- Haskell-Cafe - Haskell.org

TLS 0.9.6, question about session resumption.
by C Gosch 23 Jul '12

23 Jul '12

Hi Cafe, I am trying to use the TLS package from hackage, and it works fine so far -- except when a client wants to do session resumption (note I am not an expert in TLS, so it might be something quite simple). In that case, I get an alert, "unexpected message", during handshake. The handshake goes like this: ClientHello (with a SessionID) ServerHello (with the same SessionID) ServerHelloDone and then the server says (AlertLevel_Fatal,UnexpectedMessage) I'm not sure whether the "ServerHelloDone" should happen when resuming. Does anyone have a hint what may be going wrong? I am using TLS10 and the tls package with version 0.9.6. Thanks, Christian

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Re: [Haskell-cafe] Cabal install fails due to recent HUnit
by Simon Hengel 23 Jul '12

23 Jul '12

On Mon, Jul 23, 2012 at 12:51:32PM -0500, Stephen Paul Weber wrote: > > Currently you would have to do the upgrade manually, as `cabal-install > > cabal-install` won't work (or alternatively edit your local > > ~/.cabl/packages/hackage.haskell.org/00-index.tar). > > Pending a fix on hackage (hopefully) I've been using `cabal copy && cabal > register` to grab specific packages today. > > How would I go about fixing my 00-index.tar? I tride de-tarring, deleting > the recent versions of HUnit, and re-tarring, and when I copy that file in > cabal starts telling me no packages exist...? E.g. with vim, you can edit files in the tar file (in-place). That is what I did, and it worked fine. Cheers, Simon

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Need help with learning Parsec
by C K Kashyap 23 Jul '12

23 Jul '12

Dear gentle Haskellers, I was trying to whet my Haskell by trying out Parsec today to try and parse out XML. Here's the code I cam up with - I wanted some help with the "gettext" parser that I've written. I had to do a dummy "char ' ') in there just to satisfy the "many" used in the xml parser. I'd appreciate it very much if someone could give me some feedback. data XML = Node String [XML] | Body String deriving Show gettext = do x <- many (letter <|> digit ) if (length x) > 0 then return (Body x) else (char ' ' >> (return $ Body "")) xml :: Parser XML xml = do { name <- openTag ; innerXML <- many innerXML ; endTag name ; return (Node name innerXML) } innerXML = do x <- (try xml <|> gettext) return x openTag :: Parser String openTag = do char '<' content <- many (noneOf ">") char '>' return content endTag :: String -> Parser String endTag str = do char '<' char '/' string str char '>' return str h1 = parse xml "" "<a>A</a>" h2 = parse xml "" "<a>A</a>" h3 = parse xml "" "<a><c></c></a>" h4 = parse xml "" "<a><c></c></a>" Regards, Kashyap

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Re: [Haskell-cafe] Haskell's type inference considered harmful
by oleg＠okmij.org 23 Jul '12

23 Jul '12

> 1. Haskell's type inference is NON-COMPOSITIONAL! Yes, it is -- and there are many examples of it. Here is an example which has nothing to do with MonomorphismRestriction or numeric literals {-# LANGUAGE ExtendedDefaultRules #-} class C a where m :: a -> Int instance C () where m _ = 1 instance C Bool where m _ = 2 main = do x <- return undefined let y = x print . fst $ (m x, show x) -- let dead = if False then not y else True return () The program prints 1. If you uncomment the (really) dead code, it will print 2. Why? The dead code doesn't even mention x, and it appears after the printing! What is the role of show x, which doesn't do anything? Exercises: what is the significance of the monadic bind to x? Why can't we replace it with "let x = undefined"? [Significant hint, don't look at it] Such a behavior always occurs when we have HM polymorphism restriction and some sort of non-parametricity -- coupled with default rules or overlapping instances or some other ways of resolving overloading. All these features are essential (type-classes are significant, defaulting is part of the standard and is being used more and more).

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System.Win32.Registry... Help?
by Anonymous Void 22 Jul '12

22 Jul '12

Hi, I'm working on a project that will require me to create and possibly set registry keys. I don't have much experience with programming on Windows either, but I'm having to learn as you don't get many *nix PCs at a computer repair shop, lol. I found a mailing list post showing how to read registry keys and was able to make a function based off of it, but I have no idea what to put into some of the arguments for regSetValueEx or regCreateKeyEx, so I'm stuck. Also, what's the best way to recursively traverse trees in the registry, are there any functions for it? Can someone please help me out with this? Thank you. {-# LANGUAGE ForeignFunctionInterface #-} import System.Win32.Types import System.Win32.Registry import Foreign.Ptr (castPtr) import Foreign.Marshal.Alloc (allocaBytes) import Foreign.C.String (peekCWString, withCWString) import Control.Exception (bracket, throwIO) -- // parse a string from a registry value of certain type parseRegString :: RegValueType -> LPBYTE -> IO String parseRegString ty mem | ty == rEG_SZ = peekCWString (castPtr mem) | ty == rEG_EXPAND_SZ = peekCWString (castPtr mem) >>= expandEnvironmentStrings | otherwise = ioError (userError "Invalid registry value type") -- // FFI import of the ExpandEnvironmentStrings function needed -- // to make use of the registry values expandEnvironmentStrings :: String -> IO String expandEnvironmentStrings toexpand = withCWString toexpand $ \input -> allocaBytes 512 $ \output -> do c_ExpandEnvironmentStrings input output 256 peekCWString output foreign import stdcall unsafe "windows.h ExpandEnvironmentStringsW" c_ExpandEnvironmentStrings :: LPCTSTR -> LPTSTR -> DWORD -> IO DWORD ---- get_key :: HKEY -> String -> String -> IO String get_key cat loc key = bracket op regCloseKey $ \x -> allocaBytes 512 $ \mem -> do ty <- regQueryValueEx x key mem 512 parseRegString ty mem where op = regOpenKeyEx cat loc kEY_QUERY_VALUE set_key :: HKEY -> String -> String -> IO () set_key cat loc key = regSetValueEx cat loc rEG_SZ??? "LPTSTR? What do I put here?" magic_win32_number_here? where op = regOpenKeyEx cat loc kEY_SET_VALUE main = get_key hKEY_CURRENT_USER loc key >>= print where loc = "Software\\7-Zip" key = "Test"

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How do I marshall a pointer over SendMessage LPARAM or WPARAM?
by Simon Peter Nicholls 22 Jul '12

22 Jul '12

Cross-posted from Haskell-beginners. Apologies for not posting in the right place (though I am a beginner and have probably made a simpleton error). I'm new to Haskell, and have had some good success with FFI so far, but using Win32's sendMessage to send a pointer in LPARAM or WPARAM is resulting in access violations at the other end. Is there some issue with my pointer conversions? Am I hitting some restriction, or missing some compiler options? It's driving me pretty crazy, after a very nice start to using Haskell. Some "sending" code: Foreign.C.String.withCWString "frustrator" $ \s -> do let wParam = System.Win32.Types.castPtrToUINT s :: System.Win32.Types.WPARAM Graphics.Win32.sendMessage wnd Graphics.Win32.wM_APP wParam 0 wndProc "receiving" code: | wmsg == Graphics.Win32.wM_APP = do s <- peekCWString $ System.Win32.Types.castUINTToPtr wParam putStrLn s return 0 The string will not be seen. Some extra notes: I can get wndProc messages and integral data generally. The pointer values match textually at both ends when "shown" to stdout. At the sending side I can pass the CWString to a regular FFI function call just fine, and castUINTToPtr will give me back a functioning Ptr for that call. I have also tried sending to & receiving from a working C++ program, without success. Access violations are reported when receiving, though again the address matches up. Silence from Haskell as before, when C++ is sending. I found someone else having an issue here: http://osdir.com/ml/haskell-cafe@haskell.org/2009-11/msg00731.html but no solution unfortunately. UPDATE since prior posting: Seems if I call out via FFI on the sending side, and set the WPARAM to L"hardcoding FTW" in a wrapper C function, the SendMessage I then invoke will work. The C code sees both the original string and the new one, but only the second works when used. I must be missing something obvious, but have toasted my grey cells at this point!

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System.Win32.Registry... Help?
by Dave Buznik 22 Jul '12

22 Jul '12

---------- Forwarded message ---------- From: Anonymous Void <bitsofchaos(a)gmail.com> Date: Sun, Jul 22, 2012 at 2:10 AM Subject: Re: [Haskell-cafe] System.Win32.Registry... Help? To: Tim Matthews <tim.matthews7(a)gmail.com> I've looked into their docs, but I can't for the life of me figure out how to convert a string to lptstr, for example: set_key :: HKEY -> String -> String -> IO () set_key cat loc key = do foo <- okey regSetValueEx foo "foo" rEG_SZ "foo" 0 where okey = regOpenKeyEx cat loc kEY_ALL_ACCESS will give me an error about it expecting LPTSTR and not [char] for "foo", but I have no idea how to convert the c/win32 types using Haskell, and msdn docs aren't helping any... I know in Python the following works, but I don't want to use Python for this app... key = OpenKey(HKEY_CURRENT_USER, "Software\\7-Zip", 0, KEY_ALL_ACCESS) SetValueEx(key, "test", 0, REG_SZ, "foobar") CloseKey(key) but that's basically where I'm stuck at right now, I've searched but wasn't able to find one single use case of regSetValueEx in Haskell, otherwise I'd be done already... I've been stuck on this for days and it's a two-hour app... I'm ashamed. LOL. On Sun, Jul 22, 2012 at 1:44 AM, Tim Matthews <tim.matthews7(a)gmail.com> wrote: > > > On Sun, Jul 22, 2012 at 5:43 PM, Tim Matthews <tim.matthews7(a)gmail.com> > wrote: >> >> >> >> On Sun, Jul 22, 2012 at 5:11 PM, Anonymous Void <bitsofchaos(a)gmail.com> >> wrote: >>> >>> >>> but I have no idea what to put into some of the arguments for >>> regSetValueEx or regCreateKeyEx, >> >> >> Those are just plain bindings to windows function so refer to msdn > > -- mysql> insert into female (vagina) select male.penis from male where penis='sperm'; Query OK, 1 row affected (0.02 sec) Records: 1 Duplicates: 0 Warnings: 0 mysql> select * from female; +--------+ | vagina | +--------+ | sperm | +--------+ 1 row in set (0.00 sec) mysql> ... N' that's how programmers make babies.

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Re: [Haskell-cafe] System.Win32.Registry... Help?
by Tim Matthews 22 Jul '12

22 Jul '12

On Sun, Jul 22, 2012 at 5:43 PM, Tim Matthews <tim.matthews7(a)gmail.com>wrote: > > > On Sun, Jul 22, 2012 at 5:11 PM, Anonymous Void <bitsofchaos(a)gmail.com>wrote: > >> >> but I have no idea what to put into some of the arguments for >> regSetValueEx or regCreateKeyEx, >> > > Those are just plain bindings to windows function so refer to msdn > <http://msdn.microsoft.com/en-us/library/windows/desktop/ms724923%28v=vs.85%…> >

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Darcs / ByteString optimisation
by Luke Worth 22 Jul '12

22 Jul '12

Hi, I'm currently investigating darcs' performance issues. The first thing I looked at was ByteString.concat, which seems to allocate a lot of extra objects. Here is the standard definition: >concat :: [ByteString] -> ByteString >concat [] = empty >concat [ps] = ps >concat xs = unsafeCreate len $ \ptr -> go xs ptr > where len = P.sum . P.map length $ xs > go a b | a `seq` b `seq` False = undefined > go [] _ = return () > go (PS p s l:ps) ptr = do > withForeignPtr p $ \fp -> memcpy ptr (fp `plusPtr` s) (fromIntegral l) > go ps (ptr `plusPtr` l) I had a bad feeling about (sum . map) inside len, so I replaced it (inside darcs) with a stricter-looking one: >myconcat :: [B.ByteString] -> B.ByteString >myconcat [] = B.empty >myconcat [ps] = ps >myconcat xs = BI.unsafeCreate len $ \ptr -> go xs ptr > where len = foldl' (\a b -> a + B.length b) 0 xs > go a b | a `seq` b `seq` False = undefined > go [] _ = return () > go (BI.PS p s l:ps) ptr = do > withForeignPtr p $ \fp -> BI.memcpy ptr (fp `plusPtr` s) (fromIntegral l) > go ps (ptr `plusPtr` l) Profiling the original one gives me 2,042,215,624 bytes allocated in the heap 3,131,672,496 bytes copied during GC 363,380,328 bytes maximum residency (43 sample(s)) 2,517,336 bytes maximum slop 643 MB total memory in use (50 MB lost due to fragmentation) while the second, with no other changes, gives 1,774,403,640 bytes allocated in the heap 2,656,774,296 bytes copied during GC 358,131,264 bytes maximum residency (42 sample(s)) 1,577,080 bytes maximum slop 614 MB total memory in use (50 MB lost due to fragmentation) Is it possible that the code using ByteString.concat is preventing the compiler from doing this optimisation? Thankyou, Luke Worth

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Re: [Haskell-cafe] Monads with "The" contexts?
by oleg＠okmij.org 21 Jul '12

21 Jul '12

The bad news is that indeed you don't seem to be able to do what you want. The good news: yes, you can. The enclosed code does exactly what you wanted: > sunPerMars :: NonDet Double > sunPerMars = (/) <$> sunMass <*> marsMass > > sunPerMars_run = runShare sunPerMars > sunPerMars_run_len = length sunPerMars_run > -- 27 where earthMass, sunMass, marsMass are all top-level bindings, which can be defined in separate and separately-compiled modules. Let's start with the bad news however. Recall the original problem: > earthMass, sunMass, marsMass :: [Double] > earthMass = [5.96e24, 5.97e24, 5.98e24] > sunMass = (*) <$> [2.5e5, 3e5, 4e5] <*> earthMass > marsMass = (*) <$> [0.01, 0.1, 1.0] <*> earthMass The problem was that the computation sunPerMars = (/) <$> sunMass <*> marsMass produces too many answers, because earthMass in sunMass and earthMass in marsMass were independent non-deterministic computations. Thus the code says: we measure the earthMass to compute sunMass, and we measure earthMass again to compute marsMass. Each earthMass measurement is independent and gives us, in general, a different value. However, we wanted the code to behave differently. We wanted to measure earthMass only once, and use the same measured value to compute masses of other bodies. There does not seem to be a way to do that in Haskell. Haskell is pure, so we can substitute equals for equals. earthMass is equal to [5.96e24, 5.97e24, 5.98e24]. Thus the meaning of program should not change if we write > sunMass = (*) <$> [2.5e5, 3e5, 4e5] <*> [5.96e24, 5.97e24, 5.98e24] > marsMass = (*) <$> [0.01, 0.1, 1.0] <*> [5.96e24, 5.97e24, 5.98e24] which gives exactly the wrong behavior (and 81 answers for sunPerMars, as easy to see). Thus there is no hope that the original code should behave any differently. > I don't know if memo can solve this problem. I have to test. Is the > `memo` in your JFP paper section 4.2 Memoization, a psuedo-code? (What > is type `Thunk` ?) and seems like it's not in explicit-sharing > hackage. BTW, the memo in Hansei is different from the memo in the JFP paper. In JFP, memo is a restricted version of share: memo_jfp :: m a -> m (m a) In Hansei, memo is a generalization of share: memo_hansei :: (a -> m b) -> m (a -> m b) You will soon need that generalization (which is not mention in the JFP paper). Given such a let-down, is there any hope at all? Recall, if Haskell doesn't do what you want, embed a language that does. The solution becomes straightforward then. (Please see the enclosed code). Exercise: how does the approach in the code relate to the approaches to sharing explained in http://okmij.org/ftp/tagless-final/sharing/sharing.html Good luck with the contest! {-# LANGUAGE FlexibleContexts, DeriveDataTypeable #-} -- Sharing of top-level bindings -- Solving Takayuki Muranushi's problem -- http://www.haskell.org/pipermail/haskell-cafe/2012-July/102287.html module TopSharing where import qualified Data.Map as M import Control.Monad.State import Data.Dynamic import Control.Applicative -- Let's pretend this is one separate module. -- It exports earthMass, the mass of the Earth, which -- is a non-deterministic computation producing Double. -- The non-determinism reflects our uncertainty about the mass. -- Exercise: why do we need the seemingly redundant EarthMass -- and deriving Typeable? -- Could we use TemplateHaskell instead? data EarthMass deriving Typeable earthMass :: NonDet Double earthMass = memoSta (typeOf (undefined::EarthMass)) $ msum $ map return [5.96e24, 5.97e24, 5.98e24] -- Let's pretend this is another separate module -- It imports earthMass and exports sunMass -- Muranushi: ``Let's also pretend that we can measure the other -- bodies' masses only by their ratio to the Earth mass, and -- the measurements have large uncertainties.'' data SunMass deriving Typeable sunMass :: NonDet Double sunMass = memoSta (typeOf (undefined::SunMass)) mass where mass = (*) <$> proportion <*> earthMass proportion = msum $ map return [2.5e5, 3e5, 4e5] -- Let's pretend this is yet another separate module -- It imports earthMass and exports marsMass data MarsMass deriving Typeable marsMass :: NonDet Double marsMass = memoSta (typeOf (undefined::MarsMass)) mass where mass = (*) <$> proportion <*> earthMass proportion = msum $ map return [0.01, 0.1, 1.0] -- This is the main module, importing the masses of the three bodies -- It computes ``how many Mars mass object can we create -- by taking the sun apart?'' -- This code is exactly the same as in Takayuki Muranushi's message -- His question: ``Is there a way to represent this? -- For example, can we define earthMass'' , sunMass'' , marsMass'' all -- in separate modules, and yet have (length $ sunPerMars'' == 27) ? sunPerMars :: NonDet Double sunPerMars = (/) <$> sunMass <*> marsMass sunPerMars_run = runShare sunPerMars sunPerMars_run_len = length sunPerMars_run -- 27 -- The following is essentially Control.Monad.Sharing.Memoization -- with one important addition -- Can you spot the important addition? type NonDet a = StateT FirstClassStore [] a data Key = KeyDyn Int | KeySta TypeRep deriving (Show, Ord, Eq) -- I wish TypeRep were in Ord by default. The implementation permits that! instance Ord TypeRep where compare x y = compare (show x) (show y) data FirstClassStore = FirstClassStore { freshKey :: Int, store :: M.Map Key Dynamic } emptyStore :: FirstClassStore emptyStore = FirstClassStore { freshKey = 1, store = M.empty } getFreshKey :: MonadState FirstClassStore m => m Key getFreshKey = do key <- gets freshKey modify (\s -> s { freshKey = succ key }) return (KeyDyn key) insertVal :: (Typeable a, MonadState FirstClassStore m) => Key -> a -> m () insertVal key val = modify (\s -> s { store = M.insert key (toDyn val) (store s) }) lookupVal :: (Typeable a, MonadState FirstClassStore m) => Key -> m (Maybe a) lookupVal key = liftM (liftM (flip fromDyn err) . M.lookup key) (gets store) where err = error $ "lookupVal: bad key " ++ show key memo :: (Typeable a, MonadState FirstClassStore m) => m a -> m (m a) memo a = getFreshKey >>= \key -> return (memoKey key a) memoSta :: (Typeable a, MonadState FirstClassStore m) => TypeRep -> m a -> m a memoSta trep a = memoKey (KeySta trep) a memoKey :: (Typeable a, MonadState FirstClassStore m) => Key -> m a -> m a memoKey key a = do valM <- lookupVal key case valM of Just x -> return x Nothing -> do x <- a insertVal key $! x return x runShare :: Monad m => StateT FirstClassStore m a -> m a runShare m = evalStateT m emptyStore -- In My GHC, StateT is still not an Applicative. More bolierplate instance Monad m => Applicative (StateT s m) where pure = return (<*>) = liftM2 ($)

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Cloud Haskell Appetiser: a taste of distributed programming in Haskell
by Eric Kow 21 Jul '12

21 Jul '12

HTML version: http://www.well-typed.com/blog/68 # Cloud Haskell Appetiser: (part 2 of Parallel Haskell Digest 11) Hello Haskellers! We mentioned in the [last digest][phd-11] that we'd have just a tiny bit more to say about Parallel Haskell. As promised, here is the completed word of month on *actors* and their use in Cloud Haskell. It so happens — what a coincidence! — that Well-Typed's Edsko de Vries has recently published a beta version of the new [distributed-process][dp-announce] implementation on Hackage. We'd love it if you could give it a try let us know any trouble you ran into or ways we could improve things. To help push things along a bit, this word of month will be using the new distributed-process implementation. Also, have you had a chance to fill out the Parallel Haskell Digest Survey? It's collecting data for another couple of weeks. Anything you can tell us in the survey will inform future efforts in building the Haskell community, so if you've got a couple of minutes before Cloud Haskell Time, head over to [Parallel Haskell Digest Survey](http://goo.gl/bP2fn) Many thanks! ## Word of the month The word of the month series has given us a chance to survey the arsenal of Haskell parallelism and concurrency constructs: * some low level foundations (sparks and threads), * three ways to do parallelism (parallel arrays, strategies, dataflow), * and some concurrency abstractions (locks, transactions, channels) The Haskell approach has been to explicitly recognise the vastness of the parallelism/concurrency space, in other words, to provide a multitude of right tools for a multitude of right jobs. Better still, the tools we have are largely interoperable, should we find ourselves with jobs that don't neatly fit into a single category. The Haskell of 2012 may be in a great place for parallelism and concurrency, but don't think this is the end of the story! What we've seen so far is only a snapshot of the technology as it hurtles through the twenty-tens (How quaint are we, Future Haskeller?). While we can't say what exactly the future will bring, we can look at one of the directions that Haskell might branch into in the coming decade. The series so far has focused on things you might do with a single computer, using parallelism to speed up your software, or using concurrency abstractions to preserve your sanity in the face of non-determinism. But now what if you have more than one computer? ### Actors Our final word of the month is *actor*. Actors are not specific to distributed programming; they are really more of a low level concurrency abstraction on a par with threads. And they certainly aren't new either. The actor model has been around since the 70s at least, and has been seriously used for distributed programming since the late 80s with Erlang. So what makes an actor an actor? Let's compare with threads +----------------------------------+---------------------------------+ | **Actor** | **Thread** | +==================================+=================================+ | can create more actors | can create more threads | +----------------------------------+---------------------------------+ | can have private local state | can have private local state | +----------------------------------+---------------------------------+ | has NO shared state | has limited shared state | | (isolated from other actors!) | | +----------------------------------+---------------------------------+ | communicates with other actors | communicates with other | | via asynchronous message passing | threads via shared variables | +----------------------------------+---------------------------------+ The essential difference between actors and threads is the isolation and message passing. There aren't any holes punched into lids here, but you can always shine a message from one jam jar to another, perhaps hoping they send you one of their own. The appeal of actors is thus a kind of simplicity, where avoiding shared state eliminates a class of concurrency bugs by definition, and where each actor can be reasoned about in isolation of its brethren. This sort of thing may perhaps strike a chord with us functional programmers, and actually, there is quite a bit of actor-related work in Haskell: a handful of packages offering the actor as concurrency primitive, Martin Sulzmann's [multi-headed twist][mh-actors] on the model; [Communicating Haskell Processes][chp] exploring an actor-ish cousin known as CSP. Finally, there's [Cloud Haskell][ch-pdf], which in explicit homage to Erlang, applies the actor model to distributed programming. ### Glimpse of Cloud Haskell We'll be taking a quick look at Cloud Haskell in this word of the month, unfortunately with only the most fleeting of glimpses. If squirting money between bank accounts is the transactional hello world, playing ping pong must surely be its distributed counterpart. Before working up to that, we first start with half a hello. The following example creates three processes — “process” is the Erlang-inspired word for the actor here — one which receives `Ping` messages and just prints them to screen, one which sends a single `Ping` message, and finally one which fires up the first two processes: {-# LANGUAGE DeriveDataTypeable #-} module Main where import Control.Concurrent ( threadDelay ) import Data.Binary import Data.Typeable import Control.Distributed.Process import Control.Distributed.Process.Node import Network.Transport.TCP -- Serializable (= Binary + Typeable) data Ping = Ping deriving (Typeable) instance Binary Ping where put Ping = putWord8 0 get = do { getWord8; return Ping } server :: ReceivePort Ping -> Process () server rPing = do Ping <- receiveChan rPing liftIO $ putStrLn "Got a ping!" client :: SendPort Ping -> Process () client sPing = sendChan sPing Ping ignition :: Process () ignition = do -- start the server sPing <- spawnChannelLocal server -- start the client spawnLocal $ client sPing liftIO $ threadDelay 100000 -- wait a while main :: IO () main = do Right transport <- createTransport "127.0.0.1" "8080" defaultTCPParameters node <- newLocalNode transport initRemoteTable runProcess node ignition This little package gives us a chance to look at three big pieces of Cloud Haskell, the `Serializable` typeclass, the `Process` monad, and channels. #### Serializable Actors send messages to each other. As programmers, we see the messages in nice high-level form (eg. `Ping`), but somewhere along the way, these messages are going to have to be encoded to something we can ship around on a network. Cloud Haskell makes this encoding explicit, but reasonably convenient at the same time. Things can be messages if they implement the `Serializable` typeclass, which is done indirectly by implementing `Binary` and deriving `Typeable`. You won't be starting from scratch, as implementations are already provided for primitives and some commonly used data structures. Things which don't make sense as messages are deliberately left unserializable, for example `MVar` and `TVar`, which are only meaningful in the context of threads with a shared memory. Our Cloud Haskell program is perfectly free to use these constructs within processes (or within processes on the same machine; a bit more on that below), just not to ship them around. #### Process We use “process” to mean “actor” in a similar fashion as Erlang, in other words nothing nearly so heavy as an operating system process. One different with Erlang, however, is that Cloud Haskell allows for both actor style concurrency and the thread-based approach. The infrastructure gears you towards using the actor model when talking across machines, but on the same machine, you could also conveniently do things the old way. Want to use STM to pass notes between processes? Fine, just spawn them locally via `spawnLocal` and give them a common `TVar`. As for the `Process` monad, we see again the idea of special monad either for special kinds of sequencing. Here the idea is that things like sending/receiving messages or spawning other processes only makes sense for processes, and so you can only do these things in a “process context”. `Process` implements `MonadIO`, though, so any input/output you'd like to do within a process is merely a `liftIO` away. Going the other way, running a process from IO, you would do with the `runProcess` function. #### Channels Cloud Haskell provides a notion of channels (somewhat similar to those we introduced in the last word of the month), typed unidirectional pipelines that go from one process to another. Using them is optional (there are simpler ways to bop messages back and forth), but worth trying out for the promise of sending messages only to processes that will understand them. Below is a quick glance at channels in action: data SendPort a -- Serializable data ReceivePort a -- NOT Serializable newChan :: Serializable a => Process (SendPort a, ReceivePort a) sendChan :: Serializable a => SendPort a -> a -> Process () receiveChan :: Serializable a => ReceivePort a -> Process a A channel comes with a send and receive port, both of which are parameterised on the same type variable. Creating a `Ping` channel thus gives a `ReceivePort Ping` out of which only `Ping`'s will ever emerge, and a `SendPort Ping` into which we can only put `Ping`'s. This looks a lot more attractive when you work with multiple channels. Replying to pings with pongs, for example, would require us to create a second channel with a send a receive port of its own, which means we have now 4 ports to juggle! Having the type distinctions makes things a bit clearer: `SendPort Ping` vs `ReceivePort Ping` vs `SendPort Pong`, vs `ReceivePort Pong`. Finally, it's worth noticing that `SendPort`'s are themselves Serializable, meaning that they can be copied and shipped around to other processes possibly on other computers. This allows a channel to accept data from more than one place, and also makes for idioms like including a reply-to `SendPort` in your messages. `ReceivePort`'s on the other hand are (deliberately) left unserializable which leaves them tied to single computer. ### Ping? What happened to Pong? Our little example was more “hello wo” than “hello world”; we'd only managed to send a `Ping` without even thinking about sending `Pong`'s back. Want to try your hand at Cloud Haskell? Here's a great opportunity! 1. **[Easy]** Start with a `cabal install distributed-process` and make sure you can run this example. Note that you'll need GHC 7.4.1 and up for this 2. **[Less easy]** Next, add a new `Pong` message (as a separate data type), extending the server to send this message back, and the client to receive that reply. There are some puzzle pieces to work through here. How does the server know where to send its replies? Moreover, how do we keep the server nice and decoupled from the client? We want it to receive pings from any client, and send a reply back to the ping'er (and not just some hard-coded client). Hint: you can solve this without touching `ignition` or `main`. Remember that `SendPort` is `Serializable`! 3. **[Easy]** You now have a single ping/pong interaction. Can you make the game go back and forth indefinitely (or until the `threadDelay` ends)? Hint: have a look at `Control.Monad`; it's not essential, but it's a bit nicer. ### Conclusion Stepping back from the technology a bit, we have introduced the notion of actors as a concurrency abstraction on a par with threads. While there's nothing that makes them specific to distributed programming, they do seem to fit nicely to the problem and have been used to great effect before. Cloud Haskell is one attempt to apply this actor model, taking some of the ideas from Erlang, and combining them with Haskell's purity and type system. You might notice that in a word of the month about distributed programming, we've kept things on a single machine, alas! Indeed, we have not been able to do Cloud Haskell justice in this article, but we have hopefully laid some foundations by introducing some of the basic layers, `Serializable` messages, processes, and channels. To escape from one-machine-island, we would need to get to grips with two more concepts, nodes and closures. Nodes can basically be thought of as separate machines (you could run multiple nodes on the same machine if you wanted to, say for development purposes). This makes for three layers: nodes (machines), which contain processes (actors), which can run any number of threads they wanted. We saw how processes can communicate by sending each other messages across channels; what we've left out is the crucial detail of what happens when the processes live on different nodes. The good news here is “nothing special”, still messages across channels. The bad news is a bit of infrastructural fiddliness setting up the nodes in the first place, assigning them to roles, and spawning remote processes… for which we need to know about closures. The basic story with closures is that we need to be able send functions back and forth in order to do anything really useful with Cloud Haskell, and to send functions we need to say how they are `Serializable`. This would be easy enough — assume for now that all nodes are running the same code and just send “run function `foo`” style instructions — were it not for the fact that Haskellers do all sorts of crazy things with functions all the time (partially applying them, returning them from other function…), crazy things that introduce free variables. Expressing the serializability of function-and-its-free-variables was a source of furious head-scratching for a while until somebody hit on the old Henry T. Ford idea: You can have any free variables you want so long as they are a ByteString. Where to from here? If you're looking for more introductory stuff and have not already seen, try Simon Peyton Jones's presentation of Cloud Haskell to the Scala community ([1h video][ch-skills]). Edsko has been hard at work at the [distributed-process Haddock][dp-haddock], so it's worth checking out when you're ready to roll up your sleeves and get hacking. It'd be a very good idea to have a look at the [simplelocalnet backend][simple], which will help you get started with the nitty gritty node management issues when you start yearning to go distributed. That's the practical stuff, but don't forget to read the [Cloud Haskell paper][ch-pdf] either! The API has some slight differences (for example, `ProcessM` has since been renamed to `Process`), but it should be fairly straightforwardly transferable to the new package. It's likely we'll need a wider spectrum of documentation to bring more Cloud Haskellers into the fold (early days, eh?). Hopefully this word of the month will help you get started, and maybe in turn write a blog post of your own? Happy Distributed Haskell'ing! [ch-github]: https://github.com/jepst/CloudHaskell [ch-pdf]: http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remot… [ch-skills]: http://skillsmatter.com/podcast/home/haskell-cloud/js-4179 [chp]: http://www.cs.kent.ac.uk/projects/ofa/chp/ [dist-p]: https://github.com/haskell-distributed/distributed-process [dp-announce]: https://groups.google.com/d/topic/parallel-haskell/dw5UPEg1ePI/discussion [dp-haddock]: http://hackage.haskell.org/packages/archive/distributed-process/latest/doc/… [mh-actors]: http://sulzmann.blogspot.co.uk/2008/10/actors-with-multi-headed-receive.html [phd-11]: http://www.well-typed.com/blog/67 [simple]: http://hackage.haskell.org/packages/archive/distributed-process-simplelocal… [survey]: http://goo.gl/bP2fn -- Eric Kow <http://erickow.com>

1 0

Fwd: hackage compile failure with QuickCheck 2.5
by Alexander Foremny 20 Jul '12

20 Jul '12

Dear Levent, I think this [1] could be related. Regards, Alexander Foremny PS. Sent this to Levent directly. Here's a copy for the mailing list. Sorry for the noise. [1] http://haskell.1045720.n5.nabble.com/Bad-interface-problem-td5714184.html ---------- Forwarded message ---------- From: Alexander Foremny <alexanderforemny(a)gmail.com> Date: 2012/7/17 Subject: Re: [Haskell-cafe] hackage compile failure with QuickCheck 2.5 To: Levent Erkok <erkokl(a)gmail.com> Dear Levent, I think this [1] could be related. Regards, Alexander Foremny [1] http://haskell.1045720.n5.nabble.com/Bad-interface-problem-td5714184.html 2012/7/17 Levent Erkok <erkokl(a)gmail.com>: > [This message is more appropriate for a hackage mailing list I presume, but > that doesn't seem to exist. Let me know if there's a better place to send > it.] > > I'm having a hackage compile failure for a newly uplodaded package that has > a QuickCheck 2.5 dependence. The error message is: > > [13 of 13] Compiling Test.QuickCheck.All ( Test/QuickCheck/All.hs, > dist/build/Test/QuickCheck/All.o ) > > Test/QuickCheck/All.hs:15:1: > Bad interface file: > /usr/local/tmp/archive/install/lib/template-haskell-2.6.0.0/ghc-7.4.1/Language/Haskell/TH.hi > Something is amiss; requested module > template-haskell-2.6.0.0:Language.Haskell.TH differs from name found in the > interface file template-haskell:Language.Haskell.TH > > > The full log file is at (search for "Something is a miss" in it): > http://hackage.haskell.org/packages/archive/sbv/2.2/logs/failure/ghc-7.4 > > Needless to say, I don't see this problem when I compile this package at > home with the same compiler (ghc 7.4.1) as hackage is using; also Hackage > has a successfully compiled QuickCheck 2.5 package. > > Could it be something related to the particular cabal/ghc installation on > the hackage server? In particular, I don't understand why it picks > template-haskell 2.6.0.0 when there's a newer version (2.7.0.0). As far as I > can see, QuickCheck doesn't put an upper limit on its template haskell > version dependency. > > I'd appreciate any pointers with this. (Googling and questions on the > #haskell irc channel didn't help much, unfortunately.) > > -Levent. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

4 5

how to select random elements of a Data.Set?
by jwaldmann 20 Jul '12

20 Jul '12

Dear all, how would I quickly select an element of a Set (as in Data.Set) uniformly at random? Via the API, this can only be done in linear time? (via toList) If I could access the tree structure, then of course it could be logarithmic. But probably I'd need a weighted selection sooner or later, and this would require some specific code anyway. Or does it not? Actually I need a sequence of such selections (each selected element is deleted immediately). I don't need all elements (so, computing a random permuation might be too much). J.W.

4 3

COBOL-85 parser, anyone?
by Richard O'Keefe 20 Jul '12

20 Jul '12

Does anyone have a parser for COBOL-85 written in Haskell, or written using some freely available tool that communicates easily with Haskell? I don't need it _yet_, but I'm talking with someone who is trying to get access to a real legacy site with a bunch of, well, basically COBOL 85, but there are extensions... We're not talking about transformation at this stage, just analysis. I could probably hack up the extensions given a place to start. I've found some papers and more dead links than I care for and lots of mention of ASF+SDF which is apparently superseded by Rascal.

2 1

Bug or feature?
by Miguel Mitrofanov 20 Jul '12

20 Jul '12

Hi cafe! I'm a bit confused by the DefaultSignatures extension. It's unclear whether to consider the following an example of clever use of this extension, or an example of abuse of it: {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE DefaultSignatures #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE UndecidableInstances #-} module SubClass where import GHC.Prim(Constraint) data Void (c :: * -> Constraint) = Void data Evidence c a where Evidence :: c a => Evidence c a class c1 :<= c2 where isSubClass :: c1 a => Void c1 -> Evidence c2 a default isSubClass :: c2 a => Void c1 -> Evidence c2 a isSubClass Void = Evidence instance Show :<= Show instance Floating :<= Fractional instance Real :<= Num -- instance Fractional :<= Floating -- NO ROTTEN WAY {- Examples -} data Wrapper c where Wrapper :: c a => a -> Wrapper c instance (c :<= Show) => Show (Wrapper c) where show (Wrapper (a :: t)) = case isSubClass (Void :: Void c) :: Evidence Show t of Evidence -> show a absWrap :: (c :<= Num) => Wrapper c -> Wrapper c absWrap (Wrapper (a :: t) :: Wrapper c) = case isSubClass (Void :: Void c) :: Evidence Num t of Evidence -> Wrapper (abs a) What do you think? Also, it's a bit strange that the first example (instance (c :<= Show) => Show (Wrapper c)) requires UndecidableInstances, while the second one (absWrap) requires FlexibleContexts - although being remarkably similar.

1 0

How to add constraint to .cabal?
by Magicloud Magiclouds 19 Jul '12

19 Jul '12

Hi, Say I have a package that only appends --constraint="template-haskell==2.7.0.0" --constraint="warp-tls==1.2.1" could I install it. Now I want to release the package, then how could I have these constraint into the .cabal so the user would not get troubled? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

2 1

Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
by Alexander Foremny 19 Jul '12

19 Jul '12

Dear Levent, unfortunately I am at a loss here. As far as I understand it this should be fixed in QuickCheck's .cabal file or on Hackage. But I am not experienced enough to decide. You best wait for someone else to comment on this. Depending on template-haskell in your .cabal file is not the way to go as far as I understand it. But maybe it's a possible work-around in case you depend on the package being available on Hackage timely. Regards, Alexander Foremny 2012/7/17 Levent Erkok <erkokl(a)gmail.com>: > It builds fine locally on my box; but not on hackage. Here's the page: > http://hackage.haskell.org/package/sbv-2.2 > > Thanks for looking into this Alexander, I appreciate your help. > > -Levent. > > > On Tue, Jul 17, 2012 at 9:09 AM, Alexander Foremny > <alexanderforemny(a)gmail.com> wrote: >> >> Which package are you trying to build? Is it a local package that >> fails to build or something on Hackage? Its .cabal file or at least >> full dependencies would be of interest. >> >> Regards, >> Alexander Foremny >> >> 2012/7/17 Levent Erkok <erkokl(a)gmail.com>: >> > Thanks Alexander. However, I'm not sure how to use the workaround >> > described >> > so I can get hackage to properly compile my package. It sounds like I >> > have >> > to add a "template-haskell >= 2.7.0.0" dependency to my own cabal file, >> > which sounds like the wrong thing to do in the long-run. >> > >> > Is there something that can be done on the hackage/ghc side to avoid >> > this >> > issue? Or something less drastic than adding a template-haskell >> > dependency >> > on my own package's cabal file? >> > >> > Thanks, >> > >> > -Levent. >> > >> > >> > On Tue, Jul 17, 2012 at 7:31 AM, Alexander Foremny >> > <alexanderforemny(a)gmail.com> wrote: >> >> >> >> Dear Levent, >> >> >> >> I think this [1] could be related. >> >> >> >> Regards, >> >> Alexander Foremny >> >> >> >> PS. Sent this to Levent directly. Here's a copy for the mailing list. >> >> Sorry for the noise. >> >> >> >> [1] >> >> >> >> http://haskell.1045720.n5.nabble.com/Bad-interface-problem-td5714184.html >> >> >> >> ---------- Forwarded message ---------- >> >> From: Alexander Foremny <alexanderforemny(a)gmail.com> >> >> Date: 2012/7/17 >> >> Subject: Re: [Haskell-cafe] hackage compile failure with QuickCheck 2.5 >> >> To: Levent Erkok <erkokl(a)gmail.com> >> >> >> >> >> >> Dear Levent, >> >> >> >> I think this [1] could be related. >> >> >> >> Regards, >> >> Alexander Foremny >> >> >> >> [1] >> >> >> >> http://haskell.1045720.n5.nabble.com/Bad-interface-problem-td5714184.html >> >> >> >> 2012/7/17 Levent Erkok <erkokl(a)gmail.com>: >> >> > [This message is more appropriate for a hackage mailing list I >> >> > presume, >> >> > but >> >> > that doesn't seem to exist. Let me know if there's a better place to >> >> > send >> >> > it.] >> >> > >> >> > I'm having a hackage compile failure for a newly uplodaded package >> >> > that >> >> > has >> >> > a QuickCheck 2.5 dependence. The error message is: >> >> > >> >> > [13 of 13] Compiling Test.QuickCheck.All ( Test/QuickCheck/All.hs, >> >> > dist/build/Test/QuickCheck/All.o ) >> >> > >> >> > Test/QuickCheck/All.hs:15:1: >> >> > Bad interface file: >> >> > >> >> > >> >> > /usr/local/tmp/archive/install/lib/template-haskell-2.6.0.0/ghc-7.4.1/Language/Haskell/TH.hi >> >> > Something is amiss; requested module >> >> > template-haskell-2.6.0.0:Language.Haskell.TH differs from name found >> >> > in >> >> > the >> >> > interface file template-haskell:Language.Haskell.TH >> >> > >> >> > >> >> > The full log file is at (search for "Something is a miss" in it): >> >> > >> >> > http://hackage.haskell.org/packages/archive/sbv/2.2/logs/failure/ghc-7.4 >> >> > >> >> > Needless to say, I don't see this problem when I compile this package >> >> > at >> >> > home with the same compiler (ghc 7.4.1) as hackage is using; also >> >> > Hackage >> >> > has a successfully compiled QuickCheck 2.5 package. >> >> > >> >> > Could it be something related to the particular cabal/ghc >> >> > installation >> >> > on >> >> > the hackage server? In particular, I don't understand why it picks >> >> > template-haskell 2.6.0.0 when there's a newer version (2.7.0.0). As >> >> > far >> >> > as I >> >> > can see, QuickCheck doesn't put an upper limit on its template >> >> > haskell >> >> > version dependency. >> >> > >> >> > I'd appreciate any pointers with this. (Googling and questions on the >> >> > #haskell irc channel didn't help much, unfortunately.) >> >> > >> >> > -Levent. >> >> > >> >> > >> >> > _______________________________________________ >> >> > Haskell-Cafe mailing list >> >> > Haskell-Cafe(a)haskell.org >> >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > >> >> >> >> _______________________________________________ >> >> Haskell-Cafe mailing list >> >> Haskell-Cafe(a)haskell.org >> >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> > >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > >

7 12

Re: [Haskell-cafe] Cabal install fails due to recent HUnit
by Simon Hengel 19 Jul '12

19 Jul '12

On Wed, Jul 18, 2012 at 05:51:51PM -0600, Richard G. wrote: > What's the best way to fix this? I can see two options: > - Move the test stanzas to a different Cabal file, allowing users to > perform the tests with a little fiddling. > - Remove the conditional statements from the test stanzas, which may > break compatibility with some compilers and interpreters. > > Is there another option? I think it is preferable to have a test section in the main cabal file. Just an idea: What about having just a single test suite that *depends on the library*? I think that way the test suit would not require any conditionals, but you could not test with different optimization levels anymore. We could then add a Makefile/script that configures, builds and tests with the different optimization levels. This approach also has the advantage, that we are testing exactly the same thing (the library!) that the user is going to use. Cheers, Simon

1 0

Haskell Weekly News: Issue 236
by Daniel Santa Cruz 19 Jul '12

19 Jul '12

Welcome to issue 236 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 8 to 14, 2012. Quotes of the Week * RichardBuckminsterFuller: When I am working on a problem, I never think about beauty. But when I have finished, if the solution is not beautiful, I know that it is wrong. * Taneb: I'm doing the thing that every bored haskeller does one day and rewriting Prelude Top Reddit Stories * Haskell Performance Patterns Domain: johantibell.com, Score: 71, Comments: 34 On Reddit: [1] http://goo.gl/gZkMB Original: [2] http://goo.gl/2rj4t * Code Us Some Roguelike in Haskell! Domain: jamiltron.com, Score: 55, Comments: 19 On Reddit: [3] http://goo.gl/VYOx1 Original: [4] http://goo.gl/OOiSG * I never tested this code before releasing it Confession of a Haskell Hacker Domain: r6.ca, Score: 51, Comments: 48 On Reddit: [5] http://goo.gl/EBfT7 Original: [6] http://goo.gl/9zA0u * Breaking from a loop Domain: haskellforall.com, Score: 37, Comments: 19 On Reddit: [7] http://goo.gl/yqxqg Original: [8] http://goo.gl/NtI8j * The Classy Prelude, an alternate prelude which makes it easier to use conflicting names via typeclasses, and thereby encourages usage of more powerful libraries like text and unordered-containers. Domain: yesodweb.com, Score: 35, Comments: 46 On Reddit: [9] http://goo.gl/cZKM5 Original: [10] http://goo.gl/70d6R * The Long and Epic Journey of LambdaCase Domain: unknownparallel.wordpress.com, Score: 33, Comments: 52 On Reddit: [11] http://goo.gl/EAVXs Original: [12] http://goo.gl/ymkGy * How is this fibonacci-function memoized? Domain: stackoverflow.com, Score: 33, Comments: 1 On Reddit: [13] http://goo.gl/lo3LD Original: [14] http://goo.gl/wwHQb * errors-1.0: Simplified error handling Domain: haskellforall.com, Score: 30, Comments: 31 On Reddit: [15] http://goo.gl/tKzP1 Original: [16] http://goo.gl/O10hN * Influenced by Haskell Domain: griffsgraphs.com, Score: 30, Comments: 29 On Reddit: [17] http://goo.gl/6ptoi Original: [18] http://goo.gl/Nrz7K * Experience Report: Haskell in Computational Biology (Daniels, Gallant, Ramsey)[pdf 7 pp] Domain: eecs.tufts.edu, Score: 30, Comments: 11 On Reddit: [19] http://goo.gl/XsvMK Original: [20] http://goo.gl/TUdnL * == for Float not referentially transparent Domain: haskell.org, Score: 23, Comments: 51 On Reddit: [21] http://goo.gl/6qZ1J Original: [22] http://goo.gl/DR9TE * My second attempt at writing Haskell Domain: self.haskell, Score: 21, Comments: 27 On Reddit: [23] http://goo.gl/i7u3p Original: [24] http://goo.gl/i7u3p * Announcing split-channel « Melding Monads Domain: blog.melding-monads.com, Score: 20, Comments: 16 On Reddit: [25] http://goo.gl/XwHdl Original: [26] http://goo.gl/oh5Ce * Deriving the Y combinator from first principles Domain: wellnowwhat.net, Score: 18, Comments: 12 On Reddit: [27] http://goo.gl/5uxYN Original: [28] http://goo.gl/Q76lk * Announcing WAI 1.3! Domain: yesodweb.com, Score: 18, Comments: 11 On Reddit: [29] http://goo.gl/QD769 Original: [30] http://goo.gl/mQFBK Top StackOverflow Questions * How is this fibonacci-function memoized? votes: 43, answers: 3 Read on SO: [31] http://goo.gl/MGjr3 * What is the combinatory logic equivalent of intuitionistic type theory? votes: 39, answers: 0 Read on SO: [32] http://goo.gl/ki9nT * Does Haskell have return type overloading? votes: 15, answers: 5 Read on SO: [33] http://goo.gl/sBGWj * Is there a way to avoid unnecessary recursion? votes: 12, answers: 2 Read on SO: [34] http://goo.gl/Gk39e * Using vectors for performance improvement in Haskell votes: 12, answers: 2 Read on SO: [35] http://goo.gl/74KOH * Eliminate Haskell array bounds check for Bounded type? votes: 11, answers: 1 Read on SO: [36] http://goo.gl/CHDtJ * Having my cereal and parsing it too votes: 8, answers: 2 Read on SO: [37] http://goo.gl/1SG24 * How does HOpenGL behave with regards to other threads and TChans in Haskell? votes: 8, answers: 2 Read on SO: [38] http://goo.gl/dmoqc * Number of arguments and point-free in Haskell votes: 8, answers: 2 Read on SO: [39] http://goo.gl/8Z0B2 * Haskell: Design pattern: classes or pass functions votes: 7, answers: 3 Read on SO: [40] http://goo.gl/EO7tr * Haskell — sort list with impure function votes: 7, answers: 4 Read on SO: [41] http://goo.gl/o9kbc Until next time, Daniel Santa Cruz References 1. http://johantibell.com/files/haskell-performance-patterns.html#%281%29 2. http://www.reddit.com/r/haskell/comments/wjtms/haskell_performance_patterns/ 3. http://jamiltron.com/2012/07/Code_Us_Some_Roguelike_in_Haskell.html 4. http://www.reddit.com/r/haskell/comments/wd6p3/code_us_some_roguelike_in_ha… 5. http://r6.ca/blog/20120708T122219Z.html 6. http://www.reddit.com/r/haskell/comments/w7zd8/i_never_tested_this_code_bef… 7. http://www.haskellforall.com/2012/07/breaking-from-loop.html 8. http://www.reddit.com/r/haskell/comments/wfwyt/breaking_from_a_loop/ 9. http://www.yesodweb.com/blog/2012/07/classy-prelude 10. http://www.reddit.com/r/haskell/comments/wfiao/the_classy_prelude_an_altern… 11. http://unknownparallel.wordpress.com/2012/07/09/the-long-and-epic-journey-o… 12. http://www.reddit.com/r/haskell/comments/wbb69/the_long_and_epic_journey_of… 13. http://stackoverflow.com/q/11466284/1288 14. http://www.reddit.com/r/haskell/comments/whx2k/how_is_this_fibonaccifunctio… 15. http://www.haskellforall.com/2012/07/errors-10-simplified-error-handling.ht… 16. http://www.reddit.com/r/haskell/comments/w7xrv/errors10_simplified_error_ha… 17. http://griffsgraphs.com/2012/07/01/programming-languages-influences/ 18. http://www.reddit.com/r/haskell/comments/w9npe/influenced_by_haskell/ 19. http://www.eecs.tufts.edu/~ndaniels/Noah_files/mrfy_experience_report.pdf 20. http://www.reddit.com/r/haskell/comments/wf6xe/experience_report_haskell_in… 21. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-July/022568.html 22. http://www.reddit.com/r/haskell/comments/wbsd3/for_float_not_referentially_… 23. http://www.reddit.com/r/haskell/comments/walg1/my_second_attempt_at_writing… 24. http://www.reddit.com/r/haskell/comments/walg1/my_second_attempt_at_writing… 25. http://blog.melding-monads.com/2012/07/10/announcing-split-channel/ 26. http://www.reddit.com/r/haskell/comments/wdhf1/announcing_splitchannel_meld… 27. http://www.wellnowwhat.net/blog/?p=485 28. http://www.reddit.com/r/haskell/comments/wd5rb/deriving_the_y_combinator_fr… 29. http://www.yesodweb.com/blog/2012/07/announcing-wai-1-3 30. http://www.reddit.com/r/haskell/comments/wih94/announcing_wai_13/ 31. http://stackoverflow.com/questions/11466284/how-is-this-fibonacci-function-… 32. http://stackoverflow.com/questions/11406786/what-is-the-combinatory-logic-e… 33. http://stackoverflow.com/questions/11420126/does-haskell-have-return-type-o… 34. http://stackoverflow.com/questions/11454936/is-there-a-way-to-avoid-unneces… 35. http://stackoverflow.com/questions/11481675/using-vectors-for-performance-i… 36. http://stackoverflow.com/questions/11411797/eliminate-haskell-array-bounds-… 37. http://stackoverflow.com/questions/11424961/having-my-cereal-and-parsing-it… 38. http://stackoverflow.com/questions/11443824/how-does-hopengl-behave-with-re… 39. http://stackoverflow.com/questions/11487171/number-of-arguments-and-point-f… 40. http://stackoverflow.com/questions/11423749/haskell-design-pattern-classes-… 41. http://stackoverflow.com/questions/11469806/haskell-sort-list-with-impure-f…

1 0

HDBC nested withTransaction
by Andras Gyomrey 19 Jul '12

19 Jul '12

Hi, are there any plans of implementing nested transactions using withTransaction in HDBC? In mysql exists checkpoints and in postgres explicit subtransactions, Andras Gyomrey

1 0

strange hangs with -threaded runtime
by Joey Hess 19 Jul '12

19 Jul '12

I have a rather large program that works fine with ghc's default runtime, but now I'd like to build it with -threaded, and this causes it to hang in a way I have not been able to pin down to anything specific. It hangs at different points each time it's run; running it in strace will consistently avoid the problem. If I look at it once it's hung, I always see a pair of processes, both blocked in futex: joey@wren:~/src/git-annex>strace -p 21749 Process 21749 attached - interrupt to quit futex(0x8b03930, FUTEX_WAIT_PRIVATE, 69, NULL^C <unfinished ...> Process 21749 detached joey@wren:~/src/git-annex>strace -p 21811 Process 21811 attached - interrupt to quit futex(0x8b03df8, FUTEX_WAIT_PRIVATE, 2, NULL Are there any common gotchas with converting to the threaded runtime that might provide a hint to what's wrong? My code does not currently use Control.Concurrent or threads, although it does use some libraries like MissingH that use forkIO or forkOS. GHC 7.4.1, 7.4.2 on Debian Linux -- see shy jo

3 8

Haskell Symposium 2012 - Call for Participation (early registration closes 9th August)
by Janis Voigtländer 18 Jul '12

18 Jul '12

==================================================================== CALL FOR PARTICIPATION ACM SIGPLAN Haskell Symposium 2012 Copenhagen, Denmark 13th September, 2012 (directly after ICFP) http://www.haskell.org/haskell-symposium/2012/ ==================================================================== The purpose of the Haskell Symposium is to discuss experiences with Haskell and future developments for the language. The scope of the symposium includes all aspects of the design, semantics, theory, application, implementation, and teaching of Haskell. Accepted papers and programme: ============================== * http://haskell.org/haskell-symposium/2012/#schedule REGISTRATION IS NOW OPEN: ========================= * http://www.icfpconference.org/icfp2012/registration.html * Early registration deadline: 9th August, 2012 Local arrangements (including travel and accommodation): ======================================================== * http://www.icfpconference.org/icfp2012/local.html See you in Copenhagen, Janis Voigtlaender Haskell 2012 Programme Chair ====================================================================

1 0

Re: [Haskell-cafe] Interest in typed relational algebra library?
by oleg＠okmij.org 18 Jul '12

18 Jul '12

> And yes to first order predicate calculus too! Just two weeks ago Chung-chieh Shan and I were explaining at NASSLLI the embedding in Haskell of the higher-order predicate logic with two base types (so-called Ty2). The embedding supports type-safe simplification of formulas (which was really needed for our applications). The embedding is extensible: you can add models and more constants. http://okmij.org/ftp/gengo/NASSLLI10/course.html#semantics

2 2

lambdabot-4.2.3.3
by Dmitry Malikov 18 Jul '12

18 Jul '12

Hi. A few days ago I tried to install lambdabot package from hackage (4.2.3.2). Cabal install failed <http://hackage.haskell.org/packages/archive/lambdabot/4.2.3.2/logs/failure/…>. Then I found DanBurton's github repo <https://github.com/DanBurton/lambdabot-remedies> with some approaches to make lambdabot install fixed. All dependency packages (IOSpec, numbers) was already fixed. So I add FlexibleInstances extension to cabal file <https://github.com/dmalikov/lambdabot/commit/db7a464948d65c11b3345db524cc8f…> and upload <http://hackage.haskell.org/package/lambdabot-4.2.3.3> package to hackage. I hope that did everything right. -- Best regards, dmitry malikov !

5 8

A useful function for forking a thread, but letting the parent do setup first
by Joey Adams 18 Jul '12

18 Jul '12

Here's a useful little function: -- | Fork a thread, but wait for the main thread to perform a setup action -- using the child's 'ThreadID' before beginning work in the child thread. forkSetup :: (ThreadId -> IO (Maybe a, r)) -- ^ Setup action to be called before the thread begins working -> (a -> IO b) -- ^ What to do in the worker thread -> IO r forkSetup setup inner = mask $ \restore -> do mv <- newEmptyMVar tid <- forkIO $ join $ takeMVar mv (ma, r) <- setup tid `onException` putMVar mv (return ()) case ma of Nothing -> putMVar mv $ return () Just a -> putMVar mv $ restore $ inner a >> return () return r A question about 'mask': is it safe to use the 'restore' callback in a forked thread? Or might this be invalid in a future version of GHC? I'm aware that if forkSetup itself is called with exceptions masked, the child thread will also have exceptions masked. This seems reasonable, given that forkIO behaves the same way. Is a function like this available in some existing library? If not, where would be a good home for it? Thanks for the input!

1 0

stripSuffix
by Alvaro Gutierrez 18 Jul '12

18 Jul '12

Hi all -- Pardon me if this has been answered before: how come there's a stripPrefix in Data.List, but no matching stripSuffix? Thanks! Alvaro

4 4

What is the surefire way to handle all exceptions and make sure the program doesn't fail?
by Yifan Yu 18 Jul '12

18 Jul '12

First of all, apologise if the question is too broad. The background goes like this: I've implemented a server program in Haskell for my company intended to replace the previous one written in C which crashes a lot (and btw the technology of the company is exclusively C-based). When I chose Haskell I promised my manager (arrogantly - I actually made a bet with him), "it won't crash". Now it has been finished (with just a few hundred LOC), and my test shows that it is indeed very stable. But by looking at the code again I'm a little worried, since I'm rather new to exception handling and there're many networking-related functions in the program. I was tempted to catch (SomeException e) at the very top-level of the program and try to recursively call main to restart the server in case of any exception being thrown, but I highly doubt that is the correct and idiomatic way. There are also a number of long-running threads launched from the main thread, and exceptions thrown from these threads can't be caught by the top-level `catch' in the main thread. My main function looks like this: main :: IO () main = withSocketsDo $ do sCameraU <- socketNewPassive False 6000 sStunU <- socketNewPassive False 3478 sCmdT <- socketNewPassive True 7000 mvarCam <- newMVar M.empty mvarLog <- newMVar [] forkIO $ regCamera sCameraU mvarCam mvarLog forkIO $ updCamera mvarCam mvarLog forkIO $ stun sCameraU sStunU mvarCam mvarLog listen sCmdT 128 processCmd sCmdT mvarCam mvarLog sClose sCameraU sClose sStunU sClose sCmdT I find that I can't tell whether a function will throw any exception at all, or what exceptions will be thrown, by looking at their documentation. I can only tell if I browse the source code. So the question is, how can I determine all the exceptions that can be thrown by a given function? And what is the best way to handle situations like this, with both the long-running threads and main thread need to be restarted whenever exceptions happen. Regards, Yifan

4 6

Criterion setup/teardown functions?
by tsuraan 17 Jul '12

17 Jul '12

Is there anything in Criterion that allows for a benchmark to run some code before or after the thing that it's timing? As an example, I'd like to time a bunch of database inserts, but beforehand I want to create the target table, and afterwards I'd like to delete it. I don't really care to have the time spent on the create/delete recorded in the test run's timing, if that's possible to do.

2 2

Re: [Haskell-cafe] [haskell-cafe] Non-greedy match in Text.Regx.Posix
by Doug McIlroy 17 Jul '12

17 Jul '12

> I was exploring Text.Regex.Posix and found that I was not able to do a non-greedy match by modifying the quantifier with a "?". How can I achieve non-greedy match in Text.Regex.Posix? If it's Posix, it is greedy. Successive submatches are as long as possible subject to the constraint that a longest possible overall match be found if such exists. Doug McIlroy

1 0

Re: [Haskell-cafe] [Haskell] A riddle...
by Henning Thielemann 17 Jul '12

17 Jul '12

On Mon, 16 Jul 2012, Felipe Almeida Lessa wrote: > On Mon, Jul 16, 2012 at 12:33 PM, Vo Minh Thu <noteed(a)gmail.com> wrote: >> It seems like the infered type (and thus bounds) is different when you >> force the result to be a Color or not. Just give explicit type >> signatures and conversion functions. > > Actually, just *always* give explicit type signatures. Additionally, always compile with '-Wall' and follow the warnings. This way GHC will also warn about missing type signatures and it will even suggest type signatures. (and use haskell-cafe for riddles, please :-)

1 0

Non-greedy match in Text.Regx.Posix
by C K Kashyap 17 Jul '12

17 Jul '12

Hi all, I was exploring Text.Regex.Posix and found that I was not able to do a non-greedy match by modifying the quantifier with a "?". How can I achieve non-greedy match in Text.Regex.Posix? Regards, Kashyap

3 3

hackage compile failure with QuickCheck 2.5
by Levent Erkok 17 Jul '12

17 Jul '12

[This message is more appropriate for a hackage mailing list I presume, but that doesn't seem to exist. Let me know if there's a better place to send it.] I'm having a hackage compile failure for a newly uplodaded package that has a QuickCheck 2.5 dependence. The error message is: [13 of 13] Compiling Test.QuickCheck.All ( Test/QuickCheck/All.hs, dist/build/Test/QuickCheck/All.o ) Test/QuickCheck/All.hs:15:1: Bad interface file: /usr/local/tmp/archive/install/lib/template-haskell-2.6.0.0/ghc-7.4.1/Language/Haskell/TH.hi Something is amiss; requested module template-haskell-2.6.0.0:Language.Haskell.TH differs from name found in the interface file template-haskell:Language.Haskell.TH The full log file is at (search for "Something is a miss" in it): http://hackage.haskell.org/packages/archive/sbv/2.2/logs/failure/ghc-7.4 Needless to say, I don't see this problem when I compile this package at home with the same compiler (ghc 7.4.1) as hackage is using; also Hackage has a successfully compiled QuickCheck 2.5 package. Could it be something related to the particular cabal/ghc installation on the hackage server? In particular, I don't understand why it picks template-haskell 2.6.0.0 when there's a newer version (2.7.0.0). As far as I can see, QuickCheck doesn't put an upper limit on its template haskell version dependency. I'd appreciate any pointers with this. (Googling and questions on the #haskell irc channel didn't help much, unfortunately.) -Levent.

1 0

Haskell's type inference considered harmful [Re: [Haskell] A riddle...]
by Andreas Abel 17 Jul '12

17 Jul '12

[Is Haskell-Cafe the right place to discuss DESIGN issues of the Haskell language? This message is not about a concrete problem...] Congratulations to all that solved the riddle so quickly. (First answer in only 8 Minutes!) Now to the point of the exercise: Shocking realizations. 1. Haskell's type inference is NON-COMPOSITIONAL! In the riddle below, I am defining two things f ("rgbliste") and g ("farbliste"). Even though they are not strongly connected, but g comes after f in the definition order, the code of g influences the type of f. THAT'S WRONG! :-( It gets worse: 2. DEAD CODE changes a program's result! Yes indeed. Dead code ("farbliste") in the source cannot be removed safely. And: 3. Haskell "casts" 181935629 to Word16 value 7693 WITHOUT WARNING! Needless to say, these are serious problems for leading-edge strongly typed language. The experts are probably aware of this. But I awoke harshly from a sweet dream. Issue 3 is quite harmful and reminds me more of C than of a strongly typed language. Fixing this is not trivial. The problem is that all numeric constants have type Num a => a, and of course, type variable a can be instantiated to Int Word16 or whatever. Numeric constants need to inhabit subclasses of Num, e.g., Word16, Int and Integer /classes/ to make this behavior go away. Update: There is nothing wrong with Word16 computing modulo 2^16, but a literal like 181935629 should not get type Word16 without warning. Java asks for explicit casts in byte i = (byte) 1234876; but mindlessly also in byte i = (byte) 123; which is not a great solution either. Issues 1/2 go away with {-# LANGUAGE NoMonomorphismRestriction #-} (thx to Christian who pointed me to this). I wonder why this is not on by default?! There seems to be an unresolved dispute about this... http://hackage.haskell.org/trac/haskell-prime/wiki/NoMonomorphismRestriction But even with MonomorphismRestriction, issue 1/2 are avoidable with a more refined approach to type meta variables in the Haskell type inference: After type-checking a strongly connected component, type meta variables created for this part of the Haskell program should not be instantiated in the (topologically) later defined parts of the program. YOU DON'T WANT THE USE OF A FUNCTION INFLUENCE ITS DEFINITION! There are two alternatives what to do with remaining meta-variables of a SCC: 1. With monomorphism restriction: instantiate to the "best type". In our example below, Haskell chooses Integer. 2. Without monomorphism restriction: generalize. Choice 1 will give an error in the program below, but i rather have an error than a silent change of behavior of my program! Cheers, Andreas P.S.: In Agda, in the situation of unsolved metas in an "SCC" (a mutual block), these metas are just frozen, triggering typing errors later (since Agda cannot generalize). The user has to go back an insert more type annotations. But this is safe, whereas silent late instantiation breaks compositionality. On 16.07.2012 17:25, Andreas Abel wrote: > Today a student came to me with a piece of code that worked it executed > by itself, but produced different result in the context of his larger > problem. We cut down the example to the following: > >> import Graphics.UI.Gtk >> >> -- should produce [(26471,0,65535),... >> rgbliste = >> (map (\ i -> >> let rb = 5 * (mod (mod 181935629 (4534+i)) 100)-250+128 in >> let gb = 5 * (mod (mod 128872693 (5148+i)) 100)-250+128 in >> let bb = 5 * (mod (mod 140302469 (7578+i)) 100)-250+128 in >> let r = min 255 $ max 0 rb in >> let g = min 255 $ max 0 gb in >> let b = min 255 $ max 0 bb in >> (r*257,g*257,b*257)) [0..]) >> >> --farbliste = map (\ (r,g,b) -> Color r g b) rgbliste >> >> main :: IO () >> main = do >> print $ head rgbliste > > If you run it, it prints (26471,0,65535). > If you uncomment the def. of farbliste, it prints (44461,65535,65535). > > I was surprised. What is going on? > > Cheers, > Andreas > -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

3 2

Call to arms: lambda-case is stuck and needs your help
by Mikhail Vorozhtsov 17 Jul '12

17 Jul '12

Hi. After 21 months of occasional arguing the lambda-case proposal(s) is in danger of being buried under its own trac ticket comments. We need fresh blood to finally reach an agreement on the syntax. Read the wiki page[1], take a look at the ticket[2], vote and comment on the proposals! P.S. I'm CC-ing Cafe to attract more people, but please keep the discussion to the GHC Users list. [1] http://hackage.haskell.org/trac/ghc/wiki/LambdasVsPatternMatching [2] http://hackage.haskell.org/trac/ghc/ticket/4359

4 4

Re: [Haskell-cafe] What is the difference between runhaskell and compile?
by Magicloud Magiclouds 17 Jul '12

17 Jul '12

OK. Finally this problem totally prevents my project from working. By some guy's suggestion, I did a packet capturing, which really showed a difference. The rwdc file is for a ldap that at our IDC, which did not work with compiled binary. The rodc file is for a ldap that at local, which worked all the time. Each file contains two parts of records, at 3.* sec and 6/7.* sec. The 3.* part is for runhaskell way. The 6/7.* part is for compiled binary way. The big difference here is: the failed one sent unbindRequest before got bindResponse success. Just a clue, I have no idea what is going on.... On Fri, Jun 1, 2012 at 5:17 PM, Chris Dornan <chris(a)chrisdornan.com> wrote: > No problem -- I have attached the executable too. > > Chris > > -----Original Message----- > From: Magicloud Magiclouds [mailto:magicloud.magiclouds@gmail.com] > Sent: 01 June 2012 02:01 > To: Chris Dornan > Subject: Re: [Haskell-cafe] What is the difference between runhaskell and compile? > > I see. > Although I am using a UTF-8 environment, but all string I was using were ASCIIs. > If it is not too much trouble, may I have your LDAP compiled library files? I think since the Main.o is identical, maybe the libraries are different. > > On Thu, May 31, 2012 at 2:09 PM, Chris Dornan <chris(a)chrisdornan.com> wrote: >> I wouldn't pay too much attention to the fact that the hash tags are >> different -- they are very unlikely to be the same across two >> installations. (For example, my GHC installations are relocated under >> /usr/hs, immediately changing the contents of all of the .conf files >> and therefore the hashes.) >> >> The object files are more interesting, and they are identical! >> >> At the moment I can only explain what we are seeing by assuming that some change has affected the GHC 7.4.1 runtime system -- but only for compiled programs. >> >> Are you using non-ASCII characters in your LDAP passwords? I am thinking that maybe the compiled 7.4.1 could be using a different LOCALE from everything else, leading in some way to the passwords being encoded differently en route to the LDAP server. I don't know whether this could actually happen, but it is the kind of mechanism that seem most likely to be causing the problem. >> >> Did you try to running the variants that read address, account and passwords from standard input? Did it show the same perverse behaviour? >> >> Chris >> >> >> >> >> >> -----Original Message----- >> From: Magicloud Magiclouds [mailto:magicloud.magiclouds@gmail.com] >> Sent: 31 May 2012 02:47 >> To: Chris Dornan >> Subject: Re: [Haskell-cafe] What is the difference between runhaskell and compile? >> >> Hi, >> I have compared the files. The .o-es are exactly the same. The .hi-s are different. >> The interface hash, flag hash, and "import -/ LDAP-0.6.6:LDAP.Constants xxxxx(hash)" are different. >> I do not know why only this module (LDAP.Constants) has a different hash. I am not quite familiar with ghc compiling process. May I know your conclusion? >> >> On Wed, May 30, 2012 at 3:50 PM, Chris Dornan <chris(a)chrisdornan.com> wrote: >>> Did you carry out a standard (optimised) build to get ghc-7.4.1? >>> >>> Are you going to try the justhub rpms or compare the object/header files I sent. >>> >>> Chris >>> >>> -----Original Message----- >>> From: Magicloud Magiclouds [mailto:magicloud.magiclouds@gmail.com] >>> Sent: 30 May 2012 04:07 >>> To: Chris Dornan >>> Cc: Haskell-Cafe >>> Subject: Re: [Haskell-cafe] What is the difference between runhaskell and compile? >>> >>> A little information. >>> I did not notice the gcc/binutils versions. But in CentOS, the ghc >>> 7.2.2/7.4.1 were all compiled myself with all default configurations. >>> >>> On Tue, May 29, 2012 at 10:54 PM, Chris Dornan <chris(a)chrisdornan.com> wrote: >>>> On 29 May 2012 02:21, Magicloud Magiclouds >>>> <magicloud.magiclouds(a)gmail.com> wrote: >>>>> Interesting. I have this code tested in Debian unstable/stable, >>>>> CentOS 6.1, all 64 bit, with two different version of libldap2. >>>>> At first, Debian-s were installed with 7.4.1, CentOS with 7.2.2. >>>>> Only in CentOS the code connected after compiled. >>>>> Then I removed 7.4.1 from Debian stable and installed 7.2.2. The code worked. >>>>> At last, I installed 7.4.1 in CentOS. The code did not work. >>>>> >>>>> Could you send the .hi/.o to me, so maybe I could find out the >>>>> different? Also the exact original source. >>>>> Thank you. >>>> >>>> Interesting indeed! I am guessing that you are using the GHC-7.4.1 >>>> bindist from haskell.org. >>>> >>>> I will try and find some time to marshal the source code and >>>> intermediate files (am on the road -- will need to collect it from >>>> base, make it generic etc.). >>>> >>>> You might also like to try the http://justhub.org ghc-7.4.1-hub on >>>> your CentOS-6.1 node. It is a separate build from the haskell.org >>>> bindist and comes with it's own in-board gcc (4.6.1) and binutils >>>> (2.21) used for the build. It should work for you. >>>> >>>> (You could also try ghc-7.4.2-RC1-hub.) >>>> >>>> Chris >>> >>> >>> >>> -- >>> 竹密岂妨流水过 >>> 山高哪阻野云飞 >>> >>> And for G+, please use magiclouds#gmail.com. >>> >> >> >> >> -- >> 竹密岂妨流水过 >> 山高哪阻野云飞 >> >> And for G+, please use magiclouds#gmail.com. >> > > > > -- > 竹密岂妨流水过 > 山高哪阻野云飞 > > And for G+, please use magiclouds#gmail.com. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

2 6

Adding to the haskell wiki
by Nathan Hüsken 16 Jul '12

16 Jul '12

Hey, I have been a passive member of the haskell community for quite some time now. I have benefit from the community a lot through the mailing list, the haskell webpage and other channels. Thanks for all that! Now I am trying to give a little bit back, and I started a blog about writing javascript games in haskell using UHC and haste. I am not an expert in haskell nor javascript, but I am "collecting" information from various sources and put it together in the blog so that others with a similar goal my have a smoother start. Writing a blog is one thing, but of course I also want people who are interested in the topic to find it. Never being an active member of the community, I wonder if I should do. I guess I should add it to the haskellwiki. But I am unsure where. It certainly falls in the "games" category, but the page only lists game projects. The blog can be found here: http://jshaskell.blogspot.de/ Regards, Nathan

3 2

Re: [Haskell-cafe] [Haskell] A riddle...
by Vo Minh Thu 16 Jul '12

16 Jul '12

Oh the mailing list was haskell@ instead of café. Sorry. (Now cc'ing café.) Andreas, please post this kind of mail to the haskell-cafe instead of the haskell mailing list. 2012/7/16 Vo Minh Thu <noteed(a)gmail.com>: > It seems like the infered type (and thus bounds) is different when you > force the result to be a Color or not. Just give explicit type > signatures and conversion functions. > > Cheers, > Thu > > 2012/7/16 Andreas Abel <andreas.abel(a)ifi.lmu.de>: >> Today a student came to me with a piece of code that worked it executed by >> itself, but produced different result in the context of his larger problem. >> We cut down the example to the following: >> >>> import Graphics.UI.Gtk >>> >>> -- should produce [(26471,0,65535),... >>> rgbliste = >>> (map (\ i -> >>> let rb = 5 * (mod (mod 181935629 (4534+i)) 100)-250+128 in >>> let gb = 5 * (mod (mod 128872693 (5148+i)) 100)-250+128 in >>> let bb = 5 * (mod (mod 140302469 (7578+i)) 100)-250+128 in >>> let r = min 255 $ max 0 rb in >>> let g = min 255 $ max 0 gb in >>> let b = min 255 $ max 0 bb in >>> (r*257,g*257,b*257)) [0..]) >>> >>> --farbliste = map (\ (r,g,b) -> Color r g b) rgbliste >>> >>> main :: IO () >>> main = do >>> print $ head rgbliste >> >> >> If you run it, it prints (26471,0,65535). >> If you uncomment the def. of farbliste, it prints (44461,65535,65535). >> >> I was surprised. What is going on? >> >> Cheers, >> Andreas >> >> -- >> Andreas Abel <>< Du bist der geliebte Mensch. >> >> Theoretical Computer Science, University of Munich >> Oettingenstr. 67, D-80538 Munich, GERMANY >> >> andreas.abel(a)ifi.lmu.de >> http://www2.tcs.ifi.lmu.de/~abel/ >> >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell

2 1

lazy boxed array and builder?
by Evan Laforge 16 Jul '12

16 Jul '12

he recent discussion of whether storablevector should be deprecated in favor of vector reminded me: vector supports storable arrays, but it doesn't support lazy arrays. While storablevector has lazy arrays and a builder, it doesn't support boxed types (it would be become misnamed if it did!). So it seems the niche of boxed lazy arrays is unfilled. And if vector grew a lazy variant it could subsume storablevector and we could consolidate array libraries a little more. It seems a lot of Writer monad type stuff (logging etc.) could be made more efficient with a boxed lazy array builder, at the cost of rougher grained laziness. Does such a thing already exist? I'd check hackage, but it's down... http://www.downforeveryoneorjustme.com/http://hackage.haskell.org/

5 4

Getting a segmentation fault when starting/stopping the RTS, from C, several times.
by Captain Freako 16 Jul '12

16 Jul '12

Hi experts, Should I expect the following C code to run to completion, or am I trying to do something that was never intended? Thanks, -db C code: 1 #include <stdio.h> 2 #include <stdlib.h> 3 #include "HsFFI.h" 4 5 int main() 6 { 7 int argc = 1, i; 8 char* argv[] = {"ghcDll", NULL}; // argv must end with NULL 9 char** args = argv; 10 11 for(i=0; i<10; i++) { 12 fprintf(stderr, "Starting up the RTS...\n"); 13 hs_init(&argc, &args); 14 printf("Shutting down the RTS...\n"); 15 hs_exit(); 16 } 17 } This is what I get, when I compile and run it: dbanas@dbanas-eeepc:~/prj/haskell/amitool$ make test_rts rm -f test_rts ghc -o test_rts -lHSrts -lm -lffi -lrt test_rts.o dbanas@dbanas-eeepc:~/prj/haskell/amitool$ ./test_rts Starting up the RTS... Shutting down the RTS... Starting up the RTS... Shutting down the RTS... Segmentation fault

4 4

Plain lambda inside banana brackets in the arrow notation
by Tsuyoshi Ito 16 Jul '12

16 Jul '12

Hello, In a program, I have an arrow MyArr and a combinator called repeat of the following type: repeat :: Int -> (Int -> MyArr e a) -> MyArr e a My problem is that the code becomes messy when I use this combinator inside the arrow notation, and I am looking for a way to write the code in a more readable way. To explain the problem, first consider the following combinator repeat', which is less general than repeat: repeat' :: Int -> MyArr (e, Int) a -> MyArr e a repeat' n f = repeat n g where g i = arr (\e -> (e, i)) >>> f Combinator repeat' is nice to use in the arrow notation, thanks to banana brackets and the interpretation of lambda: test1 :: MyArr [Double] String test1 = proc xs -> do let y = func1 xs z <- job1 -< xs (|(repeat' 100) (\i -> job2 -< xs !! i + y + z)|) -- func1 :: [Double] -> Double -- job1 :: MyArr [Double] Double -- job2 :: MyArr Double String However, in my program, I often have to use repeat instead of repeat' like: test2 :: MyArr [Double] String test2 = proc xs -> do let y = func1 xs z <- job1 -< xs repeat 100 (\i -> proc (xs, y, z) -> job3 (i * 2) -< xs !! i + y + z) -< (xs, y, z) -- job3 :: Int -> MyArr Double String Note that variable i is used as an argument to function job3 outside MyArr, and this cannot be done with repeat'. The code for test2 looks messy to me because I have to write “(xs, y, z)”, that is, the list of variables used inside the subcomputation explicitly (and twice). It does not seem possible to use banana brackets here because the type of the subcomputation does not meet the requirements stated in http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/arrow-notation.html#…. How can I use combinators like repeat, which takes a plain function as an argument, in the arrow notation in a more readable way? Or am I trying to do an impossible thing? Best regards, Tsuyoshi

3 9

I can see lists "working" for matrix algorithms if the access patterns are mostly from the start or end of rows and columns.
by KC 15 Jul '12

15 Jul '12

Are there any such matrix algorithms that operate mainly from the start or end or rows and columns? Note: If the access pattern starts from the end of a row then one stores the row in reverse order in a list. -- -- Regards, KC

1 0

Re: [Haskell-cafe] [Haskell-beginners] Graph Coloring Library?
by KC 14 Jul '12

14 Jul '12

This may help: - use hMatrix to find the eigenvalues of the adjacency matrix Then by the following theorems: "An upper bound on the chromatic number in terms of eigenvalues was discovered by Wilf(1967). Chromatic Number(G) <= 1 + lambda_1(G) Hoffman (1977) discovered a lower bound on the chromatic number in terms of the smallest and largest eigenvalue. Chromatic Number(G) >= 1 - (lambda_1/lambda_n) for n>=2." >From "The Mutually Beneficial Relationship of Graphs and Matrices", Richard A. Brualdi,CBMS Series,Number 115, 2011. On Fri, Jul 13, 2012 at 1:43 PM, Alec Story <avs38(a)cornell.edu> wrote: > I have a problem where I need to find the smallest k-coloring for a graph > that represents conflicts between objects. Is there a Haskell library that > will do this for me? I'm not particularly concerned about speed, and it's > unlikely that I'll generate really bad edge cases, but I'd prefer to do > something other than write the really bad try-every-case algorithm. > > -- > Alec Story > Cornell University > Biological Sciences, Computer Science 2012 > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > -- -- Regards, KC

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ANNOUNCE: hspec-1.3.0
by Simon Hengel 14 Jul '12

14 Jul '12

Hi, hspec-1.3.0 is out [1]. This release comes with two major new features: - BDD-style combinators to make assertions [2] - Automatic test discovery [3] Test.Hspec now exports parts of the monadic API and the new BDD-style combinators. You can use Test.Hspec.Core as a drop-in replacement for legacy code. The full release notes are here at [4]. A basic introduction to Hspec is at [5]. Feedback is very welcome! Cheers, Simon [1] http://hackage.haskell.org/package/hspec [2] https://github.com/sol/hspec-expectations#readme [3] https://github.com/hspec/hspec/tree/master/hspec-discover#readme [4] https://github.com/hspec/hspec/blob/master/CHANGES.markdown [5] http://hspec.github.com/

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Re: [Haskell-cafe] [Haskell-beginners] Graph Coloring Library?
by KC 14 Jul '12

14 Jul '12

On Hackage there is the Graphalyze package but it doesn't seem to do vertex coloring. Are you talking about undirected graphs? On Sat, Jul 14, 2012 at 12:23 PM, Alec Story <avs38(a)cornell.edu> wrote: > I'd prefer an exact solution, but it doesn't have to be particularly > performant (this is research code). > > The graphs are pretty small in general, all my test cases will have < 10 > nodes, but have high connectivity. They don't have any special properties. > On Jul 14, 2012 2:50 PM, "KC" <kc1956(a)gmail.com> wrote: > >> Are you looking for an exact or approximate solution. >> >> How many nodes and edges? >> >> Any special properties that the graph has? >> >> >> On Fri, Jul 13, 2012 at 1:43 PM, Alec Story <avs38(a)cornell.edu> wrote: >> >>> I have a problem where I need to find the smallest k-coloring for a >>> graph that represents conflicts between objects. Is there a Haskell >>> library that will do this for me? I'm not particularly concerned about >>> speed, and it's unlikely that I'll generate really bad edge cases, but I'd >>> prefer to do something other than write the really bad try-every-case >>> algorithm. >>> >>> -- >>> Alec Story >>> Cornell University >>> Biological Sciences, Computer Science 2012 >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> >> >> -- >> -- >> Regards, >> KC >> > -- -- Regards, KC

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RFCQ: The Classy Prelude: a more fully polymophic, drop-in alternative prelude http://www.yesodweb.com/blog/2012/07/classy-prelude
by Max Cantor 13 Jul '12

13 Jul '12

I'd like to share Michael Snoyman's Classy Prelude: http://www.yesodweb.com/blog/2012/07/classy-prelude . The basic gist of the work is that many of the standard prelude functions are made for concrete types such as [a], [(a,b)], and [Char] but can be much more polymorphic. While this complicates error messages and makes learning Haskell more difficult, we believe experienced Haskellers will benefit from the added polymorphism. Note that we aren't saying that the standard Prelude should be replaced, rather we are offering this as a drop-in alternative. Note that it reexports most of the standard Prelude types so it should be very low impact to switch over. We love to hear your comments. Note however, that this is a RFCQ with the 'C' standing for constructive.. Thanks, Max

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Monads with "The" contexts?
by Takayuki Muranushi 13 Jul '12

13 Jul '12

tl;dr: Is there any way to pass the Supercombinator level names (the names in haskell source codes with zero indent) as context to a Monad so that it will read from "The" context? Hello, everyone. I'm thinking of representing our knowledge about our life, the universe and everything as Haskell values. I'd also like to address the uncertainties in our knowledge. The uncertainties are usually continuous (probabilistic distributions) but that's another story. Please forget about it for a while. We learned that List is for nondeterministic context. > earthMass, sunMass, marsMass :: [Double] Let's pretend that there are large uncertainty in our knowledge of the Earth mass. > earthMass = [5.96e24, 5.97e24, 5.98e24] Let's also pretend that we can measure the other bodys' masses only by their ratio to the earth mass, and the measurements have large uncertainties. > sunMass = (*) <$> [2.5e5, 3e5, 4e5] <*> earthMass > marsMass = (*) <$> [0.01, 0.1, 1.0] <*> earthMass Then, how many Mars mass object can we create by taking the sun apart? > sunPerMars :: [Double] > sunPerMars = (/) <$> sunMass <*> marsMass Sadly, this gives too many answers, and some of them are wrong because they assume different Earth mass in calculating Sun and Mars masses, which led to inconsistent calculation. *Main> length $ sunPerMars 81 We had to do this way; > sunMass' e = map (e*) [2.5e5, 3e5, 4e5] > marsMass' e = map (e*) [0.01, 0.1, 1.0] > sunPerMars' :: [Double] > sunPerMars' = do > e <- earthMass > (/) <$> sunMass' e <*> marsMass' e to have correct candidates (with duplicates.) *Main> length $ sunPerMars' 27 The use of (e <- earthMass) seems inevitable for representing that the two Earth masses are taken from the same source of nondeterminism. However, as the chain of the reasoning grows, we can easily imagine the function arguments will grow impractically large. To get the Higgs mass, we will need to feed them all the history of research that led to the measurement of it. There is "the" source of nondeterminism for Earth mass we will always use. Is there a way to represent this? For example, can we define earthMass'' , sunMass'' , marsMass'' all in separate modules, and yet have (length $ sunPerMars'' == 27) ? By the way, *Main> length $ nub $ sort sunPerMars' 16 is not 9. That's another story, I said! Thanks in advance. -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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ghci and TH cannot: unknown symbol `stat64`
by Michael Snoyman 13 Jul '12

13 Jul '12

Hi all, A quick search indicates that this problem has come up in the past, but I haven't seen any solutions yet. I'm working on the next Persistent release, and one of the changes is that the included sqlite3 C library has been updated (I believe that's the trigger here). I can compile programs against persistent-sqlite, but if there's TH code involved, or I try to runghc the file, I get an error message like: test.hs: /home/ubuntu/.cabal/lib/persistent-sqlite-1.0.0/ghc-7.4.1/HSpersistent-sqlite-1.0.0.o: unknown symbol `stat64' test.hs: test.hs: unable to load package `persistent-sqlite-1.0.0' I'm running GHC 7.4.1 on Ubuntu 12.04 64-bit. Does anyone have insight into what might be causing this? Thanks, Michael

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