` getLine print (knightTour (read a) (read b)) knightTour :: Int -> Int -> UArray (Int, Int) Int knightTour a b = runSDPSearch (SDPSearch e u s r) [((1, 1), sArray)] 2 where size = a * b range = ((1, 1), (a, b)) sArray = listArray range (1 : (replicate (size - 1) 0)) allTurns :: Array (Int, Int) [(Int, Int)] allTurns = IArr.listArray range [turns x y | x <- [1..a], y <- [1..b]] where shifts = [(1, 2),(1, -2),(2, 1),(2, -1),(-1, 2),(-1, -2),(-2, 1),(-2, -1)] turns x y = [(x+i, y+j) | (i, j) <- shifts, inRange range (x+i, y+j)] e ((x, y), arr) p = [(t, arr // [(t, p)]) | t <- changes] where changes = [t | t <- allTurns ! (x, y), arr ! t == 0] s el p | p == size = Good | otherwise = Process u = (+ 1) r l | not (null l) = snd (head l) | otherwise = error "No solutions!"

2 5

Cabal pre-compiled packages
by Diego Souza 01 Mar '10

01 Mar '10

Hi, currently when one install a cabal package it compiles it and then install generated binaries. I wonder whether or not it would be useful to have pre-compiled binaries as many package managers usually do (e.g. apt). I often think that would save some time on the expense of a busier hackage server capable of generating packages for many different platforms. I'm particularly thinking on the following scenario: suppose that you have code that is ready for production. If cabal supported pre-compiled binaries, there is no need to install ghc or eventually any other compiler, just runtime environment and eventually cabal. I must say that I have no experience in doing this in Haskell (just personal/small projects), so I suppose one have to generate binaries and use other sort of package manager to deploy code to production (which sounds reasonable as well). Thus, if the assumption is correct, cabal is a development tool, not something one could to only deploy runtime-only packages. I also would appreciate if others could share how usually this is managed. Best Regards, ~dsouza

5 5

Ghent Functional Programming Group: First Meeting on April 1, 2010
by Jeroen Janssen 01 Mar '10

01 Mar '10

Dear All, The results of the doodle are in, and the result is that the first meeting of the Ghent Functional Programming Group will be held on April 1, 2010 at 7pm in meeting room "Shannon" in the Technicum building (Sint-Pietersnieuwstraat 41, 9000 Gent) of Ghent University. The automatic doors will be closed at this time, but we will attach a notice with the telephone number of the meeting room at the front door in the outer left of the building, so you can call us to let you in. We would like to ask anyone even remotely considering to participate to fill in the following Google form: http://spreadsheets.google.com/viewform?formkey=dEtsR2ZIdVhqeVdRNkx6bmxCdF9… Filling in this form does not commit you to anything, but merely serves as an indication of the number of participants for our sake. We will also only be sending follow-up mails with the final program etc. to the persons that have registered. If you do not intend to come to this first meeting, but wish to be updated on our activities, you can follow us on Twitter: http://twitter.com/ghentfpg or you can sign-up for our Google group: http://groups.google.com/group/ghent-fpg Our current program already contains one talk by Tom Schrijvers entitled "Functional Pearl: The Monad Zipper", with the following abstract: "This pearl aims to demonstrate the application of Huet's zipper to the monad stack for the development of highly modular programs." Anyone willing to give a talk on a subject that fits the scope can e-mail us with a proposed topic, no matter what experience level you have, or no matter how short or long the talk will be. Kind Regards, Bart Coppens Jeroen Janssen

1 0

Computer Graphics and Haskell - Radiosity Methods
by Hector Guilarte 01 Mar '10

01 Mar '10

Hello cafe, While I was studying for my computer graphics test I have tomorrow I realized that maybe some of the major problems I've read so far about Radiosity Rendering Algorithms may be reduced significantly if it was implemented in Haskell and taking advantage of the lazy evaluation so that only what can be seen from the viewer's perspective point of view is calculated, and the rest of the scene just remains as thunks waiting for them to be calculated in case they are needed. I guess that probably most of the people on this list doesn't really work on the computer graphics area, but if anybody out there actually does I'd really like if he or she could explain me more of why what I'm saying could be really useful and what has already been done (or why I'm so lost that I'm probably gonna fail my exam tomorrow hahaha)... One of the things I'm studying is the Monte Carlo Radiosity Methods which takes advantages of non-determinism to create stochastic algorithms, I guess this is more difficult to implement in Haskell, please if you know any of this Monte Carlo Methods for Radiosity Algorithms share that knowledge with me (and the list). Thank you, Hector Guilarte

4 3

pddl parser
by Vojtěch Knyttl 01 Mar '10

01 Mar '10

Has anyone seen any PDDL(Planning Domain Definition Language) parser in Haskell? Thanks. VK

1 0

View patterns
by Andrew Coppin 01 Mar '10

01 Mar '10

One somewhat neat thing about Haskell is that you can say case list of [[x], [y,_], [z,_,_]] -> x + y + z _ -> 0 In Java, you'd have to write something like if (list.length() == 3) { List t1 = list.at(0); if (t1.length() == 1) { int x = t1.at(0); List t2 = list.at(1); if (t2.length() == 2) ... I can't even be bothered to finish typing all that lot! However, as somebody pointed out, the Java version is polymorphic. Assuming that length() is defined for multiple types of container, the Java version works with lists, arrays, sets, etc. If you try to do this in Haskell, you end up with case size c of 3 -> case (c ! 0, c ! 1, c ! 2) of (xs, ys, zs) | size x == 1 && size y == 2 & size z == 3 -> (xs ! 0) + (ys ! 0) + (zs ! 0) _ -> 0 _ -> 0 or similar. Which is shorter than Java, but nowhere near as nice as the original list-only version. Now I was under the impression that "view patterns" fix this problem, but it seems they don't: case c of (size -> 3) -> case (c ! 0, c ! 1, c ! 2) of (size -> 1, size -> 2, size -> 3) -> (c ! 0 ! 0) + (c ! 1 ! 0) + (c ! 2 ! 0) Any suggestions?

5 5

Is the Summer of Code TRAC relevant for this years SoC?
by Nathan Hunter 01 Mar '10

01 Mar '10

Hello. I was looking into the possibility of doing Summer of Code as a student, when I found Haskell.org's TRAC<http://hackage.haskell.org/trac/summer-of-code/report/1>. I was particularly interested in Efficient Maps using Generalised Trees and the New Purely Functional Data Structures/Containers Library. I'd love to work on either of those, but are they or any of the projects listed on the TRAC still relevant, or are they just the leftovers from SoC 2009? -Nathan Hunter

2 1

Haskell Weekly News: Issue 151 - February 28, 2010
by jfredett＠gmail.com 01 Mar '10

01 Mar '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100228 Issue 151 - February 28, 2010 --------------------------------------------------------------------------- Welcome to issue 151 of HWN, a newsletter covering developments in the [1]Haskell community. A light HWN this week, some interesting discussion about view patterns and free monads. Also announced this week was [2]TryHaskell a work-in-progress online interactive haskell interpreter and tutorial. The author (based on his reddit post) has asked us not to post this to proggit yet, but I don't think he'll mind a plug here. So after you're done reading, give it a try! Until next week, haskellers, your Haskell Weekly News. Announcements wxHaskell for ghc-6.12.1. Daniel Fischer [3]announced a new release of wxHaskell. concurrent-extra-0.2. Roel van Dijk [4]announced a new release of concurrent-extra jhc-0.7.3. John Meacham [5]announced a new version of jhc, including many bug fixes. Try Haskell! An interactive tutorial in your browser. Benjamin L. Russell [6]told us about the TryHaskell Project, available at [7]TryHaskell.org. TryHaskell is an interactive, online Haskell interpreter and tutorial. MSFP: Call for Papers. Venanzio Capretta [8]announced a call for papers for the Third Workshop on Mathematically Structured Functional Programming. HLedger 0.8. Simon Michael [9]announced a new version of hledger last week, but it slipped through my radar. This release includes bug fixes, some refactoring, and High-Res Graphical Chart generation capability. Discussion View patterns. Andrew Coppin [10]asked about making view patterns polymorphic. What are 'free' Monads? Guenther Schmidt [11]asked what exactly it meant to be a 'free monad' Multiple Interpretations for a monad? Guenther Schmidt [12]asked about monads which can have multiple levels of meaning and interpretation. Blog noise [13]Haskell news from the [14]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Joachim Breitner: [15]Exploiting sharing in arbtt. * Martijn van Steenbergen: [16]Colors in GHCi. * Don Stewart (dons): [17]Evaluation strategies and synchronization: things to watch for. * Don Stewart (dons): [18]Fusion makes functional programming fun!. * Michael Snoyman: [19]Simpler is Better. * Kevin Reid (kpreid): [20]Hvm.hs: an exercise in Haskell golf. * Bryn Keller: [21]Why Isn't Cabal Installed with GHC?. * Don Stewart (dons): [22]Modern Benchmarking in Haskell. Quotes of the Week * roconnor: sounds like you need a zygohistomorphic prepromorphism * copumpkin: These are not the monads you are looking for. * noggle: this language is like having programming super powers About the Haskell Weekly News New editions are posted to [23]the Haskell mailing list as well as to [24]the Haskell Sequence and [25]Planet Haskell. [26]RSS is also available, and headlines appear on [27]haskell.org. To help create new editions of this newsletter, please see the information on [28]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [29]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://tryhaskell.org/ 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70857 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70689 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70961 6. http://article.gmane.org/gmane.comp.lang.haskell.general/17819 7. http://tryhaskell.org/ 8. http://article.gmane.org/gmane.comp.lang.haskell.general/17818 9. http://www.haskell.org/pipermail/haskell-cafe/2010-February/073185.html 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70895 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70874 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70800 13. http://planet.haskell.org/ 14. http://haskell.org/haskellwiki/Blog_articles 15. https://www.joachim-breitner.de/blog/archives/381-Exploiting-sharing-in-arb… 16. http://martijn.van.steenbergen.nl/journal/2010/02/27/colors-in-ghci/ 17. http://donsbot.wordpress.com/2010/02/26/evaluation-strategies-and-synchroni… 18. http://donsbot.wordpress.com/2010/02/26/fusion-makes-functional-programming… 19. http://snoyberg.wordpress.com/2010/02/26/simpler-is-better/ 20. http://kpreid.livejournal.com/23060.html 21. http://www.xoltar.org/?p=33 22. http://donsbot.wordpress.com/2010/02/23/modern-benchmarking-in-haskell/ 23. http://www.haskell.org/mailman/listinfo/haskell 24. http://sequence.complete.org/ 25. http://planet.haskell.org/ 26. http://sequence.complete.org/node/feed 27. http://haskell.org/ 28. http://haskell.org/haskellwiki/HWN 29. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

1 0

ANNOUNCE: jhc-0.7.3
by John Meacham 01 Mar '10

01 Mar '10

It's been a long time coming, but jhc-0.7.3 is here. If you have been following the darcs repository, there are not a whole lot of new changes, but there have been substantial fixes since 0.7.2 for those that use the tarballs or rpms. http://repetae.net/computer/jhc/ One of the most important ones is I have begun the process of standardizing on YAML for all external interfaces that may be useful to automation, this should make interfacing jhc with external tools written in any language much easier, and it will prevent me from spending time trying to invent proprietary formats every time I think of something new to spit out. right now it is used in 3 places: - dumping dependency info with --deps - library dumping with --list-libraries -v - annotating source code, for inputting to documentation generators via --annotate-source The main place it isn't used yet that I desire too is to replace the 'cabal' library description format with a true YAML file. jhc cabal files were never quite compatible with cabal ones anyway and naming them the same thing has been a source of connfusion. some of the other changes have been: features: - a lot more regression tests, many user submitted bugs are now regressions (or fixed) - support for using 'mock' to build rpms - System.Exit, System.Cmd added to base - better compatibility with ghc in some library functions - ui improvements in showing progress and errors - extended --list-libraries info - --deps dependency dumping - more francise compatible thanks to patches from droundy - more instances for bigger tuples added - uses editline if readline not available - select libraries by hash as well as version - add System.IO.Pipe - detect version of gcc needed for cross compiling to windows performance: - storage analysis - a very basic region inference enabled by default - deadcode analysis can see through partial applications now - profiled and sped up some library routines that were producing inellegant core. - figure out when top level grin functions call themselves recursively and turn them into explicit loops. bug fixes: - do expression in infix - fix strict newtype bug - U2U bug fixed - desugaring inside of list comprehensions fix - c generator doesn't mess up on unknown values - the compiler version is stored in the cache files, so multiple versions of jhc don't clobber each others cache entries - Ord instance for lists fixed - recursive type synonyms detected properly - make sure errno.h is included when errno is referenced - don't give a parse error when seeing ghc extensions to INLINE - qualified method names fix John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/

1 0

OpenSUSE 11.2
by Andrew Coppin 28 Feb '10

28 Feb '10

OK, I imagined to install OpenSUSE 11.2. However, I can't find a way to make it install either GHC or the Haskell Platform. A quick Google search turns up a folder on download.opensuse.org which is stuffed full of Haskell stuff, yet no ammount of prodding YaST will convince it that it's possible to install anything remotely Haskell-related. (Now I remember why I don't use Linux much...) Does anybody know how to install this stuff on OpenSUSE? Alternatively, does anybody know of an alternative Linux distribution where this kind of thing is likely to work better? (I think I've got a Debian ISO somewhere - but IME everything on Debian is always 8 years old.)

4 17

Lists of Existential DT
by Yves Parès 28 Feb '10

28 Feb '10

Hello! I have a class Drawable, and some datatypes which are instances of it, and I would like to be able to draw them all at once! drawMany window [image, text, otherImage] I think the type of the function drawMany would be: drawMany :: Window -> [forall a. (Drawable a) => a] -> IO () However it doesn't work. I know one solution is to make a new datatype (e.g. DrawableObj) which will be: data DrawableObj = forall a (Drawable a) => DrawableObj a And then declare drawMany as: drawMany :: Window -> [DrawableObj] -> IO () But to use it I have to wrap up myself every drawable in a DrawableObj: drawMany window [DrawableObj image, DrawableObj text, DrawableObj otherImage] Is there another more suitable way to handle a list of drawables? ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Lists-of-Existential-DT-tp27735354p27735354.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 6

native threads vs. -threaded
by Donn Cave 28 Feb '10

28 Feb '10

I've sketched out a Haskell interface to a C++ toolkit that uses threads. It's the Haiku (BeOS) platform API (BeOS is an obscure OS from the '90s, Haiku is a pretty faithful recent open source re-implementation.) The API is fairly obvious and I imagine much like others that support windowed graphic interfaces. You have your Window class that spawns a thread, and various virtual methods of that class support program interface with the events involving that window. The curious thing is that I can get this to work with my example program only if it is NOT linked with -threaded. If linked with -threaded, when the application comes up I have 4 threads: the main thread and window thread, and two threads apparently created by the Haskell runtime. Shortly (but not immediately) after the first callback from the window thread, the window thread quietly dies. Callbacks from the main thread work fine. Linked without -threaded, I have only the 2 natural threads, and the window thread can call back apparently without problems. I imagine I'm at fault somewhere in this, since I am also responsible for the GHC port to Haiku, but just wondering if this suggests an obvious course of inquiry to anyone. I assume it's not working as intended, as from the documentation I would rather have guessed that -thread would be required in this situation. thanks! Donn Cave, donn(a)avvanta.com

2 2

Cabal will not find HaXml
by Mads Lindstrøm 28 Feb '10

28 Feb '10

Hi When I do: > cabal list --simple-output | grep -i "HaXml 1.20" I get: > HaXml 1.20 > HaXml 1.20.1 > HaXml 1.20.2 But when running: > runhaskell Setup.hs configure I get: > Configuring XrcAccessors-0.0... > Setup.hs: At least the following dependencies are missing: > HaXml -any My cabal file looks like: <snip> Executable XrcAccessors Main-is: XrcAccessors.hs Build-Depends: base==3.0.*, pretty==1.0.*, HaXml hs-source-dirs: src </snip> This is very confusing to me. Why will Cabal not find HaXml when running Setup.hs, but do find it when running "cabal list" ? My Setup.hs looks like: > import Distribution.Simple > main = defaultMain I have tried reinstalling HaXml, but that do not help. I have noticed one thing though, when doing: > ghc-pkg I get output like: <snip> /home/ghc6103/lib/ghc-6.10.3/./package.conf: Cabal-1.6.0.3, HUnit-1.2.0.3, QuickCheck-1.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), </snip> <snip> /home/ghc6103/.ghc/x86_64-linux-6.10.3/package.conf: Cabal-1.6.0.2, Cabal-1.8.0.2, Diff-0.1.2, HDBC-2.1.1, HDBC-odbc-1.1.6.0, HDBC-odbc-2.1.0.0, HTTP-4000.0.4, HUnit-1.2.2.1, HaXml-1.20.2, MetaHDBC-0.1.0, QuickCheck-2.1.0.3, SQLLib-0.0, </snip> That is, HaXml shows itself in the /home/ghc6103/.ghc/x86_64-linux-6.10.3/package.conf list, but not in the /home/ghc6103/.ghc/x86_64-linux-6.10.3/package.conf . Is this the problem and how do I fix it? Greetings, Mads Lindstrøm

2 2

listing mountpoints and getting their properties in Haskell
by Eugene Dzhurinsky 28 Feb '10

28 Feb '10

Hello! I need to list all currently mounted filesystems and get some stats like total space, free space, mount point and physical device. Is there any library capable of obtaining such information from OS itself? Parsing output of 'df' is locale-dependent and error-prone (because of locale settings, output settings etc). Thank you in advance. -- Eugene N Dzhurinsky

6 6

Haskell version of "Monad Transformers and Modular Interpreters"
by Günther Schmidt 28 Feb '10

28 Feb '10

Hello, I come across the paper "Monad Transformers and Modular Interpreters" (http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.17.268) alas the code is in Gofer and I'm not certain how all of it translates to Haskell. Are there other papers out there that translate this to Haskell? Günther

2 1

jQuery is a monad
by Graham Klyne 28 Feb '10

28 Feb '10

Nice blog post: http://importantshock.wordpress.com/2009/01/18/jquery-is-a-monad/ #g

1 0

EDSL's using Filet-O-Fish of Barrelfish project
by C K Kashyap 28 Feb '10

28 Feb '10

Hi All, I'd been looking for a mechanism to generate "C" from Haskell as EDSL - to develop an OS. In the process, I ran into the Barrelfish project ( http://www.barrelfish.org) - They seem to be using Haskell based "DSL generator" called Filet-O-Fish - http://www.barrelfish.org/fof_plos09.pdf It will take me some time understand Filet-O-Fish more but if any of the Haskell experts have already looked at it, I was wondering if I could use Filet-O-Fish to generate EDSL's for arbitrary things. For example, lets say, I wanted to write a EDSL for Linux's network drivers. Could I use Filet-O-Fish and generate a EDSL that allows me to write a driver in a really highlevel manner and then generate the "C" code for it? -- Regards, Kashyap

2 4

How to understand `|` in this code snippet ?
by zaxis 28 Feb '10

28 Feb '10

xxxMain = do timeout <- getEnv "xxx_TIMEOUT" case timeout of Just str | [(t, _)] <- reads str -> do addTimeout t (hPutStrLn stderr "*** TIMEOUT" >> _exit 1) return () _ -> return () ....... What does the `|` mean in "Just str | [(t, _)] <- reads str" ? Is it a logical `or` ? Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/How-to-understand-%60%7C%60-in-this-code-snippet---tp… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 7

redirecting ghc (as a library) output
by Martin Hilbig 27 Feb '10

27 Feb '10

hi, i'm writing a Haskell View Server for CouchDB. it communicates with couchdb over stdin and stdout. it gets JSON encoded haskell code, compiles it (like on http://www.haskell.org/haskellwiki/GHC/As_a_library) gets values, runs the given code over the given values and writes the results back (also json encoded). when there is an error in the given haskell code it should reply with the error json encoded and exit. but f.e. when the given code imports a module, which cannot be found (here Reaction) ghc just spits out a panic directly to stdin and exits like this: > ViewServer: panic! (the 'impossible' happened) > (GHC version 6.12.1 for x86_64-unknown-linux): > Could not find module `Reaction': > Use -v to see a list of the files searched for. > > > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug it puts other errors directly to stdout too, like: > Assembler messages: > Fatal error: can't create > /home/*/AlkylRadicalDecomposition.o: > Permission denied this confuses couchdb, because it expects some JSON and i cant see whats going on between them. now, how can i prevent ghc from using stdout and wrap the output in some JSON? is this even possible? with ghc 6.10 this usage of 'handle' worked for me: > main = handle (\e -> do > let e' = show (e::SomeException) > case fromException e of > Just UserInterrupt -> exitSuccess > _ -> do > let err = error2json "the impossible happened..." e' > putStrLn err > logToFile err > return []) main_loop did i got the Exception handling wrong? thanks in advance. have fun martin

1 0

save/restore of STRef state
by Job Vranish 27 Feb '10

27 Feb '10

I was using STRefs the other day and I ran across a case where I really wanted the ability to save the state of all my references, execute some action, and then restore the state later if needed. I didn't find anything that does this on hackage so I implemented a small wrapper around STRefs which does what I want. Before I put it on hackage I thought I'd run it by haskell-cafe. It works as follows: test :: (String, Bool) test = runContext $ do a <- newRef "foo" b <- newRef False restore <- save writeRef a "bar" writeRef b True when someCondition restore x <- readRef a y <- readRef b return (x, y) If someCondition, the existing state is restored and test returns ("foo", False) otherwise nothing special happens and test returns ("bar", True) Also each reference has a unique key that can make it much easier to convert structures that use STRefs to pure stuctures. Reads, writes and creating refs are still constant time operations (with only a very very small overhead) and garbage collecting behavior should be the same as with regular STRefs. My implementation is here: http://gist.github.com/316738 What do you think? Any suggestions? Does anything like this already exist in hackage? Does this seem useful to other people besides me? :) Any glaring purity issues that I overlooked? Thanks for your input, - Job

2 3

Testing and module export lists
by Magnus Therning 27 Feb '10

27 Feb '10

How do people who like unit testing / property testing deal with export lists? I often find that I do want an export list to reduce clutter in the finished code, but for testing I'd like to expose everything in a module. Is there a nice way to deal with this (using the C pre-processor would not qualify as "nice" ;-)? Maybe there's a switch that causes GHC to simply ignore the export list of a module and export everything? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

6 8

Generalizing nested list comprehensions
by Ishaaq Chandy 27 Feb '10

27 Feb '10

Hi all, If this question sounds a bit noob-ish, that would be because I am one - so apologies in advance! I have functions that look something like these: f1 :: [a] -> [b] f1 xs = [foo [x1, x2] | x1 <- xs, x2 <- bar x1, baz x1 /= baz x2] f2 :: [a] -> [b] f2 xs = [foo [x1, x2, x3] | x1 <- xs, x2 <- bar x1, x3 <- bar x2, baz x1 /= baz x2, baz x1 /= baz x3, baz x2 /= baz x3] f3 :: [a] -> [b] f3 xs = [foo [x1, x2, x3, x4] | x1 <- xs, x2 <- bar x1, x3 <- bar x2, x4 <- bar x3, baz x1 /= baz x2, baz x1 /= baz x3, baz x1 /= baz x4, baz x2 /= baz x3, baz x1 /= baz x4] Note that for the purposes of this discussion it does not matter what foo, bar and baz do. Now what I want to do is write a generalized function fn based on the pattern set by f1, f2 and f3 such that: fn :: Int -> [a] -> [b] and: (fn 1 xs) == f1 xs (fn 2 xs) == f2 xs (fn 3 xs) == f3 xs (fn 25 xs) == f25 xs - obviously if I were to implement f25 as nested list comprehensions it would be ridiculously tedious! Any ideas how I can implement fn? Thanks, Ishaaq -- View this message in context: http://old.nabble.com/Generalizing-nested-list-comprehensions-tp27719199p27… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 2

Hackage redirect loop
by Daniel Fischer 27 Feb '10

27 Feb '10

Trying to access http://hackage.haskell.org/trac/hackage I land in a redirection loop no matter which page or which browser I try. It worked earlier today. Has anybody an idea what the heck might be going on? Is it just me?

2 2

Hackage statistics
by Andrew Coppin 26 Feb '10

26 Feb '10

Is there a simple way to discover the answer to questions like "how many packages are on Hackage?" or "how many downloads per day does Hackage serve?" Or is this information not posted anywhere? (I know there was a blog article a while back with some stats in it, but what about the statistics today?)

3 2

do we need types?
by Pasqualino "Titto" Assini 26 Feb '10

26 Feb '10

Hi, just a silly question (or maybe more than one): In Haskell we have data types (Integer,[a],...) as well as type classes (Num, Ord...). But, if we have type classes do we still need types? Why shouldn't the objects that we process be defined only by their 'interfaces' (assuming that a type class is a kind of interface)? Maybe the real question is: are type classes a more primitive concept than data types? And if so, in a language that had only type classes what would a data declaration like the following map to: data List a = Cons a (List a) | Nil And what about pattern matching? Would that still be possible, and what form would it take? And finally, would having only type classes make the type system any simpler? Thanks, titto

8 9

Redirecting output
by Louis Wasserman 26 Feb '10

26 Feb '10

Is there any way I can temporarily reassign stdout? In particular, will this be handled well by the FFI? I'm hoping there's maybe something like PHP's output buffering thingy <http://php.net/manual/en/book.outcontrol.php>. I'm *binding* (not doing a process call) to an external library which outputs directly to stdout, but whose output I'd like to use... Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

2 2

Function to detect duplicates
by Rafael Gustavo da Cunha Pereira Pinto 26 Feb '10

26 Feb '10

Hi folks, While solving a puzzle, I was posed the problem of finding if there was no duplicates on a list. First I used: noneRepeated=null.(filter (>1)).(map length).group.sort But this seemed very unneficient, so I thought that I could detect the duplicates while sorting, and devised this: import Control.Monad import Data.Maybe noneRepeated=isNothing . (foldl merge (Just [])) . (map sort) . pairs pairs []=[] pairs [x]=[[x]] pairs (x:y:xs)=[x,y]:pairs xs sort []=Just [] sort [x]=Just [x] sort [x,y] | x>y=Just [y,x] | y>x=Just [x,y] | x==y=Nothing merge::(Eq a, Ord a) => Maybe [a]->Maybe [a]->Maybe[a] merge _ Nothing = Nothing merge Nothing _ = Nothing merge (Just []) (Just xs)=Just xs merge (Just xs) (Just [])=Just xs merge (Just (x:xs)) (Just (y:ys)) | x==y = Nothing | x>y = (Just y) +? (merge (Just (x:xs)) (Just ys)) | x<y = (Just x) +? (merge (Just xs) (Just (y:ys))) (+?) = liftM2 (:) My version of the merge sort returns Nothing whenever it finds two equal entries, aborting all subsequent comparisons. I have a few questions for the friendly people at this cafe: 1) Is there any improvement I can make? 2) Can it be parallelized (par, pseq)? Best regards, Rafael

7 22

ANNOUNCE: wxHaskell for ghc-6.12.1
by Daniel Fischer 26 Feb '10

26 Feb '10

Hi all, there's a new suite of wxHaskell on hackage, http://hackage.haskell.org/package/wxdirect-0.12.1.2 http://hackage.haskell.org/package/wxcore-0.12.1.3 http://hackage.haskell.org/package/wx-0.12.1.3 If you have ghc-6.12.1 and wxWidgets-2.8, a simple cabal install wx should give you wx GUI goodness. It built fine on my openSUSE, and I don't see any dependencies which look like they weren't unproblematically cross- platform. Unfortunately I was a little too restrictive with the constraints on containers, so it doesn't work with ghc-6.10, sorry.

1 0

vector to uvector and back again
by stefan kersten 26 Feb '10

26 Feb '10

hi, i've been using the vector [1] library for implementing some signal processing algorithms, but now i'd like to use the statistics [2] package on my data, which is based on the uvector [3] library. is there a (straightforward) way of converting between vectors and uvectors, preferrably O(1)? thanks, <sk> [1] http://hackage.haskell.org/package/vector [2] http://hackage.haskell.org/package/statistics [3] http://hackage.haskell.org/package/uvector

7 23

Proper round-trip HughesPJ/Parsec for Doubles?
by Andy Gimblett 26 Feb '10

26 Feb '10

Hi all, Short version: How can I pretty print and parse values of type Double such that those operations are each other's inverse? Long version: I'm writing and QuickCheck-testing a parser using the approach set out here: http://lstephen.wordpress.com/2007/07/29/parsec-parser-testing-with-quickch… That is, each syntactic category gets a pretty-printer and a parser and an Arbitrary instance, and QuickCheck checks that (parse . prettyPrint) == id, basically. Somewhat unsurprisingly, this sometimes fails for floating point values (I'm using Doubles). Now, I know that floats are in some sense imprecise, and comparing for equality is fraught with peril, but it seems that if x==x then it ought to be at least _possible_ to arrange matters such that (parse . prettyPrint x) == x as well. At worst, pretty-printing the underlying binary representation!? So my feeling is that my parser could be improved. At the moment I'm working around it by defining a type class which checks for equality within some margin of error, and using that instead of Eq - but it's messier than I'd like, so I wondered if there was something obvious I'm missing. As hpaste.org seems to be down, I'll attach a code example here instead. Thanks! -Andy -- Andy Gimblett http://gimbo.org.uk/

7 12

ErrorT instance of MonadPlus
by Dmitry Olshansky 26 Feb '10

26 Feb '10

Hello, ErrorT instance of MonadPlus define that in case of fail both arguments of mplus a value of Error will be the Error of second computation: m `mplus` n = ErrorT $ do a <- runErrorT m case a of Left _ -> runErrorT n Right r -> return (Right r) Could it be changed in this way: m `mplus` n = ErrorT $ do a <- runErrorT m case a of Left e -> do b <- runErrorT n case b of Left e' -> return $ Left (e `eplus` e') r -> return r r -> return r where `eplus` could be placed in class Error a: eplus :: a -> a -> a eplus x y = y -- default implementation In this case we could combine Errors more flexible. Best regards, Dmitry

1 0

problem loading package unix-2.3.2.0 while installing hackages on OpenSuse
by gmi 26 Feb '10

26 Feb '10

I have OpenSuse 11.2 running on VMWare and I have recently installed ghc 6.10.4 from the RPM from OpenSuse. Now I want to install cabal. I have downloaded the required packages from hackage and I try to run: runhaskell Setup configure on any of these packages. With option -v I can see an error while linking unix-2.3.2.0. I am no expert on OpenSuse yet. Does anyone know what's happening here? linux-nrvh:/home/gmi/Download/cabal/zlib-0.5.2.0 # runhaskell -f /usr/bin/ghc -v Setup configure --with-compiler=ghc-6.10.4 ... Loading package filepath-1.1.0.2 ... linking ... done. Loading package unix-2.3.2.0 ... Setup: <command line>: can't load .so/.DLL for: dl (libdl.so: cannot open shared object file: No such file or directory) -- View this message in context: http://old.nabble.com/problem-loading-package-unix-2.3.2.0-while-installing… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 2