- Haskell-Cafe - Haskell.org

(Read r) => IO (Maybe r)
by Jacek Generowicz 17 Dec '10

17 Dec '10

Hi, What are some interesting, idiomatic ways of writing something similar to the following, using a) Only standard utilities b) Non-standard utilities getValidatedInteger = do maybeInt <- maybeGet case maybeInt of Just int -> return int Nothing -> do putStrLn "That doesn't seem to be an integer. Try again." getValidatedInteger maybeGet :: (Read r) => IO (Maybe r) maybeGet = getLine >>= return . maybeReadS maybeReadS text = case reads text of [(int, rest)] | all (== ' ') rest -> Just int _ -> Nothing

2 1

bus error trying to use Network
by Marcelo Sousa 17 Dec '10

17 Dec '10

Hi, I'm getting a bus error with Network library on my Mac OS X 10.6.5. I have ghc 6.12.3, network 2.3. As an example: main = withSocketsDo $ do sock <- listenOn $ PortNumber 4244 (handle,host,port) <- accept sock hPutStr handle "Hi!" sClose sock Any idea of what the problem could be? If I use the Network.Socket library it works fine: main = withSocketsDo $ do sock <- socket AF_INET Stream 0 setSocketOption sock ReuseAddr 1 bindSocket sock (SockAddrInet 4244 iNADDR_ANY) listen sock 1 (csock,_) <- accept sock send csock "Hi!\n" sClose csock sClose sock Cheers, Marcelo

1 0

What is NoPush module?
by Magicloud Magiclouds 17 Dec '10

17 Dec '10

Hi, When I compiling happstack with ghc 7, I got an error: src/Happstack/Server/Internal/Handler.hs:29:8: Could not find module `NoPush': Use -v to see a list of the files searched for. I looked around, there is no clue for the module NoPush, except it has a function call withNoPush. What is it? -- 竹密岂妨流水过山高哪阻野云飞

2 2

syb-with-class problem.
by Magicloud Magiclouds 17 Dec '10

17 Dec '10

Hi, I am trying to make happstack work in my box, which is using ghc 7. While compiling HStringTemplate, which I did a little modification to make it work, I found that it does not work with template-haskell-2.5.0.0. I changed it to use template-haskell-2.4.0.1. This step worked fine. Then I got another error: $ cabal install Resolving dependencies... Configuring syb-with-class-0.6.1.1... Preprocessing library syb-with-class-0.6.1.1... Building syb-with-class-0.6.1.1... [1 of 4] Compiling Data.Generics.SYB.WithClass.Context ( Data/Generics/SYB/WithClass/Context.hs, dist/build/Data/Generics/SYB/WithClass/Context.o ) [2 of 4] Compiling Data.Generics.SYB.WithClass.Basics ( Data/Generics/SYB/WithClass/Basics.hs, dist/build/Data/Generics/SYB/WithClass/Basics.o ) [3 of 4] Compiling Data.Generics.SYB.WithClass.Derive ( Data/Generics/SYB/WithClass/Derive.hs, dist/build/Data/Generics/SYB/WithClass/Derive.o ) Data/Generics/SYB/WithClass/Derive.hs:23:1: Bad interface file: /home/magicloud/.cabal/lib/template-haskell-2.4.0.1/ghc-7.0.1/Language/Haskell/TH.hi Something is amiss; requested module template-haskell-2.4.0.1:Language.Haskell.TH differs from name found in the interface file template-haskell:Language.Haskell.TH cabal: Error: some packages failed to install: HStringTemplate-0.6.3 depends on syb-with-class-0.6.1.1 which failed to install. syb-with-class-0.6.1.1 failed during the building phase. The exception was: ExitFailure 1 How to resolve this? -- 竹密岂妨流水过山高哪阻野云飞

3 4

Printing of asynchronous exceptions to stderr
by Mitar 17 Dec '10

17 Dec '10

Hi! I have been spend some time to debug this in my program. ;-) Why is ThreadKilled not displayed by RTS when send to thread (and unhandled), but any other exception is? For example, I am using custom exceptions to signal different kinds of thread killing. Based on those my threads cleanup in different ways. But after they cleanup they keep "running". Not really running as their computations were interrupted. But simply hanging there. And if then new custom exception arrives it is not handled anymore (as it is after my normal exception handlers already cleaned everything) and it is printed to stderr. Now I added "`finally` throwIO ThreadKilled" at the end of my cleanup computations to really kill the thread. So that possible later exceptions are not delivered anymore. Why it is not so that after all computations in a thread finish, thread is not killed/disposed of? Because it can be reused by some other computation? But then my exceptions could be delivered to wrong (new) thread while they were meant for old one? Mitar

5 21

Re: [Haskell-cafe] [Haskell] Functor => Applicative => Monad
by Tillmann Rendel 17 Dec '10

17 Dec '10

Hi, John Smith wrote: > I would like to formally propose that Monad become a subclass of > Applicative A lot of code would break because of this change, but all problems should be reported at compile time, and are easy to fix. In most of the cases, either adding obvious Functor and Applicative instances to a library; or deleting such instances from a client, I would expect. This kind of clean-up would actually increase the quality of the library, however, and might therefore be acceptable. > The change is described on the wiki at > http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal There you write, among other things: > fail should be removed from Monad; a failed pattern match could error in the same way as is does for pure code. How is this part of the proposal related to Functor and Applicative? Since code depending on the current behavior can not be detected at compile time, this is a more serious change in a way: code keeps compiling but changes its meaning. Can you estimate how much code would break because of this change? Would it be possible to keep user-defined failure handling in do notation without keeping fail in Monad? Tillmann

9 15

TLS package server interface do not seem to allow for asyncrhonious communication
by Mads Lindstrøm 16 Dec '10

16 Dec '10

Hi Haskeleers, I got a working SSL server using the TLS package. Great. However, I really intended to use SSL for an asynchronous server. That is, the server must constantly listen for a client message, which may result in zero or more messages send _to_ the client. And the server will, without being waken up by a client message, send messages to the client. And this interface http://hackage.haskell.org/packages/archive/tls/0.3.1/doc/html/Network-TLS-… do not seem to allow for an asynchronous server. That is, whenever I call: recvPacket :: Handle -> TLSServer IO (Either TLSError [Packet]) or recvData :: Handle -> TLSServer IO ByteString The server is stuck until the client sends a message. Or am I missing something? Contrast this to Network package socket implementation. Here I can spawn two threads and simultaneously listen for client messages and send messages to the client. It seems to me that a general SSL implementation should not preclude asynchronous servers. I know, the TLS package is not more than a few months old and one can already use it for SSL servers and clients, which is quite impressive. So do not read this as a negative comment, more as suggestions for a better interface. Regards, Mads Lindstrøm

4 6

cpphs is missing from new Haskell.org server
by Dimitry Golubovsky 16 Dec '10

16 Dec '10

Hi, One more thing is missing after migration. The link found on the cpphs package page at Hackage, http://haskell.org/cpphs/ points to non-existing page. Thanks. -- Dimitry Golubovsky Anywhere on the Web

1 0

Monadic Design Patterns for the Web C9 Video Series -- Part 2 is live!
by Greg Meredith 16 Dec '10

16 Dec '10

Dear Haskellians, Just in time for the Holidays! Best wishes, --greg ---------- Forwarded message ---------- From: Charles Torre <ctorre(a)microsoft.com> Date: Tue, Dec 14, 2010 at 11:31 AM Subject: Part 2 is live! To: Meredith Gregory <lgreg.meredith(a)gmail.com> Hi Greg! Part 2 is now live on 9. Front and center: http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic… Thank you! See you in the new year to continue with this great series. Best, C -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

1 0

Infinite lists in real world programs
by Yves Parès 16 Dec '10

16 Dec '10

Hello Café, I was wondering if using infinite lists was a viable and efficient solution in haskell programs (I mean not simple prototypes) : I was considering using them to model agents in a hierarchical multi-agent application for school. A list would representate the state of an agent at a step of the program. Let's say we have two simple agents, one multiplying its input by 2 and the other dividing it by 4 : agent1 = fmap (*2) agent2 = fmap (/4) allValues = xs where ys = agent1 xs xs = 100:agen2 ys main = do mapM_ print $ take 100 allValues Of course, in a real program, an agent would rather take a list of multiple agents (i.e. a list of lists) in input, so that its ouput could depend on what several agents feed him. Some could state what I'm trying to do is FRP, and I agree. But it remains a simple goal so I'd like to keep the program simple, and not go into the whole complexity of a FRP framework (and I'm working with a non-haskeller). For instance, with my solution, I cannot dynamically connect or disconnect agents during the runtime, but I'll will not need to do that in my program. Besides, I'd like to try to implement this myself, not use an already existing framework. So is it viable or would the use of multiple infinite lists kill the performances?

3 8