- Haskell-Cafe - Haskell.org

efficient parallel foldMap for lists/sequences
by Sebastian Fischer 01 Apr '11

01 Apr '11

Hello Haskellers, in parallel programs it is a common pattern to accumulate a list of results in parallel using an associative operator. This can be seen as a simple form of the map-reduce pattern where each element of the list is mapped into a monoid before combining the results using `mconcat`. This pattern can be abstracted by the `foldMap` function of the `Foldable` class and we can give a simple parallel implementation using `par` and `pseq`: ~~~ import Data.Monoid import Control.Parallel foldMap :: Monoid m => (a -> m) -> [a] -> m foldMap f [] = mempty foldMap f [x] = f x foldMap f xs = let (ys,zs) = splitAt (length xs `div` 2) xs ys' = foldMap f ys zs' = foldMap f zs in zs' `par` (ys' `pseq` (ys' `mappend` zs')) ~~~ How can this pattern be implemented in Haskell efficiently? I plan to investigate the following options and am interested in previous work. 1. Data.Sequence (from containers) As finger trees are already balanced they look like a good candidate for parallel consumption. However, the current implementation of the `Foldable` instance is sequential. I wonder what would be the overhead and how it could be reduced if it is is too large. I think, I would first go for an implementation outside of `Foldable` and later consider the overhead of overloading. 2. Data.Array.Repa (from repa) provides a `map` and (sequential) `fold` functions. I think that the main advantage of repa is fusion of successive traversals which is not necessary for the pattern in question. Hence, I'm not sure whether it's a good candidate for the job. Also I don't know how to implement the pattern using repa. Can it be done using `slice` or should it be done by changing repa itself? 3. Data.Monoid.Reducer (from monoids) `foldMapReduce` looks promising. I wonder whether it could be used for parallel reduction and how efficient it would be. Which approach do you think is most promising? What other options are there? Best, Sebastian

1 0

Strictness is observable
by oleg＠okmij.org 01 Apr '11

01 Apr '11

Daniel Fischer wrote: > If you have a strict function, you may evaluate its argument eagerly > without changing the result^1, while eager evaluation of a non-strict > function's argument may produce _|_ where deferred evaluation wouldn't. Sadly, that is quite untrue. Strictness is observable, already in Haskell98. That distressing result has nothing to do with imprecise exceptions, seq, non-termination, lack of resources, or the use of unsafe features. Plainly, just by changing the strictness of a function one may cause the program to print different results, such as "strict" or "non-strict" in the code below. -- Haskell98! -- Strictness is observable -- With no seq, no unsafe operations import Control.Exception -- fs and fns are both essentially (const True) functions, -- but differ in strictness fs,fns :: Bool -> Bool -- non-strict fns x = True -- strict fs True = True fs x = True handler :: SomeException -> IO () handler _ = print "strict" test f = handle handler $ if f (error "Bang!") then print "non-strict" else return () main_s = test fs -- prints "strict" main_ns = test fns -- prints "non-strict"

3 3

Re: [Haskell-cafe] Strictness is observable
by John Meacham 01 Apr '11

01 Apr '11

Error is not catchable in haskell 98. Only things thrown by raiseIO are. On Apr 1, 2011 12:02 AM, <oleg(a)okmij.org> wrote:

1 0

A maintenance command of Haskell cabal packages
by Kazu Yamamoto 01 Apr '11

01 Apr '11

Hello cafe, Let me announce a maintenance command of Haskell cabal packages. http://www.mew.org/~kazu/proj/cab/en/ ---- "cab" is a MacPorts-like maintenance command of Haskell cabal packages. Some part of this program is a wrapper to "ghc-pkg" and "cabal". If you are always confused due to inconsistency of two commands, or if you want a way to check all outdated packages, or if you want a way to remove outdated packages recursively, this command helps you. ---- I'm a user of Yesod. Yesod consists of many packages and they are being updated very quickly. So, I need this command. Regards, --Kazu

5 8

Byte Histogram
by Andrew Coppin 31 Mar '11

31 Mar '11

There now follows a small grab-bag of miscelaneous but related thoughts. (Which probably means that this post will spawn a 2000 message thread discussing one tiny side-issue, rather than the main thrust of the message...) First of all, benchmarking. We have The Great Language Shootout, which tests how fast solutions implemented in various programming languages can be, given unbounded amounts of time, energy, expertise, and compiler modifications. Which is interesting, from a certain point of view. In another sense, it's less interesting. For example, a C program can be as fast as any Haskell program, since compiling Haskell entails transforming it *into* C. (Unless you're seriously going to suggest that GHC's native code generator is any match for the might of a half-decent C compiler...) That got me thinking about a more interesting question: Not *how fast* can it go, but *how easily* can it go fast? People have written fast C programs and fast Haskell programs, but how easy is it to write a "typical" program and have it go fast, without spending years optimising it? With that in mind, I set of with a plan to do some small-scale benchmarking. Pick a problem, solve it in both Haskell and C++, in the "simplest, most obvious way", and then apply "the simplest, most obvious optimisations". Measure performance and compare. There are benchmarks that just output some data. ("Find all the prime numbers less than 1000...") I dislike these for a couple of reasons, not least because you can precompute the correct answer and just write a program which outputs that. (!) So I decided that all my benchmark programs should take a variable-sized data file as input, and save the results as output. There's an old maxim of running benchmarks multiple times, to ensure that whatever time you got wasn't just a fluke. But then, maybe the first run pulls the input file into the file cache, and subsequent runs go faster. If your input data was randomly generated, perhaps you chose a particularly optimal or pessimal data set by accident. So I decided that each test run should use a different input file of approximately the same characteristics (size, random distribution, etc.) So I ended up picking benchmarks like "sort the lines of this file into ascending order", "count the number of unique words in this file", "produce a histogram of byte values", "compute some statistics of this list of numbers", etc. The tasks are all extremely simple, so that there is some hope of it being possible to also implement them in C++. One benchmark turned out to be particularly interesting: I call it "byte histogram". The task is simple: - Open a binary input file. - Read a stream of bytes from it. - Count how many times each of the 256 possible byte values appears. The test inputs are binary files of various sizes, some with a uniform distribution, some with variously skewed distributions (so that some bytes have a vastly higher count than others). Assuming we have some suitable import statements, it's quite easy to do this: bytes <- BS.readFile "Input.bin" let out = BS.foldl' (\ map byte -> MAP.insertWith (+) byte 1 map) MAP.empty bytes writeFile "Output.csv" (unlines $ map (\(k,v) -> show k ++ "," ++ show v) $ MAP.toAscList out) (There is some slight trickiness if you want bytes with zero frequency to still be listed.) All of this *works* perfectly - i.e., it produces the correct answers. It's also astronomically slow, and it's very easy to make it eat many gigabytes of RAM while it runs. (If the program starts actually swapping, then you will truly know what "slow" is!) OK, so what happens if we replace Data.Map with Data.IntMap? Well, it goes slightly faster, and consumes slightly less RAM. Inputs which didn't quite complete before will run to completion now. But performance is still abysmal. Enough with this tree sillyness. What happens if you use an array? Given that the entire program (apart from the last 0.02% of it) spends its time notionally mutating the data, a mutable array would be the logical choise. Dial up an IOArray Word8 Int and the structure of the program changes slightly, but it's still only a handful of lines of code. And the performance? Still glacial. In particular, suppose we have inc :: IOArray Word8 Int -> Word8 -> IO () inc array byte = do count <- readArray array byte let count' = count + 1 writeArray array byte count' in the main loop. The program is giving us the right answer, but using absurd amounts of time and space to get it. Now watch this: inc :: IOArray Word8 Int -> Word8 -> IO () inc array byte = do count <- readArray array byte let count' = count + 1 count' `seq` writeArray array byte count' And now, suddenly, memory usage becomes constant, regardless of input size, and run-time is slashed. The program goes from taking 50 seconds to process a certain file to taking only 0.02 seconds. And from taking 400 MB of RAM to taking... less RAM than I can actually measure properly. If we now replace the IOArray with an IOUArray then the seq call becomes superfluous, and there's a small performance increase, but nothing major. None of this should be any surprise to anybody who actually knows how Haskell works. For anybody not yet expert enough to see the problem: The "inc" function doesn't read the array, see that a given byte has a count of 5, and overwrite that with a 6. It sees a 5 and overwrites it with a 5+1. By the end of the main loop, every single array cell contains 1+(1+(1+(1+(1+(1+....)))), which takes a stackload of RAM to hold. That explains why RAM usage is so absurd. On top of all that, creating all this data takes time, and the spiralling RAM usage makes the GC go crazy trying to find garbage to get rid of (when of course there is none). And finally, at the end, you have to unwind all these expressions. (Curiously, I haven't seen a stack overflow yet...) Adding the seq call forces 5+1 to actually turn into 6, not eventualy but *right now*, and so each array cell ends up actually containing a fully-evaluated integer. (No doubt this change in strictness also triggers an avalanche of optimisation opportunities, but the basic thing of importants is that we're not uselessly delaying evaluation now.) As I say, none of this is a surprise to anybody who actually knows how to get performance out of Haskell. The strict version is actually faster than the C++ version. (I imagine I'm doing something horribly wrong with C++, of course...) Not drastically faster, but faster. Which probably just means that ByteString is doing nicer buffering than whatever the C++ standard library is called. The important obsevation is this: One tiny, almost insignificant change can transform a program from taking 50 seconds and 400 MB of RAM into one that takes 0.02 seconds and 0.1 MB of RAM. And, at least in this case, the simpler version is the slow one. To say that Haskell is "slow" is both a little bit vague, and not really backed up by facts. In about 5 minutes flat, I managed to write a Haskell program that's very simple, and yet faster than a comparably simple C++ program (and C++ is supposed to be "fast"). So it's not that Haskell is "slow". It's that Haskell is *tricky*. Tiny, tiny little changes that look innocuous can have vast effects on performance. And this is a nice little example of that effect. As we (probably?) all know, strictness is a subtle and fickle thing. Consider: or :: Bool -> Bool -> Bool or True True = True or True False = True or False True = True or False False = False Overlooking the obvious name clash, this seems like a perfectly legitimate definition for the logical-OR function. But now try this: xs `contains` x = foldr or False $ map (x ==) xs A nice, pretty Haskell definition. But oh dears - this does not have the performance you would expect at all! But if you change the definition above to the apparently equivilent or True _ = True or False x = x then "contains" becomes much faster. If you saw these two functions side by side, you might well wonder what the hell the difference is. You might even wonder if the compiler would transform one to the other (although I'm not sure in which direction). But there is, in fact, a very, very big difference: one is lazier than the other. For counting bytes, lazy = bad. For searching for matches, lazy = good. Similarly, if you take the byte histogram program and replace the lazy ByteString with a strict one, RAM usage spikes sharply. In all, laziness is a tricky, tricky little hobbitses. I wrote a C++ program, in the obvious way, and it was very, very fast. I wrote a Haskell program in several comparably obvious ways, and they were all cripplingly slow. Until I added the magic "seq", and suddenly got blinding speed. So while it is not correct to say "Haskell is slow", one could justifyably say "C++ is fast more reliably than Haskell". Consider for a moment the original implementation with Data.Map. Adding a seq or two here will do no good at all; seq reduces to WHNF. What we are wanting is NF, and I can see no way at all of doing that. (Other than doing something very expensive like looking up every possible key, which is far more operations than actually necessary.) I wrote my own little BST algorithm, customised to the fact that there are always exactly 256 keys of type Word8. It was also nausiatingly slow. And then I did something: I inserted a few strictness annotations on the constructor fields. And (as you already worked out), performance increased drastically. I can do that if *I* define the data structure. But if it's in a library, I am powerless to make it more strict (or less, if that were what I wanted). That got me thinking... What would happen if, instead of "Integer", we had two types, "evaluated Integer" and "possibly unevaluated Integer"? What if the strictness or otherwise of a data structure were exposed at the type level? The general idea here is that anything declared has having type "evaluated Integer" can never contain an unevaluated expression. It must always contain an actual integer. This of course implies that you might be able to unbox it or do other interesting things with it, but it also says something about evaluation. The principle question is "what happens if you assign an unevaluated Integer to an evaluated Integer?" Is that legal? Do you have to explicitly evaluate it first? Or does that happen implicitly? Presumably "evaluated Integer" means evaluated to WHNF only - which for an integer is the same thing as NF, but for some more complex structure it might not be. If the type system worked like this, I could do "Data.Map with evaluated integer keys and possibly evaluated integer values", and it would hold its values lazily. Or I could say "Data.Map with evaluated integer keys and evaluated integer values", and the structure would now magically be strict. Which is to say, the type system would ensure that any data passed into the structure was evaluated first, and the compiler could do any applicable optimisations based on the assumption of data being already evaluated. (For primitive types, that might be unboxing or passing in registors. For ADTs, it might just mean skipping an evaluation check.) Currently, if you want a strict list, you have to implement one yourself. But is that strict in the spine, or the elements, or what? You have to implement every combination that you want. And, let us not forget, then go implement all the functions in Data.List over your new data structure. Not fun. If, on the other hand, strictness where in the type system, data structures would be *parameterised* over the level of strictness required. I have no idea what the syntax for that would look like, and backwards compatibility looks like a nightmare. You would need to answer questions like "what does it mean for a function to return an evaluated type?" and "is evaluation implicit or explicit?" But it seems like a rather interesting idea. (Let us not forget: *Everything* is trivial for the person who doesn't have to write the implementation...)

18 59

BlockedIndefinitelyOnMVar exception
by Mitar 31 Mar '11

31 Mar '11

Hi! Is there a way to disable throwing BlockedIndefinitelyOnMVar exceptions? Because I am doing small program where I do not care if some threads block. As at the end user will have to interrupt the program anyway. Mitar

5 6

Re: [Haskell-cafe] NOT Iteratee, ghc 6.12/7.0 strange behaviour-epollControl: permission denied (Operation not permitted)
by Michael A Baikov 31 Mar '11

31 Mar '11

I am running 7.0.3. -----Original Message----- > On Thu, Mar 31, 2011 at 11:19 AM, Michael A Baikov <pacak(a)bk.ru> wrote: > > > import System.Posix.IO > > import GHC.Conc > > > > main = do > > fd <- openFd "/etc/passwd" ReadOnly Nothing defaultFileFlags > > threadWaitRead fd -- the big bang happens right here. > > closeFd fd > > > > There were a couple of bugs in event manager that we fixed some time during > the 7.0.2 or 7.0.3 release cycle. So if you're not running 7.0.3, please do. > >

1 0

Re: [Haskell-cafe] NOT Iteratee, ghc 6.12/7.0 strange behaviour -epollControl: permission denied (Operation not permitted)
by Michael A Baikov 31 Mar '11

31 Mar '11

It works fine up to the point when makefdCallback tries to use GHC.Conc.threadWaitRead on fd So my example is simplifed in to this: import System.Posix.IO import GHC.Conc main = do fd <- openFd "/etc/passwd" ReadOnly Nothing defaultFileFlags threadWaitRead fd -- the big bang happens right here. closeFd fd So iteratee is completely innocent and apoligies should be mine - I should not suspect such wonderfull library in the first place. But on the bright side - i'll play with gory details of threadWaitWrite implementation :) -----Original Message----- > Apologies, I won't have time to look at this for a few days. However, could > you try a test using the functions from Data.Iteratee.IO.Posix directly? > That would help narrow down the issue. I suspect it's either a bug in > ghc's IO, or the semantics of some IO functions changed without my noticing. > > John L. > >

2 1

Haskellers in St. Louis?
by aditya siram 31 Mar '11

31 Mar '11

Hi all, I'm looking to start a Haskell user group in St. Louis, MO. Anyone Haskellers or people interested in learning Haskell around these parts? -deech

1 0

Parallel Haskell Digest 1
by Eric Y. Kow 31 Mar '11

31 Mar '11

Parallel Haskell Digest ======================= Edition 1 2011-03-31 http://www.well-typed.com/blog/52 Hello Haskellers! If you're in the mood for a HWN chaser, I'd like to introduce the first Parallel Haskell Digest, a newsletter aiming to show off all the work that's going on using parallelism and concurrency in the Haskell community. The digest is a bit of an experiment. For the community in general, I hope to help you catch up with a monthly recap of news, interesting blog posts and discussions about parallelism in Haskell. For people (like me) who are new to parallelism and concurrency in Haskell, or maybe just have a passing interest, I hope to give you little nibbles, with regular features like the Word of Month, Featured Code and Parallel Puzzlers. Finally, as a bit of disclosure, I'm writing this digest as part of my work to promote the Parallel GHC Project. So don't be surprised if I give it a little special emphasis :-) Word of the Month ----------------- This very first edition of the PH digest is brought to you by the word /spark/. You may have heard of sparks and threads in the same sentence. What's the difference? A Haskell thread is a thread of execution for IO code. Multiple Haskell threads can execute IO code concurrently and they can communicate using shared mutable variables and channels. Sparks are specific to parallel Haskell. Abstractly, a spark is a pure computation which may be evaluated in parallel. Sparks are introduced with the par combinator; the expression (x `par` y) "sparks off" x, telling the runtime that it may evaluate the value of x in parallel to other work. Whether or not a spark is evaluated in parallel with other computations, or other Haskell IO threads, depends on what your hardware supports and on how your program is written. Sparks are put in a work queue and when a CPU core is idle, it can execute a spark by taking one from the work queue and evaluating it. On a multi-core machine, both threads and sparks can be used to achieve parallelism. Threads give you concurrent, non-deterministic parallelism, while sparks give you pure deterministic parallelism. Haskell threads are ideal for applications like network servers where you need to do lots of I/O and using concurrency fits the nature of the problem. Sparks are ideal for speeding up pure calculations where adding non-deterministic concurrency would just make things more complicated. Parallel GHC project news ---------------------------------------------------------------------- The Parallel GHC Project is an MSR-funded project to push the real-world use of parallel Haskell. Part of this project involves effort by Well-Typed to provide tools for use by the general community: Work shall soon begin working on making the "Modified Additive Lagged Fibonacci" and perhaps other random number generators from the SPRNG library available in Haskell. Current plans are to implement the algorithms directly in Haskell, and to expose them as instances of System.Random.RandomGen. These generators are attractive for use in Monte Carlo simulations because they are splittable and have good statistical quality, while providing high performance. Work is underway on extending the GHC EventLog and associated tools (ghc-events, ThreadScope). The aim is to support profiling of multi-process or distributed Haskell systems such as client/server or MPI programs. This involves incorporate some changes made in related projects (Eden, EdenTV). This work may have some benefits even for profiling single-process programs. It should also allow comparative profiling where multiple runs of the same program (e.g. different inputs or slightly different code) are viewed on the same timeline. For more information on the Parallel GHC project, see http://www.haskell.org/haskellwiki/Parallel_GHC_Project Featured Code ---------------------------------------------------------------------- The feature code for this month is hulk, an IRC server in 1250 lines of code. Hulk was coded up by Chris Done in one evening, and it was used the next morning. (Chris has done a bit of cleaning up since). Here's what Chris had to say about his experience writing Hulk: Haskell's lightweight threads make it natural (and guilt-free!) to choose the design of one-thread-per-client. With a single MVar containing a pure value for the whole server-state, and the help of a monad stack of ReaderT(connection information), WriterT (replies) and StateT (user details), it was trivial to make the bulk of the code completely pure. LineBuffering on the (Handle-wrapped) sockets tripped me up; as opposed to NoBuffering, this did not behave as expected: *many* threads would block when only *one* should have. Overall, it was textbook Haskell; keep the main code pure, and only the truly impure in the outer shell. It's up on hackage so you can install it with a quick cabal install hulk You can also fork his code on GitHub https://github.com/chrisdone/hulk Got a cool use of Parallel Haskell or an interesting problem? Nominate it for the PH digest featured code/parallel puzzler! Blog Posts ---------------------------------------------------------------------- 1. Parallelism is not Concurrency - Bob Harper https://existentialtype.wordpress.com/2011/03/17/parallelism-is-not-concurr… 2. Petri Net Concurrency - Edward Z. Yang http://blog.ezyang.com/2011/03/petri-net-concurrency Parallel-Haskell and Haskell Cafe ---------------------------------------------------------------------- 1. How does the Yesod web framework deal with concurrency? Ertugrul Soeylemez is using concurrent Haskell to serve up two related sites and spawn off some background workers would pose any problems with Yesod and WAI. No problem, according to Michael Snoyman http://www.haskell.org/pipermail/haskell-cafe/2011-January/088754.html 2. Concurrency best practices? Wren Ng Thorton is using STM to "run a lot of things in parallel without the headaches of locks." It works great, but then what if you want IO? How would you enforce atomic IO operations, eg. printing log messages in two threads without getting garbage on screen? Some suggestions were the twighlight-stm package, and using STM.TChan with potential performance penalty. http://www.haskell.org/pipermail/haskell-cafe/2011-February/088987.html 3. Possible bug in Control.Concurrent Krzysztof Skrzętnicki encountered an unexpected deadlock using Control.Concurrent. Was it a normal consequence of repeatedly reading from channel with only a single element put into it, or a bug? - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089135.html - http://hackage.haskell.org/trac/ghc/ticket/4154 4. Faster timeout but is it correct? Bas van Dijk proposed increasingly faster potential replacements for System.Timeout, including some that use the new GHC event manager. After much scrutiny, it turns out that one version is not faster and the events based one had a bug. Maybe next time! - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089360.html - http://hackage.haskell.org/trac/ghc/ticket/4963 5. Unable to get parallelism using `par`. SMP parallelism gains inferior than expected. C K Kashyap followed /A tutorial on Parallel and Concurrent programming in Haskell/ but couldn't get a speed-up. Others had better luck with different sets of inputs José Pedro Magalhães also tried using SMP parallelism without getting performance gains he was hoping for. Simon Marlow suggested analysing with Threadscope, checking for too many/few sparks, and checking to see if a lot of time was being spent in GC. - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089419.html - http://research.microsoft.com/en-us/um/people/.../parallel/AFP08-notes.pdf - http://www.haskell.org/pipermail/haskell-cafe/2011-February/089635.html 6. Auto elimination of MVars using a monad or monad transformer. Chris Dew asked if GHC eliminates MVars during compilation, or if there was a way to eliminate them automatically. Ryan Ingram suggested looking into the Adaptive package and Functional Reactive Programming. http://www.haskell.org/pipermail/haskell-cafe/2011-February/089660.html Stack Overflow ---------------------------------------------------------------------- 1. Parallelism on divide & conquer algorithm (7 Feb) http://stackoverflow.com/questions/4920077/parallelism-on-divide-conquer-al… 2. How to best synchronize game engine and network server in Haskell? (11 Feb) http://stackoverflow.com/questions/4973401/how-to-best-synchronize-game-eng… 3. Haskell concurrency and Handles (27 Feb) http://stackoverflow.com/questions/5136375/haskell-concurrency-and-handles 4. Strict evaluation techniques for concurrent channels in Haskell (6 Mar) http://stackoverflow.com/questions/5211827/strict-evaluation-techniques-for… Help and feedback ---------------------------------------------------------------------- Got something for the digest? Send me an email at eric(a)well-typed.com. I'm particularly interested in short parallelism/concurrency puzzles, cool projects for featured code. Other comments and feedback (criticism and corrections especially!) would be welcome too. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

1 0

Re: [Haskell-cafe] ANNOUNCE: enumerator 0.4.8
by John Lato 31 Mar '11

31 Mar '11

> > Message: 7 > Date: Tue, 29 Mar 2011 22:39:12 -0400 > From: wren ng thornton <wren(a)freegeek.org> > Subject: Re: [Haskell-cafe] ANNOUNCE: enumerator 0.4.8 > To: haskell-cafe(a)haskell.org > Message-ID: <4D9297D0.7060405(a)freegeek.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 3/29/11 4:40 AM, oleg(a)okmij.org wrote: > > Wren Thornton wrote: > >> This is often conflated with the iteratee throwing an error/exception, > >> which is wrong because we should distinguish between bad program > >> states and argument passing. > > > > I guess this is a matter of different points of view on exceptions. > > The problem is not so much the exceptions per se (one goto is about as > good as any other), it has more to do with the fact that important > things are being left out of the types. > > One of the great things about Haskell is that you can lean so heavily on > the type system to protect yourself when refactoring, designing by > contract, etc. However, if there's an unspoken code of communication > between specific enumerators and iteratees, it's very easy to break > things. This is why the communication should be captured in the types, > regardless of the control-flow mechanism used to implement that > communication. I'd like the static guarantee that whatever special > requests my iteratee could make, its enumerator is in a position to > fulfill those requests (or die trying). Allowing for the iteratee to be > paired with an enumerator which is incapable of handling its requests is > a type error and should be treated as such. > This has long been a goal of mine for iteratee (since before the 0.4 release), although I haven't really done any work on it. Maybe it's time to see if I can get a more satisfactory implementation. > > Wren Thornton wrote: > >> In an ideal framework the producers, transformers, and consumers of > >> stream data would have a type parameter indicating the up-stream > >> communication they support or require (in addition to the type > >> parameters for stream type, result type, and side-effect type). > > > > Very true. Currently the design of Iteratees quite resembles that of > > Control.Exception: everything can throw SomeException. Ideally one > > would like to be more precise, and specify what exceptions or sorts of > > exceptions could be thrown -- by Iteratees, and by ordinary Haskell > > functions. The design of a good effect system is still the topic of > > active research, although there are some encouraging results. > > Yeah, I'm not a big fan of extensible exceptions either. Don't get me > wrong, it's an awesome hack and it's far cleaner than the Java approach; > but it still goes against my sensibilities. > I've come around to the view that exceptions are a bad idea (in Haskell), and just making everything explicit (via Maybe, ErrorT, explicit-exceptions, or otherwise) is the best approach at present. John L.

1 0

Re: [Haskell-cafe] Iteratee, ghc 6.12/7.0 strange behaviour - epollControl: permission denied (Operation not permitted)
by John Lato 31 Mar '11

31 Mar '11

> > > Message: 15 > Date: Mon, 28 Mar 2011 21:31:39 +0400 > From: Michael A Baikov <pacak(a)bk.ru> > Subject: [Haskell-cafe] Iteratee, ghc 6.12/7.0 strange behaviour - > epollControl: permission denied (Operation not permitted) > To: haskell-cafe(a)haskell.org > Message-ID: <E1Q4GHz-00077E-00.pacak-bk-ru(a)f146.mail.ru> > Content-Type: text/plain; charset=koi8-r > > > > I am still playing with lastest iteratee and i think i found something > strange. > > > let's suppose we have a file test.hs like this: > > import Data.Iteratee > import Data.Iteratee.IO > import Data.Iteratee.Char > > main = fileDriver printLines "/etc/passwd" > > It works fine when executed via runhaskell / ghci in ghc 6.12. Compiled > version in ghc 7 > also works, but when i am trying to execute it via runhaskell / ghci i am > getting this error: > > iter.hs: epollControl: permission denied (Operation not permitted) > > Any ideas? > Apologies, I won't have time to look at this for a few days. However, could you try a test using the functions from Data.Iteratee.IO.Posix directly? That would help narrow down the issue. I suspect it's either a bug in ghc's IO, or the semantics of some IO functions changed without my noticing. John L.

1 0

cabal advice: library paths
by Michael Snoyman 31 Mar '11

31 Mar '11

Hi all, I have a package[1] which uses some system libraries (Qt to be precise). On Linux (and I'm hoping Mac), it's able to use pkg-config to determine which libraries are necessary and where to find all the files. On Windows, I've been less than successful using pkg-config. However, I *was* able to hardcode in the paths[2], and the resulting executables ran without a hitch. I still have two problems: * Obviously hardcoding paths in less than ideal. * Hackage won't allow me to upload this package anymore. It complains: 'extra-lib-dirs: c:\qt\2010.05\qt\lib' directory does not exist. Can anyone recommend a more elegant solution to the problem? Thanks, Michael [1] http://hackage.haskell.org/package/wai-handler-webkit [2] https://github.com/snoyberg/hack-handler-webkit/blob/master/wai-handler-web…

1 0

object oriented technique
by Tad Doxsee 31 Mar '11

31 Mar '11

I've been trying to learn Haskell for a while now, and recently wanted to do something that's very common in the object oriented world, subtype polymorphism with a heterogeneous collection. It took me a while, but I found a solution that meets my needs. It's a combination of solutions that I saw on the web, but I've never seen it presented in a way that combines both in a short note. (I'm sure it's out there somewhere, but it's off the beaten path that I've been struggling along.) The related solutions are 1. section 3.6 of http://homepages.cwi.nl/~ralf/OOHaskell/paper.pdf 2. The GADT comment at the end of section 4 of http://www.haskell.org/haskellwiki/Heterogenous_collections I'm looking for comments on the practicality of the solution, and references to better explanations of, extensions to, or simpler alternatives for what I'm trying to achieve. Using the standard example, here's the code: data Rectangle = Rectangle { rx, ry, rw, rh :: Double } deriving (Eq, Show) drawRect :: Rectangle -> String drawRect r = "Rect (" ++ show (rx r) ++ ", " ++ show (ry r) ++ ") -- " ++ show (rw r) ++ " x " ++ show (rh r) data Circle = Circle {cx, cy, cr :: Double} deriving (Eq, Show) drawCirc :: Circle -> String drawCirc c = "Circ (" ++ show (cx c) ++ ", " ++ show (cy c)++ ") -- " ++ show (cr c) r1 = Rectangle 0 0 3 2 r2 = Rectangle 1 1 4 5 c1 = Circle 0 0 5 c2 = Circle 2 0 7 rs = [r1, r2] cs = [c1, c2] rDrawing = map drawRect rs cDrawing = map drawCirc cs -- shapes = rs ++ cs Of course, the last line won't compile because the standard Haskell list may contain only homogeneous types. What I wanted to do is create a list of circles and rectangles, put them in a list, and draw them. It was easy for me to find on the web and in books how to do that if I controlled all of the code. What wasn't immediately obvious to me was how to do that in a library that could be extended by others. The references noted previously suggest this solution: class ShapeC s where draw :: s -> String copyTo :: s -> Double -> Double -> s -- needs {-# LANGUAGE GADTs #-} data ShapeD where ShapeD :: ShapeC s => s -> ShapeD instance ShapeC ShapeD where draw (ShapeD s) = draw s copyTo (ShapeD s) x y = ShapeD (copyTo s x y) mkShape :: ShapeC s => s -> ShapeD mkShape s = ShapeD s instance ShapeC Rectangle where draw = drawRect copyTo (Rectangle _ _ rw rh) x y = Rectangle x y rw rh instance ShapeC Circle where draw = drawCirc copyTo (Circle _ _ r) x y = Circle x y r r1s = ShapeD r1 r2s = ShapeD r2 c1s = ShapeD c1 c2s = ShapeD c2 shapes1 = [r1s, r2s, c1s, c2s] drawing1 = map draw shapes1 shapes2 = map mkShape rs ++ map mkShape cs drawing2 = map draw shapes2 -- copy the shapes to the origin then draw them shapes3 = map (\s -> copyTo s 0 0) shapes2 drawing3 = map draw shapes3 Another user could create a list of shapes that included triangles by creating a ShapeC instance for his triangle and using mkShape to add it to a list of ShapeDs. Is the above the standard method in Haskell for creating an extensible heterogeneous list of "objects" that share a common interface? Are there better approaches? (I ran into a possible limitation to this approach that I plan to ask about later if I can't figure it out myself.) - Tad

6 10

Haskell Weekly News: Issue 175
by Daniel Santa Cruz 31 Mar '11

31 Mar '11

Welcome to issue 175 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 20 - 26. You can find the HTML version at: http://bit.ly/gs9Rr8 Announcements Wren Ng Thornton [2]announced the release of unix-bytestring: Unix/Posix-specific functions for ByteStrengs. John Lato [3]announced an incremental update to the ListLike package. "This version adds newtype wrappers for strict and lazy ByteString, CharString, and CharStringLazy..." Mario Blazevic [4]announced the release of incremental-parser, which provides the usual set of Applicative and monad combinators with a few twists that make it unique. Duncan Coutts [5]announced the release of version 0.16.3 of c2hs, the FFI preprocessor tool. Jeff Wheeler [6]announced a new release of Yi, the text editor written in and extensible in Haskell. Christiaan Baaij [7]announced an incremental upgrade to 0.1.3.0 to CLaSH. Mario Blazevic [8]announced updates to monad-parallel, monad-coroutine, and SCC (Streaming Component Combinators). John Millikin [9]announced an update to enumerator to version 0.4.8. Quotes of the Week * Squarism: wo utilizing any tricks, i want 20 miljon rabbits to mate with each other once.. how many mate event will there be * mm_freak: using combinators instead of explicit recursion can be justified with about the same reasoning as using 'while' instead of explicit 'goto' in imperative programming * monochrom: <$> is pronounced Jacquelin von Brionée * geheimdienst: what does a pirate say when starting a sentence, then suddenly being hit in the nadgers? arr- ow! * monochrom: 1985 is way more than a decade ago * copumpkin: matlab is horrible as far as language design goes[.] it's fairly convenient for doing stuff in but I need to shower after every time I use it Top Reddit Stories * Debugging trick From (self.haskell), scored 60 with 11 comments. Read on [10]reddit. * Your friendly Haskell JSON library's encoding performance just got 4x faster From (serpentine.com) scored 45 with 2 comments. Read on [12]reddit. Read the [13]original post. * A monad for deterministic parallelism (ICFP'2011 submission) From (community.haskell.org) scored 41 with 8 comments. Read on [14]reddit. Read the [15]original post. * Haskell servers benchmarked against node/ruby/python/php/java. From (docs.yesodweb.com) scored 36 with 12 comments. Read on [16]reddit. Read the [17]original post. * The Yesod Framework and the Case of the Eight Level Monad Transformer Stack From (docs.yesodweb.com) scored 35 with 16 comments. Read on [18]reddit. Read the [19]original post. * Improving Persistent Performance (Yesod Blog) From (docs.yesodweb.com) scored 29 with 1 comments. Read on [20]reddit. Read the [21]original post. * Fast, clean Haskell bindings to Google's Snappy library From (hackage.haskell.org) scored 22 with 8 comments. Read on [22]reddit. Read the [23]original post. * Open Quark on Github (Haskell like Lang on the JVM) From (github.com) scored 15 with 3 comments. Read on [24]reddit. Read the [25]original post. * EclipseFP 2.0.4 released: supports GHC 7 From (jpmoresmau.blogspot.com) scored 15 with 7 comments. Read on [26]reddit. Read the [27]original post. * Demonstrating a Time Leak in Arrowized FRP From (blog.edwardamsden.com) scored 14 with 12 comments. Read on [28]reddit. Read the [29]original post. Top StackOverflow Answers * [30]Reading large file in haskell ? votes: 19 The construct "seq x x" is always useless. If y = seq x x and I force y then this forces x then returns x. This is equivalent to y=x and forcing y. Thus "seq forceEval forceEval" does nothing more than "forceEval". The error with your use of a fold is a common one. You are using a fold to perform a count of the bytes in the input. You should be using ... * [31]What to call a function that splits lists? votes: 12 I believe the function you're describing is breakBefore from the list-grouping package. Data.List.Grouping: http://hackage.haskell.org/packages/archive/list-grouping/0.1.1/doc/html/Da… st-Grouping.html ghci:: breakBefore even [3,1,4,1,5,9,2,6,5,3,5,8,9,7,9,3,2,3,8,4,6,2,6] [[3,1],[4,1,5,9],[2],[6,5,3,5],[8,9,7,9,3],[2,3],[8],[4],[6],[2],[6]] Top StackOverflow Questions * [32]Haskell record syntax votes: 16, answers: 1 * [33]Reading large file in haskell? votes: 12, answers: 1 * [34]Haskell laziness - how do I force the IO to happen sooner? votes: 9, answers: 3 * [35]case on monadic value votes: 7, answers: 2 * [36]Simplest way to find which data types are most garbage collected? votes: 7, answers: 2 About the Haskell Weekly News 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://article.gmane.org/gmane.comp.lang.haskell.libraries/15484 3. http://article.gmane.org/gmane.comp.lang.haskell.general/18590 4. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15492 5. http://article.gmane.org/gmane.comp.programming.c2hs/247 6. http://article.gmane.org/gmane.editors.yi.devel/5346 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18592 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87540 9. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15506 10. http://www.reddit.com/r/haskell/comments/gbd7w/debugging_trick/ 12. http://www.reddit.com/r/haskell/comments/g8s0t/your_friendly_haskell_json_l… 13. http://www.serpentine.com/blog/2011/03/22/a-new-week-a-new-json-performance… 14. http://www.reddit.com/r/haskell/comments/gainp/a_monad_for_deterministic_pa… 15. http://community.haskell.org/~simonmar/papers/monad-par.pdf 16. http://www.reddit.com/r/haskell/comments/g7uia/haskell_servers_benchmarked_… 17. http://docs.yesodweb.com/blog/preliminary-warp-cross-language-benchmarks 18. http://www.reddit.com/r/haskell/comments/ga40w/the_yesod_framework_and_the_… 19. http://docs.yesodweb.com/blog/hamlet-lucius-widgets 20. http://www.reddit.com/r/haskell/comments/gbz43/improving_persistent_perform… 21. http://docs.yesodweb.com/blog/improving-persistent-performance 22. http://www.reddit.com/r/haskell/comments/ga080/fast_clean_haskell_bindings_… 23. http://hackage.haskell.org/package/snappy 24. http://www.reddit.com/r/haskell/comments/g7vcj/open_quark_on_github_haskell… 25. https://github.com/levans/Open-Quark 26. http://www.reddit.com/r/haskell/comments/g8du0/eclipsefp_204_released_suppo… 27. http://jpmoresmau.blogspot.com/2011/03/eclipsefp-204-released-supports-ghc-… 28. http://www.reddit.com/r/haskell/comments/g943x/demonstrating_a_time_leak_in… 29. http://blog.edwardamsden.com/2011/03/demonstrating-time-leak-in-arrowized.h… 30. http://stackoverflow.com/questions/5410777/reading-large-file-in-haskell/54… 31. http://stackoverflow.com/questions/5438558/what-to-call-a-function-that-spl… 32. http://stackoverflow.com/questions/5367167/haskell-record-syntax 33. http://stackoverflow.com/questions/5410777/reading-large-file-in-haskell 34. http://stackoverflow.com/questions/5373883/haskell-laziness-how-do-i-force-… 35. http://stackoverflow.com/questions/5395517/case-on-monadic-value 36. http://stackoverflow.com/questions/5410345/simplest-way-to-find-which-data-…

1 0

[ghci] unknown symbol in base-unicode-symbols-0.2.1.2 ...
by John Obbele 31 Mar '11

31 Mar '11

Hi everyone ! I'm having a problem in GHCi when loading modules relying on the base-unicode-symbols package. My prompt gives me the following message: >> ghci $ GHCi, version 7.0.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m +System.USB Prelude System.USB> newCtx Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package deepseq-1.1.0.2 ... linking ... done. Loading package bytestring-0.9.1.10 ... linking ... done. Loading package text-0.11.0.5 ... linking ... done. Loading package base-unicode-symbols-0.2.1.2 ... linking ... <interactive>: /home/john/.cabal/lib/base-unicode-symbols-0.2.1.2/ghc-7.0.2/HSbase-unicode-symbols-0.2.1.2.o: unknown symbol `__stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziCategoryziUnicode_' ghc: unable to load package `base-unicode-symbols-0.2.1.2' Prelude System.USB> I thought this was due to me screwing around cabal/ghc-pkg caches but after a fresh install of Fedora15-alpha / cabal update / cabal install usb I'm still getting the same error. Moreover, a quick `nm HSbase-unicode-symbols-*.o | grep ControlziCategoryziUnicode` give me the following output. >> nm HSbase-unicode-symbols-0.2.1.2.o | grep -C 4 'ControlziCategoryziUnicode_' 0000000000000070 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziApplicativeziUnicode 0000000000000040 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziApplicativeziUnicode_ 0000000000000120 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziArrowziUnicode 00000000000000d8 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziArrowziUnicode_ U __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziCategoryziUnicode_ 00000000000001b8 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziMonadziUnicode 0000000000000188 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziMonadziUnicode_ 0000000000000250 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_DataziBoolziUnicode 0000000000000220 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_DataziBoolziUnicode_ Could someone please explain to me what's happening ? regards, /John

2 2

Accessing to a Haskell type in C
by Yves Parès 31 Mar '11

31 Mar '11

Hello, To access a haskell type from C code, do we have to make this type instance of Storable (and so to also define an homomorph structure on the C side), or does FFI specifies some C function to enable C code to access the fields, or even some GHC-specific functions? (Even if I'd prefer a portable way) I mean, if I have the type: data Character = Character { pos :: (Float, Float), size :: Float, sprite :: Image } Do I compulsorily have to double its definition on C side: stuct CCharacter { float x; float y; float size; Image sprite; } and then make Character instance of Storable, which will make the program *copy* Characters to CCharacters each time that I must call a C function to, say, draw a character?

2 1

Re: [Haskell-cafe] ArrowLoop and streamprocessors
by Patai Gergely 31 Mar '11

31 Mar '11

> So loop really doesn't seem to help here, but I couldn't find another > way either to feed outputs back into the system. > What I need is: > Either A B ~> Either C B -> A ~> C > > Does such a thing exist? At this point you don't really have enough structure to define such a feedback loop. Since you have streams of different rates, you need to find some common ground to merge them. The path taken by Reactive is to assign a timestamp to events, and ensure that merging preserves time ordering. Of course you'll immediately run into the problem of simultaneous occurrences, and it depends on the context how you want to resolve them. Also, what you're trying to achieve looks similar to Peakachu, so you might want to check it out, especially the Program abstraction [1]. It even has a feedback combinator with a slightly different signature. Gergely [1] http://hackage.haskell.org/packages/archive/peakachu/0.3.0/doc/html/FRP-Pea… -- http://www.fastmail.fm - mmm... Fastmail...

1 0

CURL and upload a file
by Evgeny Dzhurinsky 30 Mar '11

30 Mar '11

Hello! I am trying to find out the way of uploading a file using HTTP multipart/form-data. I created simple application aimed to upload an image to imgpaste.com. The source is below: ========================================================= module ImgPaste.Upload where import Control.Monad import Network.Curl import Data.ByteString import Text.Regex.PCRE import Text.Regex.PCRE.ByteString type LocalCtx = ( ByteString, ByteString, ByteString, [ByteString] ) type UploadResult = Either UploadError ByteString data UploadError = UploadError { message :: String, response :: ByteString } deriving (Show) extractResponse :: CurlResponse_ [(String,String)] ByteString -> ByteString extractResponse = respBody uploadFile :: String -> IO (ByteString) uploadFile fileName = initialize >>= withCurlDo . (flip uploadFileWithCurl fileName) uploadFileWithCurl :: Curl -> String -> IO ( ByteString ) uploadFileWithCurl curl fileName = do liftM extractResponse $ do_curl_ curl "http://imgpaste.com/" [CurlPost True, CurlHttpPost postData, CurlVerbose True ] where postData = [ HttpPost "upfile" Nothing ( ContentFile fileName ) [] Nothing, makeFormPost "submit" "Upload", makeFormPost "keep" "a" ] makeFormPost name value = HttpPost name Nothing (ContentString value) [] Nothing -- <input type="text" name="copyfield" size="31" value="http://imgpaste.com/tmp/123456.png" /> extractUrl :: ByteString -> UploadResult extractUrl src = match (src =~ "<input type=\"text\" name=\"copyfield\" size=\"31\" value=\"([^\"]+?)\" />" :: LocalCtx) where match (_,_,_,[x]) = Right x match _ = Left $ UploadError "Can not parse content." src pasteImage :: String -> IO ( UploadResult ) pasteImage = liftM extractUrl . uploadFile ========================================================= however when invoking the function: pasteImage "image.png" no data is posted to the server. The protocol dump looks like: ========================================================= POST / HTTP/1.1 Host: imgpaste.com Accept: */* Content-Length: 241 Expect: 100-continue Content-Type: multipart/form-data; boundary=----------------------------630d154aaf3b ========================================================= so the request data is not sent. Can somebody please advice, what am I doing in a wrong way? Thank you in advance! -- regards Eugene Dzhurinsky

2 2

Regarding two platforms build error.
by Serguey Zefirov 30 Mar '11

30 Mar '11

Haskell Platform 2010.1 with ghc 6.12.1 worked quite well. Problem solved. ;)

1 0

Dynamically linked executables
by Anakim Border 30 Mar '11

30 Mar '11

Hi, I'm trying to understand if it's possible to build dynamically linked executables under Mac OS X (10.6) with GHC 7. Looking here: http://hackage.haskell.org/trac/ghc/wiki/SharedLibraries/PlatformSupport it seems like the support is already present. Inside ghc-7.1.20110325 sources (the latest development snapshots as I write), I find the following: # Do we support shared libs? PlatformSupportsSharedLibs = $(if $(filter $(TARGETPLATFORM),\ i386-unknown-linux x86_64-unknown-linux \ i386-unknown-freebsd x86_64-unknown-freebsd \ i386-unknown-openbsd x86_64-unknown-openbsd \ i386-unknown-mingw32 \ i386-unknown-solaris2 \ i386-apple-darwin powerpc-apple-darwin),YES,NO) which seems to indicate the contrary. GHC bug tracker, finally, doesn't show any open ticket on the matter. Anyone has more details about this? Thanks, AB

2 1

Two Haskell Platforms on single machine.
by Serguey Zefirov 30 Mar '11

30 Mar '11

I had to use two Haskell Platforms at once in the Windows environment. We use Haskell Platform 2011.1 as our main build platform. It provide real benefits for code with GADTs so we ported most of our code there. Right now we cannot switch back or it would be quite a regress. We also have some heavy type level computation code in one of our programs so we have to use Haskell Platform 2010.2 with ghc 6.12.3 as 7.0.2 is quite slow at that. And now I have some strange error (it didn't appear when I used only one of Haskell platforms): ---------------------------------- ghc-6.12.3 -O2 -odir objs -hidir objs -stubdir objs -o CGen.exe -fcontext-stack=100 --make CGen.hs Linking CGen.exe ... realgcc.exe: USERNA: Invalid argument C:\PROGRA~1\HASKEL~1\201020~1.0\lib\..\mingw\bin\windres: "C:\PROGRA~1\HASKEL~1\ 201020~1.0\lib\..\mingw\bin\gcc exited with status 1 ---------------------------------- When I looked at the dump in hex, I encountered that "USERNA" string contains "USERNAM^H" byte sequence, actually. I cannot look at ghc -v5 as I get another error: ---------------------------------- Glasgow Haskell Compiler, Version 6.12.3, for Haskell 98, stage 2 booted by GHC version 6.10.4 Using binary package database: C:\PROGRA~1\HASKEL~1\201020~1.0\lib\package.conf.d\package.cache ... ... (I deleted much of the lines) ... Deleting: *** Deleting temp dirs: Deleting: <stderr>: hPutChar: invalid argument (character is not in the code page) ---------------------------------- I use localized version of Windows XP so it appears that ghc cannot print some name with russian characters in it. And here I stuck. I cannot find source of one error because of another error. Could anyone help me?

1 0

Re: [Haskell-cafe] object oriented technique
by Tad Doxsee 30 Mar '11

30 Mar '11

Greg, Thanks for your help. Is there any significant difference between using existential quantification data ShapeD = forall s. ShapeC => ShapeD s versus a GADT data ShapeD where ShapeD :: ShapeC s => s -> ShapeD I'm not sure I understood what you meant by "You don't need to write more typeclass instances this way." Thanks for pointing out the Control.Exception library. It was very helpful. Earlier, I was trying to figure out how to use Data.Dynamic for down-casting and couldn't get what I wanted. The Data.Typeable usage in Control.Exception is what I was looking for. Tad On Tue, Mar 29, 2011 at 12:57 AM, Gregory Collins <greg(a)gregorycollins.net> wrote: > On Tue, Mar 29, 2011 at 7:49 AM, Tad Doxsee <tad.doxsee(a)gmail.com> wrote: >> class ShapeC s where >> draw :: s -> String >> copyTo :: s -> Double -> Double -> s >> >> -- needs {-# LANGUAGE GADTs #-} >> data ShapeD where >> ShapeD :: ShapeC s => s -> ShapeD >> >> Is the above the standard method in Haskell for creating an extensible >> heterogeneous list of "objects" that share a common interface? Are there better >> approaches? (I ran into a possible limitation to this approach that I plan >> to ask about later if I can't figure it out myself.) > > The usual way to do this is: > > {-# LANGUAGE ExistentialQuantification #-} > data SomeShape = forall s . ShapeClass s => SomeShape s > > You don't need to write more typeclass instances this way. If you give > "SomeShape" a "ShapeClass" instance also, you can treat them > uniformly. The downside to these approaches is that any additional > information about the original concrete type is obliterated -- to get > OO-style downcasting you need "Typeable" support, and it isn't free. > > For an example of code which uses this idiom, see the exceptions > support from the base library: > http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exceptio… > > G > -- > Gregory Collins <greg(a)gregorycollins.net> >

3 2

Re: [Haskell-cafe] ANNOUNCE: enumerator 0.4.8
by oleg＠okmij.org 30 Mar '11

30 Mar '11

Wren Thornton wrote: > This is often conflated with the iteratee throwing an error/exception, > which is wrong because we should distinguish between bad program > states and argument passing. I guess this is a matter of different points of view on exceptions. I am a fan of the model of (effectful) computation proposed by Cartwright and Felleisen a while ago: http://okmij.org/ftp/Computation/monads.html#ExtensibleDS In their model, all the computation is done by throwing resumable exceptions -- including the pure computation such as arithmetic and CBV/CBN applications. The similarity of ExtensibleDS.hs with Iteratee should be quite noticeable, especially regarding the part about throwing `errors.' The relation with co-routines is hard to miss: in fact, Iteratees are built upon co-routines, which are being resumed by the enumerator. The error message is the additional piece of data that is being associated with `yield', telling the enumerator to do something extra rather than mere getting the next piece of data and resuming the co-routine. The co-routine is the simplest part of the iteratee; it is the plumbing that takes a long time to engineer. John A. De Goes: > 2. Error recovery is ill-defined because errors do not describe > what portion of the input they have already consumed; I'm confused about this complaint: if an iteratee encounters an unusual condition or just has a special request, it sends a message that eventually propagates to the responsible enumerator. That enumerator knows how much data it has sent down to iteratees. The RandomIO module http://okmij.org/ftp/Haskell/Iteratee/RandomIO.hs is a good illustration: when the enumerator receives the seek request, it checks if the desired stream offset corresponds to the data already in the current IO buffer. If so, no IO is performed and the iteratee is resumed with the existing buffer data. The tests in the file check for that. Iteratee knows nothing about the buffer or if there is a buffer. Wren Thornton wrote: > In an ideal framework the producers, transformers, and consumers of > stream data would have a type parameter indicating the up-stream > communication they support or require (in addition to the type > parameters for stream type, result type, and side-effect type). Very true. Currently the design of Iteratees quite resembles that of Control.Exception: everything can throw SomeException. Ideally one would like to be more precise, and specify what exceptions or sorts of exceptions could be thrown -- by Iteratees, and by ordinary Haskell functions. The design of a good effect system is still the topic of active research, although there are some encouraging results. John A. De Goes: > 3. Iteratees sometimes need to manage resources, but they're not > designed to do so which leads to hideous workarounds; Gregory Collins: > The thing which I find is missing the most from enumerator as it > stands is not this -- it's the fact that Iteratees sometimes need to > allocate resources which need explicit manual deallocation (i.e. > sockets, file descriptors, mmaps, etc), but because Enumerators are > running the show, there is no "local" way to ensure that the > cleanup/bracket routines get run on error. I used to think that processing several inputs at different paces was indeed a stumbling block. It seemed that an iteratee needed to open a separate file, which it is indeed ill-equipped to do. Fortunately, that difficulty has been overcome, surprisingly in a natural way with no changes to the library: http://okmij.org/ftp/Streams.html#2enum1iter The pleasant surprise is that we can iterate (no pun intended) Iteratee monad transformers, just as we did with (IORT s) monad transformer in the Lightweight Monadic Regions. Thus we maintain the region-like discipline of managing resources. I'm keen to hear of the example that seem to require Iteratee's allocating additional resources. I'd really like to see if any of such cases can be cast it terms of regions, implemented via iterated Iteratee transformers. John A. De Goes: > 1. It does not make sense in general to bind with an iteratee that > has already consumed input, but there's no type-level difference > between a "virgin" iteratee and one that has already consumed input; I'm not sure I follow. Why should it make a difference between a virgin iteratee and the one that consumed some input. One should think of the Iteratee as two arguments of fold (f and z) bundled together. Why the function being folded over should care how many times it has been applied to input data? It is a pure function, transforming state plus input to a new state. The useful laws of fold hold precisely because the function f is pure and doesn't care. A detailed example showing why you think you need this distinction would be appreciated. > 4. Iteratees cannot incrementally produce output, it's all or > nothing, which makes them terrible for many real world problems > that require both incremental input and incremental output. Again, a detailed example, a use case if you will, describing the desired behavior is appreciated. By detailed I mean the example that describes the desired input-output behavior, preferably including sample input or output data. For instance, the question posed by Evgeniy Permjacov last December was precise and helpful: (incrementally) merge two sorted streams. I can't promise a prompt reply this month or in April, I'm afraid.

2 1

ANNOUNCE: enumerator 0.4.8
by John Millikin 30 Mar '11

30 Mar '11

----------------------------------------------------------------------------------------------------------------------------------------------------- Enumerators are an efficient, predictable, and safe alternative to lazy I/O. Discovered by Oleg Kiselyov, they allow large datasets to be processed in near–constant space by pure code. Although somewhat more complex to write, using enumerators instead of lazy I/O produces more correct programs. http://hackage.haskell.org/package/enumerator http://john-millikin.com/software/enumerator/ ----------------------------------------------------------------------------------------------------------------------------------------------------- Hello -cafe, It's been a while since the last point release of enumerator. This one is sufficiently large that I think folks might want to know about it, and since I try not to spam too many announcements, I'll give a quick rundown on major changes in other 0.4.x versions as well. First, most of what I call "list analogues" -- enumerator-based versions of 'head', 'take', 'map', etc -- have been separated into three modules (Data.Enumerator.List, .Binary, and .Text) depending on what sorts of data they operate on. This separation has been an ongoing process throughout 0.4.x releases, and I think it's now complete. The old names in Data.Enumerator will continue to exist in 0.4.x versions, but will be removed in 0.5. Second, Gregory Collins and Ertugrul Soeylemez found a space leak in Iteratee's (>>=), which could cause eventual space exhaustion in some circumstances. If you use enumerators to process very large or infinite streams, you probably want to upgrade to version 0.4.7 or higher. Third, the source code PDF has seen some substantial improvement -- if you're interested in how the library is implemented, or have insomnia, read it at < http://john-millikin.com/software/enumerator/enumerator_0.4.8.pdf > Finally, there is a known issue in the current encoding of iteratees -- if an iteratee yields extra data but never consumed anything, that iteratee will violate the monad law of associativity. Oleg has updated his implementations to fix this problem, but since it would break a *lot* of dependent libraries, I'm holding off until the vague future of version 0.5. Since iteratees that yield extra data they didn't consume are invalid anyway, I hope this problem will not cause too much inconvenience. New features ----------------- * Range-limited binary file enumeration (requested + initial patch by Bardur Arantsson). * splitWhen , based on the "split" package < http://hackage.haskell.org/package/split > * 0.4.6: Typeable instances for most types (requested by Michael Snoyman) * 0.4.5: joinE , which simplifies enumerator/enumeratee composition (requested by Michael Snoyman)

11 30

SoC / Cabal project proposal: specify Setup.hs dependencies
by Rogan Creswick 30 Mar '11

30 Mar '11

I've been wanting to share code between cabal projects for some time, and I finally had a chance to write up the rough idea as a simple proposal. Here's the description, with links to the SoC trac and reddit haskell_proposals pages. SoC ticket: http://hackage.haskell.org/trac/summer-of-code/ticket/1602 Reddit: http://www.reddit.com/r/haskell_proposals/comments/ge1zp/cabal_dependency_s… Non-standard builds often need to implement specific build steps in Setup.hs, specifying a build-type: Custom in the project cabal file. The user hook system works reasonably well for modifying or replacing the specific sub steps of a build, but *implementing* anything more than the simplest logic in Setup.hs is very difficult. A great deal of this difficulty stems from the lack of library support for code in Setup.hs. Adding a cabal section that specifies a build-depends: for Custom (and possibly other) build types would allow developers to reuse build code between projects, to share build system modifications on hackage more easily, and to prototype new additions to cabal. Setup.hs *can* allow arbitrarily complex build system manipulations; however, it is not practical to do so because the infrastructure surrounding Setup.hs doesn't promote code reuse. The addition of dependencies that cabal-install would install prior to building setup.hs and issuing the build would enable developers to produce custom builds that perform complex operations that utilize the high-quality libraries available on hackage. Furthermore, this would provide the means to prototype (and distribute) new cabal / cabal-install features before integrating experimental code into the stable tools. I'm interested in thoughts / feedback about the idea, as well as hearing from anyone interested in pursuing this as a summer of code project :) Thanks! --Rogan

3 2

Summer of Code Mentors for Language.C
by Andrew Hirsch 30 Mar '11

30 Mar '11

Would anybody be interested in mentoring me for a summer of code project working on language.c? I want to write an internal pre-processor that saves information such as comments and potentially some specially defined annotations, so that code can be analyzed better.

3 2

hdbc-odbc adds a space to the value
by vagif.verdi＠gmail.com 29 Mar '11

29 Mar '11

I have a weird problem ? When preparing sql statements with ? placeholders, hdbc-odbc adds a space to a string value at the end. run conn "update sometable set somefield = ? where id = ?" [SqlString "bla", toSql 10] Database: Ms Sql Server 2005. Driver: FreeTds on linux Please help! Regards, Vagif Verdi

3 5

Please allow variation in the Yackage page title
by Yitzchak Gale 29 Mar '11

29 Mar '11

Could you please add a way of varying the title of the Yackage web page? That would make it much easier to work with multiple Yackages at once. For my particular setup, including its port number in the web page title would do the trick. But perhaps the easiest and most general thing would be to add a command-line option for setting the title. Using multiple Yackages is a very simple yet powerful technique to keep cabal versions straight. You can use it to group package versions in various ways - versions that you are currently hacking on, versions that have stabilized but you are not releasing yet while you work on other related packages, versions shared by various teams and subteams of developers, etc. Multiple Yackages are especially useful in combination with cabal-dev, but currently you have to fiddle a little bit to get cabal-dev to use a modified cabal config file. See: http://www.haskell.org/pipermail/haskell-cafe/2011-March/090492.html Thanks, Yitz

2 1

WANTED: Compensated Haskell Hacker for Language Project
by Bryan Edds 29 Mar '11

29 Mar '11

Hello fellow Haskellers! I have a language I would like to prototype in Haskell. It's a 'language-oriented' programming language based on Lisp, ML, and Python. It's a relatively small language, but should be powerful enough to have many of its features implemented in terms of itself (classes, actors, almost all syntax, etc.) While the project is compensated, I'm just a commercial game programmer by day and don't have a corporate budget, so part of the motivation is the joy of the project itself. Otherwise, I have a budget of $4k - $6k USD over the next several months to compensate someone to hack it up for me. I don't have much in the way of specs, and the design is ongoing (almost certainly some holes), but there is an overview of the design here - http://channel9.msdn.com/Forums/Coffeehouse/Prototyping-New-Language-With-H… I really want to churn it out as a nice working concept to prove the language design and language oriented programming is practical even for soft-real time software like console games. The project will be open source throughout its development and the language will be free to use. I really want it to take off, despite my meager resources :) Beyond that, I'm looking for charitable investors once the prototype is finished and proven. I want to find other people are so sick of writing soft-real time software in C-family languages at their work place that it's worth the donation. Perhaps there is some company who might be interested in further incubating the project if it proves commercially interesting :) I do ask for the code to be reasonably clean as I would like to be able to help once I get some time. I would be implementing the thing fully myself, but I am way too swamped at work :) Since I have to rest my hands during my off hours, I would like to do most communication by voice. Feel free to record any of the communication if you like. If anyone is interested and has the credentials to bring the project to fruition, please contact me at bryanedds(a)yahoo.com straight away. Thanks!

3 3

How to best deal with nullPtr from alloca and friends?
by Jason Dagit 29 Mar '11

29 Mar '11

I was reading up on the documentation for alloca and friends[1], which says, "If any of the allocation functions fails, a value of nullPtr<http://www.haskell.org/ghc/docs/7.0.2/html/libraries/base-4.3.1.0/Foreign-P…> is produced." It seems like every example of FFI code that I find which uses alloca ignores the possibility of a nullPtr[2, 3, 4]. So then I started trying out examples with the help of dmwit and others from #haskell. It seems that actually, alloca and friends throw exceptions: dmwit> main = allocaArray (2^30) (\ptr -> print ((nullPtr :: Ptr Double) == ptr)) <dmwit> lispy: alloca also throws an exception. <dmwit> lispy: Or rather, allocaBytes throws an exception, and alloca is implemented in terms of allocaBytes, so I'm *guessing* that alloca throws an exception. I'm on a 64bit version of windows here with more than 4GB of memory to spare for the GHC process. Unfortunately, allocaBytes takes an Int so I can't test it with a request larger than the amount of physical ram I have. Just playing around with different arguments to allocaBytes I can get different behavior: <= 2^30 bytes, works perfectly == 2^30 + 2^20 bytes, I get an "<interactive>: out of memory" message and ghci terminates == 2^31-1, I get a crash where windows pops up a little dialog saying my program (ghci) has crashed. The behavior seems to be inconsistent with the documentation. What is the correct behavior for alloca and friends and should I be checking for nullPtr? Thanks, Jason [1] http://www.haskell.org/ghc/docs/7.0.2/html/libraries/base-4.3.1.0/Foreign-M… [2] http://en.wikibooks.org/wiki/Haskell/FFI [3] http://www.haskell.org/haskellwiki/HSFFIG/Examples [4] http://book.realworldhaskell.org/read/interfacing-with-c-the-ffi.html

2 3

ANN: improve-0.3.1
by Tom Hawkins 29 Mar '11

29 Mar '11

ImProve [1] is an imperative DSL for hard realtime embedded applications. ImProve programs are verified with infinite state,unbounded model checking (k-induction, invariant strengthening, SMT). In addition to C, ImProve now supports Simulink [2] as a backend target. Simulink is a popular language for automotive and aerospace control systems. -Tom [1] http://hackage.haskell.org/package/improve [2] http://www.mathworks.com/

1 0

Re: [Haskell-cafe] ANNOUNCE: enumerator 0.4.8
by Gregory Collins 29 Mar '11

29 Mar '11

On Mar 29, 2011 10:42 AM, <oleg(a)okmij.org> wrote: > > I'm keen to hear of the example that seem to require Iteratee's > allocating additional resources. I'd really like to see if any of such > cases can be cast it terms of regions, implemented via iterated > Iteratee transformers. Hello Oleg, The first example which comes to mind - because I recently implemented this - is HTTP uploads: the input stream can hold N files (the value of N not known ahead of time), and each upload can go to a separate temporary file. If the request stream fails early (e.g. client browser closes connection), any already-created temp files and any open file handles must be cleaned up.

1 0

Re: [Haskell-cafe] Iteratee, ghc 6.12/7.0 strange behaviour -epollControl: permission denied (Operation not permitted)
by Michael A Baikov 29 Mar '11

29 Mar '11

I tried with both 7.0.2 and 7.0.3 -----Original Message----- > On Mon, Mar 28, 2011 at 7:31 PM, Michael A Baikov <pacak(a)bk.ru> wrote: > > > > > > I am still playing with lastest iteratee and i think i found something strange. > > > > > > let's suppose we have a file test.hs like this: > > > > import Data.Iteratee > > import Data.Iteratee.IO > > import Data.Iteratee.Char > > > > main = fileDriver printLines "/etc/passwd" > > > > It works fine when executed via runhaskell / ghci in ghc 6.12. Compiled version in ghc 7 > > also works, but when i am trying to execute it via runhaskell / ghci i am getting this error: > > > > iter.hs: epollControl: permission denied (Operation not permitted) > > > > Any ideas? > > Make sure you have at least GHC 7.0.2. There were some I/O manager > bugs in 7.0.1. > > Johan >

2 1

working off a Yesod example file, need help lifting values from one monad into another. (and probably other things too).
by Michael Litchard 29 Mar '11

29 Mar '11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 I'm working off of a example file from Yesod, ajax.lhs I've made an important change in types, and this has resulted in having to make the old code conform to the change. I will point out the specifics, then present my question. In the event I failed to include important information, I will paste in my code as well as the prototype. [Original] > getHomeR :: Handler () > getHomeR = do > Ajax pages _ <- getYesod > let first = head pages > redirect RedirectTemporary $ PageR $ pageSlug first [Changed] > getHomeR :: Handler () > getHomeR = do > Tframe pages _ <- getYesod > let first = head pages > redirect RedirectTemporary $ PageR $ pageSlug first Error Message test.lhs:62:4: Constructor `Tframe' should have 2 arguments, but has been given 1 In the pattern: Tframe pages In a stmt of a 'do' expression: Tframe pages <- getYesod **** This is not what I wrote ***** In the expression: do { Tframe pages <- getYesod; content <- widgetToPageContent widget; hamletToRepHtml (hamlet-0.7.1:Text.Hamlet.Quasi.toHamletValue (do { (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . preEscapedString) "<!DOCTYPE html><html><head><title>"; (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . Text.Blaze.toHtml) (Main.pageTitle content); .... })) } As far as I can tell, I only made a cosmetic change. I don't know what's going on here. [Original] > data Page = Page > { pageName :: String > , pageSlug :: String > , pageContent :: String ******** I'm going to change this ********** > } [Changed] > data Page = Page > { pageTitle :: String > , pageSlug :: String -- ^ used in the URL > , pageContent :: IO String ******** This is the change ******* > } Here's where I run into trouble [Original] > json page = jsonMap > [ ("name", jsonScalar $ pageName page) > , ("content", jsonScalar $ pageContent page) ******** I'm going to change this ******** > ] [My changes] > json page = jsonMap > [ ("name", jsonScalar $ Main.pageTitle page) > , ("content", jsonScalar $ liftIO $ pageContent page) ******* This is the change *********** > ] Here's the compiler error test.lhs:107:35: Couldn't match expected type `Char' against inferred type `[Char]' Expected type: String Inferred type: [String] In the second argument of `($)', namely `liftIO $ pageContent page' In the expression: jsonScalar $ liftIO $ pageContent page Failed, modules loaded: none. I'd appreciate a discussion about why this is wrong, and perhaps clues as to what is right. Last problem, stemming from the change in type to IO String. I don't have a clue as to what change I should make. test.lhs:100:25: No instance for (Text.Blaze.ToHtml (IO String)) arising from a use of `Text.Blaze.toHtml' at test.lhs:(100,25)-(103,3) Possible fix: add an instance declaration for (Text.Blaze.ToHtml (IO String)) In the second argument of `(.)', namely `Text.Blaze.toHtml' In a stmt of a 'do' expression: (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . Text.Blaze.toHtml) (pageContent page) In the first argument of `hamlet-0.7.1:Text.Hamlet.Quasi.toHamletValue', nam ely `do { (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . preEscapedString) "<h1>"; (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . Text.Blaze.toHtml) (Main.pageTitle page); (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . preEscapedString) "</h1><article>"; (hamlet-0.7.1:Text.Hamlet.Quasi.htmlToHamletMonad . Text.Blaze.toHtml) (pageContent page); .... }' And finally, both files can be found below, if it is necessary to look at them. [Original] <p>We're going to write a very simple AJAX application. It will be a simple site with a few pages and a navbar; when you have Javascript, clicking on the links will load the pages via AJAX. Otherwise, it will use static HTML.</p> <p>We're going to use jQuery for the Javascript, though anything would work just fine. Also, the AJAX responses will be served as JSON. Let's get started.</p> > {-# LANGUAGE ScopedTypeVariables, TypeFamilies, QuasiQuotes, TemplateHaskell, MultiParamTypeClasses #-} > import Yesod > import Yesod.Helpers.Static > import Data.Monoid (mempty) Like the blog example, we'll define some data first. > data Page = Page > { pageName :: String > , pageSlug :: String > , pageContent :: String > } > loadPages :: IO [Page] > loadPages = return > [ Page "Page 1" "page-1" "My first page" > , Page "Page 2" "page-2" "My second page" > , Page "Page 3" "page-3" "My third page" > ] loadPages :: IO [Page] loadPages = do > > data Ajax = Ajax > { ajaxPages :: [Page] > , ajaxStatic :: Static > } > type Handler = GHandler Ajax Ajax Next we'll generate a function for each file in our static folder. This way, we get a compiler warning when trying to using a file which does not exist. > staticFiles "static/yesod/ajax" Now the routes; we'll have a homepage, a pattern for the pages, and use a static subsite for the Javascript and CSS files. > mkYesod "Ajax" [$parseRoutes| > / HomeR GET > /page/#String PageR GET > /static StaticR Static ajaxStatic > |] <p>That third line there is the syntax for a subsite: Static is the datatype for the subsite argument; siteStatic returns the site itself (parse, render and dispatch functions); and ajaxStatic gets the subsite argument from the master argument.</p> <p>Now, we'll define the Yesod instance. We'll still use a dummy approot value, but we're also going to define a default layout.</p> > instance Yesod Ajax where > approot _ = "" > defaultLayout widget = do > Ajax pages _ <- getYesod > content <- widgetToPageContent widget > hamletToRepHtml [$hamlet| > \<!DOCTYPE html> > > <html> > <head> > <title>#{pageTitle content} > <link rel="stylesheet" href="@{StaticR style_css}"> > <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"> > <script src="@{StaticR script_js}"> > \^{pageHead content} > <body> > <ul id="navbar"> > $forall page <- pages > <li> > <a href="@{PageR (pageSlug page)}">#{pageName page} > <div id="content"> > \^{pageBody content} > |] <p>The Hamlet template refers to style_css and style_js; these were generated by the call to staticFiles above. There's nothing Yesod-specific about the <a href="/static/yesod/ajax/style.css">style.css</a> and <a href="/static/yesod/ajax/script.js">script.js</a> files, so I won't describe them here.</p> <p>Now we need our handler functions. We'll have the homepage simply redirect to the first page, so:</p> > getHomeR :: Handler () > getHomeR = do > Ajax pages _ <- getYesod > let first = head pages > redirect RedirectTemporary $ PageR $ pageSlug first And now the cool part: a handler that returns either HTML or JSON data, depending on the request headers. > getPageR :: String -> Handler RepHtmlJson > getPageR slug = do > Ajax pages _ <- getYesod > case filter (\e -> pageSlug e == slug) pages of > [] -> notFound > page:_ -> defaultLayoutJson (do > setTitle $ string $ pageName page > addHamlet $ html page > ) (json page) > where > html page = [$hamlet| > <h1>#{pageName page} > <article>#{pageContent page} > |] > json page = jsonMap > [ ("name", jsonScalar $ pageName page) > , ("content", jsonScalar $ pageContent page) > ] <p>We first try and find the appropriate Page, returning a 404 if it's not there. We then use the applyLayoutJson function, which is really the heart of this example. It allows you an easy way to create responses that will be either HTML or JSON, and which use the default layout in the HTML responses. It takes four arguments: 1) the title of the HTML page, 2) some value, 3) a function from that value to a Hamlet value, and 4) a function from that value to a Json value.</p> <p>Under the scenes, the Json monad is really just using the Hamlet monad, so it gets all of the benefits thereof, namely interleaved IO and enumerator output. It is pretty straight-forward to generate JSON output by using the three functions jsonMap, jsonList and jsonMap. One thing to note: the input to jsonScalar must be HtmlContent; this helps avoid cross-site scripting attacks, by ensuring that any HTML entities will be escaped.</p> <p>And now our typical main function. We need two parameters to build our Ajax value: the pages, and the static loader. We'll load up from a local directory.</p> > main :: IO () > main = do > > pages <- loadPages > let s = static "static/yesod/ajax" > warpDebug 3000 $ Ajax pages s [My changes] > {-# LANGUAGE TypeFamilies, QuasiQuotes, TemplateHaskell, MultiParamTypeClasses > #-} > import Yesod > import Yesod.Helpers.Static > import System.Environment > import System.IO > import System.Directory > import System.FilePath.Posix > import Control.Applicative > import Data.List.Split > data Page = Page > { pageTitle :: String > , pageSlug :: String -- ^ used in the URL > , pageContent :: IO String > } > loadPage :: IO [Page] > loadPage = do > let directoryPath = "/home/mlitchard/playground/webTests/files" > let processedPath = map (directoryPath </>) . filter (`notElem` [".",".."]) > pageFileNames <- processedPath <$> getDirectoryContents directoryPath > let pageFiles = map readFile pageFileNames > return $ zipWith popEntries pageFileNames pageFiles -- > return $ zipWith popEntries > popEntries :: FilePath -> IO String -> Page > popEntries pageFileName pageFile = > let pageT = last $ splitOn "/" pageFileName > pageS = "Job" ++ pageT > in Page { Main.pageTitle=pageT, > pageSlug=pageS, > pageContent=pageFile } > data Tframe = Tframe > { tframePages :: [Page] > , tframeStatic :: Static > } > type Handler = GHandler Tframe Tframe > staticFiles "static/yesod/ajax" Routes > mkYesod "Tframe" [$parseRoutes| > / HomeR GET > /page/#String PageR GET > /static StaticR Static tframeStatic > |] defining the Yesod instance > instance Yesod Tframe where > approot _ = "" > defaultLayout widget = do > Tframe pages <- getYesod > content <- widgetToPageContent widget > hamletToRepHtml [$hamlet| > \<!DOCTYPE html> > > <html> > <head> > <title>#{Main.pageTitle content} > <link rel="stylesheet" href="@{StaticR style_css}"> > <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"> > <script src="@{StaticR script_js}"> > \^{pageHead content} > <body> > <ul id="navbar"> > $forall page <- pages > <li> > <a href="@{PageR (pageSlug page)}">#{Main.pageTitle page} > <div id="content"> > \^{pageBody content} > |] > getHomeR :: Handler () > getHomeR = do > Tframe pages _ <- getYesod > let first = head pages > redirect RedirectTemporary $ PageR $ pageSlug first > getPageR :: String -> Handler RepHtmlJson > getPageR slug = do > Tframe pages _ <- getYesod > case filter (\e -> pageSlug e == slug) pages of > [] -> notFound > page:_ -> defaultLayoutJson (do > setTitle $ string $ Main.pageTitle page > addHamlet $ html page > ) (json page) > where > html page = [$hamlet| > <h1>#{Main.pageTitle page} > <article>#{pageContent page} > |] > json page = jsonMap > [ ("name", jsonScalar $ Main.pageTitle page) > , ("content", jsonScalar $ liftIO $ pageContent page) > ] > main :: IO () > main = do > > pages <- loadPage > let s = static "static/yesod/ajax" > warpDebug 3000 $ Tframe pages s If you've read to the bottom, thanks for your patience. I appreciate any illumination you can send my way. Michael

3 3

HsOpenSSL package fails to install
by Rob Nikander 29 Mar '11

29 Mar '11

Hi all, I tried `cabal install HsOpenSSL` and it died with some errors about 'i2d_of_void'. I'm on Mac OS X, gcc 4.2. The symbol 'i2d_of_void' is defined by a macro in the openssl/asn1.h header, which is indirectly included, so I don't see the problem. Any ideas? If this is a bug, is there a standard way to report it? Email the package maintainer in this case? thanks, Rob cbits/HsOpenSSL.c: In function ‘HsOpenSSL_DSAPublicKey_dup’: cbits/HsOpenSSL.c:220:0: error: ‘i2d_of_void’ undeclared (first use in this function) cbits/HsOpenSSL.c:220:0: error: (Each undeclared identifier is reported only once [... cascading errors from there ...]

1 0

Saving changes to cabal config in cabal-dev
by Yitzchak Gale 28 Mar '11

28 Mar '11

Thanks for the fantastic cabal-dev tool! Is there any convenient way to save changes to the package-specific cabal config file in cabal-dev? When I make changes to cabal-dev/cabal.config, cabal-dev seems to scribble over them next time I run it. The only solution I have found so far is to run cabal-dev install once (ignoring the errors) to create the cabal-dev directory and cabal.config, make a copy of cabal.config with a different name, edit it as needed, and then use a long-winded "--config=..." option for every subsequent run cabal-dev. The changes I need to make are as follows: - remote-repo-cache is wrong for Mac OS X - add more remote-repo lines for our local yackage servers. Note that the first is global, so it would be nice to have a way to make that change once globally for all packages. Whereas the second is, in general, package specific. We find local yackage servers much simpler to use and far more powerful than using add-source. You can organize packages by project, team, specific developers, etc. Thanks, Yitz

2 1

Iteratee, ghc 6.12/7.0 strange behaviour - epollControl: permission denied (Operation not permitted)
by Michael A Baikov 28 Mar '11

28 Mar '11

I am still playing with lastest iteratee and i think i found something strange. let's suppose we have a file test.hs like this: import Data.Iteratee import Data.Iteratee.IO import Data.Iteratee.Char main = fileDriver printLines "/etc/passwd" It works fine when executed via runhaskell / ghci in ghc 6.12. Compiled version in ghc 7 also works, but when i am trying to execute it via runhaskell / ghci i am getting this error: iter.hs: epollControl: permission denied (Operation not permitted) Any ideas?

2 1

bug in Prelude.words?
by malcolm.wallace 28 Mar '11

28 Mar '11

Does anyone else think it odd that Prelude.words will break a string at a non-breaking space? Prelude> words "abc def\xA0ghi" ["abc","def","ghi"] I would have expected this to be the obvious behaviour: Prelude> words "abc def\xA0ghi" ["abc","def\160ghi"] Regards, Malcolm

6 7

ANNOUNCEMENT: nehe-tuts 0.2.0, new release
by Jason Dagit 28 Mar '11

28 Mar '11

What is the nehe-tuts package? In 1999 Jeff Molofee, aka NeHe, wrote a series of tutorials on how to get started with basic OpenGL. In 2005, I ported the code for these tutorial pages to Haskell. You can find NeHe's original tutorials here: http://nehe.gamedev.net/default.asp Since that time, we have seen the adoption of Cabal, the rise of Hackage, a major refactoring of the Haskell OpenGL bindings, major revisions to the official OpenGL spec, and major changes to GHC. Through all of that my tutorial conversions have continue to compile with GHC by explicitly requesting Haskell98, but they are starting to bit rot and GHC complains that I use deprecated features. This release represents an effort to modernize the haskell version of these tutorials. I have stopped using the OpenGL package and now instead use the easier/cleaner OpenGLRaw bindings. Instead of GLUT, I use GLFW-b bindings. This means windows users can 'cabal install' this package and run the examples with just the Haskell Platform. No extra libraries need to be manually installed. All the dependencies can be installed from Hackage. I've also replaced some of the uglier bits that I wrote with nicer libraries like bytestring and cereal. These tutorial need further updates in order to match the latest in OpenGL specifications. The tutorials are still written using OpenGL's immediate mode, but that was deprecated in OpenGL 3.x so it's possible that in the future these examples won't be supported by your graphics card. Updating these examples to not rely on deprecated APIs should be easy, but I wanted an intermediate modernization release first. Enjoy! Jason

2 1

ANNOUNCEMENT: xmobar 0.13
by Jose A. Ortega Ruiz 28 Mar '11

28 Mar '11

I'm happy to announce the release of xmobar 0.13. xmobar is a lightweight, text-based system monitor for un*x systems. See http://projects.haskell.org/xmobar/ for the whole story. ## Version 0.13 (March 28, 2011) _New features_ - New `Volume` monitor displaying ALSA soundcards information, by Thomas Tuegel. - New `ThermalZone` plugin substituting `Thermal` and using linux's *sysfs* interface (you need this one if you're using a kernel version equal to or above 2.6.37). See [issue 44]. - xmobar app new has WM_CLASS, WM_NAME and _NET_WM_PID xprops ([issue 38]). _Incompatible changes_ - In the process of solving [issue 14], we've broken those configurations that rely on including alignment separators in the input fed to `StdinReader`. - The MPD plugin does not accept host and port options anymore: use the environment variables MPD_HOST and MPD_PORT instead. - The `Mail` plugin now takes a second parameter (a string) specifying its alias. As a side-effect, this solves [issue 30]. _Bug fixes_ - [issue 14]: `StdinReader` and other plugins accepting external input don't get confused anymore when characters from `alignSep` appear in their input. - [issue 27]: `BottomSize` placement now respects its width argument. - [issue 28]: Compilation in Mac OS X fixed. - [issue 30]: `Mail` plugin can be specified anywhere in commands list. - [issue 36]: Battery monitor now supports non-standard locations of the `/sys/class/power_supply/AC/online` file. - [issue 40]: Battery monitor now supports the new power_now, that replaces current_now in linux kernels for v. 2.36 and above. - [issue 42]: More accurate net monitor rates. - DiskIO, Cpu and MultiCpu monitors are also more accurate now. - Text is now correctly centered vertically. - `FullBM` border spec fixed. Happy monitoring, jao -- Computer games don't affect kids; I mean if Pac Man affected us as kids, we would all be running around in darkened rooms, munching magic pills, and listening to repetitive electronic music. - Kristian Wilson, Nintendo Inc.

1 0

why are trading/banking industries seriously adopting FPLs???
by Vasili I. Galchin 28 Mar '11

28 Mar '11

Hello, I am very curious about the readiness of trading and banking industries to adopt FPLs like Haskell: http://talenteze.catsone.com/careers/index.php?m=portal&a=details&jobOrderI….... I currently work in the computer security(intrusion detection). My colleagues are totally ignorant concerning the foundations/motivations of FPLs. (Ironically http://www.galois.com participates in the computer security arena!). Why are are trading/banking diving into FPLs? Regards, Vasili

8 8

Three Google Summer of Code project proposals
by Johan Tibell 27 Mar '11

27 Mar '11

Hi, I'd like to advertise three Google Summer of Code projects that I recently added to the list [1] of proposed projects: *** Build multiple Cabal packages in parallel *** http://hackage.haskell.org/trac/summer-of-code/ticket/1594 Many developers have multi-core machines but Cabal runs the build process in a single thread, only making use of one core. If the build process could be parallelized build times could be cut by perhaps a factor of 2-8, depending on the number of cores and opportunity of parallel execution available. *** Simpler support for isolated/sandboxed Cabal builds *** http://hackage.haskell.org/trac/summer-of-code/ticket/1590 cabal-dev and capri allow developers to build packages in their own sandboxes, using a separate package database for each. This allows for isolated builds and prevents breakages due e.g. package upgrades. Merging cabal-dev into Cabal allows us to share lots of code and makes the feature more accessible to developers. *** Convert the text package to use UTF-8 internally *** http://hackage.haskell.org/trac/summer-of-code/ticket/1595 When the text package was created, early benchmarks showed that using UTF-16 as the internal representation for Unicode code points was the fastest. The package still uses UTF-16 internally. The benchmarks might not have given a complete picture of the performance implications of using different internal encodings: all benchmarks were run on input data that used the same encoding as used internally, but most real world data uses UTF-8. If the benchmarks would also have taken the cost of decoding and encoding from and to UTF-8 the results might have been different. The goal of this project is to revisit the choice of internal encoding. I encourage any interested students to have a look at the three proposals (and the other proposals on the list), discuss them on this list, and sign up for GSoC (after March 28). 1. http://hackage.haskell.org/trac/summer-of-code/report/1 Cheers, Johan

3 3

Re: [Haskell-cafe] timezone-series and timezone-olson
by Yitzchak Gale 27 Mar '11

27 Mar '11

Hi Manfred, I am copying this response to the Haskell Cafe mailing list. Manfred Lotz wrote: > ...I'm trying to figure out how to use your > packages to get the time in a different timezone. > Do you have an example how to do that? What I want for example is to > provide the timezone preferably like this: US/Eastern, and get the time > information like for example this: > > US/Eastern DST EDT 2011.03.27 05:20:52 > > The pieces I like to get are: Summertime indicator, TZ abbrev and the > date resp. time in that timezone. If you are willing to hard-code the offset of the timezone, you don't need my packages. You only need them if you need your program to reflect accurately whether summer time is/was in effect at the given time at the given location, or if the timezone changed at some time in history. I assume that you are starting with a UTCTime object, constructed in one of the usual ways using the time package. Get the timezone information provided by the operating system. On anything other than Windows, that means reading the timezone file for that zone. On Windows the information is in the registry, but unfortunately we don't have an interface for reading it from there yet. The best I can suggest for now is to copy timezone files from a non-Unix computer. Assuming you have the timezone file that you need - let's say it is in the directory /usr/share/zoneinfo as typical for Linux and Mac - here is how to code it: do ... tzs <- getTimeZoneSeriesFromOlsonFile "/usr/share/zoneinfo/America/New_York" let usEastern = utcToLocalTime' tzs utc Then, to format the time as you require, use the formatTime function from the time package. Hope this helps, Yitz

2 1

NFData instance for Numeric.LinearAlgebra Matrix
by Tad Doxsee 27 Mar '11

27 Mar '11

Hello, Is there already an instance of NFData (from Control.DeepSeq in the deepseq package) for the type Matrix (from Numeric.LinearAlgebra in the hmatrix package) somewhere? If not, how does one define one? (The data declaration for Matrix is in a hidden module, Data.Packed.Internal.Matrix, if that makes any difference). Thanks, Tad

2 1

ANN: unix-bytestring 0.3.4
by wren ng thornton 27 Mar '11

27 Mar '11

-------------------------------------------- -- unix-bytestring 0.3.4 -------------------------------------------- The unix-bytestring package offers a full selection of Unix/Posix-specific functions for reading and writing ByteStrings to Fds. -------------------------------------------- -- Changes (since 0.3.2) -------------------------------------------- * For all the low-level functions (fd*Buf) there are now safer versions (tryFd*Buf) which return errors via Either Errno, instead of throwing IOError exceptions. These will be of particular interest to the maintainers of iteratee, iteratee-mtl, and liboleg. * All the low-level functions (and the appropriate higher-level analogues) now check that the ByteCount or Int arguments are positive. If they're negative or zero then they return immediately (saying that zero bytes were read/written, returning an empty ByteString, etc). Note that this behavior is different than the functions in @unix@ which only check if they're equal to zero, or don't check at all. -------------------------------------------- -- Future work -------------------------------------------- * The readv(2) and writev(2) functions have a number of corner cases that throw errors (e.g., if you have too many iovecs where "too many" could mean >16) but these can be detected beforehand, or worked around by making multiple calls. Unfortunately, finding out what the limit is (or whether the linked version of libc does its own workarounds) is pretty tricky and platform-dependent. So I'll work on figure that out and making some safer wrappers. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unix-bytestring Darcs: http://community.haskell.org/~wren/unix-bytestring Haddock (Darcs version): http://community.haskell.org/~wren/unix-bytestring/dist/doc/html/unix-bytes… -- Live well, ~wren

1 0

help with dynamic load
by Rob Nikander 27 Mar '11

27 Mar '11

Hi all, I'm trying to use the 'plugins' package. Since I already posted to stackoverflow I'll just link to that. I posted a simple program that I thought would work, but mostly doesn't. Any pointers, appreciated. http://stackoverflow.com/questions/5427777/help-with-haskell-dynamic-plugin… thanks, Rob

2 2

Functional dependence nightmare
by Edgar Gomes Araujo 27 Mar '11

27 Mar '11

Hi, I'm running in errors that, I think, is related with functional dependence. But I don't know how to interpret or solve them. I have basically 3 types: Edge, Face and Simplex (Tetrahedron) and I want to combined them in a class type such that: - Edge and Face will produce a convex hull - Face and Simplex will produce a Delaunay triangulation under the same MCB (Marriage Before Conquer) algorithm. I first tried to use Multiparameter type class with functional dependence relation: class SubUnit subUnit unit | subUnit -> unit, unit -> subUnit where buildUnit :: ActiveSubUnit subUnit -> [Point] -> Maybe unit build1stUnit :: Plane -> [Point] -> [Point] -> [Point] -> Maybe unit getAllSubUnits :: unit -> [ActiveSubUnit subUnit] subUnitPos :: BoxPair -> subUnit -> Position but then I decide to go for a GADT form (If someone wants to try to compile it, one can get the complete code on GitHub: http://github.com/EdgarGomes/DeUni) : class SubUnit subUnit where type Unit subUnit :: * buildUnit :: ActiveSubUnit subUnit -> [Point] -> Maybe (Unit subUnit) build1stUnit :: Plane -> [Point] -> [Point] -> [Point] -> Maybe (Unit subUnit) getAllSubUnits :: Unit subUnit -> [ActiveSubUnit subUnit] subUnitPos :: BoxPair -> subUnit -> Position In both cases I get a couple of errors like: src/DeUni.hs:265:39: Couldn't match type `Unit subUnit3' with `Unit subUnit2' NB: `Unit' is a type function, and may not be injective Expected type: Unit a Actual type: Unit subUnit2 In the expression: un In the expression: [un] In the first argument of `return', namely `(([un], fromList $ getAllSubUnits un) :: SubUnit a => ([Unit a], SetActiveSubUnits a))' src/DeUni.hs:265:39: Couldn't match type `Unit (ActiveSubUnit subUnit3)' with `Unit subUnit2' NB: `Unit' is a type function, and may not be injective Expected type: Unit a Actual type: Unit subUnit2 In the expression: un In the expression: [un] In the first argument of `return', namely `(([un], fromList $ getAllSubUnits un) :: SubUnit a => ([Unit a], SetActiveSubUnits a))' src/DeUni.hs:265:44: Could not deduce (a1 ~ ActiveSubUnit subUnit3) from the context (SubUnit a) bound by the type signature for mbc :: SubUnit a => [Point] -> SetActiveSubUnits a -> Box -> StateMBC a [Unit a] at src/DeUni.hs:(260,1)-(317,53) or from (SubUnit a1) bound by an expression type signature: SubUnit a1 => ([Unit a1], SetActiveSubUnits a1) at src/DeUni.hs:265:37-117 `a1' is a rigid type variable bound by an expression type signature: SubUnit a1 => ([Unit a1], SetActiveSubUnits a1) at src/DeUni.hs:265:37 Expected type: SetActiveSubUnits a Actual type: Set (ActiveSubUnit subUnit3) In the expression: fromList $ getAllSubUnits un In the first argument of `return', namely `(([un], fromList $ getAllSubUnits un) :: SubUnit a => ([Unit a], SetActiveSubUnits a))' ..... ..... Any help is more than welcome! Cheers, Edgar Gomes (LambdaSteel)

3 9

Re: [Haskell-cafe] ANNOUNCE: version 0.7 of monad-parallel, monad-coroutine and SCC
by Mario Blažević 26 Mar '11

26 Mar '11

On 11-03-26 12:27 AM, Steven Shaw wrote: > Hi Mario, > > I wondered if you had an application in mind for your incremental > parser library in Haskell? A little while ago I was following the > development of an open source text editor for Mac OS X called > Kod[.app]. They were wanting an incremental parser to help with > correct+fast syntax highlighting and the like. Looks like they decided > on gazelle written in C. I though you might find it of interest. > > https://groups.google.com/forum/#!msg/gazelle-users/RfE-lSmqb7c/vrqdPaOIoMwJ > <https://groups.google.com/forum/#%21msg/gazelle-users/RfE-lSmqb7c/vrqdPaOIo…> > http://www.reverberate.org/gazelle/ > > Is that the kind of thing you had in mind for your incremental parser > library in Haskell? The application I had in mind for the incremental parser is already using it, it's the new version of the streaming component combinators framework. More generally, though, its main purpose is the efficient communication between coroutine-like things like enumerators and iteratees. They tend to produce and consume data in chunks, but the producer's idea of proper chunk boundaries often doesn't match the consumer's. So instead of fetching raw chunks, the consumer uses incremental parser to abstract the producer's boundaries away. I did not have text editors or parsing "big" languages in mind when I wrote the library, but I suppose it could be used there as well. Among my vague plans were to provide a bridge for the enumerator and coroutine libraries, and to write a proper incremental XML parser. Thanks for the links, I wasn't aware of these projects.

1 0

hxt validation outside of IO?
by cafe-haskell＠coreland.ath.cx 26 Mar '11

26 Mar '11

Hello. Is it possible to validate XML trees against a RELAX NG schema outside of IO, using hxt? All of the functions I can see use an IOSArrow. I'm not sure what I'm missing.

1 0