December 2005 - Haskell-Cafe

Indenting issue with Haskell mode
by Joel Reymont 08 Dec '05

08 Dec '05

I believe the following is not indented correctly. It only happens with (x:xs) when you use nested cases, I think. case foo of [] -> case bar of [] -> return () (x:xs) -> -- http://wagerlabs.com/

2 1

RE: [Haskell] fptools mirror in darcs ready for testing
by Simon Marlow 08 Dec '05

08 Dec '05

On 07 December 2005 13:06, John Goerzen wrote: > On Wed, Dec 07, 2005 at 11:02:29AM +0000, Duncan Coutts wrote: >>> Also please note that these repos are READ ONLY for now. Nobody >>> will be accepting darcs patches until Simon (or someone) gives the >>> word. >> >> When that does happen, it'd be great if the repos specified a default >> 'darcs send' email address so that people do not have to guess where >> to send their patches. > > Yes, they will. Simon is thinking of having that be the appropriate > cvs-*(a)haskell.org list. Right now, this isn't happening because there > is no good place to send them. I think the cvs-* lists are fine. > Speaking of which: how long do we want to keep this interim > arrangement before switching to darcs completely? I'm not sure yet! So far I haven't used it in anger enough, though as Duncan points out it's hard to really use it until we can commit darcs changes using darcs. Certainly performance of the --partial tree seems good enough, though I don't like that I can't see the history for individual files. I can't get browsing to work using Trac: with the full darcs repository it takes too long to do anything (like 10 seconds to bring up a directory), and with the --partial one it can't browse propely, presumably because there isn't per-file history. Cheers, Simon

3 2

syntactic sugar for comonads
by Scherrer, Chad 08 Dec '05

08 Dec '05

Hi, I'm wondering about a syntactic sugar for comonads. These are still very new to me, but it seems like their usage will become much more common once manipulations are more convenient (I believe this was the case with monads and arrows, correct?). Here's an example for motivation. Consider the data type data Stream a = a :< Stream a and suppose we want to write a function movingWindow :: Stream a -> Stream [a] so that, for example, movingWindow 3 (1 :< 2 :< 3 :< ...) evaluates to [1,2,3] :< [2,3,4] :< [3,4,5] :< ... Recognizing Stream as a comonad, this is pretty easy: movingWindow n s = s =>> (take n . toList) toList :: Stream a -> [a] toList (x :< xs) = x : toList xs If the second argument of (=>>) is written in lambda form, this comes out as movingWindow n s = s =>> \x -> take n $ toList x This looks analogous to the way do is translated for monads, so a sugary alternative could be movingWindow n s = do x -< s take n $ toList x Note that the presence of "-<" tells us we're in a comonad, rather than a monad. I'm not at all stuck on this, but I think it would be good to get the ball rolling. What do you think? Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

2 1

Re: [Haskell-cafe] Can't Haskell catch up with Clean's uniqueness typing?
by Wolfgang Jeltsch 08 Dec '05

08 Dec '05

Am Donnerstag, 8. Dezember 2005 04:00 schrieb Jan-Willem Maessen: > On Dec 7, 2005, at 9:58 AM, Wolfgang Jeltsch wrote: > > Am Mittwoch, 7. Dezember 2005 14:21 schrieb Jan-Willem Maessen: > >> [...] > >> > >> The principle obstacles are the same as for any reference counting > >> scheme: It imposes more run-time overhead than GC does, unless the data > >> structures involved are large. > > > > Why? I think the point with uniqueness typing/analysis is that > > this is done at *compile-time*. > > But the email asked if there were some other way of doing this using > run-time techniques in the absence of such a type system (or at > least, that was the way I understood it, and the spirit of my answer). I thought that the original question was about using some kind of uniqueness type system at an intermediate stage during compiling. Haskell would still have no uniqueness types but the compiler would infer uniqueness types internally and use the uniqueness information it gets from this. > -Jan > [...] Best wishes, Wolfgang

2 1

Learning Haskell
by Jimmie Houchin 08 Dec '05

08 Dec '05

Hello, I am exploring the possibility of learning Haskell. I am not a professional programmer, nor do I have a CS or Math degree. I do play and program with Python, Smalltalk (Squeak), Javascript, explored Erlang some. I do not have a strong math background. Is lack of strong math background a major hindrance to learning Haskell? Also, I understand Haskell's benefits for programming larger projects. But how does it do on programming in the small? ie: using Haskell where I might have used Python for scripting? Thanks for any help and wisdom. Jimmie Houchin

14 25

String to Hash ?
by raptor 08 Dec '05

08 Dec '05

I have made a simple Split function, here is how it works : str = "k1=v1|k2=v2|k3=v3" > map (sSplit '=') (sSplit '|' str) [["k1","v1"],["k2","v2"],["k3","v3"]] Now what I want to do is to return listOfPairs (instead listOflists) so that I can give this as parameter to Data.Map.fromList and build hash-like structure. What would be the easiest way ? PS. Can anyone point me to a source/link where I can see more examples of using Hash-like structures ?

3 3

Mixing C++ and Haskell, OpenSSL thread safety, and using mmap
by Branimir Maksimovic 08 Dec '05

08 Dec '05

I wrotted some messages on fa.haskell newsgroup but then I've figured that people actually read mailing lists :) So this is digest of mine recent messages on this newsgroup. First I want to say about OpenSSL thread safety. It is not thread safe by default. Who wants to import and use OpenSLL functions with FFI, have to set locking hooks for it, or else spurious crashes with useless stack trace will result. Higher level of concurrency, more likely crash will happen. Since this hooks are called by C, it is easer to setup mutexes in C module, then call Haskell main function from it. details: http://www.openssl.org/docs/crypto/threads.html Second I've successfully linked Haskell and C++ with threaded run time. Well, I'm planning to use Haskell in C++ programs and to call Haskell functions from C++ and vice versa. So far I've tried some small test programs and it seems works ok. If I launch threads from C++ then call Haskell I have to link with -threaded flag as Haskell run time complains about entering functions unsafelly without it. Also I have another question about hs_add_root. Is this neccessary? I've tried with and without call to this function and everything seems to work same way. That means I don't notice any difference. I ask this because if for example I have lot of Haskell modules do I need to call hs_add_root for every and each one? This is just a small example that works both on linux and windows, (I have different C++ versions but Haskell module is same). Haskell makes and frees array of pointers to CStrings (char**) from argument that is CString(char*). Error checking and handling is intentionally left out for now. I would appreciate any critics about it, as I've started to learn language one month ago, but have professional experience since 92 in other languages, specially C++. -- Haskell module module MakeWords where import Foreign import Foreign.C.String import Foreign.Storable import Foreign.Ptr import Foreign.Marshal.Array foreign export ccall makeWords :: CString -> IO (Ptr CString) foreign export ccall freeWords :: Ptr CString -> IO () makeWords :: CString -> IO (Ptr CString) makeWords cs = do let lst = words $ unsafePerformIO $ peekCString cs p <- mallocArray0 $ length lst makeWords' lst p return (p) makeWords' :: [String] -> Ptr CString -> IO () makeWords' [] p = do poke p nullPtr makeWords' (s:strs) p = do poke p $ unsafePerformIO $ newCString s makeWords' strs (plusPtr p $ sizeOf p) freeWords :: Ptr CString -> IO () freeWords p = do freeWords' p free p freeWords' :: Ptr CString -> IO () freeWords' p = if nullPtr /= (unsafePerformIO $ peek p) then do free $ unsafePerformIO $ peek p freeWords' $ plusPtr p $ sizeOf p else return () -- end Haskell module // C++, windows version #include <HsFFI.h> #ifdef __GLASGOW_HASKELL__ #include "makeWords_stub.h" #endif #include <iostream> #include <ostream> #include <windows.h> using namespace std; class Mutex{ public: Mutex(){ InitializeCriticalSection(&s); } ~Mutex(){ DeleteCriticalSection(&s); } void acquire(){ EnterCriticalSection(&s); } void release(){ LeaveCriticalSection(&s); } private: Mutex(const Mutex&); Mutex& operator=(const Mutex&); CRITICAL_SECTION s; }m; DWORD WINAPI tf(LPVOID arg) { for(int i=0;i<100000;++i) { const char** p = (const char**)makeWords(arg?arg:(void*)"boring default"); const char** q = p; if(!(i%10000)) { m.acquire(); while(*q){ cout<<*q++<<' '; } cout<<endl; m.release(); } freeWords(p); } return 0; } #ifdef __GLASGOW_HASKELL__ extern "C" void __stginit_MakeWords(); #endif #ifndef NUM_THREADS #define NUM_THREADS 4 #endif int main(int argc,char* argv[]) { hs_init(&argc,&argv); #if defined(__GLASGOW_HASKELL__) && defined(STGINIT_NEDEED) hs_add_root(__stginit_MakeWords); // is this neccessary? // seems works without it, // both on linux and windows #endif DWORD t[NUM_THREADS]; HANDLE h[NUM_THREADS]; char *args[NUM_THREADS] = { "abc def", "one two three", "house on the beach", "end of the line", }; for(int i =0; i<NUM_THREADS;++i) { h[i]=CreateThread(NULL,0,tf,args[i],0,&t[i]); } DWORD waitall = WaitForMultipleObjects(NUM_THREADS,h,TRUE,INFINITE); for(int i =0; i<NUM_THREADS;++i) CloseHandle(h[i]); hs_exit(); } // end C++ If someone is interested, it is possible to use g++ to link but -u flag symbols have to be obtained by -v3 flag to ghc first. Third, can I use mmap in Haskell? I see that there is MArray whitch is base for StorableArray, but I need MMappedArray which will use mmap on linux or CreateFileMapping and MapViewOfFile on windows instead of malloc. So I would appreciate if someone knows if there is such class already, but it isn;t a problem to reuse StorableArray code and create new persistent Array based on this. Greetings, Bane. _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

2 3

Re: Verbosity of imperative code
by Scherrer, Chad 07 Dec '05

07 Dec '05

> On Tue, Dec 06, 2005 at 10:58:45PM +0300, Bulat Ziganshin wrote: > > the third-priority problem is language itself. in particular, i hate > > Haskell school of imperative manipulations: > > > > x' <- readIORef x > > y' <- readIORef y > > writeIORef z (x'*y') Here's a way to make some of this less messy: class PlusEq a b m | a -> m where (+=) :: a -> b -> m () instance (Num a) => PlusEq (IORef a) a IO where xRef += y = do x <- readIORef xRef writeIORef xRef (x + y) instance (Num a) => PlusEq (IORef a) (IORef a) IO where xRef += yRef = do y <- readIORef yRef xRef += y Then instead of do x <- xRef y <- yRef writeIORef yRef (x + y) you can just say y += x. I've started on an InPlace module to do things like this in general. -Chad Scherrer

1 0

Connecting to a running process (REPL)
by Joel Reymont 07 Dec '05

07 Dec '05

Is there a good standard way of supplying a read-eval prompt in a program? I would like to a running process with something ghci-like to be able to inspect the state and possibly modify it. The running process would be heavily multi-threaded. Thanks, Joel -- http://wagerlabs.com/

4 4

What's a thread doing / Erlang-style processes / Message passing
by Joel Reymont 07 Dec '05

07 Dec '05

I'm finding myself in dire need of monitoring the Haskell runtime. More precisely, I would like to know what each that I launch is doing at a given point in time. Is there a way to obtain this information now? I would be fine with knowing if a thread is blocking on a foreign call, MVar or something like that. The question is prompted by my current project and the issues I'm facing now. I believe I minimized the amount of garbage that I'm generating by moving to unboxed arrays from lists (thanks dcoutts) but I still have memory utilization issues. I currently have each thread associated with a TChan and I'm going to try to abstract that today by creating a special type of a thread object that is associated with two mailboxes (in and out). When starting this thread you would supply the event loop to read from the inbox and another one to write to the mailbox. I would also add stats to the TChan mailboxes so that I know the number of messages pending in each mailbox and can monitor it. This mirrors my current architecture where I have each poker bot as three threads: #1 reading messages from a socket and posting to #3, #2 reading messages sent by #3 and writing to the socket, #3 reading messages sent by #1, processing them and posting to #2. I suppose I'm trying to implement Erlang-like processes where each process has a mailbox for incoming messages and can send messages to any other process. In Erlang you can also check how many messages are pending to each process, etc. I don't think implementing message passing on top of exceptions is a good idea but please correct me if I'm wrong. In Erlang you are tasked with implementing the message loop yourself and retrieve messages by using a "receive" construct where you can pattern-match on the type of message inside. It seems that custom messages would need to be implemented on top of Dynamic but is there a way to pattern-match on that? I have messages implemented like this now but is there a better abstraction? data Event = Enter | Exit | Quit | Timeout String | Connected | Disconnected | Error String | Cmd Command | Custom Dynamic -- can't pattern-match on this? deriving Show Last but not least, to be able to send messages to any thread I would need to keep those around in some sort of a table. I would need to create records and keep the thread id, the mailbox and possibly some sort of a per-thread string so that threads can update me on their doings. Do you have any suggestions? Thanks, Joel -- http://wagerlabs.com/

2 5