June 2010 - Haskell-Cafe

Contacting Planet Haskell
by Jason Dagit 10 Jun '10

10 Jun '10

Hello, I recently tried to send an email to the planet Haskell admins. The webpage says to use planet(a)community.haskell.org. The mail was undelivered after several days and the daemon gave up trying to deliver it. My sending did overlap with the haskell.org downtown but I don't think that should have been a problem because: 1) It looks like it retried after haskell.org was back up 2) I'm pretty sure community is a different host than haskell.org 3) Shouldn't a backup mx handle this? Thanks, Jason

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Removing alternate items from a list
by R J 10 Jun '10

10 Jun '10

What's the cleanest definition for a function f :: [a] -> [a] that takes a list and returns the same list, with alternate items removed? e.g., f [0, 1, 2, 3, 4, 5] = [1,3,5]? _________________________________________________________________ The New Busy is not the old busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

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A Finally Tagless Pi Calculus
by Edward Kmett 10 Jun '10

10 Jun '10

Keegan McAllister gave a very nice talk at Boston Haskell last night about "First Class Concurrency". His slides are available online at http://t0rch.org/ His final few slides covered the Pi calculus: http://en.wikipedia.org/wiki/Pi_calculus I took a few minutes over lunch to dash out a finally tagless version of the pi calculus interpreter presented by Keegan, since the topic of how much nicer it would look in HOAS came up during the meeting. For more information on finally tagless encodings, see: http://www.cs.rutgers.edu/~ccshan/tagless/jfp.pdf<http://www.cs.rutgers.edu/%7Eccshan/tagless/jfp.pdf> Of course, Keegan went farther and defined an encoding of the lambda calculus into the pi calculus, but I leave that as an exercise for the reader. ;) -Edward Kmett > {-# LANGUAGE Rank2Types, TypeFamilies, FlexibleInstances #-} > module Pi where > import Control.Applicative > import Control.Concurrent A finally tagless encoding of the Pi calculus. Symantics is a portmanteau of Syntax and Semantics. > class Symantics p where > type Name p :: * > new :: (Name p -> p) -> p > out :: Name p -> Name p -> p -> p > (|||) :: p -> p -> p > inn :: Name p -> (Name p -> p) -> p > rep :: p -> p > nil :: p > embed :: IO () -> p Type level fixed points > newtype Nu f = Nu { nu :: f (Nu f) } > fork :: IO () -> IO () > fork a = forkIO a >> return () > forever :: IO a -> IO a > forever p = p >> forever p Executable semantics > instance Symantics (IO ()) where > type Name (IO ()) = Nu Chan > new f = Nu <$> newChan >>= f > a ||| b = forkIO a >> fork b > inn (Nu x) f = readChan x >>= fork . f > out (Nu x) y b = writeChan x y >> b > rep = forever > nil = return () > embed = id A closed pi calculus term > newtype Pi = Pi { runPi :: forall a. Symantics a => a } > run :: Pi -> IO () > run (Pi a) = a > example = Pi (new $ \z -> (new $ \x -> out x z nil > ||| (inn x $ \y -> out y x $ inn x $ \ y -> nil)) > ||| inn z (\v -> out v v nil)) A pretty printer for the pi calculus > newtype Pretty = Pretty { runPretty :: [String] -> Int -> ShowS } > > instance Symantics Pretty where > type Name Pretty = String > new f = Pretty $ \(v:vs) n -> > showParen (n > 10) $ > showString "nu " . showString v . showString ". " . > runPretty (f v) vs 10 > out x y b = Pretty $ \vs n -> > showParen (n > 10) $ > showString x . showChar '<' . showString y . showString ">. " . > runPretty b vs 10 > inn x f = Pretty $ \(v:vs) n -> > showParen (n > 10) $ > showString x . showChar '(' . showString v . showString "). " . > runPretty (f v) vs 10 > p ||| q = Pretty $ \vs n -> > showParen (n > 4) $ > runPretty p vs 5 . > showString " | " . > runPretty q vs 4 > rep p = Pretty $ \vs n -> > showParen (n > 10) $ > showString "!" . > runPretty p vs 10 > nil = Pretty $ \_ _ -> showChar '0' > embed io = Pretty $ \_ _ -> showString "{IO}" > instance Show Pi where > showsPrec n (Pi p) = runPretty p vars n > where > vars = fmap return vs ++ > [i : show j | j <- [1..], i <- vs] where > vs = ['a'..'z'] Pi> example nu a. (nu b. (b<a>. 0 | b(c). c<b>. b(d). 0) | a(b). b<b>. 0)

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Re: [Haskell-cafe] Removing alternate items from a list
by John Lato 10 Jun '10

10 Jun '10

> From: Markus L?ll <markus.l2ll(a)gmail.com> > > So out of curiosity i took the definitions given in this thread, and > tried to run timing-tests. > Here's what I ran: >> ghc -prof -auto-all -o Test Test.h >> Test +RTS -p > and then looked in the Test.prof file. I think this is a poor approach for timing tests, for two reasons: 1) -O or -O2. Reporting performance results from non-optimized builds is often highly misleading. 2) -auto-all inhibits performance of code, sometimes significantly. You'd be better off using a non-profiling build and criterion. Plus criterion does the statistics for you and warns about possibly invalid samples. John

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Execution call graph pruning
by C K Kashyap 10 Jun '10

10 Jun '10

Hi, I have a call grah which contains information of the edges in the following format caller callee count (time spent by the caller) =================================== foo bar 10 100 xxx yyy 20 10 zzz yyy 10 10 (I used pintool pintool.org to generate this call graph) Now, the problem is that the graph is huge and it take a long to render using 'dot' or use any visualizing tool. Even if they render, it's too cluttered to be useful. I wanted to prune the graph in such a way that I'd have only the edges corresponding to the top 10% of the time consumers. What would be a good way to do such a thing? Has anyone written some utility that I could use? -- Regards, Kashyap

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Strange discrepancy between Emacs interpreter and command-line on windows
by Arnaud Bailly 10 Jun '10

10 Jun '10

Hello, I have a strange issue which sprang today out of nowhere. When I load a certain file using bytestring package in Ghci using emacs, I got the following error: Couldn't match expected type `Data.ByteString.Internal.ByteString' against inferred type `bytestring-0.9.1.4:Data.ByteString.Internal.ByteString' In the second argument of `hPutStrLn', namely `(fromString msgs)' In a stmt of a 'do' expression: hPutStrLn cnx (fromString msgs) The same file compiles fine on the command-line. I have a single installation of haskell platform, using ghc 6.10.4, on windows XP. Thanks for any help, Arnaud Bailly

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Stone age programming for space age hardware?
by Michael Schuerig 09 Jun '10

09 Jun '10

A few days ago, I watched a presentation by Gerard Holzmann on the development methodology and programming techniques used at JPL for writing the software for the next Mars mission. I found the talk entertaining and learned a few things. http://www.infoq.com/presentations/Scrub-Spin Among the arsenal of methods they use to ensure correctness is model checking for the algorithms used as well as rather restrictive coding standards. Well, model checking sounds good, real formal oomph. But the coding itself? For one thing, they're using C. On top of that, the coding standards prohibit dynamic memory allocation, recursion, and loops without explicit bounds; see [*] for more details. I was dumbfounded, although I have known all this. I have no personal experience with either embedded or real time software, but I've been aware that C still is the most popular language for that purpose and that coding standards are very restrictive. The real reason behind my surprise was, that I was wondering how more modern languages could make inroads into such an environment. Haskell without recursion and dynamic memory allocation? Hard to imagine. I have a hunch that the real restrictions of this kind of software are not concerned with fixed memory, iterations, whatever, but rather with guaranteed bounds. If that is indeed the case, how feasible would it be to prove relevant properties for systems programmed in Haskell? Michael [*] http://spinroot.com/gerard/pdf/Power_of_Ten.pdf -- Michael Schuerig mailto:michael@schuerig.de http://www.schuerig.de/michael/

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strange errors with unix-compat
by Günther Schmidt 09 Jun '10

09 Jun '10

Hello, when compiling unix-compat I get this: 4 of 4] Compiling System.PosixCompat.Extensions ( dist/build/System/PosixCompat/Extensions.hs, dist/build/System/PosixCompat/Extensions.o ) cbits/HsUnixCompat.c: In function `unix_major': cbits/HsUnixCompat.c:4:0: warning: implicit declaration of function `major' cbits/HsUnixCompat.c: In function `unix_minor': cbits/HsUnixCompat.c:8:0: warning: implicit declaration of function `minor' cbits/HsUnixCompat.c: In function `unix_makedev': cbits/HsUnixCompat.c:12:0: warning: implicit declaration of function `makedev' /usr/gnu/bin/ar: creatin and an ghci -package gives me this: GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package unix-2.3.2.0 ... linking ... done. ghc: /home/guenni/.cabal/lib/unix-compat-0.1.2.1/ghc-6.10.4/HSunix-compat-0.1.2.1.o: unknown symbol `major' Loading package unix-compat-0.1.2.1 ... linking ... ghc: unable to load package `unix-compat-0.1.2.1' How can I fix this? Günther

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Monadic presentation of delimited continuations and dissection
by Greg Meredith 09 Jun '10

09 Jun '10

Dear Haskellians, After reading through the Dybvig, Jones and Sabry paper on the monadic presentation of delimited continuations, it seems like one can come up with a direct representation of the control contexts and meta continuations framework as an instance of McBride's dissection mechanism. Do either of you know if that work has already been done? McBride doesn't use that as an example in his Clowns and Jokers paper. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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HP/Cygwin and Curl
by Chris Dornan 09 Jun '10

09 Jun '10

Hi, I am trying to install the curl 1.3.5 hackage on the latest Haskell Platform (2010.1.0.0) on Windows 7. I have installed lib-curl-devel (7.19.6-1) on CygWin but I can't use Cygwin as (apparently) every attempt to install a package ends like this: C:\Program Files\Haskell Platform\2010.1.0.0\lib\..\mingw\bin\windres: can't open temporary file `\/cca01252.irc': No such file or directory Using cabal-install with MSYS generally works. Can HP be used with Cygwin? I have yet to find a way of installing Curl so that the curl hackage will recognise it-does anybody know how to do this? (I couldn't find any explanation of how HP can be used with Cygwin or how to install Curl for use with the hackage. If anyone knows where I can get these that would be fine much appreciated; the Haskell.org outage is compounding this.) Thanks in advance! Vhris

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