May 2009 - Haskell-Cafe

HDBC Oracle bindings(?/!)
by Thiago Arrais 15 Sep '09

15 Sep '09

Are there Oracle bindings for HDBC anywhere? The HDBC project page talks about Oracle backends but there isn't an Oracle-specific driver listed (I assume one can reach an Oracle backend through the ODBC interface?). I started writing Oracle bindings for HDBC based on the Takusen code. It was done mostly for playing than for anything else and certainly isn't production ready, but it is available in a darcs repo at http://thiagoarrais.com/repos/hdbc-oracle in case anyone is interested. This code isn't ready for prime time yet and I'm looking for some that *is*. Is there such a thing already? Cheers, Thiago Arrais -- Enjoy Chaos - http://blog.thiagoarrais.com Thoughts, ideas and useless ranting about software development and technology

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[ANNOUNCE] hgettext 0.1.10 - last major release
by Vasyl Pasternak 10 Aug '09

10 Aug '09

Hello, I've uploaded last (and latest) significant version on hgettext module. Currently it works fine, and has bindings to all gettext functions (from libintl.h). Next versions will be only bug fixes of this version. GNU GetText allows you only to create simple, standalone desktop applications for Linux, to make something complex you should use different trick, fortunately Haskell could do these tricks much easier than other languages (if you want to see, how to build Gtk application, which supports run time language switching, and shows information on two languages simultaneously you could download example from http://hgettext.googlecode.com/files/gtk-hello-0.0.2.tar.gz , or read about it from http://progandprog.blogspot.com/2009/04/multilingual-ui-and-dynamic-languag…) I don't see any strong reasons to write any combinators over this basic bindings. Haskell needs more powerful internationalization library, and I am plan to design it, but it will be completely different from gettext principles, so this library will be released with another name. CHANGELOG: * Added '--version' command line parameter to hgettext tool * Added support for literate haskell files to hgettext tool * Changed all functions to throw IOError when they fails instead of return Nothing * Added bindings for: ngettext, dgettext, dngettext, dcgettext and dcngettext functions -- Best regards, Vasyl Pasternak

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ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

Dear friends of Haskell and Functional Reactive Programming, its my pleasure to announce the first official release of Grapefruit, a library for Functional Reactive Programming (FRP) with a focus on user interfaces. With Grapefruit, you can implement reactive and interactive systems in a declarative style. User interfaces are described as networks of communicating widgets and windows. Communication is done via different kinds of signals which describe temporal behavior. Grapefruit consists of five packages with several interesting features: grapefruit-frp: Functional Reactive Programming core * The FRP implementation uses a hybrid push/pull-based approach. * Signals can be memoized by binding them to variables – like ordinary data structures. * Merging of event streams combines simultaneous events properly. * Signals cannot behave differently by starting them at different times. The type system is used to enforce proper aging of signals. grapefruit-records: A record system for Functional Reactive Programming * Input records can specify fields as being required or optional. * Uninteresting output fields can be ignored. The set of interesting fields is specified by pattern matching. grapefruit-ui: Declarative user interface programming * The interface for UI programming is platform-independent and can be implemented on top of different UI toolkits. * A concrete implementation can be selected at compile time or runtime. grapefruit-ui-gtk: GTK+-based implementation of the general UI interface * Gtk2Hs is used internally. grapefruit-examples: examples demonstrating features of Grapefruit * Circular communication and switching are demonstrated. Support for the following is planned for the future: * signals with incremental updates * user interfaces with changing structure * more kinds of UI components * graphical animations Further information about Grapefruit is available on its wiki page at <http://haskell.org/haskellwiki/Grapefruit>. If you have questions, applause or criticism, please get in touch with me. Wolfgang Jeltsch Principal Grapefruit developer

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Re: [Haskell] ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

[redirecting to haskell-cafe] Am Sonntag, 15. Februar 2009 00:25 schrieben Sie: > Hi Wolfgang, > > I was wondering if I can use FLTK as GUI backend for Grapefruit? This should be possible in principal. It just could be that my assumptions about how widgets are created and composed were too tight so that Grapefruit’s general interface doesn’t fit FLTK. In this case, please just tell me and I will try to make the interface more general. > I believe for this to make it happen, I would have to output FLTK's C++ > into C then create bindings for Haskell (via FFI). Is that doable or an > quite tall order? Recently, a student of mine has written a program which generates a Haskell Qt binding fully automatically from Qt header files. The generated binding consists of three layers. The first layer is C++ code which reexports Qt’s functionality as a pure C interface. The C interface is ugly for humans and not type safe (because C doesn’t know classes). The second layer consists of a couple of FFI declarations. The third layer is Haskell code which provides a nice interface similar to the original C++ interface. I still have to get the source code of the binding generator from that student but I hope this will happen soon. I want to publish it then on the web. It hope that it is possible to reuse this binding generator for other C++ libraries. > Jamie Clark Best wishes, Wolfgang

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[Haskell] ANNOUNCE: ChristmasTree 0.1 (excuses for second attempt)
by S.Doaitse Swierstra 07 Jul '09

07 Jul '09

We are pleased to announce the availability of the package "ChristmasTree", which contains the code associated with our paper at the last Haskell symposium: @inproceedings{1411296, author = {Marcos Viera and S. Doaitse Swierstra and Eelco Lempsink}, title = {Haskell, do you read me?: constructing and composing efficient top-down parsers at runtime}, booktitle = {Haskell '08: Proceedings of the first ACM SIGPLAN symposium on Haskell}, year = {2008}, isbn = {978-1-60558-064-7}, pages = {63--74}, location = {Victoria, BC, Canada}, doi = {http://doi.acm.org/10.1145/1411286.1411296}, publisher = {ACM}, address = {New York, NY, USA}, } The name of the package stands for: "Changing Haskell's Read Implementation Such That by Manipulating Abstract Syntax Trees it Reads Expressions Efficiently" which, given the time of year, seems appropriate. Feel free to download and unpack this "present" at what for the Dutch is called "Sinterklaasavond" (http://en.wikipedia.org/wiki/Sinterklaas) Arthur Baars Marcos Viera Eelco Lempsink Doaitse Swierstra PS: the package uses our library supporting transformation of typed abstract syntax, which we placed in a separate package TTTAS

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Why is Bool no instance of Num and Bits?
by Stephan Friedrichs 26 Jun '09

26 Jun '09

Hi! When looking for an xor function, I found one in Data.Bits but couldn't use it for Bool, because Bool is no instance of Bits and of Num (which would be necessary, because it's "class (Num b) => Bits b"). My question is: Why not? We could declare instance Num Bool where (+) False = id (+) True = not (*) True True = True (*) _ _ = False (-) = (+) negate = id abs = id signum = const True fromInteger = not . even which basically implements the field with 2 elements and instance Bits Bool where bitSize = const 1 isSigned = const False (.&.) = (&&) (.|.) = (||) xor = (+) complement = not shift = const shiftL = const shiftR = const rotate = const rotateL = const rotateR = const bit = (==0) setBit _ 0 = True setBit b _ = b clearBit _ 0 = False clearBit b _ = b complementBit b 0 = not b complementBit b _ = b testBit b 0 = b testBit _ _ = False quite trivial... Why is this not part of base? Or am I missing something? //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Haskell EDSL to generate SQL?
by Günther Schmidt 14 Jun '09

14 Jun '09

Hi, I tried to solve some large data processing solely in Haskell so I could avoid lots of eventually very long and complex SQL statements. Unfortunately, as was to be expected, that approach doesn't scale. So I do need an SQL backend. But I hope to be able to use an DSL from which I can automatically generate SQL-Strings instead of writing the SQL statements literally. Has anyone else taken a similar approach? Günther

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I love purity, but it's killing me.
by Tom Hawkins 09 Jun '09

09 Jun '09

I've been programming with Haskell for a few years and love it. One of my favorite applications of Haskell is using for domain specific languages. However, after designing a handful of DSLs, I continue to hit what appears to be a fundamental hurdle -- or at least I have yet to find an adequate solution. My DSLs invariably define a datatype to capture expressions; something like this: data Expression = Add Expression Expression | Sub Expression Expression | Variable String | Constant Int deriving Eq Using the datatype Expression, it is easy to mass a collections of functions to help assemble complex expressions, which leads to very concise programs in the DSL. The problem comes when I want to generate efficient code from an Expression (ie. to C or some other target language). The method I use invovles converting the tree of subexpressions into an acyclic graphic to eliminate common subexpressions. The nodes are then topologically ordered and assigned an instruction, or statement for each node. For example: let a = Add (Constant 10) (Variable "i1") b = Sub (Variable "i2") (Constant 2) c = Add a b would compile to a C program that may look like this: a = 10 + i1; b = i2 - 2; c = a + b; The process of converting an expression tree to a graph uses either Eq or Ord (either derived or a custom instance) to search and build a set of unique nodes to be ordered for execution. In this case "a", then "b", then "c". The problem is expressions often have shared, equivalent subnodes, which dramatically grows the size of the tree. For example: let d = Add c c e = Add d d -- "e" now as 16 leaf nodes. As these trees grow in size, the equality comparison in graph construction quickly becomes the bottleneck for DSL compilation. What's worse, the phase transition from tractable to intractable is very sharp. In one of my DSL programs, I made a seemingly small change, and compilation time went from milliseconds to not-in-a-million-years. Prior to Haskell, I wrote a few DSLs in OCaml. I didn't have this problem in OCaml because each "let" expression was mutable, and I could use the physical equality operator to perform fast comparisons. Unfortunately, I have grown to love Haskell's type system and its lack of side effects, and could never go back. Is there anything that can be done to dramatically speed up comparisons, or is there a better approach I can take to extract common subexpressions? I should point out I have an opportunity to get Haskell on a real industrial application. But if I can't solve this problem, I may have to resort to far less eloquent languages. :-( Thanks for any and all help. -Tom

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ANN: new version of uu-parsinglib
by S. Doaitse Swierstra 03 Jun '09

03 Jun '09

A new version of the uu-parsinglib has been uploaded to hackage. It is now based on Control.Applicative where possible. Be warned that functions like some and many will be redefined in the future. Doaitse Swierstra

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Linkage errors in scenegraph
by Gregory D. Weber 03 Jun '09

03 Jun '09

I'd like to get the scenegraph package (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/scenegraph) to work, but am encountering linkage errors. I installed scenegraph 0.1.0.1 for ghc-6.10.1 on Fedora 10 Linux. # runghc Setup configure --prefix=/opt # runghc Setup build # runghc Setup install Then I created this little program based on the example at http://www.haskell.org/haskellwiki/SceneGraph (in file SGTest.hs): module Main where import Graphics.SceneGraph import Control.Monad.Identity main :: IO () main = runScene $ osgt $ myScene myScene :: OSGT Identity SceneNode myScene = cube 0.5 `colour` Green `scale` v1x 40 `translate` vy 2 <+> camera `translate` vy (-10) <+> light `translate` vz 10 Then I compiled it and got a long list of linker errors; here are the first few lines: ghc --make SGTest Linking SGTest ... /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__210.o): In function `sdE1_info': (.text+0x8d): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__210.o): In function `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziBasic_translate_srt': (.data+0x0): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__207.o): In function `sdDq_info': (.text+0x7e): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_scaleM_info' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__207.o): In function `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziBasic_scale_srt': (.data+0x0): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_scaleM_closure' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__491.o):(.text+0x27): undefined reference to `__stginit_scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziUtils_' Also with this command, same result but a lot more detail: ghc --make -v -package scenegraph SGTest With -v, the output includes gcc -v -o SGTest -DDONT_WANT_WIN32_DLL_SUPPORT SGTest.o -L/opt/lib/scenegraph-0.1.0.1/ghc-6.10.1 -L/opt/lib/hmatrix-0.5.1.1/ghc-6.10.1 -L/opt/lib/storable-complex-0.2/ghc-6.10.1 ... -lHSscenegraph-0.1.0.1 -lHShmatrix-0.5.1.1 -lgsl -llapack -lHSstorable-complex-0.2 -lHSQuickCheck-1.2.0.0 ... which seems to show that the proper -L and -l options are included. If I'm decoding the first "undefined reference" symbol -- (.text+0x8d): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure' -- correctly, then the missing symbol is in the scenegraph-0.1.0.1 package, in module Graphics.SceneGraph.Matrix, and is associated with the translateM function defined in that module. I looked for this symbol with nm: # pwd /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1 # nm HSscenegraph-0.1.0.1.o | grep translateM U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_info # nm libHSscenegraph-0.1.0.1.a | grep translateM U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_info but I understand the "U" means that it is referenced by some other function in these files, but is undefined in them. Since the module Graphics.SceneGraph.Matrix does not declare any exports, I thought that might be the reason the symbol was undefined. But looking back at the Haskell 98 language definition, I found that the absence of an export declaration in a module means that all the symbols of the module are exported. Also, I notice that in the cabal file for scenegraph, the list of exposed modules Exposed-Modules: Graphics.SceneGraph, Graphics.SceneGraph.Basic, Graphics.SceneGraph.Vector, Graphics.SceneGraph.Render, Graphics.SceneGraph.SimpleViewport, Graphics.SceneGraph.GraphViz, Graphics.SceneGraph.Library, Graphics.SceneGraph.Dump, Graphics.SceneGraph.Textures does not include Graphics.SceneGraph.Matrix, but that should only mean that I can't call functions of that module directly -- not that the other SceneGraph modules can't call them -- right? So, why is this symbol undefined? And how can I get it to work? Oh by the way: I can actually use this package if I move into its src directory, add a main function to Examples.hs, and compile and run it right there. Just can't use it as an installed library, it seems. -- ___ ___ __ _ / _ \ / _ \| | | | Gregory D. Weber, Associate Professor / /_\// / | | | /\ | | Indiana University East / /_\\/ /__| | |/ \| | http://mypage.iu.edu/~gdweber/ \____/\_____/\___/\__/ Tel. (765) 973-8420; FAX (765) 973-8550

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