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more thoughts on "Finally tagless"
by Günther Schmidt 10 Mar '10

10 Mar '10

Hi all, and Oleg et al in particular. Yeah, subject "Finally Tagless" again, sorry, I'm just not done with it yet. In Olegs haskell implementation he is using classes mainly to model the syntax and instances to use for evaluators / compilers to allow multiple interpretations. I wonder if it'd be possible to do the same without classes using data types instead? Something similar to what Luke Palmer has done here: http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-… Günther

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PseudoTerminals and Handles (again)
by Mathijs Kwik 10 Mar '10

10 Mar '10

Hi All, A few days ago, I got started with the code found on this blogpost: http://www.serpentine.com/blog/2008/09/30/unix-hacking-in-haskell-better-ps… On my system, I found that using pseudoterminals as handles did not work. Reading output and writing input works fine, it's just that finding "the end of the stream" seems to break. This means that I can read all output up to the last character outputted by the other process, but the string doesn't end there. Instead it's throwing: fdReadBuf: hardware fault (Input/output error) Using normal pipes (no pty), this does work as expected. My guess is that I should somehow setup the fd or the pty a little more before starting the child process. Set them to nonblock/block, read/write mode, setTerminalAttribute, stuff like that. And maybe it matters when to change settings (before forking / after forking / in the child), or to change the master or the child... I don't know very much about this stuff but since the rest seems to work fine it just seems to be a problem with how the child announces it's finished writing to the fd. Is there anyone who got this to work somehow? Or is there a way to somehow trick hGetContents and friends to just treat the "hardware fault" as EOF? Thanks Mathijs PS: I'm using ghc 6.12.1 on x86_64 (ubuntu) by the way

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Re: [Haskell-cafe] Re: more thoughts on "Finally tagless"
by Bruno Oliveira 10 Mar '10

10 Mar '10

Hi Oleg, (cc'ed the haskell-cafe as it may be of interest to other readers) On Tue, 9 Mar 2010 oleg(a)okmij.org wrote: > > Hi, Bruno! > > Of course I know the EMGM paper and approach -- we discuss it > at great length in the paper we are both editing, don't we? What I had > in mind about tagless final is different from EMGM, without the mediation > by an isomorphism. The isomorphism has no role in extensibility there: it is completely orthogonal to this issue. The isomorphism is used for the generic programming part. If we apply the tecnique directly to the expression problem this is what we get: http://www.haskell.org/pipermail/haskell-cafe/2008-July/045028.html > Whether it is more direct or not is in the eye of > the beholder. > > I guess you probably won't be at the Spring School. No I am not going to be there, unfortunatelly :(. > So, I'm > sending you the code that might hopefully clarify what I meant. The > original code was written in Haskell; I am sending you the OCaml > translation. Somehow I thought you might like it better (and also tell > if it translatable to Scala). I must say that my OCaml is not as good as my Haskell or Scala :). However, if I understand the code correctly I don't see where the difference to the code in the link above is. Regarding whether it is translatable to Scala. The answer for your question is answered here: Modular Visitor Components: A Practical Solution to the Expression Families Problem Bruno C. d. S. Oliveira In Sophia Drossopoulou, editor, LNCS 5653, Proceedings of the 23rd European Conference on Object Oriented Programming (ECOOP). June 2009. Link: http://ropas.snu.ac.kr/%7Ebruno/papers/ModularVisitor.pdf > > I was a little bit surprised that you ended up in Korea. That > was quite a big jump and quite a lot of hassle moving, I imagine. On > the other hand, it makes a lot of sense: governments on that side of > the world seem to have real money, and they are intent in investing > them in science, including basic science. How is your Korean? It is worse than my OCaml :). Bruno > Cheers, > Oleg > > (* Tagless Final using dictionary passing *) > > (* Compare with Haskell's ExpSYM *) > class type ['repr] expSYM = object > method lit : int -> 'repr > method neg : 'repr -> 'repr > method add : 'repr -> 'repr -> 'repr > end;; > > (* Constructor functions *) > let lit n = fun ro -> ro#lit n;; > let neg e = fun ro -> ro#neg (e ro);; > let add e1 e2 = fun ro -> ro#add (e1 ro) (e2 ro);; > > (* Unit is for the sake of value restriction *) > (* The term is exactly the same as that in Intro2.hs *) > let tf1 () = add (lit 8) (neg (add (lit 1) (lit 2)));; > > (* We can write interepreters of expSYM *) > (* and evaluate exp in several ways. The code for the interpreters > is quite like the one we have seen already > *) > class eval = object > method lit n = (n:int) > method neg e = - e > method add e1 e2 = e1 + e2 > end;; > > let eval = new eval;; > > let 5 = tf1 () eval;; > > class view = object > method lit n = string_of_int n > method neg e = "(-" ^ e ^ ")" > method add e1 e2 = "(" ^ e1 ^ " + " ^ e2 ^ ")" > end;; > > let view = new view;; > > let "(8 + (-(1 + 2)))" = tf1 () view;; > > (* We can extend our expression adding a new expression form *) > class type ['repr] mulSYM = object > method mul : 'repr -> 'repr -> 'repr > end;; > > let mul e1 e2 = fun ro -> ro#mul (e1 ro) (e2 ro);; > > > (* Extended sample expressions *) > (* Again, the code is the same as before, modulo the occasional () *) > (* Value restriction is indeed annoying ... *) > let tfm1 () = add (lit 7) (neg (mul (lit 1) (lit 2)));; > > let tfm2 () = mul (lit 7) (tf1 ());; > > class evalM = object > inherit eval > method mul e1 e2 = e1 * e2 > end;; > > let evalM = new evalM;; > > class viewM = object > inherit view > method mul e1 e2 = "(" ^ e1 ^ " * " ^ e2 ^ ")" > end;; > > let viewM = new viewM;; > > (* can use the extended evaluator to evaluate old expressions *) > let 5 = tf1 () evalM;; > > (* Of course we can't use the old evaluator to evaluate extended > expressions > let 5 = tfm1 () eval;; > Error: This expression has type eval but an expression was expected of type > < add : 'a -> 'b -> 'c; lit : int -> 'a; mul : 'a -> 'a -> 'd; > neg : 'd -> 'b; .. > > The first object type has no method mul > *) > > let 5 = tfm1 () evalM;; > > let 35 = tfm2 () evalM;; > > let "(7 + (-(1 * 2)))" = tfm1 () viewM;; > > let "(7 * (8 + (-(1 + 2))))" = tfm2 () viewM;; > > (* The expressions are first-class: we can put them into the same list *) > > let tl1 () = [lit 1; tf1 ()];; > > (* and add the extended objects afterwards *) > > let tl2 () = tfm1 () :: tfm2 () :: tl1 ();; > > let [5; 35; 1; 5] = List.map (fun x -> x evalM) (tl2 ());; > > let ["(7 + (-(1 * 2)))"; "(7 * (8 + (-(1 + 2))))"; "1"; "(8 + (-(1 + 2)))"] > = List.map (fun x -> x viewM) (tl2 ());; > > Printf.printf "\nAll done\n";; >

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search Data.Tree.Zipper
by Jian Fan 09 Mar '10

09 Mar '10

Hi, There doesn't seem to be a function to search the tree so I come up with following function: searchTree :: (a -> Bool) -> TreeLoc a -> Maybe (TreeLoc a) searchTree pred rootLoc = if pred (getLabel rootLoc) then Just rootLoc else case firstChild rootLoc of Just loc -> case searchTree pred loc of Just loc -> Just loc Nothing -> case right loc of Just rLoc -> searchTree pred rLoc Nothing -> Nothing Nothing -> Nothing Which feels quite ugly. Any suggestions? Thanks. Jian

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Re: [Haskell-cafe] A few ideas about FRP and arbitrary access in time
by Patai Gergely 09 Mar '10

09 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) Well, my impression is that trying to unite events and behaviours in this manner is not the solution we're looking for. If you follow the WWRD principle, you have to forget about events as fundamental building blocks, because the concept of event is an abstraction over continuous phenomena. Reality has a trivial solution: throw extreme parallelism at the problem, set an extremely high sampling rate and shamelessly 'recalculate' unchanging parts of the system all the time (which is exactly why you don't want to use Yampa or Elerea for GUI programming: they are pull-based, so they'll keep the CPU churning even in the absence of events; note also that both solve the problem of unlimited access to the past). In this framework, events can only be introduced as high-level abstractions that require much more computational power than what would seem necessary. It is a bit like using a large and complex neural network to perform basic arithmetic. Also, if you think about the applicative, monad etc. type class morphisms, you'll see that what makes sense for behaviours is completely useless for events. For instance, the applicative instance for behaviours (time functions) gives us a neat point-wise function application. However, if you define events as functions that are undefined almost everywhere, point-wise application of two events is only defined at the times where both events yield a value simultaneously. Same for monads: they capture the act of sampling for behaviours, but they can't do much for temporal values that are mostly undefined if interpreted as the reader monad. This suggests that events and behaviours have a different nature. Either only behaviours have a meaningful monad instance, or their meaningful monad instances are different. The unified system you're describing very much reminds me of Lucid Synchrone, if we think of behaviours as signals synchronised to the base clock, while events as signals with a slower clock. It might be interesting to think about how clock calculus interacts with the type class instances. However, that still doesn't say anything about the fact that the set of operations meaningful for behaviours and events are different, with only some overlap. Gergely -- http://www.fastmail.fm - Does exactly what it says on the tin

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Type variables
by Giuseppe Maggiore 09 Mar '10

09 Mar '10

Hi everyone, this is my first post: I am a ph.d. student from Italy who is learning the wonderful world of Haskell :) I have encountered a problem and I cannot find a way to get past it (or even to begin to understand it). I wish to define a simple type class that defines how to convert a type function into its argument (like going from a dumb constructor like data T a = T a into a itself): class Convert rec where convert :: rec a -> a now when I try to use the conversion operator class (CNum n, HasField n (a -> (b,rec a)) l, Convert rec) => HasMethod n l a b rec where (..!) :: l -> n -> (a -> (b,a)) instance (CNum n, HasField n (a -> (b,rec a)) l, Convert rec) => HasMethod n l a b rec where l ..! n = let m = l .! n in (\x -> let (y,v) = m x in (y,convert v)) in what looks to me as a straightforward use, the compiler complains that : *References> :load Main.hs [1 of 5] Compiling Records ( Records.hs, interpreted ) [2 of 5] Compiling References ( References.hs, interpreted ) [3 of 5] Compiling Methods ( Methods.hs, interpreted ) Methods.hs:25:14: Could not deduce (HasField n (a -> (b, rec a)) l) from the context (HasMethod n l a b rec1, CNum n, HasField n (a -> (b, rec1 a)) l, Convert rec1) arising from a use of `.!' at Methods.hs:25:14-19 Possible fix: add (HasField n (a -> (b, rec a)) l) to the context of the instance declaration or add an instance declaration for (HasField n (a -> (b, rec a)) l) In the expression: l .! n In the definition of `m': m = l .! n In the expression: let m = l .! n in (\ x -> let (y, v) = ... in (y, convert v)) Failed, modules loaded: References, Records. *References> I apologize for what might look like a huge dumping of code, but I have no idea what might have caused this to further refine the code or to write a more focused example...

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Re: [Haskell-cafe] Re: more thoughts on "Finally tagless"
by Bruno Oliveira 09 Mar '10

09 Mar '10

Hi Oleg, On 9 Mar 2010, at 17:52, oleg(a)okmij.org wrote: > > Stephen Tetley wrote: >> The finally tagless style is an implementation of the TypeCase >> pattern >> (Bruno C. d. S. Oliveira and Jeremy Gibbons): >> >> http://www.comlab.ox.ac.uk/jeremy.gibbons/publications/typecase.pdf > > The finally tagless style is slightly more general. > > The TypeCase paper emphasizes that TypeCase is a pattern to define a > _closed_ type-indexed function -- the indexed family is fixed but the > collection of functions is extensible. This is in contrast to type > classes, where the collection of functions is fixed (by the class > declaration) but the indexed family is extensible (more type class > instances can be defined at any time). > > The tagless final approach permits the definition of an extensible > family of open type-indexed functions. For example, we can define a > `data type' of expressions and extend it at any time with more > expression forms *and* more processing functions (e.g., a new way to > print or compile an expression). With the tagless final approach, we > have the extensibility in both dimensions. > It is true that "Typecase" is aimed at closed type-indexed functions, although in Section 4.2 I note, in passing, that you can achieve extensibility too for the *explicit* representations variation of the typecase pattern (which is the same that is used by tagless final): "The smart datatype solution in this section is fully closed to extension. That is, in order to add another case in the formatting list, such as a constructor Chr that handles characters, we would need to modify the GADT itself. On the other hand, the solution in the previous section using the explicit version of the design pattern allows some form of extensibility. Adding a new case for printf that handles characters corresponds to adding a new function, which could even be in a different module." Fortunately, there is a whole paper devoted to the issue of extensibility for an application of the typecase pattern using *explict* representations: Extensible and Modular Generics for the Masses Bruno C. d. S. Oliveira, Ralf Hinze and Andres Loeh In Henrik Nilsson, editor, Trends in Functional Programming (TFP). Link: http://ropas.snu.ac.kr/%7Ebruno/papers/ExtensibleGM.pdf And the relation to the expression problem is pointed out there. Perhaps you'd like to have a look at the paper if you haven't seen it already. Bruno

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Re: more thoughts on "Finally tagless"
by oleg＠okmij.org 09 Mar '10

09 Mar '10

Stephen Tetley wrote: > The finally tagless style is an implementation of the TypeCase pattern > (Bruno C. d. S. Oliveira and Jeremy Gibbons): > > http://www.comlab.ox.ac.uk/jeremy.gibbons/publications/typecase.pdf The finally tagless style is slightly more general. The TypeCase paper emphasizes that TypeCase is a pattern to define a _closed_ type-indexed function -- the indexed family is fixed but the collection of functions is extensible. This is in contrast to type classes, where the collection of functions is fixed (by the class declaration) but the indexed family is extensible (more type class instances can be defined at any time). The tagless final approach permits the definition of an extensible family of open type-indexed functions. For example, we can define a `data type' of expressions and extend it at any time with more expression forms *and* more processing functions (e.g., a new way to print or compile an expression). With the tagless final approach, we have the extensibility in both dimensions. I'll be talking about this extensibility at some length in two weeks, at the Spring School on Generic and Indexed Programming.

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Real-time garbage collection for Haskell
by Luke Palmer 09 Mar '10

09 Mar '10

I have seen some proposals around here for SoC projects and other things to try to improve the latency of GHC's garbage collector. I'm currently developing a game in Haskell, and even 100ms pauses are unacceptable for a real-time game. I'm calling out to people who have seen or made such proposals, because I would be willing to contribute funding and/or mentor a project that would contribute to this goal. Also any ideas for reducing this latency in other ways would be very appreciated. Thanks, Luke

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file descriptors and handles
by Mathijs Kwik 09 Mar '10

09 Mar '10

Hi all, Today I had a look at chapter 20 of RWH. The extended example (stripped down HSH) in the end is great. I think I understand it, but I have some questions left: the master process closes the client-sided FD's. it uses fdToHandle for the other sides of the pipe to get a handle to stdIn and stdOut on the client process. it writes to the stdIn handle and closes it afterwards. - doesn't it need to close the fd as well? the stdOut handle gets used when reading the command output (using hGetContents) - doesn't the handle need to get closed? or will this happen automatically if hGetContents finds EOF ? - and also here, shouldn't the fd get closed? More general: what happens when closing a handle when there's still output left? Will reading further output throw an exception or just think it's EOF? And what happens when closing the underlying FD? will the handle "break"? or will it close automatically? Thanks for any help Mathijs

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ptys and hGetContent problem
by Mathijs Kwik 08 Mar '10

08 Mar '10

Hi all, I found this blogpost from Bryan O'Sullivan http://www.serpentine.com/blog/2008/09/30/unix-hacking-in-haskell-better-ps… and I wanted to try it out. Before moving to an interactive command (which needs pty), I just did a small test for "ls -l /" to see if it worked. I got it to compile, but when running, it throws an exception when reaching the end of the output (in this case because I evaluate the length to force reading all). Main: /dev/ptmx: hGetContents: hardware fault (Input/output error) Please have a look at the hpaste to see what I did: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=23343 What's wrong? :) My guess is that hGetContents doesn't receive a nice EOF when the process exits (ls doesn't stay around waiting for input), but I have no idea on how to fix this. And further... If I do want to use an interactive program which needs input, how do I send ctrl-d or ctrl-c? tail -f needs ctrl-c (or I need to kill the process) Thanks for any help Mathijs PS: I know about libexpect, it's not useful for what I wanna do (multiple processes, having 'answers' from 1 be used to decide what to input the other, without ending the 'answer' process)

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Books for "advanced" Haskell
by Günther Schmidt 08 Mar '10

08 Mar '10

Hi all, there seems to be a huge number of things that monads can be used for. And there are lots of papers, blog posts, etc. describing that, some more or less accessible. Apart from monads there are of course also Applicative Functors, Monoids, Arrows and what have you. But in short the Monad thingy seems to be the most "powerful" one of them all. Is there a book that specializes on Monads? A Haskell-Monad book? Günther

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Haskell Weekly News: Issue 152 - March 08, 2010
by jfredett＠gmail.com 08 Mar '10

08 Mar '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100308 Issue 152 - March 08, 2010 --------------------------------------------------------------------------- Welcome to issue 152 of HWN, a newsletter covering developments in the [1]Haskell community. Hello Haskellers, this week brings updates to some standards (like Parsec), as well as some new tunes to the Haskell songbook. WAI and Yesod were released, giving some new options for Haskell-based web applications, Don Stewart made some improvements to his Genetic Optimizer system to include options from GHC's new LLVM backend, and I made some serious progress on the new HWN software. Speaking of the new software, there have been numerous reports of warts and other unfortunate things with the current editions of the HWN. Fear not, these warts are well known and being worked on intently. Unfortunately, it's somewhat difficult to fix them given the current publication software, however, in the new-and-improved version that (hopefully) should be entering use soon, these warts will be removed. So, without further interuption about the stylistic content, Haskellers, your Haskell Weekly News! Announcements Bluetile 0.3.1. Jan Vornberger [2]announced Bluetile, an adaptation of xmonad for the masses. Bluetile aims to make tiling window managers accessible by drawing on standard window manager conventions and idioms. ED: This is not technically an announcement, but it's very cool and everyone should check it out! yesod 0.0.0 (web framework). Michael Snoyman [3]announced the release of yesod, a web framework based on WAI. wai-0.0.0 (Web Application Interface). Michael Snoyman [4]announced the first release of wai, a library implementing the WAI protocol. iteratee-parsec 0.0.2. Maciej Piechotka [5]announced iteratee-parsec, a library which allows for parsec parsers in the IterateeG monad. Parsec 3.1.0. Derek Elkins [6]announced a new release of Parsec, containing many improvments due (primarily) to Antoine Latter. Progression-0.3 (supporting benchmarking in Haskell). Neil Brown [7]announced a new release of Progression. Progression is a utility built on top of Criterion for recording benchmarks for several version of your code. Ghent Functional Programming Group: First Meeting on April 1, 2010. Jeroen Janssen [8]announced the first meeting of the Ghent Functional Programming group, see the post for location and time details. PhD position in Functional Programming at Chalmers (deadline 2010-04-07). Patrik Jansson [9]announced an opening for a PhD student at Chalmers, working in Functional Programming and DSELs. VSTTE 2010: Verified Software -- Third Call for Papers. Gudmund Grov [10]announced a third CFP for VSTTE 2010, the Third International Conference on Verified Software: Theories, Tools, and Experiments. barchart-0.1.1. Sebastian Fischer [11]announced barchart, a command line program with an associated library for generating bar charts. Call for Participation: CHR Summer School. Jon Sneyers [12]announced a call for participation in the CHR Summer School on Programming and Reasoning with Rules and Constraints. Discussion Free theorem - what do we need it for. Maciej Piechotka [13]asked about Free Theorems and what they were good for. Has anybody translated Douglas Hofstadter's Scientific American articles introducting Scheme to a general audience into Haskell? Benjamin L. Russell [14]asked if anyone had considered translating Douglas Hofstadter's SciAm articles introducting Scheme to Haskell. Evolving Faster Haskell Programs (now with LLVM!). Don Stewart [15]our resident speed demon is at it again, harnessing the new LLVM backend of GHC and the power of Natural Selection to make his programs go even faster. Blog noise [16]Haskell news from the [17]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Neil Brown: [18]Synchronous Channels using MVars. * "FP Lunch": [19]Causal Semantics of (A)FRP. * Darcs: [20]darcs weekly news #56. * Dan Piponi (sigfpe): [21]Products, Limits and Parametric Polymorphism. * Bryn Keller: [22]Real World Haskell. * Isaac Dupree: [23]Next Project, Move doc-parsing to from GHC to Haddock. * Isaac Dupree: [24]Summer of Code Wrap-Up.. * Isaac Dupree: [25]Status update. * Isaac Dupree: [26]platform progress. * Isaac Dupree: [27]another boring update, & question :-). * Isaac Dupree: [28]How To Navigate Your Code:. * Michael Snoyman: [29]Persistence. * Neil Brown: [30]Choice over Events using STM. * Michael Snoyman: [31]Tweedle Beetle Battle. * Galois, Inc: [32]The Semantics of Asynchronous Exceptions. * Bryan O'Sullivan: [33]What's in a parser? Attoparsec rewired (2/2). * Bryan O'Sullivan: [34]What's in a parsing library? (1/2). * Neil Brown: [35]Progression 0.3: Bar Charts and Normalisation. Quotes of the Week * edwardk: heh, i'm amused the ParsecT that Antoine Latter put together is in the form of a right kan extension ;) * Philippa: <TR1N1TY> I'm selling botz and backdoors <Philippa> I'm selling bans * samlee (via reddit): haha those ghc options are longer than actual program About the Haskell Weekly News New editions are posted to [36]the Haskell mailing list as well as to [37]the Haskell Sequence and [38]Planet Haskell. [39]RSS is also available, and headlines appear on [40]haskell.org. To help create new editions of this newsletter, please see the information on [41]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [42]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.bluetile.org/ 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71314 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71184 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71144 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71122 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71041 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70981 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17842 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17841 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17831 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17825 13. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/71117 14. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/71022 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/70967 16. http://planet.haskell.org/ 17. http://haskell.org/haskellwiki/Blog_articles 18. http://chplib.wordpress.com/2010/03/08/synchronous-channels-using-mvars/ 19. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=369 20. http://blog.darcs.net/2010/03/darcs-weekly-news-56.html 21. http://blog.sigfpe.com/2010/03/products-limits-and-parametric.html 22. http://www.xoltar.org/?p=36 23. http://haddock2009.wordpress.com/2009/07/21/next-project-move-doc-parsing-t… 24. http://haddock2009.wordpress.com/2009/08/26/summer-of-code-wrap-up/ 25. http://haddock2009.wordpress.com/2009/05/12/status-update/ 26. http://haddock2009.wordpress.com/2009/05/20/platform-progress/ 27. http://haddock2009.wordpress.com/2009/05/26/another-boring-update-question/ 28. http://haddock2009.wordpress.com/2009/06/23/how-to-navigate-your-code/ 29. http://www.snoyman.com/blog/entry/persistence/ 30. http://chplib.wordpress.com/2010/03/04/choice-over-events-using-stm/ 31. http://www.snoyman.com/blog/entry/tweedle-beetle-battle/ 32. http://www.galois.com/blog/2010/03/04/the-semantics-of-asynchronous-excepti… 33. http://www.serpentine.com/blog/2010/03/03/whats-in-a-parser-attoparsec-rewi… 34. http://www.serpentine.com/blog/2010/03/03/whats-in-a-parsing-library-1/ 35. http://chplib.wordpress.com/2010/03/02/progression-0-3-bar-charts-and-norma… 36. http://www.haskell.org/mailman/listinfo/haskell 37. http://sequence.complete.org/ 38. http://planet.haskell.org/ 39. http://sequence.complete.org/node/feed 40. http://haskell.org/ 41. http://haskell.org/haskellwiki/HWN 42. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Re: [Haskell] Recursive definition of fibonacci with Data.Vector
by Henning Thielemann 08 Mar '10

08 Mar '10

On Sun, 7 Mar 2010, Edgar Z. Alvarenga wrote: > Hello, > > why I can't define a recursive vector using Data.Vector, like in > the example: > > import qualified Data.Vector as V > > let fib = 0 `V.cons` (1 `V.cons` V.zipWith (+) fib (V.tail v)) Since I liked to have both element-wise lazy construction and packed data, I created a data-structure for that purpose: http://code.haskell.org/storablevector/Data/StorableVector/Cursor.hs However, those vectors/arrays are finite, but could be extended to a chunky infinite structure.

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ghc leaking memory?
by Joachim Breitner 08 Mar '10

08 Mar '10

Hi, in Debian, we are having some problems building large libraries (such as agda, highlighting-kate, xmonad-contrib) on weaker architectures (sparc, armel, s390) which compile with -fvia-C, especially when building the profiling libraries. In the case of highlighting-kate, there were really issues with the code: A huge list of words which ghc seemed to try to inline caused it consuming a lot of memory. But in the case of agda, it even fails with relatively small modules: https://buildd.debian.org/fetch.cgi?pkg=agda&arch=sparc&ver=2.2.6-3&stamp=1… Now, looking at the memory gauge on my computer while compiling such a program with Cabal (which compiles all modules in one ghc call), it looks like it is steadily increasing. I’d expect ghc to free a lot of memory when it is done with a module, but this does not seem to be the case. Is this a bug (i.e. memory leak) in ghc or expected behavior? Is there a way to make Cabal build each module on its own? Are there any fixes or work-arounds? Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

5 6

how to write a function to send a string to a tuple
by Pradeep Wickramanayake 08 Mar '10

08 Mar '10

Hi, Im having problems with sending a string to a tuple. My string contains integers and strings The whole string stay as ("test,dfdf",3,"dfsf") sortList2 :: String -> String sortList2 (x:xs) | x == ',' = "" | otherwise = [x] ++ sortList2 xs The above function separating each words from the string Now I need to put them to a tuple putList :: String -> (Int, String, String, Int, Int) putList (x:xs) |xs /="" = sortList2 ++ putList xs I wrote something like above. But I really have no clue how to continue. Can someone help me. It's a big help

2 1

Haskell platform for GHC 6.12.1?
by Peter Verswyvelen 08 Mar '10

08 Mar '10

Using GHC 6.12.1 on Windows currently is hard, since one must compile the latest version of cabal-install, which is a nightmare to do for a typical windows user (install mingw, msys, utils like wget, download correct package from hackage, compile them in correct order, etc etc) What's the status of the Haskell platform for the latest and greatest Glasgow Haskell Compiler?

13 17

Re: [Haskell] Recursive definition of fibonacci with Data.Vector
by Don Stewart 08 Mar '10

08 Mar '10

edgar: > Hello, > > why I can't define a recursive vector using Data.Vector, like in > the example: > > import qualified Data.Vector as V > > let fib = 0 `V.cons` (1 `V.cons` V.zipWith (+) fib (V.tail v)) > There's a typo: fib = 0 `V.cons` (1 `V.cons` V.zipWith (+) fib (V.tail fib)) Which let's it typecheck. But I don't think this is a sensible use of vectors. In fact, infinite vectors make no sense, as far as I can tell -- these are fundamentally bounded structures. GHC even optimizes it to: fib = fib (literally!). $ time ./A A: <<loop>> -- Don

3 4

GHC's parallel garbage collector -- what am I doing wrong?
by Michael Lesniak 07 Mar '10

07 Mar '10

Hello haskell-cafe, Sorry for this long post, but I can't think of a way to describe and explain the problem in a shorter way. I've (again) a very strange behaviour with the parallel GC and would be glad if someone could either reproduce (and explain) it or provide a solution. A similar but unrelated problem has been described in [1]. EXAMPLE CODE The following demonstration program, which is a much smaller and single-threaded version of my real problem behaves as my real program. It does some number crunching by calculating pi to a definable precision: > -- File Pi.hs > -- you need the numbers package from hackage. > module Main where > import Data.Number.CReal > import System.Environment > import GHC.Conc > > main = do > digits <- (read . head) `fmap` getArgs :: IO Int > calcPi digits > > calcPi digits = showCReal (fromEnum digits) pi `pseq` return () Compile it with ghc --make -threaded -O2 Pi.hs -o pi BENCHMARKS On my two-core machine I get the following quite strange and unpredictable results: * Using one thread: $ for i in `seq 1 5`;do time pi 5000 +RTS -N1;done real 0m1.441s user 0m1.390s sys 0m0.020s real 0m1.449s user 0m1.390s sys 0m0.000s real 0m1.399s user 0m1.370s sys 0m0.010s real 0m1.401s user 0m1.380s sys 0m0.000s real 0m1.404s user 0m1.380s sys 0m0.000s * Using two threads, hence the parallel GC is used: for i in `seq 1 5`;do time pi 5000 +RTS -N2;done real 0m2.540s user 0m2.490s sys 0m0.010s real 0m1.527s user 0m1.530s sys 0m0.010s real 0m1.966s user 0m1.900s sys 0m0.010s real 0m5.670s user 0m5.620s sys 0m0.010s real 0m2.966s user 0m2.910s sys 0m0.020s * Using two threads, but disabling the parallel GC: for i in `seq 1 5`;do time pi 5000 +RTS -N2 -qg;done real 0m1.383s user 0m1.380s sys 0m0.010s real 0m1.420s user 0m1.360s sys 0m0.010s real 0m1.406s user 0m1.360s sys 0m0.010s real 0m1.421s user 0m1.380s sys 0m0.000s real 0m1.360s user 0m1.360s sys 0m0.000s THREADSCOPE I've additionally attached the threadscope profile of a really bad run, started with $ time pi 5000 +RTS -N2 -ls real 0m15.594s user 0m15.490s sys 0m0.010s as file pi.pdf FURTHER INFORMATION/QUESTION Just disabling the parallel GC leads to very bad performance in my original code, which forks threads with forkIO and does a lot of communications. Hence, using -qg is not a real option for me. Do I have overlooked some cruical aspect of this problem? If you've read this far, thank you for reading ... this far ;-) Cheers, Michael [1] http://osdir.com/ml/haskell-cafe@haskell.org/2010-02/msg00850.html -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

5 8

GPL answers from the SFLC (WAS: Re: ANN: hakyll-0.1)
by Tom Tobin 07 Mar '10

07 Mar '10

After politely pestering them again, I finally heard back from the Software Freedom Law Center regarding our GPL questions (quoted below). I exchanged several emails to clarify the particular issues; in short, the answers are "No", "No", "N/A", and "N/A". The SFLC holds that a library that depends on a GPL'd library must in turn be GPL'd, even if the library is only distributed as source and not in binary form. They offered to draft some sort of explicit response if we'd find it useful. Maybe it would be useful if Cabal had some sort of licensing check command that could be run on a .cabal file, and warn an author if any libraries it depends on (directly or indirectly) are GPL'd but the .cabal itself does not have the license set to GPL. On Fri, Dec 11, 2009 at 10:21 PM, Tom Tobin <korpios(a)korpios.com> wrote: > I'd like to get these questions out to the SFLC so we can satisfy our > curiosity; at the moment, here's what I'd be asking: > > Background: X is a library distributed under the terms of the GPL. Y > is another library which calls external functions in the API of X, and > requires X to compile. X and Y have different authors. > > 1) Can the author of Y legally distribute the *source* of Y under a > non-GPL license, such as the 3-clause BSD license or the MIT license? > > 2) If the answer to 1 is "no", is there *any* circumstance under which > the author of Y can distribute the source of Y under a non-GPL > license? > > 3) If the answer to 1 is "yes", what specifically would trigger the > redistribution of a work in this scenario under the GPL? Is it the > distribution of X+Y *together* (whether in source or binary form)? > > 4) If the answer to 1 is "yes", does this mean that a "BSD-licensed" > library does not necessarily mean that closed-source software can be > distributed which is based upon such a library (if it so happens that > the library in turn depends on a copylefted library)? > > By the way, apologies to the author of Hakyll — I'm sure this isn't > what you had in mind when you announced your library! I'm just hoping > that we can figure out what our obligations are based upon the GPL, > since I'm not so sure myself anymore. >

23 42

cabal install darcs fails on Mac OS X Snow Leopard
by George Colpitts 07 Mar '10

07 Mar '10

I'm unable to install darcs using cabal. It seems to be getting an error configuring haskeline.Can anybody help? Thanks cabal -V cabal-install version 0.6.4 using version 1.6.0.3 of the Cabal library bash-3.2$ ghc -V The Glorious Glasgow Haskell Compilation System, version 6.10.4 bash-3.2$ cabal install darcs Resolving dependencies... [1 of 1] Compiling Main ( /var/folders/Gj/Gj20S2zmFESpNdOqFfUlq++++TI/-Tmp-/haskeline-0.6.2.26747/haskeline-0.6.2.2/Setup.hs, /var/folders/Gj/Gj20S2zmFESpNdOqFfUlq++++TI/-Tmp-/haskeline-0.6.2.26747/haskeline-0.6.2.2/dist/setup/Main.o ) Linking /var/folders/Gj/Gj20S2zmFESpNdOqFfUlq++++TI/-Tmp-/haskeline-0.6.2.26747/haskeline-0.6.2.2/dist/setup/setup ... Configuring haskeline-0.6.2.2... checking whether to use -liconv... using -liconv. *Preprocessing library haskeline-0.6.2.2...* */usr/bin/hsc2hs: line 16: unexpected EOF while looking for matching `"'* */usr/bin/hsc2hs: line 17: syntax error: unexpected end of file* Configuring zlib-0.5.2.0... Preprocessing library zlib-0.5.2.0... /usr/bin/hsc2hs: line 16: unexpected EOF while looking for matching `"' /usr/bin/hsc2hs: line 17: syntax error: unexpected end of file cabal: Error: some packages failed to install: darcs-2.4 depends on zlib-0.5.2.0 which failed to install. hashed-storage-0.4.7 depends on zlib-0.5.2.0 which failed to install. haskeline-0.6.2.2 failed during the building phase. The exception was: exit: ExitFailure 2 zlib-0.5.2.0 failed during the building phase. The exception was: exit: ExitFailure 2 When I installed ghc on Snow Leopard I followed the instructions at http://www.haskell.org/haskellwiki/Mac_OS_X

2 2

Re: [Haskell-cafe] Books for "advanced" Haskell
by Günther Schmidt 06 Mar '10

06 Mar '10

Hi John, please, do try. Günther

1 0

How to obtain unique integers
by Robert Rothenberg 06 Mar '10

06 Mar '10

I'm implementing a variant of Prolog in Haskell (yes, I know others already exist...) and am looking to solve the following issues: (1) I need to translate anonymous variables "_" into unique variable names. Data.Unique does not give me a printable identifier. Is there another package that does this? (2) Is there a cabal package similar to the Atom.hs module form ginsu, that associates strings with unique hashes? The alternative of course is to maintain state variables and pass them along to display and parsing functions, but that makes it difficult to make the terms instances of Read/Show without reference to a global variable anyhow. Thanks, Rob

3 3

Re: Gut Build System
by Achim Schneider 06 Mar '10

06 Mar '10

Matthias Kilian <kili(a)outback.escape.de> wrote: > On Fri, Mar 05, 2010 at 09:26:08PM +0000, Simon Marlow wrote: > > >Switching GHC to a completely new build system written completely > > >in Haskell would be the most stupid idea ever. (You know why) > > > > You're referring to bootstrapping, I presume? > > Yes. > As it stands, GHC can't be bootstrapped without a GHC, anyway. I'd love to see GHC bootstrapping from C, e.g. via hugs, though. (A man is allowed to dream) > > I did think about mentioning that. Of course any Haskell build > > system would have to support cross-compilation or some other > > bootstrapping method, e.g. generation of a build script. > > That's a noble goal. But it has to be kept in mind. > This may be fairly easy, if the generated script does not have to come with major build-system capabilities: Leaving out most of dependency analysis and support -- at the utmost -- restarting a failed build from the rule whose source file has last recently changed, otherwise compiling in a linear order deduced at script generation time should reduce the problem to serializing the actions generated for a clean build to a shell script. I'm cross-posting this to the cafe and suggest that most if not all of this discussion be done there as most of it doesn't specifically relate to GHC. And there might be volunteers amidst the crowd. That said, I'd very much like to see a make-replacement done in Haskell, there's not only place for a properly-scriptable make, but also sufficient room for general innovation: Omake[1] would be even better if it wasn't scripted in some ad-hoc, impure, incomplete ocaml dialect but in a decently pure functional language; IBB[2] sound like a feature we'd definitely want to have (thanks for the link, Thomas) I'd also like to point to Nix[3] and Cabal[4] for ideas, and I bet most of the dependency analysis could be ripped from the latter. I think we can do vastly better than the whole SCons/CMake/Jam crowd. [1] http://omake.metaprl.org/index.html [2]http://chadaustin.me/2010/03/your-version-control-and-build-systems-dont-scale-introducing-ibb/ [3] http://nixos.org/nix/ [4] You know where. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.

3 3

Using Haskell's FFI to send ancillary data over Unix domain sockets
by ihope 05 Mar '10

05 Mar '10

Unix domain sockets are a type of socket created between two programs on a single Unix system. They are useful in part because over them you can send so-called ancillary data: file descriptors and credentials (i.e. a proof of who the process on the other end is). The thing is, Haskell doesn't have a nice way of sending ancillary data. Network.Socket does have these really opaque functions for sending and receiving ancillary data: sendAncillary :: Socket -> Int -> Int -> Int -> Ptr a -> Int -> IO () sendAncillary sock level ty flags datum len = do ... recvAncillary :: Socket -> Int -> Int -> IO (Int, Int, Ptr a, Int) recvAncillary sock flags len = do ... return (lev,ty,pD,len) Looking in the man page UNIX(7), which describes Unix domain sockets, some enlightening information is given. It says that ancillary data is sent and received using sendmsg(2) and recvmsg(2). Those two man pages say that sock and flags are arguments to sendmsg and recvmsg. UNIX(7) says that level is always SOL_SOCKET, and ty is either SCM_RIGHTS or SCM_CREDENTIALS depending on whether the ancillary data is file descriptors or credentials. In the former case, datum is "an integer array of the file descriptors"; in the latter case, it's the following struct: struct ucred { pid_t pid; /* process ID of the sending process */ uid_t uid; /* user ID of the sending process */ gid_t gid; /* group ID of the sending process */ }; As for len, this struct seems to be enlightening: struct cmsghdr { socklen_t cmsg_len; /* data byte count, including header */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by unsigned char cmsg_data[]; */ }; So, sock is a Socket, level is the constant SOL_SOCKET, ty is either SCM_RIGHTS or SCM_CREDENTIALS (strangely, Network.Socket contains the former but not the latter), flags is the flags passed to sendmsg or recvmsg (whatever those are--what are they?), datum is either a C integer array or that struct, and len is the length in bytes of all that. For me, this presents a few problems. I don't know where to get the SCM_CREDENTIALS constant, I have no idea what flags to use (does the Network module help with that?), I don't know how to get from a list of file descriptors or a tuple to a Ptr, and perhaps most importantly, I have no idea how to get the lengths of pid_t, uid_t, gid_t, and socklen_t. Can anyone offer assistance?

2 1

GPL answers from the SFLC (WAS: Re: ANN: hakyll-0.1)
by Kevin Jardine 05 Mar '10

05 Mar '10

I'm a Haskell newbie but long time open source developer and I've been following this thread with some interest. The GPL is not just a license - it is a form of social engineering and social contract. The idea if I use the GPL is that I am releasing free and open source software to the community. You are welcome to use it for any purpose but in exchange you must also agree to release any software you create that uses my software as free and open source. That is the difference between GPL and BSD type licenses. The GPL very deliberately creates an obligation. Yes, that can be inconvenient. It is meant to be inconvenient. Actually the GPL reminds me of a Haskell concept that I am struggling with right now - the monad. When I started writing Haskell code I was always trying to mix pure and IO code and I soon learned that once I used the IO monad I was stuck within it. The monad creates an inconvenient obligation and any IO code can only be used within other IO code. There are good reasons for monads (just as, in my view, there are good reasons for the GPL) but using them means that I need to make a lot of changes to the way I write software. Kevin

6 5

How to put data from a string to a tuple
by Pradeep Wickramanayake 05 Mar '10

05 Mar '10

As luke requested Here is the codes and some more brief explanation. Please help me getItemFile :: IO String getItemFile = do test <- readFile "input.txt" return test im taking a file data to a string it contains string like "hello,world,I,am,a,new,developer" so I need to put these each to a tuple. Because the content have Integers and Strings sortList2 :: String -> String sortList2 (x:xs) | x == ',' = "" | otherwise = [x] ++ sortList2 xs Im breaking word by word from this above function Now I need to send it to a tuple. Can someone help me how to do it. Is this possible. Using recursion im checking each word till "," occurs And taking that string and passing it to the tuple. Can someone please help me to do it. Please

2 1

New OpenGL package: efficient way to convert datatypes?
by Peter Verswyvelen 05 Mar '10

05 Mar '10

The newest package seems to require using GLdouble/GLfloat. What is the most efficient way to convert Double/Float to GLdouble/GLfloat? I'm currently using realToFrac. But essentially the operation should be a nop on my machine. I haven't looked at the core code yet (on Windows, last time I checked, the ghc-core util did not work, and without it, reading core is even harder) Thanks, Peter

7 13

Hackage vs. legacy support in Cabal files
by John D. Ramsdell 05 Mar '10

05 Mar '10

I'd like to support Hackage and GHC 6.8 with one source distribution. The enclosed Cabal file would do the trick if Hackage would simply evaluate the Cabal file using the default flag values. Instead, it chokes on a hack required to support GHC 6.8. The directory containing the autogen files has to be on the source path. if flag(old) Hs-Source-Dirs: src dist/build/autogen else Hs-Source-Dirs: src Is there a way to force Hackage to respect flag values? John

1 0

Re: A few ideas about FRP and arbitrary access in time
by Patai Gergely 05 Mar '10

05 Mar '10

> I'm not sure I want to follow WWRD all the way. I do want events, for > example mouse clicks (for which there doesn't seem to be any logical > behavior representation). As you note the pull-based approach does a > lot more work in those cases than seems necessary. Events are indispensable for practically all applications, even those that have continuous output, so leaving them out would be no option. I only did so for Elerea to be able to quickly get a working system running, so I can actually experiment with application-side design. But yeah, it can't be used for serious purposes without some support for events. > In any case, what I'm discussing in that report is mostly centered on > the continuous-time values, so I'm more concerned with what we call > behaviors. The question is how to model memory-full operations with > point-wise operations. I think it would help if you tried to solve more specific problems first and see what common structure you can factor out from the solutions. It's easy to get into a dead end if you go for the big picture right away. > Right. You'll agree that it would be nice to have a general approach? > How will that work? That's what I'd like to find out. And I do hope you'll manage to find a useful result. My intuition says, however, that looking for a sensible interface between events and behaviours is more fruitful than trying to unify these two notions. The reason is the difference in TCMs, as noted previously. > report. I'd like things to be unified, so maybe playing with the idea > of a total function to 'Maybe a' for events is the right direction. I > think this was already explored by some FRP incarnations, but I don't > recall what came out of it. Yes, that's the Yampa way. The problem is that in this case you'll need an awkward operator to refer to 'the time of last defined point'. > But isn't Lucid Synchrone essentially discrete-timed? Also, events > shouldn't be semantically constrained to multiples of some basic > clock, they are defined over continuous time. Of course, it's more like a possible model for operational semantics (I know you're interested in denotation, but inspiration can come from anywhere). I just thought that clock calculus might be an interesting topic to throw into the hat. Gergely -- http://www.fastmail.fm - Email service worth paying for. Try it for free

1 0

How to put data from a string to a tuple
by Pradeep Wickramanayake 05 Mar '10

05 Mar '10

Hi, Im self learner in Haskell. And im stuck in a small place which I tried searching in google but couldn't find Proper answer I have some values in a string, Im removing them one by one from the string (word by word) and I want to put them in a tuple. Because it contain Integers and Strings. Can someone help me __________ Information from ESET NOD32 Antivirus, version of virus signature database 4917 (20100305) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com

3 2

How do you rewrite your code?
by Sean Leather 05 Mar '10

05 Mar '10

There are numerous threads on the Haskell Café involving rewriting, refactoring, refining, and in general improving code (for some definition of improve). I am interested in seeing examples of how Haskell code can be rewritten to make it better. Some general examples are: - Eta-reduce - Make more pointfree - Introduce monadic operators or do-notation - e.g. for Maybe, lists, State - Eliminate monadic operators or do-notation - Generalize types - e.g. change map to fmap, (++) to mappend - Use instances of Functor, Applicative, Alternative, Category, Arrow, Monoid, Traversable, etc. - Use library functions from Data.List, Data.Map, Data.Set, etc. - Use some form of generic programming (e.g. SYB, Uniplate, EMGM, Alloy) - Use other libraries not included in the Platform My question is simple: *How do you rewrite your code to improve it?* You can answer this in any way you like, but I think the most useful answer is to show a reasonably small, concrete example of what your code looked like before and after. Also, please describe how you think the rewrite improves such code. - Is it better style? More useful? More efficient? - Are the types (before and after) the same? - Are the semantics the same? - How did you prove or test equivalence? (e.g. Can you use equational reasoning to confirm the rewrite is valid? Did you use QuickCheck?) Here is an example that I find myself doing occasionally. For all x, f: x >>= return . f --> fmap f x or f <$> x -- requires importing Control.Applicative I think the right-hand side (RHS) is more concise and simpler. The types here do change: the type constructor has a Monad constraint in the left-hand side and a Functor constraint in the RHS. Types that are Monad instances are generally also Functor instances, so this is often possible. I'm convinced the semantics are preserved, though I haven't proven it. What's an example of a rewrite that you've encountered? Thanks, Sean

13 14

QOTM: It's a series of 3 TLAs designed to make people who know what they are feel more in-the-know than those who don't.
by caseyh＠istar.ca 05 Mar '10

05 Mar '10

CBV, CBN, CBL

2 1

ANNOUNCE: wai-0.0.0 (Web Application Interface)
by Michael Snoyman 04 Mar '10

04 Mar '10

Hello all, I'd like to announce the first release of the Web Application Interface package[1]. The WAI is a common protocol between web server backends and web applications, allowing application writers to target a single interface and have their code run on multiple server types. There are two previous implementations of the same idea: the Network.Wai module as implemented in the Hyena package, and Hack. Some distinguishing characteristics of the wai package include: * Unlike Hack, this does not rely upon lazy I/O for passing request and response bodies within constrained memory. * Unlike Hyena, the request body is passed via a "Source" instead of Enumerator, so that it can easily be converted to a lazy bytestring for those who wish. * Unlike both, it attempts to achieve more type safety by have explicit types of request headers, response headers, HTTP version and status code. * It also removes any variables which are not universal to all web server backends. For example, scriptName has been removed, since it has no meaning for standalone servers. This package also contains separate modules for conversions to and from Sources and Enumerators. I am also simultaneously releasing two other packages: web-encodings 0.2.4[2] includes a new function, parseRequestBody, which directly parses a request body in a WAI request. It handles both URL-encoded and multipart data, and can store file contents in a file instead of memory. This should allow dealing with large file submits in a memory-efficient manner. You can also receive the file contents as a lazy bytestring. Finally, wai-extra 0.0.0 [3] is a collection of middleware and backends I use regularly in web application development. On the middleware side, there are five modules, including GZIP encoding and storing session data in an encrypted cookie (ClientSession). On the backend side, it includes CGI and SimpleServer. The latter is especially useful for testing web applications, though some people have reported using it in a production environment. All of the code is directly ported from previous packages I had written against Hack, so they are fairly well tested. As far as stability, I don't expect the interface to change too drastically in the future. I am happy to hear any suggestions people have for moving forward, but expect future versions to be mostly backwards compatible with this release. Also, future versions will respect the Package Versioning Policy[4]. Thanks to everyone who contributed their input. Michael [1] http://hackage.haskell.org/package/wai [2] http://hackage.haskell.org/package/web-encodings [3] http://hackage.haskell.org/package/wai-extra [4] http://www.haskell.org/haskellwiki/Package_versioning_policy

1 0

How do you set up windows enviroment
by Maciej Piechotka 04 Mar '10

04 Mar '10

How do you set up Windows environment? I'd like to develop few platform-specific code or rather port it to this platform. I tried to set up environment but I failed (darcs hanging on copy via ssh, command line is... well slightly better then in W98 etc.). Does anyone have a tips for setting Haskell environment on Windows 7. Regards

6 10

Prelude.undefined
by Tom Hawkins 04 Mar '10

04 Mar '10

How do I track down an reference to an undefined value? My program must not be using a library correctly because the program makes no direct use of 'undefined'. Running with +RTS -xc yields: <GHC.Err.CAF>Test: Prelude.undefined

8 12

ANNOUNCE: Parsec 3.1.0
by Derek Elkins 04 Mar '10

04 Mar '10

Parsec is a monadic combinator library that is well-documented, simple to use, and produces good error messages. Parsec is not inherently lazy/incremental and is not well-suited to handling large quantities of simply formatted data. Parsec 3 adds to Parsec the ability to use Parsec as a monad transformer and generalizes the input Parsec accepts. Parsec 3 includes a compatibility layer for Parsec 2 and should be a drop-in replacement for code using Parsec 2. Code using the features of Parsec 3 should use the modules in Text.Parsec. Due almost entirely to the work of Antoine Latter there is a new version of Parsec 3 available. He documented some of his thoughts on this in this series of blog posts: http://panicsonic.blogspot.com/2009/12/adventures-in-parsec.html The main features of this release are: - the performance should be much better and comparable to Parsec 2 - notFollowedBy's type and behavior have been generalized Changes: - the changes to the core of Parsec lead to some changes to when things get executed when it is used as a monad transformer "In the new version bind, return and mplus no longer run in the inner monad, so if the inner monad was side-effecting for these actions the behavior of existing code will change." - notFollowedBy p now behaves like notFollowedBy (try p) which changes the behavior slightly when p consumes input, though the behavior should be more natural now. - the set of names exported from Text.Parsec.Prim has changed somewhat

4 3

How to break strings and put to a tuple
by Pradeep Wickramanayake 04 Mar '10

04 Mar '10

Hi, sortList2 :: String -> String sortList2 (x:xs) | x == ',' = "" | otherwise = [x] ++ sortList2 xs im breaking a one specific string and putting them to each word. But I need to put them to a tuple. Can someone help me with the code Please.

2 1

Profiling help (Warning: Euler spoilers)
by Louis Wasserman 04 Mar '10

04 Mar '10

James, Which version of Control.Monad.State are you importing? If you're just importing vanilla Control.Monad.State, that's actually sending you to Control.Monad.State.Lazy. Using Control.Monad.State.Strict might solve your problems, srsly. Laziness can result in epically failing memory leaks. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

2 3

ANN: iteratee-parsec 0.0.2
by Maciej Piechotka 04 Mar '10

04 Mar '10

Iteratee-parsec is a library which allows to have a parsec (3) parser in IterateeG monad. It contains 2 implementations: - John Lato's on public domain. It is based on monoid and design with short parsers in mind. - Mine on MIT. It is based on single-linked mutable list. It seems to be significantly faster for larger parsers - at least in some cases - but it requires a monad with references (such as for example IO or ST). Version 0.0.2 does not differ much from 0.0.1 except that it is up-to-date with parsec 3.1.0 (version 0.0.1 is not). Regards

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type class constraints headache
by muneson 04 Mar '10

04 Mar '10

When writing a command-line interface I ran into type class conflicts I don't understand. Could anyone explain why ghc 6.10.4 compiles this > methods :: (Eq a) => [(String, a)] > methods = > [ ("method1", undefined ) > , ("method2", undefined) > ] but not the following? > methods :: (Eq a) => [(String, a)] > methods = > [ ("method1", undefined ) > , ("method2", undefined) > ] > > enumerateMethodNames :: [String] > enumerateMethodNames = map fst methods thanks, Marcus -- View this message in context: http://old.nabble.com/type-class-constraints-headache-tp27752745p27752745.h… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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idioms ... for using Control.Applicative.WrapMonad or Control.Arrow.Kleisli?
by Nicolas Frisby 04 Mar '10

04 Mar '10

Each time I find myself needing to use the wrapping functions necessary for this embeddings, I grumble. Does anyone have a favorite use-pattern for ameliorating these quickly ubiquitous conversions? For runKleisli, I was considering something like okKleisli :: (Control.Arrow.Kleisli m a b -> Control.Arrow.Kleisli m' a' b') -> (a -> m b) -> (a' -> m' b') onKleisli f = Control.Arrow.runKleisli . f . Control.Arrow.Kleisli but haven't really tested its usefulness. My most frequent use cases usually include Arrow.first, Arrow.second, &&&, ***, or +++. E.g. somefun :: (Monad m, Num a) => (a, d) -> m (a, d) somefun = onKleisli Control.Arrow.first (\ a -> return (a + 1)) Perhaps a Control.Arrow.Kleisli, which would export (same-named) Kleisli specializations of all the Control.Arrow methods? And an analogous Control.Applicative.Monad? (Data.Traversable does this a little bit to specialize its interface for monads, such as Data.Traversable.sequence.) While writing this, I realized that you can't leave the Arrow-ness of Kleisli arrows implicit, since (->) a (m b) is two kinds of arrows depending on context -- which is precisely what the Kleisli newtype resolves. So I'm not seeing a reason to bring up the 'class Applicative m => Monad m where' dispute. Thanks for your time.

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Re: [Haskell-cafe] Free theorem - what do we need it for
by Janis Voigtländer 04 Mar '10

04 Mar '10

Maciej Piechotka wrote: > I am afraid that I fail to see practical benefits (I mean that I don't > know all implication not that I think there is no benefits). > What are the benefits of free theorem except large chance that data will Lots of applications. See page 34 (along with the references starting from page 218) of http://www.iai.uni-bonn.de/~jv/ppl2010-slides.pdf for a list of some of them. Ciao, Janis -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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ANN: uuid-0.1.2
by Mark Lentczner 04 Mar '10

04 Mar '10

Antione and I are please to announce the release of uuid-0.1.2. CHANGES: - added functions toByteString and fromByteString - added 'nil' UUID - added unit tests and benchmarks, built when configured -ftest - major speed up of to/from functions (as well as in general) - added version-3 generation (deterministic based on MD5) - major changes to internal representation - now uses four strict Word32 values - internal ByteSource classes for easy construction (see Builder.hs) - Storable instance now stores in memory as system libraries in C do: 16 bytes derived from the network order of the fields, no matter what the host native endianess is. - fixed bugs in V1 time and clock stepping, and V1 generated values - builds cleanly under GHC's -Wall - added CHANGES file String conversions were sped up 4x to 7x! - Mark Mark Lentczner (MtnViewMark) http://www.ozonehouse.com/mark/

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Profiling help (Warning: Euler spoilers)
by James Jackson 04 Mar '10

04 Mar '10

I have written the program below, which solves problem 14 from Project Euler, which asks us to find the hailstone number below 1 million that takes the longest to get to 1. The program solves the problem using dynamic programming using a Data.Map. It completes in under a minute (barely), but allocates a ridiculous amount of memory and actually runs slower than the naive one-liner it was supposed to replace. I have an intuition on what the problem is, but would like to verify it by profiling. Unfortunately, I don't know enough about how profiling works in GHC to diagnose the problem. Hopefully the Cafe can help. Here's the code. scratch.hs: This is the main program, which just calls the problemFourteen function. The EP datatype just contains an int representing the problem number and an (IO ()) which prints out the solution --------------------------------------------------------------------------------------------------------------------------- import Euler1to20 -- contains problems 1 through 20 import Euler -- Contains various utility functions including the EC type which represents Euler Problems main = proc where (EP num proc) = problemFourteen --------------------------------------------------------------------------------------------------------------------------- Euler1to20.hs: contains the following lines, among others --------------------------------------------------------------------------------------------------------------------------- problemFourteen = EP 14 $ print $ (key, fromJust $ findIndex (==key) keys) where (keys, results) = runState (mapM findChainLength [1..10^6]) $ M.fromList [(1,1)] key = maximum keys findChainLength::Integer->State (M.Map Integer Integer) Integer findChainLength n = do m <- get case M.lookup n m of Nothing -> let nextLink = if even n then div n 2 else 3*n+1 in do len <- liftM (1+) (findChainLength nextLink) m2 <- get m2 `seq` put (M.insert n len m2) return len Just n'-> return n' --------------------------------------------------------------------------------------------------------------------------- My guess is that the problem is in the recursive call to findChainLength, which is not in tail position. I think that this is causing the code to keep too many old copies of the map around. Before I go rewriting everything, however, I'd like to verify that this is indeed the problem, so the next step is to compile and run with profiling turned on. After quickly looking at the Real World Haskell chapter on profiling, I do the following. Compiling and running: --------------------------------------------------------------------------------------------------------------------------- ghc -prof --make -auto-all -fforce-recomp scratch.hs [1 of 3] Compiling Euler ( Euler.hs, Euler.o ) [2 of 3] Compiling Euler1to20 ( Euler1to20.hs, Euler1to20.o ) [3 of 3] Compiling Main ( scratch.hs, scratch.o ) Linking scratch ... ./scratch +RTS -sstderr -hc -p -K100M (525,837798) 9,503,907,112 bytes allocated in the heap 42,785,764,376 bytes copied during GC (scavenged) 15,697,610,688 bytes copied during GC (not scavenged) 421,548,032 bytes maximum residency (193 sample(s)) 17787 collections in generation 0 (650.00s) 193 collections in generation 1 (1748.37s) 782 Mb total memory in use INIT time 0.00s ( 0.00s elapsed) MUT time 18.68s ( 19.19s elapsed) GC time 2398.37s (2401.47s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 50.48s ( 50.50s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2467.53s (2471.17s elapsed) %GC time 97.2% (97.2% elapsed) Alloc rate 508,742,769 bytes per MUT second Productivity 0.8% of total user, 0.8% of total elapsed --------------------------------------------------------------------------------------------------------------------------- I can see from the above that the maximum resident memory at one time is 782Mb, which seems excessive, since I don't expect the Map to contain more than around 2M entries. Also, I had to use "-K 100M" to increase the stack space or the program bombs out with a stack overflow exception. Since I compiled with profiling information, running the program produced the following scratch.prof: --------------------------------------------------------------------------------------------------------------------------- Wed Mar 3 19:41 2010 Time and Allocation Profiling Report (Final) scratch +RTS -sstderr -hc -p -K100M -RTS total time = 18.68 secs (934 ticks @ 20 ms) total alloc = 5,327,238,992 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc findChainLength Euler1to20 93.8 90.5 CAF Euler1to20 5.2 8.0 problemFourteen Euler1to20 1.0 1.5 individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 1 0 0.0 0.0 100.0 100.0 CAF Main 212 2 0.0 0.0 0.0 0.0 main Main 218 1 0.0 0.0 0.0 0.0 CAF GHC.Handle 164 4 0.0 0.0 0.0 0.0 CAF Euler1to20 139 21 5.2 8.0 100.0 100.0 findChainLength Euler1to20 220 3168610 93.8 90.5 93.8 90.5 problemFourteen Euler1to20 219 1 1.0 1.5 1.0 1.5 CAF Control.Monad.State.Lazy 137 1 0.0 0.0 0.0 0.0 --------------------------------------------------------------------------------------------------------------------------- The above tells me that almost all the CPU time and memory allocation happens in findChainLength, but that isn't really a surprise. I think the next step is to start adding cost centers to get a more detailed profile, but here is where I hit the end of my ghc profiling knowledge. Can anyone help with some general strategies for how to use the profiling tools in GHC to track this down? Of course once the culprit has been found, I have to figure out what to *do* about it, but one thing at a time... James

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Free theorem - what do we need it for
by Maciej Piechotka 04 Mar '10

04 Mar '10

Some time ago I get knew about free theorem that states (omitting undefined & co.) that there is 1 haskell function forall a. a -> a. The property seems to be workaround by Typeable but it stills not work for all types so it cannot be used in, for example, Monad or Functor. I am afraid that I fail to see practical benefits (I mean that I don't know all implication not that I think there is no benefits). What are the benefits of free theorem except large chance that data will obey he functor/monad laws? I.e. what advantage it gives over the 'reflection' from other languages? Regards

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Has anybody translated Douglas Hofstadter's Scientific American articles introducting Scheme to a general audience into Haskell?
by DekuDekuplex＠Yahoo.com 04 Mar '10

04 Mar '10

There is an interesting, if somewhat dated, suggestion on "Lambda the Ultimate" (see http://lambda-the-ultimate.org/node/1748) that "someone translate Doug Hofstadter's Scientific American columns introducing Scheme to a general audience into Haskell." (I came across this link while adding full titles and links to the HaskellWiki "Books and tutorials" page (see http://www.haskell.org/haskellwiki/Books_and_tutorials) where I clicked on the link to "Tutorials" (see http://www.haskell.org/haskellwiki/Tutorials) which contained a link to a "Haskell vs. Scheme" (see http://www.reddit.com/r/programming/tb/nq1k) article, which described the post containing the suggestion.) According to a comment by Ehud Lamm (see http://lambda-the-ultimate.org/node/1748#comment-21292) on the above post, the columns are in Hoftstadter's book _Metamagical Themas: Questing For The Essence Of Mind And Pattern_ [1] (see http://www.amazon.com/Metamagical-Themas-Questing-Essence-Pattern/dp/046504…) Has anybody translated Hofstadter's articles from Scheme into Haskell? -- Benjamin L. Russell [1] Hofstadter, Douglas. _Metamagical Themas: Questing For The Essence Of Mind And Pattern._ New York: Basic Books, 1996. <http://www.amazon.com/Metamagical-Themas-Questing-Essence-Pattern/dp/046504…>. -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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how to get a string from file
by Pradeep Wickramanayake 03 Mar '10

03 Mar '10

getItemFile :: IO String getItemFile test = do test <- readFile "input.txt" return test this is the program I did to take a file and get all the data from that file to a string. Can someone help me. I get this error Type error in explicitly typed binding *** Term : getItemFile *** Type : a -> IO [Char] *** Does not match : IO String What I need is to get the file to a string and then get it to a tuple by checking each places where the file contain a blank space Please help me Thanks

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minimal interface
by Sean McLaughlin 03 Mar '10

03 Mar '10

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. Thanks, Sean

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segmentation fault with ghc-6.10.4 on linux-x86_x64
by Wim Vanderbauwhede 03 Mar '10

03 Mar '10

Hello, I'm writing a compiler and it segfaults when compiled with ghc-6.10.4 on linux-x86_x64. It runs fine when compiled with ghc-6.10.4 on Mac OS X 10.5 (64-bit Intel) and with ghc-6.8.2 on Mac OS X 10.4 (32-bit PPC). I use the Haskell platform, the ghc -v output is Glasgow Haskell Compiler, Version 6.10.4, for Haskell 98, stage 2 booted by GHC version 6.8.2 Using package config file: /usr/local/lib/ghc-6.10.4/./package.conf Using package config file: /users/wim/.ghc/x86_64-linux-6.10.4/package.conf hiding package base-3.0.3.1 to avoid conflict with later version base-4.1.0.0 hiding package network-2.2.1.2 to avoid conflict with later version network-2.2.1.4 hiding package time-1.1.2.4 to avoid conflict with later version time-1.1.4 wired-in package ghc-prim mapped to ghc-prim-0.1.0.0 wired-in package integer mapped to integer-0.1.0.1 wired-in package base mapped to base-4.1.0.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0.1.0 wired-in package syb mapped to syb-0.1.0.1 wired-in package template-haskell mapped to template-haskell-2.3.0.1 wired-in package dph-seq mapped to dph-seq-0.3 wired-in package dph-par mapped to dph-par-0.3 The offending code is (simplified) mapM emit (sc_args sc) with sc::ServiceCall data ServiceCall = MkServiceCall { sc_args::[Expr], ... } data Expr = PureE (PString String) | ... and emit defined via a type class EmitG: emitExpr (PureE e) = emit e instance EmitG Expr where emit = emitExpr emitPureExpr (PString s) = return $ show s instance EmitG PureExpr where emit = emitPureExpr So it actually maps "show" over a list of strings. The compiler parses a test program, the segfault occurs when emitting code for lines like ImgBlock() img1 = img.in("img.raw"); (a "service call" in my language) Replacing the string by a number of any other type of the AST makes no difference. It looks like a bug in ghc's memory allocation. Any suggestions on how to debug this? Thanks! Wim

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