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Philip Wadler video on Howard-Curry Correspondence ???
by Galchin, Vasili 27 Nov '08

27 Nov '08

Hello, I am reading re-reading Prof. Wadler paper Proofs are Programs: 19th Century Logic and 21st Century Computing but also want to re-read watch his video on same subject..... ??? Very kind thanks, Vasili

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Problem getting code from AFP08 parallel tutorial to run in parallel
by Olivier Boudry 27 Nov '08

27 Nov '08

Hi all, I'm reading the following tutorial: http://research.microsoft.com/~simonpj/papers/parallel/AFP08-notes.pdf "A Tutorial on Parallel and Concurrent Programming in Haskell" and have problems getting the expected speed improvement from running two tasks in parallel. With any version of the code present in pages 1-7 of the tutorial I keep getting the CPU stick to 50%. I did not forget to compile the code with `-threaded` and run it with `+RTS -N2` and it runs on a dual core machine on which I already used the Control.Parallel.Strategies.parMap function and got 100% CPU usage. The first version of the parallel function in the tutorial (page 6) is: parSumFibEuler :: Int −> Int −> Int parSumFibEuler a b = f 'par' (f + e) where f = fib a e = sumEuler b In the tutorial, swapping f and e on the 3rd line does the job, but in my case it doesn't change anything. C:\Temp\haskell>ghc --make -threaded SumEulerP6.hs [1 of 1] Compiling Main ( SumEulerP6.hs, SumEulerP6.o ) Linking SumEulerP6.exe ... C:\Temp\haskell>SumEulerP6 +RTS -N1 sum: 119201850 time: 36.890625 seconds C:\Temp\haskell>SumEulerP6 +RTS -N2 sum: 119201850 time: 36.859375 seconds Next page of the tutorial the tasks are explicitly sequenced so the code does not depend on the ordering of the two `+` operands: parSumFibEuler :: Int -> Int -> Int parSumFibEuler a b = f `par` (e `pseq` (f + e)) where f = fib a e = sumEuler b With once again a disappointing result: C:\Temp\haskell>ghc --make -threaded SumEulerP7.hs [1 of 1] Compiling Main ( SumEulerP7.hs, SumEulerP7.o ) Linking SumEulerP7.exe ... C:\Temp\haskell>SumEulerP7 +RTS -N1 sum: 119201850 time: 36.875 seconds C:\Temp\haskell>SumEulerP7 +RTS -N2 sum: 119201850 time: 36.75 seconds I tried this on a Windows XP Dell Dual Core with GHC 6.10.1 and on a iMac Dual Core with GHC 6.8.3 and got the same result on both. I'm probably missing something really stupid but I'm lacking the "parallel thinking" skills to understand how to look at the problem and resolve it. Any pointers? Thanks, Olivier.

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Need machine for DPH benchmarking
by Dominic Steinitz 27 Nov '08

27 Nov '08

> we, the DPH team, are at the moment in the very unfortunate situation > of not having a proper machine for running our benchmarks on. Could a > kind soul maybe give us (i.e., me) access to a quadcore or 2xquadcore > x86 Linux or OS X machine? I only need to build ghc on it and run > small benchmarks which never take more than a couple of minutes, maybe > once every couple of days or so. We do need to use all cores, though, > so no other CPU-intensive processes can be running during > benchmarking. This is only for a week or two, until we get our own > machine. We would be eternally grateful and won't forget you when DPH > takes over the world. > > Roman > Roman, Graeme can set you up on an 8 core machine if you email him. Dominic.

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Instances that shouldn't overlap
by oleg＠okmij.org 27 Nov '08

27 Nov '08

Paul Johnson wrote: > class (Eq a) => AppEq f a where > instance (Applicative f, Eq a) => AppEq f a where > instance (Ord a) => AppEq Interval a where In Haskell, instances are selected based solely on the types in the head. Constraints like `Applicative f' are not consulted when the instance is being selected. Rather, constraints are checked after the instance has been selected. The above two instances are overlapping since Interval is a particular case of `f'. That said, it is possible to select an instance based on constraints. The approach is described at http://haskell.org/haskellwiki/GHC/AdvancedOverlap

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Finalisers
by Andrew Coppin 27 Nov '08

27 Nov '08

Here's an interesting question... Is it possible to attach finalisers to a value? (That is, have some Haskell code executed when the item in question is reclaimed by the GC.) I'm interested in knowing whether a particular data structure is shared (i.e., whether it's safe to mutate it or whether it must be copied first), and a simple reference-counting scheme looks like the easiest option.

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build tools and Cabal
by John Lato 26 Nov '08

26 Nov '08

Hello, Cabal allows specifying arguments for tools it recognizes on the command line, e.g. runhaskell Setup.hs configure --c2hs-option=some_option Unfortunately, I can't find a way to make this work with .cabal (or .buildinfo) files, except for the specific cases of ghc, hugs, and nhc98 options. Am I missing something? If not, could this be added easily? It would be very helpful as I'm trying to work around the broken cpp Apple provides. Thanks, John

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Re: [Haskell] Wait for *either* MVar to be set
by Isaac Dupree 26 Nov '08

26 Nov '08

Peter Verswyvelen wrote: > ... by spawning and killing two threads (which might be an expensive operation, I'm not sure) pretty cheap in GHC -- they're not system threads > Am I wrong in this? If so, is this something that might be considered as a future enhancement in the GHC libraries and runtime? Also, look at STM (e.g. TVars), which is designed to do what you want more directly, I think. (timeouts might still use the GHC-thread thing. Don't worry about its performance unless you're measurably suffering from it...) -Isaac

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monads with take-out options
by Greg Meredith 26 Nov '08

26 Nov '08

Haskellians, Some monads come with take-out options, e.g. - List - Set In the sense that if unit : A -> List A is given by unit a = [a], then taking the head of a list can be used to retrieve values from inside the monad. Some monads do not come with take-out options, IO being a notorious example. Some monads, like Maybe, sit on the fence about take-out. They'll provide it when it's available. Now, are there references for a theory of monads and take-out options? For example, it seems that all sensible notions of containers have take-out. Can we make the leap and define a container as a monad with a notion of take-out? Has this been done? Are there reasons for not doing? Can we say what conditions are necessary to ensure a notion of take-out? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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template haskell overly conservative during splicing?
by Nicolas Frisby 26 Nov '08

26 Nov '08

When using template haskell (via Derive) to generate this (exact) instance: instance Foldable ((->) Int) => Foldable Data.Derivable.InterpreterLib.Test.List where foldMap f (Cons x0 x1) = (const mempty Cons `mappend` foldMap f x0) `mappend` foldMap f x1 foldMap f (Nil) = const mempty Nil I realize the context is insatisfiable. My issue, is that I can't even reach that "challenge". I get this error instead: Malformed type AppT ArrowT (ConT GHC.Base.Int) When splicing generated code into the program I couldn't find an existing ticket or discussion for this issue relying on the phrase "malformed type". I couldn't even find the source that generates that string in haskell-src, template-haskell, or ghc-6.8.2. Can anybody help? Thanks for your time.

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IRC question
by Galchin, Vasili 26 Nov '08

26 Nov '08

Hello, I am using Ubuntu Linux and I want to get the Haskell IRC feed. What IRC client can I use and how to configure? Thanks, Vasili

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