- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] iteratee-compress space leak?
by Michael A Baikov 22 Feb '11

22 Feb '11

-----Original Message----- > Hi Maciej, > > Thanks for looking in to this. > > > After looking into problem (or rather onto your code) - the problem have > > nothing to do with iteratee-compress I believe. I get similar behaviour > > and results when I replace "joinIM $ enumInflate GZip > > defaultDecompressParams chunkedRead" by chunkedRead. (The memory is > > smaller but it is due to decompression not iteratee fault). > > > > This is due to "printLines". Whether it's a bug depends on what the correct > behavior of "printLines" should be. > > "printLines" currently only prints lines that are terminated by an EOL > (either "\n" or "\r\n"). This means that it needs to hold on to the entire > stream received until it finds EOL, and then prints the stream, or drops it > if it reaches EOF first. In your case, the stream generated by "convStream > consChunk printLines" is just a stream of numbers without any EOL, where the > length is dependent on the specified block size. This causes the space > leak. > > If I change the behavior of "printLines" to print lines that aren't > terminated by EOL, the leak could be fixed. Whether that behavior is more > useful than the present, I don't know. Alternatively, if you insert some > newlines into your stream this could be improved as well. > > As a result of investigating this, I realized that > Data.Iteratee.ListLike.break can be very inefficient in cases where the > predicate is not satisfied relatively early. I should actually provide an > enumeratee interface for it. So thanks very much for (indirectly) > suggesting that. Actually i can give you full sorce code - it uses also attoparsec-iteratee. it leaks with iteratee-compress and works fine without it. Whole idea - get bytestring from access.log, convert it to stream of data object with usernames and bytes downliaded and then feed this stream into iteratee which will collect all data into one big Map ByteString Integer.

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Faster timeout but is it correct?
by Bas van Dijk 22 Feb '11

22 Feb '11

Dear all, I wrote a faster implementation for System.Timeout.timeout but wonder whether it's correct. It would be great if someone can review the code. The implementation comes with a tiny Criterion benchmark: darcs get http://bifunctor.homelinux.net/~bas/bench_timeouts/ On ghc-7.0.1 with -O2 the following benchmark, which always times out, doesn't show a difference with the current timeout: timeout 1 (threadDelay 1000000) However the following benchmark, which never times out, shows nice speedups for both a non-threaded and a threaded version: timeout 1000000 (return ()) non-threaded: 3.6 faster threaded: 10.8 faster I suspect the reason why my timeout is faster is that I use only one 'catch' while the original uses two: handleJust and bracket. I have to admit that my implementation is more complex and more fragile than the original. That's why I'm not sure of it's correctness yet: newtype Timeout = Timeout Unique deriving (Eq, Typeable) instance Show Timeout where show _ = "<<timeout>>" instance Exception Timeout timeout :: Int -> IO a -> IO (Maybe a) timeout n f | n < 0 = fmap Just f | n == 0 = return Nothing | otherwise = do myTid <- myThreadId timeoutEx <- fmap Timeout newUnique uninterruptibleMask $ \restore -> do tid <- restore $ forkIO $ threadDelay n >> throwTo myTid timeoutEx let handle e = case fromException (e :: SomeException) of Just timeoutEx' | timeoutEx' == timeoutEx -> return Nothing _ -> killThread tid >> throwIO e mb <- restore (fmap Just f) `catch` handle killThread tid return mb If nobody proves it incorrect I will make a patch for the base library. Regards, Bas

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Infinite types should be optionally allowed
by Job Vranish 22 Feb '11

22 Feb '11

There are good reasons against allowing infinite types by default (mostly, that a lot of things type check that are normally not what we want). An old haskell cafe conversation on the topic is here: http://www.nabble.com/There%27s-nothing-wrong-with-infinite-types!-td771373… However, I think infinite types should be allowed, but only with an explicit type signature. In other words, don't allow infinite types to be inferred, but if they are specified, let them pass. I think it would be very hard to shoot yourself in the foot this way. Newtype is the standard solution to situations where you really need an infinite type, but in some cases this can be a big annoyance. Using newtype sacrifices data type abstraction and very useful type classes like Functor. You can use multiparameter type classes and functional dependencies to recover some of the lost abstraction, but then type checking becomes harder to reason about and the code gets way more ugly (If you doubt, let me know, I have some examples). Allowing infinite types would fix this. I'm imagining a syntax something like this: someFunctionThatCreatesInfiniteType :: a -> b | b = [(a, b)] Thoughts? Opinions? Am I missing anything obvious? - Job

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Is there something better than tags?
by Magnus Therning 22 Feb '11

22 Feb '11

I've been using haskellmode for vim for a while now and I'm really happy with it. It does however have some shortcomings. These are the things I find missing at times: 1. Fully functional go-to-definition. With that I mean that go-to-definition is aware of local definitions introduced in 'let' and 'where' clauses and ideally also for names bound in 'do' clauses. I've found there are several ways to generate a tags file, GHCi can generate them, and there are two tools for it, hasktags and hothasktags. haskellmode currently uses GHCi to generate tags. However, the limitation seems to be inherent in the tags format itself. AFAICS there is no way to get a fully functional go-to-definition with that format. 2. Retrieve type information for local defintions ('let', 'where', 'do'), and ideally also for sub expressions (like ':t filter (== 1)'. I believe haskellmode uses GHCi to get the type information out, but it is only able to get it for top-level definitions. Are there any options to address these two shortcomings in haskellmode? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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#haskell-game & Haskellers Game Interest Group
by Korcan Hussein 22 Feb '11

22 Feb '11

Some of us interested in Haskell GameDev have formed chat room & Haskellers Interest Group, you can out find more info here. Just spreading the word around.

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Re: [Haskell-cafe] Please review my Xapian foreign function interface
by Edward Z. Yang 21 Feb '11

21 Feb '11

Excerpts from Oliver Charles's message of Mon Feb 21 08:53:48 -0500 2011: > Yes, this is a concern to me as well. The only places I've used > unsafePerformIO is with Query objects, which I am mostly treating as > immutable data, and never exposing anyway to modify query > objects. That is a good way to start thinking about it. If there is an efficient mechanism for copying query objects, you can also implement "persistent update" (e.g. copy the structure and then mutate it). > However, what is better? Should I avoid taking this > risk/assumption of immutability and use this within the IO monad also? I > guess my biggest fear is that this entire library is only usable in the > IO monad, which from what I understand limits my ability to test easily. Don't take the risk: verify for yourself that there is no risk! Note that putting things in IO doesn't get you off the concurrency hook: things in IO can be run in different threads and you need to synchronize them. Indeed, as the Xapian faq states: If you want to use the same object concurrently from different threads, it's up to you to police access (with a mutex or in some other way) to ensure only one method is being executed at once. It is admittedly more annoying to test things in IO. One thing you can do is if database objects are completely isolated from one another (which seems to be the case) you can build up a custom monad for manipulating this object in a single-threaded and/or thread safe manner. I did something like this (actually, I needed to enforce more complex invariants about when what functions could get called), but unfortunately it was for work and the code hasn't been cleared for publication yet. Cheers, Edward

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Re: [Haskell-cafe] iteratee-compress space leak?
by John Lato 21 Feb '11

21 Feb '11

Hi Maciej, Thanks for looking in to this. > From: Maciej Piechotka <uzytkownik2(a)gmail.com> > > On Fri, 2011-02-18 at 17:27 +0300, Michael A Baikov wrote: > > I am trying to play with iteratee making parser for squid log files, but > found that my code do not run in constant space when it tries to process > compressed log files. So i simplified my code down to this snippet: > > > > import Data.ByteString (ByteString) > > import Data.Iteratee as I > > import Data.Iteratee.Char > > import Data.Iteratee.ZLib > > import System > > > > main = do > > args <- getArgs > > let fname = args !! 0 > > let blockSize = read $ args !! 1 > > > > fileDriver (leak blockSize) fname >>= print > > > > leak :: Int -> Iteratee ByteString IO () > > leak blockSize = joinIM $ enumInflate GZip defaultDecompressParams > chunkedRead > > where > > consChunk :: Iteratee ByteString IO String > > consChunk = (joinI $ I.take blockSize I.length) >>= return . show > > > > chunkedRead :: Iteratee ByteString IO () > > chunkedRead = joinI $ convStream consChunk printLines > > > > > > First argument - file name (/var/log/messages.1.gz will do) > > second - size of block to consume input. with low size (10 bytes) of > consumed blocks it leaks very fast, with larger blocks (~10000) it works > almost without leaks. > > > > So. Is it bugs within my code, or iteratee-compress should behave > differently? > > After looking into problem (or rather onto your code) - the problem have > nothing to do with iteratee-compress I believe. I get similar behaviour > and results when I replace "joinIM $ enumInflate GZip > defaultDecompressParams chunkedRead" by chunkedRead. (The memory is > smaller but it is due to decompression not iteratee fault). > This is due to "printLines". Whether it's a bug depends on what the correct behavior of "printLines" should be. "printLines" currently only prints lines that are terminated by an EOL (either "\n" or "\r\n"). This means that it needs to hold on to the entire stream received until it finds EOL, and then prints the stream, or drops it if it reaches EOF first. In your case, the stream generated by "convStream consChunk printLines" is just a stream of numbers without any EOL, where the length is dependent on the specified block size. This causes the space leak. If I change the behavior of "printLines" to print lines that aren't terminated by EOL, the leak could be fixed. Whether that behavior is more useful than the present, I don't know. Alternatively, if you insert some newlines into your stream this could be improved as well. As a result of investigating this, I realized that Data.Iteratee.ListLike.break can be very inefficient in cases where the predicate is not satisfied relatively early. I should actually provide an enumeratee interface for it. So thanks very much for (indirectly) suggesting that. Cheers, John L

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PhD Studentships in Functional Programming
by Henrik Nilsson 21 Feb '11

21 Feb '11

Apologies for multiple copies. -- Henrik Nilsson School of Computer Science The University of Nottingham nhn(a)cs.nott.ac.uk

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Re: [Haskell-cafe] Haskell for children? Any experience?
by John Lato 21 Feb '11

21 Feb '11

> > > Message: 7 > Date: Sun, 20 Feb 2011 04:34:56 +0900 > From: "Benjamin L. Russell" <DekuDekuplex(a)Yahoo.com> > Subject: Re: [Haskell-cafe] Haskell for children? Any experience? > To: haskell-cafe(a)haskell.org > Message-ID: <ijp613$9q5$1(a)dough.gmane.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Actually, I've been wishing for a high-level way of creating an > interactive three-dimensional virtual world in Haskell that doesn't > require explicit knowledge of linear algebra. Ideally, I'm looking for > a Haskell way of creating a functional counterpart to, say, Open Cobalt > (see http://www.opencobalt.org/) that is high-level-enough not to > require explicit manipulation of row and column vectors. > > One of the main problems, however, is the lack of reflection. Ideally, > I would like the project to be able to modify its own framework in real > time, so that, for example, within the virtual world, users would be > able to create portals to other virtual worlds, and then write code > while the project was running to change the configuration without > restarting the project. Then users would be able to write code in the > functional style to change the virtual environment _in situ._ > It's not Haskell, but this sounds a lot like Newspeak: http://bracha.org/Site/Newspeak.html John L.

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Releasing head of lazy ByteString
by tsuraan 21 Feb '11

21 Feb '11

I have a streaming network protocol where each message in the stream is prefixed by a 64-bit message length marker. Lazy ByteStrings seem to be an elegant way to wrap network communications, so I'm using them. I have one concern though: how can I prevent my program from hanging on to the beginning of the stream? My code looks roughly like this: lazy <- getContents clientSock let (lenBS, rest1) = splitAt 8 lazy let length = runGet getWord64be lenBS let (msg, rest2) = splitAt (fromIntegral length) rest1 -- process msg The program never uses that initial "lazy" again, but it's there, and it's been assigned, so I assume that reference to the head of the stream will always be around, and thus always consuming memory. Is there a way to indicate to haskell that I want to "forget" about that assignment, so the runtime doesn't need to keep the stream around? In this case, I think I could do the stream processing in a tail-recursive loop, and thus lose the previous references to the stream, but is there a more specific way to tell haskell that I'm done with some data?

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Please review my Xapian foreign function interface
by Oliver Charles 21 Feb '11

21 Feb '11

Hello! I've finally came up with some motivation for a project to get my feet wet using Haskell, and for this little pet project I need an interface to Xapian. After reading various documents on FFI in general, I've got a brief working implementation, and I'm now looking for how to better structure the public API. First, a quick bit of background if you're not familiar with Xapian. Xapian is a search engine, and provides a C++ API. You store documents in a database (handled by Xapian), and index documents by adding terms to them. Xapian provides stemming algorithms to help generate these terms from other data. Xapian also has an interface to queries (through a Xapian::Enquire object), and also a query parser to allow for natural language queries to be parsed and ran. For more information, you can check out the API at [1] - it's fairly small. As Xapian is C++, it seems my best option is to create my own simple C wrapper, which also lets me tailor my FFI to be easy to use from Haskell. You can see my C api on Github [2] - for now it's very stripped down; I've been wrapping stuff on a need-to-use basis. * * * Currently what I have is functional (in the sense that it works), but it's extremely tied to I/O and very little of the code is pure. For example, to create and index a document, you need to do something along the lines of: do document <- newDocument setDocumentData document "Document data" addPosting document "search_term" 1 addDocument database document (Assuming you already have an open database handle). How horrible imperative this all looks! :-) A document *feels* like it should be quite pure, however retrieving properties of a document performs I/O. For example, I'd like to have something like: data Document = Document { data :: String, postings :: [String] } do document <- getDocument database 123 -- Get doc #123 and have `document` refer to a pure Document object. I'm still stuck in the IO monad a bit, but at least I can write pure functions to operate on `Document` values now. The problem I see with this, is that I believe I'd have to retrieve all parts of document in my `getDocument` function (include the data and all postings), and I can't benefit from being lazy here. >From what I gather, all the methods on Xapian documents are lazy (such as getting the document data, and getting terms associated with documents), which would mean that my foreign imports would have to be `IO String`, for example. This tends to fairly quick cause the IO monad to propogate everywhere. * * * I think that's enough information to explain my current progress, and my concerns. It could well be that I'm overly worrying about everything being in the IO monad, but as I said - Haskell is new to me. All of my work is at [3], and I'd love any advice you have. Haddock documents have been exported to ocharles.org.uk [4]. Thanks for your time, Oliver Charles / ocharles -- [1]: http://xapian.org/docs/apidoc/html/annotated.html [2]: https://github.com/ocharles/Xapian-Haskell/blob/master/c/cxapian.h [3]: https://github.com/ocharles/Xapian-Haskell [4]: http://ocharles.org.uk/tmp/search-xapian/Search-Xapian.html

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Alex -g
by Mihai Maruseac 20 Feb '11

20 Feb '11

Hi, When running Alex -g I get several warning telling me that a bang pattern is required and that the warning will be an error in GHC 6.14. It is something along the following lines: Warning: Bindings containing unlifted types must use an outermost bang pattern: { (base) = alexIndexInt32OffAddr alex_base s } *** This will be an error in GHC 6.14! Fix your code now! In the expression: let (base) = alexIndexInt32OffAddr alex_base s ((I# (ord_c))) = ord c (offset) = (base +# ord_c) .... I'm running it without the -g flag for now. Don't know yet where too look to solve it or if it is really needed. -- Mihai

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iteratee-compress space leak?
by Michael A Baikov 20 Feb '11

20 Feb '11

I am trying to play with iteratee making parser for squid log files, but found that my code do not run in constant space when it tries to process compressed log files. So i simplified my code down to this snippet: import Data.ByteString (ByteString) import Data.Iteratee as I import Data.Iteratee.Char import Data.Iteratee.ZLib import System main = do args <- getArgs let fname = args !! 0 let blockSize = read $ args !! 1 fileDriver (leak blockSize) fname >>= print leak :: Int -> Iteratee ByteString IO () leak blockSize = joinIM $ enumInflate GZip defaultDecompressParams chunkedRead where consChunk :: Iteratee ByteString IO String consChunk = (joinI $ I.take blockSize I.length) >>= return . show chunkedRead :: Iteratee ByteString IO () chunkedRead = joinI $ convStream consChunk printLines First argument - file name (/var/log/messages.1.gz will do) second - size of block to consume input. with low size (10 bytes) of consumed blocks it leaks very fast, with larger blocks (~10000) it works almost without leaks. So. Is it bugs within my code, or iteratee-compress should behave differently?

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Sub-optimal
by Andrew Coppin 20 Feb '11

20 Feb '11

I have a small program that fills a file with random numbers. If I compile it without optimisation, it runs in constant space. And yet, if I supply -O2 (or even just -O1), for large output files the program gobbles large amounts of RAM. Is this a known bug? (GHC 6.10.x)

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Unable to get parallelism using `par`
by C K Kashyap 20 Feb '11

20 Feb '11

Hi, I tried the first example from "A tutorial on Parallel and Concurrent programming in Haskell" but I cant seem to get sparks to get converted to OS threads. Below is the program I am using. I did ghc --make -threaded program.hs then ./program +RTS -N2 I don't see any speed gain compared to N1. Am I missing something? import System.Time import Control.Parallel fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) mkList :: Int -> [Int] mkList n = [1..n-1] relPrime :: Int -> Int -> Bool relPrime x y = gcd x y == 1 euler :: Int -> Int euler n = length (filter (relPrime n) (mkList n)) sumEuler :: Int -> Int sumEuler = sum . (map euler) . mkList sumFibEuler :: Int -> Int -> Int sumFibEuler a b = fib a + sumEuler b parSumFibEuler :: Int -> Int -> Int parSumFibEuler a b = f `par` (e `pseq` (e + f)) where f = fib a e = sumEuler b secDiff :: ClockTime -> ClockTime -> Float secDiff (TOD secs1 psecs1) (TOD secs2 psecs2) = fromInteger (psecs2 - psecs1) / 1e12 + fromInteger (secs2 - secs1) r1 :: Int r1 = sumFibEuler 38 5300 main :: IO () main = do t0 <- getClockTime pseq r1 (return()) t1 <- getClockTime putStrLn ("Sum: " ++ show r1) putStrLn ("time: " ++ show (secDiff t0 t1) ++ " seconds")

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Update2
by Patrick M 19 Feb '11

19 Feb '11

Ok so I wrote and update function update :: Node -> [Node] It takes in a Node and returns a list of the Nodes that result from assigning True to an unassigned atom in one case and False in the other (ie. a case split). The list returned has two nodes as elements. One node contains the formula with an atom assigned True and the model updated with this assignment, and the other contains the formula with the atom assigned False and the model updated to show this. The lists of unassigned atoms of each node are also updated accordingly. This function makes use of an assign function to make the assignments. It also uses the chooseAtom function to select the literal to assign. update :: Node -> [Node] update (formula, (atoms, model)) = [(assign (chooseAtom atoms, True) formula, (remove (chooseAtom atoms) atoms, ((chooseAtom atoms,True)) `insert` model)) , (assign (chooseAtom atoms, False) formula, (remove (chooseAtom atoms) atoms, ((chooseAtom atoms, False) `insert` model)) )] Now I have to do the same thing but this time I must implement a variable selection heuristic.this should replace the chooseAtom and I'm supposed to write a function update2 using it type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) update2 :: Node -> [Node] update2 = undefined So my question is how can I create a heurestic and to implement it into the update2 function ,that shoud behave identical to the update function ?

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Re: [Haskell-cafe] Haskell for children? Any experience?
by Chris Smith 19 Feb '11

19 Feb '11

Manuel, Wow, that gloss package is really cool, and exactly the sort of thing I was looking for. As I've said before, I don't think I can prevent this from becoming about how to write games eventually. Gloss looks like provides a nice way to approach graphics programming in a simple functional style, with a clean interface consisting entirely of high-level ideas, and which easily switches over to the game interface later. Awesome! Thank you so much for pointing it out. -- Chris Smith

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Haskell for children? Any experience?
by Chris Smith 19 Feb '11

19 Feb '11

So I find myself being asked to plan Haskell programming classes for one hour, once a week, from September through May this coming school year. The students will be ages 11 to 13. I'm wondering if anyone has experience in anything similar that they might share with me. I'm trying to decide if this is feasible, or it I should try to do something different. To be honest, as much as I love Haskell, I tried to push the idea of learning a different language; perhaps Python. So far, the kids will have none of it! This year, I've been teaching a once-a-week exploratory mathematics sort of thing, and we've made heavy use of GHCi... and they now insist on learning Haskell. (By the way, GHCi is truly amazing for exploratory mathematics. We really ought to promote the idea of Haskell for elementary / junior-high level math teachers! It's so easy to just try stuff; and there are so many patterns you can just discover and then say "Huh, why do you think that happens? Can you write it down precisely? ...") -- Chris Smith

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skin info of haskellwiki
by hanjoosten 19 Feb '11

19 Feb '11

Hi, I would like to use a copy of the hawiki skin for http://ampersand.sf.org my projects mediawiki . I like the way the front page of www.haskell.org works. It uses a mediawiki skin, that is customized for the haskellwiki. However, it used to be http://www.haskell.org/haskellwiki/HaskellWiki:Site_software published , but I find broken links. Is there anyone that could help me out? thanks! -- View this message in context: http://haskell.1045720.n5.nabble.com/skin-info-of-haskellwiki-tp3392503p339… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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linear and dependent types
by Vasili I. Galchin 19 Feb '11

19 Feb '11

Hello, Does Haskell currently have support for linear types and dependent types? If so, is it necessary to specify a "pragma" to use and if so, what is the pragma(s)? Kind regards, Vasili

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Re: [Haskell-cafe] linear and dependent types
by oleg＠okmij.org 19 Feb '11

19 Feb '11

There has been extensive discussion of whether Haskell has dependent types, made larger by the fact that the definition of dependently-typed languages is a bit fuzzy. On some accounts, type functions or GADTs are sufficient for calling a language dependently-typed, and Haskell got both features. The situation is clearer with linear types. First of all, Haskell has all facilities for statically ensuring the proper use of resources. The simplest example is enforcing the locking protocol: a lock can be acquired no more than once; only acquired locks may be released, all acquired locks must be released at the end. This simple example is described in Sec 5.2 of http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fu… That example can be made more complex: a program with two locks choosing the lock to acquire based on the data it reads from a file. We can still statically guarantee that all acquired locks are released and no lock is acquired or released twice -- even if we cannot statically tell which particular lock will be acquired during a program run. Other examples of resource control include session types and regions. There are several Hackage packages with these keywords. Generally, Haskell can easily embed languages with dependent or linear types -- just as easily as Haskell embeds languages with IO, global mutable state, non-determinism. The latter languages go by the name of monads. One can easily embed typed linear lambda-calculus, with Haskell type-checker ensuring that each bound variable is referenced exactly once. http://okmij.org/ftp/tagless-final/course/index.html#linear The linear lambda-calculus turns out not very useful; no wonder Girard had to add exponentials.

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WGP 2011: Call for Papers
by Shin-Cheng Mu 19 Feb '11

19 Feb '11

====================================================================== CALL FOR PAPERS WGP 2011 7th ACM SIGPLAN Workshop on Generic Programming Tokyo, Japan Sunday, September 18th, 2011 http://flolac.iis.sinica.edu.tw/wgp11/ Collocated with the International Conference on Functional Programming (ICFP 2011) ====================================================================== Goals of the workshop --------------------- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, and, for at least 20 years, generic programming techniques have been a specific focus of research in the functional and object-oriented programming communities. Generic programming has gradually spread to more and more mainstream languages, and today is widely used in industry. This workshop brings together leading researchers and practitioners in generic programming from around the world, and features papers capturing the state of the art in this important area. We welcome contributions on all aspects, theoretical as well as practical, of * generic programming, * programming with (C++) concepts, * meta-programming, * programming with type classes, * programming with modules, * programming with dependent types, * polytypic programming, * adaptive object-oriented programming, * component-based programming, * strategic programming, * aspect-oriented programming, * family polymorphism, * object-oriented generic programming, * and so on. Organizers ---------- Co-Chair Jaakko Järvi, Texas A&M University, USA Co-Chair Shin-Cheng Mu, Academia Sinica, Taiwan Programme Committee ------------------- Dave Abrahams, BoostPro Computing, USA Magne Haveraaen, Universitetet i Bergen, Norway Akimasa Morihata, Tohoku University, Japan Pablo Nogueira, Universidad Politécnica de Madrid, Spain Ulf Norell, Chalmers University of Technology and University of Gothenberg, Sweden Ross Paterson, City University London, UK Rinus Plasmeijer, Radboud University Nijmegen, The Netherlands Sibylle Schupp, Technische Universität Hamburg-Harburg, Germany Andrew Sutton, Kent State University, USA Tarmo Uustalu, Institute of Cybernetics, Estonia Important Information --------------------- We plan to have formal proceedings, published by the ACM. Submission details Deadline for submission: Monday 2011-06-06 Notification of acceptance: Tuesday 2011-07-01 Final submission due: Monday 2011-07-25 Workshop: Sunday 2011-09-18 Authors should submit papers, in postscript or PDF format, formatted for A4 paper, to the WGP11 EasyChair instance by the above deadline. The length should be restricted to 12 pages in standard (two-column, 9pt) ACM format. Accepted papers are published by the ACM and will additionally appear in the ACM digital library. History of the Workshop on Generic Programming ---------------------------------------------- This year: * Tokyo, Japan 2011 (affiliated with ICFP11) Earlier Workshops on Generic Programming have been held in * Baltimore, Maryland, US 2010 (affiliated with ICFP10) * Edinburgh, UK 2009 (affiliated with ICFP09) * Victoria, BC, Canada 2008 (affiliated with ICFP), * Portland 2006 (affiliated with ICFP), * Ponte de Lima 2000 (affiliated with MPC), * Marstrand 1998 (affiliated with MPC). Furthermore, there were a few informal workshops * Utrecht 2005 (informal workshop), * Dagstuhl 2002 (IFIP WG2.1 Working Conference), * Nottingham 2001 (informal workshop), There were also (closely related) DGP workshops in Oxford (June 3-4 2004), and a Spring School on DGP in Nottingham (April 24-27 2006, which had a half-day workshop attached). Additional information: The WGP steering committee consists of J. Gibbons, R. Hinze, P. Jansson, J. Järvi, J. Jeuring, B. Oliveira, S. Schupp, and M. Zalewski

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Status update on {code, trac, projects, planet, community}.haskell.org
by Duncan Coutts 18 Feb '11

18 Feb '11

All, As you will be aware, some of the *.haskell.org websites have been down recently, specifically: code.haskell.org trac.haskell.org projects.haskell.org planet.haskell.org community.haskell.org These are all hosted on the community server. The community server was hacked on the 26th January and we took it offline. The server was running an old version of debian that was no longer supported with security updates. (Ironically, two days previously the infrastructure team had been discussing the fact that nobody seemed to have any time available to do the planned migration to a new host). The hacker replaced sshd which we noticed because the ssh host signature changed and it started prompting for passwords (we use key-based rather than password based logins). MSR kindly allowed Ian to take time off from the GHC 7.0.2 release to work on migrating the services to the new host (that had previously been partially prepared). Thanks to Ian's efforts and some help from other members of the infrastructure team, the migration is now nearly complete. planet.haskell.org should now be working, along with email, project mailing lists and trac instances. We have not yet re-enabled user login accounts, nor re-enabled access to code repositories. The support ticket system is not yet enabled. We will send a further update when these are re-enabled, or procedures for people to re-enable them are finalised. On a more positive note, the new VM that we are using is a lot more powerful than the old one, in particular about 5x more memory. As many of you will have experienced, services on the old server tended to go AWOL which was primarily due to running up against memory limits, causing services to get killed off. In the case of the web server it was mainly due to having to use a very small number of concurrent connections, again to minimise memory use. So all in all we expect the new server to be a good deal more reliable. Duncan (On behalf of the Haskell infrastructure team)

6 11

`cabal install darcs` fail !
by z_axis 18 Feb '11

18 Feb '11

>cabal update >cabal install darcs ...... Installing library in /home/sw2wolf/.cabal/lib/hashed-storage-0.5.5/ghc-6.10.4 Registering hashed-storage-0.5.5... Reading package info from "dist/installed-pkg-config" ... done. Writing new package config file... done. cabal: Error: some packages failed to install: darcs-2.5.1 depends on haskeline-0.6.3.2 which failed to install. haskeline-0.6.3.2 failed during the configure step. The exception was: ExitFailure 1 >uname -a FreeBSD mybsd.zsoft.com 8.1-RELEASE FreeBSD 8.1-RELEASE #1: Wed Sep 8 09:07:54 CST 2010 root@mybsd.zsoft.com:/media/G/usr/obj/media/G/usr/src/sys/MYKERNEL i386 It seems that the cabal is NOT very strong, isnot it ? ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/cabal-install-darcs-fail-tp3390542p339… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

Haskell Web Framework Happstack 6 Released
by Jeremy Shaw 18 Feb '11

18 Feb '11

Hello, I am pleased to announce the release of Happstack 6. We fully recommend that all Happstack users migrate to the new release. Happstack is a high-performance Haskell web framework. It has high- level functionality for routing requests, extracting parameters, manipulating responses, serving static file contents, compression responses, and more. It includes an integrated HTTP server and a native RAM-cloud database system known as MACID. This allows you to deploy your web application with out having to configure apache, php, mysql, memcached, etc! Install instructions can be found here: http://happstack.com/download If you are porting existing applications you should check out the Happstack 6 migration guide: http://code.google.com/p/happstack/wiki/Happstack6Migration Porting should be pretty straight-forward. Despite the massive reorganization, the exports from Happstack.Server are largely the same. What's Exciting! ---------------- The focus of this release has been happstack-server. The primary goal was to make happstack-server much easier to learn and to use. This includes: 1. improving the overall organization of the library 2. greatly improving the documentation 3. making it easier to install 4. ensuring that the server provides fast, reliable performance 5. making it easier for new (and old) developers to understand and get started If you have looked at Happstack before and found it too difficult to understand, I highly recommend you take another look. The new organization and documentation make it far easier to understand. More Logical Organiation ------------------------ In prior versions, the Happstack.Server.SimpleHTTP module contained just about everything, with little organization, and no real differentiation between the external API and the library internals. In Happstack 6, SimpleHTTP has been refactored into numerous, smaller modules. The internals have been moved into Happstack.Server.Internal.* so that they are not leaking into the external API. This should make it much easier to find what you are looking for, and prevent you from accidently running across internal things like the WebT monad, which you do not really need to know anything about. Better Documentation -------------------- The API documentation for happstack-server is also much better. All external API functions should now have good haddock documentation with many inline examples. However, learning a new library by studying the API documentation can still be pretty frustrating. So we also have the brand new Happstack Crash Course. http://happstack.com/docs/crashcourse/index.html. The Happstack Crash Course covers a vast majority of the happstack- server API. It covers the API in a logical manner and includes many downloadable, runnable examples. It is intended to be read start to finish, and to also be usable as a reference guide for specific "How do I do XXX" questions. Additionally, the guestbook example has been updated to a cleaner, more modern style of Happstack coding. It also has more comments/ haddock documentation. Better API ---------- During the process of documenting Happstack, many small improvements where made to the API. Once you try to document something stupid you realize how stupid it is and decided it would be good to fix it first ;) For example, the Cookie API now makes it more obvious how to create session cookies vs persistent cookies. And makes it obvious how to expire a cookie. The functions for looking up values in the query string and require body no longer require the use of the RqData monad. You can use the look* functions directly in the ServerPart monad. If you do choose to use RqData, it now has an Applicative instance which can accummulate and report lookup errors. (http://happstack.blogspot.com/2010/10/is-rqdata-monad-still-needed.html ) There are a bunch of other improvements to request data handling documented here, http://happstack.com/docs/crashcourse/RqData.html. Easier Install -------------- In order to make Happstack easier to install we have removed as many dependencies as possible. happstack-data no longer depends on happstack-util, which makes it easier to install happstack-data or happstack-state with out the rest of the framework. HSP and HStringTemplate support has been moved into *optional* happstack-hsp and happstack-hstringtemplate packages. New Features ------------ Happstack also has a number of new features including: - support for Heist templates (http://happstack.com/docs/crashcourse/Templates.html#helloheist ) - support for Hamlet templates - improved environment for extracting query string and form values (http://happstack.blogspot.com/2010/10/is-rqdata-monad-still-needed.html ) - support for recompilation and reloading of templates in a running server (http://happstack.blogspot.com/2010/10/recompile-your-haskell-based-template… ) - directory browsing when using serveDirectory And numerous other improvements including enhancements to IxSet, file locking, and much more. Bigger, Better Version Number ----------------------------- We have also upgraded our version numbers! We have decided to call this release Happstack 6.0 instead of Happstack 0.6. There are two reasons for this: 1. the amount of changes we are making really do represent a new major release 2. it will make it easier to comply with the Haskell package version policy. Now that Happstack 6 is out, we plan to make more frequent releases. The new version scheme will allow us to talk about the next major release (Happstack 7), but still put out changes to Happstack 6 which change the API (aka, Happstack 6.2, etc). GHC 7 - mostly -------------- Happstack mostly works with GHC 7. Due to some compiler bugs, HSP does not work with GHC 7.0.1. These bugs were reported and are fixed in the upcoming GHC 7.0.2 release. If you are not using the optional happstack-hsp package, then GHC 7 is fine. If you are using happstack- hsp, then you will need to use GHC 6.12, a recent version of GHC from darcs, or wait until GHC 7.0.2 comes out. The Future ========== The current future plan looks like this: Happstack 7 ----------- The next major release of Happstack will be Happstack 7. The primary focus of Happstack 7 will be improving happstack-state (also known as MACID). Big features include: - sharding support - a rewrite of IxSet based on kdtrees with much better RAM and CPU performance - performance testing to show how awesome MACID is - much better documentation and examples - better tools for examining and modifying state in running applications There are also a bunch of lower-level API improvements planned which should make it nicer to use happstack-state. Happstack 8 ----------- Happstack 8 will finally feature an enumerator-based HTTP backend. This is a feature we have been hoping to add since January of 2009. http://code.google.com/p/happstack/issues/detail?id=29 Rather than develop a competing HTTP backend, Happstack will put its efforts into improving an existing HTTP backend. We currently have our eye on WAI/warp. However, we will also consider snap and hyena when the time comes. It should be noted that the current lazy I/O based backend is not the relentless nightmare of space leaks that some people might lead you to believe it is. The current happstack-server can easily handle uploads and downloads of large or streaming files with out leaks. But, enumerators *are* nice. How You can Help ================ You are encouraged to get involved in Happstack. The easiest way to get involved is to complain about how horrible Happstack is. If you don't voice your complaints, then we might not know something things to be fixed. If you are looking to make code contributions, please feel free to ask on #happs or the mailing list. There are tasks that are easy for novices, tasks that are worthy of a Ph.D, and everything in between. If you want to work, we can certainly find a way to use your talents! There are also plenty of non-coding related tasks including documentation, graphic design, user surveys, etc. You are also encouraged to blog about your Happstack projects! Thanks! - jeremy shaw

2 1

upgrading mtl1 to mtl2
by Evan Laforge 18 Feb '11

18 Feb '11

I just got started on this because packages are starting to use mtl2. I had hoped it would be simple, and hopefully it is, but... Do I really have to add (Functor m) to the 300 or so functions with (Monad m) on them? Or just not use fmap or applicative? Ow. Not what I would call "small incompatibilities." Is there a more graceful way to get around this? So I thought if it's going to be this much of a hassle I might as well just port to transformers, which I gather is supposed to be the future anyway. But transformers is lacking the classes, and I gather they're in monads-tf and monads-fd. But monads-fd says it's now deprecated because of the existence of mtl-2. So what's the story? Has transformers now turned around and been deprecated in favor of mtl-2? Should I just keep using mtl but port to mtl-2? Or is there some other non-obsolete package for transformers? This page: http://haskell.org/haskellwiki/Monad_Transformers Says that the only benefit of 'transformers' is that it's "haskell 98 and thus more portable," but doesn't that come with the caveat that "only if you don't use classes and do all the lifting manually"?

6 9

Help needed for converting IOArray to ByteString
by C K Kashyap 18 Feb '11

18 Feb '11

Hi, I need to convert IOArray to bytestring as shown below - import Data.Array.IO import Data.Binary.Put import qualified Data.ByteString.Lazy as BS import Data.Word main = do arr <- newArray (0,9) 0 :: IO (IOArray Int Int) let bs=toByteString arr return () How can I implement the 'toByteString' function? Regards, Kashyap

5 7

Re: [Haskell-cafe] Alex monadUserState question
by Mihai Maruseac 17 Feb '11

17 Feb '11

On Thu, Feb 17, 2011 at 2:42 PM, Simon Marlow <simonmar(a)microsoft.com> wrote: > >> Just downloaded the latest Alex package from Hackage (different from >> the one in Ubuntu's repositories by 2 minor versions) and found that >> the monadUserState wrapper still misses 2 functions for dealing with >> changes in the user supplied state. Is this intended or it was an >> unwanted omission? > > Could you tell me what's missing? Is this a regression? > Yes, there are two functions (this is the type that I guess they should have) getUserData :: Alex AlexUserState setUserData :: AlexUserState -> Alex () I've used them in a particular implementation today in this way but maybe there are other ways to handle them too. He're the code I've used: getUserData :: Alex AlexUserState getUserData = Alex $ \s@AlexState{alex_ust=ud} -> Right (s, ud) setUserData :: AlexUserState -> Alex () setUserData ust = Alex $ \s -> Right (s{alex_ust=ust}, ()) Thanks for the quick input, -- Mihai

1 0

happy + alex parsing question
by Roman Dzvinkovsky 17 Feb '11

17 Feb '11

Hi, using alex+happy, how could I parse lines like these? > "mr <username> says <message>\n" where both <username> and <message> may contain arbitrary characters (except eol)? If I make lexer tokens > "mr " { T_Mr } > " says " { T_Says } > \r?\n { T_Eol } > . { T_Char $$ } and parser > 'mr ' { T_Mr } > ' says ' { T_Says } > eol { T_Eol } > char { T_Char } ... > line :: { (String, String) } > : 'mr ' string ' says ' string eol { ($2, $4) } > string :: { String } > : char { [ $1 ] } > | char string { $1 : $2 } then I get error when <username> or <message> contain "mr " substrings, because parser encounters T_Mr token. Workaround is mention all small tokens in my <string> definition: > string :: { String } > : { [] } > | 'mr ' string { "mr " ++ $2 } > | ' says ' string { " says " ++ $2 } > | char string { $1 : $2 } but that is weird and I'm sure there is a better way. Thanks for advance, Roman.

3 3

Using MonadFix to tie the knot with STM
by Sebastiaan Visser 17 Feb '11

17 Feb '11

Hi all, During a little experiment I discovered there is no MonadFix instance available for the STM monad. Is this absence the consequence of some deep restriction of how STM works or 'just accidental'? Is there some way I could derive this instance? For those who are interested and may know other ways to solve my problem, I'm trying to tie the knot in a graph by using transactional variables as the edges between nodes. I'm trying to do this in a rather generic way: > {-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, StandaloneDeriving, DoRec #-} > module Graph where > > import Control.Applicative > import Control.Concurrent.STM > import Control.Monad.Fix > import Data.Maybe > import Data.Foldable > import Data.Traversable > import Prelude hiding (mapM) > import qualified Data.Map as M > > instance MonadFix STM where > mfix = error "I need this instance!" -- What to do? > > -- A single node is a graph, has a list of both incoming and outgoing edges. > > data Node edge = Node > { nodeId :: NodeId > , payload :: String > , incoming :: [edge] > , ougoing :: [edge] > } deriving (Functor, Foldable, Traversable) > > type NodeId = String > > -- A graph with an index on the nodes by NodeId, parametrized with the type we > -- want to used as the edge pointer. > > type Graph edge = M.Map NodeId (Node edge) > > -- Type level fixed point combinator with a TVar annotation. > > newtype TFix f = TIn { tout :: TVar (f (TFix f)) } > > -- Take a graph that uses NodeIds as edges and recursively transform it into a > -- graph with TVars to the neighbouring nodes in the edges. > > tieTheKnot :: Graph NodeId -> STM (Graph (TFix Node)) > tieTheKnot untied = > do rec tied <- (mapM . mapM) (\nodeid -> TIn <$> newTVar (tryLookup nodeid tied)) untied > return tied > > -- Helper function to lookup a pre-tied node from a graph, throws an error when > -- the edge could not be resolved. This should, of course, not happen! > > tryLookup :: NodeId -> Graph (TFix Node) -> Node (TFix Node) > tryLookup i = fromJust (error msg) . M.lookup i > where msg = "tieTheKnot: Lookup error, input is an incomplete graph." Thanks in advance, -- Sebastiaan Visser http://www.silkapp.com

3 3

Alex monadUserState question
by Mihai Maruseac 17 Feb '11

17 Feb '11

Hi, Just downloaded the latest Alex package from Hackage (different from the one in Ubuntu's repositories by 2 minor versions) and found that the monadUserState wrapper still misses 2 functions for dealing with changes in the user supplied state. Is this intended or it was an unwanted omission? Thanks, Mihai

1 0

Haskell Weekly News: Issue 169
by Daniel Santa Cruz 17 Feb '11

17 Feb '11

Welcome to issue 169 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of February 6 - 12, 2011. Congrats to Bryan O'Sullivan and Mark Lentczner for pulling off a successfull [2]BayHac 2011! Announcements Joao Fernandes [3]sent out a call for papers for the 4th International Conference on Software Language Engineering to be held in Braga, Portugal, 3-6 July 2011. Bas van Dijk annouced [4]version 0.1 and [5]version 0.2 of his monad-control library. "monad-control is a rewrite of monad-peel than uses CPS style operations and exploits RankNTypes language extensions to simplify and speed up most functions." Gregory Collins and the Snap team [6]proudly announced the release of Snap 0.4 "containing a whole bunch of nifty new features." Peter Scott [7]announced pwstore, a library that aims to "handle all the details of password storage for you, in a way that should be so easy to use that there's no reason not to use it." Bas van Dijk [8]released case-insensitive 0.1. "The package provides the module Data.CaseInsensitive which exports the CI type constructor which can be parameterised by a string-like type... Comparisons of values of the resulting type are then insensitive to cases." Ryan Newton [9]released a package to do accelerated AES and RNG. Brent Yorgey [10]sent in a second call for submissions to the Monad.Reader -- special poetry and fiction edition. John Lato [11]released version 3.0.1 of ListLike, which "provides polymorphic functions with Data.List-style api for many different data types, including list, arrays, and bytestrings. John also [12]released version 0.7.0.2 and 0.8.0.1 of iteratee, after discovering a memory leak which affects all current versions of iteratee. He states that he would "strongly encourage all users of earlier versions to upgrade to one of these releases." Ganesh Sittampalam and the darcs team are [13]pleased to announce version 2.5.1, a minor upgrade for the 2.5 release. "The main focus of this release is support for the GHC 7.0 series and the upcoming February 2011 Haskell Platform, but the release also includes bug fixes and minor usability improvements." Trystan Spangler [14]announced the release of hspec: Behavior Driven Development for Haskell. "hspec ties together requirements and code that verifies the requirements are met. The verifying code can be a boolean expression, a QuickCheck property, or an explination of why it's pending. The requirements can then be turned into documentation of what the code does and what remains to be implemented." Wren Ng Thornton [15]announce bytestring-trie 0.2.3, a "long awaited release for efficient finite maps from (byte)strings to values." Brent Yorgey would like to invite you to [16]save the date for the Haskell hackathon in Philadelphia, July 29-31. Quotes of the Week * smerdyakov: Some might argue that you can derive False from a sentence containing both "dependent types" and "contract programming." * Einstein: Make everything as simple as possible, but not simpler. * Einskell: Make everything as monad as possible, but not monadder. * kmc: it's important to show that Haskell is usable not just for useless maths, but also for real-world tasks such as rendering teapots * ddarius: Attempting to join #not-not-math sent me to #math. Freakin' Boole. * gwern: [quoting a hypothetical edwardk] 'I HAVE CONSUMED YOUR KNOWLEDGE; LEAVE ME LEST I CONSUME YOUR FLESH AS WELL' * monochrom: @let germanize = concat . words Top Reddit Stories * Announcing: Snap Framework v0.4 Domain: snapframework.com, Score: 40, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/fgc38/announcing_snap_framework_v0… Original: http://snapframework.com/blog/2011/02/05/snap-0.4-released * Haskell Summers of Code retrospective (updated for 2010) Domain: gwern.net, Score: 31, Comments: 7 On Reddit: http://www.reddit.com/r/haskell/comments/fid5w/haskell_summers_of_code_retr… Original: http://www.gwern.net/Haskell%20Summer%20of%20Code.html * How to write type-level lambda-abstractions at the value level Domain: haskell.org, Score: 26, Comments: 7 On Reddit: http://www.reddit.com/r/haskell/comments/fjbxw/how_to_write_typelevel_lambd… Original: http://www.haskell.org/pipermail/haskell-cafe/2011-February/089184.html * Storing passwords securely: don't roll your own Domain: finger-tree.blogspot.com, Score: 25, Comments: 22 On Reddit: http://www.reddit.com/r/haskell/comments/fgm26/storing_passwords_securely_d… Original: http://finger-tree.blogspot.com/2011/02/storing-passwords-securely-in-haske… * #haskell-game & Haskellers Game Interest Group Domain: self.haskell, Score: 25, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/fjzz0/haskellgame_haskellers_game_… Original: /r/haskell/comments/fjzz0/haskellgame_haskellers_game_interest_group/ * Type class generics to be replaced in GHC 7.2 or 7.4 Domain: haskell.org, Score: 24, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/fiq7o/type_class_generics_to_be_re… Original: http://www.haskell.org/pipermail/glasgow-haskell-users/2011-February/020006… * What can I do with Haskell? Domain: self.haskell, Score: 22, Comments: 33 On Reddit: http://www.reddit.com/r/haskell/comments/fgfh9/what_can_i_do_with_haskell/ Original: /r/haskell/comments/fgfh9/what_can_i_do_with_haskell/ * There seems to be lots of interest in SMT solvers in the Haskell community these days, so I started ##smt on freenode Domain: self.haskell, Score: 20, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/fh2zw/there_seems_to_be_lots_of_in… Original: /r/haskell/comments/fh2zw/there_seems_to_be_lots_of_interest_in_smt_solvers/ * darcs 2.5.1 released Domain: lists.osuosl.org, Score: 16, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fj46y/darcs_251_released/ Original: http://lists.osuosl.org/pipermail/darcs-users/2011-February/026018.html * monad-control Domain: mail-archive.com, Score: 11, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/fgbao/monadcontrol/ Original: http://www.mail-archive.com/haskell@haskell.org/msg23278.html Top StackOverflow Answers * How does Data.MemoCombinators work? votes: 20 Read on SO: http://stackoverflow.com/questions/4980102/how-does-data-memocombinators-wo… This library is a straightforward combinatorization of the well-known technique of memoization. Let's start with the canonical example: fib = (map fib' [0..] !!) where fib' 0 = 0 fib' 1 = 1 fib' n = fib (n-1) + fib (n-2) I interpret what you said to mean that you know how and why this works. So I'll focus on the combinatorization. We ... * What's similar to fmap for monadic values? votes: 16 Read on SO: http://stackoverflow.com/questions/4927014/whats-similar-to-fmap-for-monadi… pelotom's answer is the straightforward one. But not the fun one! sequence is the Haskell function that one can think of as flipping the order of type constructors between a list and a monad. sequence :: (Monad m) => [m a] -> m [a] Now what you want is, so to speak, to flip the order of type constructors between a Maybe and a monad. Data.Traversable ... * Does there exist a monad instance for Data.Map / Data.IntMap? votes: 15 Read on SO: http://stackoverflow.com/questions/4973335/does-there-exist-a-monad-instanc… Is there any particular reason you want to avoid using monad transformers? if you get the MaybeT package from Hackage you can achieve what you want like this: import Control.Monad import Control.Monad.Maybe import Control.Monad.State import qualified Data.Map as Map type MapM k v a = MaybeT (State (Map.Map k v)) a lookupM k = MaybeT $ Map.lookup k `liftM` ... * How to split a string in Haskell? votes: 11 Read on SO: http://stackoverflow.com/questions/4978578/how-to-split-a-string-in-haskell… There is a package for this called split. cabal install split Use it like this: ghci> splitOn "," "my,comma,separated,list" ["my","comma","separated","list"] It comes with a lot of other functions for splitting on matching delimiters or having several delimiters. * Haskell (n+1) in pattern matching votes: 10 Read on SO: http://stackoverflow.com/questions/4913588/haskell-n1-in-pattern-matching/4… It's a specific case of what is called "n+k patterns", which is generally disliked, and will be has been removed from the language. See here for more information. Here is a good note on n+k patterns, which quotes the following from the Haskell 98 Report (emphasis mine): Matching an n+k pattern (where n is a variable and k is a positive integer ... * Setting upper limit to the input set according to the output function votes: 10 Read on SO: http://stackoverflow.com/questions/4927387/setting-upper-limit-to-the-input… The check fib x < 1000000 in the list comprehension filters away the fib x values that are less than 1000000; but the list comprehension has no way of knowing that greater values of x imply greater value of fib x and hence must continue until all x have been checked. Use takeWhile instead: takeWhile (< 1000000) [ fib x | x <- [0..35]] Top StackOverflow Questions * How does Data.MemoCombinators work? votes: 13, answers: 4 Read on SO: http://stackoverflow.com/questions/4980102/how-does-data-memocombinators-wo… * How to best synchronize game engine and network server in Haskell? votes: 10, answers: 3 Read on SO: http://stackoverflow.com/questions/4973401/how-to-best-synchronize-game-eng… * How to choose right Haskell C type? votes: 9, answers: 1 Read on SO: http://stackoverflow.com/questions/4924994/how-to-choose-right-haskell-c-ty… * How to split a string in Haskell? votes: 9, answers: 5 Read on SO: http://stackoverflow.com/questions/4978578/how-to-split-a-string-in-haskell * Haskell (n+1) in pattern matching votes: 7, answers: 1 Read on SO: http://stackoverflow.com/questions/4913588/haskell-n1-in-pattern-matching About the Haskell Weekly News You can find an html version of this newsletter at: http://bit.ly/h1ICad To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://mtnviewmark.wordpress.com/2011/02/13/bayhac-2011-wrap-up/ 3. http://article.gmane.org/gmane.comp.lang.haskell.general/18473/match=call+p… 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18474 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18485 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86087 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18476 8. http://article.gmane.org/gmane.comp.lang.haskell.general/18475 9. http://article.gmane.org/gmane.comp.lang.haskell.general/18481 10. http://themonadreader.wordpress.com/2011/01/19/call-for-copy-special-poetry… 11. http://article.gmane.org/gmane.comp.lang.haskell.general/18486 12. http://article.gmane.org/gmane.comp.lang.haskell.general/18490 13. http://article.gmane.org/gmane.comp.version-control.darcs.user/25902 14. http://article.gmane.org/gmane.comp.lang.haskell.general/18493 15. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15294 16. http://article.gmane.org/gmane.comp.lang.haskell.general/18501

1 0

open datatypes
by rodrigo.bonifacio 16 Feb '11

16 Feb '11

2 1

How to use http-enumerator with hoauth?
by Jeremy Fitzhardinge 16 Feb '11

16 Feb '11

Hi, I'm trying to use http-enumerator with Twitter's streaming API, which requires OAuth authentication. I was hoping to use the hoauth package for this, but it seems that combining with with http-enumerator is pretty awkward. In principle, it should be straightforward since hoauth defines a HttpClient typeclass, and so I would just need to write an instance for http-enumerator. But in practice it is pretty awkward for a few reasons. One is that both packages define their own Request and Response types - however they are semantically identical, so writing conversion functions is tedious but possible. However, I can't work out how to implement error handling; http-enumerator uses the Control.Failure exception mechanism for returning errors, but I can't work out how to fit that into HttpClient's runClient function. If I use "http" rather than "httpRedirect", I can avoid this to some extent (since http doesn't use Failure HttpException m), but I still can't get it to typecheck. I can get a simple non-streaming instance to typecheck using httpLbs (but again, not httpLbsRedirect): data HttpOAuth = HttpOAuth { } instance O.HttpClient HttpOAuth where runClient c r = (HE.httpLbs . http_cvt_request) r >>= return . cvt where cvt :: HE.Response -> Either String O.Response cvt r(a)(HE.Response st _ b) | 200 <= st && st < 300 = Right $ http_cvt_response r | otherwise = Left $ "HTTP status" ++ show st However, I can't get the streaming version to type check at all: newtype HttpOAuthStream a m b = HttpOAuthStream { iter :: W.Status -> W.ResponseHeaders -> DE.Iteratee a m b } instance O.HttpClient (HttpOAuthStream a m b) where -- runClient :: (MonadIO m) => c -> Request -> m (Either String Response) runClient c r = liftM cvt $ DE.run $ HE.http (http_cvt_request r) (iter c) cvt :: Show a => Either a HE.Response -> Either String O.Response cvt (Left a) = Left $ show a cvt (Right r) = Right $ http_cvt_response r (Full code below) And since I'm still trying to get my head around enumerators, I may have that aspect of things completely wrong. I haven't even tried running any of this yet, so I don't know if I've made any non-type errors. Am I even barking up the right tree at all? Should I try to be using hoauth this way at all, or should I just hack it up to work within http-enumerator? That seems counter-productive. A general comment: There are many partially functional http packages in hackage. It seems to me that rather than requiring one package be a complete http library, we would get further by allowing different packages to implement different aspects of http, so long as they can all be composed in a reasonable way. At the very least, is it really necessary for hoauth to define its own Request/Response types? Would it be worthwhile trying to define general types which all http packages could use? Is that the goal of Network.Wai? What about the HTTP package? Should I just use that instead? What about Network.Curl? Thanks, J import qualified Data.Enumerator as DE import qualified Data.ByteString.Char8 as C8 import Control.Monad (liftM) import Control.Applicative ((<$>), (<*>), (<|>), empty, pure) import Control.Arrow (first, second, (***)) import qualified Network.OAuth.Consumer as O import qualified Network.OAuth.Http.HttpClient as O import qualified Network.OAuth.Http.Request as O import qualified Network.OAuth.Http.Response as O import qualified Network.HTTP.Enumerator as HE import qualified Network.Wai as W import Data.List (intercalate) import Control.Failure import Control.Exception (SomeException) -- Convert a Network.OAuth.Http.Request into a Network.HTTP.Enumerator.Request -- What. A. Pain. http_cvt_request :: O.Request -> HE.Request http_cvt_request oar = HE.Request method secure host port path query headers body where method = C8.pack . show . O.method $ oar secure = O.ssl oar host = C8.pack . O.host $ oar port = O.port oar path = C8.pack . intercalate "/" $ O.pathComps oar query = packpair <$> (O.toList . O.qString $ oar) headers = (first W.mkCIByteString) . packpair <$> (O.toList . O.reqHeaders $ oar) body = O.reqPayload oar -- Convert a Network.HTTP.Enumerator.Response into a Network.OAuth.Http.Response -- See above. http_cvt_response :: HE.Response -> O.Response http_cvt_response her = O.RspHttp status reason headers payload where status = HE.statusCode her reason = "" -- ? headers = O.fromList $ (unpackpair . first W.ciOriginal) <$> HE.responseHeaders her payload = HE.responseBody her mappair f (a,b) = (f a, f b) packpair = mappair C8.pack unpackpair = mappair C8.unpack newtype HttpOAuthStream a m b = HttpOAuthStream { iter :: W.Status -> W.ResponseHeaders -> DE.Iteratee a m b } instance O.HttpClient (HttpOAuthStream a m b) where -- runClient :: (MonadIO m) => c -> Request -> m (Either String Response) runClient c r = liftM cvt $ DE.run $ HE.http (http_cvt_request r) (iter c) cvt :: Show a => Either a HE.Response -> Either String O.Response cvt (Left a) = Left $ show a cvt (Right r) = Right $ http_cvt_response r data HttpOAuth = HttpOAuth { } instance O.HttpClient HttpOAuth where runClient c r = (HE.httpLbs . http_cvt_request) r >>= return . cvt where cvt :: HE.Response -> Either String O.Response cvt r(a)(HE.Response st _ b) | 200 <= st && st < 300 = Right $ http_cvt_response r | otherwise = Left $ "HTTP status" ++ show st

4 14

Cabal && license combinations
by Vivian McPhail 16 Feb '11

16 Feb '11

Dear All, There was recently a discussion on haskell-cafe ( http://www.mail-archive.com/haskell-cafe@haskell.org/msg86472.html) about licenses of libraries such as hmatrix and the combination of various different licences. One question was about per-package versus by-file licenses: In Haskell the compilation unit is the module, and the per-file cabal header allows for a license field. It seems then that a package should be the least restrictive combination of all the licenses in all the contained modules. If this has to become a hand-coded fancy function of various GPLx, BSDy, OpenSource, and other licenses then so be it. That is the legal reality. And use of a BSD3 module in hmatrix that does not depend upon GPL'd GSL modules would be acceptable. In short, I argue for a per-file(module) license regime. Vivian

11 15

Alex question
by Mihai Maruseac 16 Feb '11

16 Feb '11

Hi, I want to make Alex to parse a file using states. I wrote a simple basic wrapped .x file but it complaints that it doesn't know the "begin" symbol. As listed here[1], my code does something like this: <0> \/{2} { begin italic } Am I doing something wrong? Should I manage myself the states? Should I use another parser generator? [1]: http://www.haskell.org/alex/doc/html/alex-files.html#startcodes Thanks, -- Mihai

1 2

Unit propagation
by PatrickM 16 Feb '11

16 Feb '11

This is my last part from a project and I need some help with the following function: "If a clause in a propositional formula contains only one literal, then that literal must be true (so that the particular clause can be satisfied). When this happens, we remove the unit clauses (the ones that contain only one literal), all the clauses where the literal appears and also, from the remaining clauses, we delete the negation of the literal (because if P is true, -P will be false). For example, in the formula (P v Q v R) ^ (-P v Q v-R) ^ (P) we have one unit clause (the third clause (P) ). Because this one has to be true for the whole formula to be true we assign True to P and try to find a satisfying assignment for the remaining formula. Finally because -P cannot be true (given the assigned value of P) then the second clause is reduced by eliminating the symbol -P . This simplification results in the revised formula (Q v -R). The resulting simplification can create other unit clauses. For example in the formula (-P v Q) ^ (P) is simplified to (Q) when the unit clause (P) is propagated. This makes (Q) a unit clause which can now also be simplified to give a satisfying assignment to the formula. Your function should apply unit propagation until it can no longer make any further simplifications. Note that if both P and -P are unit clauses then the formula is unsatisfiable. In this case the function" type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) ropagateUnits :: Formula -> Formula propagateUnits = filter.something---here I need help Thanks in advance -- View this message in context: http://haskell.1045720.n5.nabble.com/Unit-propagation-tp3384635p3384635.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 4

Haskell GUI
by Mihai Maruseac 16 Feb '11

16 Feb '11

Hi, I'd like to start a new project in Haskell, this time using an user interface. Looking at [1] I found that there are several of them listed there. However, the list there is very long. Right now, I am unsure on what is best to use. Can someone give me any hints on which is the most kept-to-date and most supported GUI library? Basically, I want to display some graphs in a dynamic way, reacting to user input, nothing fancy right now. Thanks, -- Mihai

5 6

coding a queue with reactive
by sam.roberts.1983＠gmail.com 16 Feb '11

16 Feb '11

Hi all, I hope someone is interested in helping me out with the reactive library. I am trying to implement a function "queue" in reactive: queue :: Double -> Event a -> Event a This is a simple queue: events from the event stream coming into the queue, queue up waiting to be processed one by one. Processing an event takes a constant amount of time for every event. The output of the queue function is the stream of processed events. My current (deficient) implementation of the queue function is: queue dt eventsIn = do (a,exitT) <- withExitTime eventsIn _ <- atTime exitT return a where withExitTime = scanlE calcExitTime (undefined, -1/0) . withTimeE calcExitTime (_,prevExitT) (a,inT) = (a, (max inT prevExitT) + dt) I am having three problems. 1 - I find my implementation of the queue is less clear then an imperative description of a queue. 2 - I rely on being able to calculate the exit time of an event when it first arrives at the queue, whereas an imperative queue would simply store the event in queue and only need to calculate the output time once the event was popped off the queue. If I want to do something similar with my function, I think I need to make some sort of recursive definition of a queue which responds to it's own exit events. I've tried to code this up, but have not managed to wrap my brain around the concept. 3 - The code performs horribly! I am guessing that this is because I have not told reactive that the exit events preserve the ordering of the input events, but I'm not sure how to encode that relationship in reactive. (It's worth noting here that I actually have a fourth problem too: I get linker errors while trying to compile a profiling version of the program ... but that's a separate topic.) It's also worth noting, from a performance point of view, that a much simpler "delay" function with similar use of the bind function performs badly as well: delay :: Double -> Event a -> Event a delay dt es = do (e,t) <- withTimeE es _ <- atTime (t+dt) return e I'd appreciate any light that anyone could shed on any of these problems. If there's a better way of structuring my queue function, or if there's a better way of changing event times in reactive, I am open to all suggestions. Many thanks, Sam

2 4

Noob question about list comprehensions
by Tako Schotanus 16 Feb '11

16 Feb '11

Hello, I was going through some of the tuturials and trying out different (syntactic) alternatives to the given solutions and I I got to this line: *length [chain x | x <- [1..100] , length (chain x) > 15]* Now, there's nothing wrong with it, it works of course. But the application of chain x is repeated twice and I wondered if there was a way for a guard in a list comprehension to refer to the item being produced? Like this for example (invented syntax): *length [@c(chain x) | x <- [1..100] , length c > 15]* NB: Just to make clear, I'm not asking if there is an alternative way of preventing the repetition, of course there is, I'm just wondering about this very specific case within list comprehensions. -Tako

4 4

Proving correctness
by C K Kashyap 16 Feb '11

16 Feb '11

Hi Folks, I've come across this a few times - "In Haskell, once can prove the correctness of the code" - Is this true? I know that static typing and strong typing of Haskell eliminate a whole class of problems - is that related to the proving correctness? Is it about Quickcheck - if so, how is it different from having test sutites in projects using mainstream languages? Regards, Kashyap

21 25

On hGetContents semi-closenesscloseness
by Rafael Cunha de Almeida 16 Feb '11

16 Feb '11

From http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html: Computation hGetContents hdl returns the list of characters corresponding to the unread portion of the channel or file managed by hdl, which is put into an intermediate state, semi-closed. In this state, hdl is effectively closed, but items are read from hdl on demand and accumulated in a special list returned by hGetContents hdl. What state is that? It seems to be something related to Haskell. I couldn't find a definition for it in the unix documentation I have laying around. Continuing reading System IO documentation I read: Any operation that fails because a handle is closed, also fails if a handle is semi-closed. The only exception is hClose. A semi-closed handle becomes closed: * if hClose is applied to it; * if an I/O error occurs when reading an item from the handle; * or once the entire contents of the handle has been read. So it looks like hGetContents sets some flag in the handler saying it is in that semi-closed state. So no other operations are valid, but I think the file descriptor to the file is actually kept open. It's only when the contents are entirely consumed that the descriptor gets closed. Is hGetContents responsible for closing the descriptor? Or is it the garbage collector? Who closes the descriptor when the contents are read? Looking at hGetContents function definition, it uses lazyRead to read the contents, but it calls a wantReadableHandle which might or might not close the handle after lazyRead. By looking at the documentation it seems like the only way for us to actively close the descriptor is either reading the whole thing or calling hClose. But one has to be very carefully when to call the handler, because it doesn't matter if it looks like it was consumed, it really has to be consumed. The following code prints the contents of foo file to the screen: openFile "foo" ReadMode >>= \handle -> (hGetContents handle >>= (\s -> putStr s >> hClose handle)) [1] The following code does not: openFile "foo" ReadMode >>= \handle -> (hGetContents handle >>= (\s -> hClose handle >> putStr s)) [2] It is common knowledge that haskell is very lazy, so it only does things when absolutely necessary, otherwise it prefers to write it off in the TODO list. It does that even if writing to the TODO takes longer than the computation would, that's how lazy it is. That's the origin of the often used expression "he is quite a haskell". The question most people doesn't have a good answer is: when does Haskell thinks it is necessary to do something? In [2] the lazyRead inside of hGetContents (or perhaps hGetContents all together) only gets executed after hClose handle. Why is that? How do I figure out the ordering of computation?

6 5

Review request for "platform independent interactive graphics" with VNC
by C K Kashyap 16 Feb '11

16 Feb '11

Hi, I've been working on a Haskell based platform independent graphics rendering using VNC. I'd like it very much if you could take a look at it and give me feedback. Using it is straight forward - git clone git@github.com:ckkashyap/Chitra.git cd Chitra make ./Main 100 100 5900 Main starts off a vncserver listening on port 5900. A standard vncviewer can be used to connect to 'localhost' and you can see a 100 x 100 screen. Clicking on the screen will set the pixel on the click location. What I eventually want to do is somehow integrate this piece with GHCI in such a manner that one can use GHCI to draw things on the VNC buffer. Regards, Kashyap

4 14

Abstracting a Genetic Algorithm
by Alex Good 16 Feb '11

16 Feb '11

I've copied this from http://stackoverflow.com/questions/5010267/haskell-abstracting-a-genetic-al… as someone suggested that it might spark an interesting discussion I'm new to the world of Haskell programming and I'm cutting my teeth on a simple genetic algorithm for finding good solutions to the Travelling Salesman problem. I am representing the solutions as permutations on Integers and so I have this type synonym type Genome = [Int] The algorithm itself is a set of functions which operate on the solutions: mutation :: Genome -> Genome selectParents :: [Genome] -> [Genome] -> [Genome] crossover :: Genome -> Genome -> (Genome, Genome) selectSurvivors :: [Genome] -> [Genome] -> [Genome] I'm not sure how much of my code is relevant to my question so please ask if more details are needed. One thing that might be worth mentioning is that the type signatures above are actually simplified, I am in fact using the State monad to carry around an StdGen so all of these functions actually return stateful computations. There are several things which I would like to do with this but can't quite get my head around. I want to make it possible to choose different representations for the solutions, it seems to me that this would be a natural place to use a type class, so that Genome would be the type class and [Int] a specific instance of this Genome. Now, I want to be able to experiment with the implementations, and I want to be able to use the code in other projects. Using a type class like this would require that every new algorithm I create would require me to create another instance of Genome, is this a good way to go about creating a library? One bonus question, just a thing that's been bothering me, is there any way to create something like a type synonym for a function so that if I'm writing a function which takes functions as arguments I can write the synonym rather than the whole type signature of the function i.e so that something like the following would work. type someFunc = [Int] -> [Int] -> Int someOtherFunc :: someFunc -> [Int] -> Int Right, hopefully that's a lucid enough explanation of the problem, feel like I've missed the really obvious answer but it hasn't jumped out at me. Cheers

2 1

Re: [Haskell-cafe] ANNOUNCE: hspec-0.1.0 (Behavior Driven Development for Haskell)
by Simon Michael 16 Feb '11

16 Feb '11

On 2/11/11 8:42 PM, trystan.s(a)comcast.net wrote: > Any advice, comments, or questions are welcome. Hi Trystan.. it looks great. I like the rubyish brevity and readability. Please do publish on hackage. What should we read to find out more about this style of tests, http://rspec.info ? When you get a chance, some examples using quickcheck properties would be interesting. I suppose it could also be made to accept hunit tests ? I currently use test-framework partly for it's nice concurrent test runner. I assume hspec tests currently run in sequence ? Any thoughts on how this fits in with the existing haskell test tools, when to use it ? Thanks for working on it. -Simon

2 1

unicode filenames advice
by Joey Hess 15 Feb '11

15 Feb '11

I've been trying to deal with how my haskell program handles unicode filenames. Been dealing with problems like those described here: http://hackage.haskell.org/trac/ghc/ticket/3307 Or, simply demonstrated by feeding unicode to getLine >>= readFile My approach currently is to carefully identify any point where a FilePath is output, and run it through a filePathToString (which varies by OS), as suggested in the above bug report. Is there a less tedious and error-prone way? What is the best approach to use now, assuming that these issues will be dealt with in some way in Haskell, eventually? -- see shy jo

2 1

Re: [Haskell-cafe] Froglingo – a functional programming language
by kevin＠froglingo.com 15 Feb '11

15 Feb '11

Upon a few requests, a brief view about how you would do with Froglingo is given as following: With Froglingo, users' main task is to organize business data and logic in the EP data model, i.e., the way that higher-order functions are related to each others. The remaining task is to specify constraints for data entries and to generate reports (queries). Developing data-intensive software applications in Froglingo is analogous to database management in a relational DBMS, where users only need to define data schemes, enter data, and specify data queries. The main difference is the expressive powers: the relational DBMS is for tables and the relational albegra, and the Froglingo is for total recursive functions and a class of total recursive functions. Note that a class of total recursive functions get rid of non-termination processes and keep the complete semantics for all the meaningful software applications. Depending on business needs, users can specify different report formats such as Froglingo expressions, HTML/XML, or any textual based formats. Froglingo has its own web server such that users can send requests through HTTP over internet. It manages files as data. It has built-in access control operators to protect data as if a file system has its own operators to protect files. The roles of programmers in Froglingo are consolidated with DBAs' because there is no longer "source code" separated from database (every data or logic is in database). Type-free is the most important difference of Froglingo from traditional programming languages in practice. In other words, users don't need to define their own types (e.g., data structures or classes) for software applications. The subtypes of applications are specified as constraints via Froglingo built-in operators as if employee data and salary data were specified in tables of a relational DBMS. Unlike other type-free systems such as the lambda-calculus, the EP data model guarantees bug-free (non-termination). Please let me know if you have any questions. Thank you. Kevin -----Original Message----- From: kevin(a)froglingo.com [mailto:kevin@froglingo.com] Sent: Tuesday, February 8, 2011 07:21 AM To: 'Haskell Cafe' Subject: [Haskell-cafe] Froglingo – a functional programming language Hi, I have recently finished a paper about Froglingo, a programming language designed to make it easier to write data-intensive business applications. It is a monolith consolidating programming, database/file management, data exchange, web server, and access control. What makes Froglingo unique is a sub language that is the first known as semantically equivalent to a class of TOTAL recursive functions. Froglingo shares with Haskell a declarative approach to programming, and strong mathematical foundations, so I thought that people on this list might find it of some interest. You can find the paper at: http://www.froglingo.com/FroglingoPL.pdf. If you have any thoughts about it, I’d love to hear from you. Thank you for your attention. Kevin

1 0

Headlines on haskell.org
by Michael Snoyman 15 Feb '11

15 Feb '11

Hi everyone, I'm not quite sure to whom to address this, but it is with regards to the "Headlines" section at the bottom of haskell.org. It has not been updated for 2011 yet, which I can't imagine looks very good to new users. Is anyone interested in/able to update it? If I may be so bold, I would like to offer up Warp as a possible headline. Are there any objections to this? Michael

5 4

rewrite rules to specialize function according to type class?
by Patrick Bahr 15 Feb '11

15 Feb '11

Hi all, I am trying to get a GHC rewrite rule that specialises a function according to the type of the argument of the function. Does anybody know whether it is possible to do that not with a concrete type but rather a type class? Consider the following example: > class A a where > toInt :: a -> Int > {-# NOINLINE toInt #-} > class B a where > toInt' :: a -> Int The idea is to use the method of type class A unless the type is also an instance of type class B. Let's say that Bool is an instance of both A and B: > instance A Bool where > toInt True = 1 > toInt False = 0 > instance B Bool where > toInt' True = 0 > toInt' False = 1 Now we add a rule that says that if the argument to "toInt" happens to be an instance of type class B as well, use the method "toInt'" instead: > {-# RULES > "toInt" forall (x :: B a => a) . toInt x = toInt' x > #-} Unfortunately, this does not work (neither with GHC 6.12 or GHC 7.0). Expression "toInt True" gets evaluated to "1". If the rewrite rule is written with a concrete type it works as expected: > {-# RULES > "toInt" forall (x :: Bool) . toInt x = toInt' x > #-} Now "toInt True" is evaluated to "0". Am I doing something wrong or is it not possible for GHC to dispatch a rule according to type class constraints? Thanks, Patrick

6 13

problems with HDBC
by Kurt Norwood 15 Feb '11

15 Feb '11

Hi, I am getting some very cryptic error messages while trying to compile some simple code using hdbc. I am able to run the following lines in ghci without any problems. I am using ghc 6.12.1 on Ubuntu 10.10. Thanks in advance, Kurt ################## # Contents of the file ################## import Database.HDBC.Sqlite3 import Database.HDBC main = dosomething dosomething :: IO () dosomething = do conn <- connectSqlite3 "testdb" run conn "create table test (id integer)" [] run conn "insert into test (id) values (6)" [] commit conn disconnect conn ############## # Error message ############## > ghc testsql.hs testsql.o: In function `s1eF_info': (.text+0x2e): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1eG_info': (.text+0x6e): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1eJ_info': (.text+0x16e): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1eM_info': (.text+0x272): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1ho_info': (.text+0x379): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnection_connectSqlite3_closure' testsql.o: In function `s1ho_info': (.text+0x489): undefined reference to `__stginit_HDBCzm2zi2zi6zi1_DatabaseziHDBC_' testsql.o: In function `s1ho_info': (.text+0x493): undefined reference to `__stginit_HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3_' testsql.o: In function `s1eF_info': (.text+0x36): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_disconnect_info' testsql.o: In function `s1eG_info': (.text+0x76): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_commit_info' testsql.o: In function `s1eJ_info': (.text+0x176): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_run_info' testsql.o: In function `s1eM_info': (.text+0x27a): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_run_info' testsql.o: In function `s1eN_srt': (.data+0x8): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1ho_srt': (.data+0x18): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnection_connectSqlite3_closure' collect2: ld returned 1 exit status

2 2