- Haskell-Cafe - Haskell.org

Lambda's
by Henk-Jan van Tuyl 17 Feb '10

17 Feb '10

I love lambda's: http://hawtness.com/2009/12/30/wtf-girl-photo-more-reasons-why-half-life-is… -- Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

11 11

RFC: concurrent-extra
by Roel van Dijk 17 Feb '10

17 Feb '10

Hello, We wrote a small library (1) which offers a few extra synchronization primitives. These primitives are found in the standard libraries of languages like Java and Python, but not in Haskell. Before releasing concurrent-extra on hackage, we would like some comments on its name, design, implementation, documentation (2) and general usefulness. All primitives are implemented with MVars and IORefs, except for Control.Concurrent.STM.Event. There is a small test suite which tests some properties: cabal configure -ftest cabal build cabal test The test suite still needs to be expanded a great deal. We found that testing concurrent algorithms is really hard. Now we test things like "this shouldn't deadlock" by checking if it doesn't take too long. But that doesn't feel very formal. Is there a more principled approach to testing concurrent algorithms in Haskell? After writing our version of a read-write lock we noticed the package rwlock (3) on hackage. But there is a difference: rwlock is implemented with STM while our version is implemented entirely with MVars. Regards, Roel & Bas van Dijk 1 - http://code.haskell.org/~roelvandijk/code/concurrent-extra/ 2 - http://code.haskell.org/~roelvandijk/code/concurrent-extra/doc/html/concurr… 3 - http://hackage.haskell.org/package/rwlock

3 6

Example for RVarT?
by Kemps-Benedix, Torsten 17 Feb '10

17 Feb '10

Hello folks, is there an example (simpe or not) for the use of Data.Random.RVarT with an underlying monad other than Identity, e.g. StateT, ReaderT etc.? Thx Torsten

1 0

Notes on migrating from uvector to vector
by Don Stewart 17 Feb '10

17 Feb '10

I've summarised my notes on how to migrate array code from the uvector package to Roman's vector package: http://donsbot.wordpress.com/2010/02/15/migrating-from-uvector-to-vector/ And I'll take this opportunity to declare that uvector is now in official maintainance-only mode. Enjoy the new decade of flexible, fusible, fast arrays for Haskell! -- Don

8 12

Efficient M to N conversion on fusion?
by iquiw 17 Feb '10

17 Feb '10

Hi, Is there any efficient way to convert M-length ByteString, Text or Vector into N-length one? For example, (a) converting byte array into array of their hex representations. ("abc" => "616263") (b) escaping some specific characters in string. ("a\bc" => "a\\bc") The only way I found is to use concatMap... Thanks, iquiw

3 3

pandoc code bocks not working
by Walter De Jonge 16 Feb '10

16 Feb '10

http://johnmacfarlane.net/pandoc/README.html#delimited-code-blocks "Like regular code blocks, delimited code blocks must be separated from surrounding text by blank lines." goodluck walter

3 2

Linguistic hair-splitting
by Andrew Coppin 16 Feb '10

16 Feb '10

Here's one for you to ponder. 7 is a number. 7 is an integer, and integers are numbers. 7 is not a field. 7 is an element of [at least one] field, but 7 itself is not a field. 7 is not a group. 7 is a member of the set of integers, but the set of integers is not a group either. The set of integers form a group when taken together with the addition operator. (And, actually, forms another, different, group when taken with the multiplication operator.) Now, here's the question: Is is correct to say that [3, 5, 8] is a monad? Is it correct to say that lists are a monad? Or would it be more correct to say that lists "form" a monad?

14 17

Microsoft Research PhD Scholarship, University of Leeds
by David Duke 16 Feb '10

16 Feb '10

I'm pleased to announce a PhD Scholarship within the Visualization and VR group at Leeds, generously funded by Microsoft Research. We are seeking a student to investigate how visualization techniques can be further developed to understand and improve the performance of parallel Haskell programs running on multi-core CPUs. Closing date for applications is 22 March 2010. Could you please pass on this call to anyone who might be interested in applying. Further details are included below, and are also available on the web, at http://www.engineering.leeds.ac.uk/pg/studentships/duke.shtml thanks, David Duke PROJECT TITLE: "Visualizing Performance for Multicore Haskell" SUPERVISION: Dr. David Duke, School of Computing, University of Leeds, UK MRL CO-SUPERVISION: Dr. Satnam Singh Prof. Simon Peyton Jones Dr. Simon Marlow Microsoft Research Laboratory, Cambridge, UK OUTLINE: The GHC Haskell compiler now provides high-level abstractions that allow the programmer to benefit from multi-core CPUs. These abstractions support implicit parallelism - the programmer can indicate expressions that could usefully be evaluated in parallel, but is freed from concerns about when parallel evaluation takes place. However, it now becomes more difficult to isolate and resolve performance issues. Visualization of run-time behaviour can help. The Haskell community already have the benefit of a tool, ThreadScope, developed at Microsoft Research Cambridge (research.microsoft.com/en-us/labs/ Cambridge/) that provides insight into resource utilisation. However ThreadScope, like other performance visualization tools, presents the programmer with a view of low-level resources that differs from the level of abstraction at which Haskell programmers are encouraged to work. This studentship will investigate visualization techniques for linking high-level program abstractions with low-level runtime performance data. It should allow Haskell programmers to exploit parallelism more effectively. The student will join the Visualization and VR group at Leeds (www.comp.leeds.ac.uk/vvr) . Work on the PhD will involve: understanding how information about run-time resources can be extracted and visualized, and how programmers use these representations to reason about design and implementation choices; building or extending visualization tools; conducting user evaluations; modify components of the GHC compiler and runtime system. Work will be undertaken with support from the GHC/ ThreadScope team at Microsoft Research (Satnam Singh, Simon Peyton Jones, and Simon Marlow). At the discretion of Microsoft Research, the student may also be offered an internship at MRL Cambridge. FUNDING: The Scholarship provides a bursary of £20,000 per year for three years, covering fees and maintenance. A further £3000 per annum has been reserved for travel expenses to attend conferences and visits to Microsoft Research in Cambridge. Applicants from outside of the EU should note that higher student fees (currently £13,300) will necessarily reduce the amount available for maintenance. Home/EU fees stand at £3,390 currently. Microsoft Research will provide the successful candidate with a laptop equipped with a suite of software. ENTRY REQUIREMENTS: Applicants must hold a First-Class BSc (Hon) degree in Computer Science or equivalent. It is essential that applicants have excellent knowledge of functional programming and significant practical experience writing software in Haskell, Clean, or ML. Experience with GUI programming and/or information visualization is highly desirable. Knowledge of programming technologies, in particular compilers and implementation techniques for functional languages is also desirable. The position is open to students of all nationalities. SCHOOL OF COMPUTING, LEEDS: The School of Computing (www.comp.leeds.ac.uk) is among the 10 best Computing departments in the UK according to the 2008 Research Assessment Exercise (RAE). An impressive 80% of staff is rated internationally excellent or world leading. This clearly confirms the School’s position as one of the leading computing departments in the UK and a leader in the field internationally. APPLICATION PROCEDURE: Formal applications for research degree study must be made either online through the University website, or on the University’s application form. Detailed information of how to apply on line can be found at: www.leeds.ac.uk/students/apply_research.htm The paper application form is available at: www.leeds.ac.uk/rds/Admissions/Admis_home.htm Please return the completed application form to: Research Degrees & Scholarships Office, University of Leeds, LS2 9JT. Please provide all the documents required as soon as possible, either included with your paper application or sent directly to the School of Computing secretary (rsadmit(a)comp.leeds.ac.uk) if you apply online. Scanned copies are acceptable for a conditional offer; however you will need to provide originals or certified copies at registration. These will include your degree certificate(s), transcripts of marks achieved in previous degrees, plus evidence of English language qualifications if your first language is not English and you do not hold a degree from an English-speaking country. Please note, if you intend to send academic references we can only accept them if they are on official letter headed paper and contain an original signature and stamp; they must arrive in sealed envelopes. Alternatively, the School will contact your named academic referees directly. CLOSING DATE: Applications must be received by 22 March 2010. ACADEMIC CONTACT: For questions about the research topic, please contact Dr. David Duke, University of Leeds; Tel: +44 113 343 6800; Email: D.J.Duke(a)leeds.ac.uk; Web: www.comp.leeds.ac.uk/djd/

1 0

on finding abstractions ...
by Günther Schmidt 16 Feb '10

16 Feb '10

Hello, I've got a problem, in short my haskell code sucks. While it does work and I do manage to use higher-orderish aspects quite extensively to make my code more concise it still is nowhere abstract, always concrete and thus always with lots of boilerplate. Oh I have gotten better compared to when I started but on the abstraction slope I'm still stuck. So fellows, what is the next stop on my road to enlightenment? I really think I need best to start from scratch. I think I'm sufficiently familiar now with most of Haskell's technicalities but how do I climb the ladder of abstraction? Günther

7 7

Interpreting profiling results
by Patrick LeBoutillier 16 Feb '10

16 Feb '10

Hi, This question didn't get any replies on the beginners list, I thought I'd try it here... I've written (and improved using other solutions I've found on the net) a simple sudoku solver which I'm trying to profile. Here is the code: import Array import List (transpose, nub, (\\)) import Data.List data Sudoku = Sudoku { unit :: Int, cells :: Array (Int, Int) Int, holes :: [(Int, Int)] } cell :: Sudoku -> (Int, Int) -> Int cell s i = (cells s) ! i instance Read Sudoku where readsPrec _ s = [(Sudoku unit cells holes, "")] where unit = length . words . head . lines $ s cells = listArray ((1, 1), (unit, unit)) (map read . words $ s) holes = [ c | c <- indices cells, (cells ! c) == 0] instance Show Sudoku where show s = unlines [unwords [show $ cell s (x,y) | x <- [1 .. unit s]] | y <- [1 .. unit s]] genNums :: Sudoku -> (Int, Int) -> [Int] genNums s c@(i,j) = ([1 .. u] \\) . nub $ used where used = (row s u i j) ++ (col s u i j) ++ (square s sq u i j) u = unit s sq = truncate . sqrt . fromIntegral $ u row s u i j = [cell s (i, y) | y <- [1 .. u]] col s u i j = [cell s (x, j) | x <- [1 .. u]] square s sq u i j = [cell s (x, y) | y <- [1 .. u], x <- [1 .. u], f x i, f y j] where f a b = div (a-1) sq == div (b-1) sq solve :: Sudoku -> [Sudoku] solve s = case holes s of [] -> [s] (h:hs) -> do n <- genNums s h let s' = Sudoku (unit s) ((cells s) // [(h, n)]) hs solve s' main = print . head . solve . read =<< getContents When I compile as such: $ ghc -O2 --make Sudoku.hs -prof -auto-all -caf-all -fforce-recomp and run it on the following puzzle: 0 2 3 4 3 4 1 0 2 1 4 0 0 3 2 1 I get the following profiling report: Fri Jan 1 10:34 2010 Time and Allocation Profiling Report (Final) Sudoku +RTS -p -RTS total time = 0.00 secs (0 ticks @ 20 ms) total alloc = 165,728 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc CAF GHC.Handle 0.0 10.7 CAF Text.Read.Lex 0.0 2.1 CAF GHC.Read 0.0 1.2 square Main 0.0 2.8 solve Main 0.0 1.3 show_aVx Main 0.0 3.7 readsPrec_aYF Main 0.0 60.6 main Main 0.0 9.6 genNums Main 0.0 5.0 cell Main 0.0 1.2 individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 1 0 0.0 0.3 0.0 100.0 main Main 186 1 0.0 9.6 0.0 85.6 show_aVx Main 196 2 0.0 3.7 0.0 3.7 cell Main 197 16 0.0 0.0 0.0 0.0 solve Main 188 5 0.0 1.3 0.0 11.8 genNums Main 189 8 0.0 5.0 0.0 10.4 square Main 194 88 0.0 2.8 0.0 3.2 cell Main 195 16 0.0 0.4 0.0 0.4 col Main 192 4 0.0 0.7 0.0 1.1 cell Main 193 16 0.0 0.4 0.0 0.4 row Main 190 4 0.0 0.7 0.0 1.1 cell Main 191 16 0.0 0.4 0.0 0.4 readsPrec_aYF Main 187 3 0.0 60.6 0.0 60.6 CAF GHC.Read 151 1 0.0 1.2 0.0 1.2 CAF Text.Read.Lex 144 8 0.0 2.1 0.0 2.1 CAF GHC.Handle 128 4 0.0 10.7 0.0 10.7 CAF GHC.Conc 127 1 0.0 0.0 0.0 0.0 Does the column 'entries' represent the number of times the function was called? If so, I don't understand how the 'square' function could be called 88 times when it's caller is only called 8 times. Same thing with 'genNums' (called 8 times, and solve called 5 times) What am I missing here? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

5 6