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Random ramblings by me.

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Jan
27th
Fri
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Free, nicely presented textbooks with good distribution

Free, nicely presented textbooks with good distribution have got quite an appeal.

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Nov
21st
Mon
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Seminar by John Hughes & Simon Peyton Jones @ UNSW

As part of YOW! 2011 (the Australian Software Developer Conference), Simon Peyton Jones and John Hughes will be in Sydney on the 7th and 8th of December 2011. On the evening of the 7th, they will appear at the YOW! Night Sydney.

On Thursday (8 December), John and Simon will give two presentations at the School of Computer Science and Engineering (CSE) of the University of New South Wales. The details are as follows.

Time: 8 December 2011, 10AM

Location: CSE Seminar room (K17_113), Level 1, CSE Building (K17)

 

Talk #1

Title: Accelerating race condition detection through procrastination

Speaker: John Hughes (Chalmers University & Quviq AB)

Abstract

Race conditions are notoriously frustrating to find, and good tools can help. The main difficulty is reliably provoking the race condition. In previous work we presented a randomising scheduler for Erlang that helps with this task.

In a language without pervasive shared mutable state, such as Erlang, performing scheduling decisions at random uncovers race conditions surprisingly well. However, it is not always enough. We describe a technique, procrastination, that aims to provoke race conditions more often than by random scheduling alone. It works by running the program and looking for pairs of events that might interfere, such as two message sends to the same process. Having found such a pair of events, we re-run the program but try to provoke a race condition by reversing the order of the two events.

We apply our technique to a piece of industrial Erlang code. Compared to random scheduling alone, procrastination allows us to find minimal failing test cases more reliably and more quickly.

John Hughes

John Hughes has worked with functional programming since 1980, was one of the designers of Haskell, and has been heavily involved with Erlang in recent years. In 2000 he and Koen Claessen published the first version of QuickCheck, a software testing tool which recently won the ACM SIGPLAN award for Most Influential Paper from ICFP in that year. He has focussed more and more on testing since then, co-founding Quviq AB in 2006 to market a commercial version of QuickCheck. He is currently both a Professor at Chalmers University, Sweden, and CEO of Quviq AB.

Talk #2

Title: Termination Combinators Forever

Speaker: Simon Peyton Jones (Microsoft Research)

Abstract

Nobody wants their compiler, or theorem prover, to go into an infinite loop, but making sure that never happens usually means applying some over-conservative heuristic like “never unfold a recursive function”. Approaches like that don’t work at all when you are doing partial evaluation or supercompilation, which fundamentally depend on inlining recursive functions. 

What we need is an online termination test. That is easier said than done; it is easy to make a mistake. In this talk I’ll describe a new combinator library that lets you build complex termination tests by combining simpler ones, while guaranteeing that that the result really is a termination test. The library elegantly encapsulates some clever mathematical ideas on well-quasi orders the cunning details are hidden from the customer.

I’ll use Haskell as the language of exposition, but you don’t need to be a Haskell guru to understand it, and the library would work equally well in other languages.

Simon Peyton Jones

Simon Peyton Jones, MA, MBCS, CEng, graduated from Trinity College Cambridge in 1980. After two years in industry, he spent seven years as a lecturer at University College London, and nine years as a professor at Glasgow University, before moving to Microsoft Research (Cambridge) in 1998.

His main research interest is in functional programming languages, their implementation, and their application. He has led a succession of research projects focused around the design and implementation of production-quality functional-language systems for both uniprocessors and parallel machines. He was a key contributor to the design of the now-standard functional language Haskell, and is the lead designer of the widely-used Glasgow Haskell Compiler (GHC). He has written two textbooks about the implementation of functional languages.

More generally, he is interested in language design, rich type systems, software component architectures, compiler technology, code generation, runtime systems, virtual machines, and garbage collection. He is particularly motivated by direct use of principled theory to practical language design and implementation — that’s one reason he loves functional programming so much.

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Sep
3rd
Sat
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Video and slides of “Data Parallelism in Haskell” @ Brisbane FP Group

Video @ vimeo

The Brisbane FP Group (BFPG) kindly invited me to give a talk about our work on data parallel programming in Haskell. The talk motivates the use of functional programming for parallel, and in particular, data parallel programming and explains the difference between regular and irregular (or nested) data parallelism. It also discusses the Repa library (regular data parallelism for multicore CPUs), the embedded language Accelerate (regular data parallelism for GPUs), and Data Parallel Haskell (nested data parallelism for multicore CPUs).  The slides of the presentation are available in two formats: HTML5 slideshow and PDF. Thank you to everybody who attended. Special thanks go to OJ Reeves and Tony Morris for organising everything, to Rob Manthey for producing the video, to Mincom for the venue, and to Functional IO for the sponsorship.

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Aug
12th
Fri
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Data Parallel Haskell and Repa for GHC 7.2.1

As an add-on to the just released GHC 7.2.1, Ben just uploaded the Data Parallel Haskell packages (version 0.5) to Hackage. This version is still largely a technology preview and not a production-ready system. Nevertheless, it is significantly more robust than previous versions, especially for programs with a statically fixed depth of nesting of parallelism. For further information on how to install and use Data Parallel Haskell (DPH), see the DPH documentation.
Ben simultaneously released the companion library Repa (version 2.1.1.6) for use with GHC 7.2.1. In contrast to the DPH libraries, which enable nested data parallelism, Repa implements parallel, shape-polymorphic, regular multi-dimensional arrays.  See this previous post for some more details.

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Apr
17th
Sun
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Data.Array.Accelerate now on GitHub

Prompted by GHC’s move to Git and the unreliability of the community.haskell.org infrastructure, I decided to move Data.Array.Accelerate to GitHub: mchakravarty/accelerate. The GitHub repository is now the main public repository for the project. I will also move the tickets of the old Trac bug tracker over to GitHub Issues.

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Mar
30th
Wed
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Real-time edge detection with the latest release of the parallel array library Repa

We just released version 2.0.0.1 of the Repa array library for Haskell, which includes Ben’s recent work on high-performance, parallel stencil computations. The work on stencil computations is described in detail in a draft paper entitled 
Efficient Parallel Stencil Convolution in Haskell. Be sure to check out Ben’s screencast of a real-time edge detection application, written in Objective-C and Haskell, using the new Repa library. For more details, see Ben’s blog post.

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Simon Peyton Jones: parallel = functional

Simon Peyton Jones recently gave a talk on parallel programming in Haskell at Functional Programming eXchange 2011. It provides a comprehensive overview of the state of the art of parallel programming in Haskell. It covers Software Transactional Memory, Erlang-style communicating processes, parallel strategies as well as our Data Parallel Haskell, Repa, and Accelerate projects. Watch the video.

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Jan
21st
Fri
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Moving mail archives to MobileMe (or other Sun Java System Messaging Servers) (tag: ipad, mac, mobileme, mail, imap)

I recently consolidated all my email on MobileMe (for easy access from both my Mac and iPad).  However, I ran into a problem with the MobileMe IMAP server when I tried to move my old mail archives to the MobileMe IMAP server.  MobileMe uses the Sun Java System Messaging Server (which has since become the Oracle Communications Messaging Exchange Server).  It turns out that this IMAP server (like some others as well) is rather strict about the format of mail headers; hence, uploading old mail archives usually aborts with the following error message: “The IMAP command “APPEND” (to …) failed with server error: Message contains invalid header”.
Querying the Internets and some experimentation revealed that the problem are lines in email message headers that start with “From “, “>From “, and “»From “.  These are not proper message headers fields (whose names need to be delimited by a colon), but remnants from storage of these mail messages in the mbox format.  The remedy is to delete or edit these invalid lines in all affected message headers.

Apple Mail stores mail messages in “$USER/Library/Mail/Mailboxes/”.  Each mailbox directory (suffix “.mbox”) contains a directory “Messages” with one file per message.  These files can easily be modified using your favourite command line tools that support matching of regular expressions or by loading them, en masse, into TextMate for project-wide search and replace.  This is somewhat naughty as the files containing individual messages contain the message header and text as well as a property list (plist), used by Mail.app to store a few attributes.  The start of that property list in the text file is marked by a character count in the first line of each message.  By eliminating or modifying the offending lines, you invalidate that character count.  However, Mail.app seems to cope just fine with those slightly malformed messages.  After fixing the headers, all my messages uploaded without problems (and no attributes seemed to get lost).

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Nov
26th
Fri
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Final version of the Accelerate paper (GPGPU in Haskell)

We will present our paper Accelerating Haskell Array Codes with Multicore GPUs at ACM SIGLAN Declarative Aspects of Multicore Programming (DAMP 2011), which is co-located with POPL’11 in Austin, TX, in January. The final version of our paper is now available, and we plan to soon release a significantly improved version of the Accelerate library (matching the API used in the paper), which enables high-level GPGPU programming in Haskell based on NVIDIA’s CUDA environment.

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Nov
25th
Thu
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Final version of the Singleton paper

Our paper Singleton: A General-Purpose Dependently-Typed Assembly Language will be presented at the ACM SIGPLAN Workshop on
Types in Language Design and Implementation (TLDI’11)
 co-located with POPL’11 in Austin, TX, in January.  The final version of the paper is now available.

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