- Haskell-Cafe - Haskell.org

cabal update error
by Daniel Díaz 13 Nov '10

13 Nov '10

Hi all, I just installed the Haskell Platform, and when I try "cabal update", I have the following output: > Downloading the latest package list from hackage.haskell.org > cabal: failed This error does not give me information about what is wrong. Has anyone had this error before? I would appreciate an explanation (or possible explanation), rather than an "update it manually". Thanks in advance. -- Daniel Díaz

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Serialization of (a -> b) and IO a
by Jesse Schalken 13 Nov '10

13 Nov '10

Is it possible to serialize and deserialize a function to/from binary form, perhaps using Data.Binary, for example? What about an IO action? If so, is there a way the serialized representation could be architecture-independent? I have been shown how useful it can be to store functions inside data structures, and while looking at data serialization for the purpose of persistence I wondered "since functions are just values in Haskell, why can't we persist them, too?". To me the idea has interesting implications. For example, an arbitrary program could simply be represented by a serialization of `IO ()`. In fact, you could load any program into memory from a file and use Control.Concurrent.forkIO to run it, and later kill it, giving you the beginnings of an operating environment. If such a serialization is architecture independent then to my understanding you have the beginnings of a virtual machine. You could break your program into pieces and store them in a database and load them when needed, or even pull updates to each piece individually from down the web etc, enabling interesting methods of software distribution. It would make very cool stuff possible. I have had a look at hs-plugins, but it is unclear how to derive a simple pair of functions `(a -> b) -> ByteString` and `ByteString -> Either ParseError (a -> b)`, for example, from the functionality it provides, if it is possible at all. I guess such a thing requires thorough digging into the depths of GHC, (or maybe even LLVM if an architecture independent representation is sought, but I don't know enough to say.). Perhaps this is more a question for those interested and knowledgable in Haskell compilation (and, to some extent, decompilation). If not Haskell, are there any languages which provide a simple serialization and deserialization of functions?

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an evil type hangs GHC
by Petr Pudlak 13 Nov '10

13 Nov '10

Hi, I was playing with the following example I found in D.A.Turner's paper Total Functional Programming: > data Bad a = C (Bad a -> a) > > bad1 :: Bad a -> a > bad1 b@(C f) = f b > > bad2 :: a > bad2 = bad1 (C bad1) To my surprise, instead of creating a bottom valued function (an infinite loop), I managed to send the GHC compiler (ver. 6.12.1) to an infinite loop. Could anybody suggest an explanation? Is this a GHC bug? Or is this "Bad" data type so evil that type checking fails? Thanks, Petr

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"Haskell is a scripting language inspired by Python."
by Jonathan Geddes 12 Nov '10

12 Nov '10

This is off topic (almost regardless of the topic), but It gave me a laugh. Hope you all enjoy it, too. I was telling a friend about the power and elegance of Haskell. When I mentioned that it has influenced many other programming languages, including his favorite language (Python) he retorted by saying that I was mistaken and it was, in fact, the other way around: Python inspired Haskell. The following link was his source of information (or FUD, as the case may be). http://www.datarecoverylabs.com/ultimate-computer-language-guide.html It's called "The *Ultimate* Computer Language Guide," and it's on the internets, so it must be correct, right? --Jonathan

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ANNOUNCE: The Fibon benchmark suite (v0.2.0)
by David Peixotto 12 Nov '10

12 Nov '10

I'm pleased to announce the release of the Fibon benchmark tools and suite. Fibon is a set of tools for running and analyzing benchmark programs in Haskell. Most importantly, it includes an optional set of benchmark programs including many programs taken from the Hackage open source repository. The source code for the tools and benchmarks are available on github https://github.com/dmpots/fibon http://github.com/dmpots/fibon-benchmarks The Fibon tools (without the benchmarks) are available on hackage. http://hackage.haskell.org/package/fibon The package needs to be unpacked and built in place to be able to run any benchmarks. It can be used with the official Fibon benchmarks or you can create your own suite and just use Fibon to run and analyze your benchmark programs. Some more documentation is available on the fibon wiki https://github.com/dmpots/fibon/wiki Fibon Tools =================================================================== Fibon is a pure Haskell framework for running and analyzing benchmark programs. Cabal is used for building the benchmarks. The benchmark harness, configuration files, and benchmark descriptions are all written in Haskell. The benchmark descriptions and run configurations are all statically compiled into the benchmark runner to ensure that configuration errors are found at compile time. The Fibon tools are not tied to any compiler infrastructure and can build benchmarks using any compiler supported by cabal. However, there are some extra features available when using GHC to build the benchmarks: * Support in config files for using an inplace GHC HEAD build * Support in `fibon-run` for collecting GC stats from GHC compiled programs * Support in `fibon-analyse` for reading GC stats from Fibon result files The Fibon Benchmark Suite =================================================================== The Fibon benchmark suite currently contains 34 benchmarks from a variety of sources. The individual benchmarks and lines of code are given below. Dph _DphLib 316 Dotp 308 Qsort 236 QuickHull 680 Sumsq 72 ------------------------------ TOTAL 1612 Hackage Agum 786 Bzlib 432 Cpsa 11582 Crypto 4486 Fgl 3834 Fst 4532 Funsat 16085 Gf 23970 HaLeX 4035 Happy 5833 Hgalib 819 Palindromes 496 Pappy 7313 QuickCheck 4495 Regex 6873 Simgi 5134 TernaryTrees 722 Xsact 2783 ------------------------------ TOTAL 104210 Repa _RepaLib 8775 Blur 77 FFT2d 89 FFT3d 103 Laplace 274 MMult 133 ------------------------------ TOTAL 9451 Shootout BinaryTrees 63 ChameneosRedux 96 Fannkuch 27 Mandelbrot 68 Nbody 192 Pidigits 26 SpectralNorm 97 ------------------------------ TOTAL 569

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Manatee - The Haskell/Gtk+ Integrated Live Environment first version release!
by Andy Stewart 12 Nov '10

12 Nov '10

Hi all, I am proud to announce the release my gtk2hs project : Manatee - The Haskell/Gtk+ Integrated Live Environment http://hackage.haskell.org/package/manatee Screenshots at : http:goo.gl/MkVw Code at https://patch-tag.com/r/AndyStewart/ beginning with manatee-* Manatee is Haskell integrated environment written in Haskell. The goal of the Manatee project is to provide a fast, safe and flexible integrated environment for haskell hacking. You can consider it is new environment mix Gnome and Emacs. Like Gnome to provide friendly graphics interface and work efficient like Emacs. Manatee use multi-processes framework, any sub-module running in separate process to protected core won't crash. So it minimize your losses when some unexpected exception throw in extension. Now i have implement below sub-modules in Manatee: Editor Webkit Browser File Manager Image Viewer IRC Client Multimedia Player PDF Viewer Process Manager RSS/Atom reader >From some friends feedback, manatee can't work in XMonad, i will fix it soon. You can play it in Gnome. Enjoy! :) Below are steps to build Manatee: 1) Install C library: In Debian use below command: sudo aptitude install libgtksourceview2.0-dev libgconf2-dev libwebkit-dev libcurl4-openssl-dev libgtkimageview-dev libpoppler-glib-dev poppler-data libtagc0-dev -y 2) Install Gtk2hs: cabal install gtk2hs-buildtools gtk And make sure HOME.cabalbin/ in your PATH. 3) Install Manatee: cabal install manatee-core manatee-anything manatee-browser manatee-editor manatee-filemanager manatee-imageviewer manatee-ircclient manatee-mplayer manatee-pdfviewer manatee-processmanager manatee-reader manatee That's all, then type command manatee to play it! :) "manatee-core" "manatee-anything" "manatee" are core packages, must be install, other extension package you can choose you want. Example, if you not install manatee-imageviewer, when you open Image file, manatee will call default image-viewer in your system instead. Manatee will show you "search interface" when you startup it. You can type some filepath or url to open it, example, you can type Haskell file to open in editor, and type url to open in browser. Below are quick play keys: "F2" ==> startProcessManager "F3" ==> startFeedReader "F4" ==> startFileManager "F5" ==> startBrowser "F6" ==> loginIrcDefaultChannel "F7" ==> startIrc Manatee project still in early develop stage, just core framework finish, many details still not perfect. But i think it's good start to build Real-World application in Haskell. Below are high task in my TODO list: Perfect current sub-module: IDE features, code completion browser JavaScript framework graphics custom system etc. Terminal emulator: support MVC design, not like VTE widget Mail-client BT-Client Proxy bridge: to build uniform proxy interface to fighting GFW!!! Jabbar client: video support etc. Spell checker CHM viewer DVI viewer LaTex editor PS viewer Multi-thread download manager Org-Mode : http://orgmode.org/ Twitter client Network toolkit: sniffer etc. Multi-Language translater offline support Too many ideas lying in my TODO list.... Any suggestion and contribution are welcome! :) -- Andy

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Opportunity for Haskell porting to java at R&D labs in Bay Area, CA
by Padma 12 Nov '10

12 Nov '10

We are looking for a entry level Haskell programmer who has experience in porting from Haskell to java. Please contact me by Email or you can call me at 408-207-9367. LOCATION: SUNNYVALE, CA DURATION: 6 MONTHS Degree: Bs or Ms or Ph.D Start immediately Good experience in porting particularly from Haskell to java environment. Check and validate smooth functioning of the system.(After porting is done) This is a R&D project. (prior experience is desired) Good experience in testing and compiling. Regards, Padma SRAOSS INC. 5300 Stevens Creek Blvd Suite 460 San Jose,CA 95129 Direct:(408) 207-9367 Tel: (408) 855-8200 x 321 Fax: (408) 855-8206 <blocked::mailto:padma@sraoss.com> padma(a)sraoss.com <blocked::http://www.sraoss.com/> www.sraoss.com <blocked::http://www.sra.co.jp/> www.sra.co.jp

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Re: Haskell-cafe] Serialization of (a -> b) and IO a
by John Lato 12 Nov '10

12 Nov '10

> > Message: 3 > Date: Thu, 11 Nov 2010 18:56:00 +0100 > From: Sjoerd Visscher <sjoerd(a)w3future.com> > > On Nov 11, 2010, at 6:34 PM, John Lato wrote: > > > I don't know to what extent it would apply in this hypothetical > situation, but ghc (and probably other compilers) rely upon Haskell's > semantics in performing various code transformations. If you break the > semantics some transformations become invalid, resulting in incorrect code. > > > > I've experienced this with code that violated ref. transparency. The > program behavior changed depending on the compiler's optimization settings. > I'm not keen to go back to that. > > Then don't do that. Being able to serialize functions is just as dangerous > as having unsafePerformIO. If you don't use it, you don't have problems. > If you do use "serialize", I think it would cause problems most of the time. A pure serialization would violate referential transparency. GHC relies upon referential transparency for some transformations to be valid. Therefore it's not possible to use a pure serialization function safely unless the user can guarantee that it's only used in a referentially transparent manner, which is an extremely high barrier IMO (higher than for unsafePerformIO). John

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Re: [Haskell-cafe] Serialization of (a -> b) and IO a
by John Lato 12 Nov '10

12 Nov '10

> > From: Sjoerd Visscher <sjoerd(a)w3future.com> > > On Nov 11, 2010, at 3:36 PM, Dan Doel wrote: > > > On Thursday 11 November 2010 6:22:06 am Sjoerd Visscher wrote: > > > >> The reasoning above is used regularly to shoot down some really useful > >> functionality. So what would go wrong if we chose to take the practical > >> path, and leave aside the theoretical issues? > > > > You would lose many uses of equational reasoning in your programs. Have > you > > every substituted 'x * 2' for the expression 'x + x' in one of your > programs, > > or vice versa? You can no longer do that, because someone may be > serializing > > the function you're writing, checking how it's implemented, and relying > it. > > > Yes, but it would not break any existing code. It would only break code > that knowingly did the wrong thing. > > > We already have a weak case of this, since (\x -> undefined x) can be > > distinguished from undefined using seq, but that can be hand-waved away > by not > > worrying about bottoms so much. That isn't going to work for serialize. > > Why not? > I don't know to what extent it would apply in this hypothetical situation, but ghc (and probably other compilers) rely upon Haskell's semantics in performing various code transformations. If you break the semantics some transformations become invalid, resulting in incorrect code. I've experienced this with code that violated ref. transparency. The program behavior changed depending on the compiler's optimization settings. I'm not keen to go back to that. I think the only way this would work would be if you consider functions to be equal only to themselves, i.e. "x+x" and "2*x" are not equal. That's not a trade-off I would be willing to make. John

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Fwd: Re: [Haskell-cafe] Type Directed Name Resolution
by John Lask 12 Nov '10

12 Nov '10

> On Thu, Nov 11, 2010 at 8:16 PM, John Lask<jvlask(a)hotmail.com> wrote: >> consider "length" ... >> >> I have records with the attribute length, length can be given as an Int, >> Double, Float or maybe as a constructed type "Length", length's use as a >> record selector would also clash with List.length. All these have the same >> denotation. >> >> should I then seporate into int_length, float_length, or use rec1_length, >> rec2_length etc etc... > > class Lengthy a where > type LengthType a > length :: a -> LengthType a > > This extends easily to lenses if you want setters. > to make use of the class Lengthy I still need to define record selectors with different names, which is exactly the point I am making... ....ie data Record = RecLen { rec_length :: ... } instance Lengthy Record where length = rec_length >> This is easily handled in C, Pascal, PL/1, Cobol why not in Haskell ? > > By this argument, Haskell should provide global mutable state and > allow side-effects universally. > no, but these languages have their strengths as well, for example Cobol PIC strings format currency values very nicely and it would be great if there were a Haskell library that could do the same. Not to mention currency values! The point is that languages are often constructed with a purpose in mind, at which they tend to be particularly good. Haskell has traditionally been a test bed for type system innovation, which is why we all like it so much. As and if, the usage of Haskell broadens, then domains of application stray into areas of application for which it is not ideally suited, in those circumstances why not consider features of other languages which handle those use cases well. (for some definition of "well") By the way I am not arguing for TDNR, merely that all is not well with haskell records.

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Rebindable record syntax
by Philippa Cowderoy 12 Nov '10

12 Nov '10

I stood up and suggested rebindable record syntax at Anglohaskell earlier this year, but never got round to posting a proposal. Given the TDNR discussion, it seems timely to link everyone to what I'd got round to writing: http://flippac.org/RebindableRecordSyntax.html Apologies for the lack of concrete examples: if anyone's interested I'll try to crank some out soon, specific requests would help. I probably don't have the energy to champion this properly on my own, so speak up if you think it's a good idea! Incidentally, I believe something similar has been proposed elsewhere - it's also possible I'd seen it before and forgotten before "rediscovering" the idea. -- flippa(a)flippac.org

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Fwd: Re: [Haskell-cafe] Type Directed Name Resolution
by John Lask 12 Nov '10

12 Nov '10

> > On 12/11/2010, at 2:16 PM, John Lask wrote: > >> On 12/11/2010 9:22 AM, Richard O'Keefe wrote: >>> >>> I'm afraid it's not a *convincing* use case. >>> It's not convincing because here "owner" *means different things*. >> >> >> consider "length" ... >> >> I have records with the attribute length, length can be given as an Int, Double, Float or maybe as a constructed type "Length", length's use as a record selector would also clash with List.length. All these have the same denotation. > > "All these have the same denotation"? I am extremely confused here. > AH! You mean you call them all by the same name. Well yes, that's > the problem, right there. > > I remind readers once again that in SML record selectors *don't* clash with > names of functions. I am not concerned here to argue either for or against > SML-style records and their selectors, only to point out that wanting > *record fields* whose significance depends on the record they select from > is *NOT* the same thing as TDNR in principle, so that arguments for that > don't even come close to being arguments for TDNR as such. > >> >> should I then seporate into int_length, float_length, or use rec1_length, rec2_length etc etc... > > No, the differing result types are an epiphenomenon of their differing > semantics. > > The length of a piece of string is a physical length. length of string measured in CM's, in MM, in Inches, as an Int, float ... they all denote the same thing modulo some theory. You could argue that a length in CM's measured as a float is semantically different to that of CM's as an Int, which it is, but somewhere along the line those semantic differences become irrelevant to the abstract domain of application, your model, and it is that model that you wish to represent as a program, although you need to deal with the concrete differences as an implementation detail. > The length of a lecture is a time. > The length of a queue is a natural number (counting people). > The length of a book is a positive integer (counting say pages, or words) > The length of a vowel is a relative time. > > These quantities are measured differently, combined differently, and assessed > differently. Calling them all "length" is a METAPHOR. (There are at least > three metaphors in the list above: time-is-space, codex-is-scroll, and > a form of metonomy.) > >> for proper name space management why should I have to define each record >> that defines a length field with different representation in distinct modules, or with different names for the field label when they denote the same thing? > > But you just explained that while they may have the same denotation (the > identifier), what it DENOTES is NOT the same (otherwise they could not > be different types). > you are right > As SML proves, having record-sensitive field names is a *different* > question from Type Directed Name Resolution applied to plain function names. >> agreed >> This is easily handled in C, Pascal, PL/1, Cobol why not in Haskell ? > > But it *isn't* handled *AT ALL* in any of those languages, let alone > handled easily. > > In C, Pascal, PL/I, and COBOL field names are *not* values. > C, Pascal, and PL/I let you pass functions as parameters to functions. > But you cannot pass field names that way. The only thing you can do > with a field name is apply it *immediately* to a record. And those > languages don't have type variables, so when you apply a selector to > a record, you know then and there what the type is. > > If what you want is some sort of record facility at least as good as > SML's, fine. What I am arguing against is anything resembling Type > Directed Name Resolution (alias ad hoc overloading) being applied to > plain ordinary functions. that's something like what I want. I am not argiung in favor of TDNR, only that haskell record system needs improvement, particularly the scoping of field labels. > > (By the way, if the field names of a record type are visible outside its > module, I start wondering why.) > > depends on what your using them for .. >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > >

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[ANNOUNCE] csound-expression - csound combinator library
by Anton Kholomiov 12 Nov '10

12 Nov '10

Hi, I'm glad to announce csound combinator library. It features liberation from id-style csound code, haskore-like composition structures, type-safe composable opcodes and simple instrument interface (no interface at all, instrument is just a function from some note representation to signal). http://hackage.haskell.org/package/csound-expression Anton

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build-type: configure on windows
by Nils Schweinsberg 12 Nov '10

12 Nov '10

Hi, I'm having a few problems with cabals build-type "configure" on windows, especially with the packages "curl" and "pcre-light". Both fail either with: configure: error: curl libraries not found, so curl package cannot be built Configuring pcre-light-0.4... cabal: Missing dependency on a foreign library: * Missing C library: pcre Both libraries (pcre3.dll and libcurl.dll) are in my PATH variable, and I can build other non-haskell-applications which are using these via mingws gcc compiler. But still, somehow cabal seems unable to find them. Using the recommanded "--extra-include-dirs" and "--extra-lib-dirs" won't help either. Or is cabal looking for different files than those two .dlls? Can anyone help me on this? Did anyone manage to build curl/pcre-light on windows? My current configuration is MinGW/msys with gcc 4.5, ghc 6.12.3, cabal-install 0.8.2, cabal 1.8.0.6. libcurl.dll and pcre.dll are the latest builds from the official webpages. Thanks for any Help, Nils PS: Sorry for the previous (incomplete) mail.

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Stack Overflow?
by Ben Christy 11 Nov '10

11 Nov '10

I have implemented a scene graph in Haskell and I have a problem. I walk down the scenegraph and at each node I recalculate translation matrix and pass it to each child. Well it seems to be causing a stack overflow and I am lost as to how to resolve the issue without issue a state variable of some kind to hold a "matrix stack" as is common in imperative implementations of scene graphs. When I run the program fromghci I get "drawingcamera" "drawingground" "drawinggroundModel" Matrix4x4 {i1j1 = -4.371139e-8, i1j2 = 0.0, i1j3 = 1.0, i1j4 = -3.9999998, i2j1 = 0.0, i2j2 = 1.0, i2j3 = 0.0, i2j4 = -1.5, i3j1 = -1.0, i3j2 = 0.0, i3j3 = -4.3 71139e-8, i3j4 = 3.0, i4j1 = 0.0, i4j2 = 0.0, i4j3 = 0.0, i4j4 = 1.0} "a model" "classic matrix operation" "drawingcarousel" "drawingcarouselbase" "drawingcarouselbaseModel" Matrix4x4 {i1j1 = -4.371139e-8, i1j2 = 1.0, i1j3 = -4.371139e-8, i1j4 = 0.0, i2j 1 = 0.0, i2j2 = -4.371139e-8, i2j3 = -1.0, i2j4 = 0.0, i3j1 = -1.0, i3j2 = -4.37 1139e-8, i3j3 = 1.9106855e-15, i3j4 = -1.0, i4j1 = 0.0, i4j2 = 0.0, i4j3 = 0.0, i4j4 = 1.0} "classic matrix operation" "drawingcarouselRoof" "drawingcarouselRoofModel" Matrix4x4 {i1j1 = -4.371139e-8, i1j2 = 1.0, i1j3 = -4.371139e-8, i1j4 = 0.0, i2j 1 = 0.0, i2j2 = -4.371139e-8, i2j3 = -1.0, i2j4 = 0.3, i3j1 = -1.0, i3j2 = -4.37 1139e-8, i3j3 = 1.9106855e-15, i3j4 = -1.0, i4j1 = 0.0, i4j2 = 0.0, i4j3 = 0.0, i4j4 = 1.0} "classic matrix operation" "drawingcarouselFloor" "drawingcarouselFloorModel" Matrix4x4 {i1j1 = -4.371139e-8, i1j2 = 1.0, i1j3 = -4.371139e-8, i1j4 = 0.0, i2j 1 = 0.0, i2j2 = -4.371139e-8, i2j3 = -1.0, i2j4 = 0.0, i3j1 = -1.0, i3j2 = -4.37 1139e-8, i3j3 = 1.9106855e-15, i3j4 = -1.0, i4j1 = 0.0, i4j2 = 0.0, i4j3 = 0.0, i4j4 = 1.0} "classic matrix operation" "drawingpolls" "drawingpoll1" "drawingpoll1Model" Matrix4x4 {i1j1 = <interactive>: <<loop>> <interactive>: interrupted As you can see there are only a handful of nodes at this point so I am confused as to why it is happening here, any thoughts?

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ghc dump the code for derived instances
by Ozgur Akgun 11 Nov '10

11 Nov '10

Café, Is there a way to make GHC dump the code for auto-derived type class instances, say for Show, Eq and such? Thanks, Ozgur

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Gödel's System T
by Petr Pudlak 11 Nov '10

11 Nov '10

Hi, I was reading the paper "Total Functional Programming" [1]. I encountered an interesting note on p. 759 that primitive recursion in a higher-order language allows defining much larger set of function than classical primitive recursion (which, for example, cannot define Ackermann's function). And that this is studied in in Gödel's System T. It also states that this larger set of primitive functions includes all functions whose totality can be proved in first order logic. I was searching the Internet but I couldn't find a resource (a paper, a book) that would explain this in detail, give proofs etc. I'd be happy if someone could give me some directions. Thanks, Petr [1] http://www.jucs.org/jucs_10_7/total_functional_programming/jucs_10_07_0751_…

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ANN: network-2.2.3, merger with network-bytestring
by Johan Tibell 11 Nov '10

11 Nov '10

Hi all, I like to announce a new version of the network package, network-2.2.3. You can install the latest version by running: cabal update && cabal install network This version marks the end of the network-bytestring package, which has now been merged into the network package. This means that efficient and correct networking using ByteStrings is available as part of the standard network package. As part of the merger, two new modules have been added: Network.Socket.ByteString and Network.Socket.ByteString.lAzy This release is backwards compatible with the previous release. Johan

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build-type: configure on windows
by Nils Schweinsberg 11 Nov '10

11 Nov '10

Hi, I'm having a few problems with cabals build-type "configure" on windows, especially with the packages "curl" and "pcre-light". Both fail either with: * Missing C library: pcre

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Haskell Weekly News: Issue 158 - November 10, 2010
by Daniel Santa Cruz 11 Nov '10

11 Nov '10

Welcome to issue 158 of the HWN, a newsletter covering developments in the [1]Haskell community in the week of October 31 - November 06. I'd like to start things off with a quote from Jeremy O'Donoghue this past week on a thread that produced plenty of laughs: Haskell was developed by Isaac Newton in the 15th Century as a tool to help his investigations into the Alchemic arts. It was rediscovered in the 1980s by three Cambridge undergraduates who were browsing through Newton's laboratory notebooks looking for smutty jokes in the margins, and has since developed into an elaborate joke perpetrated by elite computer scientists who believe that predictable order of execution is contrary to natural law. The current version of Haskell is Haskell 1714, which adds syntactic sugar for Zygohistomorphic Premorphisms to the original language definition of 1693. This week we had 378 messages posted to Haskell-Cafe (up 76 from last week!), 30 stories posted to Reddit (1 more than last week), 40 questions posted to StackOverflow (15 more than last week), and 96 updates to Hackage (11 more than last week). Busy busy week! From all the noise in the intertubes, it sounds like BelHac was awesome. There were plenty of kudos to Jasper Van der Jeugt for how well things went. Jasper posted a [2]summary blog, and we also have some [3]pictures from the event. Announcements Will Sonnex [4]updated Zeno, a fully automated inductive theorem proving tool for Haskell programs. Gregory D. Weber [5]updated Sifflet, a visual, functional programming language and support system for students learning about recursion. Mario BlaÅ¾eviÄ/= [6]updated monad-coroutine and SCC to version 0.6. The monad-coroutine package exports a generic monad transformer: Coroutine: Functor s => MonadTrans (Coroutine s). Johan Tibell informed us that he has [7]merged network-bytestring into the network package. This means that efficient and correct networking using ByteString is available as part of the standard network package. By far, the most popular thread this week in -cafe was a thread entitled [8]"Haskell is a scripting language inspired by Python." As you can imagine, many could not resist! The thread diverted into some interesting historical notes about Haskell. Top Reddit Stories * Learn You a Haskell: Zippers Domain: learnyouahaskell.com Score: 91, Comments: 44 On Reddit: http://www.reddit.com/r/haskell/comments/e0qqz/learn_you_a_haskell_zippers/ Original: http://learnyouahaskell.com/zippers * Neat Haskell commenting trick Domain: self.haskell Score: 78, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/dyvsi/neat_haskell_commenting_tric… Original: /r/haskell/comments/dyvsi/neat_haskell_commenting_trick/ * Haskell Halloween! Domain: i.imgur.com Score: 53, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/dyznc/haskell_halloween/ Original: http://i.imgur.com/t7qWf.jpg * Circular wavefronts Domain: claudiusmaximus.goto10.org Score: 39, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/e02vp/circular_wavefronts/ Original: http://claudiusmaximus.goto10.org/cm/2010-11-02_circular_wavefronts.html * Optimizing Haskell Loops With LLVM Domain: dmpots.com Score: 35, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/e1yfn/optimizing_haskell_loops_wit… Original: http://www.dmpots.com/blog/2010/11/05/optimizing-haskell-loops-with-llvm.ht… * Obtaining the name of a function in Haskell Domain: mainisusuallyafunction.blogspot.com Score: 24, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/e1i3q/obtaining_the_name_of_a_func… Original: http://mainisusuallyafunction.blogspot.com/2010/11/obtaining-name-of-functi… * Generative Type Abstraction and Type-level Computation Domain: cis.upenn.edu Score: 17, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/dzx80/generative_type_abstraction_… Original: http://www.cis.upenn.edu/~sweirich/newtypes.pdf * Relations rather than functions - where to learn about them? Domain: self.haskell Score: 17, Comments: 17 On Reddit: http://www.reddit.com/r/haskell/comments/e1520/relations_rather_than_functi… Original: /r/haskell/comments/e1520/relations_rather_than_functions_where_to_learn/ * Haskellers.com: Special Interest Groups feature active Domain: haskellers.com Score: 16, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/dzkel/haskellerscom_special_intere… Original: http://www.haskellers.com/news/6/ * The Utrecht Haskell Compiler now in github Domain: github.com Score: 16, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/e26ar/the_utrecht_haskell_compiler… Original: https://github.com/uhc/uhc Top StackOverflow Questions * how to do this in haskell? Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4106247/how-to-do-this-in-haskell * Haskell define Merge using mapReduce Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4099627/haskell-define-merge-using-mapre… * Real GHC RTS memory usage Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4088351/real-ghc-rts-memory-usage * reading file and accessing in haskell (functional programming language) Question (votes: 0, answers: 2): Read on SO: http://stackoverflow.com/questions/4074342/reading-file-and-accessing-in-ha… * Converting tree types Question (votes: 0, answers: 2): Read on SO: http://stackoverflow.com/questions/4073091/converting-tree-types * Printing Binary Tree per Layer in a List Question (votes: 0, answers: 3): Read on SO: http://stackoverflow.com/questions/4068681/printing-binary-tree-per-layer-i… * Comparing Bool Value in Matrix Question (votes: 0, answers: 2): Read on SO: http://stackoverflow.com/questions/4063553/comparing-bool-value-in-matrix * Converting Int to Integer Question (votes: 0, answers: 3): Read on SO: http://stackoverflow.com/questions/4061777/converting-int-to-integer * Integrating Haskell with .Net? Question (votes: 1, answers: 3): Read on SO: http://stackoverflow.com/questions/4111467/integrating-haskell-with-net * Putting existentials into monad Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4106793/putting-existentials-into-monad Quotes of the week * Luke Palmer:Also they "don't scale well", which I guess means that they don't make it inconvenient to design badly. * merijn: <dankna> someday we'll need meta-software that converts from obsolete languages into modern ones <merijn> Don't we call that compilers? * dolio: Let's spawn a thread every time we want to and two booleans. * ray: how about a language where you program with unit vectors: orientation oriented programming * Axman6: Haskell is easy, you just apply functions to arguments, and then you apply them to other arguments. Axman6's girlfriend: I don't like conflict * monochrom: in soviet haskell, functions run you * geheimdienst: in soviet russia, i remember lambdabot's quotes * Funktorsalat: sum is just the free monoid functor applied to the mediating arrow of the terminality of () * geheimdienst: ah, yes. now we have to make a lambdacat captioned "kittehs no bery gud at speling. we says leksah fur lexer and monad fur moniod in catagory of ennobunktors" * Vanadium: That tripped my C++ highlight. * jmcarthur: what you just typed isn't even well typed About the Haskell Weekly News New editions are posted to [9]the Haskell mailing list as well as to [10]the Haskell Sequence and [11]Planet Haskell. [12]RSS is also available, and headlines appear on [13]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://jaspervdj.be/posts/2010-11-09-belhac-summary.html 3. http://www.flickr.com/groups/1566670@N23/ 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18277 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18279 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18281 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/82798 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/82995 9. http://www.haskell.org/mailman/listinfo/haskell 10. http://sequence.complete.org/ 11. http://planet.haskell.org/ 12. http://sequence.complete.org/node/feed 13. http://haskell.org/

1 0

Fwd: Questions about lambda calculus
by Patrick LeBoutillier 11 Nov '10

11 Nov '10

Hi all, Sorry for cross-posting this, but I didn't get anything on the beginners list so I thought I'd give it a try here. Thanks, Patrick ---------- Forwarded message ---------- From: Patrick LeBoutillier <patrick.leboutillier(a)gmail.com> Date: Thu, Nov 4, 2010 at 2:02 PM Subject: Questions about lambda calculus To: beginners <beginners(a)haskell.org> Hi all, I've been reading this article: http://perl.plover.com/lambda/tpj.html in which the author talks about the lambda calculus and uses examples in Perl (here's a link directly to the code: http://perl.plover.com/lambda/lambda-brief.pl) I'm a total newbie with respect to the lambda calculus. I tried (naively) to port these examples to Haskell to play a bit with them: fTRUE = \a -> \b -> a fFALSE = \a -> \b -> b fIF = \b -> \x -> \y -> b x y fPAIR = \a -> \b -> \f -> f a b fFIRST = \p -> p fTRUE fSECOND = \p -> p fFALSE fZERO = fPAIR fTRUE fTRUE fSUCC = \x -> fPAIR fFALSE x fIS_ZERO = \x -> fFIRST x fPRED = \x -> fSECOND x fONE = fSUCC fZERO fTWO = fSUCC fONE fADD = \m -> (\n -> fIF (fIS_ZERO m) n (fADD (fPRED m) (fSUCC n))) but I couldn't get fADD to compile: Occurs check: cannot construct the infinite type: t = (t1 -> t1 -> t1) -> t Probable cause: `fADD' is applied to too many arguments In the third argument of `fIF', namely `(fADD (fPRED m) (fSUCC n))' In the expression: fIF (fIS_ZERO m) n (fADD (fPRED m) (fSUCC n)) I think its because in these Perl examples all functions are treated as being of the same type (number or type of args doesn't matter), but this is not the case in Haskell. Is there any way to create code similar to this in Haskell? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

5 5

Typeable state map
by Vladimir Matveev 11 Nov '10

11 Nov '10

Hi, I want to use something like (Control.Monad.StateT (Data.Map.Map String Dynamic) m). So I wrote VarStateT datatype and corresponding classes (Control/Monad/VarState.hs in [1]). Then I tried to use it in my program (Math/Eigenvalues.hs in [1]). But compiler refused to compile it with many type errors. I thought that it would be sufficient to use ScopedTypeVariables and write all type annotations. It helped slightly, many messages were gone, but now compiler throws an error I don't know how to solve. ghc --make -o out/eigenvalues -outputdir build Main.hs [3 of 4] Compiling Math.Eigenvalues ( Math/Eigenvalues.hs, build/Math/Eigenvalues.o ) Math/Eigenvalues.hs:14:30: Could not deduce (MutableMatrix a e m) from the context (MutableMatrix a2 e2 m, Typeable a2, Typeable e2, Floating e2, Ord e2) arising from a use of `getPhi' at Math/Eigenvalues.hs:14:30-35 Possible fix: add (MutableMatrix a e m) to the context of the type signature for `compute' In the second argument of `(>>)', namely `getPhi' In the first argument of `(>>)', namely `maxElem a >> getPhi' In the first argument of `(>>)', namely `maxElem a >> getPhi >> buildB' Math/Eigenvalues.hs:14:40: Could not deduce (MutableMatrix a1 e1 m) from the context (MutableMatrix a2 e2 m, Typeable a2, Typeable e2, Floating e2, Ord e2) arising from a use of `buildB' at Math/Eigenvalues.hs:14:40-45 Possible fix: add (MutableMatrix a1 e1 m) to the context of the type signature for `compute' In the second argument of `(>>)', namely `buildB' In the first argument of `(>>)', namely `maxElem a >> getPhi >> buildB' In the first argument of `(>>=)', namely `maxElem a >> getPhi >> buildB >> buildA' How to sort this out? [1] http://dl.dropbox.com/u/1415321/prog/eigenvalues.tar.gz

2 3

Finding the contents of haskell platform?
by Magnus Therning 10 Nov '10

10 Nov '10

I know there's a .cabal file for the latest version of HP somewhere, but I can't coerce Google into finding me a link that actually works. Furthermore, the following page: http://hackage.haskell.org/platform/contents.html does list all the contents, but to my big surprise it doesn't link to the specific versions of the packages for HP, instead it links to the latest version found on Hackage. Would someone with the power to make changes on the HP pages *please* make it as easy as possible to find the *exact* specification of what HP contains? Please, pretty please with sugar on top. No, a changelog entry (http://hackage.haskell.org/platform/changelog.html) is not very helpful (why the HP front page links to it I can't understand). Going via the Haskell wiki (http://www.haskell.org/haskellwiki/Haskell_Platform#What.27s_in_the_platform) to find a link to the .cabal (http://code.haskell.org/haskell-platform/haskell-platform.cabal) is not that user friendly. It's even worse that the latter link doesn't seem to work at all at the moment. (The short irritated tone in this email accurately shows my desperation with the situation: I thought I would be able to find this information with only 5 minutes to spare before my next meeting.) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Is there a better way to convert from UTCTime to EpochTime ?
by David Virebayre 10 Nov '10

10 Nov '10

I want to set a file's modification time to the time I got from exif data. To get the time from exif, I found : Graphics.Exif.getTag :: Exif -> String -> IO (Maybe String) To set the file modification time, I found : System.Posix.Files.setFileTimes :: FilePath -> EpochTime -> EpochTime -> IO () Assuming I do find a Time in Exif, I need to convert a String to an EpochTime. - With parseTime I can get a UTCTime. - With utcTimeToPOSIXSeconds I can get a POSIXTime - With a POSIXTime I can more or less get an EpochTime To convert from a UTCTime to EpochTime this typechecks, but I'm not sure it's correct : fromIntegral . fromEnum . utcTimeToPOSIXSeconds $ etime This is part of a function getTime that will return the time from Exif data, if present, otherwise the file's modification time : getTime (path,stat) = do let ftime = modificationTime $ stat err (SomeException _) = return ftime time <- liftIO $ handle err $ do exif <- Exif.fromFile path let getExifTime = MaybeT . liftIO . Exif.getTag exif res <- runMaybeT $ do tmp <- msum . map getExifTime $ [ "DateTimeOriginal", "DateTimeDigitized", "DateTime" ] MaybeT . return . parseTime defaultTimeLocale "%Y:%m:%d %H:%M:%S" $ tmp case res of Nothing -> return ftime Just etime -> return . fromIntegral . fromEnum . utcTimeToPOSIXSeconds $ etime return (path,time) Questions : 1) is there a better way to convert the time ? 2) any general comments on getTime ? Thanks, David.

1 0

HTTP 4000.1.0 release
by Ganesh Sittampalam 10 Nov '10

10 Nov '10

Hi, I've just released HTTP 4000.1.0 to hackage: - Fixed a bug that caused infinite loops for some URLs on some platforms (whether the URL was a trigger is probably related to the size of the returned data, and the affected platforms. Based on a patch by Daniel Wagner. - This is technically a breaking change (and hence there is an API version bump) as the fix introduces a new member to the HStream class which can be used to define a new payload type instead of String/ByteString, but I scanned hackage and noone actually seems to be doing that. - Applied a patch by Antoine Latter to fix a bug in the handling of 301 and 307 response codes. Here's his description of the change: *** The Network.Browser module is a convenience layer over the Network.Http modules offering many of the niceties one would expect in a web-browser - cookies, user authentication, and transparent handling of redirects. When using Network.Browser, previously (since version 4000.0.8) a redirect response code of 301 or 307 from the server would force the redirected request to be sent as a GET, even if the original request was of some other HTTP method. This behavior is only appropriate for a response code of 302 or 303 and has been fixed. *** The upstream repo is now on github: https://github.com/haskell/HTTP, though I may well switch back to darcs if I remain maintainer for long. I'd like to emphasise that I have no particular desire to maintain HTTP and if anyone else would like to take over they would be very welcome! Cheers, Ganesh

1 0

ActionScript Byte Code backend ?
by Aaron Gray 10 Nov '10

10 Nov '10

Is there a Flash ActionScript Byte Code generating backend for Haskell ? I know there was an older SWF 3 backend :- http://www.n-heptane.com/nhlab/repos/haskell-swf/ But is there anything more up to date ? Aaron

3 2

change in overlapping instance behavior between GHC 6.12 and GHC 7 causes compilation failure
by Jeremy Shaw 09 Nov '10

09 Nov '10

Hello, I have a module, XMLGenerator, which has some overlapping instances. I have a second module, Test, which imports that module and also adds some more overlapping instances. Both modules contain {-# LANGUAGE OverlappingInstances #-} at the top. Under some old version of 6.13 (and probably 6.12), if I put both modules in the same directory and try to load Test.hs, it gets the error: Test.hs:16:15: Overlapping instances for EmbedAsChild (M IO) (XMLGenT m (XML m)) arising from a use of `asChild' at Test.hs:16:15-21 Matching instances: instance (m1 ~ m, EmbedAsChild m c) => EmbedAsChild m (XMLGenT m1 c) -- Defined at XMLGenerator.hs:16:10-68 instance (XML m ~ x, XMLGen m) => EmbedAsChild m x -- Defined at XMLGenerator.hs:19:10-51 In the first argument of `($)', namely `asChild' In the expression: asChild $ (genElement "foo") In the definition of `asChild': asChild b = asChild $ (genElement "foo") If I put the XMLGenerator module in a separate package, dummy-hsx, and the Test modules links against it, I still get the error. *but* if I add: Extensions: OverlappingInstances to the dummy-hsx.cabal file, then Test.hs compiles just fine! So, for starters, I do not understand why that happens. Under GHC 7.0rc1, modifying the .cabal file has no effect. Instead I always get the error: Test.hs:16:15: Overlapping instances for EmbedAsChild (M IO) (XMLGenT m (XML m)) arising from a use of `asChild' Matching instances: instance [overlap ok] (m1 ~ m, EmbedAsChild m c) => EmbedAsChild m (XMLGenT m1 c) -- Defined in XMLGenerator (The choice depends on the instantiation of `m' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) Adding the IncoherentInstances flag does make it compile -- but I have never enabled that flag and not regretted it. What changed between GHC 6.12 and GHC 7.0? Is there a some solution besides using IncoherentInstances in every module that imports XMLGenerator? I have attached XMLGenerator.hs, Test.hs, and dummy-hsx.cabal. thanks! - jeremy

5 6

Re: [Haskell-cafe] Error installing hp2any-graph on Snow Leopard
by John Lato 09 Nov '10

09 Nov '10

> > From: Mark Spezzano <mark.spezzano(a)chariot.net.au> > > Hi, > > I get the following error when doing a 'cabal install hp2any-graph' (the > profiling tool) > > Does anyone know why? (I'm running Snow Leopard and I've tried using other > flags suggested...but with no success). GLUT is already installed via > MacPorts and via cabal. > Have you tried to build any Haskell programs using GLUT? Do they work? John

2 1

Combining wl-pprint and ByteString?
by Mark Spezzano 09 Nov '10

09 Nov '10

Hi all, I want to do the following tasks in this order: 1. Read text from standard input (should this be stored internally in my program as a ByteString or as a String?) 2. Process the text via left justifying it and making it word-wrap (again, internally, should I store this as a ByteString or String?) 3. Format the text using the Wadler-Leijen Pretty Printer (to the best of my knowledge this only processes Strings, but not ByteStrings) 4. Print to Standard output the formatted (i.e. colourised, and justified) text (again, should this be as a ByteString or as a String?) I thought that the reason that most people use ByteString was for _performance_, but wl-pprint can only process Strings to the best of my knowledge. I would, ideally, like to use ByteStrings everywhere...all throughout my code. Is this possible with coloured text? I want to boost up the performance of my application dramatically, so in theory I could process everything as a String, then as the last second pack everything processed into a ByteString and print it out...I think that this might work. Then again it might not because of the types expected. Any help with this would be appreciated. Cheers, Mark Spezzano

3 4

ANNOUNCE: Haskell Communities and Activities Report (19th ed., November 2010)
by Janis Voigtländer 09 Nov '10

09 Nov '10

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (19th edition, November 2010) is now available in PDF and HTML formats: http://haskell.org/communities/11-2010/report.pdf http://haskell.org/communities/11-2010/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2011: target deadline for contributions to the May 2011 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

1 0

iMAP library
by A Smith 09 Nov '10

09 Nov '10

I'm looking for a Haskell library to allow me to access an IMAP mailbox. Anyone know of one I could use ? -- Andrew

2 1

Francisco Vieira Souza deixou uma mensagem para você no Badoo!
by Badoo 09 Nov '10

09 Nov '10

Você tem uma nova mensagem em Badoo! Francisco Vieira Souza deixou uma mensagem pra você. Siga este link para abrir: http://us1.badoo.com/01121329442/in/nYfwm9FNeVQ/?lang_id=61 Mais gente que espera pacientemente por você: Douglas Levita (Teresina, Brasil) Naikely (Teresina, Brasil) Paula Hullek (Teresina, Brasil) http://us1.badoo.com/01121329442/in/nYfwm9FNeVQ/?lang_id=61 Se os links desta mensagem não funcionam, copie e cole-os na barra de endereços do seu browser. Este email é parte do sistema de entrega de mensagens enviadas por Francisco Vieira Souza. Se você recebeu este email por engano, por favor, ignore. Em breve a mensagem será removida do sistema. Divirta-se! A Badoo Equipe Você recebeu este email, porque um membro do Badoo deixou uma mensagem para você no Badoo. Esta é uma mensagem de somente envio. Respostas a esta mensagem não são monitoradas ou respondidas. Se você não quer receber mais mensagens do Badoo, por favor nos notifique: http://us1.badoo.com/impersonation.phtml?lang_id=61&mail_code=21&email=hask…

1 0

Decoupling type classes (e.g. Applicative)?
by Dominique Devriese 09 Nov '10

09 Nov '10

Hi all, I have a problem with the design of the Applicative type class, and I'm interested to know people's opinion about this. Currently, the Functor and Applicative type class are defined like this: class Functor f where fmap :: (a -> b) -> f a -> f b class Functor f => Applicative f where pure :: a -> f a (<*>) :: f (a -> b) -> f a -> f b My problem is that in the grammar-combinators library [1], the pure combinator is too general for me. I would propose a hierarchy like the following: class Pointed f where pure :: a -> f a class ApplicativeC f where (<*>) :: f (a -> b) -> f a -> f b The original type class Applicative can then be recovered as follows, and the applicative laws can be specified informally in this class's definition. class (Pointed f, ApplicativeC f, Functor f) => Applicative f where This would allow me to restrict injected values to stuff I can lift into Template Haskell later on: class LiftablyPointed f where pureL :: (Lift a) -> a -> f a pureL' :: a -> Q Exp -> f a class (LiftablyPointed f, ApplicativeC) => LiftablyApplicative f where This problem currently makes it impossible for me to use the (<*>) combinator and I have to redefine it under a different name (I currently use (>>>)). To me the problem seems similar to the well known example of the inclusion of the fail primitive in the monad class, where the general opinion seems to be that it was a bad idea to include fail in the Monad class (see e.g. the article on the haskell wiki about handling failure [2]). I've been thinking about the following type class design principles: * Only include two functions in the same design class if both can be implemented in terms of each other. * Only introduce a dependency from type class A to type class B if all functions in type class B can be implemented in terms of the functions in type class A or if type class A is empty. (Disclaimer: I currently do not follow these principles myself ;)) I would like to know people's opinions about this. Are there any issues with this advice that I don't see? Have other people encountered similar problems? Any interesting references? Thanks, Dominique Footnotes: [1] http://projects.haskell.org/grammar-combinators/ [2] http://www.haskell.org/haskellwiki/Failure

7 7

new version of the mtl package
by Ross Paterson 09 Nov '10

09 Nov '10

After lengthy discussion on the libraries list, the mtl package has been updated to depend on the transformers package, from which mtl re-exports the monad transformers and the MonadTrans and MonadIO classes. This makes the two packages compatible. The monads-fd package is now a deprecated stub re-exporting the mtl modules. Ths transformers/mtl-2 combination is under consideration for inclusion in the Haskell Platform: http://trac.haskell.org/haskell-platform/wiki/Proposals/transformers Discussion is on the libraries list. So far there have been no objections. The new version has a few incompatibilities with the old, which will break a few packages, though the great majority build unchanged. The above web page lists the incompatibilities and suggested fixes. You might also consider whether the portable transformers package suffices for your package. As packages move to the new version, there will be increasing dependency clashes with packages that currently specify a bound on their mtl dependency excluding the new version. In some cases the dependency can be simply relaxed with no other change, or with small changes obtain compatibility with both versions. In other cases one must choose a version, or use cpp hackery to support both.

1 0

Mysterious fact
by Andrew Coppin 09 Nov '10

09 Nov '10

The other day, I accidentally came up with this: |{-# LANGUAGE RankNTypes #-} type Either x y= forall r. (x -> r) -> (y -> r) -> r left :: x -> Either x y left x f g= f x right :: y -> Either x y right y f g= g y | This is one example; it seems that just about any algebraic type can be encoded this way. I presume that somebody else has thought of this before. Does it have a name?

12 14

[issue1002] darcs support for ohloh: call for help
by Simon Michael 09 Nov '10

09 Nov '10

Many of us are interested, and Ohloh is interested, in getting ohloh.net to support darcs repositories directly. It doesn't look hard to get good-enough support working; it just requires hacking 10 or so simple, well-organised ruby files. I've made a start on github[1], and set up a collaborative how-to & notes page here: http://etherpad.osuosl.org/pCIn3uhOeW Please help! And forward this to appropriate places. Best, -Simon [1] will be merged with prior work as needed

1 3

Haskell meeting in Berlin on Tuesday Nov 9th
by Daniel van den Eijkel 09 Nov '10

09 Nov '10

Hi everybody, just wanted to send this again to the list - it's tomorrow. Cheers Daniel Am 10/31/10 6:30 PM, schrieb Sönke Hahn: > I can confirm the next meeting: > > Date: Tuesday, November 9th > Time: from 20:00 > Location: c-base, Rungestrasse 20, 10179 Berlin > > I was asked by some people from c-base, if we could do a talk about Haskell > for non-haskellers, which is a good idea, IMHO. We could discuss the details > in the mext meeting. > > Sönke >

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ANN: cil-0.1.0
by Tom Hawkins 08 Nov '10

08 Nov '10

CIL (C Intermediate Language) [1], not to be confused with the Common Intermediate Language, is a mature OCaml library that parses and reduces C programs down to a simplified subset of the C language, making it easier to analyze and compile C programs. This library [2, 3] parses these results, providing a Haskell interface to CIL. -Tom [1] http://cil.sourceforge.net/ [2] http://hackage.haskell.org/package/cil [3] http://github.com/tomahawkins/cil

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ANN: Copilot 0.25
by Lee Pike 08 Nov '10

08 Nov '10

Copilot[1] is a dataflow stream language built on Atom[2] for writing hard real-time C monitors of embedded systems. Copilot is a Haskell eDSL. The current release includes the following improvements: * Added true & false Specs (Spec Bool). * Removed generic const -- unneeded, since Spec is an instance of Num. * Change casting to only allow casts that (1) are guaranteed not to change the sign and (2) are to a larger type. * Added triggers---C functions to call when a monitor fails. * Removed libs from Copilot.hs. You must import these explicitly. * Fixed bugs with LTL and ptLTL libraries & examples and added documentation. * Added external arrays for sampling. * Fixed a bug in external array analysis for computing the minimum period size. * Added the ability to specify a HW clock for Atom. [1] <http://hackage.haskell.org/package/copilot> and <http://leepike.github.com/Copilot/> [2] <http://hackage.haskell.org/package/atom> Regards, Lee

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Re: internship opportunities in France
by Lee Pike 08 Nov '10

08 Nov '10

Hi, On a (possibly-related) note: Galois, Inc. and the National Institute of Aerospace (NIA) will likely be hosting a Visiting Scholar this summer for a joint NASA-sponsored project, located in Virginia, on the East coast of the U.S. (Since this particular opportunity is a visiting scholar position, we pay all travel/living expenses, but no salary, so one does not need a visa/work permit to work in the U.S.) We had two visiting scholars last summer, from Germany and France respectively, publishing a paper and even flying some aircraft <http://www.galois.com/blog/2010/09/22/copilot-a-dsl-for-monitoring-embedded…>. The project is a Haskell DSL project focused on monitoring embedded systems. Interested folks can contact Alwyn Goodloe (Cc'ed) at the NIA and myself, off the list. Regards, Lee > Hello folks ! > I am a Computer Science student looking for an internship of 6 months here > in France. Does anybody know of any company working with Haskell ? > Thanks in advance :D, > > Lorenzo Fundaró García

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How can I use MACID in my existing application?
by Dupont Corentin 08 Nov '10

08 Nov '10

Hello, I'm wondering how can I use Happstack's MACID in my application without breaking everything. I have a monad like that: type Comm = StateT Communication IO type GameState a = StateT Game Comm a and many functions like: foo :: GameState () foo = do lift $ putComm <some message to player's channel> modify <someAction> The state of the game is stored in Game. Comm is used as an abstraction to communicate over several channels with players. Whereas MACID asks to use: type *Update* state = Ev<http://happstack.com/docs/0.5.0/happstack-state/Happstack-State.html#t%3AEv>( StateT<http://happstack.com/docs/0.5.0/mtl/Control-Monad-State-Lazy.html#t%3AStateT>state STM <http://happstack.com/docs/0.5.0/base/GHC-Conc.html#t%3ASTM>) How can I use this without modifying everything?? I understand that MACID must record the <someAction> from above but the message should not. Thanks for help! Corentin

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"Best practices" for Text
by Ivan Lazar Miljenovic 08 Nov '10

08 Nov '10

With the recent discussions on the libraries mailing list about the API of Bryan O'Sullivan's text library, and the fact that I'm looking at using it soon for the first time, are there any recommended best practices or tips & tricks people have come up with? For starters, I'm considering writing a polyparse [1] instance for Text. However, even with the current Bytestring instances for polyparse there seems to be an emphasis on character-based parsing. Is that the correct way of doing things? For example, what would be the best way to try to parse a text value when you don't care about case? At the moment, with my String-based parser, I'm doing repeated comparisons on Char values [2,3]. [1]: http://hackage.haskell.org/package/polyparse [2]: http://hackage.haskell.org/packages/archive/graphviz/2999.10.0.1/doc/html/s… [3]: http://hackage.haskell.org/packages/archive/graphviz/2999.10.0.1/doc/html/s… -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANNOUNCE: seal-module
by Joachim Breitner 08 Nov '10

08 Nov '10

Hi Cafe, for those who are not following planet haskell: I uploaded a module providing a template haskell solution to the configuration problem to hackage: http://hackage.haskell.org/package/seal-module It turns the style you prefer to write in (pure code that uses configuration values as if they were global constants) into the style that is semantically correct (pure code with configuration values as an additional parameter). The module is well documented, so as soon as hackage has built the documentation you can read how it works on http://hackage.haskell.org/packages/archive/seal-module/0.1.0.1/doc/html/La… BTW: Is there a way to avoid having to indent a declarations passed to a top-level splice? Currently, one has to write: splice [d| some = declaration |] but splice [d| some = declaration |] would be nicer. Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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hmatrix's fitModel function crashes ghc(i)
by Vivian McPhail 08 Nov '10

08 Nov '10

> Message: 29 > Date: Sat, 6 Nov 2010 13:22:10 +0100 > From: Roel van Dijk <vandijk.roel(a)gmail.com> > Subject: [Haskell-cafe] hmatrix's fitModel function crashes ghc(i) > To: Haskell Caf? <haskell-cafe(a)haskell.org> > Message-ID: > <AANLkTim5eGSL_BZ+RUv-=d-Z3DB65Sc8o=cKQLTcyKT4(a)mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > Hello, > > I would like to use hmatrix to do some function fitting with the > Levenberg Marquardt algorithm. As an example I would like to fit the > very simple function "f x = a*x + b" on some data points. The problem > is that executing the 'fitModel' function crashes GHC(i) with a > segmentation fault. This makes debugging difficult. Can anyone spot > what I am doing wrong? Given all the lists of Double's it seems very > easy to make an error regarding the number of arguments with the model > function or the derivative. > > Try to evaluate the 'test' function in the small program listed below. > I would expect an output of [1, 0] (y = 1*x + 0) instead of a > segmentation fault. > > Relevant versions: > - hmatrix-0.10.0.0 > - gsl-1.14 > - ghc-6.12.3 (64 bit) > > Is that the 64 bit Linux ghc? I think the problem is with the GSL random number generation through GHCi. Try: > module Test where > > Import Numeric.Container(RandDist,randomVector) > > seed = 0 > size = 100 > > main = putStrLn $ show $ randomVector seed Gaussian size This should work when compiled with `ghc --make` and crash when invoked in `ghci`. I think the problem is with linking static data in GHCi on x86_64. Hope this helps. I seem to recall there might be a ghc trac ticket related to this but a quick search turned up nothing. Cheers, Vivian

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ANNOUNCE: arrow-list. List arrows for Haskell.
by Sebastiaan Visser 08 Nov '10

08 Nov '10

Hi all, Live from the Hackaton in Ghent, Belgium, I present the first release of the arrow-list[1,2] package. List arrows are a powerful tool when processing XML, building query languages and lots of other domains that build on functions that might return more than one value as their output. This package is inspired by the arrow combinators provided by the HXT package, but in my opinion list arrows deserve to be on Hackage on their own. Cheers, Sebastiaan [1] http://hackage.haskell.org/package/arrow-list [2] https://github.com/sebastiaanvisser/arrow-list ------------------------------------------------------------------------ (package description) List arrows for Haskell. This small Haskell library provides some type class, types and functions to work with list arrows. List arrows represent computations that may return multiple outputs. Making functions that return lists an instance of both the `Category` and `Arrow` type classes allow you to easily compose multiple computations into one with standard building blocks. This package provides: - A type class `ArrowList` for embedding functions that produce a list of outputs into _some_ list arrow. - A list of utility functions for working with list-arrows, these functions are based on the `ArrowList` type class so they are not tied one specific instance. - A concrete list arrow type that is implemented as a `Kleisli` arrow over the `ListT` list monad transformer. In short, you can both build pure list arrows and list arrows that produce tributary effects. - Not list arrow specific: A type class `ArrowKleisli` for embedding monadic computations into an arrow.

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hmatrix's fitModel function crashes ghc(i)
by Roel van Dijk 08 Nov '10

08 Nov '10

Hello, I would like to use hmatrix to do some function fitting with the Levenberg Marquardt algorithm. As an example I would like to fit the very simple function "f x = a*x + b" on some data points. The problem is that executing the 'fitModel' function crashes GHC(i) with a segmentation fault. This makes debugging difficult. Can anyone spot what I am doing wrong? Given all the lists of Double's it seems very easy to make an error regarding the number of arguments with the model function or the derivative. Try to evaluate the 'test' function in the small program listed below. I would expect an output of [1, 0] (y = 1*x + 0) instead of a segmentation fault. Relevant versions: - hmatrix-0.10.0.0 - gsl-1.14 - ghc-6.12.3 (64 bit) Small program: module Test where -- from base: import Control.Arrow ( second ) import Control.Applicative ( pure ) -- from hmatrix: import Data.Packed.Matrix ( Matrix ) import Numeric.GSL.Fitting ( FittingMethod(LevenbergMarquardt), fitModel ) -- input list of (x, y) pairs, output coefficients of "f x = a x * b" fitLinear :: [(Double, Double)] -> ([Double], Matrix Double) fitLinear samples = fitModel 1 1 10 (linearModel, linearDer) (map (second pure) samples) [0, 0] linearModel :: [Double] -> Double -> [Double] linearModel [a, b] x = [a*x + b, 0] linearModel _ x = error "wrong arguments" linearDer :: [Double] -> Double -> [[Double]] linearDer [_, _] x = [[x, 0]] linearDer _ _ = error "wrong arguments" test = fitLinear [(0,0), (1,1), (2,2)]

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Fwd: [Haskell-cafe] DSL libraries
by Dupont Corentin 07 Nov '10

07 Nov '10

Nobody had the compilation messages I had? ---------- Forwarded message ---------- From: Dupont Corentin <corentin.dupont(a)gmail.com> Date: Tue, Nov 2, 2010 at 2:30 PM Subject: [Haskell-cafe] DSL libraries (Was: Map constructor in a DSL) To: steffen <steffen.siering(a)googlemail.com>, haskell-cafe(a)haskell.org Hello Steffen, can you compile AwesomePrelude? I've got error messages (see below). By the way, I've looked at some DSLs made in Haskell, if I don't mistake there are lots of similarities between them. There similarities could be put in a library to help the implementors of a DSL, more or less like AwesomePrelude. Is there already packages on Hackage dealing with that? Cheers, Corentin AwesomePrelude compilation error: I'm using GHC 6.12.1. > cd tomlokhorst-AwesomePrelude-9819315 > cabal install (...) Warning: Lang.Haskell: Instances of type and data families are not yet supported. Instances of the following families will be filtered out: H Then, when trying with a very simple GATD in GHCI, I've got: <interactive>: HSAwesomePrelude-0.1.0.o: unknown symbol `AwesomePreludezm0zi1zi0_ CompilerziLiftDefinitions_inline_closure' ghc: unable to load package `AwesomePrelude-0.1.0' On Thu, Oct 28, 2010 at 2:02 PM, steffen <steffen.siering(a)googlemail.com>wrote: > I think you would love to have a look at AwesomePrelude[1] or a fork > of AwesomePrelude using associated types[2] > Some more background information by Tom Lokhorst [3][4]. > > [1] http://github.com/tomlokhorst/AwesomePrelude > [2] http://github.com/urso/AwesomePrelude > [3] http://tom.lokhorst.eu/2009/09/deeply-embedded-dsls > [4] http://tom.lokhorst.eu/2010/02/awesomeprelude-presentation-video > > On 28 Okt., 12:09, Dupont Corentin <corentin.dup...(a)gmail.com> wrote: > > Thank you for your rich responses. > > > > Indeed I think I miss some thinks in my DSL, that would make things > easier > > to deal with lists and first class functions. > > I don't really know what for now. > > Perhaps a List Constructor? Or a constructor on functions like yours > Ryan? > > EAp :: Exp ref (a -> b) -> Exp ref a -> Exp ref b > > It's from which DSL? It is accessible on the net? > > > > Chris suggested me that I can only define the Foldr constructor and > deduce > > Map from it. > > But maybe I have to add a List constructor for that. > > > > But in the suggestions from Ryan and Brandon I don't understand why I > should > > add an extra type parameter and what it is! > > > > Steffen: Wow nice. I'll integrate that ;) > > > > I'm also looking at the Atom's DSL to get inspiration. > > Something I don't understand in it is that it has two languages, on > typed: > > > > data E a where > > VRef :: V a -> E a > > Const :: a -> E a > > Cast :: (NumE a, NumE b) => E a -> E b > > Add :: NumE a => E a -> E a -> E a > > etc. > > > > And, along with it, an untyped counterpart: > > > > -- | An untyped term. > > data UE > > = UVRef UV > > | UConst Const > > | UCast Type UE > > | UAdd UE UE > > etc. > > > > What that for? What's the use of having beautiful GADT if you have to > > maintain an untyped ADT aside?? > > > > Corentin > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-C...@haskell.orghttp:// > www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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ANNOUNCE: language-cil
by Tom Lokhorst 07 Nov '10

07 Nov '10

Hi all, I've just uploaded language-cil [1], a library for manipulating a Common Intermediate Language (CIL) [2]. CIL is the lowest level language used by Microsoft .NET and Mono on Mac an Linux. With language-cil, you can construct an AST and pretty print it to concrete syntax. The assembler `ilasm`, can create a .exe file which can be run by Mono or .NET. The package is not finished, it only supports a subset of the full language, but it's already usable. For more information, as well as an example to quickly get started, see the github page [3]. Cheers, Tom Lokhorst [1]: http://hackage.haskell.org/package/language-cil [2]: http://en.wikipedia.org/wiki/Common_Intermediate_Language [3]: https://github.com/tomlokhorst/language-cil#readme

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Review request for my baby steps towards a "platform independent interactive graphics" using VNC
by C K Kashyap 07 Nov '10

07 Nov '10

Hi, I started with the implementation of a VNC server library intended to be used as a library for rendering graphics and interacting with the user(mouse/keyboard). I'd appreciate it very much if I could get some feedback on my approach to binary parsing and Haskellism. Also, any reference/suggestion on how I could go about using a state machine to deal with the RFB protocol. http://hpaste.org/41131/vnc_server It's really early - but just wanted to get some advice on the approach. -- Regards, Kashyap

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internship opportunities in France
by Lorenzo Fundaró 07 Nov '10

07 Nov '10

Hello folks ! I am a Computer Science student looking for an internship of 6 months here in France. Does anybody know of any company working with Haskell ? Thanks in advance :D, Lorenzo Fundaró García

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