- Haskell-Cafe - Haskell.org

HDF5
by Tim Sears 27 Oct '10

27 Oct '10

Does anybody know of any utilities in haskell for reading and writing HDF5 files? Other number crunchy formats? Thanks.--Tim

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Re: [Haskell-cafe] Map constructor in a DSL
by Brandon Moore 27 Oct '10

27 Oct '10

You have to figure out here how you can use the function which is the argument to map. It seems you want to end up with a function of type a -> Evaluator b, to be used as an argument to mapM. One idea is to add a new constructor Var to represent variables, and something like evalVar :: Obs b -> a -> Evaluator b which handles Var by "return"ing the second argument, and everything else as in evalObs. Then you can evaluate Map like evalObs (Map f) as = evalObs as >>= mapM (evalVar (f Var)) Except, this won't quit work because the type for evalVar is wrong. That can be fixed by adding an extra parameter for the variable type: data Obs varType a where ... Var :: Obs v v Map :: (Obs a a -> Obs a b) -> Obs v [a] -> Obs v [b] or something like that. Brandon

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problem with happy
by Roman Dzvinkovsky 26 Oct '10

26 Oct '10

Hi, I've tried to make simple monadic happy parser with monadic lexer, but even trivial parser (attached) outputs: Ok "\r\n" instead of string from 'sample' file. Seems I can't spot error by myself, so I need your help, guys. Thanks in advance, Roman.

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Parsec in Haskell platform
by Roman Cheplyaka 26 Oct '10

26 Oct '10

Since Parsec 2 and Parsec 3 are usually considered to be different packages, it's ambiguous to say that 'parsec' is included in Haskell Platform [1]. Could someone please: 1. Clarify which version(s) are included? 2. Fix the page [1] It would be convenient to have a page which would list all the HP packages with their versions. The release page [2] only has a list of packages whose versions has changed since the last release, as I understood. [1] http://hackage.haskell.org/platform/contents.html [2] http://hackage.haskell.org/platform/changelog.html -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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[ANNOUNCE] MissingPy 0.10.5
by Matt Brown 26 Oct '10

26 Oct '10

Hello all, I've recently taken over maintenance of MissingPy, and pushed version 0.10.5 to hackage[1]. This includes two notable improvements: - fix build errors for python >= 2.5 - allow calling into multi-threaded python code (requires python >= 2.3). I'm trying to provide support for as many python versions as possible, though I still have more testing to do. Help in this area would be most appreciated. Please report any bugs via my github repo [2]. Examples can be found in the testsrc directory. A good introduction by Tim Lopez is also available [3]. withGIL provides the easiest way to call into threaded python code. I haven't added any examples of its usage yet, but the gist of it is: > withGIL $ do <some python stuff> I'll try to add some actual examples soon. Thanks, -matt [1]: http://hackage.haskell.org/package/MissingPy [2]: http://github.com/softmechanics/missingpy/issues [3]: http://www.brool.com/index.php/using-missingpy

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Functional Programming in Industry
by Jeroen Janssen 26 Oct '10

26 Oct '10

Dear all, I am pleased to announce to you that, on the occasion of BelHac, the first Belgian Haskell Hackathon from 5-7 Nov 2010, the Ghent Functional Programming Group is organizing a symposium on "Functional Programming in Industry", to which you are all cordially invited. These talks will be held on Friday, November 5th at 17:00 in the Jozef Plateauzaal in the Plateau building of Ghent University (Jozef Plateaustraat 22, 9000 Gent). The program is as follows: - Duncan Coutts* (Well-Typed, http://www.well-typed.com/, BelHac sponsor) who will be talking on how Well-Typed are providing consultancy services for Haskell, a pure functional programming language. - Romain Slootmaekers (Incubaid, http://www.incubaid.com, BelHac sponsor) who will be talking on the use of functional programming in Incubaid, a company based in the Ghent area. - Don Stewart (Galois, http://www.galois.com) who will be talking on how Galois is using Haskell in many of their projects, including government contracts. The talks will be followed by a reception to which all attendees are cordially invited. * Actual speaker has yet to be confirmed, but the talk will take place. Kind Regards, The GhentFPG Organizing Committee.

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Working Generic/Polytypic Haskell extension
by mulhern 26 Oct '10

26 Oct '10

Hi all, I would like to teach a small section on polytypism/genericity in the "functional programming using Haskell" course I'm teaching. I won't, though, unless I can assign an actual programming exercise in polytypic programming, however brief. Can anybody recommend a functioning compiler that I can reasonably require the students to use in doing this assignment? I am not up-to-date on the current state of research in this topic in Haskell and hope that some people on the list can help me out. Thanks! - mulhern

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Call for Contributions - Haskell Communities and Activities Report, November 2010 edition
by Janis Voigtländer 26 Oct '10

26 Oct '10

Dear all, It is time to collect contributions for the 19th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 1 November 2010 (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 an existing project related to Haskell that has not previously been mentioned in the HCAR, 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 May 2010 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/11-2010/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2010/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 already have received your old entry as a template (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: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. If you have several entries to submit, it helps if you send them in separate emails. 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 up entries. There is no minimum length of an entry! The report aims for 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 about 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 typically 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.

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Re: [Haskell-cafe] running and understanding a lifting program
by Patrick Browne 26 Oct '10

26 Oct '10

Patrick, Thanks for taking the time to get the program running. It seems fine, but I cannot get the *md* to print out, probably missing the Show class somewhere. Thanks again, Pat Patrick LeBoutillier wrote: > Patrick, > > I found this program interesting and decided to spend a bit of time on > it, even though I'm still a newbie. > I did a few things to simplify the code, here are some comments: > > 1) I chose to rename the arithmetic functions in the Number class > instead of trying to overload the "real" ones, I'm not that good at > Haskell yet... > > 2) The program had some errors, namely I think the definition of the > Point type should be: > > data Point a = Point a a > > to allow for different types of Points. > > 3) The Points class seemed useless in the end, I simply defined the > dist function at the top level. > > 4) If you import Control.Monad, it makes functions (and therefore > "Changing v") into a Monad (maybe my terminology is off here...) and > allows you to use the general liftM and liftM2 lifting functions > instead of defining your own. > > > Here's the complete program: > > {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} > > data Point a = Point { x ::a, y :: a } > type Time = Float > > -- Functor Changing, which adds time parameter t to its input value. > -- For example, Changing Float indicates a changing floating number > -- (i.e. a function of time). > type Changing v = Time -> v > > -- Lifting functions > lift1 op a = \t -> op (a t) > lift2 op a b = \t -> op (a t) (b t) > > class Number a where > add, sub, mul :: a -> a -> a > square, squareRoot :: a -> a > square a = a `mul` a > > instance Number Float where > add = (+) > sub = (-) > mul = (*) > squareRoot = sqrt > > instance Number (Changing Float) where > add = lift2 add > sub = lift2 sub > mul = lift2 mul > squareRoot = lift1 squareRoot > > -- The distance operation is defined as follow > dist :: Number a => Point a -> Point a -> a > dist a b = squareRoot $ square((x a) `sub` (x b)) `add` square ((y a) > `sub` (y b)) > > -- Running the code > -- If p1 and p2 are two 2D static points, > -- their distance d is calculated as follows: > p1, p2 :: Point Float > p1 = Point 3.4 5.5 > p2 = Point 4.5 4.5 > > -- distance between p1 and p2 --> 1.55 > d = dist p1 p2 > > -- For 2D moving points mp1 and mp2, their distance md, > -- which is a function of time, is calculated as follows: > mp1, mp2 :: Point (Changing Float) > mp1 = Point (\t -> 4.0 + 0.5 * t) (\t -> 4.0 - 0.5 * t) > mp2 = Point (\t -> 0.0 + 1.0 * t) (\t -> 0.0 - 1.0 * t) > -- distance between mp1 and mp2 > md = dist mp1 mp2 > -- distance md for time 2 ----> 5.83 > > >> >> This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > Patrick > This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Re: [Haskell-cafe] Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)
by caseyh＠istar.ca 25 Oct '10

25 Oct '10

Quoting Aleksandar Dimitrov <aleks.dimitrov(a)googlemail.com>: > Hi, > >> Is it possible to "easily" connect Haskell to JavaScript/JavaFX in >> the browser and use a browser as a Windows GUI? :) > > I don't exactly know what you mean by "connect," but here's an LtU > thread that might give you some hints: > http://lambda-the-ultimate.org/node/1836 > > Regards, > Aleks > I meant call Haskell functions from a browser; so the browser handles the GUI widgets and Haskell handles the processing. :)

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type level program
by Patrick Browne 25 Oct '10

25 Oct '10

Hi, I hypothesize that at type level Haskell offers a form of equational logic. At type level the following program[1] could be interpreted as a first order program in equational logic where; 1)Data types represent declarations of constructors (both constants and functions) 2)Type synonyms represent equations assigning constructors to variables 3)Each class contains a non-constructor operation signature with dependencies 4) instances contain equations that define the operations (similar the term rewriting systems (TRS) of Maude/CafeOBJ). 5):t acts as a reduction or evaluation at type level Is the a reasonable description of this program? Regards, Pat [1] From Fritz Ruehr http://www.willamette.edu/~fruehr/haskell/evolution.html -- static Peano constructors and numerals data Zero data Succ n type One = Succ Zero type Two = Succ One type Three = Succ Two type Four = Succ Three -- dynamic representatives for static Peanos zero = undefined :: Zero one = undefined :: One two = undefined :: Two three = undefined :: Three four = undefined :: Four -- addition, a la Prolog class Add a b c | a b -> c where add :: a -> b -> c instance Add Zero b b instance Add a b c => Add (Succ a) b (Succ c) -- multiplication, a la Prolog class Mul a b c | a b -> c where mul :: a -> b -> c instance Mul Zero b Zero instance (Mul a b c, Add b c d) => Mul (Succ a) b d -- factorial, a la Prolog class Fac a b | a -> b where fac :: a -> b instance Fac Zero One instance (Fac n k, Mul (Succ n) k m) => Fac (Succ n) m -- try, for "instance" (sorry): -- -- :t fac four This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Re: [Haskell] Specification and prover for Haskell
by Dominique Devriese 25 Oct '10

25 Oct '10

Romain, 2010/10/25 Romain Demeyer <rde(a)info.fundp.ac.be>: > I'm working on static verification in Haskell, and I search for existing > works on specification of Haskell programs (such as pre/post conditions, for > example) or any other functional language. It would be great if there exists > a prover based on this kind of specifications. I already found the > ESC/Haskell. Do you know some other works which could be interesting? I found the paper "Verifying Haskell using constructive type theory" [1] interesting... Dominique Footnotes: [1] http://www.mimuw.edu.pl/~ben/Papers/monadic.pdf

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Converting Values Between Lua And Haskell
by aditya siram 25 Oct '10

25 Oct '10

Hi all, The HsLua page [1] says that Int,Double,String,Bool,[a] and [(a,b)] types can be converted to and from Lua values. However the on hslua API page I don't see a StackValue instance [2] for [a] or [(a,b)]. Am I missing something? -deech [1] http://www.haskell.org/haskellwiki/HsLua#Exchanging_data_between_Haskell_an… [2] http://hackage.haskell.org/packages/archive/hslua/latest/doc/html/Scripting…

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ANNOUNCE: iteratee-compress 0.1.1
by Maciej Piechotka 25 Oct '10

25 Oct '10

Iteratee-compress provides compressing and decompressing enumerators including flushing. Currently only gzip is provided but at least bzip is planned. Changes from previous version: - Independent from zlib library (Haskell one, not C) - Allow hand-flushing the contents (from outside). - Fix potential memory-leak Next goals: - BZip support - Generic interface for flushing To think about: - Should inner iteratee be able to request flushing? Regards PS. It did change API by removing dependency on zlib but I home such breakage in 0.1.x will be allowed

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Eta-expansion and existentials (or: types destroy my laziness)
by Max Bolingbroke 24 Oct '10

24 Oct '10

Hi Cafe, I've run across a problem with my use of existential data types, whereby programs using them are forced to become too strict, and I'm looking for possible solutions to the problem. I'm going to explain what I mean by using a literate Haskell program. First, the preliminaries: > {-# LANGUAGE ExistentialQuantification #-} > import Control.Arrow (second) > import Unsafe.Coerce Let's start with a simple example of an existential data type: > data Stream a = forall s. Stream s (s -> Maybe (a, s)) This is a simplified version of the type of streams from the stream fusion paper by Coutts et al. The idea is that if you have a stream, you have some initial state s, which you can feed to a step function. The step function either says "Stop! The stream has ended!", or yields an element of the stream and an updated state. The use of an existential quantifier is essential to this code, because it means that we can write most functions on Streams in a non-recursive fashion. This in turn makes them amenable to inlining and simplification by GHC, which gives the us the loop fusion we need. An example stream is that of natural numbers: > nats :: Stream Int > nats = Stream 0 (\n -> Just (n, n + 1)) Here, the state is just the next natural number to output. We can also build the list constructors as functions on streams: > nil :: Stream a > nil = Stream () (const Nothing) > cons :: a -> Stream a -> Stream a > cons a (Stream s step) = Stream Nothing (maybe (Just (a, Just s)) (fmap (second Just) . step)) List functions can also easily be expressed: > taken :: Int -> Stream a -> Stream a > taken n (Stream s step) = Stream (n, s) (\(n, s) -> if n <= 0 then Nothing else maybe Nothing (\(a, s) -> Just (a, (n - 1, s))) (step s)) To see this all in action, we will need a Show instance for streams. Note how this code implements a loop where it repeatedly steps the stream (starting from the initial state): > instance Show a => Show (Stream a) where > showsPrec _ (Stream s step) k = '[' : go s > where go s = maybe (']' : k) (\(a, s) -> shows a . showString ", " $ go s) (step s) We can now run code like this: > test1 = do > print $ (nil :: Stream Int) -- [] > print $ cons 1 nil -- [1, ] > print $ taken 1 $ cons 1 $ cons 2 nil -- [1, ] Now we may wish to define infinite streams using value recursion: > ones :: Stream Int > ones = cons 1 ones Unfortunately, 'ones' is just _|_! The reason is that cons is strict in its second argument. The problem I have is that there is no way to define cons which is simultaneously: 1. Lazy in the tail of the list 2. Type safe 3. Non-recursive If you relax any of these constraints it becomes possible. For example, if we don't care about using only non-recursive functions we can get the cons we want by taking a roundtrip through the skolemized (existiental-eliminated - see http://okmij.org/ftp/Computation/Existentials.html) version of streams: > data StreamSK a = StreamSK (Maybe (a, StreamSK a)) > > skolemize :: Stream a -> StreamSK a > skolemize (Stream s step) = StreamSK (fmap (\(a, s') -> (a, skolemize (Stream s' step))) $ step s) > > unskolemize :: StreamSK a -> Stream a > unskolemize streamsk = Stream streamsk (\(StreamSK next) -> next) > > instance Show a => Show (StreamSK a) where > showsPrec _ (StreamSK mb_next) k = maybe (']' : k) (\(a, ssk) -> shows a . showString ", " $ shows ssk k) mb_next Now we can define: > cons_internally_recursive x stream = cons x (unskolemize (skolemize stream)) This works because unskolemize (skolemize stream) != _|_ even if stream is bottom. However, this is a non-starter because GHC isn't able to fuse together the (recursive) skolemize function with any consumer of it (e.g. unskolemize). In fact, to define a correct cons it would be sufficient to have some function (eta :: Stream a -> Stream a) such that (eta s) has the same semantics as s, except that eta s /= _|_ for any s. I call this function eta because it corresponds to classical eta expansion. We can define a type class for such operations with a number of interesting instances: > class Eta a where > -- eta a /= _|_ > eta :: a -> a > > instance Eta (a, b) where > eta ~(a, b) = (a, b) > > instance Eta (a -> b) where > eta f = \x -> f x > > instance Eta (StreamSK a) where > eta ~(StreamSK a) = StreamSK a If we had an instance for Eta (Stream a) we could define a lazy cons function: > cons_lazy :: a -> Stream a -> Stream a > cons_lazy x stream = cons x (eta stream) As we have already seen, one candidate instance is that where eta = unskolemize . skolemize, but I've already ruled that possibility out. Given that constraint, the only option that I see is this: > instance Eta (Stream a) where > -- Doesn't type check, even though it "can't go wrong": > --eta stream = Stream (case stream of Stream s _ -> s) (case stream of Stream _ step -> step) > eta stream = Stream (case stream of Stream s _ -> unsafeCoerce s :: ()) (case stream of Stream _ step -> unsafeCoerce step) I've had to use unsafeCoerce to make this go through, because I can't see any type-correct way of defining eta-expansion of existential data types - and indeed GHC complains if you try to use the ~pattern notation to define it. Nonetheless, it is "obvious" that 'eta' can't actually go wrong (in Milners's sense of the term). Now we can define the infinite stream we originally wanted: > lazy_ones :: Stream Int > lazy_ones = cons_lazy 1 lazy_ones > > test2 = print $ taken 2 $ lazy_ones -- [1, 1, ] Finally, here is some code to actually run the tests: > main = do > test1 > test2 So, cafe: is there any way to define a cons that satisfies my three earlier conditions: 1. Lazy in the tail of the list 2. Type safe 3. Non-recursive Or am I going to have to stick with my slightly unsafe eta that uses unsafeCoerce? Cheers, Max

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"back doors" into the IO monad
by Manlio Perillo 24 Oct '10

24 Oct '10

Hi. What are the available methods to execute IO actions from pure code? I know only unsafePerformIO and foreign import (to call a non pure foreign function). Assuming I want to execute external untrusted code using plugins (via the `plugins` package), is it possible to completely forbid execution of IO actions from user code? Thanks Manlio

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Haddock: patch to generate single-index page in addition to perl-letter indexes indices.
by Ryan Newton 24 Oct '10

24 Oct '10

When I encounter a split-index (A-Z) page it can be quite frustrating if I don't know the first letter of what I'm searching for. I want to use my browser find! For example, tonight I wanted to look at all the functions that END in "Window" in the Chart package -- no luck: * http://hackage.haskell.org/packages/archive/Chart/0.13.1/doc/html/doc-index… * * * *Therefore I propose that even when generating the "A-Z" individual pages that there also be an "All" option for the single-page version. Attached is a patch against haddock's HEAD (darcs get http://code.haskell.org/haddock/right?) that implements this behavior. As an example, here is FGL's documentation built with the patched haddock:* * * * http://people.csail.mit.edu/newton/fgl_example_doc/doc-index.html* The great thing about hackage being centralized is that if people are happy with this fix it can be widely deployed where it counts, and quickly! *Cheers,* * -Ryan* * * P.S. At the other end of the spectrum, when considering a central index for all of hackage (as in the below ticket) maybe it would be necessary to have more than 26 pages, I.e. Aa-Am | An-Az or whatever. http://hackage.haskell.org/trac/hackage/ticket/516#comment:6

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Re: [Haskell-cafe] concurrency vs. I/O in GHC
by John Lato 24 Oct '10

24 Oct '10

> > From: Donn Cave <donn(a)avvanta.com> > > [I wrote initially, ...] > > > As I have migrated more of my application into Haskell, I find that > > I/O in one thread effectively blocks other threads. > > Resolved - the SSL_read external I/O function needs to be "safe". If anyone is listening, I would very much like for there to be a mechanism by which external functions can be called "unsafe"-ly, but without blocking all other Haskell threads. I have code that does this: calcUpdate a b c = do expensiveComputation a b expensiveComputation a c "expensiveComputation" mutates the second argument (b or c here), but does not call back into Haskell or update "a" and I would very much like to run these two in parallel, however it's not possible if "expensiveComputation" is unsafe. If I make "expensiveComputation" safe, the extra time necessary for safe-ness outweighs the gains from parallelization for even reasonably-sized inputs (the expensiveComputation is called 100k's of times or more). My current plan is to handle the parallelization in C, but I'd prefer to do it in Haskell if possible. John

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Splay tree in a pattern matching way
by Xinyu LIU 24 Oct '10

24 Oct '10

Hi, I checked the Hackage Splay Tree implementation from. http://hackage.haskell.org/packages/archive/TreeStructures/0.0.1/doc/html/s… Basically it's based on the strategy mentioned in Okasaki's ``Purely Functional Programming'', Chapter 5.4. That in case we traverse twice in left (or right), we rotate the tree, so in long term of view, the tree turns more and more balanced. There are 3 cases for splay: zig-zig case, zig-zag case and zig case according to http://en.wikipedia.org/wiki/Splay_tree So I wonder if Splay tree can also be implemented by using pattern matching in a similar way as red black tree. Here is a draft source code: >>> data STree a = E -- Empty | Node (STree a) a (STree a) -- left, element, right deriving (Eq, Show) -- splay by pattern matching splay :: (Eq a) => STree a -> a ->STree a -- zig-zig splay t@(Node (Node (Node a x b) p c) g d) y = if x == y then Node a x (Node b p (Node c g d)) else t splay t@(Node a g (Node b p (Node c x d))) y = if x == y then Node (Node (Node a g b) p c) x d else t -- zig-zag splay t@(Node (Node a p (Node b x c)) g d) y = if x == y then Node (Node a p b) x (Node c g d) else t splay t@(Node a g (Node (Node b x c) p d)) y = if x == y then Node (Node a g b) x (Node c p d) else t -- zig splay t@(Node (Node a x b) p c) y = if x == y then Node a x (Node b p c) else t splay t@(Node a p (Node b x c)) y = if x == y then Node (Node a p b) x c else t -- otherwise splay t _ = t -- insert by using pattern matching insert :: (Ord a) => STree a -> a -> STree a insert E y = Node E y E insert (Node l x r) y | x < y = splay (Node (insert l y) x r) y | otherwise = splay (Node l x (insert r y)) y <<< I am not quite sure if the solution is correct and what about the efficiency of this code. Regards. -- Larry. LIU Xinyu http://sites.google.com/site/algoxy<http://sites.google.com/site/algoxy/home>

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Haskellers design
by Michael Snoyman 24 Oct '10

24 Oct '10

I've gotten two offers for designs for Haskellers. The one uses the Haskell color theme choices we seem to be standardizing on as a community, the other uses a more custom theme. There is a screenshot of the former[1] and a demo page for the latter[2]. So the questions are: * How important is adhering to a "standard" look? * Which theme is overall more visually appealing? * Which theme gives a more professional feel? Please note that questions 1 and 2 may have different answers. Keep in mind when evaluating these themes, especially the first, that it is *not* in finalized form, just an basic idea of what could be. A third option that I have not pursued yet due to time constraints would be to leave the layout as-is currently but replace it with the Haskell color scheme. Yet another option would be to take the second theme mentioned above and try to force the Haskell color theme on it; I don't find that a good idea. Options 5 and 6 are leaving the site as it is (both layout and design), or proposing a totally different layout. Michael [1] http://imgur.com/lnkE7.png [2] http://previasports.com/haskellers_website/

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Error Calling Lua Function
by aditya siram 24 Oct '10

24 Oct '10

Hi all, I'm having some issues calling Lua functions from Haskell. I have the following in "Haskell2Lua.lua": function hello () return "hello world" end And my Haskell file "Haskell2Lua.hs" looks like this: import qualified Scripting.Lua as Lua main = do l <- Lua.newstate Lua.openlibs l succ <- Lua.loadfile l "/Haskell2Lua.lua" print succ Lua.callproc l "hello" "" Lua.close l When I compile and run this file I get: 0 Haskell2Lua: user error (attempt to call a nil value) I have tried some variations on calling the function including: Lua.callproc l "hello" Lua.callfunc l "hello" Lua.callfunc l "hello" "" I know the Lua bridge is working because the following works: Lua.callproc l "print" "Hello from Lua" Any help is appreciated! -deech

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Re: [Haskell-cafe] concurrency vs. I/O in GHC
by Brandon Moore 24 Oct '10

24 Oct '10

On Oct 23, 2010, at 2:57 PM, Claude Heiland-Allen <claudiusmaximus(a)goto10.org> wrote: On 23/10/10 17:42, Gregory Crosswhite wrote: On 10/23/10 7:54 AM, John Lato wrote: On Fri, Oct 22, 2010 at 6:16 PM, Bulat Ziganshin <bulat.ziganshin(a)gmail.com <mailto:bulat.ziganshin@gmail.com>> wrote: Hello John, Monday, October 18, 2010, 8:15:42 PM, you wrote: > If anyone is listening, I would very much like for there to be a > mechanism by which external functions can be called "unsafe"-ly, but > without blocking all other Haskell threads. I have code that does this: +RTS -N2 This doesn't work, which was why the OP asked in the first place. When a thread calls an unsafe foreign function, it blocks everything until that function returns. Is that true? The last time we discussed this in Haskell Cafe the conclusion I drew from the discussion was that unsafe foreign functions block the current thread but not any other thread. The conclusion I drew was that "unsafe" foreign functions block the current "capability" (OS thread) and any "threads" (Haskell forkIO etc) currently scheduled on that capability, but other capabilities and threads continue executing as normal. I haven't tested it recently, but I think that is mostly correct except that an unsafe call will also prevent major collections, which will eventually tie up the rest of the capabilities as well.

1 0

Redy to release gtk2hs-0.12.0!
by Andy Stewart 24 Oct '10

24 Oct '10

Dear gtk2hs user: We have ready to release next version : gtk2hs-0.12.0 Please pull gtk2hs darcs to test, if no bug report, we will release. You can use below commands (Debian) to install gtk2hs darcs: darcs get --lazy http://code.haskell.org/gtk2hs/ cd ./gtk2hs sudo chmod +x ./bootstrap.sh ./bootstrap.sh Then recompile your gtk2hs program to test. Of course, any successful feedback are welcome! Thanks, -- Gtk2hs Team

3 3

Compiling and installing glib and gtk+ on Snow Leopard
by Mark Spezzano 23 Oct '10

23 Oct '10

Hi, I'm trying to install the Leksah IDE and I've run into the problem that it requires glib and gtk+. Whenever I type 'configure' for glib-2.26.0 everything goes fine. But when I type 'make' it goes through necessary actions and then suddenly stops halfway through stating the following (annoying) error message: error: #error GNU libiconv not in use but included iconv.h is from libiconv What, exactly is happening here? I've compiled libiconv and put it under /usr/bin (so iconv is there). Yet it still complains...I don't get it. I've spend the best part of a day mucking around with this to no avail. Also: I used to have Leopard installed on my machine and then I upgraded to Snow Leopard--does this mean that I would need to rebuild from scratch most of the packages on the entire system? Could this change from 32-bit to 64-bit be causing an issue? Thanks, Mark

2 1

[ANNOUNCE] text-xml-qq - XML quasiquoter for Text.XML.Light
by Oscar Finnsson 23 Oct '10

23 Oct '10

Hi, I've just released the first version of text-xml-qq, It's a Template Haskell quasiquoter that converts XML code into Text.XML.Light.Element compile time. Feature requests, bug fixes etc are welcomed. The package only got one backend (xml-package) at the moment but it should be trivial to add more backends (e.g. for a bytestring-based xml package). You can embed Haskell variables in the xml code. Inside xml-elements you use the syntax {foo} for embedded variables. If you wish to embed an element or text you use the syntax <<foo>>. See example or the test-file http://github.com/finnsson/text-xml-qq/blob/master/src/Text/XML/TestQQ.hs at the project site http://github.com/finnsson/text-xml-qq. -- Oscar === Example === Given the variables url = "google.se" elem = "gmail" attrNs = "something" attrName = "Pelle" attrValue = "Arne" elemCont = CRef "testing" cont1 = Elem $ blank_element { elName = QName "hej" Nothing Nothing } cont2 = CRef "other test" the code [$xmlQQ| <{url}:{elem} {attrNs}:{attrName}={attrValue} attr="cool"> <elem ns1:elem1="1" ns2:elem2="2"><<elemCont>></elem> <elem /> <el /> <<cont1>> <<cont2>> </{url}:{elem}> |] will generate the data structure element { elName = QName elem Nothing (Just url), elAttribs = [Attr (QName attrName Nothing (Just attrNs)) attrValue, Attr (qname "attr") "cool"], elContent = [ (Elem $ blank_element { elName = QName "elem" Nothing Nothing, elAttribs = [Attr (QName "elem1" Nothing (Just "ns1")) "1", Attr (QName "elem2" Nothing (Just "ns2")) "2"], elContent = [elemCont] }), (Elem $ blank_element { elName =QName "elem" Nothing Nothing}), (Elem $ blank_element { elName = QName "el" Nothing Nothing}), cont1, cont2] } === End example ===

1 0

The Haskell theme
by Christopher Done 23 Oct '10

23 Oct '10

To kick off discussion about Haskell's general theme, as discussed recently, here's some random ideas. Going with the original colours of the nominated Haskell logo, and kinda what's been done here: http://new-www.haskell.org/haskellwiki/Haskell http://img840.imageshack.us/img840/3577/ideasv.png To download the Inkscape SVG grab it here: http://chrisdone.com/designs/haskell-theme-1.svg I don't know what I was thinking here but it seemed like fun: http://img412.imageshack.us/img412/1827/rect5935.png I get bored really quickly, I was alright for 15 minutes and then I lost the will to point and click. Anyway, I was thinking rather than trying to come up with a brilliant all-encompassing design, we could agree on conventions: * colours * headings * spacing * links * the particular incarnation of the logo etc. I'm pretty happy to go with the colour theme of http://new-www.haskell.org/haskellwiki/Haskell Haskellers.com also makes me think we need an umbrella theme that encompasses all Haskell sites. Maybe we should do a theme poll like the logo poll. I don't know. Regardless, I think we do need to decide on something and stick with it. Should this discussion be taken to the web devel mailing list? Is that appropriate?

6 7

readProcess exception
by Leskó Dániel 23 Oct '10

23 Oct '10

Hi everyone! I run into a problem with readProcessWithExitCode (from System.Process module). Basically what i want is to start an exe file, giving it some input on stdin and receiving the results on stdout. But id the stdin and out is bigger that 5000-7000 char (nondeterministically), i get commitAndReleaseBuffer exception and sometimes hFlush exception. I'm getting these errors with GHCi 6.12.3, if it matters. Have you guys any idea what causes the problem, to how to avoid it? Thanks Daniel Here is a dummy example, which produces the exception: Haskell code: import System.Process test = do result <- sequence $ Prelude.map (\x -> runOnce x) [1..10000] return () where runOnce :: Int -> IO [Int] runOnce i = do putStrLn (show i) a <- caller i b <- caller i return (a++b) caller a = do (exit_code,stdout) <- readProcessWithExitCode "a.exe" [show a] (unlines (map show [1..850])) return (read stdout) C code (for executable) #include <stdio.h> #include <complex.h> #include <stdlib.h> int main(int argc, char *argv[]) { printf("[%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d]",100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100); }

2 1

Re: Scrap your rolls/unrolls
by Max Bolingbroke 23 Oct '10

23 Oct '10

Forgot to reply to list On 22 October 2010 12:14, Dan Doel <dan.doel(a)gmail.com> wrote: > Another solution, though, is SHE. With it, you can write: > > data ListF a r = NilF | ConsF a r > newtype List a = Roll (ListF a (List a)) > > pattern Nil = Roll NilF > pattern Cons x xs = Roll (ConsF x xs) > > And not worry about Rolls anymore. Ah yes, pattern synonyms. This solution is somewhat unsatisfying because you will also need some smart constructors: """ nil = Roll NilF cons x xs = Roll (ConsF x xs) """ Now the names of the smart constructors for building the data type are not the same as the things you pattern match on, which is a slightly annoying asymmetry. It's certainly less complicated than the TypeFamilies based solution though! Cheers, Max

2 2

HSCurses - Hello World? (can't find much documentation)
by Anonymous Void 23 Oct '10

23 Oct '10

Hi everyone, Haven't posted here since I was a kid, when this language first warped my poor imperative brain. My main work environment is a three-split terminal session using `Yakuake' as a dropdown-terminal (one for vim, one for interactive interpreter, one for MySQL). I don't mind using the MySQL cli *that* much, but I find typing my queries all the time much takes up precious time when I'm developing things, and it annoys me. Since there is currently no TUI for mysql administration (besides links/phpadmin? o.0). I decided to make one, choosing Haskell because, well, it's awesome. I've made curses apps before, but never in Haskell (only wx, cgi, cli, etc). Couldn't find anything besides an api reference; Which was impossible to find before typing haddock specifically, even after an exhausting search on google, krugle, archives, etc... It's livable (I've learned more complex things with less). But since I've never seen how the HSCurses functions really integrate together, or any tutorials/examples (besides hsFishEx), I fear toying with it is going to be annoying until I figure it out on my own, especially since I'm already doing a project for someone... So, I was wondering if anyone would be kind enough to point me to (or write me) a basic example of how HSCurses works, maybe w/ some very simple example of interactivity (e.g. getch processing, screen refresh, ..) support. Also, maybe warn me of some of the common pitfalls and differences in hscurses vs ncurses in other languages. Having found the haddock, and having done curses apps in other languages, that should be enough for me to get a handle on it. I'm just dreading the "read the haddock, assume it works this way, find out you do it this way, repeat" loop of learning w/o a starting point. Any help would be greatly appreciated, and will save me some (anticipated) hassle. TYIA, -Pete

2 1

A rant against the blurb on the Haskell front page
by Ben Franksen 22 Oct '10

22 Oct '10

This is a critique of the current 'Haskell Blurb', the first paragraph on www.haskell.org. This blurb should, IMO, give a concise description of what Haskell, the programming language, is, what makes it different from other languages, and why I should be interested in it. What it does, instead, is to make me scratch my head and ask myself: what marketing idiot has written this inclonclusive mumble-jumble of buzz-words? Let me explain. "Haskell is an advanced purely functional programming language." Good start, if only the "advanced" were replaced with something more characteristic, like "lazy", or "statically typed". Which, BTW, both do not appear in the whole blurb, even though they are *the* characteristics of Haskell, lazyness being even something that sets it apart from most other languages. I hear the marketeers crying "but the average visitor has no idea what lazyness means". So what? Give them a link to the wiki with an explanation. So, a better introductory sentence would be -> "Haskell is a lazily evaluated, purely functional programming language with a very flexible and powerful static type system." Next sentence: "An open source product of more than twenty years of cutting edge research, it allows rapid development of robust, concise, correct software." This really gets me every time I read it. How can anyone write such a nonsense? Haskell is not an "open source product"! It is no product at all. That most (maybe all) implementations are opens source is certainly an interesting fact, but IMO not something that should appear at the top of the page right under the header "The Haskell Programming Language". The second and third sentences deliberately conflate language and implementation(s). This is a well known falacy and I am ashamed that it appears on the front page of my favourite programming language. The blurb talks about "robust, concise, correct software", but misses itself most of these goals: it is imprecise, incorrect, and not robust (because implementations vary), and therefore not a good advertisement, though quite possibly "rapidly developed". The blurb promises "rapid development of robust, concise, correct software" lest one think this were something akin to Perl which certainly allows rapid development, yet typically neither robust nor correct, especially if done rapidly. So, how does Haskell differ from that? Well, I'd say this is where lazyness and a static yet flexible type system come into play. But no, I forgot, we don't want to explain anything or even be logical, dear reader, we want to pound slogans into your head! That cutting edge research is done for Haskell as well as for its implementations is of course good to know, but just stating it is not nearly enough: such a statement must be corroberated with evidence, otherwise it is just idle marketing. (Not that there wouldn't be evidence amass, it's just that none is given.) On we go: "With strong support for integration with other languages, built-in concurrency and parallelism, debuggers, profilers, rich libraries and an active community, Haskell makes it easier to produce flexible, maintainable high-quality software." Let us take that apart: (1) Fact: Haskell has a good and very easy to use FFI. To the C language. I have never heard of integration with any other langauge being directly supported. (2) Fact: Built-in concurrency and parallelism is not exactly part of the langauage, although purity makes it possible to support them in a more precise and less error-prone way; which is exploited by ghc's concurrency and parallelism extensions. (3) Fact: Traditional debuggers are practically useless in Haskell, due to lazy evaluation. But, oh, we forgot to mention the small fact that Haskell is lazy. Too bad. Profiling is supported by ghc only (AFAIK), but is supposed to be very useful (never seriously used it, so I can't judge.) (4) Fact: there are libraries. Some of them are good, others are not so good. Most leave a lot to be desired, but I would be hard pressed to come up with something better myself. What really distinguishes Haskell libraries from what is found in other languages is the level of abstraction. I know no other language where stuff like Monad, (Applicative) Functor, Monoid, Category etc. is *at the heart* of all the libraries. But, oh, I forgot, we don't want to scare people off, so better not talk about this in public. (5) Fact: Haskell has an active community. No question. (6) Fact: Haskell makes certain things easier (than other languages). Other things that are easy in other langauges are hard in Haskell. Or at least very non-obvious. Whether Haskell programs are more flexible, when compared with dynamically typed OO languages like Python, I seriously doubt. Maintainable high-quality software is the real selling point, IMO; or rather *reliably correct* software. However, I cannot see how this dirtectly follows from the previous points, which is what the sentence is saying. What has an FFI do to with high quality? What has the supposed availability of debuggers (where? which?) to do with it? Enough. I think I have made my point. Cheers Ben

17 30

tried to use the example given in the source of network.browser
by Michael Litchard 22 Oct '10

22 Oct '10

This is what is in HTTPbis/Network/Browser.hs > do > rsp <- Network.Browser.browse $ do > setAllowRedirects True -- handle HTTP redirects > request $ getRequest "http://google.com/" > fmap (take 100) (getResponseBody rsp) And how I changed it slightly to test it out > import Network.HTTP > import Network.Browser > main = > do > rsp <- Network.Browser.browse $ do > setAllowRedirects True -- handle HTTP redirects > request $ getRequest "http://google.com/" > fmap (take 100) (getResponseBody rsp) but I got this errortest.lhs:10:39: Couldn't match expected type `Network.Stream.Result (Response [a])' against inferred type `(Network.URI.URI, Response String)' In the first argument of `getResponseBody', namely `rsp' In the second argument of `fmap', namely `(getResponseBody rsp)' In the expression: fmap (take 100) (getResponseBody rsp) how did I get this error? I'm perplexed as this came right from the source.

2 1

ICFP 2011: Call for Workshop Proposals
by Wouter Swierstra 22 Oct '10

22 Oct '10

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2011 16th ACM SIGPLAN International Conference on Functional Programming September 19 - 21, 2011 Tokyo, Japan http://www.icfpconference.org/icfp2011 The 16th ACM SIGPLAN International Conference on Functional Programming will be held in Tokyo, Japan on September 19-21, 2011. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as tutorials) to be affiliated with ICFP 2011 and sponsored by SIGPLAN. These events should be more informal and focused than ICFP itself, include sessions that enable interaction among the attendees, and be fairly low-cost. The preference is for one-day events, but other schedules can also be considered. NEW THIS YEAR: The workshops are scheduled to occur on September 18 (the day before ICFP) and September 22-24 (the three days after ICFP). ---------------------------------------------------------------------- Submission details Deadline for submission: November 19, 2010 Notification of acceptance: December 17, 2010 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2011 workshop co-chairs (Gabriele Keller and Derek Dreyer), via email to icfp11-workshops at mpi-sws.org by November 19, 2010. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by December 17, 2010, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2011/icfp11-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://acm.org/sigplan/sigplan_workshop_proposal.htm ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2010 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Gabriele Keller (University of New South Wales) Workshop Co-Chair: Derek Dreyer (MPI-SWS) General Co-Chair: Manuel Chakravarty (University of New South Wales) General Co-Chair: Zhenjiang Hu (National Institute of Informatics) Program Chair: Olivier Danvy (Aarhus University) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Gabriele Keller and Derek Dreyer), via email to icfp11-workshops at mpi-sws.org.

1 0

Eta-expansion and existentials (or: types destroy my laziness)
by oleg＠okmij.org 22 Oct '10

22 Oct '10

Max Bolingbroke wrote: > Let's start with a simple example of an existential data type: > > data Stream a = forall s. Stream s (s -> Maybe (a, s)) > > ones :: Stream Int > > ones = cons 1 ones > > Unfortunately, 'ones' is just _|_! The reason is that cons is strict > in its second argument. The problem I have is that there is no way to > define cons which is > simultaneously: > > 1. Lazy in the tail of the list > 2. Type safe > 3. Non-recursive Really? Here are two 'cons' that seem to satisfy all the criteria > {-# LANGUAGE ExistentialQuantification #-} > > data Stream a = forall s. Stream s (s -> Maybe (a, s)) > > nil :: Stream a > nil = Stream () (const Nothing) > > -- One version > -- cons :: a -> Stream a -> Stream a > -- cons a str = Stream Nothing (maybe (Just (a, Just str)) run) > -- where run (Stream s step) = > -- step s >>= (\ (a,s) -> return (a, Just (Stream s step))) > > -- the second version > cons :: a -> Stream a -> Stream a > cons a str = Stream (Just (a,str)) step > where step Nothing = Nothing > step (Just (a, (Stream s step'))) = Just (a, > case step' s of > Nothing -> Nothing > Just (a',s') -> Just (a',(Stream s' step'))) > > > instance Show a => Show (Stream a) where > showsPrec _ (Stream s step) k = '[' : go s > where go s = maybe (']' : k) > (\(a, s) -> shows a . showString ", " $ go s) (step s) > > taken :: Int -> Stream a -> Stream a > taken n (Stream s step) = > Stream (n, s) (\(n, s) -> > if n <= 0 then Nothing else maybe Nothing > (\(a, s) -> Just (a, (n - 1, s))) (step s)) > > ones :: Stream Int > ones = cons 1 ones > > test2 = taken 5 $ ones > -- [1, 1, 1, 1, 1, ] Finally, if one doesn't like existentials, one can try universals: http://okmij.org/ftp/Algorithms.html#zip-folds http://okmij.org/ftp/Haskell/zip-folds.lhs The code implements the whole list library, including zip and zipWith. None of the list operations use value recursion. We still can use value recursion to define infinite streams, which are processed lazily. In fact, the sample stream2 of the example is the infinite stream.

3 5

Convert Either to Tree - Occurs check
by André Batista Martins 22 Oct '10

22 Oct '10

hello, i want convert "Either" to a tree. Example: Either ( Either 1 2 ) ( Either 3 4) ---- > Branch ( Branch (Leafl 1) (Leafr2) ) ( Branch (Leafl 3) (Leafr4)) ) Code: data TreeE a b = Empty |Leafl a | Leafr b | Branch (TreeE a b ) (TreeE a b) deriving Show f3 (Right b) (Branch l r) =case( isRight(b) || isLeft(b) )of true -> Branch l (f3 b r) false -> Branch l (Leafl b) f3 (Left b) (Branch l r) = case( isRight(b) || isLeft(b) )of true -> Branch (f3 b l) r false -> Branch (Leafl b) r Error: Occurs check: cannot construct the infinite type: b = Either a b Expected type: Either a b -> TreeE t t1 -> TreeE t t1 Inferred type: Either a (Either a b) -> TreeE t t1 -> t2 In the second argument of `Branch', namely `(f3 b r)' In the expression: Branch l (f3 b r) I don't understand why this happen... Can anyone help me?

3 4

Haskell Weekly News: Issue 155 - October 20, 2010
by Daniel Santa Cruz 22 Oct '10

22 Oct '10

Welcome to issue 155 of the HWN, a newsletter covering developments in the [1]Haskell community in the week of October 10 - 16. This time around we again have 87 posts to HackageDB. Instead of posting the individual packages, we get to see and celebrate the 43 people behind these efforts. Want to keep a close eye on the haskellers that lurk twitter? Don Steward made [2]a twitter list of tweeting haskellers! Let him know if you'd like to be added to the list. There were a total of 24 new stories posted to the Haskell Reddit channel, 27 new questions taged with Haskell in StackOverflow, and 408 messages posted to Haskell-Cafe. So, what was hot last week? Announcements Gregory Crosswhite is pleased to [3]announce the release of a family of packages for type-level natural numbers. He also [4]announced tagged-list, a package which provides fixed-length lists that are tagged with a phantom type-level natural number corresponding to the length. Janis Voigtlander [5]announced that it is time to collect contributions for the 19th edition of the Haskell Communities & Activities Report. The submission deadline is November 1, 2010. Alexander Solla [6]announced his new "Facts" library. The Facts hierarchy is meant to contain commonly used, relatively static facts about the "real world." Kevin Jardine [7]announced polyToMonoid: a library that supplies two very general polyvariadic functions that can map their arguments into any monoid you specify. Simon Hengel [8]announced a new version of DocTest. DocTest now uses Haddock for parsing of comments. Interesting Threads on Haskell-Cafe Michael Snoyman [9]reported that Haskellers.com has become popular a lot falter that he anticipated. Read up on what changes are planned for Haskellers.com. Michael is very interested in some help with running the site. Jacek Generowicz [10]asked how to deal with dynamic dispatch on extensible sets of types. Uwe Schmidt [11]replied to a question about why HXT uses arrows as opposed to using monads. Good comments followed. Jason Dusek [12]asked if there is a way to write a Haskell data structure that is necessarily only one or two or seventeen items long; but that is nonetheless statically guaranteed to be of finite length? Twenty-three messages followed. Simon Thompson [13]annouced the availability of books for review for the Journal of Functonial Programming. Andrew Copping [14]summarized a report produced by Google Szwitzerland on taking a Python system and rewriting bits of it in Haskell, some of which is now in production use. Ben Franksen [15]ranted about the current "Haskell Blurb" on the first paragraph of haskell.org. Quite the read to end the week :) Top Reddit Stories * Using Haskell’s ‘newtype’ in C Domain: blog.nelhage.com Score: 37, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/dptzh/using_haskells_newtype_in_c/ Original: http://blog.nelhage.com/2010/10/using-haskells-newtype-in-c/ * The Haskell theme: consistent visual branding for Haskell Domain: haskell.org Score: 34, Comments: 11 On Reddit: http://www.reddit.com/r/haskell/comments/dqiej/the_haskell_theme_consistent… Original: http://www.haskell.org/pipermail/haskell-cafe/2010-October/084781.html * Haskellers: Survey results and new site features Domain: haskellers.com Score: 22, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/dr37b/haskellers_survey_results_an… Original: http://www.haskellers.com/news/1/ * My Experience Learning Haskell Domain: blog.virtucal.com Score: 21, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/drgcz/my_experience_learning_haske… Original: http://blog.virtucal.com/cyclical/2010/10/14/my-experience-learning-haskell… * Invertible monads for exception handling and memory allocations Domain: docs.yesodweb.com Score: 21, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/drkdj/invertible_monads_for_except… Original: http://docs.yesodweb.com/blog/invertible-monads-exceptions-allocations/ * Accelerating Haskell Array Codes with Multicore GPUs Domain: justtesting.org Score: 19, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/dq1yo/accelerating_haskell_array_c… Original: http://justtesting.org/accelerating-haskell-array-codes-with-multico * Unicode text performance improvements Domain: serpentine.com Score: 17, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/drinz/unicode_text_performance_imp… Original: http://www.serpentine.com/blog/2010/10/15/unicode-text-performance-improvem… * Like a coding challenge? Check out Code Quarterly! Domain: serpentine.com Score: 16, Comments: 9 On Reddit: http://www.reddit.com/r/haskell/comments/dpjgv/like_a_coding_challenge_chec… Original: http://www.serpentine.com/blog/2010/10/11/like-a-coding-challenge-check-out… * C9 Lecture: Ralf Lämmel on Monads Domain: channel9.msdn.com Score: 16, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/dq614/c9_lecture_ralf_lämmel_on_mo… Original: http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-AFP-The… * Take the Haskellers.com survey Domain: spreadsheets.google.com Score: 15, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/dq7xj/take_the_haskellerscom_surve… Original: https://spreadsheets.google.com/viewform?formkey=dG8tcVhBdzRSS1lkdVV6U281em… Top StackOverflow Questions * When is memoization automatic in GHC Haskell? Question (votes: 14, answers: 3) http://stackoverflow.com/questions/3951012/when-is-memoization-automatic-in… * Why is my code using monadic lists from the List package so slow? Question (votes: 12, answers: 3) http://stackoverflow.com/questions/3916444/why-is-my-code-using-monadic-lis… * Haskell: lift vs liftIO Question (votes: 8, answers: 2) http://stackoverflow.com/questions/3921237/haskell-lift-vs-liftio * Can anyone suggest a good constraint library for Haskell? Question (votes: 8, answers: 2) http://stackoverflow.com/questions/3926656/can-anyone-suggest-a-good-constr… * Library function to compose a function with itself n times Question (votes: 6, answers: 4) http://stackoverflow.com/questions/3911060/library-function-to-compose-a-fu… * Evaluation of a small math type language that supports one variable Question (votes: 5, answers: 1) http://stackoverflow.com/questions/3906231/evaluation-of-a-small-math-type-… * What's the difference of a “class” in Haskell and in an “abstract class” in OO language? Question (votes: 5, answers: 4) http://stackoverflow.com/questions/3938779/whats-the-difference-of-a-class-… * Haskell and State Question (votes: 5, answers: 4) http://stackoverflow.com/questions/3944170/haskell-and-state * Can Haskell guard on abstract data types in a polymorphic function? Question (votes: 5, answers: 3) http://stackoverflow.com/questions/3944756/can-haskell-guard-on-abstract-da… Quotes of the Week * pumpkin: sounds like a lot of work! it's easier for me to just bitch about it * OlegKiselyov: Haskell is inconsistent in yet another way. * monochrom: just add #! /usr/bin/runghc to your haskell file. then type errors occur at runtime only. * IIcke: [haskell] is like wearing a corset, at the beginning is annoying, but then your tits look bigger and the world becomes a better place Contributors to HackageDB Who contributed, and how many uploads did they make? MichaelSnoyman (14), BryanOSullivan (11), GregoryCrosswhite (5), AndyStewart (5), GregoryCollins (4), JosePedroMagalhaes (3), StephenBlackheath (2), SjoerdVisscher (2), PaulTalaga (2), MichaelZuo (2), DanielDiaz (2), ConalElliott (2), AristidBreitkreuz (2), AlexanderSolla (2), YuriyIskra (1), VivianMcPhail (1), TristanSkudlik (1), TonyMorris (1), StephenTetley (1), SimonHengel (1), ReinierLamers (1), OlegBelozeorov (1), NeilMitchell (1), MaxBolingbroke (1), MatthiasBartsch (1), MatthewSottile (1), MasatakeDaimon (1), MarkSnyder (1), ManuelChakravarty (1), LeePike (1), KiYungAhn (1), KevinJardine (1), JustinBailey (1), IlyaPortnov (1), GeoffHulette (1), EugeneKirpichov (1), DonaldStewart (1), DanielSeidel (1), DanielSchoepe (1), CarlHowells (1), BenFranksen (1), ArieMiddelkoop (1), AlexSuraci (1) About the Haskell Weekly News New editions are posted to [16]the Haskell mailing list as well as to [17]the Haskell Sequence and [18]Planet Haskell. [19]RSS is also available, and headlines appear on [20]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://twitter.com/#!/donsbot/haskellers 3. http://groups.google.com/group/haskell-cafe/browse_thread/thread/c007154763… 4. http://groups.google.com/group/haskell-cafe/browse_thread/thread/ff95d6c78d… 5. http://groups.google.com/group/haskell-cafe/msg/d9d227b0f38bcf51 6. http://groups.google.com/group/haskell-cafe/browse_thread/thread/fc86bd03d0… 7. http://groups.google.com/group/haskell-cafe/browse_thread/thread/4ba5741441… 8. http://groups.google.com/group/haskell-cafe/browse_thread/thread/7982f0ad3b… 9. http://groups.google.com/group/haskell-cafe/browse_thread/thread/2440a1683f… 10. http://groups.google.com/group/haskell-cafe/browse_thread/thread/f0a05dd5b5… 11. http://groups.google.com/group/haskell-cafe/browse_thread/thread/38185e7ba6… 12. http://groups.google.com/group/haskell-cafe/browse_thread/thread/7ba4b3b038… 13. http://www.cs.kent.ac.uk/people/staff/sjt/JFP/available.html 14. http://groups.google.com/group/haskell-cafe/browse_thread/thread/81707fea2b… 15. http://groups.google.com/group/haskell-cafe/browse_thread/thread/a7c7b569dd… 16. http://www.haskell.org/mailman/listinfo/haskell 17. http://sequence.complete.org/ 18. http://planet.haskell.org/ 19. http://sequence.complete.org/node/feed 20. http://haskell.org/

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[ANNOUNCE] text 0.10.0.0 - fast Unicode text handling
by Bryan O'Sullivan 22 Oct '10

22 Oct '10

[Blog copy of the announcement here<http://www.serpentine.com/blog/2010/10/22/text-0-10-0-0-is-here/> .] I just pushed it to bitbucket <http://bitbucket.org/bos/text> and github<http://github.com/bos/text>, and you can install it from the text site on Hackage<http://hackage.haskell.org/package/text> in the usual way: cabal update cabal install text What's in this release? - New functions for reading integers and floating point numbers<http://hackage.haskell.org/packages/archive/text/0.10.0.0/doc/html/Data-Tex…>, an oft-requested feature. They're fast, too: they range from parity with their bytestring counterparts, to up to 4 times faster. You can expect to parse 3 to 4 million Int values per second out of a text file, or up 2 million Double values per second. They're also easy to use, give error messages, and come in strict and lazy variants. - UTF-8 decoding and encoding are now very fast<http://www.serpentine.com/blog/2010/10/15/unicode-text-performance-improvem…>. They're up to 9x faster than they were, and close to the performance of pure C UTF-8 decoding and encoding. - The Eq and Ord instances are also now very fast, up to 5x faster than before<http://www.serpentine.com/blog/2010/10/19/a-brief-tale-of-faster-equality/>. They're now faster than the bytestring instances. - Several other common functions received drive-by performance improvements too. - Better protection against rare crashes on really huge volumes of data.

3 2

suspected problem with network.curl version 1.3.5
by Michael Litchard 22 Oct '10

22 Oct '10

Here's my code, I'm pretty sure I am doing this right. The problem seems to be with method_POST. I tried to duplicate manually, but I'm not sure I used command line curl correctly. Take a look at the output below. > import Network.Curl > import System (getArgs) > import Text.Regex.Posix > -- | Standard options used for all requests. Uncomment the @CurlVerbose@ > -- option for lots of info on STDOUT. > opts = [ CurlCookieJar "cookies" , CurlVerbose True ] > -- | Additional options to simulate submitting the login form. > loginOptions user pass = > CurlPostFields [ "login=" ++ user, "password=" ++ pass ] : method_POST > main = withCurlDo $ do > -- Get username and password from command line arguments (will cause > -- pattern match failure if incorrect number of args provided). > [user, pass] <- getArgs > -- Initialize curl instance. > curl <- initialize > setopts curl opts > -- POST request to login. > r <- do_curl_ curl "https://github.com/login" (loginOptions user pass) > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed to log in: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else do > -- GET request to fetch account page. > r <- do_curl_ curl ("https://github.com/session") method_GET > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed to retrieve account page: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else putStrLn $ extractToken $ respBody r > -- | Extracts the token from GitHub account HTML page. > extractToken body = head' "GitHub token not found" xs > where > head' msg l = if null l then error msg else head l > (_,_,_,xs) = body =~ "github\\.token (.+)" > :: (String, String, String,[String]) * About to connect() to github.com port 443 (#0) * Trying 207.97.227.239... * connected * Connected to github.com (207.97.227.239) port 443 (#0) * found 142 certificates in /etc/ssl/certs/ca-certificates.crt * server certificate verification SKIPPED * common name: *.github.com (matched) * server certificate expiration date OK * server certificate activation date OK * certificate public key: RSA * certificate version: #3 * subject: O=*.github.com,OU=Domain Control Validated,CN=*.github.com * start date: Fri, 11 Dec 2009 05:02:36 GMT * expire date: Thu, 11 Dec 2014 05:02:36 GMT * issuer: C=US,ST=Arizona,L=Scottsdale,O=GoDaddy.com\, Inc.,OU=http://certificates.godaddy.com/repository,CN=Go Daddy Secure Certification Authority,serialNumber=07969287 * compression: NULL * cipher: AES-128-CBC * MAC: SHA1 > GET /login HTTP/1.1 Host: github.com Accept: */* <snip> See, the code set a GET, where I thought I was doing a POST here is my attempt to do a POST manually, it failed but I'm not sure it's right. curl --data "authenticity_token=BigAssToken&login=UserName&password=Password&commit=Log+in" https://github.com/login > login_out anyway, could someone try to replicate this problem?

2 1

Regression test utility suggestions?
by Peter Schmitz 22 Oct '10

22 Oct '10

I am seeking suggestions for a regression test utility or framework to use while developing in Haskell (in a MS Windows environment). I am developing a Haskell application that parses an input text file, and outputs some information about what was parsed. The application is currently in a GUI, but the parsing/analysis code does not require any user interaction, so it would be simple to implement a command-line based version (that could be invoked by a regression test utility -- http://en.wikipedia.org/wiki/Regression_testing) that takes as args: the input and output file pathnames. As I develop, I have a growing set of input files that are used to test various aspects of the grammar. I am seeking a regression test utility that I could point at a dir of test input files, and it would process each input file using my app, creating a corresponding output file. The utility would then compare each output file with a known good output file, and let me somehow review the results (i.e., the differences), and for each test file (or all of them) allow me to decide one of: -- there were no diffs; all is well -- the diff shows the output changed as I expected, due to coding enhancements or fixes; take the new output file and use it as the new good output file -- oops, unexpected diffs; I need to fix my code and then re-run the utility afterwards I have reviewed http://www.haskell.org/haskellwiki/Introduction_to_QuickCheck and http://hackage.haskell.org/packages/archive/pkg-list.html#cat:testing but I did not see anything that seems to fit. I'm not sure whether (or how) QuickCheck would lend itself to this type of testing. I don't want to generate any test cases; I have a manually created collection of test cases that I want to use (which is all I am interested in, at this point). In the past I have coded something like this using shell or Perl scripts, etc. I am wondering if there is a commonly used utility available for this purpose that you like. (I use a MS Windows environment, but I have a Bash and various gnu shell utils installed.) (It does not matter to me whether it is written in Haskell, although I suppose that would make it more enjoyable to hack on. :) Thanks in advance very much for any suggestions. -- Peter

3 2

playing around with network.curl - redub
by Michael Litchard 22 Oct '10

22 Oct '10

My current problem is how to pass around the cookie jar. I need to gather cookies, while establishing a session. Could someone provide an example? I've messed about with CurlCookieFile and CurlCookieJar, to no avail. I'll provide my failed attempt if needed. I'm using https, if it makes a difference.

1 0

HELP
by Yaadallah Khan 22 Oct '10

22 Oct '10

I am Studying for an exam, and i have just come accross the following 3 questions, i am not familiar with the functions, therefore i would appreciate any help. !! i have already created painting 2 & 3 all i need is the 3 functions, for the three different tasks as shown below. the functions are the showPic, sizeRaw and isPic. questions are: 1) – Define showPic :: Canvas -> IO () for displaying pictures on theterminal. Examples: Cw2010> showPic (painting 2)+---+---+---+---+---+| a | n | e | x | a |+---+---+---+---+---+| m | p | l | e | t |+---+---+---+---+---+| e | x | t | i | s |+---+---+---+---+---+| s | h | o | w | n |+---+---+---+---+---+ Cw2010> showPic (painting 3)+---+---+---+---+---+---+---+| A | n | o | t | h | e | r |+---+---+---+---+---+---+---+| | t | e | s | t | ! | |+---+---+---+---+---+---+---+ 2) – Furthermore, define sizeRaw :: Canvas -> (Int, Int) for findingout the size of a raw picture as demonstrated here.Cw2010> sizeRaw ["anexa","mplet","extis","shown"](5,4) 3) – Finally, define isPic :: Canvas -> Bool for checking a particularnecessary condition which items of type Canvas must satisfy (full orraw). The condition you should check for is whether all the ‘rows’ ofthe input have the same length. (It may be assumed that the inputof isPic is of type Canvas.) Examples: Cw2010> isPic (painting 3)TrueCw2010> isPic (painting 4)False Suggestion. I found it useful in my implementation to define and usehere the auxiliary function isEqual :: [Int] -> Bool; example: Cw2008> isEqual [8,8,8,8]TrueCw2008> isEqual [8,8,4,8]FalseYou may wish to define and use isEqual in your implementation ofisPic too.

2 1

Haskellers.com skills list moderation?
by Daniel Peebles 22 Oct '10

22 Oct '10

Hi all, Might it be worthwhile to take the elected "superusers" on haskellers.comand let them police the skills list? It's become rather messy, with overly broad terms like "Mathematics" in it, as well as overly specific ones like "Other languages I know: C# .NET, XSLT, Microsoft SQL Server, XML, SQL, CSS, C, C++, Java, HTML, Visual Basic Script, Pascal, Rexx, Basic and assembler". Dan

18 26

Should Yesod.Mail be a separate package?
by Michael Snoyman 21 Oct '10

21 Oct '10

Hey all, I wrote a simple email module in Yesod[1] that handles such things as multipart messages and Base64 encoding. It's still missing some features (multipart/alternative, for instance), but it can be useful for throwing together emails. It's currently part of the yesod package, but I'm going to be moving it to a separate package to free me up to make breaking API changes more frequently. As of right now, I'm just going to move it into the yesod-auth package (also being split off from the main yesod package), and therefore it will still have all the dependencies on Yesod. My question is whether people would find this package useful outside the scope of Yesod. There are no dependencies from this module onto any Yesod-specific stuff, so this separation could easily be done. I just don't feel like adding another package to maintain if no one is interested. So if anyone wants this offered up as a separate package, and/or has any API suggestions, please let me know. Michael [1] http://hackage.haskell.org/packages/archive/yesod/0.5.4/doc/html/Yesod-Mail…

8 21

Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)
by Dimitry Golubovsky 21 Oct '10

21 Oct '10

Hi, Atze Dijkstra wrote: [skip] >but to provide a proper interface to XML/HTML DOM (see http://www.haskell.org/haskellwiki/Haskell_in_web_browser) or a GUI abstraction around it (perhaps a subset of wxHaskell?) >will take more work & time. I hope this can be done as part of a studentproject, or maybe picked up during a Hackathon. I think a big portion of this task might be done using this package: http://hackage.haskell.org/package/webidl or its newer (not published on Hackage, but recommended) version split into two (you need both): http://code.haskell.org/yc2js/webidl/ -- lexer (FFI based), parser, and a test program.* http://code.haskell.org/yc2js/webidlsyn/ -- just AST and pretty-printer I must admit: I haven't touched this project for more than a year, and have little plans to do so: it is up for grabs by anyone interested. But if it can parse something like this: http://es-operating-system.googlecode.com/svn/trunk/include/w3c/html5.idl or http://es-operating-system.googlecode.com/svn/trunk/include/w3c/svg.idl it's likely still intact. Back in those days it could. A WebIDL converter helps derive type signatures for DOM methods and attributes (just like HSFFIG does this for C function prototypes). Have fun ;) and don't hesitate to ask questions. PS I'm glad that the idea lives on. With more GHC flrxibility on backends than it was in 2006-2007, and with faster Javascript engines in browsers, this can get a second life... ------------------------------ * the whole repo has to be downloaded: darcs get http://code.haskell.org/yc2js/ -- Dimitry Golubovsky Anywhere on the Web

1 0

data.time.zoneinfo
by Anatoly Yakovenko 21 Oct '10

21 Oct '10

I am trying to figure out how to use this library, seems like this should work Prelude Data.Time.ZoneInfo> ctx <- newContext Nothing Prelude Data.Time.ZoneInfo> name <- zoneInfoName (utcOlsonZone ctx) True Prelude Data.Time.ZoneInfo> newOlsonZone ctx name *** Exception: Data.Time.ZoneInfo.newOlsonZone: invalid argument (Invalid argument) but it doesn't, neither does Prelude Data.Time.ZoneInfo> newOlsonZone ctx "America/New_York" *** Exception: Data.Time.ZoneInfo.newOlsonZone: invalid argument (Invalid argument) and my zoneinfo seems to be kosher on my machine $ ls /usr/share/zoneinfo/America/New_York /usr/share/zoneinfo/America/New_York $ ls /usr/share/zoneinfo/GMT /usr/share/zoneinfo/GMT what am i missing?

2 2

Announce: splot-0.1.1
by Eugene Kirpichov 21 Oct '10

21 Oct '10

Hi cafe, I wrote yet another tiny visualization tool for my purposes and uploaded it on hackage. splot is a tool for visualizing the lifecycle of many concurrent multi-stage processes. For example, I'm using it to find the performance bottlenecks in a distributed application: processes are tasks and tasks have stages like "task in queue", "validating", "running", "response in queue". http://www.haskell.org/haskellwiki/Splot The tool is very basic but sufficient for my purposes. I'll be happy if anyone else also finds it useful (or not useful, but tells me in which exact way). P.S. Remember, there's also http://www.haskell.org/haskellwiki/Timeplot - a tool with similar purpose but a different (and bigger) set of features. There's a new version coming in a few days. However, I wasn't able to quickly implement the functionality of splot in Timeplot, and I needed it quickly, so I wrote a separate tool. -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

1 0

Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)
by caseyh＠istar.ca 21 Oct '10

21 Oct '10

Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)

5 5

playing around with network.curl
by Michael Litchard 21 Oct '10

21 Oct '10

I'm using this tutorial as a guide http://flygdynamikern.blogspot.com/2009/03/extended-sessions-with-haskell-c… github has changed since this was posted, but I have managed a successful login. Now I am faced with dealing with a re-direct. I found this constructor CurlFollowLocation Bool on this page http://hackage.haskell.org/packages/archive/curl/1.3.5/doc/html/Network-Cur… It seems to do what I want, But I am not clear on how to use it. Could someone provide an example? Thanks. End goal is to snarf the cookie that establishes the session.

2 3

hs-plugins and memory leaks
by Evan Laforge 21 Oct '10

21 Oct '10

I was happy to see the recent announcement about hs-plugins being updated to work with newer ghc. I have a project and had always been planning to use it. However, there are some questions I've had about it for a long time. The 'yi' paper mentions both 'yi' and 'lambdabot' as users of hs-plugins. However, both those projects have long since abandoned it. I can't find any documentation on why, or even any documentation at all for Yi wrt its dynamic code execution system, but from looking at the source it looks like it uses hint for dynamic code execution and dyre for configuration. Dyre in turn uses serialization to pass the old state to the reconfigured app. So we have retreated from the idea of hotswapping the application state. It seems to me that the advantages as put forth in the 'yi' paper still hold. Changing the configuration in yi is rather heavyweight. Relinking the entire editor takes a long time, and yi is still a relatively small program. Editors can keep most of their state on disk and can have very simple GUI state, so perhaps the serialization and deserialization isn't such a problem, but this doesn't hold for other programs. It seems to me the loss is significant: there's a big difference between being able to experiment with a command by editing and rerunning it immediately, and having to wait 10s or more for the app to recompile, relink, shut down the ui, serialize all state, and restart. And if you add hint, you are linking in large parts of ghc, with an even slower link time. So, yi is no longer a dynamically reconfigurable application, and is now merely a configurable application. The apparent loss of such a useful feature (you might even say a defining feature) would presumably only happen if keeping it was untenable. And of course that makes me reluctant to make any kind of design that relies on it without first knowing why all existing users jumped ship. I can think of one possible reason, and that's a memory leak. In ghc/rts/Linker.c:unloadObj there's a commented out line '// stgFree(oc->image);'. In a test program I wrote that behaves like 'plugs', every executed line increases the size of the program by 12-16k. I have to remove the resolveObjs call from plugs for it to work, but once I do it displays the same leak. So my questions are: Why did lambdabot and yi abandon plugins? Is unloadObj a guaranteed memory leak? As far as I can tell, it's never called within ghc itself. If the choices are between a memory leak no matter how you use it and dangerous but correct if you use it right, shouldn't we at least have the latter available as an option? E.g. a reallyUnloadObj function that also frees the image. If I uncomment that line will it fix the problem? Is it safe to do so if I first force all thunks that might contain unloaded code? Long shot, but are there any more principled ways to guarantee no pointers to a chunk of code exist? The only thing I can think of is to have the state be totally strict and consist only of types from the static core. Would it be possible to hand responsibility for the memory off to the garbage collector? GHC now supports dynamic libraries. Given that plugins may need to link large portions of the static core "library", can it be loaded as a dynamic library so both the core and the plugins can share the same code? I haven't been able to find many references to ghc's support for dynamic linking.

3 5

How to make cabal pass flags to happy?
by Niklas Broberg 21 Oct '10

21 Oct '10

Hi all, I want to do something I thought would be quite simple, but try as I might I can't find neither information nor examples on how to achieve it. What I want specifically is to have happy produce a GLR parser from my .ly file, and I want this to happen during 'cabal install'. Which in turn means I want cabal to pass the "--glr" flag to happy during setup. My best guess is that I might want to use 'ppHappy' [1], or something in the vicinity, but there's no documentation for the combinator and it's far from obvious how to pass arguments to it. ... help? :-) Cheers, /Niklas

3 5

libefence useful for debugging ghc+ffi programs?
by Johannes Waldmann 21 Oct '10

21 Oct '10

Hi. I have an application compiled with ghc and using C via FFI (and lots of Haskell libraries as well) and it keeps annoying me with segfaults and (worse) unpredictable behaviour. I guess a C programmer would now apply electric-fence or valgrind. Is this a reasonable approach for ghc+ffi as well? E.g., this: Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x4114f950 (LWP 8343)] 0x00007f5af4e273b5 in free () from /usr/lib/libefence.so.0 (gdb) where #0 0x00007f5af4e273b5 in free () from /usr/lib/libefence.so.0 #1 0x00007f5af4059519 in ?? () from /usr/lib/libcurl.so.4 #2 0x00007f5af4059748 in ?? () from /usr/lib/libcurl.so.4 #3 0x00007f5af4059abd in ?? () from /usr/lib/libcurl.so.4 #4 0x00007f5af405f080 in ?? () from /usr/lib/libcurl.so.4 #5 0x00000000007bef95 in ?? () #6 0x00007f5aefb66628 in ?? () #7 0x00007f5af0df9f00 in ?? () #8 0x0000000000000000 in ?? () Is this a reason to distrust libcurl (the C libs) or curl (the Haskell package)? Thanks - J.W.

2 7