- Haskell-Cafe - Haskell.org

Re: Are there standard idioms for lazy, pure error handling?
by Henning Thielemann 10 Dec '09

10 Dec '09

Somehow I missed this thread. I want to say that I have implemented a general way to add the information of an exception to the "end" of an arbitrary data structure. http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/ht… Duncan already mentioned it: > data Exceptional e a = Exceptional { > exception :: Maybe e > result :: a > } > However it's pretty clear from the structure of this type that it cannot > cope with lazy error handling in sequences. If you try it you'll find > you cannot do it without space leaks. Actually you spotted the space leak - but how is it pretty clear, that it has the space leak? I wondered why 'unzip' does not have the space leak, although when I sequence a lot of Exceptional values using mconcat, I'm doing something pretty similar. I also agree with you, that the possibility to access the result directly, and thus easily ignore the exception is bad. I thought it could be improved by removing the field accessors and instead provide the function: switch :: (Maybe e -> c) -> (a -> c) -> Exceptional e a -> c

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Parsec-like parser combinator that handles left recursion?
by Adam Cigánek 10 Dec '09

10 Dec '09

Hello there, Is there some other parser library, with similar nice API than Parsec, but which somehow handles left-recursive grammars? Ideally if it has at least rudimentary documentation and/or tutorial :)

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Re: OpenAL and Hsndfile
by jean legrand 10 Dec '09

10 Dec '09

a while ago, I wrote a minimal and inefficient code to generate and play an audible 4-second-sine sound http://hpaste.org/fastcgi/hpaste.fcgi/view?id=1538 Can you hear something with it ?

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Nano-Languages
by John D. Earle 10 Dec '09

10 Dec '09

What I wrote under "Allowing hyphens in identifiers" and what Richard O'Keefe wrote earlier under "A new form of newtype" caused me to think. It was pointed out that hyphens are not permitted in Haskell identifiers much as they are in Lisp for good cause even though such a feature is regarded by some as especially desirable. It is technically feasible (or at least appears to be). It is a matter of what constitutes good language design. My suggestion was to whip up a makeshift preprocessor to make the necessary substitutions. It might be better to go about it in a fashion similar to how one would use Haskell to create a new language using Happy, but instead of making a full fledged language create a language that makes only minor adjustments to the official language where most of the original source code and command line options are copied verbatim with a handful of things caught such as the Richard O'Keefe newtype, a nano-language if you will. Actually I have known about this possibility for years. There is always something you would like to change about whatever language you are working with and so I have put thought into it. It would make prototyping of new ideas easier. It would furthermore make it possible to make changes that would be unacceptable for an official version of the language such as hyphens in place of underscores for those who feel the benefit of being able to use hyphens this way outweigh the problems it may cause. The official language remains intact. Broadly speaking I am talking about a preprocessor, but one tailored for the needs of Haskell. For related art see Ciao Prolog (see http://ciaohome.org) They are using their preprocessor to do some rather sophisticated things which include program analysis and optimization. Something so elaborate may not be necessary, however. Just some Happy code that has been cobbled together that address some of the issues involved that is reworked to satisfy a special need.

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Re: [Haskell-cafe] forkSequence, runPar, parallelize
by Mario Blažević 10 Dec '09

10 Dec '09

> > > I can't test it right now, but wouldn't the > > following do the job in the Identity monad? > > > forkExec :: Identity a -> Identity (Identity a) > > forkExec k = let result = runIdentity k > > in result `par` return (Identity result) > > > Since Identity is a newtype, would that be equivalent to "result `par` > result"? The forkExec in the IO monad let's other computations keep > going until I need the result from the forked computation. You're right, it doesn't seem to work the way I hoped. The equivalent function on Maybe monad works, though, so it is possible to write forkExec in monads other than IO. > In a pure computation, I can already get the same result with `par` > and laziness, right? Yes. The goal is to enable writing monadic parallel computations which work under any parallelizable monad. For example, I'm using it to run two trampolining producer/consumer coroutines in parallel. A large majority of interesting coroutines I have are completely agnostic with respect to the underlying monad.

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You are in a twisty maze of concurrency libraries, all different ...
by Patrick Caldon 10 Dec '09

10 Dec '09

I'm looking for the "right" concurrency library/semantics for what should be a reasonably simple problem. I have a little simulator: runWorldSim :: MTGen -> SimState -> IO SimState it takes about a second to run on a PC. It's functional except it whacks the rng, which needs IO. I run 5-10 of these jobs, and then use: mergeWorld :: [SimState] -> SimState to pick the best features of the runs and build another possible world (state). Then I use this new world to run another 5-10 jobs and so on. I run this through ~20000 iterations. It's an obvious place for parallelism. I'm looking for a concurrency library with something like: forkSequence :: Int -> [IO a] -> IO [a] which I could call with something like this: forkSequence 4 (take 10 (repeat (runWorldSim g ss))) this would construct 4 threads, then dispatch the 10 jobs onto the threads, and pack up the results into a list I could run through my merger. It strikes me as something someone would already have done, but I can't find anything in hackage. Probably I've missed something obvious? Any pointers? If not, what would be the best/easiest existing package to write an extension to? Thanks, Patrick.

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can there be (hash-table using) O(n) version of this (I think currently) n log n algo?
by Thomas Hartman 09 Dec '09

09 Dec '09

The code below is, I think, n log n, a few seconds on a million + element list. I wonder if it's possible to get this down to O(N) by using a hashtable implemementation, or other better data structure. Further, is there a hashtable implementation for haskell that doesn't live in IO? Maybe in ST or something? import Data.HashTable import qualified Data.Set as S import Data.List (foldl') testdata = [1,4,8,9,20,11,20,14,2,15] ++ [1..(10^6)] wantedsum = 29 -- set data structure -- findsums locates pairs of integers in a list that add up to a wanted sum. findsums :: [Int] -> Int -> S.Set (Int,Int) findsums xs wanted = snd . foldl' f (S.empty,S.empty) $ xs where f (candidates,successes) next = if S.member (wanted-next) candidates then (candidates, S.insert (next,wanted-next) successes) else (S.insert next candidates,successes) -- hashtable data structure -- result: t -- fromList [(15,14),(16,13),(17,12),(18,11),(19,10),(20,9),(21,8),(22,7),(23,6),(24,5),(25,4),(26,3),(27,2),(28,1)] -- probably O(n log n) complexity since using tree based Data.Set (a few seconds on million+ element list) t = findsums testdata wantedsum

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ANNOUNCE: usb-safe-0.1
by Bas van Dijk 09 Dec '09

09 Dec '09

Hello, My usb library provides a standard Haskell abstracting layer over bindings-libusb providing: abstract types instead of Ptrs, automatic marshalling and unmarshalling, automatic garbage collection, exceptions instead of integer return codes, etc.. While all that is very nice there are still some things that you can do wrong. For example doing I/O with a closed device or reading from or writing to an endpoint which doesn't belong to the claimed interface. Or reading from an Out endpoint or writing to an In endpoint. I released the usb-safe package to prevent you making these errors. See: http://hackage.haskell.org/package/usb-safe-0.1 usb-safe provides the following guarantees: - You can't reference handles to devices that are closed. In other words: no I/O with closed handles is possible. - The programmer specifies the region in which devices should remain open. On exit from the region the opened devices are automatically closed. - You can't reference handles to configurations that have not been set. - You can't reference handles to interfaces that have not been claimed. - You can't reference handles to alternates that have not been set. - You can't reference endpoints that don't belong to a setted alternate. - You can't read from an endpoint with an Out transfer direction. - You can't write to an endpoint with an In transfer direction. - You can't read from or write to endpoints with the unsupported transfer types Control and Isochronous. Only I/O with endpoints with the Bulk and Interrupt transfer types is allowed. The primary technique used in usb-safe is called "Lightweight monadic regions" which was invented by Oleg Kiselyov and Chung-chieh Shan. See: http://okmij.org/ftp/Haskell/regions.html#light-weight Note that I consider this a preview release. In fact, I haven't tested the package at all. I can only guarantee you that it will pass the type-checker. As always: questions, comments and patches are more than welcome. Please don't look at the hscolour generated source code from the haddock documentation because it screws up the nice unicode symbols I used. If you want to read the source download the package or go straight to the darcs repos: darcs get http://code.haskell.org/~basvandijk/code/usb-safe regards, Bas

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pexports Was: Re: [Haskell-cafe] binding to C libraries on Windows was Low Level Audio - Writing bytes to the sound card?
by Stephen Tetley 09 Dec '09

09 Dec '09

Hi All, Would a pure Haskell version of pexports be useful to the Haskell community? For a Sunday afternoon hack that turned out to take a bit more effort (its now Wednesday), I thought I'd code up a tool that extracts function symbols from .dll's (also when I first looked at the C pexports it seemed somewhat unadopted, though checking today it appears to in MinGW, so my local MinGW must be out-of-date). If there are compelling uses that aren't covered by pexports and would ease Haskell C binding problems on Windows, I don't mind polishing up my tool, but otherwise I've exhausted my natural interest. Best wishes Stephen

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Haskell job opportunity
by siki 09 Dec '09

09 Dec '09

I've posted this before but did not get a whole lot of responses, so here it is again: Principal investment firm based in Manhattan is looking for an outstanding software developer to develop and maintain the firm's proprietary valuation models as well as accounting and portfolio management systems. You should have at least a bachelor’s degree in computer science from a top university and impeccable coding style. This is a high-impact, high-visibility job opportunity where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. This is a very small hedge fund with me being the only developer right now. The next hire will have a very direct say into how this side of our business progresses from here. We are a growth oriented firm that values people who take a craftsman’s pride in their work. We always like to take a contrarian view of things, value experimenting and using often overlooked technologies. That is one of the main reasons why we currently started using Haskell in developing our quantitative models. We are looking for a candidate who shows strong analytical, organizational, and communication skills. Attention to detail is essential, i.e. we fully document everything we do and pay a lot of attention to coding style. The majority of our existing code was written in Java, so you should have at least some experience with Java and related technologies, in order to be able to maintain the existing code. But as I mentioned above, Haskell has now become my choice of language and eventually all of the existing code will have to be re-written in Haskell. Background in accounting is not necessary but is definitely a plus. Please send your CV and cover letter to recruitment at karamaan.com, or reply to this post if you have any questions. Thanks -- View this message in context: http://old.nabble.com/Haskell-job-opportunity-tp26697169p26697169.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: binding to C libraries on Windoww
by John Lato 09 Dec '09

09 Dec '09

To reply to an earlier point of Andrew's (I can't find the quote now, sorry), one of the biggest difficulties developers face on Windows is the lack of common install locations/practices. Windows software is usually distributed as a binary, which may or may not include header files. These files may be installed in any of numerous locations, such as C:\Program Files\, the user's home directory, the system directory, or directly off the drive root. Defaults are not common among different programs or even versions of programs, and install locations are frequently changed by users anyway. Libraries and headers are usually not located on the PATH environment variable, and there's no standard INCDIR or LIBDIR variable either. While it is Cabal's job to find the location of necessary libraries, header files, and tools, there simply is no feasible way to do so on Windows without searching most of the hard drive. Even then, there's no guarantee the right version will be found (google "DLL Hell" for examples of the chaos that ensues). This is particularly true for developers, who frequently have multiple versions of libraries installed. The only workable approach is to have users specify the locations of these files, which unfortunately requires more sophistication than can be expected of most Windows users (and even some Windows developers). On a related matter, many packagers resort to using configure-style configuration setups to assist with binding to C libraries. Cabal is now sophisticated enough to handle many of these steps (although not all), however packagers may not have had an opportunity to update their build process to remove the dependency on "configure". These packages will continue to require cygwin or mingw for the "configure" step. One last wrinkle: many distributors of C libraries do not have Windows machines available to test, and (understandably) have little interest in supporting a platform they don't have and aren't familiar with. As a result, many C libraries are not well supported on Windows and require somebody familiar with gnu build tools (i.e. gcc, make, and autoconf) to be usable on Windows *at all*. This means that, as a Haskell developer wanting to use these libs, you need to use gnu build tools because you're not just binding to the library, you're maintaining a Windows port of it. C and unix are pretty much designed to work together, and have been for decades, so I think it's reasonable to expect that Windows is the side that needs to adapt to support their model. MS hasn't done so, which means that C developers would need to unacceptably compromise unix support in their packages in order to better support Windows. Cross-platform support precludes windows-native solutions, so you're left making the best of it with Linux tools (mingw and cygwin) on Windows. John

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fgetc and fputc equivalents
by John D. Earle 09 Dec '09

09 Dec '09

Although much work has apparently gone into providing support for manipulation binary data I was unable to uncover a Haskell equivalent of the C fgetc and fputc functions. The Haskell equivalents work with Unicode character streams, not bytes words etcetera. I did find a library for handling arrays of bytes and such, but I did not find it to be convenient for the application I have in mind. I resolved myself to emitted Unicode 0s and 1s that are later converted to binary 0s and 1s by means of a C program. Is this functionality missing in Haskell? or did I miss something?

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diff implementation in haskell
by Victor Nazarov 09 Dec '09

09 Dec '09

Are there pure haskell implementations of diff and diff3 algorithms. Like: > data Diff a = Deleted [a] | Common [a] | Added [a] > > diff :: Eq a => [a] -> [a] -> [Diff a] > > diff3 :: Eq a => [a] -> [a] -> [a] -> [a] or smth more general like: > class Diff a where > type Difference a > diff :: a -> a -> Difference a > patch :: a -> Difference a -> a > > prop_diffpatch a b = patch a (diff a b) == b ? If not, where can I start towards the implementation? What's the way to do it more functionally? -- Victor Nazarov

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Re: fgetc and fputc equivalents
by John D. Earle 09 Dec '09

09 Dec '09

My interest isn't actually to push around characters. I do enough of that. What I really want to do is push bits and not bytes.

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Low Level Audio - Writing bytes to the sound card?
by M Xyz 09 Dec '09

09 Dec '09

What is the most minimal (preferably platform independent) library available for writing bytes to the sound card? I see 60 wonderful libraries on Hackage, but I really just need the Haskell equivalent of an audio.write(byte[]) method. What sound api are these 60 libraries using? I think the portaudio library is the only contender but when I try to install it I get: >cabal install portaudio Resolving dependencies... Downloading portaudio-0.0.1... Configuring portaudio-0.0.1... cabal: Missing dependency on a foreign library: * Missing C library: portaudio This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: portaudio-0.0.1 failed during the configure step. The exception was: exit: ExitFailure 1

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Re: A new form of newtype
by John Lato 09 Dec '09

09 Dec '09

> From: "Richard O'Keefe" <ok(a)cs.otago.ac.nz> > Subject: Re: [Haskell-cafe] A new form of newtype > > People are now writing EDSLs using Haskell to generate code for > all sorts of interesting things. What if you want to use Haskell > as a host for an EDSL targeted at a 24-bit DSP? *plug* for this specific case, I would use the word24 package, http://hackage.haskell.org/package/word24 which I wrote for a very similar problem. It was surprisingly difficult to get right (N.B. I have only tested the code, not proven it correct). I would have much preferred to let the compiler handle the details. > > There may not be MACHINES with 42-bit integers, > but that doesn't mean there aren't PROBLEMS that need them. > This whole idea of "let the machine dictate the sizes" is > precisely what I'm complaining of. > <snip> > > Sorry, but creating instances of Bounded is what a compiler is for. > If I don't write it, I won't wrong it. I'm pretty sure I don't understand the syntax of your proposal (which is my fault), but I definitely see the usefulness. I'll leave the syntax wrangling for the language lawyers, however. John

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Checking dependencies from C packages in cabal
by Maurício CA 09 Dec '09

09 Dec '09

Hi, all, When pkg-config info is not available for a library (when I could use pkgconfig-depends), what should I use to check if needed libraries are installed and to give the compiler all needed info about include files and library location? Also, is it possible to do that without configure and make files? I have no experience with them. I also tried reading the Distribution module, but wasn't able to recognize what could be a natural way to handle this problem. Thanks, Maurício

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ANNOUNCE: usb-0.3
by Bas van Dijk 09 Dec '09

09 Dec '09

Hello I made a new release of my usb library for high-level communication with usb devices from Haskell. See: http://hackage.haskell.org/package/usb-0.3 I made the following changes compared to the previous 0.2.0.1: - Moved the enumeration of usb devices in its own module: System.USB.Enumeration - Moved the device descriptor inside the device type so that you can retrieve it without doing IO, e.g: deviceDesc :: Device -> DeviceDesc - Moved the configuration descriptors inside the device descriptor so you can retrieve them without doing IO, e.g: deviceConfigs :: DeviceDesc -> [ConfigDesc] - Implemented standard device requests that are missing from libusb. - Made the timeout of I/O operations explicit. Now all I/O operations return an additional Bool that indicates if the operation has timed out. If an operation timed out does not mean there is no result, it just means that the result may be incomplete. - Added experimental (and still untested) support for iteratees for doing predictable, high-performance, safe, and elegant input processing using the iteratee package. See module: System.USB.IO.Synchronous.Enumerator - Fixed some asynchronous exception related bugs (put a needed bracket, block and unblock here and there). - Finally some functions, constructors and types got renamed, documentation got updated and extended and some refactoring has taken place. As always: questions, comments and patches are more than welcome. darcs get http://code.haskell.org/~basvandijk/code/usb regards, Bas

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Announcing a summer internship for a NASA-sponsored project
by Lee Pike 09 Dec '09

09 Dec '09

************************************************ * ANNOUNCING: * * SUMMER INTERNSHIP FOR NASA-SPONSORED PROJECT * ************************************************ The National Institute of Aerospace (NIA) and Galois, Inc. would like to announce a Summer Visitor position for a joint in the summer of 2010 (and the summer of 2011). The position will be located at the NIA, located in the historic Hampton Roads area of Virginia, U.S. The visitor will work with researchers at both the NIA and Galois. PROJECT OVERVIEW: We are looking for a summer visitor to work on a NASA-sponsored project investigating runtime monitoring applied to hard real-time distributed systems such as avionics. Runtime monitoring is an approach to check a system's conformance to safety specifications at runtime. The work is being directed by Dr. Alwyn Goodloe (NIA) and Dr. Lee Pike (Galois, Inc.)---contact below. The visitor's work will depend on the skills and interests of the student, but it will likely focus on the development and application of of a monitor synthesis tool, CoPilot. Copilot takes property specification and synthesizes monitors for the properties to embedded C. Copilot is a Haskell library built on top of the Haskell eDSL Atom <http://hackage.haskell.org/package/atom>. The work is expected to lead to one or more conference publications. QUALIFICATIONS: * MUST-HAVES: * The candidate should have significant experience in programming in modern typed functional programming languages such as Haskell, OCAML, or Clean (programming will be done mainly in Haskell). * Some experience and exposure to writing and debugging C programs. * Some experience with using and configuring *nix systems. * NICE-TO-HAVES: * Experience with real-time systems, RTOSes, scheduling, distributed systems, control theory, or avionics. * Experience or interest in in configuring a small distributed hardware-based system (e.g., using Arduinos). * Experience or interest in formal methods (e.g., model-checking, temporal logics, runtime monitoring). * Good writing skills/experience in writing technical papers. * Interest in continuing collaboration at his or her home institution during the remainder of the year. * Interest in an internship for the summers of 2010 *and* 2011. * Interest in NASA. * DO *NOT* NEED: * A degree (we're interested in hearing from post-docs, graduate students, and undergrads). * U.S. citizenship or work visa (all expenses are paid, but we anticipate this being an unpaid internship). ABOUT THE NIA: The National Institute of Aerospace is a nonprofit institution with both research and educational components. We have both full-time research staff members as well as faculty and graduate students from our six member universities. Research carried out at NIA includes fluid mechanics, space vehicle design, atmospheric science, material science, and computer science. Within computer science, the primary focus is in formal methods, and we work closely with the formal methods group at NASA's Langley Research Center, located less than one mile away. The visitor will have an opportunity for frequent interaction with the NASA Langley Formal Methods Group. The NIA is located in Hampton, Virginia. The cities of Norfolk and Virginia Beach are nearby as is historic Colonial Willamsburg. Numerous attractions and activities are available within an hour's drive. Within a couple of hours are the Outer Banks of North Carolina and Washington DC, which are popular weekend getaways for our summer visitors. NASA and NIA both host a large number of summer visitors (undergraduate and graduate students) from around the world. ABOUT GALOIS: Galois, Inc. is located in Portland, Oregon with a mission to create trustworthiness in critical systems. We’re in the business of taking blue-sky ideas and turning them into real-world technology solutions. We've been developing real-world systems for the past 10 years using Haskell. BENEFITS: This is anticipated to be an unpaid but internship in which all living expenses are covered. These include * travel expenses to and from the NIA and your home * housing allowance * food allowance * rental car and gas TO APPLY: Please email a C.V. (PDF or plain text) and a brief plain text note statting your interest and experience to Dr. Alwyn Goodloe at <alwyn.goodloe(a)nianet.org >. There is flexibility regarding the dates of the visit. *************************************** * Application due date: Jan. 15, 2010 * *************************************** LEARN MORE: * NIA website: http://www.nianet.org/ * Galois website: http://www.galois.com/ * Hampton Roads: http://en.wikipedia.org/wiki/Hampton_Roads * NASA Langley Formal Methods Group: http://shemesh.larc.nasa.gov/fm/ Please email either Dr. Alwyn Goodloe <alwyn.goodloe(a)nianet.org> and/ or Dr. Lee Pike <leepike(a)galois.com> with any questions.

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Re: Nano-Languages
by John D. Earle 09 Dec '09

09 Dec '09

To help clarify my meaning Haskell would be used as an assembler language or put in another way an intermediate language for those constructs that are caught by the preprocessor for special processing. Since Haskell is a functional language I would imagine that this would be unproblematic.

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OpenAL and Hsndfile
by Matthew 09 Dec '09

09 Dec '09

Yesterday, I set out to accomplish the challenge of loading and playing a sound from a file. So far, my attempts have resulted only in silence... rather disheartening after all the effort it took to get everything to build and run cleanly. I currently take the following steps: - Load samples from file into an IOCArray using hsndfile - Create an OpenAL buffer which points to the IOCArray - Play sound using OpenAL code is at: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=13836 I'm wondering if anyone who is familiar with these libraries can tell me if I'm using them sanely or not. Alternatively, if anyone has a favorite sound library and can help me get started with that, I'd be very grateful. -Matt

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Re: [Haskell-cafe] Re: ANN: hakyll-0.1
by Robert Greayer 09 Dec '09

09 Dec '09

sigh -- to the list this time. On Wed, Dec 9, 2009 at 9:16 AM, Tom Tobin <korpios(a)korpios.com> wrote: > On Wed, Dec 9, 2009 at 4:59 AM, Ketil Malde <ketil(a)malde.org> wrote: > > Tom Tobin <korpios(a)korpios.com> writes: > >> If it turns out that Hakyll *is* okay to be BSD3 licensed so > >> long as neither any binary nor the GPL'd work's source is distributed > >> under non-GPL terms, well ... I'll say that the meaning of "BSD > >> licensed" will have become much less reliable, since it means you > >> actually have to trace the genealogy of the libraries you use *all* > >> the way back in order to understand the situation for certain. > > > > How so? To me it's the exact converse: if the author of Hakyll may > > *not* distribute his work under the BSD license, just because it is > > intended to be linked with some GPL code, this complicates issues > > tremendously. > > For instance, it would mean that businesses which may be writing > proprietary software can't assume they can freely use a liberally > licensed (e.g., BSD3) library — which would *completely* go against > the prevailing understanding of liberally licensed software. Tainting > your software with a GPL dependency without realizing it is a > terrifying prospect (and certainly one of the questions I'd now like > to pose to the SFLC). > I don't think I follow your reasoning here: certainly, a business can use, freely, the Hakyll library (meaning that they can redistribute, in binary form, their executable built with it, without distributing the Hakyll source or their own source). They cannot freely use the Pandoc library in the same way, because it is GPL. Since the Hakyll library depends on the Pandoc library, they will of course have some trouble with *building* an executable that contains Hakyll but not Pandoc. Both there's no hidden dependency, no 'tainting' of Hakyll involved. There is a danger, of course, that when installing Hakyll (via cabal) the user won't realize they've also installed Pandoc, even though the dependency is clearly specified. The lesson here is that someone packaging an executable for distribution has to be aware of everything they are building into it. It's possible to make a mistake here, if one is not careful. But it doesn't require much diligence to get it right (if you use cabal to build your executable, you have to specify your dependencies. Check the licensing terms of each, and comply).

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ANNOUNCE: ls-usb-0.1.0.2
by Roel van Dijk 09 Dec '09

09 Dec '09

Hello everyone, I have released a minor update of my ls-usb program: http://hackage.haskell.org/package/ls-usb This small utility lists USB devices connected to your system. It can also show you the device descriptors, interface descriptors, vendor and product identifiers and various other things. All in wonderful colours of course. The only major change is that it now depends on usb-0.3.* Regards, Roel van Dijk

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ANNOUNCE: bindings-libusb-1.4.2
by Bas van Dijk 09 Dec '09

09 Dec '09

Hello, I just uploaded a new release of Maurício C. Antunes' bindings-libusb to hackage. See: http://hackage.haskell.org/package/bindings-libusb-1.4.2 bindings-libusb is a bindings-DSL based, low-level binding to libusb. This release adds support for the recent libusb-1.0.5. BTW, there is work being done to create a Windows port for libusb. This means that in the near future libusb and therefor bindings-libusb and my usb packages will work on all major platforms, hooray! regards, Bas

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Re: Allowing hyphens in identifiers
by John D. Earle 09 Dec '09

09 Dec '09

Oh yes I forgot. You will also need to map not merely those places where hyphens are used by the language, but also those circumstances where, wondering how to state it abstractly, anyway in string literals and such where substitutions should not occur.

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Re: Allowing hyphens in identifiers
by John D. Earle 09 Dec '09

09 Dec '09

A makeshift preprocessor written say in Perl could be used to replace the embedded hyphens with underscores thereby making them synonyms. A list of ways in which hyphens are used in the language will need to be developed so they may be distinguished. You don't want a single line comment sign being taken as containing an embedded hyphen. If done properly it shouldn't cause any problems. It will allow you to side step having to actually modify the language and may be portable across language implementations. I use the C language preprocessor to correct a number of features of the C language syntax that I find ugly.

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A new form of newtype
by Richard O'Keefe 09 Dec '09

09 Dec '09

Haskell is very nearly a high level language. One rather unpleasant way in which it lets the the underlying machine show through is integral types. Aside from the unbounded Integer type, which is fine, there are integral types bounded by machine sizes: Int, size unspecified, Data.Int.Int{8,16,32,64}, and Data.Word.Word{8,16,32,64}. There are a number of problems with these. The principal one is that in order to figure out which size is needed, a programmer must do arithmetic. I still remember a student's bitter complaint about being required to find the base 2 logarithm of 32 in an examination where calculators were forbidden... I propose a small addition to 'newtype' syntax: 'newtype' id '=' '[' expr {',' expr}... ']' [deriving-part] The expressions are Integral expressions made from constants and suitable Prelude functions. Each Haskell implementation supports some sublist of the types [Int1,Word1,Int2,Word2,Int3,Word3,...,Int64,Word64, ...,Int256,Word256,...,Integral, _except (It's almost interesting that the Natural type missing from Haskell would go after Integral in this list, and so would never be used.) The effect of the declaration would be to make "id" a copy of the first type in the list of supported types that is big enough to include the value of every expression on the right hand side, except that minBound = the smallest value on the right hand side maxBound = the largest value on the right hand side rather than being the bounds for the underlying type. The instances that could be derived would include Bits, Bounded, Data, Enum, Eq, Integral, Ix, Num, Ord, PrintfArg, Read, Real, Show, Storable, Typable. I'd actually prefer that the default list of derived instances be short, but it's hard to draw an arbitrary line, so the default instances should be the same as the default instances for Int in the same Haskell system. Suppose I knew that I had a 2TB disc (I don't, and wish I did) and wanted to be able to hold not only any file size but the sums and differences of up to 10,000,000 file sizes. (There could be one giant file, and all the file sizes could be the size of that file.) newtype FileSize = [-10000000*2^41,10000000*2^41] I don't see any simple way to get named values into the expressions defining a new integral type, but the use of cpp with Haskell has been common practice for a long time, so #define Tera 2^40 #define Many 10000000 newtype FileSize = [-Many*2*Tera,Many*2*Tera] In this case, FileSize would be a copy of Integer.

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ANNOUNCE: dbus-core 0.6 and dbus-client 0.2
by John Millikin 09 Dec '09

09 Dec '09

This is the second public release of my D-Bus implementation. dbus-core is an implementation of the D-Bus protocol, and dbus-client is a set of wrappers and utility computations to simplify writing basic clients. Interesting changes to dbus-core ================================ * Performance improvements: sending and receiving large structures should now be significantly faster. My thanks and appreciation go to Bryan O'Sullivan, whose excellent Criterion library made benchmarking the serialisation code much easier than it had any right to be. * Better support for byte arrays: byte strings (strict and lazy) are now directly supported types, with special cases to avoid most conversion overhead. * Implemented the TCP/IP transport, though it's untested because I can't figure out how to make the server listen on a socket. If anybody cares about remote D-Bus, and this doesn't work, drop me a mail with your configuration and I'll find out why. * Added module to represent "match rules", required for full signal support. * Added module for name reservations -- this is really just moving the bulk of the code from dbus-client into dbus-core, where it belongs. * Fixed possible concurrency issue when receiving messages from a connection in multiple threads. This is still a really bad idea, and it will never work correctly, but at least now it won't corrupt the connection state. Interesting changes to dbus-client ================================== * Fixed dumb signature to mkClient (thanks, Max). Its purpose should now be more obvious. * Shifted around some of the method call APIs -- it's now possible to send method calls without a proxy. * Uses the new MatchRule work to fully support receiving signals. Documentation ============= PDFs of the source; these are the most useful and complete documentation currently available: https://dl.dropbox.com/u/1947532/dbus-core_0.6.pdf https://dl.dropbox.com/u/1947532/dbus-client_0.2.pdf An API listing is also available, good for a quick overview: https://dl.dropbox.com/u/1947532/dbus-core_0.6/index.html https://dl.dropbox.com/u/1947532/dbus-client_0.2/index.html Hackage is currently experiencing some dependency conflicts with the "bytestring" library[1], so it's not generating its own documentation. I'll keep the dropbox pages up until Hackage is in a more usable state. Downloads ========= Available from the usual places: http://hackage.haskell.org/packages/archive/dbus-core/0.6/dbus-core-0.6.tar… http://hackage.haskell.org/packages/archive/dbus-client/0.2/dbus-client-0.2… [1] "binary" uses bytestring-0.9.1.4, "text" uses bytestring-0.9.1.5, kaboom

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How to understand such a `case` ?
by zaxis 09 Dec '09

09 Dec '09

findHelper (x:xs) = do -- not lazy, but that's not really important here filex <- fileExists (file x) filex' <- fileExists (file' x) case () of _ | filex -> return $ Just $ file x | filex' -> return $ Just $ file' x | otherwise -> findHelper xs file x = foldl1 joinFileName (x ++ [helper]) file' x = (file x) ++ (getConfig "exe_ext") Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/How-to-understand-such-a-%60case%60---tp26703526p2670… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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"Rebox Package" or "To Hackage or not to Hackage"
by John Van Enk 08 Dec '09

08 Dec '09

Hi List, I've recently had a situation where I used the same pattern quite a bit while reducing and evaluating an AST. I quickly wrapped the operation in a package and stuck it on github. http://github.com/sw17ch/rebox/blob/master/src/Data/Rebox.hs I'm wondering two things: 1) Does any one recognize the pattern I'm using here as something with a different name? 2) Is this worth sticking on Hackage? It's trivial, but we have plenty of trivial concepts on Hackage (and it's not a bad thing). Any feedback would be great. /jve

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Re: [Haskell-cafe] Interactive chatbot
by Ben Millwood 08 Dec '09

08 Dec '09

Oops, I clicked "reply" instead of "reply to all". Duplicating the message below. I suppose this means someone is going to get two copies of this. Sorry someone! On Thu, Nov 5, 2009 at 12:56 AM, Ben Millwood <haskell(a)benmachine.co.uk> wrote: > On Wed, Nov 4, 2009 at 10:21 PM, Torsten Otto <t-otto-news(a)gmx.de> wrote: >> >> When we read the user's input through >>> t <- getLine >> it is not possible to delete typos before hitting enter and thereby sending >> the input off to the system (at least in OS X, bash). I didn't find that >> terribly problematic, but of course it is a bit of a show stopper from their >> point of view. >> > > As people have said it's worth checking what buffering settings you > are using (especially note that ghci changes some interesting settings > in relation to how input is handled, and compiled code may behave > differently), but it might also be worth checking the terminal > application's preferences to see if there are settings related to the > interpretation of the backspace key that you need to twiddle one way > or the other. In particular, if you are finding that pressing delete > makes ^H appear on the input line instead of deleting things, or if > pressing ctrl-H deletes stuff where the delete key fails to do so, it > might be a problem with your terminal rather than with your program. > This is only based on what I vaguely remember from faffing with the > Mac Terminal application some time ago when it wouldn't co-operate > with screen, but it may be worth a look. > > yours, > Ben Millwood >

2 1

The Transient monad
by Alberto G. Corona 08 Dec '09

08 Dec '09

Hi haskell cafe: concerning Stable Names http://www.haskell.org/ghc/docs/6.10.4/html/libraries/base/System-Mem-Stabl… makeStableName :: a -> IO (StableName a) I Did not test fully my proposal, and I´m thinking aloud, Just to inpire others and fish some ideas; The IO in makeStableName suggest more side effects than makeStableName really do. But still the call isn't pure. For calls such are makeStableName that gives a different result the FIRST time they are called but return the same result every time in the same session, I suggest to use a Transient monad: makeStableName :: a -> Transient (StableName a) The key here is to maintain the programmer aware that it is not pure, but there are no IO and that the results have no meaning from session to session. Instance Monad Transient where Transient x ↠ f = f x return x = Transient x We can Transient`ize IO calls by means of an implicit memoization: liftT:: IO a -> Transient a liftT= < whatever memoization code> liftT2=.... liftT3=.... Memorization then is embedded in the monad transformation This may be more elegant than IO plus unsafePerformIO and is more informative for the programmer. The transition form IO to pure can be done in two steps, see below Instead of unsafePerformIO, we can use: unsafePurifyTransient :: Transient a -> a unsafePurifyTransient (Transient x) = x for the inherently transient calls A safer version of unsafePerformIO using implicit memoization could be: unsafePerformIOT :: IO a -> a unsafePerformIOT = unsafePurifyTransient . liftT unsafePerformIOT guatantee that it returns the same value in the same session. 2009/12/8 Vladimir Zlatanov <vlado(a)dikini.net> > <jeanchristophe.mincke(a)gmail.com> wrote: > > I think lisp like symbols could be quite useful in the context of > embedded > > DSL to create ... well... symbols that can be interpreted as variables in > > that DSL. > > Well for such use-case you could use either stable names, or recode > them into a state monad- you will get either a global i.e. lisp like > unique symbols, or their equivalent within a state context. Or some > variation of the code posted previously in this thread. > > > http://www.haskell.org/ghc/docs/6.10.4/html/libraries/base/System-Mem-Stabl… > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Lisp like symbols in haskell
by jean-christophe mincke 08 Dec '09

08 Dec '09

Hello, Has there already been attempts to introduce lisp like symbols in haskell? Thank you Regards J-C

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ANNOUNCE: new installment of failure framework
by Michael Snoyman 08 Dec '09

08 Dec '09

We'd like to announce the next installment of the Failure Framework. Based on feedback, our main goals in this release is to reduce the number of external dependencies and avoid complications with monad transformer libraries at the core of the framework. We have made the following changes: * Failure is not tied directly to monads anymore. The main typeclass is Failure, which provides the failure function. MonadFailure, ApplicativeFailure and FunctorFailure are provided for convenience. * The Failure typeclass is now located in the failure package, which has no dependencies. All code dealing with transformers and mtl has been factored into control-monad-failure and control-monad-failure-mtl, respectively. In addition, we are simultaneously releasing the following packages: * safe-failure provides safe versions of dangerous, non total functions, such as head, by returning failures. It depends exclusively on failure. * attempt provides a concrete data type for handling extensible exceptions as failures. It depends exclusively on failure. * control-monad-attempt provides a monad transformer for attempt using the transformers library. We do not provide currently an mtl version of this package, but can provide one if there is demand. * control-monad-exception, explicitly typed, checked exceptions with monadic stack traces. Below are the links to the packages just released. Enjoy! http://hackage.haskell.org/package/failure-0.0.0 http://hackage.haskell.org/package/control-monad-failure-0.6.0 http://hackage.haskell.org/package/control-monad-failure-mtl-0.6.0 http://hackage.haskell.org/package/safe-failure-0.4.0 http://hackage.haskell.org/package/attempt-0.2.0 http://hackage.haskell.org/package/control-monad-attempt-0.0.0 http://hackage.haskell.org/package/control-monad-exception-0.8.0 http://hackage.haskell.org/package/control-monad-exception-mtl-0.8.0 http://hackage.haskell.org/package/control-monad-exception-monadsfd-0.8.0 http://hackage.haskell.org/package/control-monad-exception-monadstf-0.8.0 Michael Snoyman, Pepe Iborra, Nicolas Pouillard

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Re: binding to C libraries on Windoww
by John Lato 08 Dec '09

08 Dec '09

> From: Andrew Coppin <andrewcoppin(a)btinternet.com> > > John Lato wrote: > >> The only workable approach is to have users specify the >> locations of these files, which unfortunately requires more >> sophistication than can be expected of most Windows users (and even >> some Windows developers). >> > > Well, I don't know. It's going to vary from package to package, but most > things that have a semi-official Windows version either come as a > Windows Installer package (which, one presumes records where it put > everything *somewhere* in the Windows registry), or possibly the > *developer* package comes as a simple Zip file that you just unzip to > wherever you fancy. Just tell the user the URL to grab the Zip file > from, unzip it and tell Cabal where the root is now. Shouldn't be too > hard. (Famous last words...) Indeed, in theory it would work this way. In practice, not so much. > >> On a related matter, many packagers resort to using configure-style >> configuration setups to assist with binding to C libraries. Cabal is >> now sophisticated enough to handle many of these steps (although not >> all), however packagers may not have had an opportunity to update >> their build process to remove the dependency on "configure". These >> packages will continue to require cygwin or mingw for the "configure" >> step. >> > > Ah yes, this is pretty much guaranteed to make stuff not work on > Windows. :-) Still, all Unix systems have Automake, configure, etc., so > I guess most people don't even think twice before using it. > > I gather it's supposed to be Cabal's job to figure this stuff out in > Haskell land? > AFAIK, this is true to an extent. I think that someone like Duncan could explain the role of Cabal better than I can. Of course Cabal was missing much of the necessary functionality until only recently. As an example, the possibility of listing C library dependencies was only added in 1.6 IIRC. >> One last wrinkle: many distributors of C libraries do not have Windows >> machines available to test, and (understandably) have little interest >> in supporting a platform they don't have and aren't familiar with. As >> a result, many C libraries are not well supported on Windows and >> require somebody familiar with gnu build tools (i.e. gcc, make, and >> autoconf) to be usable on Windows *at all*. This means that, as a >> Haskell developer wanting to use these libs, you need to use gnu build >> tools because you're not just binding to the library, you're >> maintaining a Windows port of it. >> > > This seems to be kind of the catch-22. Nobody uses Windows, so there > isn't much support for Windows, so we don't attract many Windows > users... QED. :-} > Agreed (by users I presume you mean Haskell developers). I remember that many readers of this list were shocked by the download numbers for the HP, particularly the high number of Windows downloads. I think it goes to show that there is interest among Windows developers, as long as they have the sense that their platform is equally-supported compared to Linux. There are many more potential windows developers than linux developers due to the size of the Windows install base, so good Windows support is one of the easiest ways to expand the audience. >> C and unix are pretty much designed to work together, and have been >> for decades, so I think it's reasonable to expect that Windows is the >> side that needs to adapt to support their model. > > In other words, "Windows needs to become just like Unix". Not going to > happen. ;-) Argue about whether this is good or bad amoungst yourselves. > But it's not happening. I don't mean that Windows needs to become just like Unix. I mean that if Windows supported a common installation location for .dll's and .h files (and binary packages regularly provided them), much of this difficulty would be easily resolved. It wouldn't be "becoming like Unix", it would mean adopting to a C distribution model that Unix happens to share. Regardless, I think I have a better chance of winning the lottery jackpot than seeing MS support this in my lifetime. > >> MS hasn't done so, >> which means that C developers would need to unacceptably compromise >> unix support in their packages in order to better support Windows. >> > > I don't know about that... Lots of commercial and open-source products > are available for Windows and Unix, seemingly without any problem. (GTK, > for example...) I don't deny that adding Windows support is _a lot more > work_ than adding support for just another Unix, though. > This goes back to the catch-22 you listed above, but I wouldn't say Windows support is "without any problem." First you need active Windows developers for this. Commercial products have a big advantage here. Second, Windows support means a host of platform-specific bugs to be dealt with in every part of the package. These are in addition to any other bugs that you'd have anyway. FWIW, I had great difficulty in building hCsound on Windows, and my primary development box at the time was WinXP. I did eventually make it work with the binary csound distribution, but not without a lot of pain derived from the specifics of the binaries in ways that cabal can't (and probably shouldn't) be expected to handle. John

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Re: New Hackage category: Error Handling
by Henning Thielemann 08 Dec '09

08 Dec '09

On Mon, 7 Dec 2009, Michael Snoyman wrote: > I actually *did* read your article, and don't know what you are referring to. If this is true, sorry, I didn't had the impression. I also think that in an earlier mail I answered, that errors can leave you with corrupt data, say invalid file handles, memory pointers, or data structures that violate invariants. You won't like to close files from invalid file handles and free memory from corrupt pointers or run into infinite loops by traversing the corrupt data structures. That's the reason why it is better to stop execution of the program and hand over control to the next higher level, say the shell or the web browser, that can free resources safely.

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Lots of Haskell at PEPM 2010 - Call for Participation
by Janis Voigtländer 08 Dec '09

08 Dec '09

Hi all, If you are planning to go to Madrid in January, for POPL, don't forget registering for PEPM as well! If you haven't yet been planning to go, maybe you want to reconsider, for PEPM if not for POPL. Why, you ask? Well, the PEPM program has *lots* of Haskell this year. Indeed, no fewer than 10 of the 21 presentations are directly related to Haskell. And the other papers are great, too, and many of them will be interesting to Haskell folk as well. Scan the speakers and titles below, and you will know why I think you won't want to miss out on that one. Best regards, Janis. =============================================================== CALL FOR PARTICIPATION ACM SIGPLAN 2010 Workshop on Partial Evaluation and Program Manipulation (PEPM'10) Madrid, January 18-19, 2010 (Affiliated with POPL'10) http://www.program-transformation.org/PEPM10 =============================================================== Abstracts of all papers and presentations are available from the above web site. INVITED TALKS: * Lennart Augustsson (Standard Chartered Bank, UK) Title: O, Partial Evaluator, Where Art Thou? * Jeremy Siek (University of Colorado at Boulder, USA) Title: General Purpose Languages Should be Metalanguages. CONTRIBUTED TALKS: * Nabil el Boustani and Jurriaan Hage. Corrective Hints for Type Incorrect Generic Java Programs. * Johannes Rudolph and Peter Thiemann. Mnemonics: Type-safe Bytecode Generation at Run Time. * Elvira Albert, Miguel Gomez-Zamalloa and German Puebla. PET: A Partial Evaluation-based Test Case Generation Tool for Java Bytecode. * Martin Hofmann. Igor2 - an Analytical Inductive Functional Programming System. * José Pedro Magalhães, Stefan Holdermans, Johan Jeuring and Andres Löh. Optimizing Generics Is Easy! * Michele Baggi, María Alpuente, Demis Ballis and Moreno Falaschi. A Fold/Unfold Transformation Framework for Rewrite Theories extended to CCT. * Hugh Anderson and Siau-Cheng KHOO. Regular Approximation and Bounded Domains for Size-Change Termination. * Évelyne Contejean, Pierre Courtieu, Julien Forest, Andrei Paskevich, Olivier Pons and Xavier Urbain. A3PAT, an Approach for Certified Automated Termination Proofs. * Fritz Henglein. Optimizing Relational Algebra Operations Using Generic Equivalence Discriminators and Lazy Products. * Adrian Riesco and Juan Rodriguez-Hortala. Programming with Singular and Plural Non-deterministic Functions. * Martin Hofmann and Emanuel Kitzelmann. I/O Guided Detection of List Catamorphisms. * Andrew Moss and Dan Page. Bridging the Gap Between Symbolic and Efficient AES Implementations. * Christopher Brown and Simon Thompson. Clone Detection and Elimination for Haskell. * Stefan Holdermans and Jurriaan Hage. Making Stricterness More Relevant. * Arun Lakhotia, Davidson Boccardo, Anshuman Singh and Aleardo Manacero Júnior. Context-Sensitive Analysis of Obfuscated x86 Executables. * Xin Li and Mizuhito Ogawa. Conditional Weighted Pushdown Systems and Applications. * Ivan Lazar Miljenovic. The SourceGraph Program. * Florian Haftmann. From Higher-Order Logic to Haskell: There and Back Again. SPECIAL FEATURE: * Andy Gill, Garrin Kimmell and Kevin Matlage. Capturing Functions and Catching Satellites. IMPORTANT DATES: * Early registration deadline: December 22, 2009 * Hotel registration deadline: December 28, 2009

1 0

How to deal with C buf in Haskell?
by Magicloud Magiclouds 08 Dec '09

08 Dec '09

Hi, I am warping some C libs. In one function, I do this: 183 allocaBytes bufLen $ \buf -> do 184 ret <- {# call buf_read #} 185 bluh 186 bala 187 buf 188 bufLen 189 if ret < 0 190 then userError "bufRead error" 191 else -- what should I do here? I am thinking about bytestring, to pass the content of the buf to the outside of the function. But I am not sure. What should I do? -- 竹密岂妨流水过山高哪阻野云飞

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New Hackage category: Error Handling
by Gregory Crosswhite 08 Dec '09

08 Dec '09

Hey everyone, When I uploaded my new package, "error-message", I also went ahead and created a new category: "Error Handling". I did this because there are a number of packages related to this issue, so it might be helpful to have them presented together in one place. Thus, if you have or are in the process of writing a package related to this, please consider adding it to this category sometime so that my package doesn't get lonely. :-) Cheers, Greg

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Hayoo and Hoogle (beginner question)
by drostin77 08 Dec '09

08 Dec '09

Hello Hopefully Helpful Haskell Community! (I really wanted that to be alliteration... couldn't come up with an h word for community) I've just started learning Haskell (working my way through "Real World Haskell" and really liking it)! I have started to appreciate that there are a lot of Haskell libraries I will want to search and access quite regularly when I work on Haskell projects, so I had the following questions: 1. Hoogle and Hayoo: I'm a bit confused by the difference here. From initial impressions it seems that Hayoo is linked to Hackage, but Hoogle is not? And it seems that Hayoo is, well, "Similar to Hoogle, but with less focus on type search." Should I check both of these when I want a library, or choose one and go with it...? Or is it indeed better to search Hayoo when I'm thinking of a named library and Hoogle when I think I know the type signature for just the function I need? 2. I much prefer a command line search interface to a browser, I see Hoogle offers such a tool, does Hayoo? P.S. If these questions are in the wrong place, or if they are already answered in detail somewhere that my Googling didn't find please feel free to paste a link and tell me to search better next time! -- View this message in context: http://old.nabble.com/Hayoo-and-Hoogle-%28beginner-question%29-tp26669924p2… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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OpenCL target for DPH?
by Marcus G. Daniels 07 Dec '09

07 Dec '09

Hi, I'm wondering if anyone has looked at OpenCL as target for Data Parallel Haskell? Specifically, having Haskell generate CL kernels, i.e. SIMD vector type aware C language backend, as opposed to just a Haskell language binding. Thanks, Marcus

3 2

Is anyone reading Pattern Calculus by Barry Jay?
by Duane Johnson 07 Dec '09

07 Dec '09

I just bought a copy of Pattern Calculus [1] by Barry Jay and I would like to discuss the lambda- and pattern-calculus with anyone who is interested. Is there anyone else here who is reading the book and would like to discuss here (if it is appropriate) or take the discussion elsewhere? My knowledge of types has come primarily through reading this Haskell Cafe list, so I am by no means an expert. Just a tinkerer :) Regards, Duane Johnson [1] http://lambda-the-ultimate.org/node/3695

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Re: [Haskell-cafe] Re: [Haskell] ANNOUNCE: control-monad-exception 0.5 with monadic call traces
by klondike 07 Dec '09

07 Dec '09

Luke Palmer escribió: > On Fri, Nov 6, 2009 at 6:54 PM, klondike > <klondikehaskellcafe(a)xiscosoft.es> wrote: > >> Henning Thielemann escribió: >> >>> That's what I meant with my post: Programming errors (like "head []") >>> are not handled by control-monad-exception. As far as I understand, >>> control-monad-exception makes _exceptions_ explicit in the type >>> signatures, not programming errors. Why and how would you make >>> possible programming errors explicit in the type? But for exceptions >>> (e.g. "file could not be found") a detailed stack trace is not of much >>> use. >>> >> I think you have overlooked a few things. First, not every developer >> knows each one of the lines in the code well enough as to see where a >> exception comes from, specially when you are not the author of that code. >> >> Of course, that wouldn't mind so much unless you see another thing, if >> we don't know which exceptions can be launched by a operation then you >> will get it on the upper frame and rendered unable to solve it. Use >> typed exceptions (as this library intends to) you may say. Ok, now we >> have another problem, the strange habit of coders to keep the exceptions >> they don't know/can't treat going up and up and up, until then usually >> hit the top frame and you are screwed. You can check some Java code (to >> see an example on how this happens) as some of these exceptional >> conditions are put on the method's signature. >> > > You sound like you are expecting something that works exactly like the > imperative exception handling mechanisms you are used to, and are not > willing to accept anything else. > No, I sound like I am expecting programmers comming from the imperative world to do ugly things, maybe because I once was one. > This "strange habit" of programmers is simply bad practice, and leads > to just as brittle code as no exception handling, except that you get > inexplicable error message boxes with OK buttons instead of crashes. > You might as well just put the whole program in a catch block in IO > and bail with something noncomittal. > Now comes the time when I have to show you that not every exception could be handled, IE a file not found exception when looking for the config file can be fatal and force the program to stop. But what if this is on a library? How do you suggest that the programmer knows that the File Not Found exception is due to the lack of that config file, specially when the code is badly (or not) documented. > IMO, Haskell's typed exceptions (via eg. explicit-exception) *are* the > way to go. They keep the number of exceptional conditions that can > occur in a body of code small -- if some code has many exceptional > conditions, it forces you to *compose* them somehow, to come up with a > more precise idea of what your code is doing and how it can fail. > Yeah, the thing comes when you get the same kind of exceptions from different places, you need a way to tell them appart. > If you are just blindly passing exceptions up from the code below, the > interface to your code is getting more and more complex. With typed > exceptions, the types are reflecting the complexity of the interface > (which is precisely the purpose of types). > Well, it's not like blindly pass exceptions up but more like passing exceptions I don't know how to handle up.

3 3

A bug of groupBy implement
by L.Guo 07 Dec '09

07 Dec '09

Hi there: My friend asked me a question, and i suppose he has found a bug of `groupBy'. Here is the code piece: > List.groupBy (\a b -> Foreign.unsafePerformIO (Text.Printf.printf "\t%d <= %d ?: %s\n" a b (show (a<=b)) >> return (a<=b))) [7,3,5,9,6,8,3,5,4] I have tested it in GHC 6.10.4 (Win XP) and GHC 6.8.3 (Linux), both give the wrong result (categaried): 7 <= 3 ?: False 3 <= 5 ?: True 3 <= 9 ?: True 3 <= 6 ?: True 3 <= 8 ?: True 3 <= 3 ?: True 3 <= 5 ?: True 3 <= 4 ?: True [[7],[3,5,9,6,8,3,5,4]] Regards -------------- L.Guo 2009-12-08

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The Haskell Web News: December 2009 Edition
by Don Stewart 07 Dec '09

07 Dec '09

The Haskell Web News is a monthly summary of the hottest news about the Haskell programming language, as found in our online communities. If you want to catch up with what’s been happening in Haskell, this might be the journal for you. http://haskellwebnews.wordpress.com/2009/12/05/whats-new-in-haskell-decembe… What’s been happening with the Haskell programming language for the past month, as voted by readers of The Haskell Reddit. This is the first edition of the Web News, so feedback on the content, schedule and goals is welcome. -- Don

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Is it possible: Haskell platform - portable edition
by Rafael Gustavo da Cunha Pereira Pinto 07 Dec '09

07 Dec '09

Hello, Has anyone considered the possibility of making a *Portable* Haskell Platform distribution (portable meaning on a USB stick) for Windows? This worked form me: I installed GHC on one machine, copied it to a USB stick and created a batch to set the environment path and open a command prompt. This gives me the freedom to use Haskell wherever I am! The only downside is that I have to edit packages.conf by hand everytime I install a package. I would like to improve this, by making it more automatic. Do you haskellers think it is possible? Regards, Rafael Gustavo da Cunha Pereira Pinto

1 0

ANNOUNCE: PortAudio Windows Tutorial and Binaries
by M Xyz 07 Dec '09

07 Dec '09

I got a lot of great help this weekend from Haskell-Cafe, thanks. Now that I have portaudio up and running I put up a tutorial and a 103 kb download of all the windows binaries and files. I hope this helps people exploring digital audio with Haskell. http://www.subreddits.org/misc/haskell/portaudio.html

2 1

computing lists of pairs
by Christian Maeder 07 Dec '09

07 Dec '09

Wizards, I've the following small piece of code \begin{code} pairs :: [a] -> [b] -> [[(a, b)]] pairs l1 = map (zip l1) . takeKFromN l1 takeKFromN :: [b] -> [a] -> [[a]] takeKFromN s l = case s of [] -> [[]] _ : r -> [ a : b | a <- l, b <- takeKFromN r l] \end{code} I have a predicate: p :: (a, b) -> Bool and I want to keep only those results of pairs which fulfill "all p". I do so currently by "filter (all p) (pairs l1 l2)", but I want to generate the beginning of this pair lists more efficiently, because the result list of pairs may become very large, soon: length (pairs l1 l2) == length l2 ^ length l1 Any ideas (or other hints) to improve the code? "pairs" computes all different mappings from all elements of l1 to some elements of l2. "takeKFromN" computes all possible sequences of length l1 with elements from l2. I somehow want to integrate the predicate into the generation. Cheers Christian

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Hoogle Down
by Elliot Wolk 07 Dec '09

07 Dec '09

hello! im not sure that this is the correct mailing list for saying so, and also whether or not today's down-ness is just scheduled maintenance, but hoogle appears to be down again. sorry if this is known/redundant/not the right place! thanks, elliot

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Re: [Haskell-cafe] Re: [Haskell] ANNOUNCE: control-monad-exception 0.5 with monadic call traces
by Henning Thielemann 07 Dec '09

07 Dec '09

klondike schrieb: > Henning Thielemann escribió: > >> It seems again to me, that mixing of (programming) errors and >> exceptions is going on, and I assumed that the purpose of >> control-monad-exception is to separate them in a better way. > You know, could you tell me when using head on an empty list is a > programming error and when it is a exception, I have seen both cases... The case of (head []) is simple: It is a programming error, since the precondition for calling 'head' is that the argument list is non-empty. The caller of 'head' is responsible to check this. If the list comes from user input, then the program part that receives this list from the user is responsible to check for the empty list before calling 'head'. If there is no such check, this is a programming error, and it will not be possible to handle this (like an exception). Before you answer: "What about web servers?", please read on the article I have written recently to sum up the confusion about errors and exceptions: http://www.haskell.org/haskellwiki/Error_vs._Exception

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