- Haskell-Cafe - Haskell.org

ANNOUNCE: uuid-1.2.6
by Antoine Latter 21 Sep '12

21 Sep '12

Hi folks, I'm happy to announce a new point release of the uuid library, 1.2.6: http://hackage.haskell.org/package/uuid-1.2.6 The 'uuid' package implements most of RFC 4122[1] including random generation and generation based on hardware MAC addresses. I haven't announced a point-release in a while. The changes since 1.2.1 include: * When generating UUIDs from the hardware MAC address, if the MAC address is not available we now use a random seed for our (hidden, global) state machine * The 'Read' instance now drops leading spaces * Added the functions 'toWords' and 'fromWords', primary to support the package uuid-quasi[2]. Take care, Antoine 1: http://tools.ietf.org/html/rfc4122 2: http://hackage.haskell.org/package/uuid-quasi

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simple servers
by brad clawsie 21 Sep '12

21 Sep '12

Hi cafe looking at http://www.haskell.org/haskellwiki/Simple_Servers The last two solutions compared are forkIO vs. explicit event support (based on what was System.Event). Further reading appears to indicate that event support has been integrated into the runtime. Is it true that writing a simple server using forkIO now integrates native event loops implicitly? Or do I still need to create code that explicitly uses (what is now) GHC.Event? One last question. When writing C code, using epoll apis explicitly can impose some blocking. Is the same to be said for GHC.Event? I know all of the changes I am discussing seemed to happen in ghc more than a year ago, sorry, I just can't find anything to explicitly guide me here. Thanks! Brad

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progress reporting in a backtracking algorithm
by Harald Bögeholz 21 Sep '12

21 Sep '12

Dear Haskell Cafe, I am playing around with a backtracking algorithm that can take quite a while to compute all solutions to a problem. I'd like to use Debug.Trace.trace to provisionally output something that lets me estimate the total time it might take. But I just can't wrap my head around it. This is how I think I'd write it in a C-like language: backtrack(int level, double position, double stepsize, misc...) { // with variations = number of variations to try on this level double part = stepsize / variations // split time on this level for (i=0; i<variations; ++i) { double current = position + part*i // do the actual work backtrack(level+1, current, part); if (level < not_too_much_detail) printf("progress: %f%%\n", current); } } and start with backtrack(1, 0.0, 100.0). And now for something completely Haskell: I have a function step :: State -> Index -> [State] that on a certain level tries all allowable varaiants and returns a list of those that can be further pursued on deeper levels. Then solving the problem involves applying the step on all levels (whicht are indexed by some array indices here): solve :: Problem -> [State] solve problem = foldM step start grid where start = stateFromProblem problem grid = indices (sLines start) I am totally at loss at how I could accomplish some kind of progress reporting in this lazily evaluated (I hope) backtracking scheme. If anybody would like to review the full code (about 80 lines total vor the solver, not counting I/O), this is where I am right now: https://github.com/ctbo/slitherlink/tree/c8951ca1eaf83ce9de43f0483740ce339f… and this is the branch I am working on right now: https://github.com/ctbo/slitherlink/tree/2lines Or is there maybe a totally different and better way to approach this kind of tree search in Haskell? I'm eager to learn. Thanks for your attention Harald -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

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Haskell Weekly News: Issue 244
by Daniel Santa Cruz 20 Sep '12

20 Sep '12

Welcome to issue 244 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 9 to September 15, 2012. Inbox As you might have heard, GHC 7.6.1 is out for all platforms. I would say "get it while it is still hot", but I guess I missed my opportunity to do that last week. [1] http://goo.gl/0fwDQ Sonke Hahn wrote in to announce that on Monday "Story Episodes of Nikki and the Robots" was was released. [2] http://goo.gl/dBifT Malcolm Wallace has uploaded videos from ICFP 2012, for those of us that didn't have the opportunity to be there. Big thanks for sharing these with the community! [3] http://goo.gl/6ZhYV Quotes of the Week * copumpkin: when in doubt, blame ski * shachaf: You can call the greengrocer and place an order. That'll convert their unordered pears into ordered pears! * shachaf: Those who would give up essential type independence for a little temporary type safety deserve neither independence nor safety. * ddarius: edwardk: So your plan for Haskell adoption is to write Haskell in languages that aren't Haskell, say "Man, these languages suck, this would be super easy in Haskell", and then use the Haskell you started with reproducing functionality at an "unbelievable" rate. * cmccann: personally I'm just waiting for an extension that demotes types to the value level, so that we can finally have natural numbers. Top Reddit Stories * Yet another Haskell IDE in the works Domain: yesodweb.com, Score: 71, Comments: 94 On Reddit: [4] http://goo.gl/N1nfs Original: [5] http://goo.gl/z47p8 * The case of the mysterious explosion in space Domain: serpentine.com, Score: 65, Comments: 9 On Reddit: [6] http://goo.gl/O782g Original: [7] http://goo.gl/eQxOc * Haskell vs. F# vs. Scala: A High-level Language Features and Parallelism Support Comparison Domain: macs.hw.ac.uk, Score: 47, Comments: 30 On Reddit: [8] http://goo.gl/XWdPX Original: [9] http://goo.gl/daRy2 * The functor design pattern Domain: haskellforall.com, Score: 42, Comments: 45 On Reddit: [10] http://goo.gl/FpJL4 Original: [11] http://goo.gl/pZ4yu * How To Exclude Women From Your Technical Community: A Tutorial Domain: tim.dreamwidth.org, Score: 42, Comments: 309 On Reddit: [12] http://goo.gl/iKN7H Original: [13] http://goo.gl/GKMjb * A simple library that allows logic programming in Haskell Domain: github.com, Score: 38, Comments: 10 On Reddit: [14] http://goo.gl/J9Pbp Original: [15] http://goo.gl/1bRw5 * Larry Wall: "You should probably know about it [Haskell] if only to be able to say 'is this like Haskell?', and, if so, then you know you'll have to hire some really smart people to program in it." Domain: youtube.com, Score: 34, Comments: 130 On Reddit: [16] http://goo.gl/z1PTW Original: [17] http://goo.gl/5bCN9 * Malcolm Wallace is broadcasting videos from ICFP Domain: m.youtube.com, Score: 33, Comments: 9 On Reddit: [18] http://goo.gl/O6uuy Original: [19] http://goo.gl/Kyg5O * Darcs hub alpha: a darcsden fork to “turn the dogfooding knobs up to 11” Domain: hub.darcs.net, Score: 30, Comments: 10 On Reddit: [20] http://goo.gl/5D614 Original: [21] http://goo.gl/pDHL9 * The Architecture of the Mighttpd High-Speed Web Server Domain: iij.ad.jp, Score: 28, Comments: 4 On Reddit: [22] http://goo.gl/7yRbw Original: [23] http://goo.gl/oCkBt * Tying the knot on a Rubik's cube Domain: self.haskell, Score: 25, Comments: 7 On Reddit: [24] http://goo.gl/0XPiY Original: [25] http://goo.gl/0XPiY * Kazu Yamamoto: Improving the performance of Warp Domain: yesodweb.com, Score: 24, Comments: 1 On Reddit: [26] http://goo.gl/yvRfR Original: [27] http://goo.gl/gEQlA * Taking advantage of "Theorems for Free"? Domain: self.haskell, Score: 20, Comments: 9 On Reddit: [28] http://goo.gl/cqy6U Original: [29] http://goo.gl/cqy6U Top StackOverflow Questions * Is there a nice way to make function signatures more informative in Haskell? votes: 37, answers: 6 Read on SO: [30] http://goo.gl/vOY98 * Purely functional data structures for text editors votes: 26, answers: 7 Read on SO: [31] http://goo.gl/ciJQ3 * Lazy Pattern matching in Data.List votes: 15, answers: 1 Read on SO: [32] http://goo.gl/T2YPC * Can I provide the type-checker with proofs about inductive naturals in GHC 7.6? votes: 14, answers: 1 Read on SO: [33] http://goo.gl/zoMDr * Why does Haskell's “flip id” has this type? votes: 12, answers: 1 Read on SO: [34] http://goo.gl/HWuyZ * How long pauses can occur in a Haskell program due to garbage collection? votes: 11, answers: 1 Read on SO: [35] http://goo.gl/Stnq7 * Good introduction to free theorems [closed] votes: 11, answers: 1 Read on SO: [36] http://goo.gl/2OVYJ * Int vs Word in common use? votes: 10, answers: 3 Read on SO: [37] http://goo.gl/JR9xK * Haskell collections with guaranteed worst-case bounds for every single operation? votes: 9, answers: 1 Read on SO: [38] http://goo.gl/Oc4L9 * Zipper data structure for graphical model editor votes: 8, answers: 1 Read on SO: [39] http://goo.gl/BF865 Until next time, Daniel Santa Cruz References 1. http://comments.gmane.org/gmane.comp.lang.haskell.general/19477 2. http://joyridelabs.de/blog/?p=1435 3. http://www.youtube.com/playlist?list=PL4UWOFngo5DV0sAM9qsfnPzNtRaDyZ7MD 4. http://www.yesodweb.com/blog/2012/09/building-haskell-ide 5. http://www.reddit.com/r/haskell/comments/zrm9z/yet_another_haskell_ide_in_t… 6. http://www.serpentine.com/blog/2012/09/12/the-case-of-the-mysterious-explos… 7. http://www.reddit.com/r/haskell/comments/zrxns/the_case_of_the_mysterious_e… 8. http://www.macs.hw.ac.uk/~dsg/gph/papers/abstracts/fhpc12.html 9. http://www.reddit.com/r/haskell/comments/zxcks/haskell_vs_f_vs_scala_a_high… 10. http://www.haskellforall.com/2012/09/the-functor-design-pattern.html 11. http://www.reddit.com/r/haskell/comments/zxl49/the_functor_design_pattern/ 12. http://tim.dreamwidth.org/1762846.html 13. http://www.reddit.com/r/haskell/comments/zxmzv/how_to_exclude_women_from_yo… 14. https://github.com/smichal/hs-logic 15. http://www.reddit.com/r/haskell/comments/zq1gb/a_simple_library_that_allows… 16. http://www.youtube.com/watch?v=LR8fQiskYII&feature=player_detailpage#t=169s 17. http://www.reddit.com/r/haskell/comments/zpff3/larry_wall_you_should_probab… 18. http://m.youtube.com/channel/UCP9g4dLR7xt6KzCYntNqYcw?&desktop_uri=%2Fchann… 19. http://www.reddit.com/r/haskell/comments/zsjag/malcolm_wallace_is_broadcast… 20. http://hub.darcs.net/ 21. http://www.reddit.com/r/haskell/comments/zx4th/darcs_hub_alpha_a_darcsden_f… 22. http://www.iij.ad.jp/en/company/development/tech/mighttpd/ 23. http://www.reddit.com/r/haskell/comments/zrydv/the_architecture_of_the_migh… 24. http://www.reddit.com/r/haskell/comments/znt85/tying_the_knot_on_a_rubiks_c… 25. http://www.reddit.com/r/haskell/comments/znt85/tying_the_knot_on_a_rubiks_c… 26. http://www.yesodweb.com/blog/2012/09/improving-warp 27. http://www.reddit.com/r/haskell/comments/ztvu7/kazu_yamamoto_improving_the_… 28. http://www.reddit.com/r/haskell/comments/zydcs/taking_advantage_of_theorems… 29. http://www.reddit.com/r/haskell/comments/zydcs/taking_advantage_of_theorems… 30. http://stackoverflow.com/questions/12416723/is-there-a-nice-way-to-make-fun… 31. http://stackoverflow.com/questions/12358083/purely-functional-data-structur… 32. http://stackoverflow.com/questions/12429449/lazy-pattern-matching-in-data-l… 33. http://stackoverflow.com/questions/12442858/can-i-provide-the-type-checker-… 34. http://stackoverflow.com/questions/12339822/why-does-haskells-flip-id-has-t… 35. http://stackoverflow.com/questions/12404031/how-long-pauses-can-occur-in-a-… 36. http://stackoverflow.com/questions/12421085/good-introduction-to-free-theor… 37. http://stackoverflow.com/questions/12432154/int-vs-word-in-common-use 38. http://stackoverflow.com/questions/12393104/haskell-collections-with-guaran… 39. http://stackoverflow.com/questions/12339805/zipper-data-structure-for-graph…

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foldl vs. foldr
by Jan Stolarek 20 Sep '12

20 Sep '12

Hi list, I have yet another question about folds. Reading here and there I encountered statements that foldr is more important than foldl, e.g. in this post on the list: http://www.haskell.org/pipermail/haskell-cafe/2012-May/101338.html I want to know are such statements correct and, if so, why? I am aware that foldl' can in some circumstances operate in constant space, while foldr can operate on infinite lists if the folding function is lazy in the second parameter. Is there more to this subject? Properties that I mentioned are more of technical nature, not theoretical ones. Are there any significant theoretical advantages of foldr? I read Bird's and Wadler's "Introduction to functional programming" and it seems to me that foldl and foldr have the same properties and in many cases are interchangeable. Greets, Janek

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[ANNOUNCE] Fmark markup language
by José Lopes 19 Sep '12

19 Sep '12

Hello everyone, I just wanted to share a package I created called Fmark, now available on HackageDB. Feedback both on the project and on the code is greatly appreciated :) Fmark (Friendly Markup) is a very simple markup language without syntax and simple but sophisticated document styling, capable of producing PDF and XML files. The key philosophy behind this markup language is to eliminate the strange syntactic characters seen in most markup languages, but at the same time try to maintain a high level of expressiveness, using only document reconstruction. Check it out http://hackage.haskell.org/package/fmark https://github.com/jabolopes/fmark Best regards, José -- José António Branquinho de Oliveira Lopes 58612 - MEIC-A Instituto Superior Técnico (IST), Universidade Técnica de Lisboa (UTL) jose.lopes(a)ist.utl.pt

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ANNOUCE: one-liner-0, SYB-like generics with constraint kinds
by Sjoerd Visscher 19 Sep '12

19 Sep '12

Hi all, I am pleased to announce the first release of One-Liner, a package for writing short and concise generic instances of type classes. It works a bit like Scrap-Your-Boilerplate, but it uses the new constraint kinds instead of the Typeable class. On hackage: http://hackage.haskell.org/package/one-liner-0 On github: https://github.com/sjoerdvisscher/one-liner For example, this is how to write generic equality (using the All monoid): eqADT :: (ADT t, Constraints t Eq) => t -> t -> Bool eqADT s t = ctorIndex s == ctorIndex t && getAll (mbuilds (For :: For Eq) (\fld -> All $ s ! fld == t ! fld) `at` s) The code works like this: "Constraints t Eq" means it requires that all subcomponents of type t have an Eq instance, and then values s and t are equal if they are built by the same constructor and each subcomponent is equal. The package is called One-Liner because the generic functions often end up as short as eqADT, especially if there's already an appropriate Monoid or Applicative functor available. Here are two more examples, generic put and get for the Binary package (after adding the missing Monoid instance for Put) putADT :: (ADT t, Constraints t Binary) => t -> Put putADT t = putWord8 (toEnum (ctorIndex t)) >> gfoldMap (For :: For Binary) put getADT :: (ADT t, Constraints t Binary) => Get t getADT = do ix <- fromEnum <$> getWord8 buildsA (For :: For Binary) (const get) !! ix Finally, to give a complete sense of what's involved, here's an example data type and its ADT implementation: data T a = A Int a | B a (T a) instance ADT (T a) where ctorIndex A{} = 0 ctorIndex B{} = 1 type Constraints (T a) c = (c Int, c a, c (T a)) buildsRecA For sub rec = [ (ctor "A", A <$> sub (FieldInfo (\(A i _) -> i)) <*> sub (FieldInfo (\(A _ a) -> a))) , (ctor "B", B <$> sub (FieldInfo (\(B a _) -> a)) <*> rec (FieldInfo (\(B _ t) -> t))) ] If you want to learn more, you can find an introductory blog post here: https://github.com/sjoerdvisscher/blog/blob/master/2012/2012-09-06%20constr… Some complete examples are here: https://github.com/sjoerdvisscher/one-liner/tree/master/examples Some more generic functions, including generic Show and Read: https://github.com/sjoerdvisscher/one-liner/blob/master/src/Generics/OneLin… -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog

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How to implement nested loops with tail recursion?
by sdiyazg＠sjtu.edu.cn 19 Sep '12

19 Sep '12

I need to implement fast two-level loops, and I am learning using seq to make calls tail-recursive. I write programs to compute main = print $ sum [i*j|i::Int<-[1..20000],j::Int<-[1..20000]] This program (compiled with -O2) runs twenty times slower than the unoptimized (otherwise the loop gets optimized out) C version. But it seems to run in constant memory, so I assume that it has been turned into loops. #include <stdio.h> int main(){ int s=0; for(int i=1;i<=20000;++i){ for(int j=1;j<=20000;++j){ s+=i*j; } } printf("%d\n",s); return 0; } Then I write main = print $ f 1 where f i = let x = g 1 in x `seq` (x + if i<20000 then f (i+1) else 0) :: Int where g j = let x = i*j in x `seq` (x + if j<20000 then g (j+1) else 0) :: Int This version runs out of memory. When I scale the numbers down to 10000, the program does run correctly, and takes lots of memory. Even if I change the seqs into deepseqs, or use BangPatterns (f !i =... ; g !j = ...), the situation doesn't change. A monadic version import Control.Monad.ST.Strict import Control.Monad import Data.STRef.Strict main = print $ runST $ do s <- newSTRef (0::Int) let g !i !j = if (j<=10000) then modifySTRef s (+1)>>(g i (j+1)) else return () let f !i = if (i<=10000) then g i 1>>(f $ i+1) else return () f 1 readSTRef s also runs out of memory. So how can I write a program that executes nested loops efficiently?

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ANNOUNCE: som-1.0
by Amy de Buitléir 19 Sep '12

19 Sep '12

Do you have some data that you'd like to understand better? I'm happy to announce a new package called som that may help: http://hackage.haskell.org/package/som https://github.com/mhwombat/som/wiki (wiki) A Kohonen Self-organising Map (SOM) maps input patterns onto a regular grid (usually two-dimensional) where each node in the grid is a model of the input data, and does so using a method which ensures that any topological relationships within the input data are also represented in the grid. This implementation supports the use of non-numeric patterns. In layman's terms, a SOM can be useful when you want to discover the underlying structure of some data. I have a brief tutorial in the wiki, which I hope to expand over the next few weeks.

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ANNOUNCE: grid-2.0
by Amy de Buitléir 19 Sep '12

19 Sep '12

I'm happy to announce a new major release of the grid package: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) WHAT'S NEW: The GridMap module provides ordered maps from tiles on a grid to values. This module is a wrapper around Grid and Map, in order to combine the functionality of grids and maps into a single type. This may be of interest to anyone developing a game based on a grid. ABOUT GRID: Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, and hexagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome.

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Choosing color of the track
by Manish Trivedi 18 Sep '12

18 Sep '12

Hi Folks, I am trying to plot two tracks with different colors. Following is my input file, ------ 2012-09-18 00:10:48,166 @CurrentPerHour13057 Red 2012-09-18 00:10:48,166 =CurrentPerHour13057 0.0 2012-09-18 00:10:58,155 =CurrentPerHour13057 0.0 2012-09-18 00:11:08,203 =CurrentPerHour13057 0.0 2012-09-18 00:11:18,166 =CurrentPerHour13057 0.0 2012-09-18 00:11:28,159 =CurrentPerHour13057 0.0 2012-09-18 00:11:38,170 =CurrentPerHour13057 0.0 2012-09-18 00:11:48,175 =CurrentPerHour13057 0.0 2012-09-18 00:11:58,174 =CurrentPerHour13057 0.0 2012-09-18 00:12:08,216 =CurrentPerHour13057 0.0 2012-09-18 00:12:18,218 =CurrentPerHour13057 0.0 2012-09-18 03:58:28,159 @PacingReqPerHr13057 Blue 2012-09-18 03:58:28,159 =PacingReqPerHr13057 2375.0534412521242 2012-09-18 03:58:38,161 =PacingReqPerHr13057 2375.383005739316 2012-09-18 03:58:48,175 =PacingReqPerHr13057 2375.713057258422 2012-09-18 03:58:58,160 =PacingReqPerHr13057 2376.0422443063935 2012-09-18 03:59:08,192 =PacingReqPerHr13057 2376.3730727321126 2012-09-18 03:59:18,207 =PacingReqPerHr13057 2375.9038854561304 2012-09-18 03:59:28,168 =PacingReqPerHr13057 2376.2324444615497 2012-09-18 03:59:38,156 =PacingReqPerHr13057 2376.5619853019184 2012-09-18 03:59:48,160 =PacingReqPerHr13057 2376.892145685344 2012-09-18 03:59:58,160 =PacingReqPerHr13057 2377.222265743067 ----- tplot -o /root/color.png -or 1024x768 -k 'CurrentPerHour' 'lines' -k 'PacingReqPerHr' 'lines' -if /root/color.input -tf 'date %Y-%m-%d %H:%M:%OS' I cant get to print plots in different color. Could anyone point out what am I doing wrong? Thanks, Manish

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ANNOUNCE: MFlow-0.1.5
by Alberto G. Corona 18 Sep '12

18 Sep '12

Hi haskellers and specially the web developers. http://hackage.haskell.org/package/MFlow-0.1.5.3 MFlow is a is a Web framework with some unique, and I mean unique, characteristics that I find exciting: - It is a Web application server that start and restart on-demand stateful web server processes (not request.-response) This means that all the page navigation can be coded in a single procedure. This increases readability of the programmer code. I woul call it a anti-node.js. Buit usual request-response (stateless) server processes are also allowed - When the process is invoqued as result of an URL request, the Web app server not only restart the process but also recover its execution state. The enclosing Workflow monad provides the thread state persistence. There are state timeouts and process timeouts defined by the programmer. Processes with no persistent state (transient) are possible. -The user interface is made of widgets. They are formlets with added formatting, attributes, validations, modifiers and callbacks, that are composable, so the pieces are reusable and return type safe responses to the calling process. Even the links are part of widgets and return back type safe inputs at compile time to the calling server process. Tho glue these components, ordinary applicative combinators and other extra combinators are used. - The widgets and the communication don´t make assumptions about the architecture, so it can be adapted to non-web environments. This versions has interface for WAI-warp, Hack, Text.XHtml (xhtml) , and Haskell Server Pages. -The widget rendering can be converted to ByteStrings automatically with special combinators. A mix of widgets with different formats can be combined in the same source file. For example Text.Html and HSP (Haskell server pages) -These widgets can be cached, to avoid widget rendering on every interaction. -To handle the back button, and because the processes are stateful, they can run backwards until the response match. This is transparent for the programmer, thanks to the embedded FlowM monad. -All the code is in pure Haskell. No deployment, special scripts, formats etc are necessary. -Besides automatic state persistence, TCache provides transactions and user data persistence, that can be configured for SQL databases. Default persistence in files permit very rapid prototyping. Just code and run it with runghc. -Has AJAX support All of this sounds very complicated, but really it is simple!. Most of these things are transparent. The resulting code is quite readable and has very little plumbing! There is a non trivial example that some of these functionalities embedded here that you can run: http://hackage.haskell.org/packages/archive/MFlow/0.1.5.3/doc/html/MFlow-Fo… Take a look and tell me your opinion. I hope that you find it as exciting as me. I´m looking for people to collaborate in the development of MFlow. Although still it is experimental, it is being used in at least one future commercial project. So I have te commitment to continue its development. There are many examples in the documentation and in the package. Alberto

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Type error with Type families
by Marco Túlio Pimenta Gontijo 18 Sep '12

18 Sep '12

Hi. I cannot make this program type check: {-# LANGUAGE TypeFamilies, FlexibleContexts #-} import qualified Data.ListLike as LL class LL.ListLike full (Item full) => ReplaceOneOf full where type Item full :: * replaceOneOf :: [Item full] -> full -> full -> full replaceOneOf from to list | LL.null list = list | x `element` from = LL.concat [to, replaceOneOf from to $ LL.dropWhile (`element` from) xs] | otherwise = LL.cons x $ replaceOneOf from to xs where x = LL.head list xs = LL.tail list element :: Item full -> [Item full] -> Bool The error message is: Line 9: 1 error(s), 0 warning(s) Could not deduce (Item full0 ~ Item full) from the context (ReplaceOneOf full) bound by the class declaration for `ReplaceOneOf' at /home/marcot/tmp/test_flymake.hs:(4,1)-(15,45) NB: `Item' is a type function, and may not be injective Expected type: [Item full0] Actual type: [Item full] In the second argument of `element', namely `from' In the expression: x `element` from I have tried using asTypeOf, but it did not work: {-# LANGUAGE TypeFamilies, FlexibleContexts, ScopedTypeVariables #-} import qualified Data.ListLike as LL class LL.ListLike full (Item full) => ReplaceOneOf full where type Item full :: * replaceOneOf :: Item full -> [Item full] -> full -> full -> full replaceOneOf xt from to list | LL.null list = list | (x `asTypeOf` xt) `element` from = LL.concat [to, replaceOneOf xt from to $ LL.dropWhile (`element` from) xs] | otherwise = LL.cons x $ replaceOneOf xt from to xs where x = LL.head list xs = LL.tail list element :: Item full -> [Item full] -> Bool Can someone point me to a solution? Greetings. -- marcot http://marcot.eti.br/

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cost-centre names
by Marco Túlio Pimenta Gontijo 18 Sep '12

18 Sep '12

Hi. I have a question about cost-centre names, as shown on .hp files produced by +RTS -hc. They have the form A/B/C/D/E/F/G/... Usually, it seems that it means A, called by B, called by C, etc, but that's not the case sometimes. I have a case here (called with -L200): replaceOneOf’/clean/tagsText/anyTag/dropTagClose/parseObservations/specificTagText/tagText/parseOab/dropTagText/dropTill/tag/tagOpen... tagsText calls clean which calls replaceOneOf', but anyTag does not call tagsText. parseObservations calls dropTagClose which calss anyTag, but specificTagText does not call parseObservations. They seem to be grouped by three. Is this correct? How should I interpret it? Greetings. -- marcot http://marcot.eti.br/

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Either Monad and Laziness
by oleg＠okmij.org 18 Sep '12

18 Sep '12

> I am currently trying to rewrite the Graphics.Pgm library from hackage > to parse the PGM to a lazy array. Laziness and IO really do not mix. > The problem is that even using a lazy array structure, because the > parser returns an Either structure it is only possible to know if the > parser was successful or not after the whole file is read, That is one of the problems. Unexpected memory blowups could be another problem. The drawbacks of lazy IO are well documented by now. > The behaviour I want to achieve is like this: I want the program when > compiled to read from a file, parsing the PGM and at the same time > apply transformations to the entries as they are read and write them > back to another PGM file. Such problems are the main motivation for iteratees, conduits, pipes, etc. Every such library contains procedures for doing exactly what you want. Please check Hackage. John Lato's iteratee library, for example, has procedure for handling sound (AIFF) files -- which may be very big. IterateeM has the TIFF decoder -- which is incremental and strict. TIFF is much harder to parse than PGM.

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Dynamic Programming with Data.Vector
by Myles C. Maxfield 18 Sep '12

18 Sep '12

Hello, I've been writing dynamic programming (dp) algorithms in imperative languages for years, and I was thinking recently about how to use it in a Haskell context. In particular, I want to write a function that takes an ordered collection of items and produces a new item to insert into the ordered collection. The most straightforward way to do this would be to use a list, something like the following: recurse :: [Integer] -> [Integer] recurse l = newValue : recurse (take (length l + 1) infiniteList) where newValue = ... infiniteList :: [Integer] infiniteList = initialList ++ recurse initialList where initialList = ... solution :: Integer solution = infiniteList !! 5 I'm assuming that this can run fast because I'm assuming the 'take' function won't actually duplicate the list ([1] doesn't actually list the running time of 'take') Is this a correct assumption to make? Secondarily, and most importantly for me, I'm curious about how to make this fast when the computation of 'newValue' requires random access to the inputted list. I'm assuming that I would use Vectors instead of lists for this kind of computation, and [2] describes how I can use the O(1) 'slice' instead of 'take' above. However, both of Vector's cons and snoc functions are O(n) which defeats the purpose of using this kind of algorithm. Obviously, I can solve this problem with mutable vectors, but that's quite inelegant. Overall, I'm looking for a function, similar to Data.Vector's 'generate' function, but instead of the generation function taking the destination index, I'd like it to take the elements that have previously been constructed. Is there such a function? If there isn't one, is this kind of function feasible to write? If such a function doesn't exist and is feasible to write, I'd be happy to try to write and contribute it. [1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html#g… [2] http://hackage.haskell.org/packages/archive/vector/0.9.1/doc/html/Data-Vect…

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Re: [Haskell-cafe] What is the good way to work with list comprehension and UTCTime?
by Chris Heller 18 Sep '12

18 Sep '12

You might want to have a look at the time-recurrence package: http://hackage.haskell.org/package/time-recurrence For your simple cases you would do something like: Each second: starting (UTCTime ...) $ recur secondly Each minute: starting (UTCTime ...) $ recur minutely The rules can get quite a bit more complex than that. I am the author of time-recurrence, so if you have questions feel free to ping me. -Chris On Fri, Sep 14, 2012 at 6:00 AM, <haskell-cafe-request(a)haskell.org> wrote: > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Fwd: How Type inference work in presence of Functional > Dependencies (Dan Doel) > 2. Re: [ANN] Cumino 0.2 - Now supports pretty indentation > through stylish-haskell (Brandon Allbery) > 3. Re: [ANN] Cumino 0.2 - Now supports pretty indentation > through stylish-haskell (Alfredo Di Napoli) > 4. ANNOUNCE: Groundhog 0.1.0.1 - high-level database library > (Boris Lykah) > 5. Re: ANNOUNCE: Groundhog 0.1.0.1 - high-level database library > (Tom Murphy) > 6. Re: Either Monad and Laziness (wren ng thornton) > 7. How do I generate random numbers using random-fu, with > platform-agnostic code? (Andrew Pennebaker) > 8. Is Hackage down? (C K Kashyap) > 9. Re: Is Hackage down? (damodar kulkarni) > 10. What is the good way to work with list comprehension and > UTCTime? (Magicloud Magiclouds) > 11. Re: What is the good way to work with list comprehension and > UTCTime? (Karl Voelker) > 12. Re: [ANN] Cumino 0.2 - Now supports pretty indentation > through stylish-haskell (Alfredo Di Napoli) > 13. Re: Is Hackage down? (John Wiegley) > 14. Re: [ANN] Cumino 0.2 - Now supports pretty indentation > through stylish-haskell (Alfredo Di Napoli) > 15. Re: What is the good way to work with list comprehension and > UTCTime? (Roman Cheplyaka) > 16. Re: What is the good way to work with list comprehension and > UTCTime? (Magicloud Magiclouds) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 13 Sep 2012 10:44:17 -0400 > From: Dan Doel <dan.doel(a)gmail.com> > Subject: [Haskell-cafe] Fwd: How Type inference work in presence of > Functional Dependencies > To: Haskell Caf? List <haskell-cafe(a)haskell.org> > Message-ID: > < > CAHEA9tPsGTUX4Ke9eGCpA+-f_b_Ky9--y6RZkHPAmN9LW8MxHA(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > Copying the mailing list, because I forgot. > > On Thu, Sep 13, 2012 at 5:18 AM, satvik chauhan <mystic.satvik(a)gmail.com> > wrote: > > Consider the code below : > > > > {-# LANGUAGE > > > MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts > > #-} > > class Foo a c | a -> c > > instance Foo Int Float > > f :: (Foo Int a) => Int -> a > > f = undefined > > > > Now when I see the inferred type of f in ghci > > > >> :t f > > > >> f :: Int -> Float > > > > Now If I add the following code > > > > g :: Int -> Float > > g = undefined > > > > h :: (Foo Int a) => Int -> a > > h = g > > > > > > I get the error > > > > Could not deduce (a ~ Float) > > > > > > I am not able to understand what has happened here ? The restriction "Foo > > Int a" should have restricted the type of h to "Int -> Float" as shown in > > the inferred type of f. > > The answer, I believe, is that the difference between the fundep > implementation and type families is local constraint information. > Fundeps do no local propagation. > > So in your first definition, you've locally provided 'Int -> a', which > is acceptable to GHC. Then it figures out externally to the function > that '(Foo Int a) => Int -> a' is actually Int -> Float. > > In the second definition, you're trying to give 'Int -> Float', but > GHC only knows locally that you need to provide 'Int -> a' with a > constraint 'Foo Int a' which it _won't_ use to determine that a ~ > Float. > > This is not inherent to fundeps. One could make a version of fundeps > that has the local constraint rules (easily so by translating to the > new type families stuff). But, the difference is also the reason that > overlapping instances are supported for fundeps and not type families. > But I won't get into that right now. > > -- Dan > > > > ------------------------------ > > Message: 2 > Date: Thu, 13 Sep 2012 11:20:51 -0400 > From: Brandon Allbery <allbery.b(a)gmail.com> > Subject: Re: [Haskell-cafe] [ANN] Cumino 0.2 - Now supports pretty > indentation through stylish-haskell > To: Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> > Cc: Ray <i(a)maskray.tk>, haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: > <CAKFCL4WAtx6R0NVtP7nxnj1ZFp= > KHDx1dGeO-s9EsG4H9nMsvQ(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > On Thu, Sep 13, 2012 at 2:22 AM, Alfredo Di Napoli < > alfredo.dinapoli(a)gmail.com> wrote: > > > urxvtc -e sh -c 'echo a' > >> xterm -e echo a > >> > > > > I would like to, and in fact I've already tried, but urxvt is trickier > > than other shells. Using the command you gave me does not create a new > > window, tested both on XMonad and Gnome. This is > > the error you can see using the option "-hold", this way: > > > > urxvt --hold -e "echo a" > > > > This opens a new window with the error: *urxvt: Unable to exec child. > > * > > > > I think you misunderstood; as I read it (and as I would expect it to work > given the above descrtiption) that would be > > urxvt --hold -e sh -c 'echo a' > > -- > brandon s allbery allbery.b(a)gmail.com > wandering unix systems administrator (available) (412) 475-9364 vm/sms >

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Re: [Haskell-cafe] Either Monad and Laziness
by John Lato 18 Sep '12

18 Sep '12

> Subject: Re: [Haskell-cafe] Either Monad and Laziness > > On 9/14/12 5:16 PM, Eric Velten de Melo wrote: >> But now I'm kinda lost. Is there an easy way to explain the difference between: >> -iteratee >> -conduit >> -enumerator I tend to group them into three families. 'iteratee' and 'enumerator' are fairly directly drawn from Oleg's code, with mostly implementation differences (at least when compared to the other families). They've tended to keep Oleg's original names (iteratee, enumerator, enumeratee). The biggest user-visible difference between iteratee and enumerator is the level of datastream abstraction. iteratee abstracts over the stream, and enumerator abstracts over elements of the stream. The stream is explicitly a list of elements. This exposes some of the details of data chunking to the user, which has both advantages and disadvantages (iteratee exposes this also, but it's not necessary as is the case for enumerator). The second family (chronologically) includes conduit and (maybe) iterIO. I've written a little about this group at http://johnlato.blogspot.sg/2012/06/understandings-of-iteratees.html Although they serve the same goals in spirit, the implementation may or may not necessarily be an iteratee/enumerator arrangement (e.g. conduit). This is a technical observation, not a criticism, depending on exactly what you consider to define the style in the first place. This group has usually renamed functions. I discuss some of the other differences on my blog. The third familiy is all the pipes-* stuff. This group tends towards emphasizing the relationship between iteratee/enumerator pairs and coroutines, and also emphasizing (to use Oleg terminology) composition of enumeratees. I've been meaning to write more about this group, but thus far have been unable to do so. I'd rather not hijack by evangelizing, but obviously I think iteratee provides several important advantages over the other options. John L.

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guards in applicative style
by felipe zapata 17 Sep '12

17 Sep '12

Hi Haskellers, Suppose I have two list and I want to calculate the cartesian product between the two of them, constrained to a predicate. In List comprehension notation is just result = [ (x, y) | x <- list1, y <-list2, somePredicate x y ] or in monadic notation result = do x <- list1 y <- list2 guard (somePredicate x y) return $ (x,y) Then I was wondering if we can do something similar using an applicative style result = (,) <$> list1 <*> list2 (somePredicate ???) The question is then, there is a way for defining a guard in applicative Style? Thanks in advance, Felipe Zapata.

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Transforming a ADT to a GADT
by Florian Lorenzen 17 Sep '12

17 Sep '12

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hello cafe, I have a question wrt. to GADTs and type families in GHC (7.6.1). I'd like to transform a value of an ADT to a GADT. Suppose I have the simple expression language > data Exp = Lit Int > | Succ Exp > | IsZero Exp > | If Exp Exp Exp and the GADT > data Term t where > TLit :: Int -> Term Int > TSucc :: Term Int -> Term Int > TIsZero :: Term Int -> Term Bool > TIf :: Term Bool -> Term ty -> Term ty -> Term ty that encodes the typing rules. Now, I'd like to have a function > typecheck :: Exp -> Maybe (Term t) > typecheck exp = <...> that returns the GADT value corresponding to `exp' if `exp' is type correct. I found a solution at http://okmij.org/ftp/tagless-final/TypecheckedDSLTH.hs but this has as type (slightly adapted) > typecheck :: Exp -> Maybe TypedTerm with > data TypedTerm = forall ty. (Typ ty) (Term ty) > data Typ ty = > TInt Int | TBool Bool That is, the GADT value is hidden inside the existential and cannot be unpacked. Therefore, I cannot write a type preserving transformation function like > transform :: Term t -> Term t that gets the result of the `typecheck' function as input. The solution mentioned above uses Template Haskell to extract the `Term t' type from the existential package and argues that type errors cannot occur during splicing. Is there a possibility to avoid the existential and hence Template Haskell? Of course, the result type of typecheck depends on the type and type correctness of its input. My idea was to express this dependency by parameterizing `Exp' and using a type family `ExpType' like: > typecheck :: Exp e -> Maybe (Term (ExpType e)) > typecheck (ELit > i) = Just (TLit i) > typecheck (ESucc exp1) = case typecheck exp1 of > Nothing -> Nothing > Just t -> Just (TSucc t) > <...> with > data TEXP = L | S TEXP | IZ TEXP | I TEXP TEXP TEXP -- datakind > > > data Exp (e::TEXP) where > ELit :: Int -> Exp L > ESucc :: Exp e1 -> Exp (S e1) > EIsZero :: Exp e1 -> Exp (IZ e1) > EIf :: Exp e1 -> Exp e2 -> Exp e3 -> Exp (I e1 e2 e3) It seems to me that `ExpType' then would be a reification of the type checker for `Exp' at the type level. But I did not know how to define it. Does it make sense at all? Is it possible in GHC? All the examples on the net I looked at either start with the GADT right away or use Template Haskell at some point. So, perhaps this `typecheck' function is not possible to write in GHC Haskell. I very much appreciate any hints and explanations on this. Best regards, Florian - -- Florian Lorenzen Technische Universität Berlin Fakultät IV - Elektrotechnik und Informatik Übersetzerbau und Programmiersprachen Sekr. TEL12-2, Ernst-Reuter-Platz 7, D-10587 Berlin Tel.: +49 (30) 314-24618 E-Mail: florian.lorenzen(a)tu-berlin.de WWW: http://www.user.tu-berlin.de/florenz/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://www.enigmail.net/ iEYEARECAAYFAlBTCr8ACgkQvjzICpVvX7ZfTgCeOPflPtaEW/w1McjYWheaRaqq oUQAnRXSrGP3se+3oiI3nnd+B/rK8yx8 =X1hR -----END PGP SIGNATURE-----

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Re: [Haskell-cafe] Transforming a ADT to a GADT
by oleg＠okmij.org 17 Sep '12

17 Sep '12

Florian Lorenzen wrote: > Now, I'd like to have a function > > typecheck :: Exp -> Maybe (Term t) > > typecheck exp = <...> > that returns the GADT value corresponding to `exp' if `exp' is type > correct. Let us examine that type: typecheck :: forall t. Exp -> Maybe (Term t) Do you really mean that given expression exp, (typecheck exp) should return a (Maybe (Term t)) value for any t whatsoever? In other words, you are interested in a type-*checking* problem: given an expression _and_ given a type, return Just (Term t) if the given expression has the given type. Both expression and the type are the input. Incidentally, this problem is like `read': we give the read function a string and we should tell it to which type to parse. If that is what you mean, then the solution using Typeable will work (although you may prefer avoiding Typeable -- in which case you should build type witnesses on your own). > that returns the GADT value corresponding to `exp' if `exp' is type > correct. Your comment suggests that you mean typecheck exp should return (Just term) just in case `exp' is well-typed, that is, has _some_ type. The English formulation of the problem already points us towards an existential. The typechecking function should return (Just term) and a witness of its type: typecheck :: Exp -> Sigma (t:Type) (Term t) Then my > data TypedTerm = forall ty. (Typ ty) (Term ty) is an approximation of the Sigma type. As Erik Hesselink rightfully pointed out, this type does not preclude type transformation functions. Indeed you have to unpack and then repack. If you want to know the concrete type, you can pattern-match on the type witness (Typ ty), to see if the inferred type is an Int, for example. Chances are that you wanted the following typecheck :: {e:Exp} -> Result e where Result e has the type (Just (Term t)) (with some t dependent on e) if e is typeable, and Nothing otherwise. But this is a dependent function type (Pi-type). No wonder we have to go through contortions like Template Haskell to emulate dependent types in Haskell. Haskell was not designed to be a dependently-typed language.

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[ANNOUNCE] Juicy.Pixels v2.0
by Vincent Berthoux 16 Sep '12

16 Sep '12

Hello, I'm pleased to announce the release of Juicy.Pixels (a pure Haskell library for image loading and writing) in version 2.0. The new features of this version are a Gif file decoder, and a rework of the plane extraction part to be type safe github: https://github.com/Twinside/Juicy.Pixels hackage: http://hackage.haskell.org/package/JuicyPixels Thanks Vincent Berthoux

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Could not find module `Text.Regex'
by zaxis 16 Sep '12

16 Sep '12

import Text.Regex date_by_ntday dateStr ntday = do let [y,m,d] = map (\x -> read x::Int) $ splitRegex (mkRegex "-") dateStr .... %ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 Which package(s) do i need to use Text.Regex ? Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/Could-not-find-module-%60Text.Regex%27-tp27366745p273… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Invitation to connect on LinkedIn
by Sakari Joinen via LinkedIn 16 Sep '12

16 Sep '12

LinkedIn ------------ Sakari Joinen requested to add you as a connection on LinkedIn: ------------------------------------------ Steve, I'd like to add you to my professional network on LinkedIn. - Sakari Accept invitation from Sakari Joinen http://www.linkedin.com/e/uc6lxc-h70llpqy-2x/XvIdBwmueHfd6vFMPXXdLaqreCbl5o… View profile of Sakari Joinen http://www.linkedin.com/e/uc6lxc-h70llpqy-2x/rso/203924390/GIO8/name/259079… ------------------------------------------ You are receiving Invitation emails. This email was intended for Steve Severance. Learn why this is included: http://www.linkedin.com/e/uc6lxc-h70llpqy-2x/plh/http%3A%2F%2Fhelp%2Elinked… (c) 2012, LinkedIn Corporation. 2029 Stierlin Ct, Mountain View, CA 94043, USA.

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ANN: darcsden-1.0
by Simon Michael 15 Sep '12

15 Sep '12

I'm pleased to announce the release of darcsden 1.0! darcsden is a web application for browsing and managing darcs repositories (and issue trackers, and their owners), and an SSH server which lets users push changes to the app without needing a full system login. It can be used for browsing your local repositories, for publishing your repos on the web, or for running a public repo hosting site. It is available under the BSD license. darcsden was created by Alex Suraci. The last release was 0.5, in 2010. I've been tinkering with it and using it to publish repos on my site for a while. In recent months, with help from the Darcs community I made a serious attempt to make it more robust and to run a reliable instance at http://hub.darcs.net , with promising results. Alex and I feel this is a good time to release an official darcsden 1.0, containing hub's reliability/functionality updates (but not the UI style tweaks). Home: http://hackage.haskell.org/package/darcsden Release notes: -------------- Changes in this release fall into several themes: Deployability/Hackability * dependencies have been updated; darcsden now uses the latest GHC, darcs and snap * cabal build flags have been added: - darcs28: build with released darcs 2.8.2+ instead of latest darcs-screened ? default: true - ssh, closing, highlighting: build the ssh server, issue-closing script, and highlighting features ? default: true (disable these for easier building on windows) * settings allowing easier custom deployment are available in src/DarcsDen/Settings.hs: base hostname, path and URL, home link, front page content, root directory, log files, maximum request time, maximum displayable patch/file size, database hosts/ports, etc. * the db setup required for issue trackers has been added * the register form has a security question to reduce junk signups * issue trackers can be enabled/disabled per-repo * a robots.txt file has been added (default: allow all bots) * non-content pages now have NOINDEX meta tag to discourage search indexing * user repos and log files have moved to subdirectories for easier management * a makefile has been added for developer convenience Reliability * error messages are more user-friendly * intermittent browsing errors and eventual stylesheet loss due to non-thread-safe working directory in libdarcs have been fixed * viewing a patch or file with binary data no longer gives an error * patch content larger than 200K is elided, avoiding some timeouts * viewing a patch shows the page right away and fetches the patch data asynchronously, with a more friendly message if it times out * viewing a non-existent patch id shows a 404 instead of an exception * source code highlighting is faster (using highlighting-kate); viewing a complex file (like minified jquery.js) no longer times out * the error-prone repo bootstrap feature has been dropped * patch author names are now replaced with user links reliably (when the email address matches) * repo links now work when the current url ends in / * . (period) is supported in repo names * merge now redirects properly when it completes * the login button in the issue tracker now works * private repos are now listed for their owners * private repos are no longer darcs gettable by non-owners * patch author names now link to local users and hide email addresses reliably * input fields now accept at most 1000 characters * ssh access now supports DSA public keys Usability * the site favicon now works in Chrome * the register form now disables browser autofill and is readable in Chrome * navigation buttons are clearer and more context-sensitive * the front page remains accessible after login * users are listed on the front page * repos are listed alphabetically * page headings are more consistent when browsing files * files view displays directory names in bold * when viewing file content, long lines now use all space to window edge * all markdown files are now rendered as markdown, not just README * the patches view has been renamed branches, and shows all related repos including upstream * merging patches can now also be done by repo members * pagination controls also appear at the top and have a stable layout for easy clicking * patch view lists files alphabetically * patch view now also displays added/removed file content * all issues, closed issues views have been added * code blocks on issue pages no longer overflow their background * list styles have been improved in markdown * button transition, border radius and box shadow styles have been updated for modern browsers Credits: -------- Alex Suraci created darcsden. Simon Michael led this release, with help from Ganesh Sittampalam, Guillaume Hoffmann, Niklas Haas, Daniel Patterson, Owen Stephens and Scott Lawrence. darcsden depends on Darcs, Snap, GHC, and the rest of the Haskell ecosystem. Contributing: ------------- I think darcsden fills a strategic niche in the Darcs ecosystem and deserves wider use and more active development. It has a small, clean codebase and is fun to hack on, not least because (if you contribute to hub's darcsden branch) useful changes will quickly be deployed at hub.darcs.net, providing a tight dogfooding/feedback loop. Here's how to contribute a patch there: - register at hub.darcs.net, add your ssh key in settings - fork http://hub.darcs.net/simon/darcsden - darcs get http://hub.darcs.net/yourname/darcsden - (edit, record) - darcs push yourname@hub.darcs.net:darcsden - your change appears at http://hub.darcs.net/simon/darcsden/patches ; ping me by IRC or email to merge it Best, -Simon, Alex and the Darcs team

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ANNOUNCE: Groundhog 0.1.0.1 - high-level database library
by Boris Lykah 14 Sep '12

14 Sep '12

I am happy to announce a new version of Groundhog, a library for fast high-level database access: http://hackage.haskell.org/package/groundhog http://hackage.haskell.org/package/groundhog-th http://hackage.haskell.org/package/groundhog-postgresql http://hackage.haskell.org/package/groundhog-sqlite Groundhog has been completely overhauled since the last release. Notably, it got support for PostgreSQL and natural foreign keys. I believe that it is a big step forward as this brings more flexibility to the design of the relational schemas while keeping the applications independent of the storage layer. Some of the solutions, particularly schema migration were based on Persistent code. Please see examples at http://github.com/lykahb/groundhog/tree/master/examples. Features: * Support for Sqlite and PostgreSQL. * Natural and composite foreign keys. Earlier it was possible to reference an entity only by the mandatory integer primary key. Now an entity can have several keys including autoincrement primary key (optional) and unique keys which have one or more columns. * Full support of embedded datatypes. You can access a field that contains an embedded datatype as a whole, or access some of the inner subfields individually. This powerful mechanism has allowed implementation of the composite keys, and can be used in future to work with PostgreSQL composite types or MongoDB embedded documents. Instead of serializing value to string, the Groundhog backends flatten tree of embedded datatypes to db columns. * Projections. You can choose what columns to query from a table in a type-safe manner. * Initialization and migration of database schema. * Sum types and polymorphic types. * Expression DSL for use in queries. * Basic list support. * YAML-based settings mechanism. It separates datatype definition and description which facilitates modularity. The settings are inferred from the analysis of the difinition, and overridden with values set by user. The Criterion benchmarks are available at http://lykahb.github.com/groundhog/SqliteBench.html and http://lykahb.github.com/groundhog/PostgreSQLBench.html. Future plans: * Support for joins * Database indexes * Investigate options for implementing MongoDB and MySQL backends Your feedback, suggestions for improvement and criticism are welcome. -- Regards, Boris

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ANN: Cumino 0.3 - Now supports sandboxed environments (e.g, Hsenv ones)
by Alfredo Di Napoli 14 Sep '12

14 Sep '12

Hi Cafè, I'm glat to annouce the third major improvement of my VIm plugin Cumino: https://github.com/adinapoli/cumino This one is a juicy one. If you have a sandboxed environment enabled (e.g. one created with Hsenv) and you start vim from the same shell, Cumino will automatically uses at startup the right ghci instance. This means that if you have installed only some packages in your sandbox env, you will have them available inside ghci :) Feedback are welcome, as always. A.

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What is the good way to work with list comprehension and UTCTime?
by Magicloud Magiclouds 14 Sep '12

14 Sep '12

Hi, Simple usage, I could make an instance of Enum to UTCTime, so [utcTime..] could work. But that is so stiff. How if sometimes I want to step by 1 min, sometimes I want to step by 1 sec? So I think some way like [ t | addUTCTime last 60 ] could be nice. But I cannot figure it out.... Any idea? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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[ANN] Cumino 0.2 - Now supports pretty indentation through stylish-haskell
by Alfredo Di Napoli 14 Sep '12

14 Sep '12

Hi everyone, in case you have missed it, I've released a Vim plugin called Cumino: http://adinapoli.github.com/cumino/ It does one simple thing: It allows communication between Vim and tmux, in particular to a ghci session. With Cumino you can fire-up Vim, load a ghci session and interact with it with only few keystrokes. The plugin also supports visual selection: you can select for example a function (even with all its signature!) and you can send it to ghci. The visual selection supports imports, custom types and typeclasses. It's a simple idea but so damn useful, imho. This release also adds the possibility to prettify the code using the excellent stylish-haskell: select a snippet, simply indent in the usual way ( = ) and voilà, now your code is indented! Feedback are highly appreciated, as well as contributions. There are still some issues with some terminals (for example urxvt does not work right now) but the plugin has been tested against gnome-terminal, xterm and mlterm. I'll post in reddit too for completeness! Bye! Alfredo

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Is Hackage down?
by C K Kashyap 14 Sep '12

14 Sep '12

Is it just me or is Hackage indeed been going down more frequently of late? Regards, Kashyap

3 2

How do I generate random numbers using random-fu, with platform-agnostic code?
by Andrew Pennebaker 14 Sep '12

14 Sep '12

I can't figure out how to use Data.Random.Source.IO to generate random numbers in a multiplatform way. I can generate random numbers in Unix using Data.Random.Source.DevRandom, and there is an example in the GitHub documentation for Windows using Data.Random.Source.MWC, but there is no example code for using Data.Random.Source.IO. Cheers, Andrew Pennebaker www.yellosoft.us

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Fwd: How Type inference work in presence of Functional Dependencies
by Dan Doel 13 Sep '12

13 Sep '12

Copying the mailing list, because I forgot. On Thu, Sep 13, 2012 at 5:18 AM, satvik chauhan <mystic.satvik(a)gmail.com> wrote: > Consider the code below : > > {-# LANGUAGE > MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts > #-} > class Foo a c | a -> c > instance Foo Int Float > f :: (Foo Int a) => Int -> a > f = undefined > > Now when I see the inferred type of f in ghci > >> :t f > >> f :: Int -> Float > > Now If I add the following code > > g :: Int -> Float > g = undefined > > h :: (Foo Int a) => Int -> a > h = g > > > I get the error > > Could not deduce (a ~ Float) > > > I am not able to understand what has happened here ? The restriction "Foo > Int a" should have restricted the type of h to "Int -> Float" as shown in > the inferred type of f. The answer, I believe, is that the difference between the fundep implementation and type families is local constraint information. Fundeps do no local propagation. So in your first definition, you've locally provided 'Int -> a', which is acceptable to GHC. Then it figures out externally to the function that '(Foo Int a) => Int -> a' is actually Int -> Float. In the second definition, you're trying to give 'Int -> Float', but GHC only knows locally that you need to provide 'Int -> a' with a constraint 'Foo Int a' which it _won't_ use to determine that a ~ Float. This is not inherent to fundeps. One could make a version of fundeps that has the local constraint rules (easily so by translating to the new type families stuff). But, the difference is also the reason that overlapping instances are supported for fundeps and not type families. But I won't get into that right now. -- Dan

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How Type inference work in presence of Functional Dependencies
by satvik chauhan 13 Sep '12

13 Sep '12

Consider the code below : {-# LANGUAGE MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts #-} class Foo a c | a -> c instance Foo Int Float f :: (Foo Int a) => Int -> a f = undefined Now when I see the inferred type of f in ghci > :t f > f :: Int -> Float Now If I add the following code g :: Int -> Float g = undefined h :: (Foo Int a) => Int -> a h = g I get the error Could not deduce (a ~ Float) I am not able to understand what has happened here ? The restriction "Foo Int a" should have restricted the type of h to "Int -> Float" as shown in the inferred type of f. - Satvik

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Re: [Haskell-cafe] [ANN] Cumino 0.2 - Now supports pretty indentation through stylish-haskell
by Roman Cheplyaka 13 Sep '12

13 Sep '12

Ah, okay. I was just confused by the fact that it uses tmux, and thought that I was misusing it. Yes, I also usually keep ghci in a separate window (and I am an xmonad user, too). I just thought that this offers a different experience and wanted to try it out. Anyway, thanks for clarifying. On Wed, Sep 12, 2012 at 5:24 PM, Alfredo Di Napoli < alfredo.dinapoli(a)gmail.com> wrote: > Hi Roman, > > Cumino was thought to operate in another terminal. Personally, but this is > only a personal taste, having to switch between terminal tabs or tmux panes > is not the fastest workflow. Being an Xmonad user, i can easily swap > between terminals (read ghci and vim) simply with mod + j or mod + k. > So the answer to your answer is: You can't, Cumino will always start in > another terminal window. Is the same behaviour of Slime, though. > > Sent from my iPad > > On 12/set/2012, at 17:06, Roman Cheplyaka <roma(a)ro-che.info> wrote: > > So, suppose that I'm in a terminal vim session, and I want to start ghci > (in the current terminal). What do I do? > > <localleader>cc starts a new terminal, which is not what I want. > > On Wed, Sep 12, 2012 at 3:03 PM, Alfredo Di Napoli < > alfredo.dinapoli(a)gmail.com> wrote: > >> Hi everyone, >> >> in case you have missed it, I've released a Vim plugin called Cumino: >> >> http://adinapoli.github.com/cumino/ >> >> It does one simple thing: It allows communication between Vim and tmux, >> in particular to a ghci session. With Cumino you can fire-up Vim, >> load a ghci session and interact with it with only few keystrokes. The >> plugin also supports visual selection: you can select for example a >> function (even with all its signature!) >> and you can send it to ghci. The visual selection supports imports, >> custom types and typeclasses. >> >> It's a simple idea but so damn useful, imho. >> >> This release also adds the possibility to prettify the code using the >> excellent stylish-haskell: select a snippet, simply indent in the usual way >> ( = ) and voilà, now >> your code is indented! >> >> Feedback are highly appreciated, as well as contributions. >> There are still some issues with some terminals (for example urxvt does >> not work right now) but the plugin has been tested against gnome-terminal, >> xterm and mlterm. >> >> I'll post in reddit too for completeness! >> >> Bye! >> Alfredo >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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[Hackage] Bug report and proposal
by Stayvoid 13 Sep '12

13 Sep '12

Hello, I found a bug: "source" points to the wrong page. [1] I know that bugs should be reported via Github. But I don't have an account and don't want to create one. There might be others who don't want to use Github. Why rely on external bug tracker? I assume that it has been chosen because of its features, but it doesn't have the most important one (i.e. anonymous posting). What's better: to have more features or to have less bugs? Is it possible to move away from Github? What about a separate mailing list for bugs? I'm sorry if this sounds to harsh; I'm not good at writing. I don't want to attack or blame anyone. I'm also sorry if this message is not appropriate, but I haven't found a better place. [1] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… Cheers

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Re: [Haskell-cafe] [ANN] Cumino 0.2 - Now supports pretty indentation through stylish-haskell
by Alfredo Di Napoli 12 Sep '12

12 Sep '12

Could you please tell me what your desired behaviour would be? In praticular, do you want a ghci session in another tab or in a tmux pane perhaps? Otherwise I can't see any viable way to let vim and ghci cooperate inside the SAME window. Bye, Alfredo Sent from my iPad On 12/set/2012, at 17:05, Roman Cheplyaka <roma(a)ro-che.info> wrote: > So, suppose that I'm in a terminal vim session, and I want to start ghci (in the current terminal). What do I do? > > <localleader>cc starts a new terminal, which is not what I want. > > On Wed, Sep 12, 2012 at 3:03 PM, Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> wrote: > Hi everyone, > > in case you have missed it, I've released a Vim plugin called Cumino: > > http://adinapoli.github.com/cumino/ > > It does one simple thing: It allows communication between Vim and tmux, in particular to a ghci session. With Cumino you can fire-up Vim, > load a ghci session and interact with it with only few keystrokes. The plugin also supports visual selection: you can select for example a function (even with all its signature!) > and you can send it to ghci. The visual selection supports imports, custom types and typeclasses. > > It's a simple idea but so damn useful, imho. > > This release also adds the possibility to prettify the code using the excellent stylish-haskell: select a snippet, simply indent in the usual way ( = ) and voilà, now > your code is indented! > > Feedback are highly appreciated, as well as contributions. > There are still some issues with some terminals (for example urxvt does not work right now) but the plugin has been tested against gnome-terminal, xterm and mlterm. > > I'll post in reddit too for completeness! > > Bye! > Alfredo > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] type variable in class instance
by oleg＠okmij.org 12 Sep '12

12 Sep '12

Corentin Dupon wrote about essentially the read-show problem: > class (Typeable e) => Event e > > data Player = Player Int deriving (Typeable) > data Message m = Message String deriving (Typeable) > > instance Event Player > > instance (Typeable m) => Event (Message m) > > viewEvent :: (Event e) => e -> IO () > viewEvent event = do > case cast event of > Just (Player a) -> putStrLn $ show a > Nothing -> return () > case cast event of > Just (Message s) -> putStrLn $ show s > Nothing -> return () Indeed the overloaded function cast needs to know the target type -- the type to cast to. In case of Player, the pattern (Player a) uniquely determines the type of the desired value: Player. This is not so for Message: the pattern (Message s) may correspond to the type Message (), Message Int, etc. To avoid the problem, just specify the desired type explicitly > case cast event of > Just (Message s::Message ()) -> putStrLn $ show s > Nothing -> return () (ScopedTypeVariables extension is needed). The exact type of the message doesn't matter, so I chose Message (). BTW, if the set of events is closed, GADTs seem a better fit > data Player > data Message s > > data Event e where > Player :: Int -> Event Player > Message :: String -> Event (Message s) > > viewEvent :: Event e -> IO () > viewEvent (Player a) = putStrLn $ show a > viewEvent (Message s) = putStrLn $ show s

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IO vs MonadIO
by Sergey Mironov 12 Sep '12

12 Sep '12

Hi. Just a brief question. System.IO functions are defined in IO monad and have signatures like Foo -> IO Bar. Would it be better to have all of them defined as (MonadIO m) => Foo -> m Bar? What are the problems that would arise? Sergey

2 2

Pattern matching: multi constructors to same branch
by Thiago Negri 12 Sep '12

12 Sep '12

Is it possible to capture more than one constructor in a single pattern matching? I mean, is it possible to generalize the following pattern matching of A and B to a single branch? g f C = [f C] g f v@(A _ n) = f v : g n g f v@(B _ n) = f v : g n For example: g f C = [f C] g f v@(A|B _ n) = f v : g n Or: g f v = case v of C -> [f C] A|B -> f v : g (n v) Thanks.

2 1

Either Monad and Laziness
by Eric Velten de Melo 11 Sep '12

11 Sep '12

Hello, I am currently trying to rewrite the Graphics.Pgm library from hackage to parse the PGM to a lazy array. The current implementation parses it straight to UArray, which is strict. The behaviour I want to achieve is like this: I want the program when compiled to read from a file, parsing the PGM and at the same time apply transformations to the entries as they are read and write them back to another PGM file. The problem is that even using a lazy array structure, because the parser returns an Either structure it is only possible to know if the parser was successful or not after the whole file is read, therefore requiring it to read the entire file before applying the transformations, ruining the property of laziness. Is there some way to keep this from happening? Should I even want to make it like this? Not really a real life situation, but imagine I want to read a reaaaaally large PGM file which does not fit into RAM memory and I don't want to be forced to have the whole array in the memory at once. One alternative I thought was parsing only the PGM header and then read the rest of the input without using Parsec and the Either Monad. In the event the data is corrupted, though, I would not know how to recover from it. Any thoughts? Hopefully I'm not saying anything really stupid. Eric

3 2

Destructive updates to plain ADTs
by Milan Straka 11 Sep '12

11 Sep '12

Hi all, is there any way to perform a destructive update on a plain ADT? Imagine I have a simple data Tree a = Nil | Node a (Tree a) (Tree a) I would like to be able to modify right subtree of an existing tree. I can do that for example when using IORefs by changing the datatype to data Tree a = Nil | Node a (IORef (Tree a)) (IORef (Tree a)) and use unsafePerformIO + writeIORef. But the IORefs cause additional complexity when working with the data type. At the moment I am interested in any GHC solution, be it non-portable or version specific. I would like just to run some benchmarks and see the results. Cheers, Milan

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ANNOUNCE: hsqml-0.1.1 (and hsqml-morris-0.1.0)
by Robin KAY 11 Sep '12

11 Sep '12

Dear All, I would like to announce a new package called 'hsqml' [1]. HsQML provides a Haskell binding to the Qt Quick framework. It allows you to write graphical applications where the front-end is written in Qt Quick's QML language (incorporating JavaScript) and the back-end is written in Haskell. The two layers are coupled together via a facility to define custom JavaScript objects through which QML code can call into Haskell. HsQML requires an installation of Qt 4.7 (including QtDeclarative) or later on your path. It has been tested with GHC 7.4 on both Linux and Windows. There is also a simple example program available using HsQML on Hackage called 'hsqml-morris'. It implements a human v computer game of Nine Men's Morris with game logic and AI written in Haskell and the presentation layer in QML. The library is currently limited by a lack of support for Qt signals, so calls can only be made from JavaScript to Haskell and not the other way. I plan to add support for these in a future release. Migration over to Qt 5.x is also likely once it's released. [1] http://hackage.haskell.org/package/hsqml-0.1.1 [2] http://hackage.haskell.org/package/hsqml-morris-0.1.0 Regards, -- Robin KAY

1 0

The Architecture of the Mighttpd High-Speed Web Server
by Kazu Yamamoto 11 Sep '12

11 Sep '12

Hello from Copenhagen, In last may, I wrote an article about Mighttpd in Japanese because a PR staff of IIJ asked me to do so. Since this article got popular in Japan, the PR staff decided to translate it into English. The English version is now open to the public: http://www.iij.ad.jp/en/company/development/tech/mighttpd/ # Mighttpd is based on Yesod's Warp but I did touch it to make the # article simple. I guess that we can use this article to explain why Haskell is suitable for network programming to other programming language communities. FYI, the original Japanese article can be found: http://www.iij.ad.jp/company/development/tech/activities/mighttpd/ Regards, --Kazu

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type variable in class instance
by Corentin Dupont 11 Sep '12

11 Sep '12

Hello everybody! I'm soliciting once again your help! It's been several days I'm blocked by this problem: *{-# LANGUAGE DeriveDataTypeable #-} import Data.Typeable class (Typeable e) => Event e data Player = Player Int deriving (Typeable) data Message m = Message String deriving (Typeable) instance Event Player instance (Typeable m) => Event (Message m) viewEvent :: (Event e) => e -> IO () viewEvent event = do case cast event of Just (Player a) -> putStrLn $ show a Nothing -> return () case cast event of Just (Message s) -> putStrLn $ show s Nothing -> return () * gives me a: * Ambiguous type variable `t0' in the constraint: (Typeable t0) arising from a use of `cast' Probable fix: add a type signature that fixes these type variable(s) In the expression: cast event In the expression: case cast event of { Just (Message s) -> putStrLn $ show s Nothing -> return () }* This is because *Message* has a type variable, while *Player* has not... How to get this to work? I tried everything, existential types, scoped type variables etc. without success... Thanks!! Corentin

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extensible-exceptions no longer a part of GHC 7.6.1?
by Peter Simons 10 Sep '12

10 Sep '12

Hi, 'extensible-exceptions' used to be a part of GHC, but it appears that the package has been dropped from 7.6.1. Yet, the release notes on haskell.org don't say anything about this subject (other than TODO). Was that change intentional? Take care, Peter

2 1

ANNOUNCE: grid-1.1
by Amy de Buitléir 10 Sep '12

10 Sep '12

I'm happy to announce a new package called grid: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, and hexagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome.

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ANNOUCE: Haddock 2.10.0 and 2.11.0
by David Waern 09 Sep '12

09 Sep '12

-------------------------------------------- -- Haddock 2.10.0 and 2.11.0 -------------------------------------------- Two new versions of Haddock have been uploaded to Hackage: version 2.10.0 which comes with GHC 7.4.2 and 2.11.0 which comes with the new GHC 7.6.1! -------------------------------------------- -- Changes in version 2.12.0 -------------------------------------------- * Labeled URLs (e.g <http://example.net/ some label>) * Improved memory usage (new dependency: deepseq) -------------------------------------------- -- Changes in version 2.11.0 -------------------------------------------- * Show deprecation messages for identifiers * List identifiers declared on the same line (with a common type) separately * Don't crash on unicode strings in doc comments * Fix reporting of modules safe haskell mode * Fix a case where we were generating invalid xhtml * Improved --qual option (no crashes, proper error messages) * A new --qual option "aliased" which qualifies identifers by the module alias used in the source code * The Haddock API restores GHC's static flags after invocation * Access to unexported identifiers through the Haddock API again -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/package/haddock-2.12.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://darcs.haskell.org/haddock.git -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Paolo Capriotti Simon Hengel Ian Lynagh Simon Peyton Jones Iavor S. Diatchki David Terei Henning Thielemann David Waern -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://darcs.haskell.org/haddock.git Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

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package categories
by Sergey Mironov 09 Sep '12

09 Sep '12

Hello list. Do we have a guideline for selecting correct category for a package being developed? I'd like to choose between Web.MyLib and Network.MyLib. Or probably just MyLib if I feel ambitious? Sergey.

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ANNOUNCE vkhs-0.0.3
by Sergey Mironov 08 Sep '12

08 Sep '12

I'm happy to announce a new package called VKHS: http://hackage.haskell.org/package/VKHS https://github.com/ierton/vkhs VKHS provides access to Vkontakte [1] (popular Russian social network) API methods. Library can be used to login into the network as a standalone application (OAuth implicit flow as they call it). Interaction with user is not required. For now, vkhs offers limited error detection and no captcha support. Here is an example code: import Web.VKHS.Login import Web.VKHS.Api main = do let client_id = "111111" -- application id, register first let user_of_interest = "222222" let e = env client_id "user(a)example.com" "password" [Photos,Audio,Groups] (Right at) <- login e (Right ans) <- api e at "users.get" [ ("uids",user_of_interest) , ("fields","first_name,last_name,nickname,screen_name") , ("name_case","nom") ] putStrLn ans Internally, library uses small curl-based HTTP automata and tagsoup for jumping over relocations and submitting various 'Yes I agree' forms. Sergey. [1] - http://vk.com

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Haskell with all the safeties off
by David Feuer 08 Sep '12

08 Sep '12

I have no plans to do such a thing anytime soon, but is there a way to tell GHC to allow nasal demons to fly if the program forces bottom? This mode of operation would seem to be a useful optimization when compiling a program produced by Coq or similar, enabling various transformations that can turn bottom into non-bottom, eliminating runtime checks in incomplete patterns, etc.

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