- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] [Haskell] ANNOUNCE: set-monad
by George Giorgidze 18 Jun '12

18 Jun '12

Hi Derek, On 16 June 2012 21:53, Derek Elkins <derek.a.elkins(a)gmail.com> wrote: > The law that ends up failing is toList . > fromList /= id, i.e. fmap g . toList . fromList . fmap h /= fmap g . > fmap h This is not related to functor laws. The property that you desire is about toList and fromList. The toList and fromList functions are not required to satisfy toList . fromList /= id. This does not hold for Data.Set and it does not hold for the Data.Set.Monad wrapper either. Duplicates are lost when lists are converted into sets. Other instances of fromList and toList also fail to satisfy the property that you desire (e.g., Map). Cheers, George

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Re: [Haskell-cafe] Haskell (GHC 7) on ARM
by Ben Gamari 18 Jun '12

18 Jun '12

Joshua Poehls <joshua(a)poehls.me> writes: > Hello Ben, > Hello, Sorry for the latency. I'm currently on vacation in Germany so I haven't had terribly consistent Internet access. I've Cc'd haskell-cafe@ as I've been meaning to document my experiences anyways and your email seems like a good excuse to do this. > I just got a Raspberry Pi and I'm interested in running Haskell on it. So > far I'm running Arch Linux ARM and I noticed there is is a GHC package > available, but it is version 6.12.3. > > Doing research online I saw that you did some work on adding GHCi support > for ARM in GHC 7.4. I was wondering if you had anything to share related to > running GHC/GHCI 7.4 on ARM, specifically Arch Linux ARM. > Indeed my ARM linker patch was merged into GHC 7.4.2. This means that ARM is now a fully supported target platform for GHC, complete with GHCi support (which is pretty much necessary for many ubuiquitous packages). My ARM development environment consists of a PandaBoard (and before this a BeagleBoard) running Linaro's Ubuntu distribution. For this reason, I'm afraid I won't be of terribly much help in the case of Arch. That being said, GHC has a wonderful build system which has never given me trouble (except in cases of my own stupidity). Just installing the required development packages and following the build instructions on Wiki[1] is generally quite sufficient. I've also have some notes of my own[2] where I've collected my own experiences. As you likely know, GHC doesn't have a native ARM code generator of its own, instead relying on LLVM for low-level code generation. For this reason, you'll first need to have a working LLVM build. I have found that the ARM backend in the 3.0 release is a bit buggy and therefore generally pull directly from git (HEAD on my development box is currently sitting at 7750ff1e3cbb which seems to be stable). That being said, I haven't done much work in this space recently and it's quite likely that the 3.1 release is better in this respect. Compiling LLVM is one case where I have found the limited memory of my BeagleBoard (which is I believe is similar to the Raspberry Pi) to be quite troublesome. There are cases during linking where over a gigabyte of memory is necessary. At this stage the machine can sit for hours thrashing against its poor SD card making little progress. For this reason, I would strongly recommend that you cross-compile LLVM. The process is pretty straightforward and I have collected some notes on the matter[3]. The size of GHC means that in principle one would prefer to cross-compile. Unfortunately, support[4] for this is very new (and perhaps not even working/complete[5]?) so we have to make due with what we have. The build process itself is quite simple. You will want to be very careful to ensure that the ghc/libraries/ trees are sitting on the same branch as your ghc/ tree (with the ghc/sync-all script). Inconsistency here can lead to very ambiguous build failures. Otherwise, the build is nearly foolproof (insofar as this word can describe any build process). The build takes the better part of a day on my dual-core Pandaboard. I can't recall what the maximum memory consumption was but I think it might fit in 512MB. Lastly, if I recall correctly, you will find that you'll need to first build GHC 7.0 bootstrapped from 6.12 before you can build 7.4 (which requires a fairly new stage 0 GHC). Let the list know if you encounter any issues. I'll try to dust off my own development environment once I get back to the states next week to ensure that everything still works. I've been meaning to setup the PandaBoard as a build slave as Karel's has been failing for some time now (perhaps you could look into this, Karel?). Moreover, perhaps I can find a way to produce redistributable binaries to help others get started. Cheers, - Ben [1] http://hackage.haskell.org/trac/ghc/wiki/Building [2] http://bgamari.github.com/posts/ghc-llvm-arm.html [3] http://bgamari.github.com/posts/cross-compiling_llvm.html [4] http://hackage.haskell.org/trac/ghc/wiki/CrossCompilation [5] http://www.haskell.org/pipermail/cvs-ghc/2012-February/070791.html

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Single board computers on HaskellWiki
by Henk-Jan van Tuyl 18 Jun '12

18 Jun '12

L.S., As more and more people start to develop software in Haskell for single board computers, I created the HaskellWiki pages Arduino[0], ARM[1], Raspberry Pi[2], BeagleBoard[3] and PandaBoard[4]. I hope experts on the matter will give these pages more body (for example, describe what is necessary to get Haskell programs running on these platforms) and add links to relevant web pages. The PandaBoard page needs an image, I couldn't find one with public domain license. If you don't know how to edit HaskellWiki pages, read the contribute page[5] first. To get your appetite wet, if you are not involved in this yet, take a look at the Arduino Weather Clock[6]; this is really a fun project. Regards, Henk-Jan van Tuyl [0] http://www.haskell.org/haskellwiki/Arduino [1] http://www.haskell.org/haskellwiki/ARM [2] http://www.haskell.org/haskellwiki/Raspberry_Pi [3] http://www.haskell.org/haskellwiki/BeagleBoard [4] http://www.haskell.org/haskellwiki/PandaBoard [5] http://www.haskell.org/haskellwiki/HaskellWiki:Contributing [6] http://www.geeky-gadgets.com/arduino-weather-clock-13-01-2010/ -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Receiving UDP messages on Windows 7
by Edward Amsden 18 Jun '12

18 Jun '12

Hi all, I have the following program, which I'm running using runghc 7.4.1 with HP2012.2 on Windows 7: == {-# LANGUAGE OverloadedStrings #-} module Main where import Network.Socket hiding (send, sendTo, recv, recvFrom) import Network.Socket.ByteString import qualified Data.Text as T import qualified Data.Text.Encoding as T import qualified Data.Text.IO as T port :: String port = show (5555 :: Int) main :: IO () main = withSocketsDo $ do addrInf:_ <- fmap (filter ((== AF_INET) . addrFamily)) $ getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just port) putStrLn "Address info: " print addrInf sock <- socket (addrFamily addrInf) Datagram defaultProtocol putStrLn "Socket created" bindSocket sock (addrAddress addrInf) putStrLn "Socket bound" let procMessages = do (msg, addr) <- recvFrom sock 1024 let addrTxt = T.pack $ show addr msgTxt = T.decodeUtf8 msg outputTxt = T.concat [addrTxt, " says ", msgTxt] T.putStrLn outputTxt procMessages procMessages == I'm trying to receive incoming UDP packets on port 5555. Unfortunately, when I run the program it does not receive packets. It prints the address info, and the messages that the socket has been created and bound. When I run Wireshark I can see that there are indeed incoming UDP packets arriving on port 5555 (from another computer on the local network running a proprietary program). The other bit of information that may be useful is that the machine has 2 network interfaces. However, when I replace the Nothing parameter of getAddrInfo with (Just "192.168.1.3") which is the address of the correct NIC, the behavior is as before. Is there something I'm missing?

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Noticed this change about infix decls in GHC 7.4.2
by Christopher Done 18 Jun '12

18 Jun '12

So I have some module, in a work project that I'm portnig from GHC 6.12.3 to GHC 7.4.2, module Data.Monoid.Operator where import Data.Monoid (++) :: Monoid a => a -> a -> a (++) = mappend infixr 5 ++ This compiles happily on GHC 6.12.3, but on 7.4.2 says: src/Data/Monoid/Operator.hs:9:10: Ambiguous occurrence `++' It could refer to either `Data.Monoid.Operator.++', defined at src/Data/Monoid/Operator.hs:8:1 or `Prelude.++', imported from `Prelude' at src/Data/Monoid/Operator.hs:3:8-27 (and originally defined in `GHC.Base') It seems that it used to assign higher priority to the declared thing in the current module over the imported one. Is this intentional? I'd suspect not, given that if I comment the binding out: -- (++) :: Monoid a => a -> a -> a -- (++) = mappend I get: src/Data/Monoid/Operator.hs:9:10: The fixity signature for `++' lacks an accompanying binding (The fixity signature must be given where `++' is declared) Which seems to contradict the previous error message. Bug? Ciao! P.S. Yes, I know I can fix this by not importing Prelude. P.P.S Yes, I know there's a <> or <+> defined now, but I prefer (++), thanks.

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lightweight alternative to mtl?
by Anton Kholomiov 18 Jun '12

18 Jun '12

What are you using instead of mtl? I need simple type for State. The more classy it gets the harder error messages are to understand. I've installed new package. Silently it installed new mtl. And here I'm staring into three lines of code for half an hour trying to understand where I misused the types. Compiler tells me long story about functional dependencies. After downgrade it takes me a second to find the mistake. Ohh type mismatch, forgot to pass an argument. It's class for strict and lazy states. Maybe it's better to take approach of containers (the same interface and different modules)? Anton

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Re: [Haskell-cafe] Safe Haskell at the export symbol granularity?
by Ryan Newton 18 Jun '12

18 Jun '12

Thanks David. I'm glad to see it was discussed in the wiki. (Btw, my 2 cents is that I like the comment pragmas more than new keywords.) The issue that I think doesn't make it into the wiki is of splitting, not modules, but* type-classes*. That's where I think it becomes a more serious issue. Do you think a symbol-level Safe Haskell would be able to distinguish one method of a type class as unsafe, while the others are safe? -Ryan P.S. In my two examples -- There's only one "Acc" type and Accelerate's "fold" can pretty easily be moved into an .Unsafe module, though it breaks the one-giant-module-for-the-whole-programming-model thing it has going now. In the Par example on the other hand type classes are used to abstract over different implementations, so that's where we run into the safe/unsafe factoring problem.

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Data.Binary hanging when used with socket - feature?
by Paul Brenman 17 Jun '12

17 Jun '12

I'm using the 0.5.1.0 binary package (i.e. Data.Binary/Data.Binary.Get) to encode/decode a lazy bytestring. Unfortunately, decode/get are hanging, possibly due to the underlying chunking logic in Data.Binary.Get. The lazy bytestring is being populated from a socket (via "socketTohandle sock ReadWriteMode . hGetContents"). Periodically, encoded data structures are received from the socket. When the encoded data structure happens to end exactly on a chunk boundary, and in my specific case the final encoded item is an Int(64), the Data.Binary.Get.getBytes function wants to force the next chunk to be read into the lazy bytestring before returning the decoded item (see comment in file at line 333 that reads "forces the next chunk before this one is returned"). Unfortunately, since the socket is not yet ready to send anything the call to decode/get hangs. The assumption seems to be that the lazy bytestring isn't populated from a source that can potentially block. Is this reading correct? If so, are there any workarounds? Appreciate any advice - production code.

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event handler
by Corentin Dupont 17 Jun '12

17 Jun '12

Hi folks, I'm trying to make a simple event driven engine. It simply consists of two functions: - "addEvent", where you pass the event name with a callback, - "triggerEvent" where you pass the event name with the data. the data shall be passed to the callback of the corresponding event. I have trouble making it correctly typed. Here is my try: * type Player = Int --dummy types for the example type Rule = Int data EventEnum = NewPlayer | NewRule deriving Eq data Data = P Player | R Rule data Handler = H (Data -> IO ()) addEvent :: EventEnum -> Handler -> [(EventEnum, Handler)] -> [(EventEnum, Handler)] addEvent e h es = (e,h):es triggerEvent :: EventEnum -> Data -> [(EventEnum, Handler)] -> IO () triggerEvent e d es = do let r = lookup e es case r of Nothing -> return () Just (H h) -> h d* The trouble is that I want the user to be only able to add an event that is compatible with its handler: For example the event NewPlayer should have a handler of type Player -> IO (). The data passed when triggering this event should be only of type Player. How can I do that? It sound like dependant typing... Thanks! Corentin

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Haskell is rapidly approaching the top 20
by Henk-Jan van Tuyl 17 Jun '12

17 Jun '12

L.S., From the TIOBE web site[0]: -----✂---- TIOBE Programming Community Index for June 2012 June Headline: Haskell is rapidly approaching the top 20 Last month we asked ourselves the question what language could become the next big new programming language. We suggested several candidates such as Scala, Erlang and Clojure. Clearly, the new thing was expected to come from the functional programming field. A functional language not explicitly mentioned was Haskell. And this month it was Haskell that jumped from #35 to #25. Looking at the TIOBE trend graph of Haskell (starting in 2003) it shows a constant rise, with peaks in 2006, 2010 and now in 2012. This certainly sounds promising. -----✂---- Regards, Henk-Jan van Tuyl [0] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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