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Adaptive transactions
by Ryan Ingram 07 Jul '09

07 Jul '09

It seems to me that the Adaptive framework[1][2] is well-suited to STM transactions. Consider this snippet: > expensive_transaction :: STM Int -- does a bunch of work > > example :: TVar Int -> STM Int > example t = (+) <$> readTVar t <*> expensive_transaction If the TVar passed to example is rapidly changing (perhaps it represents the current time), then this transaction will starve; it never gets a chance to complete, even if the values used by expensive_transaction aren't changing at all, we still re-evaluate the entire action each time we "retry" this transaction. But STM has a wonderful property: what we need to commit a transaction is just the result value and a transaction log; both of these depend only on the values of TVars read by that transaction. So, if "expensive_transaction" doesn't refer to the TVar "t", then even if "t" is rapidly changing, the transaction log generated by "expensive_transaction" must not change, and we can just re-generate the part of the transaction log for "readTVar t" and recompute the (+). This gets even better when considering big processes that are blocked by an explicit "retry"; if some change in input is just going to cause the transaction to retry again (and go back to sleep), we'd like that to happen as quickly as possible. Has anyone looked down this path? It seems quite promising to me. -- ryan [1] http://hackage.haskell.org/package/Adaptive [2] Acar et al, Adaptive Functional Programming, POPL 2002 http://citeseer.ist.psu.edu/old/752721.html

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Re: what about moving the record system to an addendum?
by John Meacham 07 Jul '09

07 Jul '09

Well, without a replacement, it seems odd to remove it. Also, Haskell currently doesn't _have_ a record syntax (I think it was always a misnomer to call it that) it has 'labeled fields'. None of the proposed record syntaxes fit the same niche as labeled fields so I don't see them going away even if a record syntax is added to haskell in the future. I would like to see the simple modifications to the record syntax listed on this page though http://hackage.haskell.org/trac/haskell-prime/wiki/ExistingRecords and a reworking of the standard to not refer to the current system as a 'record syntax' but rather a 'labeled fields' syntax. John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/

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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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Implementing Las Vegas algorithms in Haskell
by Matthias Görgens 07 Jul '09

07 Jul '09

A Las Vegas algorithm, like randomized quicksort, uses a source of randomness to make certain decisions. However its output is unaffected by the randomness. So a function > f :: RandomGen g => g -> a -> b implementing a Las-Vegas-Algorithm 'looks' like a pure function, ignoring its first argument and depending solely on its second argument. What is an idiomatic way to implement such a function? I believe, Monads are too linear and strict.

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Optimizing Compiler for the ICFP 09 contest's VM
by Matthias Görgens 07 Jul '09

07 Jul '09

The byte code for the virtual machine of this years ICFP specified a language with single assignment per simulation step. Interestingly most memory locations get overwritten each simulation step before they are read. That means, those locations don't have to be remember between steps. Also locations that never get overwritten (e.g. location associated with Noops), are constant. Thus the variables state of the simulation is orders of magnitude smaller than the naive 2^16 * 32 bit + 1 bit. I wrote a small program that analyses the dataflow of a byte code program (and initial memory setup) for the VM. After analyzing my program emits Haskell code to run the given byte code. If anyboby is interested, I can document my program and put it online somewhere. I also made pretty graphs of the dataflow with graphviz.

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ANN: bloxorz clone
by Patai Gergely 07 Jul '09

07 Jul '09

Hello all, This post is not about my own creation, it's just a little fun program written by a student of mine. You can install the bloxorz package to try it out, and read more about its background on my blog: http://just-bottom.blogspot.com/2009/07/playing-and-learning.html Gergely -- http://www.fastmail.fm - Access all of your messages and folders wherever you are

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DHUG: meeting tomorrow (July 7th) in Utrecht
by Chris Eidhof 07 Jul '09

07 Jul '09

Hi everyone, The Dutch HUG [1] will meet again tomorrow. This time we'll meet in Utrecht again, at the Stairway To Heaven. The meeting will start at 19h. The venue is a sometimes a bit noisy, but there's beer and nice big tables. If you want to hear everything about the Dutch HUG straight from the horse's mouth: make sure you're at the meetings and subscribed to the mailing list [2]. We already have more than 40 members after two meetings! See you tomorrow! -chris [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug

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How to read safely?
by Magicloud Magiclouds 07 Jul '09

07 Jul '09

Hi, Read often throws runtime errors, which breaks the robust of the problem. How to deal with it? Without lost too much proformance (so reads is a no). At least, if its error could be catched, that'd be better. -- 竹密岂妨流水过山高哪阻野云飞

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Could FFI support pass-by-value of structs?
by Maurício 07 Jul '09

07 Jul '09

It's not usual, but it is allowed to have values of structs passed between functions directly instead of using pointers: /*****/ struct ex { int x; int y; int z; }; ex example_functions (ex p) { (...) } /*****/ Would it be possible to allow that in Haskell FFI by, say, allowing any instance of Storable to be used in a 'foreign' declaration? Like: -- data Ex = (...) instance Storable Ex where sizeOf _ = ... alignment = sizeOf (...) foreign import ccall "example_functions" exampleFunction :: Ex -> IO Ex -- Thanks, Maurício

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Where can I get GHC for Solaris?
by John Ky 07 Jul '09

07 Jul '09

Hi, Anyone know where I can get the GHC compiler and libraries for Solaris?: SunOS sun05 5.9 Generic_118558-30 sun4u sparc SUNW,Sun-Fire-280R I tried to compile GHC myself and got the following error: $ ./configure --enable-hc-boot checking build system type... sparc-sun-solaris2.9 checking host system type... sparc-sun-solaris2.9 checking target system type... sparc-sun-solaris2.9 Canonicalised to: sparc-sun-solaris2 checking for ghc... no checking for nhc... no checking for nhc98... no checking for hbc... no checking for ld... /usr/ccs/bin/ld checking for path to top of build tree... ./configure: -v0: command not found ./configure: utils/pwd/pwd: No such file or directory configure: error: cannot determine current directory Thanks -John

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