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Definition of the Haskell standard library
by Chris Smith 03 Sep '07

03 Sep '07

Can someone clarify what's going on with the standard library in Haskell? As of right now, I can download, say, GHC from haskell.org/ghc and get a set of libraries with it. I can visit http://haskell.org/ghc/docs/latest/html/libraries/, linked from the haskell.org home page, and see descriptions of all of those libraries. I can build with --make (or if I'm feeling masochistic, add several lines of -package options) and it works. That's all great. I've seen some stuff lately on -libraries and this list indicating that there's an effort to change this. People asking whether something should be included in the standard library are being told that there is no standard library really. I'm hearing that the only distinction that matters is "used by GHC" or "not used by GHC", and that being on hackage is as official as it gets. Am I misunderstanding? Is there something awesome about Hackage that I'm not seeing? I hope one of those two is the case. Otherwise, there's a serious mistake being made here. Having a web site where people can download any of hundreds of independent libraries is not the same thing as having a good standard library for the language. I don't want to see the day when setting up Haskell involves a day-long effort of figuring out what libraries to download and install from Hackage, and in what order to do it to satisfy all the dependencies, and new Haskellers poring over web sites for the thousandth time before realizing that so-and-so's GUI library hasn't actually been touched since they finished their class project in 1998 and doesn't build with the latest version of Qt or whatever. -- Chris Smith

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APLAS07 - Call for Participation
by Shin-Cheng Mu 03 Sep '07

03 Sep '07

Call for Participation The Fifth Asian Symposium on Programming Languages and Systems November 29 - December 1, 2007 Singapore http://www.comp.nus.edu.sg/~aplas07/ http://flint.cs.yale.edu/aplas2007/ Scope of the Conference ----------------------- APLAS aims at stimulating programming language research by providing a forum for the presentation of recent results and the exchange of ideas and experience in topics concerned with programming languages and systems. APLAS is based in Asia, but is an international forum that serves the worldwide programming languages community. The APLAS series is sponsored by the Asian Association for Foundation of Software (AAFS), which has recently been founded by Asian researchers in cooperation with many researchers from Europe and the USA. The past formal APLAS symposiums were successfully held in Sydney (2006, Australia), Tsukuba (2005, Japan), Taipei (2004, Taiwan) and Beijing (2003, China) after three informal workshops held in Shanghai (2002, China), Daejeon (2001, Korea) and Singapore (2000). Proceedings of the past symposiums were published in Springer-Verlag's LNCS 2895, 3302, 3780, and 4279. Conference Location ------------------- APLAS'07 will be held at Kent Ridge Guild House, National University of Singapore. Singapore is a dynamic city rich in contrast and color where you'll find a harmonious blend of culture, cuisine, arts and architecture. A bridge between the East and the West for centuries, Singapore, located in the heart of fascinating Southeast Asia, continues to embrace tradition and modernity today. Brimming with unbridled energy and bursting with exciting events, the city offers countless unique, memorable experiences waiting to be discovered. Registration ------------ The early registration deadline is set on 28Sept07. The registration fees will cover conference proceeding, banquet dinner, and lunches/teas. Deadline Full Fees Student Fees early 28Sep07 S$600 S$400 online 23Nov07 S$650 S$425 on-site 29Nov07 S$700 S$450 Payment can be made with credit cards. Exchange rate (subject to fluctuation) is approx. US$1 = S$1.53 Online registration can be made at: http://www.comp.nus.edu.sg/~aplas07/local.html#registration Accommodation ------------- A special rate S$155 per nite (excluding taxes but inclusive of daily breakfast) is applicable to attendees of APLAS 2007 at Rendezvous Hotel Singapore: http://www.rendezvoushotels.com/singapore/ The hotel is located in city center. A free daily shuttle bus will be provided between this hotel and the conference site (at NUS) during the conference period. Our block of rooms is guaranteed till 28Sept07, so advanced booking is encouraged as some rooms may be released after this date. For reservation, please use a special form that can be found at: http://www.comp.nus.edu.sg/~aplas07/Hotel.APLAS07.pdf We have also reserved a limited number of cheaper twin/double shared rooms (S$280 for 4-nights) on NUS campus. Priority will be given to students and poster participants. Deadline for application is 20Sep07. Details will be given on our web in early Sept. Poster Session -------------- APLAS07 will include a poster session. Submission deadline is 14 Sept 2007. For details, please see: http://www.comp.nus.edu.sg/~aplas07/posters.html Conference Program ------------------ 29 Nov (Thu) ------------ 915- 930 Opening note 930-1030 Invited Talk X10: Programming Parallel Machines, Productively Vijay Saraswat (IBM TJ Watson Research Lab) 1100-1230 Session 1 The Nuggetizer: Abstracting Away Higher-Orderness for Program Verification Paritosh Shroff (Johns Hopkins University), Christian Skalka (University of Vermont) and Scott F. Smith (Johns Hopkins University) Local Reasoning for Storable Locks and Threads Alexey Gotsman (University of Cambridge), Josh Berdine (Microsoft Research), Byron Cook (Microsoft Research), Noam Rinetzky (Tel Aviv University) and Mooly Sagiv (Tel Aviv University) Monadic, Prompt Lazy Assertions in Haskell Frank Huch (CAU Kiel) and Olaf Chitil (University of Kent) 1345-1515 Session 2 Translation Correctness for First-Order Object-Oriented Pattern Matching Burak Emir (EPFL Lausanne), Qin Ma (OFFIS) and Martin Odersky (EPFL Lausanne) Persistent Oberon: A Programming Language with Integrated Persistence Luc Blaser (ETH Zurich) More Typed Assembly Languages for Confidentiality Dachuan Yu (DoCoMo Communication Laboratories USA) 1515-1630 Poster Session 1630-1730 Session 3 A Novel Test Case Generation Method for Prolog Programs Based on Call Patterns Semantics Lingzhong Zhao (Guilin University of Electronic Technology), Tianlong Gu (Guilin University of Electronic Technology), Junyan Qian (Guilin University of Electronic Technology) and Guoyong Cai (Guilin University of Electronic Technology) On a Tighter Integration of Functional and Logic Programming Frank Huch (CAU Kiel) and Bernd Brassel (CAU Kiel) 1800-2000 Reception 30 Nov (Fri) ------------ 930-1030 Invited Talk Scalable Simulation of Biological Signaling Networks Vincent Danos (University of Paris VII & CNRS) 1100-1230 Session 4 Timed, Distributed, Probabilistic, Typed Processes Martin Berger (Imperial College London) and Nobuko Yoshida (Imperial College London) A Probabilistic Applied Pi-Calculus Jean Goubault-Larrecq (ENS Cachan), Catuscia Palamidessi (Ecole Polytechnique) and Angelo Troina (ENS Cachan & Ecole Polytechnique) Type-Based Verification of Correspondence Assertions for Communication Protocols Daisuke Kikuchi (Tohoku University) and Naoki Kobayashi (Tohoku University) 1400-1530 Session 5 Deriving Compilers and Virtual Machines for a Multi-Level Language Atsushi Igarashi (Kyoto University) and Masashi Iwaki (Kyoto University) Finally Tagless, Partially Evaluated (Tagless Staged Interpreters for Simpler Typed Languages) Jacques Carette (McMaster University), Oleg Kiselyov (FNMOC) and Chung-chieh Shan (Rutgers University) Polymorphic Delimited Continuations Kenichi Asai (Ochanomizu University) and Yukiyoshi Kameyama (University of Tsukuba) 1600-1730 Session 6 Adjunct Elimination in Context Logic for Trees Cristiano Calcagno (Imperial College London), Thomas Dinsdale-Young (Imperial College London) and Philippa Gardner (Imperial College London) Positive Arithmetic without Exchange is a Subclassical Logic Stefano Berardi (University of Torino) and Makoto Tatsuta (National Institute of Informatics, Japan) Mixed Inductive/Coinductive Types and Strong Normalization Andreas Abel (University of Munich) 1900-2100 Banquet 1 Dec (Sat) ------------ 930-1030 Invited Talk Static and Dynamic Analysis : Better Together Sriram Rajamani (Microsoft Research India) 1100-1230 Session 7 The Semantics of "Semantic Patches" in Coccinelle: Program Transformation for the Working Programmer Neil Jones (University of Copenhagen) and Rene Rydhof Hansen (University of Copenhagen) An Efficient SSA-based Algorithm for Complete Global Value Numbering Jiu-Tao Nie (Peking University) and Xu Cheng (Peking University) A Systematic Approach to Probabilistic Pointer Analysis Alessandra Di Pierro (University of Verona), Chris Hankin (Imperial College London) and Herbert Wiklicky (Imperial College London) 1400-1530 Session 8 Complete Lattices and Up-to Techniques Damien Pous (ENS Lyon) A Trace Based Bisimulation for the Spi Calculus: An Extended Abstract Alwen Tiu (Australian National University) CCS with Replication in the Chomsky Hierarchy: The Expressive Power of Divergence Frank Valencia (Ecole Polytechnique), Cinzia Di Giusto (University of Bologna), Jesus Aranda (Ecole Polytechnique) and Mogens Nielsen (University of Aarhus) 1600-1700 Session 9 Call-by-Name and Call-by-Value in Normal Modal Logic Yoshihiko Kakutani (University of Tokyo) Call-by-Value is Dual to Call-by-Name, Extended Daisuke Kimura (National Institute of Informatics, Japan) 1700-1715 Closing note Organization ------------- GENERAL CHAIR Joxan Jaffar (National University of Singapore, Singapore) PROGRAM CHAIR Zhong Shao (Yale University, USA) PROGRAM COMMITTEE Lars Birkedal (IT University of Copenhagen, Denmark) Martin Hofmann (Univ of Munich, Germany) Kohei Honda (Queen Mary, University of London, UK) Atsushi Igarashi (Kyoto University, Japan) Suresh Jagannathan (Purdue University, USA) Annie Liu (State University of New York at Stony Brook, USA) Shin-Cheng Mu (Academia Sinica, Taiwan) Henrik Nilsson (University of Nottingham, UK) Michael Norrish (NICTA, Australia) Jens Palsberg (University of California, Los Angeles, USA) G. Ramalingam (Microsoft Research, India) Zhendong Su (University of California, Davis, USA) Martin Sulzmann (National University of Singapore, Singapore) Eijiro Sumii (Tohoku University, Japan) Jerome Vouillon (CNRS, France) Kwangkeun Yi (Seoul National University, Korea) Jian Zhang (Chinese Academy of Sciences, China) POSTER SESSION CHAIR Eijiro Sumii (Tohoku University, Japan) LOCAL ARRANGEMENT CHAIR Wei-Ngan Chin (National University of Singapore, Singapore)

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Bi-directional Maps
by Andrew Wagner 03 Sep '07

03 Sep '07

Hi folks! So, in writing my chess engine, I've been trying to maintain 2 Map objects. One maps squares on the board to Ints, the other maps Ints to actual Pieces. It occurred to me that it would be useful to explicitly have a Bi-directional Map, which does the maintenance of keeping the Maps synchronized behind the scenes. Thus, Bimap was born! I've taken the API for Data.Map (which you can find at ), and cannibalized it for Bimap. The new API is at http://hpaste.org/2344 . The idea is that if you have a Bimap k a, and you want to treat the k's as keys, and use a function like Data.Map.foo, it will be called Data.Map.left_foo in Bimap. And if they want to do the same thing, but using the a's as keys, then they simply use right_foo. The metaphor is that we can view it as a Map in 2 directions, manipulating it from the left (on the k's), or from the right (on the a's). Is this useful? Is there a better way? Is the API too big, and if so, how can it be pared down?

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GHC-API: a few questions
by Andrea Rossato 03 Sep '07

03 Sep '07

Hi, I'm trying to acquire some confidence with the GHC-API and I'm having some problems, related to error handling, I seem not be able to solve. Basically there are 3 functions to (interactively) compile/run Haskell expressions: compileExpr, dyCompileExpr and runStmt. The first 2 will return the value of the compiled expression, as a Maybe HValue the first, and a Maybe Dynamic the second, while the third can bind names and report them, or report exceptions. But my problem is related to the first two. I run expressions (string_expression) wrapped around some let x = string_expression in take 2048 (show x) to avoid infinite list or loops. According to the documentation one should be able to control how error messages are dealt with, by manipulation the GHC session's state. One component of the session state, indeed, is the log_action which, by default, prints errors and warnings to the standard error (compiler/main/DynFlags.hs). Now, no matter what I do, I seem not to be able to change the log_action to collect error messages and warnings. In any case everything seem to be printed to standard error. This way I can only collect exceptions raised by the compiler. For instance: "Prelude.last: empty list" for string_expression = "last []" But if my expression raises a type error, this is printed to stderr. Like in the case of string_expression = "last" which would produce a "No instance for (Show ([a] -> a))" What am I doing wrong? Thanks for your kind attentions. Andrea ps: there are quite a lot of other questions I'd like to ask about some programming techniques I've seen applied in GHC. For instance, sometime a list of actions is given as a string, to be compiled to a HValue which is written to a IORef. Running those actions is just a matter of reading the IORef (you can see this in compiler/ghci/InteractiveUIhs, when it comes to buffer management. For instance: flushInterpBuffers). I ask to myself: Why? Just to amaze the reader? ;-)

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Ideas
by Andrew Coppin 03 Sep '07

03 Sep '07

How easy would it be to make / would anybody care / has somebody already made ... in Haskell? - Blogging software. (Because there isn't enough of it in the world yet.) - A wiki program. (Ditto.) - A general CMS. - An interactive function plotter. (GNUplot is nice, but it can't plot recursive functions...) - A "graphical programming tool". (You add boxes and put in lines, it constructs a "program" that you can run.)

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Array.Diff, strange oversight or myopia ?
by Chaddaï Fouché 02 Sep '07

02 Sep '07

Data.Array.Diff don't have an instance for DiffUArray Bool, which is strange by itself since IOUArray Bool exists and it's the only IOUArray that is not mirrored in Diff. But ok, why not, I guess it might be a small oversight, so I go on to create the missing instance IArray (IOToDiffArray IOUArray) Bool on the model of the instances in the source of Array.Diff. Ok, Diff exports readDiffArray, newDiffArray and replaceDiffArray for this exact purpose, seems it will be straightforward... except that boundsDiffArray is not exported and you can't rewrite it since it needs to take apart the DiffArrayData type which is not exported either ! So how can I do this (short of copying and renaming an alternative Array.Diff or changing the library in my installation of GHC) ? And is there something obvious I missed due to a sudden case of myopia ? -- Jedaï

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Re: [Haskell-cafe] Learn Prolog...
by Peter Verswyvelen 02 Sep '07

02 Sep '07

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HApps API Documentation?
by James Britt 02 Sep '07

02 Sep '07

Are there alternative sites for HAppS API docs? There are two links on http://happs.org/#documentation but both give "File not found!" messages. Thanks, James

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Looking for suggestions to improve my algorithm
by David Frey 02 Sep '07

02 Sep '07

Hello Haskellers, I have been trying to learn a bit about Haskell by solving Project Euler problems. For those of you who don't know what Project Euler is, see http://projecteuler.net After solving problem 21, which is related to amicable pairs, I decided to attempt problem 95 since it is an extension of the same topic. The full description of problem 95 is here: http://projecteuler.net/index.php?section=problems&id=95 This is the summary: "Find the smallest member of the longest amicable chain with no element exceeding one million." I have attempted to solve this problem, but my solution is too resource hungry to search the full set of 1000000 numbers. I am hoping someone reading this list can suggest : - How I can improve my algorithm - An alternative algorithm that will be more efficient - Ways to profile a Haskell program to help me understand why my implementation is performing poorly. In addition to the question above, I would also be interested in comments on the style of the code. If there is a more idiomatic way to write portions of the code, I would like to see what it is. My solution is at the bottom of this e-mail. The program will probably run obscenely slow or die from stack overflow unless you replace [1..999999] with [1..somethingSmaller] in main. Thanks, David Frey --- BEGIN Main.hs --- module Main where import ProjectEuler (takeUntil, divisors) import qualified Data.Map as M import qualified Data.IntSet as I main = let initialContext = Context (I.fromList []) 0 0 in print $ cycleStart $ foldl checkForChain initialContext [1..999999] {- Idea: * Put all the numbers that have been visited into a map regardless of whether they are a part of a chain or not. * Store the min element in the cycle and the number of elements in the cycle * As you process, from 1->n if the stopping conditions for a sumOfDivisors result are: * has already been seen before * number is less than the start of this chain attempt * >= 1,000,000 -} data Context = Context {seenNum::I.IntSet, cycleStart::Int, cycleLength::Int} hasBeenSeen :: Int -> Context -> Bool hasBeenSeen n context = I.member n (seenNum context) markSeen :: Int -> Context -> Context markSeen n context = context { seenNum = (I.insert n (seenNum context)) } deleteFromSeen :: Int -> Context -> Context deleteFromSeen n context = context { seenNum = (I.delete n (seenNum context)) } {- - Examines the context to see if the input has potential to be a chain or not - based on whether the input number has been visited before. If it could be a - chain, an attempt is made to build the chain. -} checkForChain :: Context -> Int -> Context checkForChain context n = if hasBeenSeen n context then deleteFromSeen n context else buildChain context (sum $ divisors n) n [n] {- - Builds a chain until ones of the 3 stopping conditions are met or a chain is - found. If a chain is found the context will be updated with the new chain if - it is the longest. - - Stopping Conditions: - * Number has already been seen before - * Number is less than the start of this chain attempt - * Number >= 1,000,000 -} buildChain :: Context -> Int -> Int -> [Int] -> Context buildChain context candidate first cycleList = if elem candidate cycleList then foundChain (takeUntil ((==) candidate) cycleList) context else if candidate < first || candidate >= 1000000 || hasBeenSeen candidate context then context else buildChain (markSeen candidate context) (sum $ divisors candidate) first (candidate : cycleList) {- - Updates the context with the new longest chain and the start value if the - chain input parameter is longer than the one currently in the context. -} foundChain :: [Int] -> Context -> Context foundChain ls context = let l = length ls m = minimum ls in if l > (cycleLength context) then context { cycleLength = l, cycleStart = m } else if l == (cycleLength context) then let m = minimum ls in if m < (cycleStart context) then context { cycleStart = m } else context else context --- END Main.hs --- I put a bunch of common functions in ProjectEuler.hs. Here are the relevant functions for this problem: {- - Gets all of the proper divisors of a number. That is all divisors starting - from 1, but not including itself. -} divisors :: (Integral a) => a -> [a] divisors n = let p1 = [x | x <- [1 .. floor $ sqrt $ fromIntegral n], n `mod` x == 0] p2 = map (div n) (tail p1) in p1 `concatNoDupe` (reverse p2) where {- - Concatenate two lists. If the last element in the first list and the - first element in the second list are ==, produce only the value from - the first list in the output. -} concatNoDupe :: (Eq a) => [a] -> [a] -> [a] concatNoDupe [] ys = ys concatNoDupe [x] (y:ys) = if x == y then (y : ys) else (x : y : ys) concatNoDupe (x:xs) ys = x : (concatNoDupe xs ys) {- - Similar to takeWhile, but also takes the first element that fails the - predicate. -} takeUntil :: (a -> Bool) -> [a] -> [a] takeUntil pred (x:xs) = (x : if pred x then [] else takeUntil pred xs) takeUntil _ [] = []

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defining mapPairs function
by Alexteslin 01 Sep '07

01 Sep '07

Hello, I just came across with this question on the exam and can not think of implementing it. mapPair :: (a -> a -> a) -> [a] -> [a] such that mapPairs f [x1, x2, x3, x4...] = [f x1 x2, f x3 x4,...] and if the list contains an odd number of elements, the last one is kept unchanged, for example mapPairs f [x1, x2, x3] = [f x1 x2, x3] Any ideas will be appreciated, thanks -- View this message in context: http://www.nabble.com/defining-mapPairs-function-tf4350356.html#a12395628 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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