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Lists of Existential DT
by Yves Parès 28 Feb '10

28 Feb '10

Hello! I have a class Drawable, and some datatypes which are instances of it, and I would like to be able to draw them all at once! drawMany window [image, text, otherImage] I think the type of the function drawMany would be: drawMany :: Window -> [forall a. (Drawable a) => a] -> IO () However it doesn't work. I know one solution is to make a new datatype (e.g. DrawableObj) which will be: data DrawableObj = forall a (Drawable a) => DrawableObj a And then declare drawMany as: drawMany :: Window -> [DrawableObj] -> IO () But to use it I have to wrap up myself every drawable in a DrawableObj: drawMany window [DrawableObj image, DrawableObj text, DrawableObj otherImage] Is there another more suitable way to handle a list of drawables? ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Lists-of-Existential-DT-tp27735354p27735354.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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native threads vs. -threaded
by Donn Cave 28 Feb '10

28 Feb '10

I've sketched out a Haskell interface to a C++ toolkit that uses threads. It's the Haiku (BeOS) platform API (BeOS is an obscure OS from the '90s, Haiku is a pretty faithful recent open source re-implementation.) The API is fairly obvious and I imagine much like others that support windowed graphic interfaces. You have your Window class that spawns a thread, and various virtual methods of that class support program interface with the events involving that window. The curious thing is that I can get this to work with my example program only if it is NOT linked with -threaded. If linked with -threaded, when the application comes up I have 4 threads: the main thread and window thread, and two threads apparently created by the Haskell runtime. Shortly (but not immediately) after the first callback from the window thread, the window thread quietly dies. Callbacks from the main thread work fine. Linked without -threaded, I have only the 2 natural threads, and the window thread can call back apparently without problems. I imagine I'm at fault somewhere in this, since I am also responsible for the GHC port to Haiku, but just wondering if this suggests an obvious course of inquiry to anyone. I assume it's not working as intended, as from the documentation I would rather have guessed that -thread would be required in this situation. thanks! Donn Cave, donn(a)avvanta.com

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Cabal will not find HaXml
by Mads Lindstrøm 28 Feb '10

28 Feb '10

Hi When I do: > cabal list --simple-output | grep -i "HaXml 1.20" I get: > HaXml 1.20 > HaXml 1.20.1 > HaXml 1.20.2 But when running: > runhaskell Setup.hs configure I get: > Configuring XrcAccessors-0.0... > Setup.hs: At least the following dependencies are missing: > HaXml -any My cabal file looks like: <snip> Executable XrcAccessors Main-is: XrcAccessors.hs Build-Depends: base==3.0.*, pretty==1.0.*, HaXml hs-source-dirs: src </snip> This is very confusing to me. Why will Cabal not find HaXml when running Setup.hs, but do find it when running "cabal list" ? My Setup.hs looks like: > import Distribution.Simple > main = defaultMain I have tried reinstalling HaXml, but that do not help. I have noticed one thing though, when doing: > ghc-pkg I get output like: <snip> /home/ghc6103/lib/ghc-6.10.3/./package.conf: Cabal-1.6.0.3, HUnit-1.2.0.3, QuickCheck-1.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), </snip> <snip> /home/ghc6103/.ghc/x86_64-linux-6.10.3/package.conf: Cabal-1.6.0.2, Cabal-1.8.0.2, Diff-0.1.2, HDBC-2.1.1, HDBC-odbc-1.1.6.0, HDBC-odbc-2.1.0.0, HTTP-4000.0.4, HUnit-1.2.2.1, HaXml-1.20.2, MetaHDBC-0.1.0, QuickCheck-2.1.0.3, SQLLib-0.0, </snip> That is, HaXml shows itself in the /home/ghc6103/.ghc/x86_64-linux-6.10.3/package.conf list, but not in the /home/ghc6103/.ghc/x86_64-linux-6.10.3/package.conf . Is this the problem and how do I fix it? Greetings, Mads Lindstrøm

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listing mountpoints and getting their properties in Haskell
by Eugene Dzhurinsky 28 Feb '10

28 Feb '10

Hello! I need to list all currently mounted filesystems and get some stats like total space, free space, mount point and physical device. Is there any library capable of obtaining such information from OS itself? Parsing output of 'df' is locale-dependent and error-prone (because of locale settings, output settings etc). Thank you in advance. -- Eugene N Dzhurinsky

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Haskell version of "Monad Transformers and Modular Interpreters"
by Günther Schmidt 28 Feb '10

28 Feb '10

Hello, I come across the paper "Monad Transformers and Modular Interpreters" (http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.17.268) alas the code is in Gofer and I'm not certain how all of it translates to Haskell. Are there other papers out there that translate this to Haskell? Günther

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jQuery is a monad
by Graham Klyne 28 Feb '10

28 Feb '10

Nice blog post: http://importantshock.wordpress.com/2009/01/18/jquery-is-a-monad/ #g

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EDSL's using Filet-O-Fish of Barrelfish project
by C K Kashyap 28 Feb '10

28 Feb '10

Hi All, I'd been looking for a mechanism to generate "C" from Haskell as EDSL - to develop an OS. In the process, I ran into the Barrelfish project ( http://www.barrelfish.org) - They seem to be using Haskell based "DSL generator" called Filet-O-Fish - http://www.barrelfish.org/fof_plos09.pdf It will take me some time understand Filet-O-Fish more but if any of the Haskell experts have already looked at it, I was wondering if I could use Filet-O-Fish to generate EDSL's for arbitrary things. For example, lets say, I wanted to write a EDSL for Linux's network drivers. Could I use Filet-O-Fish and generate a EDSL that allows me to write a driver in a really highlevel manner and then generate the "C" code for it? -- Regards, Kashyap

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How to understand `|` in this code snippet ?
by zaxis 28 Feb '10

28 Feb '10

xxxMain = do timeout <- getEnv "xxx_TIMEOUT" case timeout of Just str | [(t, _)] <- reads str -> do addTimeout t (hPutStrLn stderr "*** TIMEOUT" >> _exit 1) return () _ -> return () ....... What does the `|` mean in "Just str | [(t, _)] <- reads str" ? Is it a logical `or` ? Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/How-to-understand-%60%7C%60-in-this-code-snippet---tp… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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redirecting ghc (as a library) output
by Martin Hilbig 27 Feb '10

27 Feb '10

hi, i'm writing a Haskell View Server for CouchDB. it communicates with couchdb over stdin and stdout. it gets JSON encoded haskell code, compiles it (like on http://www.haskell.org/haskellwiki/GHC/As_a_library) gets values, runs the given code over the given values and writes the results back (also json encoded). when there is an error in the given haskell code it should reply with the error json encoded and exit. but f.e. when the given code imports a module, which cannot be found (here Reaction) ghc just spits out a panic directly to stdin and exits like this: > ViewServer: panic! (the 'impossible' happened) > (GHC version 6.12.1 for x86_64-unknown-linux): > Could not find module `Reaction': > Use -v to see a list of the files searched for. > > > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug it puts other errors directly to stdout too, like: > Assembler messages: > Fatal error: can't create > /home/*/AlkylRadicalDecomposition.o: > Permission denied this confuses couchdb, because it expects some JSON and i cant see whats going on between them. now, how can i prevent ghc from using stdout and wrap the output in some JSON? is this even possible? with ghc 6.10 this usage of 'handle' worked for me: > main = handle (\e -> do > let e' = show (e::SomeException) > case fromException e of > Just UserInterrupt -> exitSuccess > _ -> do > let err = error2json "the impossible happened..." e' > putStrLn err > logToFile err > return []) main_loop did i got the Exception handling wrong? thanks in advance. have fun martin

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save/restore of STRef state
by Job Vranish 27 Feb '10

27 Feb '10

I was using STRefs the other day and I ran across a case where I really wanted the ability to save the state of all my references, execute some action, and then restore the state later if needed. I didn't find anything that does this on hackage so I implemented a small wrapper around STRefs which does what I want. Before I put it on hackage I thought I'd run it by haskell-cafe. It works as follows: test :: (String, Bool) test = runContext $ do a <- newRef "foo" b <- newRef False restore <- save writeRef a "bar" writeRef b True when someCondition restore x <- readRef a y <- readRef b return (x, y) If someCondition, the existing state is restored and test returns ("foo", False) otherwise nothing special happens and test returns ("bar", True) Also each reference has a unique key that can make it much easier to convert structures that use STRefs to pure stuctures. Reads, writes and creating refs are still constant time operations (with only a very very small overhead) and garbage collecting behavior should be the same as with regular STRefs. My implementation is here: http://gist.github.com/316738 What do you think? Any suggestions? Does anything like this already exist in hackage? Does this seem useful to other people besides me? :) Any glaring purity issues that I overlooked? Thanks for your input, - Job

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