June 2009 - Haskell-Cafe

Breaking ghci
by Günther Schmidt 30 Jun '09

30 Jun '09

Hi folks, I would like to announce that I have not merely managed to make the RTS choke during runtime on stack overflows like lesser programmers, no, *I* have managed to write code that ghc is not even able to compile due to exhausting virtual memory! Top that! Günther

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Re: BerkeleyDBXML linking problems.
by Sebastiaan Visser 30 Jun '09

30 Jun '09

(crosspost from haskell-libraries) Hello all, again. Meanwhile I've been able to install this package without too much trouble on a Linux system. Unfortunately MacOSX is still a problem. Both on Linux and Mac I installed the Berkeley DB and DBXML packages with `--prefix=/usr' so all include files and libraries end up in a standard place. Also both on Linux and Mac I installed the Haskell BerkeleyDB bindings just with: cabal install BerkeleyDB Because the DBXML include files are installed under /usr/include/dbxml/ *.hpp and the BerkeleyDBXML binding accesses them without this prefix I did: cabal install BerkeleyDBXML --extra-include-dirs=/usr/include/dbxml On both system installation is not a problem. Compiling the demo `adventure' included in de bindings distribution works fine on Linux but fails on MacOSX. The errors are the ones in my previous mail below. A full hpaste of it can be found here[1]. Any idea where the difference in behaviour comes from on these platforms and how to fix this? Unfortunately, my knowledge of the c++ toolchain and Haskell FFI bindings using these is _very_ limited. Thanks, Sebastiaan [1] http://www.hpaste.org/fastcgi/hpaste.fcgi/view?id=6389#a6389 On Jun 26, 2009, at 12:56 PM, Sebastiaan Visser wrote: > Hello all, > > After manually specifying where to find the appropriate include and > library directories for the dbxml distribution, I succeeded to > install both the BerkeleyDB and the BerkeleyDBXML Haskell bindings. > They both nicely show up in `ghc-pkg list'. > > But when I try to compile my own test or the demo program included > in the BerkeleyDBXML package it will kindly present me a dozen or so > pages of linking errors. The top lines are: > > Undefined symbols: > "std::basic_string<char, std::char_traits<char>, > std::allocator<char> >::_Rep::_M_destroy(std::allocator<char> > const&)", referenced from: > __xmlDocument_setContent in > libHSBerkeleyDBXML-0.6.2.a(dbxml_helper.o) > ... > > Do I have to tell GHC where to find the original dbxml libraries > when compiling my BerkeleyDBXML aware Haskell programs? > > Small system overview: > - Mac OS X 10.5.7 > - GHC 6.10.3 > - BerkeleyDB 0.7.1 > - BerkeleyDBXML 0.6.2 > - gcc/g++ 4.0.1 > > Any clue on how to fix this? > > Thanks, > > -- > Sebastiaan Visser

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coding standard question
by Vasili I. Galchin 30 Jun '09

30 Jun '09

Hello, I am working with some existing code. "where/let" functions use the same name for function parameters as the outer function and hence there is a "shadow" warning from the compiler. To me it doesn't see totally unreasonable to code like this .... the downside is the nasty ghc warnings. Is there a coding consensus on this issue? Vasili

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Wondering about c type in haskell.
by Magicloud Magiclouds 30 Jun '09

30 Jun '09

Hi, There are times, when we need to make some system calls, or call C library. So we have to deal with C data types. For example, CPid, which is an integer, actually. So we can do "fromIntegral pid". Then why do not we define "type CPid = Integer", and convert Haskell Integer with C Int internally. So the "user" does not have to care about the type convert (everywhere, which is ugly). And, specially, when doing something like serialisation, for Haskell Integer, the user does not have to do things with precision. But for CPid, without the fromIntegral, we have to specify its precision, well, on different machine/OS, the precision may not be the same. -- 竹密岂妨流水过山高哪阻野云飞

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Making a strict (non-lazy) GCL Interpreter
by Hector Guilarte 30 Jun '09

30 Jun '09

Hi everyone! (First of all, I don't know Monads!) I made a GCL (Guarded Command Language) Compiler and Interpreter for my Languages and Machines course in my University with alex, happy and ghc. I asked about a week ago about the use of unsafePerformIO for that same project, now I have two new doubts: 1) When more than a Condition is true in a if cond1 -> instruction1 | cond2 -> instruction2 ... | condN -> instructionN fi I need to randomly select ONE of the valid conditions and execute it's instruction. I know there is a Random Monad, but it returns an IO Int, Is there anyway I can do some Random that doesn't involve IO? or any other solution? That part of the code is: (My code is in Spanish. TablaOutput is a type and it stands for the tupe of the Symbol Table (Data.Map) and the Output [String], obtenerValidos checks wich pair of (condition,instruction) are true and makes a list out of them, ejecutarUnoRand is the Instruction that executes one of the instructions randomly) evalSeleccion:: [(BoolY,Instruccion)] -> TablaOutput -> TablaOutput evalSeleccion lista tabla = let validos = obtenerValidos lista [] tabla in if (isEmpty validos) then error "Las guardias del If estan incompletas" else ejecutarUnoRand validos tabla obtenerValidos:: [(BoolY,Instruccion)] -> [(BoolY,Instruccion)] -> TablaOutput -> [(BoolY,Instruccion)] obtenerValidos [] validos tabla = validos obtenerValidos ((boolY,instruccion):bloqueIns) validos tabla = if (evalBoolY boolY (fst tabla)) then obtenerValidos bloqueIns (validos ++ [(boolY,instruccion)]) tabla else obtenerValidos bloqueIns validos tabla ejecutarUnoRand:: [(BoolY,Instruccion)] -> TablaOutput -> TablaOutput ejecutarUnoRand validos tabla = let rand = 0 --getStdRandom (randomR (0,(length validos))) HERE'S WHERE I'M SUPPOSED TO DO RANDOM, RIGHT NOW I'M JUST CHOSING THE FIRST in evalInstruccion (snd (validos !! rand)) tabla 2) Since Haskell is Lazy, and my GCL program is being interpreted in Haskell then my GCL is Lazy too (I know is not as simple as that but believe me, somehow it is behaving lazy). The problem is that it can't be lazy (said to me by my teacher today) so I tried using seq, I'll paste the code after this: Programs in GCL like: a) var i : value main i <- 1 / 0 end b) var i : value main i <- 1 / 0; show i end c) var i : value var foo : array of 2 main i <- foo[42] end d) var i : value main i <- foo[42]; show i end acts like this: a and c finish interpretation b throws division by zero error and finish interpretation d throws index out of bounds error and finish interpretation Now the code: (again, it is in Spanish. ListLValue is a List of L-Values for the assigments, ListExpr is the list of Expressions to be assigned, Tabla is the Symbol Table (Data.Map), actualizarVar updates a Variable in the Symbol Table with the new value "valor", ActualizarArray updates the position "indice" of an array in the Symbol Table. evalExpr evaluates an arithmetic Expression and returns an Int. Inside evalExpr are the verifications for division by zero of modulo by zero.) evalAsignacion:: ListLvalue -> ListExpr -> Tabla -> Tabla evalAsignacion [] [] tabla = tabla evalAsignacion ((Lid id):valueList) (expr:exprList) tabla = let valor = (evalExpr expr tabla) in valor `seq` evalAsignacion valueList exprList (actualizarVar id valor tabla) evalAsignacion ((LArrayPosition id exprArray):valueList) (expr:exprList) tabla = let valor = (evalExpr expr tabla) indice = (evalExpr exprArray tabla) in valor `seq` indice `seq` evalAsignacion valueList exprList (actualizarArray id indice valor tabla) evalExpr:: Expr -> Tabla -> Int evalExpr expr tabla = let salida = (snd (evalAritmetico expr tabla)) in salida `seq` if (isLeft salida) then error (getLeft salida) else getRight salida --((Int,Int) is the Line and Colum, that's for error reporting) evalAritmetico :: Expr -> Tabla -> ((Int,Int),(Either String Int)) --LET ME KNOW IF YOU NEED THIS PART TOO Thanks in advance, Hector Guilarte

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[: Where the bracket things are? :]
by Paul Keir 30 Jun '09

30 Jun '09

I'd like to add my own custom list delimiters to ghc; such as the [: and :] of Data Parallel Haskell. The purpose is mainly to learn a little about GHC's internals. Any suggestions on the GHC files I should look at first? Alternatively, maybe this is actually possible from outside the compiler. Cheers, Paul

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What's in a text API?
by Bryan O'Sullivan 30 Jun '09

30 Jun '09

I don't normally post blog links here, but this one is near and dear to my heart: what can we do to make Haskell a more hospitable language for text processing? http://www.serpentine.com/blog/2009/06/30/python-and-haskell-text-apis-comp… If you have comments or suggestions, please feel welcome to respond via the blog's comments or on this list. I'll be following both.

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Reflections on Haskell and ICFP Contest 2009
by Rafael Gustavo da Cunha Pereira Pinto 29 Jun '09

29 Jun '09

Hi All, As the ICFP 2009 contest (http://www.icfpcontest.org) approaches its end, I decided to write down some thoughts I had while trying. On Friday I downloaded the task (http://www.icfpcontest.org/task-1.9.pdf) read it for a while, and, since my goal is to finish by Xmas, went to the swimming pool. :-D While swimming, I started to think on the problem, and realized that the task is almost too easy for Haskell. Actually the task was so easy, that they added a final, non-scoring, task for those who have finished early. This year's task is to build a VM that runs a (simple) program that simulates the physics of a satellite. The VM code has no branches, acting like a huge pocket calculator. The mission is to create a simulator that interact with this VM and maneuver the sattelite(s) on predefined patterns. Here is how it went: 1) Reading the orbiter data file: The orbiter data file is a stream of 32-bit unsigned integers, which are commands, and 64-bit double precision IEEE-754 floats, which are data, forming a 96-bit frame. For this part I decided to use the Data.Binary module from hackage. The module is great, and it is very simple to use. But that took me 3 hours to learn how to use it (which, with my "honey-do" list ended up to be in Sunday morning) By Saturday night I was cursing "The Binary Strike Force" because a) get :: Word32 reads it in little-endian b) get :: Double reads the result of an decodeFloat, which has no resamblance at all with IEEE-754 standards c) I spent the entire Saturday (2 hours, honey-do excluded) thinking I was doing something wrong!! I was really pissed of, until I found "getWord32le" and "getWord64le", which solved 80% of the problem. The final blow was to decode the Word64 encoded IEEE-754 float into a Haskell float, which took me another honey-do-free hour. First step solved. (BTW: I am attaching the first version to this e-mail. I will upload to Hackage upon completion, when getIEEE754float64le and putIEEE754float64le functions are done!) 2) Running the VM: This is the step I am working on. A really good and obvious approach to run the VM is to use a RWS monad, since: a) I have a fixed environment (the program and the input ports) b) I have a state (the program counter, the status register and the data memory contents) c) I have a monoid output (the output ports) This is my recurrent complaint (a rant, maybe): this really important monad is completely undocumented, left for the developer to figure out how to use it. Even each of the singular monads (Reader, Writer and State) is poorly documented. I will do some toy programs to understand each one. Let's hope I can create a hands-on tutorial on the RWS monad. 3) Controlling the sattelites This is going to be a piece of cake (aheeem! Just kidding!!). After computing the VM iteration, another computation will take the output data time series and estimate the next move. It will be a piece of cake, considering I would have learned enough of Haskell state monads on step 2 to have just to worry about the physics of the problem!!! I'll keep you updated as my tests go, and hopefully I will have a working demo of the task written in Haskell by Xmas... 2010! ;-) Best regards, Rafael Gustavo da Cunha Pereira Pinto

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INAP 2009: DEADLINE EXTENSION and Final Call for Papers
by Pedro Salgueiro 29 Jun '09

29 Jun '09

------------------------------------------------------------------- (PLEASE DISTRIBUTE -- **DEADLINE EXTENDED** -- PLEASE DISTRIBUTE) ------------------------------------------------------------------- Final Call for Papers INAP 2009 18th International Conference on Applications of Declarative Programming and Knowledge Management November 5-7, 2009 Evora, Portugal http://www.di.uevora.pt/inap2009/ http://inap.dialogengines.com/ ------------------------------------------------------------ Organized by the Portuguese AI Society (APPIA), the INAP Committee and the Society of Logic Programming (GLP e.V.) == Overview == Declarative Programming is a family of advanced paradigms for the modeling and solving of complex problems. These specification and implementation methods have attracted more and more attention over the past years, e.g. in the domains of databases and natural language processing, for modeling and the processing of combinatorial problems, and for establishing systems for the web. == INAP 2009 == INAP is a communicative and dense forum for intensive discussion of applications of important technologies related to Prolog, Logic and Constraint Programming as well as closely related advanced software. It comprehensively covers the impact of programmable logic solvers in the Internet Society, its underlying technologies, and leading edge applications in industry, commerce, government, and societal services. INAP 2009 continues a tradition of successful workshops cast around the applications of declarative programming, which were held in Kobe (1997), Tokyo (1995, 1996, 1998 - 2001), Potsdam (2004), Fukuoka (2005) and Wuerzburg (2007). We invite the submission of high quality papers on the described topics, especially, but not exclusively, on different aspects of Declarative Programming, Constraint Processing and Knowledge Management as well as their use for Distributed Systems and the Web: - Knowledge Management, e.g. Data Mining, Decision Support, Deductive Databases - Distributed Systems and the Web, e.g. Agents and Concurrent Engineering, Semantic Web - Constraints, e.g. Constraint Systems, Extensions of Constraint (Logic) Programming - Theoretical Foundations, e.g. Deductive Databases, Nonmonotonic Reasoning - Systems and Tools for Academic and Industrial Use - Knowledge-based Web Services - Logic Solvers and Applications == Workshop Format == The technical program of the workshop will include invited presentations (to be announced), regular technical sessions with presentations of the accepted papers, system demonstrations and a panel discussion. == Conference Venue == The conference will be held at the University of Evora, Portugal in November 5-7, 2009. Evora is a nice and quiet historical city located in the south of Portugal that can be reached from Lisbon by train or coach in under 2 hours. It is a small city of 60.000 inhabitants, 120 km inland from Lisbon and classified by Unesco as World Heritage. The University of Evora was established in the 16th Century and is the 2nd oldest Portuguese University. The social program is promising since the region is very rich in historical sites (Stone Age, Roman, Medieval and Renaissance remains) and also offers a very special gastronomy. See http://en.wikipedia.org/wiki/%C3%89vora for more information. == Important Dates == Paper Submission Deadline: July 13, 2009 (EXTENDED!) Notifications to Authors: August 17, 2009 Camera-ready Version Deadline: September 14, 2009 INAP 2009 Workshop: November 5-7, 2009 == Submission Guidelines == Participants should submit a paper (maximum 15 pages, PDF format), describing their work in topics relevant to the workshop. Accepted papers will be presented during the workshop. At least one author of an accepted contribution is expected to register for the workshop, and present the paper. All submissions should include the author's name(s), affiliation, complete mailing address, and email address. Authors are requested to prepare their submissions, following the LNCS/LNAI Springer format. Please see: http://www.springer.de/comp/lncs/authors.html for further details. The submission should be submitted through the electronic submission site: http://www.easychair.org/conferences/?conf=inap2009 The deadline for receipt of submissions is July 13, 2009. Papers received after this date will not be reviewed. Eligible papers will be peer-reviewed by at least three members of the Program Committee. Authors will be notified via email of the results by August 17, 2009. Authors of accepted papers are expected to improve their paper based on reviewers' comments and to send a camera ready version of their manuscripts by September 14, 2009. Accepted papers will be included in the workshop proceedings, which will be distributed to the participants. As in previous editions, we plan to publish selected papers in a proceedings volume in the Springer Lecture Notes in Artificial Intelligence (LNAI) series. == Organizing Committee == Vitor Nogueira vbn AT di.uevora.pt Salvador Abreu spa AT di.uevora.pt Pedro Salgueiro pds AT di.uevora.pt Universidade de Evora Portugal == Program Committee == Salvador Abreu, University of Evora, Portugal (co-chair) Sergio Alvarez, Boston College, USA Philippe Codognet, CNRS/JFLI, Tokyo, Japan Vitor Santos Costa, University of Porto, Portugal Daniel Diaz, University of Paris I, France Ulrich Geske, University of Potsdam, Germany Gopal Gupta, UT Dallas, USA Petra Hofstedt, Technical University of Berlin, Germany Ulrich Neumerkel, Technical University of Vienna, Austria Vitor Nogueira, University of Evora, Portugal Enrico Pontelli, New Mexico State University, USA Irene Rodrigues, University of Evora, Portugal Carolina Ruiz, Worcester Polytechnic Institute, USA Dietmar Seipel, University of Wuerzburg, Germany (co-chair) Terrance Swift, CENTRIA, Portugal Hans Tompits, Technical University of Vienna, Austria Masanobu Umeda, Kyushu Institute of Technology, Japan Armin Wolf, Fraunhofer FIRST, Berlin, Germany Osamu Yoshie, Waseda University, Japan == Contact Information == inap2009(a)di.uevora.pt Universidade de Evora Departamento de Informatica Largo dos Colegiais, 2 7004-516 Evora - PORTUGAL

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FFI and heap memory usage limit
by Marcin Kosiba 29 Jun '09

29 Jun '09

Hello, Recently I've come across a certain GC/FFI-related problem. I've googled a bit, but didn't find anything specific. I'm running certain simulations, which tend to allocate a lot of garbage in memory. Since this causes the OOM-killer to kill my simulation at 98% completion, I used the -M switch, and all was well. But because my simulation results are fairly big, I needed to compress them with bz2 before sending them over the network. So I used bzlib. Now this took an odd turn, because the simulation started crashing with out-of-memory errors _after_ completing (during bz2 compression). I'm fairly certain this is a GC/FFI bug, because increasing the max heap didn't help. Moving the bz2 compression to a separate process provided a reasonable solution. What I think is happening is that after the simulation completes, almost all of the available memory (within the -M limit) is filled with garbage. Then I run bzlib which tries to allocate more memory (from behind FFI?) to compress the results, which in turn causes an out-of-memory error instead of triggering a GC collection. I'm writing to ask if this is a known/fixed issue. I'm using ghc 6.10.3, bzlib 0.5.0.0. If this is something new then I'll try to come up with a small program which demonstrates the problem. -- Marcin Kosiba

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