- Haskell-Cafe - Haskell.org

Character I/O
by Andrew Coppin 02 Apr '09

02 Apr '09

I ran the following program: >module Main where > >main = do > putStrLn "Key" > c <- getChar > print c > main Unfortunately, it doesn't do what I was hoping. Specifically, it only returns any data when I hit [return]. It also seems to be giving me a command history somehow. Special keys (e.g., arrow keys, backspace, escape) don't get through to the application. I'm trying to write a program that uses individual keys as commands. What's the correct way to get this to work?

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HXT: desperatedly trying to "concat"
by Steffen Schuldenzucker 02 Apr '09

02 Apr '09

Hi. I've got a problem with the Haskell XML Toolkit (hxt). I want to write a little app that performs REST requests from a certain (rather simple) XML format. A example procedure Call looks like testData defined below. What I'd like to do is to transform this xml tree into a GET variable string using an XmlArrow. The task sounds easy, and it has to be easy, but I've been sitting here for about a day now, staring at my code. It looks like this (the transformation should be done by the arrow "mkGetStr"): -- Rest.hs -- This is also on HPaste: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3210#a3210 {-# LANGUAGE NoMonomorphismRestriction #-} module Rest where import Text.XML.HXT.Arrow import Data.List getParamsA = hasName "param" >>> getChildren >>> isElem >>> (getName &&& (getChildren >>> getText)) >>> arr2 mkGetPair mkMethodStr = ("method=" ++) mkGetPair k v = k ++ "=" ++ v getMethodA = hasName "method" >>> getChildren >>> getText >>> arr mkMethodStr mkGetStr = isElem >>> (getMethodA <+> getParamsA) >>. intercalate "&" -- Try it with: runX (testData >>> mkGetStr) >>= print testData = xread <<< constA ( "<method>my.Method</method>" ++ "<param>" ++ "<foo_arg>Foo</foo_arg>" ++ "<bar_arg>Bar</bar_arg>" ++ "</param>" ) -- End of Rest.hs What I get out of it is this (in ghci): *Rest> runX (testData >>> mkGetStr) >>= print "method=my.Methodfoo_arg=Foo&bar_arg=Bar" There is an "&" missing right after "method=my.Method"! Why? I've tried many variants of this and they give me either this or a similar result or multiple results (what I don't want either). I'd be really happy if someone could save my day and help me with this issue. Thanks in advance, Steffen ________________________________________________________________________ Neu bei WEB.DE: Kostenlose maxdome Movie-FLAT! https://register.maxdome.de/xml/order/LpWebDe?ac=OM.MD.MD008K15726T7073a

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SciMark2 benchmark
by Jon Harrop 02 Apr '09

02 Apr '09

Has anyone ported the SciMark2 benchmark suite to Haskell? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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ZipList monad, anyone?
by Patai Gergely 02 Apr '09

02 Apr '09

Does ZipList have any useful monad instance? The thought came up while thinking about higher order dataflows and an ArrowApply interface for Yampa. As a ZipList can be thought of as a function with a discrete domain, I figured its monadic form could be analogous to the reader monad, hence: instance Monad ZipList where return = ZipList . repeat ZipList xs >>= f = ZipList $ zipWith ((!!) . getZipList . f) xs [0..] Correct me if I'm wrong, but it seems to me that f always returning an infinite list is a sufficient condition for the monad laws to hold. However, this is painfully inefficient. I assume that bringing FRP-like switching and some clever data structure (which optimises stateless streams for starters) into the picture can potentially lead to constant-time access at least in the case of sequential sampling. Is there any recent work going in this direction? Gergely -- http://www.fastmail.fm - One of many happy users: http://www.fastmail.fm/docs/quotes.html

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Re: [Haskell-beginners] Re: making translation from imperative code
by Michael Mossey 02 Apr '09

02 Apr '09

Thanks very much for the help... I will look at this over the next couple of days. Your code actually addresses a different problem, the one of merging separates lists of timed events. I do need to write code to do that eventually, so I will try to understand what you have written here. However, the original problem concerns visual layout, which actually takes place *after* creating a merged list. In layout, items do have times associated with them, but also take up physical space. Different items takes up different amounts of space, and at any given "time," there may be items on all the staves or just some of them. I will try to come up with (1) a more succinct explanation of the problem (with textual graphics as a visual aid) (2) a more succinct algorithm. For example, you are right that I'm mixing concerns. The system layout can terminate for two reasons: (1) reached the end of the score (2) reached the right edge of the page. There might be a way to simplify the loop or fold so that these concerns look more unified. -Mike PS a question below: Heinrich Apfelmus wrote: > Michael Mossey wrote: >> Heinrich Apfelmus wrote: >>> Can you elaborate on what exactly the algorithm is doing? Does it just >>> emit notes/chords/symbols at given positions or does it also try to >>> arrange them nicely? And most importantly, "where" does it emit them to, >>> i.e. what's the resulting data structure? >>> >>> So far, the problem looks like a basic fold to me. >> Here is some Haskell code that explains the problem in >> more detail. >> [...] > > Thanks for the elaboration. > > I think the code doesn't separate concerns very well; mixing information > about widths and times, page size and the recursion itself into one big > gnarl. > > Also, there is one important issue, namely returning a special value > like -1 as error code in > >> tryAgain state = >> case scoreNextTime score (time state) of >> -1 -> indicateNoMoreChunks state >> t -> layoutSystem' (setTime state t) > > Don't do this, use Maybe instead > > tryAgain state = case scoreNextTime score (time state) of > Nothing -> indicateNoMoreChunks state > Just t -> layoutSystem' (state { time = t }) > > where Nothing indicates failure and Just success. > > > Back to the gnarl in general, I still don't have a good grasp on the > problem domain, which is key to structuring the algorithm. Therefore, > I'll expand on toy model and you tell me how it differs from the real thing. > > The model is this: we are given several lists of notes (f.i. a piano > part and a vocal line) where each note is annotated with the time it is > to be played at. We abstract away the fact that we are dealing with > musical notes and simply consider a list of *events* > > type Time = Integer > type Events a = [(Time, a)] > > with the invariant that the timestamps are (strictly) increasing: > > valid :: Events a -> Bool > valid xs = all $ zipWith (\(t1,_) (t2,_) -> t1 < t2) xs (drop 1 xs) > > Now, the toy task is to merge several lists of similar events into one > big list that is ordered by time as well. > > merge :: [Events a] -> Events [a] > > Since some events may now occur simultaneously, the events of the > results are actually lists of "primitive" events. > > One possibility for implementing merge is to start with a function to > merge two event lists > > merge2 :: Events [a] -> Events [a] -> Events [a] > merge2 [] ys = ys > merge2 xs [] = xs > merge2 xs@((tx,x):xt) ys@((ty,y):yt) = case compare tx ty of > LT -> (tx,x ) : merge2 xt ys > EQ -> (tx,x++y) : merge2 xt yt > GT -> (ty, y) : merge2 xs yt > > and to apply it several times > > merge = foldr merge2 [] . map lift > where lift = map $ \(t,x) -> (t,[x]) > > > Another possibility is to simply concatenate everything first and then > sort by time > > merge = map (\((t,x):xs) -> (t,x:map snd xs)) > . groupBy ((==) `on` fst) > . sortBy (comparing fst) > . concat > > > The code above can be made more readable by choosing nice names like > > time = fst > event = snd > > or avoiding pairs altogether and implementing these names as record > fields. Also, the (&&&) combinator from Control.Arrow is very handy. > > merge = map (time . head &&& map event) > . groupBy ((==) `on` time) > . sortBy (comparing time) > . concat > > > I hope this gives you a few ideas to think about. How does this toy > model differ from the real thing? > > > Regards, > apfelmus > > > PS: If some parts of my example code give you trouble, it's probably > fastest to ask around on the #haskell IRC channel. > > -- > http://apfelmus.nfshost.com > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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GSoC proposal
by Csaba Hruska 02 Apr '09

02 Apr '09

Hi! I've submitted my proposal on gsoc portal. (3D Rendering Engine) What's your opinion?

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Leksah patch release 0.4.4.1
by Jürgen Nicklisch-Franken 02 Apr '09

02 Apr '09

I've pushed a patch with a minor fix for the recurring query to open a file: * Doesn't attempt a background build, when there is no open package. * Doesn't keep bugging the user if the session they select does not have a valid package * Improved title on package open dialog * Updated the manual for better Ubuntu installation description Please note that Leksah doesn't work with GHC version 6.10.2 currently, because gtk2hs has problems with the changes on finalizers. Hope they can fix this soon. Jürgen

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ANNOUNCE: vacuum: extract graph representations of ghc heap values.
by Matt Morrow 02 Apr '09

02 Apr '09

I am pleased to announce the release of vacuum, a Haskell<http://haskell.org/>library for extracting graph representations of values from the GHC <http://haskell.org/ghc/> heap, which may then be further processed and/or translated to Graphviz <http://graphviz.org/> dot format to be visualized. The package website is at http://moonpatio.com/vacuum/, which contains a gallery section (which i intend to add to over time). The most recent version is 0.0.6, which is available on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vacuum/. Feedback, comments, and/or gallery submissions will be gratefully received via this email address (contrary to the email address currently on the vacuum Hackage page (i fail at setting up email :)). Matt

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Re: [Haskell-cafe] uvector package appendU: memory leak?
by Manlio Perillo 02 Apr '09

02 Apr '09

wren ng thornton ha scritto: > Manlio Perillo wrote: >> Since ratings for each customers are parsed "at the same time", using >> a plain list would consume a lot of memory, since stream fusion can >> only be executed at the end of the parsing. >> >> On the other hand, when I want to group ratings by movies, stream >> fusion seems to work fine. >> > [...] > > For the problem as you've discussed it, I'd suggest a different > approach: You can't fit all the data into memory at once, so you > shouldn't try to. You should write one program that takes in the > per-movie grouping of data and produces a per-user file as output. Well, creating 480189 files in a directory is not a very nice thing to do to a normal file system. I should arrange files in directory, but then this starts to become too complex. The solution I'm using now just works. It takes about 950 MB of memory and 35 minutes, but it's not a big problem since: 1) Once loaded, I can serialize the data in binary format 2) I think that the program can be parallelized, parsing subsets of the files in N threads, and then merging the maps. Using this method, should optimize array copying. The problem is that unionWith seems to be lazy, and there is no no strict variant; I'm not sure. > Then > have your second program read in the reorganized data and do fusion et al. > > This reduces the problem to just writing the PerMovie -> PerUser > program. Since you still can't fit all the data into memory, that means > you can't hope to write the per-user file in one go. The data *do* fit into memory, fortunately. > [...] > > Best of luck. > Thanks Manlio

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Zippers from any traversable [Was: Looking for practical examples of Zippers]
by oleg＠okmij.org 02 Apr '09

02 Apr '09

wren ng thornton wrote: > > how, for instance, turn a nested Map like > > > > Map Int (Map Int (Map String Double) > > > > into a "zipped" version. > You can't. Or rather, you can't unless you have access to the > implementation of the datastructure itself; and Data.Map doesn't provide > enough details to do it. Actually Data.Map does provide enough details: Data.Map is a member of Traversable and anything that supports Traversable (at the very least, provides something like mapM) can be turned into a Zipper. Generically. We do not need to know any details of a data structure (or if it is a data structure: the collection may well be ephemeral, whose elements are computed on the fly). Please see the enclosed code; the code defines a function tmod to interactively traverse the collection, displaying the elements one by one and offering to modify the current element, or quit the traversal. The enclosed code implements the zipper that can only move forward. Chung-chieh Shan has well described how to turn any one-directional zipper into bi-directional. Again generically. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/WalkZip3/ Although the enclosed code demonstrates the possibility of turning a Data.Map into a Zipper, one may wonder about the merits of that endeavour. Data.Map is a very rich data structure, with efficient means to focus on any given element and replace it (e.g., elemAt, replaceAt) and to incrementally deconstruct the map (deleteMax, deleteMin, minView, etc). Triple-nested maps can be processed just as effectively. The case for a tree of maps (which is essentially a file system) is described in http://okmij.org/ftp/Computation/Continuations.html#zipper-fs module ZT where import qualified Data.Traversable as T import Control.Monad.Cont import qualified Data.Map as M -- In the variant Z a k, a is the current, focused value -- evaluate (k Nothing) to move forward -- evaluate (k v) to replace the current value with v and move forward. data Zipper t a = ZDone (t a) | Z a (Maybe a -> Zipper t a) make_zipper :: T.Traversable t => t a -> Zipper t a make_zipper t = reset $ T.mapM f t >>= return . ZDone where f a = shift (\k -> return $ Z a (k . maybe a id)) zip_up :: Zipper t a -> t a zip_up (ZDone t) = t zip_up (Z _ k) = zip_up $ k Nothing reset :: Cont r r -> r reset m = runCont m id shift :: ((a -> r) -> Cont r r) -> Cont r a shift e = Cont (\k -> reset (e k)) -- Tests -- sample collections tmap = M.fromList [ (v,product [1..v]) | v <- [1..10] ] -- extract a few sample elements from the collection trav t = let (Z a1 k1) = make_zipper t (Z a2 k2) = k1 Nothing (Z a3 k3) = k2 Nothing (Z a4 k4) = k3 Nothing in [a1,a3,a4] travm = trav tmap -- Traverse and possibly modify elements of a collection tmod t = loop (make_zipper t) where loop (ZDone t) = putStrLn $ "Done\n: " ++ show t loop (Z a k) = do putStrLn $ "Current element: " ++ show a ask k ask k = do putStrLn "Enter Return, q or the replacement value: " getLine >>= check k check k "" = loop $ k Nothing check k "\r" = loop $ k Nothing check k ('q':_) = loop . ZDone . zip_up $ k Nothing check k s | [(n,_)] <- reads s = loop $ k (Just n) -- replace check k _ = putStrLn "Repeat" >> ask k testm = tmod tmap

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Parsing with Proof
by muad 02 Apr '09

02 Apr '09

Hi everyone on haskell-cafe, I am wondering about how to give a correctness prove of a simple parsing algorithm. I tried to think of a simple example but even in this case I don't know how. I have an abstract syntax: > data Parens = Empty | Wrap Parens | Append Parens Parens assumption (A), in Append x y, x and y are never Empty assumption (B), Strings only contain '(' and ')' characters and a printing function > print :: Parens -> String > print Empty = "" > print (Wrap m) = "(" ++ print m ++ ")" > print (Append x y) = print x ++ print y So the idea is that I would like to write parse :: String -> Maybe Parens, such that if it gives Nothing then the String was malformed, if it gives Just x then the string should be equal to print x. I think I can write such a function in a few different ways, but I don't know how to prove this code works, could anybody show me how? Thank you for any replies. -- View this message in context: http://www.nabble.com/Parsing-with-Proof-tp22814576p22814576.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: Definition of "tail recursive" wrt Folds
by Ben Franksen 02 Apr '09

02 Apr '09

Mark Spezzano wrote: > Just looking at the definitions for foldr and foldl I see that foldl is > (apparently) tail recursive while foldr is not. > > Why? > > Is it because foldl defers calling itself until last whereas foldr > evaluates itself as it runs? > > What, strictly speaking, is the definition of ”tail recursive” as opposed > to just “recursive”? An application of some function f inside another function g is in 'tail position' (or a 'tail call') if the result of applying f is the result of g. Operationally speaking, calling f is the very last thing g does. Tail calls can be optimized by a compiler (or interpreter) so that the call does not use additional stack; that is, the call can be replaced by a jump. A function is called ”tail recursive” (as opposed to just “recursive”) if the recursive call to itself is in tail position. If the compiler performs tail call optimization, tail recursive functions can work with constant stack space, similar to a (imperative) loop. Looking at a definition of foldl, e.g. foldl f z0 xs0 = lgo z0 xs0 where lgo z [] = z lgo z (x:xs) = lgo (f z x) xs you see that lgo calls itself in tail position, thus is tail recursive. In contrast, foldr can be defined as foldr k z xs = go xs where go [] = z go (y:ys) = y `k` go ys where you see that the result of go is not the recursive call to go. Instead, the result is y `k` go ys . Thus, foldr is not tail recursive. So, if you are saying that "foldl defers calling itself until last whereas foldr evaluates itself as it runs" then you got the right idea, I think. The point is that foldr still needs to do something (namely to apply (y `k`)) to the result of applying itself. It needs to remember to do so, and thus the stack grows linearly with the size of the list. Cheers Ben

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ANN: cmonad 0.1.1
by Lennart Augustsson 02 Apr '09

02 Apr '09

I've uploaded my CMonad package to Hackage. It allows you to write Haskell code in a C style. Unfortunately, GHC lacks certain optimizations to make efficient code when using CMonad, so instead of C speed you get low speed. Example: Computing some Fibonacci numbers: fib = do { a <- arrayU[40]; i <- auto 0; a[0] =: 1; a[1] =: 1; for (i =: 2, (i :: EIO Int) < 40, i += 1) $ do { a[i] =: a[i-1] + a[i-2]; }; retrn (a[39]); } Example: Copying stdin to stdout: cat = do { c <- auto 0; while ((c =: getchar()) >= 0) $ do { putchar(c); }; return (); } -- Lennart

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[SoC] XML Schema Implementation
by Vlad Dogaru 02 Apr '09

02 Apr '09

Hello everyone, I am writing to judge interest in a Summer of Code proposition: an XML Schema[1] implementation, described as a possbile application here[2]. As both a tool and an inspiration, I intend to use HaXML[3]. More specifically, I would be interested in the degree the Haskell community uses XML Schema, and if you were tempted to use it if we had an implementation. To further expand the question, how useful do you consider each of these components: * a validator * a pretty-printer * a translator from XML Schema to Haskell, similar to DtdToHaskell[4] For the latter item, some runtime checking will definitely be required: restrictions such as value ranges for integers or minimum and maximum occurences of an element cannot, to my knowledge, be enforced through type definitions. Finally, I kindly ask anyone with relevant experience to point out other possible pitfalls. Vlad Dogaru [1] http://www.w3schools.com/Schema/ [2] http://hackage.haskell.org/trac/summer-of-code/ticket/1120 [3] http://www.cs.york.ac.uk/fp/HaXml/ [4] http://www.cs.york.ac.uk/fp/HaXml/Xml2Haskell.html

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Re: [Haskell] Marketing Haskell
by Maarten Hazewinkel 01 Apr '09

01 Apr '09

That's not you usual Koala face. Must be a special Simon^H^H^H^H^HHaskell-Koala species. On 1 Apr 2009, at 10:07, Simon Peyton-Jones wrote: > > Dear Haskell enthusiasts, > > Now that the logo issue finally has been settled, it is time to select > the proper Haskell mascot. As you are no doubt aware, Microsoft's > involvement in Haskell means that we have moved from avoiding success > at all cost to actively marketing the language, and any language > striving for success is entirely dependent on a cute and distinctive > mascot. Where would Perl be today without its camel? > > Since the recent logo discussion has demonstrated once and for all the > futility of attempting a democratic process in the Haskell community - > to be quite honest, the elected logo looks like an error message > from an IBM > mainframe - I have decided to decide on a mascot myself. > > So I hereby declare the official Haskell mascot to be the koala, in > the form of the image attached below. Please ensure that this image > accompanies any material published on the web or on paper. > > Simon > > <haskell-mascot.jpg>_______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell Maarten Hazewinkel maarten.hazewinkel(a)gmail.com Tel: 06-53 692 432 (+31-653 692 432 from outside the Netherlands)

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DEFUN09: Call for Talks & Tutorials (co-located w/ ICFP09)
by Matthew Fluet (ICFP Publicity Chair) 01 Apr '09

01 Apr '09

Call for Talks and Tutorials ACM SIGPLAN 2009 Developer Tracks on Functional Programming http://www.defun2009.info/ Edinburgh, Scotland, September 3 and 5, 2009 The workshop will be held in conjunction with ICFP 2009 http://www.cs.nott.ac.uk/~gmh/icfp09.html Important dates Proposal Deadline: June 5, 2009, 0:00 UTC Notification: June 19, 2009 DEFUN 2009 invites functional programmers and researchers who know how to solve problems with functional progamming to give talks and lead tutorials at the The ICFP Developer Tracks. We want to know about your favorite programming techniques, powerful libraries, and engineering approaches you've used that the world should know about and apply to other projects. We want to know how to be productive using functional programming, write better code, and avoid common pitfalls. We invite proposals for presentations in the following categories. Lightning talks 5- to 10-minute talks that introduce exciting and promising research or techniques that may be in progress or not yet ready for widespread use, but that offer a glimpse into the near future of real world functional programming. Examples: * Clustered high performance computing in a functional language * Making advanced type systems more accessible to working programmers * How and why we're infiltrating category theory info industry How-to talks 45-minute "how-to" talks that provide specific information on how to solve specific problems using functional programming. These talks focus on concrete examples, but provide useful information for developers working on different projects or in different contexts. Examples: * "How I use Haskell for oilfield simulations." * "How I replaced /sbin/init by a Scheme program." * "How I hooked up my home appliances to an Erlang control system." * "How I got an SML program to drive my BMW." General language tutorials Half-day general language tutorials for specific functional languages, given by recognized experts for the respective languages. Technology tutorials Half-day tutorials on techniques, technologies, or solving specific problems in functional programming. Examples: * How to make the best use of specific FP programming techniques * How to inject FP into a development team used to more conventional technologies * How to connect FP to existing libraries / frameworks / platforms * How to deliver high-performance systems with FP * How to deliver high-reliability systems with FP Remember that your audience will include computing professionals who are not academics and who may not already be experts on functional programming. Presenters of tutorials will receive free registration to CUFP 2009. Submission guidelines Submit a proposal of 150 words or less for either a 45-minute talk with a short Q&A session at the end, or a 300-word-or-less proposal for a 3-hour tutorial, where you present your material, but also give participants a chance to practice it on their own laptops. Some advice: * Give it a simple and straightforward title or name; avoid fancy titles or puns that would make it harder for attendees to figure out what you'll be talking about. * Clearly identify the level of the talk: What knowledge should people have when they come to the presentation or tutorial? * Explain why people will want to attend: o Is the language or library useful for a wide range of attendees? o Is the pitfall you're identifying common enough that a wide range of attendees is likely to encounter it? * Explain what benefits attendees are expected to take home to their own projects. * For a tutorial, explain how you want to structure the time, and what you expect to have attendees to do on their laptops. List what software you'll expect attendees to have installed prior to coming. Submit your proposal in plain text electronically to defun-2009-submissions(a)serpentine.com by the beginning of Friday, June 5 2009, Universal Coordinated Time. Organizers * Yaron Minsky (Jane Street Capital) * Ulf Wiger (Erlang Training and Consulting) * Mike Sperber - co-chair (DeinProgramm) * Bryan O'Sullivan - co-chair (Linden Lab)

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Call for Contributions - Haskell Communities and Activities Report, May 2009 edition
by Janis Voigtlaender 01 Apr '09

01 Apr '09

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, I would very much like to collect contributions for the 16th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 1 May 2009 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the November 2008 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2009/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2009/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should receive your old entry as a template soon (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or ``historic'' overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep entries up. There is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports over distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write ``wishlist'' items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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[ANN] salvia-0.1, salvia-extras-0.1
by Sebastiaan Visser 01 Apr '09

01 Apr '09

Hi all, I am pleased to announce a new version of Salvia, the lightweight Haskell Web Server Framework with modular support for serving static files, directories indices, default error responses, connection counting and logging, HEAD and PUT requests, keep-alives, custom banner printing, default handler environments for parsing request and printing responses, dispatching based on request methods, URI, paths and filename extension, URI rewriting and redirection, virtual hosting, cookie, session and user management and more... Changes since previous version: - Some more advanced, non-fundamental handlers have been moved to their own package salvia-extras. This helps pruning the dependencies a bit. - The package now has some default handler environments that simplify setting up a server application. - The server now has support for keep-alive, significantly increasing the performance. - The library functions are now almost fully documented instead of no- documentation-at-all in the previous versions. - Salvia now also works on windows (I heard). - Lots of code cleanups throughout the code. - Lots of minor bug fixes. To install: use cabal. Thanks to the people that helped me with suggestions and bug-reports! -- Sebastiaan.

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Is there a way to see the equation reduction?
by Daryoush Mehrtash 01 Apr '09

01 Apr '09

I am trying to write out the execution of the recursive calls in mkDList example<http:/http://www.haskell.org/haskellwiki/Tying_the_Knot/www.haskell.org/haskellwiki/Tying_the_Knot>for different size lists. Is there a way in ghc, or ghci where for a given list I can see the intermediate recursive and evaluation steps? thanks, Daryoush

4 4

Building/Prerequisites->Linux->glibc-devel libedit-devel ?
by Windoze 01 Apr '09

01 Apr '09

Greetings, am considering learning how to do a build for my slackware-based distro. The doc sited below say's it requires: glibc-devel libedit-devel ncurses-devel gmp-devel .etc. Must I used the dev versions, or will it work with the lastest, stable release versions of same? thanks much for the assistance http://hackage.haskell.org/trac/ghc/wiki/Building/Prerequisites http://hackage.haskell.org/trac/ghc/wiki/Building/Prerequisites -- View this message in context: http://www.nabble.com/Building-Prerequisites-%3ELinux-%3Eglibc-devel--libed… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

QuesoGLC / cabal extra-libraries not used?
by Peter Verswyvelen 01 Apr '09

01 Apr '09

I'm busy writing my first library for Hackage - a wrapper for QuesoGLC, yet another OpenGL font renderer using Freetype2. So if someone else is already doing this, stop doing so :-) I've succesfully build the libquesoglc.a library from C source, and can build my Haskell tutorial file using ghc --make -lquesoglc Tutorial1.hs But I can't drop the -lquesoglc, then I get linker errors. However I did specify extra-libraries: quesoglc in the cabal file, and the package file got configured, build and installed So I must be doing something wrong, but can't figure out what. Thanks a lot, Peter PS: I'm on Windows

2 2

Re: [Haskell-cafe] about the Godel Numbering for untyped lambda calculus
by John Tromp 01 Apr '09

01 Apr '09

> I am reading the book "The lambda calculus: Its syntax and Semantics" in the > chapter about Godel Numbering but I am confused in some points. > > We know for Church Numerals, we have Cn = \fx.f^n(x) for some n>=0, > i.e. C0= \fx.x and C > 1 = \fx.fx. > > >From the above definition, I could guess the purpose of this kind of > encoding is trying to encode numeral via terms. > > How about the Godel Numbering? From definition we know people say #M is the > godel number of M and we also have [M] = C#M to enjoy the second fixed point > theorem : for all F there exists X s.t. F[X] = X. > > What the mapping function # is standing for? How could I use it? What the #M > will be? How to make use of the Godel Numbering? > > Thank you very much! My Wikipedia page on Binary Lambda Calculus. http://en.wikipedia.org/wiki/Binary_lambda_calculus shows a simple encoding of lc terms as bitstrings, which in turn are in 1-1 correspondence with the natural numbers. It also shows how to represent bitstrings as lambda terms, and gives an interpreter that extracts any closed term M from its encoding #M. It also give several examples of how all that is applicable to program size complexity. regards, -John

2 1

uvector package appendU: memory leak?
by Manlio Perillo 01 Apr '09

01 Apr '09

Hi. As with a previous post, I think I have found a possible memory problem with the uvector package. I have this data structure (for, again, my Netflix Prize project): IntMap (UArr (Word16 :*: Word8)) I was adding elements to the map using something like: v = map singletonU (a :*: b) insertWith appendU k v m However doing this eats a *lot* of memory. Today I have rewritten my program to use `alter` and `snocU`: append Nothing = Just $ singletonU (a :*: b) append (Just u) = Just $ snocU u (a :*: b) alter append k m This, finally, works. Unfortunately I'm still not able to load the entire Netflix Prize training data set, grouping ratings by customers, because my PC has only 2 GB of RAM. The required memory is about 2 GB, but when the system start to swap, I have to kill the program. So the question is: why appending an array of only one element to an existing array causes memory problems? This should be pratically the same as adding an element. Thanks Manlio

4 16

TMVar's are great but fail under ghc 6.10.1 windows
by Alberto G. Corona 01 Apr '09

01 Apr '09

Control.Concurrent.STM.TMVar's combine the best of MVars and TVars: -Unlike TVars, they permit blocking/early retry when the TMVar is being used by other process, so that complete processes are not retried at the end when happens that the TVars have been modified in the meantime. - Unlike MVars, they have no deadlocks, specially when a process is trying to adquire exclusive access to more than one TMVar. however, It happens that fails in my windows box with ghc 6.10.1 , single core here is the code and the results: -----------begin code: module Main where import Control.Concurrent.STM import Control.Concurrent import System.IO.Unsafe import GHC.Conc mtxs= unsafePerformIO $ mapM newTMVarIO $ take 5 $ repeat 0 proc i= atomically $ do unsafeIOToSTM $ putStr $ "init of process "++ show i++"\n" xs<- mapM takeTMVar mtxs mapM (\(mtx,x) ->putTMVar mtx (x+1)) $ zip mtxs xs xs' <- mapM readTMVar mtxs unsafeIOToSTM $ putStr $ "End of processs "++show i ++ " result= "++ show xs'++"\n" main=do mapM (forkIO . proc) [1..5] threadDelay 100000000 -------------end code the rigth result must be (occasionally with some "init of process x" repeated) init of process 1 init of process 2 End of processs 1 result= [1,1,1,1,1] init of process 2 -- retried End of processs 2 result= [2,2,2,2,2] init of process 3 End of processs 3 result= [3,3,3,3,3] init of process 4 End of processs 4 result= [4,4,4,4,4] init of process 5 End of processs 5 result= [5,5,5,5,5] under windows the program produces strange results for example init of process 1 init of process 2 init of process 3 init of process 4 init of process 5 End of processs 1 result= [1,1,1,1,1] End of processs 2 result= [1,1,1,1,1] (deadlock) or this other: init of process 1 init of process 2 init of process 3 init of process 4 init of process 5 End of processs 1 result= [1,1,1,1,1] End of processs 2 result= [1,1,1,1,1] init of process 3 init of process 4 init of process 5 init of process 2 End of processs 4 result= [2,2,2,2,2] End of processs 3 result= [2,2,2,2,2] End of processs 5 result= [2,2,2,2,2] End of processs 2 result= [2,2,2,2,2] init of process 3 init of process 5 init of process 2 End of processs 3 result= [3,3,3,3,3] End of processs 5 result= [3,3,3,3,3]

3 2

Google Summer of Code student application deadline approaches
by Johan Tibell 01 Apr '09

01 Apr '09

Hi, The Summer of Code student application deadline is April 3rd and we need more applications! If you have an idea that you would like to hack on this summer hurry up and apply. Here's an idea that could be implemented in a summer: A Sawzal [1]l like library that processes large volumes of data using monoids to compute aggregate statistics. Foldable makes this possible for smaller data sets than can fit in memory but many interesting data sets are tens or hundreds of gigabytes in size. A simple API with a high performance implementation would make Haskell a nice data analysis tool. Here's a strawman interface for such a library: -- | Given a file of log records compute aggregate statistics by converting each record -- to a monoid @m@ and combine the resulting monoids using 'mappend'. fold :: (Record r, Monoid m) => (r -> m) -> FilePath -> IO m There are lots of interesting optimizations that could be done. Starting with an efficient single threaded implementation using ByteString you could add the ability to either process many files in parallel or splitting one file into many chunks and process each chunk in parallel. The Wide Finder 2 [2] challenge has a fast Ocaml implementation of the latter strategy. One could take the library further by running the processing on multiple machines like in the Google Sawzall implementation. 1. http://research.google.com/archive/sawzall.html 2. http://www.tbray.org/ongoing/When/200x/2008/05/01/Wide-Finder-2 Cheers, Johan

1 0

satchmo: monadic SAT encoding library
by Johannes Waldmann 01 Apr '09

01 Apr '09

Dear all, I am releasing a preliminary version of "satchmo", a monadic library for encoding boolean and integral number constraints to CNF-SAT. It uses minisat (http://minisat.se/) as a backend solver. http://dfa.imn.htwk-leipzig.de/satchmo/ http://hackage.haskell.org/cgi-bin/hackage-scripts/package/satchmo Enjoy - J.W. PS: what would be the proper top level hackage module name for this? I guess it could be Constraint.SAT.Encoding ...

1 0

Re: [Haskell-cafe] abou the Godel Numbering for untyped lambda calculus
by Dan Weston 01 Apr '09

01 Apr '09

DISCLAIMER: The last I studied this was over 20 years ago, so my brain is seriously rusty. You are warned... > you mean the godel function (denoted by # and generally will be in type, > #:term -> number ?) is a kind of functions instead of a particular > function? No. The Godel mirror function (as defined by Godel) is a single function, the product of primes (whose ordering gives the position in the string) with exponents (giving the symbol at that position). However, I think any total injective function with partial injective inverse will do. Don't get hung up on the details of the mirroring (Godel doesn't). Once you accept that it can be done, there is no need to actually do it. There is nothing magic about integers, beyond the fact that elementary arithmetic is "well understood" by humans and should not have inconsistencies (it probably doesn't, but we can't prove it within any language more intuitive than arithmetic, so why should be believe the proof in that language?). Importantly, all but the last step of the proof can be encoded as primitive recursive functions, meaning that they always halt within a predetermined number of steps (satisfying the Finitists). Only the last step cannot be so limited. This is a clear example where non-finite mathematics has demonstrably more power in a fundamentally important way. Why bother going through this encoding? The main reason for the encoding is to overcome the fundamental limitation of structural induction whereby there is no "this" smaller than this. I.e. quoting a sentence only makes it longer. With functions on integers, this limitation is removed, so that although there is no string saying "This statement is not true", (since "This statement" != "\"This statement is not true\""), there *is* some function taking the Godel number of the unquoted This to the Godel number of the string to which it refers. Doing so takes some bit of cleverness, but the original proof in German is only about 8 pages long, as I recall. If we don't go with mirroring and just use strings, we need to introduce some equivalent trick, namely named functions. In a language without pointers and only lambda expressions, there is no way to refer to oneself without repeating the quote in infinite regression. For instance "\x -> 3333333" is longer than "333333". You cannot talk about a string without embedding it as a substring. However, once you add named functions to your metalanguage (Haskell): f 3 = 33333333 Then the string "f 3" is shorter than the string "xxxxxxx". Suppose f 4 = "~xxxxxxx". Now one can talk about the other with the shorter string on the left: length "f 3" < length "~xxxxxxx" but also length "f 4" < length "xxxxxxx". A longer string can talk about a shorter string (but not vice versa), so this mapping is critical to overcoming the limitation about talking about oneself. After that, the rest is easy: encode the Liar's Paradox and voila. once we have the encoding of a language, and construct functions to encode proof transformations, we can reduce derivability to a simple predicate derivable :: String -> Bool and look for some formula f such that derivable (f) == False and derivable("~" ++ f) == False. Godel's First Incompleteness Theorem merely says that there is some integer (which mirrors some well-formed formula) not in the set of integers that mirror "derivable theorems", making your system incomplete. Moreover, if you remedy this situation by adding this integer to the set as an axiom, you immediately can derive another integer which mirrors its negated formula, making your system inconsistent. The metasystem (with symbols for derivable, transformation rules like modus ponens, and the like), are not part of the formal system, but in the metalanguage. By encoding this metalanguage as well and redoing the above, you get Godel's Second Incompleteness Theorem, which says that no system can prove its own consistency, and if an axion is added to assert consistency, you can immediately also prove its negation, making it unsound. Don't look for a hat trick. Encoding the meta-meta-system buys you nothing extra. That's about all I can remember without hypnosis, and I'm afraid of what I might say under hypnosis... :) Dan Algebras Math wrote: > you mean the godel function (denoted by # and generally will be in type, > #:term -> number ?) is a kind of functions instead of a particular > function? If so, what kind of property this kinds of functions have? any > limitation? how capable they will be? > > Thanks > > alg > > On Tue, Mar 31, 2009 at 4:01 AM, Dan Weston <westondan(a)imageworks.com > <mailto:westondan@imageworks.com>> wrote: > > I can't tell exactly what you're asking, but I'll give it a try! :) > > primes :: [Integer] > primes = [2,3,5,7,11,13,17,19,23,29,31,undefined] > > godel :: String -> Integer > godel = product . zipWith (^) primes . map (toInteger . ord) > > -- Here is the identity function (note that double backslash is a > single character > godel "\\x -> x" > 1629920514342441477851613634029704631135430980107335428017064836156091059806450516131002598452000651374890570022191128551421788559667251656959733380229796871976693176094558797489489057311763922507760911447493016261397754567695234219623056037447490713525815311253273691703346455275115486717580778238094808661626709709766328915159206002675716287759792707200037683200000000000000000000000000000000 > > -- = 2^92 * 3^120 * 5^32 * 7^45 * 11^62 * 13^32 * 17^120 > > ungodel :: Integer -> String > -- ungodel . godel = id > > (See http://en.wikipedia.org/wiki/Fundamental_theorem_of_arithmetic > for why this works). > > These numbers get large quickly, but are finite for any finite > string of finite alphabet. > > godel is a total function (every string has a unique integer), and > its inverse (an exercise to the reader involving prime factor > decomposition) would also be total if the alphabet were infinite (so > that ord would be total). > > Frankly, I wouldn't know how to begin encoding the godel numbering > in the type system (shudder!), but being that it is a total > primitive recursive function, I suppose there is no theoretical > impediment. > > In contrast, Godel's Theorems themselves cannot be so encoded > because the last step involves a general recursive function. > > Dan > > > Algebras Math wrote: > > Hi all, > > I am reading the book "The lambda calculus: Its syntax and > Semantics" in the chapter about Godel Numbering but I am > confused in some points. > > We know for Church Numerals, we have Cn = \fx.f^n(x) for some > n>=0, i.e. C0 = \fx.x and C1 = \fx.fx. > > From the above definition, I could guess the purpose of this > kind of encoding is trying to encode numeral via terms. > > How about the Godel Numbering? From definition we know people > say #M is the godel number of M and we also have [M] = C#M to > enjoy the second fixed point theorem : for all F there exists X > s.t. F[X] = X. > > What the mapping function # is standing for? How could I use it? > What the #M will be? How to make use of the Godel Numbering? > > Thank you very much! > > alg > > >

1 0

ANNOUNCE: vacuum: extract graph representations of ghc heap values. (take 2)
by Matt Morrow 01 Apr '09

01 Apr '09

I am pleased to announce the release of vacuum, a library for extracting graph representations of values from the GHC heap, which may then be further processed and/or translated to Graphviz dot format to be visualized. The package website is at http://moonpatio.com/vacuum/ , which contains a gallery section (which i hope to add to over time). The most recent version is 0.0.6, which is available on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vacuum/ . Feedback, comments, and gallery submissions will be gratefully received via this email address (contrary to the email address currently on the vacuum Hackage page (i fail at setting up email :)). Matt

2 1

[Probably a dumb question] Quasiquoting
by Andrew Coppin 01 Apr '09

01 Apr '09

Can somebody explain to me how [$expr| 1 + 2 |] is different from ($expr "1 + 2") Other than a superficially different type signature, I'm not seeing what the fundamental difference is...

2 3

Haddock: inserting interactive sessions in the documentation
by Manlio Perillo 01 Apr '09

01 Apr '09

Hi. What is the suggested (if any) convention for inserting an interactive session in the documentation? Right know I'm doing (in random-shuffle package): -- |Convert a sequence @(e1...en)@ to a complete binary tree. -- -- @ -- System.Random.Shuffle> buildTree ['a','b','c','d','e'] -- Node 5 (Node 4 (Node 2 (Leaf 'a') (Leaf 'b')) -- (Node 2 (Leaf 'c') (Leaf 'd'))) -- (Leaf 'e') -- @ buildTree :: [a] -> Tree a As an example, in Python we use """ >>> 3 + 2 5 """ One nice feature of this special syntax is that it can be parsed, using doctest. Thanks Manlio

2 2

type-level programming support library
by spoon 31 Mar '09

31 Mar '09

I've been doing some basic work on a support library for type level programming ( see http://hackage.haskell.org/trac/summer-of-code/ticket/1541 ). I know there have been similar attempts using fundeps ( Edward Kmett showed me some of his work, but I've lost the address... ) but this approach uses type families. Anyway, I would like to hear your critique! Currently I have higher order type functions and ad-hoc parametrized functions. Here's what foldl looks like: type family Foldl ( func :: * -> * -> * ) val list type instance Foldl func val ( Cons first rest ) = Foldl func ( Eval ( func val first ) ) rest type instance Foldl func val Nill = val Notice the use of Eval - this is a trick to enable us to pass around data with kind * -> *, or whatever, and then trip this into becoming a value. Here's an example, using this trick to define factorial: -- multiplication type family Times x y type instance Times x Zero = Zero type instance Times x ( Succ y ) = Sum x ( Times x y ) -- The "first order" function version of Times data TimesL x y -- Where what Eval forced TimesL to become. type instance Eval ( TimesL x y ) = Times x y -- multiplies all the elements of list of Nat together type Product l = Foldl TimesL ( Succ Zero ) l -- here list to creates a list from ( Succ Zero ) to the given number type Factorial x = Product ( ListTo x ) We can now use the function like this: *TPrelude> result ( ) :: Factorial ( Succ ( Succ ( Succ ( Zero ) ) ) ) Succ (Succ (Succ (Succ (Succ (Succ Zero))))) Using the parametrized types kinda reminds me of programming in Erlang: -- What would conventionally be the monad type class, parametized over m type family Bind m ma ( f :: * -> * ) type family Return m a type family Sequence m ma mb Here's the maybe monad: -- Monad instance type instance Bind ( Maybe t ) Nothing f = Nothing type instance Bind ( Maybe t ) ( Just a ) f = Eval ( f a ) type instance Sequence ( Maybe t ) Nothing a = Nothing type instance Sequence ( Maybe t ) ( Just a ) b = b type instance Return ( Maybe t ) a = Just a type instance Eval ( Just x ) = Just x Here's an example: *TPrelude> result ( ) :: Bind ( Maybe Nat ) ( Just Zero ) Just Just Zero For more information and to download the loose collection of module implementing this please see: http://www.killersmurf.com/projects/typelib John Morrice

7 11

GHC on AIX
by nitralime 31 Mar '09

31 Mar '09

Hi, I have tried to install the newest available binary distribution of GHC for AIX (6.8.2). Unfortunately, it didn't work properly. Following the trivial installation steps as described in INSTALL file I executed the following commands: 1) ./configure --prefix=/home/nik/local 2) gmake install But I got the following error message: -------------------------------------------------------------------------------------------------------------------------------------- .... Reading package info from stdin ... done. warning: can't find GHCi lib HSrts.o Saving old package config file... done. Writing new package config file... done. /tmp/ghc-6.8.2/utils/mkdirhier/mkdirhier /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2 for i in libHSrts*.a ; do \ case $i in \ *.a) \ /tmp/ghc-6.8.2/install-sh -c -m 644 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2; \ : /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/`basename $i` ;; \ *.dll) \ /tmp/ghc-6.8.2/install-sh -c -m 644 -s $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2 ;; \ *.so) \ /tmp/ghc-6.8.2/install-sh -c -m 755 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2 ;; \ *.dylib) \ /tmp/ghc-6.8.2/install-sh -c -m 755 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2; \ install_name_tool -id /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/`basename $i` /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/`basename $i` ;; \ *) \ /tmp/ghc-6.8.2/install-sh -c -m 644 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2; \ esac; \ done gmake[1]: Leaving directory `/tmp/ghc-6.8.2/rts' gmake -C libraries install DOING_BIN_DIST=YES gmake[1]: Entering directory `/tmp/ghc-6.8.2/libraries' if ifBuildable/ifBuildable base; then \ cd base && \ ../installPackage/installPackage '/tmp/ghc-6.8.2/utils/ghc-pkg/ghc-pkg.bin' '/home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/package.conf' '/home/nik/local/ghc-6.8.2' '/home/nik/local/ghc-6.8.2' '/home/nik/local/ghc-6.8.2/bin' '/home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/lib' '/home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/lib' '/home/nik/local/ghc-6.8.2/share/ghc-6.8.2' '/home/nik/local/ghc-6.8.2/share/doc/ghc/libraries/$pkg' '/home/nik/local/ghc-6.8.2/share/doc/ghc/libraries/$pkg' '/home/nik/local/ghc-6.8.2/share/doc/ghc/libraries/$pkg' ; \ fi Installing: /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/lib/base-3.0.1.0 gmake[1]: *** [install.library.base] Error 127 gmake[1]: Leaving directory `/tmp/ghc-6.8.2/libraries' gmake: *** [install] Error 2 -------------------------------------------------------------------------------------------------------------------------------------- Am I missing something? Any help is very much appreciated! Regards Nik

1 0

Converting -fglasgow-exts into LANGUAGE pragmas
by Martijn van Steenbergen 31 Mar '09

31 Mar '09

Hello café, I want to replace the -fglasgow-exts in the snippet below by LANGUAGE pragmas. Rank2Types alone doesn't suffice. Which do I need to get the snippet to compile? > {-# OPTIONS -fglasgow-exts #-} > > module RunMud where > > import Control.Monad.State > > type Mud = StateT MudState IO > data MudState = MudState { mRunMud :: RunMud } > type RunMud = forall a. Mud a -> IO a > > getRunMud :: Mud RunMud > getRunMud = do > s <- get > return (mRunMud s) Thanks in advance, Martijn.

2 1

Re: Exception handling in numeric computations
by John Lato 31 Mar '09

31 Mar '09

> From: Donn Cave <donn(a)avvanta.com> > Quoth John Lato <jwlato(a)gmail.com>, > >> An exception is caused by some sort of interaction with the run-time >> system (frequently a hardware issue). The programmer typically can't >> check for these in advance, but can only attempt to recover after >> they've happened. >> >> An error is some sort of bug that should be fixed by the programmer. > > I have never felt that I really understood that one. Honestly, me neither, until recently. I'm only barely starting to understand it, and I do think there's a great deal of overlap. Even if an error is a bug that can be fixed by the programmer, certain exceptional situations can also be fixed by the programmer by handling the exception, even if they can't be detected in advance. In general I would say that I agree with JCC's responses. > > What about invalid inputs? Say someone encounters a disk full error, > and the resulting partly written file is now unreadable data for its > intended application because of an invalid file encoding? Is that > an exception, or a bug that should be fixed? > > My guess is that you'll say it's a bug, i.e., that application's > file decoding result should be an Either type that anticipates that > the file encoding may be invalid. Not necessarily. I would be satisfied with a Maybe :) I've encountered malformed data frequently enough that any decoders I write (and I have done so recently) would anticipate invalid data. > > I will also guess if the file is unreadable because of an external > I/O problem like no read access to file or filesystem, you would > similarly expect this to be treated like that - I mean, ideally, e.g., > hGetLine :: Handle -> IO (Either IOError String) Not necessarily, but possibly. The big difference, of course, is that decoding can be a pure operation, while reading never is. I personally wouldn't mind if hGetLine had the type you give. The way I see it, there are two advantages to exceptions in this case. The first is that it's very easy for exceptions to trickle up and be handled at a higher level. The second 'advantage' is that the programmer doesn't need to explicitly handle exceptions, whereas an Either would require at least a pattern match to use the resulting value. John

4 7

ghc: Var/Type length mismatch message - what does it mean?
by John Lask 31 Mar '09

31 Mar '09

consider the following types (from the paper: "Stream Fusion From Lists to Streams to Nothing at All") > data Stream a = forall s. Stream ( s ->(Step a s)) s > data Step a s = > Done > | Yield a s > | Skip s an instance of this data type is: > stream0 :: Stream () > stream0 = Stream (\ s -> Yield () s) () now consider: > data StreamM m a = forall s. StreamM ( s ->m (Step a s)) s and the following instance: > stream0IO :: StreamM IO () > stream0IO = StreamM (\ s -> return (Yield () s)) () loads ok, but as soon as we introduce the following constraint: > data Monad m => StreamM' m a = forall s. StreamM' ( s ->m (Step a s)) s > stream0IO' :: StreamM' IO () > stream0IO' = StreamM' (\ s -> return (Yield () s)) () we get the following message when we load into ghci (6.8.2) Var/Type length mismatch: [] [base:GHC.Base.(){(w) tc 40}] is this a bug? if not, what is this message telling us ?

3 2

Unique monad?
by Andrew Coppin 31 Mar '09

31 Mar '09

Maybe I'm just being blind here, but I don't see a monad transformer (or even a monad) in the standard libraries for producing "unique" values. Have I missed something? Related, I don't see a monad for gradually consuming input. We've got the Reader monad, but that appears to just give you global access to a single monolithic value. That value could be some kind of collection, but there's no way to track where you've read up to (or efficiently access that point). We've got Parsec, which is a pretty efficient and powerful text parser. But what if you don't need full parsing capabilities? You just need to consume input, but you're not exactly sure where you're going to consume it? The Writer monad seems to let you incrimentally write values out, but there's no matching monad for incrimentally reading stuff back in. (I guess ultimately you can build all these specialised monads out of the general State monad if you want. I'm just surprised they're not already defined somewhere...)

10 10

ANN: new release of HTTP, version 4000.0.5
by Sigbjorn Finne 31 Mar '09

31 Mar '09

Hi, a new version of the HTTP package - 4000.0.5 has been uploaded, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP Includes a bunch of fixes and cleanups (thanks to all that have reported&contributed), along with some (shock, horror) API documentation. Let me know if it appears to be well behaved and dependable. thanks --sigbjorn

1 0

abou the Godel Numbering for untyped lambda calculus
by Algebras Math 31 Mar '09

31 Mar '09

Hi all, I am reading the book "The lambda calculus: Its syntax and Semantics" in the chapter about Godel Numbering but I am confused in some points. We know for Church Numerals, we have Cn = \fx.f^n(x) for some n>=0, i.e. C0= \fx.x and C 1 = \fx.fx. >From the above definition, I could guess the purpose of this kind of encoding is trying to encode numeral via terms. How about the Godel Numbering? From definition we know people say #M is the godel number of M and we also have [M] = C#M to enjoy the second fixed point theorem : for all F there exists X s.t. F[X] = X. What the mapping function # is standing for? How could I use it? What the #M will be? How to make use of the Godel Numbering? Thank you very much! alg

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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 31 Mar '09

31 Mar '09

Thanks for the information. Michael --- On Mon, 3/30/09, Ryan Ingram <ryani.spam(a)gmail.com> wrote: From: Ryan Ingram <ryani.spam(a)gmail.com> Subject: Re: [Haskell-cafe] Rational and % operator remix To: "michael rice" <nowgate(a)yahoo.com> Cc: "Ketil Malde" <ketil(a)malde.org>, haskell-cafe(a)haskell.org Date: Monday, March 30, 2009, 1:21 PM 2009/3/30 michael rice <nowgate(a)yahoo.com>: > I'm still not sure what some of the error messages I was getting were about. > As I wrote the function I tried to be aware of what "mixed mode" operations > were kosher ala This is a mistake, but understandable given your lispy background; there aren't really "mixed mode" operations in Haskell. One thing that might be confusing you: Numeric literals are really calls to "fromInteger"; that is, "5" is really "fromInteger (5 :: Integer)" Generally you will find that :t in the interpreter is your friend: ghci> :t 1 1 :: (Num t) => t This says that the expression "1" can be of any type that is an instance of Num. ghci> :t (\a b -> a - b) (\a b -> a - b) :: (Num a) => a -> a -> a "-" takes two arguments that are of the same type, and returns something of that type. ghci> :m Data.Ratio ghci> :t (%) (%) :: (Integral a) => a -> a -> Ratio a "%" lifts objects from an integral type into a type that represents the ratio of two integers. ghci> :t floor floor :: (RealFrac a, Integral b) :: a -> b So "floor" is a "cast" operation that converts between any fractional type to any integral type. This is why you need to either use "fromInteger" or "%" on the result of "floor" to get it back as a Rational. -- ryan

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Re: [Haskell-cafe] Internationalization of Haskell code
by Henning Thielemann 31 Mar '09

31 Mar '09

Sorry, my reply got lost in a private mailbox. :-) On Sat, 28 Mar 2009, Henning Thielemann wrote: > On Sat, 28 Mar 2009, Martijn van Steenbergen wrote: > >> Hi Henning, >> >> Henning Thielemann wrote: >>> Thanks for working on that issue! The German translation is fine. I have >>> recently written a Wiki article on how to avoid IO in Haskell code without >>> resorting to unsafePerformIO where I incidentally chose gettext as an >>> example: >>> http://progandprog.blogspot.com/2009/03/i18n-and-haskell.html >> >> I don't think this is the link you meant to post. :-) > > Right, this would be too much of an incidence. I meant this one: > http://www.haskell.org/haskellwiki/Avoiding_IO

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Really need some help understanding a solution
by Günther Schmidt 31 Mar '09

31 Mar '09

Hi guys, I tried for days now to figure out a solution that Luke Palmer has presented me with, by myself, I'm getting nowhere. He has kindly provided me with this code: import Data.Monoid newtype IntTrie a = IntTrie [a] deriving Show singleton :: (Monoid a) => Int -> a -> IntTrie a singleton ch x = IntTrie $ replicate ch mempty ++ [x] ++ repeat mempty lookupTrie :: IntTrie a -> Int -> a lookupTrie (IntTrie xs) n = xs !! n instance (Monoid a) => Monoid (IntTrie a) where mempty = IntTrie (repeat mempty) mappend (IntTrie xs) (IntTrie ys) = IntTrie (infZipWith mappend xs ys) infZipWith f ~(x:xs) ~(y:ys) = f x y : infZipWith f xs ys test = mconcat [singleton (n `mod` 42) [n] | n <- [0..]] `lookupTrie` 10 It's supposed to eventually help me group a list of key value pairs and then further process them in a linear (streaming like) way. The original list being something like [('a', 23), ('b', 18), ('a', 34) ...]. There are couple of techniques employed in this solution, but I'm just guessing here. The keywords I've been looking up so far: Memmoization, Deforestation, Single Pass, Linear Map and some others. Can someone please fill me in? Günther

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Record updates
by Andrew Coppin 31 Mar '09

31 Mar '09

Haskell's record syntax is quite nice, for a number of reasons. However, suppose I have some record: data Foobar = Foobar {foo1, foo2, foo3...} Now suppose that foo3 :: [Int], and I want to prepend 5 to it. I end up having to write something like let v1 = v0 {foo3 = 5 : (foo3 v0)} If the field name isn't "foo" but something more descriptive, and the transformation to be applied to it is more intricate, you end up with quite a bit of code. In summary, record syntax gives you a nice way of replacing the value of one field with something else, but no easy way to *modify* the existing value somehow. Does anybody know of a way around this? Is there some trick I'm not seeing? Is there an extension or proposal that fixes this?

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Re: [Haskell-cafe] Re: Exception handling in numeric computations
by John Lato 31 Mar '09

31 Mar '09

On Wed, Mar 25, 2009 at 7:02 PM, Gregory Petrosyan <gregory.petrosyan(a)gmail.com> wrote: > First of all, thanks everybody for the discussion -- very interesting to read! > > Apologies if this is off-topic a bit. > > While reading, I have a feeling that proposed solutions are somewhat similar > to checked exceptions. And IMO they turned out to be more harmful than useful. Do you mean checked exceptions, or the proposed solutions? IMO the problem with checked exceptions is that they conflate two ideas by trying to use the existing exception framework to do something that can't otherwise be done by the language. This leads to something that doesn't work well for either use. Languages with checked exceptions usually use them for two purposes: 1. Exceptional conditions - disk full, CPU on fire, etc. 2. Error handling - invalid arguments to a function, attempt to invert non-invertible matrix, etc. In the first case, checked exceptions are a pain because there are so many different possible exceptional conditions to enumerate. Or alternatively you have an IOException that can be one of hundreds of actual exceptional conditions. This is exactly what regular exceptions are for, and in general there's little that can be done except terminate the program, except for certain special cases explicitly handled by the programmer. For the second case, checked exceptions actually work okay IMO. The syntax is a bit clunky, but they serve the purpose. That is, they specifically indicate what functions may fail and the failure mode. They also allow calling code to either handle it there or pass the error up to a higher level as appropriate. The syntax is usually ugly, though. Combining these two functions into one tool gives suboptimal results. If you're using checked exceptions for error handling, then you also end up using them for exception handling. That means you have IOException thrown by just about everything (and in theory any function in an imperative language could have an IOException due to hardware fault), leading to complex exception handlers with a half dozen case statements. There are a few reasons why Haskell's approach is (or at least can be) different. Compare the error handling models. Checked exceptions are bolted on to an existing tool attempting to make it serve a different purpose. It seems like a good idea, but ends up being painful because the two uses are at cross purposes. Instead of using a specific implementation and trying to alter it for another use, Haskell uses a very general facility, the type system, to solve one problem, and keeps the exception handling facility separate. Haskell's syntax for error handling is also much nicer. Nobody likes using methods that could possibly throw a dozen different exceptions, some of which are IOException and some are not. Actually, Haskell IO makes a big difference to this. Conceptually, you could think of Haskell functions as purely mathematical constructs. In particular, they can't interact with the physical world. This is true even for the IO monad. Exceptional conditions only arise when the Haskell runtime actually tries to *evaluate* an IO function. As a consequence exceptional conditions only arise as part of IO actions, and can only be handled by IO actions (because they're interacting with the run-time environment). This distinction makes it possible to enforce a separation between exceptions and error conditions, which is generally not possible in imperative languages where a function could fail either for a computational reason (divide by 0) or a hardware/runtime fault. Even in Haskell this separation isn't absolute. Programmer errors, such as dividing by 0, can and do lead to exceptional conditions. The proper way to handle dividing by 0 is to not do it in the first place, but if it happens because of a programming error, you've got an exception. Unfortunately this encourages programmers to think that handling the exception is the proper way to deal with this condition, but it isn't. I've only recently come around to the camp of treating exception handling and errors separately, so some of these thoughts may be a bit loose for the moment. In particular my thoughts from the above paragraph have only recently become clear. Henning T., FYI your constant advocacy has gotten at least one person around to this view. Cheers, John Lato

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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 31 Mar '09

31 Mar '09

My query was in response to an earlier post claiming that ai was a Rational, which post he almost simultaneously said to ignore. I've used GHC and like it better than Hugs, but the Fedora Linux package is humongous and I'm only on dialup right now. I'm still not sure what some of the error messages I was getting were about. As I wrote the function I tried to be aware of what "mixed mode" operations were kosher ala Main> 1 / (6%7) 7 % 6 Main> 1 - (6%7) 1 % 7 Main> (6%7) - 1 (-1) % 7 Main> but still kept getting kicked out. Better error messages certainly couldn't hurt. Thanks. Michael --- On Mon, 3/30/09, Ketil Malde <ketil(a)malde.org> wrote: From: Ketil Malde <ketil(a)malde.org> Subject: Re: [Haskell-cafe] Rational and % operator remix To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, haskell-cafe(a)haskell.org Date: Monday, March 30, 2009, 2:48 AM michael rice <nowgate(a)yahoo.com> writes: > cf2 :: Rational -> [Int] > cf2 a = let ai = floor a <-- Doesn't this make ai an Int? -Michael > in > if a == (toRational ai) > then [ai] > else ai : cf2 (1 / (a - ai)) One thing that you could try, is ghci in addition to (or instead of) Hugs. That will give you another take on the error messages, where Hugs says: > Main> :load cf.hs > ERROR "cf.hs":7 - Type error in application > *** Expression : ai : cf2 (1 / (a - ai)) > *** Term : ai > *** Type : Ratio Integer > *** Does not match : Int ghci will tell you: Couldn't match expected type `Rational' against inferred type `Int' In the second argument of `(-)', namely `ai' In the second argument of `(/)', namely `(a - ai)' In the first argument of `cf2', namely `(1 / (a - ai))' So while this is (almost) right: > cf2 a = let ai = floor a <-- Doesn't this make ai an Int? -Michael The problem is the subtraction of ai from a, which forces them to be the same type. I tend to find ghc's messages more informative than Hugs's (but perhaps it is just that I am more familiar with ghc?). -k -- If I haven't seen further, it is by standing in the footprints of giants

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Darcs - dependencies between repositories (aka forests)
by Peter Verswyvelen 30 Mar '09

30 Mar '09

With Cabal we have this nice dependency system: when you install a package, it installs missing but requires packages. How do you solve this with Darcs? I hate it that I must manually figure out the dependencies, and then do a "darcs get" on them individually.

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Problem using #define in .hsc files
by Johan Tibell 30 Mar '09

30 Mar '09

Hi, I'm trying to have hsc2hs interpret this snippet that picks a calling convention to use for FFI imports: #if !defined(CALLCONV) #ifdef WITH_WINSOCK #define CALLCONV stdcall #else #define CALLCONV ccall #endif #endif foreign import CALLCONV unsafe "send" c_send :: CInt -> Ptr a -> CSize -> CInt -> IO CInt Compiling results in: parse error on input `CALLCONV' Inspecting the generated .hs file shows that CALLCONV appear in the source code instead of being replaced with either stdcall or ccall as expected. What's the correct way to use #define in .hsc files? Changing the file extension to .cpphs solves the problem, by using plain CPP instead of hsc2hs I presume. Cheers, Johan

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A guess on stack-overflows - thunks build-up and tail recursion
by Günther Schmidt 30 Mar '09

30 Mar '09

Hi all, I've been running into stack-overflow problems for some time now. Here is what I gathered so far. I used to think that the build up of thunks caused the stack overflow when, as it turns out, it does not. I apparently can have a huge thunk build up eventhough I use a supposedly accumulative, tail-recursive algorithm. Apparently it is the evaluation of this huge build-up that causes the stack-overflow but not the thunk-build-up *as such*. Do I understand this correctly? Günther

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ANNOUNCE: wxAsteroids 1.0
by Henk-Jan van Tuyl 30 Mar '09

30 Mar '09

Your space ship enters an asteroid belt, try to avoid collisions! wxAsteroids is a game demonstrating the wxHaskell GUI. More about this at: http://www.haskell.org/haskellwiki/wxAsteroids -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 30 Mar '09

30 Mar '09

cf2 :: Rational -> [Int] cf2 a = let ai = floor a <-- Doesn't this make ai an Int? -Michael in if a == (toRational ai) then [ai] else ai : cf2 (1 / (a - ai)) --- On Sun, 3/29/09, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: From: Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> Subject: Re: [Haskell-cafe] Rational and % operator remix To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "Lennart Augustsson" <lennart(a)augustsson.net>, haskell-cafe(a)haskell.org Date: Sunday, March 29, 2009, 4:27 PM On 2009 Mar 29, at 16:19, michael rice wrote:but I'm still at least one error short of a clean run Main> :load cf.hs ERROR "cf.hs":7 - Type error in application *** Expression : ai : cf2 (1 / (a - ai)) *** Term : ai *** Type : Ratio Integer *** Does not match : Int Where did I go wrong or what did I leave out? cf2 returns [Int], but you're using it as the tail of a list starting with ai, which is a Rational. Need to decide which you actually want. At a guess: > cf2 a = let ai = floor a> rai = toRational ai> in> if a == rai> then [ai]> else rai : cf2 (1 / (a - ai)) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.comsystem administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.eduelectrical and computer engineering, carnegie mellon university KF8NH

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Is Text.XHtml.Table usable?
by Thomas Hartman 30 Mar '09

30 Mar '09

I was playing with Text.XHtml.Table but couldn't use it to output tables. ( cell . toHtml $ " a " ) `beside` (cell . toHtml $ " b " ) <tr > a b </tr > already seems wrong -- should be two cells, right? And the result doesn't get embedded in a table tag? Is there something I'm missing? Working code samples would be great.

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