- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Marlow 04 Dec '05

04 Dec '05

On 29 November 2005 12:08, Tomasz Zielonka wrote: > On Tue, Nov 29, 2005 at 12:00:03PM -0000, Simon Marlow wrote: >> threadDelay is IO-only; there's no way to use threadDelay in an STM >> transaction. For example, if you want to wait for a TVar to go from >> Nothing to Just x with a timeout, you could do this: >> >> readOrTimeout :: TVar (Maybe a) -> Int -> STM (Maybe a) >> readOrTimeout t secs = do timeout <- registerTimeout secs >> let check_timeout = do >> b <- readTVar timeout >> if b then return Nothing else retry >> check_t = do >> m <- readTVar t >> case m of >> Nothing -> retry >> Just x -> return x >> atomically $ check_timeout `orElse` check_t > > Wouldn't it be > readOrTimeout :: TVar (Maybe a) -> Int -> IO (Maybe a) > ^^ > ? Sorry, yes. > Alternatively, it would be nice to have a new STM primitive: > > wailUntil :: ClockTime -> STM () > > so you would wait until some time-point passes, not for a number of > time-units (waiting for a number of time-units wouldn't work because > of retries). I think it could be efficiently implemented, wouldn't it? Interesting. You could use that do wait for idle time, for example: atomically $ do t <- readTVar last_mouse_click waitUntil (t+1000) ... so this transaction only completes when some idle time has passed since the last mouse click. But you could also implement this using registerTimeout, albeit with some more code and an extra thread, and waitUntil requires an implementation in the runtime which is not entirely trivial. Cheers, Simon

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Bug: Invalid object in isRetainer
by Joel Reymont 04 Dec '05

04 Dec '05

./randomplay +RTS -hr ... <- some time passes and then randomplay: internal error: Invalid object in isRetainer(): 67 Please report this as a bug to glasgow-haskell-bugs(a)haskell.org, or http://www.sourceforge.net/projects/ghc/ The combination of this and the failure with -hc -hbdrag,void is preventing me from optimizing the program and it has a huge space leak somewhere. Thanks, Joel -- http://wagerlabs.com/

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Wierd activation record found on stack
by Joel Reymont 04 Dec '05

04 Dec '05

randomplay: internal error: scavenge_stack: weird activation record found on stack: 0 Please report this as a bug to glasgow-haskell-bugs(a)haskell.org, or http://www.sourceforge.net/projects/ghc/ I installed my own signal handlers with the intent of having my profile reports written out. The program runs forever and otherwise generates no profile reports when I interrupt it. I install the handlers like this: installHandler sigPIPE Ignore Nothing flip mapM_ [sigINT, sigHUP, sigABRT, sigTERM] $ \sig -> do installHandler sig handler Nothing where handler = Catch $ exitWith (ExitFailure 1) P.S. Is there a prize of some sorts for the highest number of wierd bug reports per time frame? :D -- http://wagerlabs.com/

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Invalid object in processHeapClosureForDead()
by Joel Reymont 04 Dec '05

04 Dec '05

Has anyone seen this before? ./randomplay +RTS -hc -hbdrag,void -sgcstat randomplay: internal error: Invalid object in processHeapClosureForDead(): 68 Please report this as a bug to glasgow-haskell-bugs(a)haskell.org, or http://www.sourceforge.net/projects/ghc/ -- http://wagerlabs.com/

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Re: :t main
by Scherrer, Chad 03 Dec '05

03 Dec '05

> From: Bulat Ziganshin <bulatz(a)HotPOP.com> > Hello Chad, > > Wednesday, November 30, 2005, 5:07:02 AM, you wrote: > > SC> I've been reading some of the articles about comonads, > and I thought > SC> the idea of giving main the type OI () -> () was pretty > interesting. > > it looks interesting. what are you reading? > > -- > Best regards, > Bulat mailto:bulatz@HotPOP.com > > I'm reading the articles by Kieburtz and Uustalu linked from this page http://haskell.org/hawiki/CoMonadArticles On checking this again, I can't find the reference to the types I mentioned -- It may have just been the result of my reading the articles and letting the ideas simmer in my head, combined with a lack of rereading them before posting. I still need to read things through more carefully to be sure I understand it all correctly. In any case, here's the idea (still evolving, of course). There are safe functions of the form a -> IO b but not IO a -> b Dually, you should be able to write functions OI a -> b but not a -> OI b Now, suppose you want to read stdin, apply f to it, and print the result. The current standard is something like main = do s <- getContents print $ f s or main = getContents >>= (print . f) But it's simpler to write main = print . f . coGetContents where coGetContents :: OI () -> String f :: String -> a print :: a -> IO () Of course, changing the type of main alltogether is probably a bad idea, because it would break too much code. But maybe allowing constructions like this would be reasonable? Alternatively, there could be a nice way to write toMain :: (OI () -> IO ()) -> IO () My first thought is toMain f = f $ OI () but somehow this seems to easy, and I'm worried about the type safety. --------------------------------------------- Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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Re: [Haskell-cafe] Thread blocked indefinitely
by Joel Reymont 02 Dec '05

02 Dec '05

I got a copy, thanks! On Dec 2, 2005, at 6:47 PM, Greg Woodhouse wrote: > Did anyone ever send you a copy of this paper? I'm an ACM member, so I > could forward it to you if need be. -- http://wagerlabs.com/

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Thread blocked indefinitely
by Joel Reymont 02 Dec '05

02 Dec '05

Folks, I'm trying to learn more about this exception. Would someone with a subscription to ACM kindly send me this paper: http://portal.acm.org/ft_gateway.cfm?id=507655&type=pdf? Thanks in advance, Joel -- http://wagerlabs.com/

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RE: Examining the Haskell stack (read at your own risk ; -))
by Simon Marlow 02 Dec '05

02 Dec '05

On 02 December 2005 14:17, Joel Reymont wrote: > On Dec 2, 2005, at 2:08 PM, Simon Marlow wrote: > >> It looks like your crash happened in the SSL library, and you have a >> useful stack trace there. > > This is contrived in that I already know where the error is and it > clearly points to SSL_free. I'm trying to figure out how I would have > gotten to that call to getHostByName. > >> r22 is a pointer to the stack, not a pointer to code, so you can't >> disassemble it, you need to display memory (as I described in >> separate mail). > > Quoting you: > > --- > gdb> p16 $r22 > > which prints 16 words of memory backwards (the way I like it) starting > at the addresss in $r22. when displaying memory this way, gdb very > handily prints the symbol name for words that point into the program. > You can then pick things off the stack that look like return addresses > and disassemble them, if you want. > --- > > And p16 is defined in .gdbinit as: > > define p16 > pmem $arg0 16 > end > > Printing the 16 words gives me the printout below but where do I find > my Haskell function? The code tha causes the crash looks like this: > > maybeFreeSSL :: MaybeSSL -> IO () > maybeFreeSSL tmv = > do putStrLn $ "maybeFreeSSL invoked" > mssl <- atomically $ swapTMVar tmv Nothing > case mssl of > Nothing -> return () > Just (ssl, _, _) -> do sslFree ssl > sslFree ssl > > Is there a way to have maybeFreeSSL in the trace? I compiled the app > with -debug but the libraries and the above maybeFreeSSL code was > compiled without it. > > Thanks, Joel > > P.S. > > Program received signal EXC_BAD_ACCESS, Could not access memory. > Reason: KERN_PROTECTION_FAILURE at address: 0x00000019 > 0x00568fe0 in sk_pop_free () > (gdb) p16 $r22 > 0x137a40c: 0x1ce63c <stg_NO_TREC_closure> > 0x137a408: 0x0 > 0x137a404: 0x0 > 0x137a400: 0x18fd0 <stg_TREC_HEADER_info> > 0x137a3fc: 0xd6dbc <stg_stop_thread_info> > 0x137a3f8: 0xd6f38 <stg_noforceIO_info> > 0x137a3f4: 0x13ef148 > 0x137a3f0: 0x0 > 0x137a3ec: 0x2605c <stg_catch_frame_info> > 0x137a3e8: 0x25c80 <stg_unblockAsyncExceptionszh_ret_info> > 0x137a3e4: 0x13ef150 > 0x137a3e0: 0x27030 <s3BN_info> > 0x137a3dc: 0x1d22cc <GHCziConc_lvl7_closure> > 0x137a3d8: 0x26f6c <s3BK_info> > 0x137a3d4: 0x13f38f4 > 0x137a3d0: 0x243f4 <s7Fy_info> > 0x137a3cc: 0xc3270 <s7FI_info> Ok, you want a crash course in reading the Haskell stack. Each xxx_info symbol is a return address. The other values are the contents of stack frames: values saved for use at the return address. A return address also has an associated info table (use pinfo in the .gdbinit I sent you for disaplying the info table, eg. pinfo s7FI_info). Understanding info tables is beyond the scope of this message; please see InfoTables.h in the GHC sources. You probably want to know what each return address corresponds to: - the ones beginning "stg_" are in the runtime, you can see the code for these in the GHC sources (grep in ghc/rts). - the ones that look like "s7FI_info" are local symbols. You might be able to find which module it comes from by grepping the .o files in your program and the .a library files. If you find it in a library, you can use 'nm' on the library to get a better idea of what function the symbol is associated with. One problem is that because these are local symbols, you might find the same symbol in multiple places, and you have to make an educated guess as to which is the appropriate one (or use the disassembly to distinguish). To map this back to Haskell code, you need to recompile the original module and dump the intermediate code with -ddump-stg. Understanding the output of -ddump-stg is beyond the scope of this message :-) Cheers, Simon

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Re: returning lists
by Scherrer, Chad 02 Dec '05

02 Dec '05

raptor <raptor(a)tvskat.net> writes, > ]- that is the problem :"), 'cause such functions should accept Char and > String, but return a Tuple of Strings, but it is recursive (so that it fetch > next,next,next char :)) i.e. it is own "consumer" and should return String > --this is wrong, just tring grab char (x:xs) You might also have a look at http://haskell.org/hawiki/WorkerWrapper This idea should apply here. -Chad Scherrer

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RE: Examining the Haskell stack (read at your own risk ; -))
by Simon Marlow 02 Dec '05

02 Dec '05

On 02 December 2005 14:03, Joel Reymont wrote: > You told me a bit about how to examine the Haskell stack by looking > at R22 on the PowerPC and $ebx on Intel architectures. I looked at > your .gdbinit but could not figure out which macros are to be used. > > The example below is a bit contrived in that I'm freeing the SSL > context twice, on purpose. I tried getting a disassembler dump using > the contents of R22 without luck: > > (gdb) info registers r22 > r22 0x137a3cc 20423628 > (gdb) disas 0x137a3cc > No function contains specified address. > > This is my stack trace: > > Program received signal EXC_BAD_ACCESS, Could not access memory. > Reason: KERN_PROTECTION_FAILURE at address: 0x00000019 > 0x00568fe0 in sk_pop_free () > (gdb) where > #0 0x00568fe0 in sk_pop_free () > #1 0x0059f128 in X509_VERIFY_PARAM_free () > #2 0x003c4d5c in SSL_free () > #3 0x000c3254 in r7cH_info () > #4 0x000ccd04 in schedule (mainThread=0x13f3f18, > initialCapability=0x578df0) at Schedule.c:932 > #5 0x000cdcac in waitThread_ (m=0x1100360, initialCapability=0x0) at > Schedule.c:2156 > #6 0x000cdb90 in scheduleWaitThread (tso=0x13c0000, ret=0x0, > initialCapability=0x0) at Schedule.c:2050 > #7 0x0001ff0c in rts_evalLazyIO (p=0x1ce0c8, ret=0x0) at RtsAPI.c:459 > #8 0x0000495c in main (argc=25, argv=0x578df0) at Main.c:104 > (gdb) info registers r22 > r22 0x137a3cc 20423628 > (gdb) disas 0x137a3cc > No function contains specified address. It looks like your crash happened in the SSL library, and you have a useful stack trace there. r22 is a pointer to the stack, not a pointer to code, so you can't disassemble it, you need to display memory (as I described in separate mail). Cheers, Simon

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RE: Threaded runtime (was Re: [Haskell-cafe] Project postmortem)
by Simon Marlow 02 Dec '05

02 Dec '05

On 02 December 2005 13:32, Joel Reymont wrote: > I was going on this quote by Simon PJ: > > -- > It should be find to have lots of threads, esp if most of them are > asleep. The only thing to watch out for is that GHC's runtime system > will consume one *OS* thread for each *blocked* foreign call. So if > you have 10k threads each making a separate call to the OS to read > from 10k sockets, and they all block, you'll use 10k OS threads, and > that will probably fail. > -- > > Is this correct and if so how does it mesh with what you said above? It's correct, but not the whole story. When you do a blocking I/O operation, it is not implemented in terms of a blocking foreign call, so it doesn't create an OS thread(*). In -threaded mode, blocking I/O is implemented by sending a reguest to the I/O manager thread, which returns a response when I/O is available. In non-threaded mode, blocking I/O is implemented by returning to the runtime, which occasionally checks for available I/O and wakes up the appropraite threads. Either way, as I said, the programmer doesn't see any difference. (*) except on Windows, where everything is done differently and blocking I/O currently gets a real OS thread. Cheers, Simon

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RE: [Haskell-cafe] Project postmortem
by Simon Marlow 02 Dec '05

02 Dec '05

On 02 December 2005 12:25, Joel Reymont wrote: > I thought that if -threaded is not used then all the blocking IO is > assigned a separate OS thread. No - the runtime is completely single-threaded without -threaded. Blocking I/O is managed by the runtime. With -threaded, blocking I/O is managed by a Haskell thread. The programmer shouldn't see any difference in the behaviour of I/O. Is the documentation for -threaded lacking? I realise it's a bit terse, but do you have any concrete suggestions for improving it? Cheers, Simon

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wxHaskell and do statements
by mempko 02 Dec '05

02 Dec '05

Hello, I have a program that just will not compile and I cannot figure out why. I am starting out with wxHaskell but keep getting certain errors. Here is the source: ------------------------- module Main where import Graphics.UI.WX main :: IO () main = start hello hello :: IO () hello = do f <- frame [text := "Super Window"] lab <- staticText f [text:= "Hello"] quit <- button f [text:= "Quit", on command := close f] touch <- button f [text:= "Touch Me", on command := set quit [text:= "Yay"]] set f [layout:= floatCenter (widget lab) $ floatCenter (widget touch) $ floatCenter (widget quit)] ------------------ I keep the following error **** Compiling Main ( test1.hs, test1.o ) test1.hs:9:11: The last statement in a 'do' construct must be an expression ***** I looked at source code for the do statement and can not understand why I get that error.

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RE: [Haskell-cafe] Project postmortem
by Simon Marlow 02 Dec '05

02 Dec '05

On 02 December 2005 12:49, Tomasz Zielonka wrote: > On Fri, Dec 02, 2005 at 12:39:25PM -0000, Simon Marlow wrote: >>> Do I understand correctly that another workaround is >>> - don't compile your programs with -threaded >>> ? >> >> No, the bug isn't related to -threaded. It still occurs without >> -threaded. > > Let's check that now I understand - so the sequence > > call gethostbyname > read the returned hostent > > is written in Haskell, and many such sequences can be interleaved when > using Concurrent Haskell? Yes, exactly. Cheers, Simon

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RE: [Haskell-cafe] Project postmortem
by Simon Marlow 02 Dec '05

02 Dec '05

On 02 December 2005 12:11, Tomasz Zielonka wrote: > On Fri, Dec 02, 2005 at 11:20:54AM -0000, Simon Marlow wrote: >> If anyone is interested, this turned out to be a bug in the >> Network.BSD module, namely that getHostByName isn't thread safe >> because it is based on the C library function gethostbyname(), which >> returns data in a single static area. >> >> Workarounds are: >> >> - do your own mutual exclusion locking around getHostByName and any >> function that calls it (eg. connectTo). >> >> - use Network.Alt >> (http://www.cs.helsinki.fi/u/ekarttun/network-alt/) which has a >> thread-safe implementation of getHostByName. >> >> - wait for 6.4.2, which will contain a fix for this bug (we don't >> have a fix committed yet, Einar Karttunen has kindly offered to >> look into it). > > Do I understand correctly that another workaround is > - don't compile your programs with -threaded > ? No, the bug isn't related to -threaded. It still occurs without -threaded. Cheers, Simon

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RE: [Haskell-cafe] Project postmortem
by Simon Marlow 02 Dec '05

02 Dec '05

On 21 November 2005 10:16, Joel Reymont wrote: > Is Wolfgang still around? > > Would you guys be willing to guide me through this? I could then > possibly become the next Mac OSX expert :-). > > I have the disassembler dumps, etc. I do not know how to approach > this problem. I read up a bit on the GHC internals, STG, code > generation, etc. If anyone is interested, this turned out to be a bug in the Network.BSD module, namely that getHostByName isn't thread safe because it is based on the C library function gethostbyname(), which returns data in a single static area. Workarounds are: - do your own mutual exclusion locking around getHostByName and any function that calls it (eg. connectTo). - use Network.Alt (http://www.cs.helsinki.fi/u/ekarttun/network-alt/) which has a thread-safe implementation of getHostByName. - wait for 6.4.2, which will contain a fix for this bug (we don't have a fix committed yet, Einar Karttunen has kindly offered to look into it). Cheers, Simon

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RE: Re[2]: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Marlow 02 Dec '05

02 Dec '05

On 01 December 2005 13:27, Bulat Ziganshin wrote: > Hello Simon, > > Wednesday, November 30, 2005, 3:26:44 PM, you wrote: > >> It's a modification of GHC.Conc, so I can't supply a standalone >> implementation I'm afraid. > >>> I could use Tomasz's workaround in the meantime but isn't GHC 6.6 >>> supposed to be out in a year? > >> Rather less than a year, I hope. > > Joel, you can download GHC 6.6 beta builds (from > http://www.haskell.org/ghc/dist/head/dist ?), so you only > need to wait until Simon actually merges this update to the main code http://www.haskell.org/ghc/dist/current/dist/ and registerTimeout is already in. Cheers, Simon

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Problem with continuations and typing
by oleg＠pobox.com 02 Dec '05

02 Dec '05

Jerzy Karczmarczuk wrote: > I tried to cook-up a simple version of nondeterministic computations > using the two-continuation model. I got stuck on an 'infinite type' > error, and my question is: have you seen a *concrete* (really > concrete) Haskell implementation of a similar model, I could inspire > myself on? (I know a few theoretical works on that). Yes, of course: the code for the LogicT paper http://pobox.com/~oleg/ftp/packages/LogicT.tar.gz (please see SFKT.hs) and http://www.haskell.org/pipermail/haskell/2005-October/016577.html http://www.haskell.org/pipermail/haskell/2005-October/016696.html As we can see, FR stream indeed takes success and the failure continuations. This is emphasized at the beginning of the first article. > But I would like to continue this exercice along these lines, without > too much exotism (no monads, yet...), for my students. Do you have any > simple work-around Introduce some algebraic constructors? Perhaps > higher-rank polymorphism could do something (but then I would have to > explain it to my folk...) You're correct on both counts. We can emulate equi-recursive types with iso-recursive ones: e.g., plain Haskell list. Or, you may choose to use higher-ranked type (the FR stream). There is a third solution: use a small typing hole in Haskell to sneak in the real equi-recursive type. I have a feeling though you might not wish to explain the latter method to your students. This present question is actually related to your previous inquiry about expressing a predecessor in a simply-typed lambda-calculus. It cannot be expressed. We need either a recursive or higher-ranked type: predecessor is 'car'.

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returning lists
by raptor 01 Dec '05

01 Dec '05

hi, i'm having this problem, tring to implement split (for learning purposes). Here is one sample of my tries : split _ [] = [] split char (x:xs) | x /= char = x : split char xs | otherwise = [] it is ok i.e. it returns the first of the splits, but I want to get all of the results. str = "testingxsplitxtoxseexhowxitxwork" *Main> split 'x' str "testing" I want : ["testing","split",...] How do i do this.

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RE: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Marlow 01 Dec '05

01 Dec '05

On 29 November 2005 13:01, Joel Reymont wrote: > Would you share your implementation for us advanced users? It's a modification of GHC.Conc, so I can't supply a standalone implementation I'm afraid. > I could use Tomasz's workaround in the meantime but isn't GHC 6.6 > supposed to be out in a year? Rather less than a year, I hope. Cheers, Simon

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:t main
by Scherrer, Chad 01 Dec '05

01 Dec '05

I've been reading some of the articles about comonads, and I thought the idea of giving main the type OI () -> () was pretty interesting. So I was wondering, would it be possible to allow the type of main to be inferred? It seems like IO () OI () -> () OI () -> IO () all make sense (at least I think they do). One particularly nice side effect of this (pardon the pun) is that a good number of useful programs can be written as getContentsW :: OI () -> a doSomeStuff :: (Show b) => a -> b print :: (Show b) => b -> IO () main = print . doSomeStuff . getContentsW so much less understanding about monads et al is required to do some basic stuff. I think it would be much easier in this case to make the transition to Haskell. Also, has anyone given any thought to syntactic sugar for comonads? Since arrows and monads share the (<-) symbol, it seems like a nice unification could be possible... Maybe (-<) alone could be used similarly? Thanks, Chad Scherrer

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RE: [Haskell-cafe] Detecting Cycles in Datastructures
by Simon Peyton-Jones 01 Dec '05

01 Dec '05

| Andrew Pimlott: | //about my highly spiritual essay on lazy computing of PI//: | | > In addition to being clever and terribly funny, the conclusion | > foreshadows (inspired?) later work on Enron [1]. | | Come on, it is improbable that Master Simon ever read my essay... I hadn't before, but I have now. Cool stuff. I once wrote a continued-fraction package with the something of the same flavour (since overtaken by David Lester's work, which to my deep sadness remains unpublished to this day). Your code clearly belongs in the nofib suite, as a splendid way to consume compute cycles and test compilers. Do send me a Main.hs to compute pi, with a changeable runtime (presumably just adjust 'take'). Simon

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first post
by raptor 01 Dec '05

01 Dec '05

hi, I'm trying to figure out how to parse a text file, but these type-conversations are just killing me! Whats wrong with following code : module Main where main = putStrLn "hi" parse str = take 10 str load = do content <- readFile "parse.hs" return content when try : Prelude> parse load <interactive>:1:6: Couldn't match `[a]' against `IO String' Expected type: [a] Inferred type: IO String In the first argument of `parse', namely `load' In the definition of `it': it = parse load i tried different combinations, but still cant figure out... How do u use an "IO String" in normal "String" functions.. I want to particulary slurp file in a string and then make funny things with it...spliting to lines,parsing them and building hash like strucutre of it...etc. later i want to try spliting,regex-es..etc. It is very hard to figure out how to use these modules.. :"(

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first post
by Chad Scherrer 01 Dec '05

01 Dec '05

Because the load function interacts with the outside world, its type involves the IO monad. You can get the result you're looking for using something like liftM parse load liftM takes an ordinary function and applies it to a monadic value (here the monad is IO), returning another monadic value. Or, if you're using ghci, you could just do s <- load parse s This is because ghci executes "in the IO monad" (maybe there are some details of this I'm missing, but that's how I think of it anyway. BTW, your load function could be equivalently written simply as load = readFile "parse.hs" -Chad Scherrer

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Oops, Hsc license is clearly GPL in the Hsc.cabal file was Re: Hacking Haskell in Nightclubs?
by rd＠slavepianos.org 30 Nov '05

30 Nov '05

> My mistake, the Hsc.cabal file clearly states GPL. (I assume I can choose > either GPL v1 or v2 ?) The SuperCollider server is GPL v2. As this is just an interface to the server, I think it should have the same license. I haven't worked out about the Cabal yet, but it's on my todo list, and then I'll make the version number explicit. Rohan

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Hacking Haskell in Nightclubs?
by rd＠slavepianos.org 30 Nov '05

30 Nov '05

Real-time audio is much simpler these days due to SuperCollider, a truly excellent cross platform audio synthesis server by James McCartney. http://www.audiosynth.com http://supercollider.sf.net To communicate with the server one only needs to implement the most basic aspects of the Open Sound Control (OSC) byte protocol (in particular none of the pattern matching is required). http://www.opensoundcontrol.org/ To define 'instruments' there is a SuperCollider specific byte protocol for describing Unit Generator (UGen) graphs. I have an initial Haskell implementation of both these protocols at: http://www.slavepianos.org/rd/f/207949/ darcs get http://www.slavepianos.org/rd/sw/sw-69/ If SuperCollider is running on the local host at the usual port then: Hsc*> play' sc ab Hsc*> stop' sc should play (and then stop playing) the famous 'Analog Bubbles' test sound: ab = out AR 0 $ combn AR s 0.2 0.2 4 where s = sinosc AR (midicps f) 0 * 0.1 f = lfsaw KR 0.4 0 * 24 + o o = lfsaw KR (MCE [8, 7.23]) 0 * 3 + 80 The Hsc module can also write UGen graphs to the 'dot' language, so graphs can be drawn automatically, which can be useful for debugging. I have been working with scheme and SuperCollider for some time, but have been using haskell recently and generally like it very well. I don't think static typing is an issue. OSC messages can be timestamped, and SuperCollider has a sample accurate scheduling queue, so language timing jitter can easily be worked around. I think that the SuperCollider model is an excellent fit with languages like Haskell. Regards, Rohan On Mon Nov 28 21:35:38 EST 2005 Paul Hudak wrote: > Although Haskore (haskell.org/haskore) doesn't currently support > real-time music, it's something I've thought about numerous times in the > past, and wish I had the time to do it... > > -Paul Hudak > > > Echo Nolan wrote: > > Hello all, > > I read an article on using perl for live improvised synthesis a > > while ago (http://www.perl.com/pub/a/2004/08/31/livecode.html) Does > > anyone have thoughts in doing this is haskell? Would strong typing make > > "jazzy" programming too difficult?

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type inference algorithm for type classes
by robert wong 30 Nov '05

30 Nov '05

Hi All, I have been developing a type inference system which is very similar to type classes' (by Wadler and Blott). However, I cannot find a detailed description of the algorithm. In "Type Classes in Haskell", implementation issues are discussed briefly. However, it is too brief for me to grasp. Does anybody have any suggestion on where to look? Thanks, Robert _________________________________________________________________ Get an advanced look at the new version of MSN Messenger. http://messenger.msn.com.sg/Beta/Default.aspx

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RE: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Marlow 29 Nov '05

29 Nov '05

On 29 November 2005 12:47, Joel Reymont wrote: > Where is this "registerTimeout" in the GHC code? Well, it's not there yet :-) But I've hacked up an implementation this morning so it might be in GHC 6.6. Cheers, Simon

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R: [Haskell-cafe] Hacking Haskell in Nightclubs?
by Santoemma Enrico 29 Nov '05

29 Nov '05

Doing music is a hard low level problem, a concurrency problem and is also an interesting problem for intelligent and behavioural computation. If it's true that Haskell can do its best on the second and third aspect, the undertaking seems to wrap with foreing interface one of the many good C libraries for low level audio/midi management. Btw, about the third aspect I liked your approach to model animations through behaviour in your book School of expressions. So now the questions seem to be: how difficult is today to wrap a C library? Which is the right tool to do it? C->Haskell? Salus, Enrico > -----Messaggio originale----- > Da: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org]Per conto di Paul Hudak > Inviato: martedì 29 novembre 2005 3.36 > A: Echo Nolan > Cc: haskell-cafe(a)haskell.org; paul.hudak(a)yale.edu > Oggetto: Re: [Haskell-cafe] Hacking Haskell in Nightclubs? > > > Although Haskore (haskell.org/haskore) doesn't currently support > real-time music, it's something I've thought about numerous > times in the > past, and wish I had the time to do it... > > -Paul Hudak > > > Echo Nolan wrote: > > Hello all, > > I read an article on using perl for live improvised synthesis a > > while ago (http://www.perl.com/pub/a/2004/08/31/livecode.html) Does > > anyone have thoughts in doing this is haskell? Would strong > typing make > > "jazzy" programming too difficult? > > Regards, > > Echo Nolan > > -- > Professor Paul Hudak > Department of Computer Science Office: (203) 432-1235 > Yale University FAX: (203) 432-0593 > P.O. Box 208285 email: paul.hudak(a)yale.edu > New Haven, CT 06520-8285 WWW: www.cs.yale.edu/~hudak > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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RE: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Marlow 29 Nov '05

29 Nov '05

On 29 November 2005 06:29, Tomasz Zielonka wrote: > On Mon, Nov 28, 2005 at 11:27:46PM +0000, Joel Reymont wrote: >> Folks, >> >> How would you implement a timed read from a channel with STM? I would >> like to return Timeout if nothing was read from a TChan in X ms. >> >> Is this a basic two-thread timeout implementation or is there a more >> elegant way of implementing this using `orElse`? > > Here is a basic two-thread implementation: > > http://www.haskell.org/pipermail/haskell-cafe/2005-January/008303.html > > The other approach I see is to create a TVar that would be updated > with current time every 0.1 s (or something like that), but that would > be rather inefficient. > > You could also create a time-out manager thread (with a priority > queue, etc), so you don't have to spawn a thread for every timeout. Interestingly, GHC already has a timeout thread - the I/O manager thread handles threadDelay too. It wouldn't be too hard to adapt it to do STM timeouts too, with a function like registerTimeout :: Int -> STM (TVar Bool) and you wait for your timeout by waiting for the TVar to contain True. Cheers, Simon

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STM, orElse and timed read from a channel
by Joel Reymont 29 Nov '05

29 Nov '05

Folks, How would you implement a timed read from a channel with STM? I would like to return Timeout if nothing was read from a TChan in X ms. Is this a basic two-thread timeout implementation or is there a more elegant way of implementing this using `orElse`? Thanks, Joel -- http://wagerlabs.com/

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Hacking Haskell in Nightclubs?
by Echo Nolan 29 Nov '05

29 Nov '05

Hello all, I read an article on using perl for live improvised synthesis a while ago (http://www.perl.com/pub/a/2004/08/31/livecode.html) Does anyone have thoughts in doing this is haskell? Would strong typing make "jazzy" programming too difficult? Regards, Echo Nolan

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Lazier I/O?
by Dimitry Golubovsky 28 Nov '05

28 Nov '05

This may be a stupud question, but how to make I/O in Haskell really lazy? Here is a simple program: ==== module Main where import System.IO import Foreign import Data.Word import Data.Char s2c :: String -> [Word8] s2c s = map (fromIntegral . ord) s sendstr :: Handle -> String -> IO Int sendstr h s = do let c = s2c s ln = length c allocaBytes ln $ \buf -> do pokeArray buf c hPutBuf h buf ln return ln main = do hSetBuffering stdout NoBuffering l1 <- sendstr stdout "abcde\n" l2 <- sendstr stdout "defghij\n" putStrLn $ "sent " ++ (show l2) ++ " bytes" putStrLn $ "sent " ++ (show l1) ++ " bytes" ==== It prints: abcde defghij sent 8 bytes sent 6 bytes i. e. the first string is output first although the result from that output (how many bytes sent) is needed second. What is desired is to have the IO actions perform as their results are needed. I am assuming some knowledge that those actions have only limited scope of side effects (e. g. order of outputs within a window is significant, but order of appearance of those windows on the screen may not be). I see some way to do this by writing regular non-monadic Haskell stuff, representing each side effects scope (i. e. where ordering of actions is necessary) with its own instance of an I/O like monad (but runnable from an outside non-monadic code), and then using unsafePerformIO as needed. But there may be some framework already developed (albeit with unsafePerformIO, but hiding it from application developers). Any ideas, pointers? Thanks Dimitry

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Other languages using monads?
by Gregory Woodhouse 28 Nov '05

28 Nov '05

My knowledge of functional programming is pretty much limited to Haskell, Scheme, and a smattering of Common Lisp. Are there languages other than Haskell that explicitly use monads? How about "not so explicitly"? === Gregory Woodhouse gregory.woodhouse(a)sbcglobal.net "The universe is not required to be in perfect harmony with human ambition." --Carl Sagan

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RE: [Haskell-cafe] STM, IO and event loops
by Simon Marlow 28 Nov '05

28 Nov '05

On 28 November 2005 11:47, Joel Reymont wrote: > On Nov 28, 2005, at 11:32 AM, Simon Marlow wrote: > >> On Unix with -threaded, all blocking I/O using Handles or sockets >> goes via the I/O manager thread, which multiplexes all the blocked >> I/O requests into a single select() call. If you have 20k blocked >> socket reads, there will be 20k lightweight Haskell threads, and >> probably 2 or 3 real OS threads. Without -threaded the outcome is >> pretty much the same, except the runtime does the select(). > > I'm not well-versed on the differences between select in the I/O > manager and the runtime. What is the difference? Should I just plug - > threaded into my cabal file and not worry about it? How big of an > impact is select() in the runtime? When the runtime is doing select(), you'll get more calls to select() because the runtime calls it for every thread switch, whereas with -threaded select() is only called when something changes - we need to wait on a new descriptor, or I/O is available on one or more descriptors. So you should get better performance with -threaded for large numbers of descriptors. Also you might get a small amount of parallelism on a multi-processor because the select() will run on a different CPU. I'm aware that there are various better alternatives to select(), but we don't use any of them yet. Cheers, Simon

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RE: [Haskell-cafe] STM, IO and event loops
by Simon Marlow 28 Nov '05

28 Nov '05

On Unix with -threaded, all blocking I/O using Handles or sockets goes via the I/O manager thread, which multiplexes all the blocked I/O requests into a single select() call. If you have 20k blocked socket reads, there will be 20k lightweight Haskell threads, and probably 2 or 3 real OS threads. Without -threaded the outcome is pretty much the same, except the runtime does the select(). On Windows, unfortunately you'll get 20k OS threads for this scenario (regardless of -threaded), because we haven't implemented the I/O manager on Windows yet. So, to answer the question: >> Are there any facilities in Haskell that can poll a set of 10k >> descriptors and retrieve input from the first available? Yes, you have one forkIO thread per descriptor, and you do your I/O using standard Handle I/O or Network.Socket.recvfrom/sendto. Unfortunately it'll be expensive on Windows right now, but the best way to fix this would be to implement the I/O manager (in GHC.Conc) rather than doing the multiplexing explicitly in your app code. Cheers, Simon On 28 November 2005 08:57, Simon Peyton-Jones wrote: > A good design pattern, I think, is to have a Haskell thread dedicated > to each socket. It can suck on the socket and dump what it finds > into an STM channel or buffer. Then other threads (the ones doing > the work) can read it transactionally. > > It should be find to have lots of threads, esp if most of them are > asleep. The only thing to watch out for is that GHC's runtime system > will consume one *OS* thread for each *blocked* foreign call. So if > you have 10k threads each making a separate call to the OS to read > from 10k sockets, and they all block, you'll use 10k OS threads, and > that will probably fail. > > Simon > >> -----Original Message----- >> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Joel >> Reymont >> Sent: 27 November 2005 11:12 >> To: Haskell Cafe >> Subject: [Haskell-cafe] STM, IO and event loops >> >> Folks, >> >> I'm trying to build an event-driven system that includes multiple >> event channels. I would have one channel for network events and >> another one for events sent by other threads. I would like to use STM >> and `orElse` to poll the various event channels. >> >> Is there a reasonably safe way to do this with STM and IO? >> >> The network side of things currently times out after a given number >> of seconds if no input is available but can also throw exceptions. >> I'm having a hard time figuring out how to wrap this in STM. >> >> There's another alternative that I can think of... >> >> My poker bots are launched in separate threads but do not talk to >> each other right now. They just receive events from the network. I >> would like a poker bot to tell others to stop playing and exit, for >> example. I guess I could build poker bots with an event loop that >> reads from a TChan but... I would then need twice as many threads >> since each bot has a socket associated with it and one thread per bot >> would need to read from the socket and push events into the TChan. >> >> Are there any facilities in Haskell that can poll a set of 10k >> descriptors and retrieve input from the first available? >> >> I don't know if I should be concerned with launching 20k threads vs. >> 10k threads but it seems that disassociating bots from the network >> has some positive effects. I could compose packets by hand in the >> interpreter and feed them to the TChan by hand to test individual >> bots, for example. >> >> Any advice is appreciated! >> >> Thanks, Joel >> >> -- >> http://wagerlabs.com/ >> >> >> >> >> >> _______________________________________________ >> 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

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Haskell GUI on top of Xlib?
by Dimitry Golubovsky 28 Nov '05

28 Nov '05

Do there exist Haskell graphics/UI toolkits implemented on top of the X11 library (Xlib) without any intermediate C/C++ libraries (i. e. not WxHaskell for example)? I have a very low level client-side interface to the X11 protocol implemented in pure Haskell (layout of protocol packets obtained from Xlib header files preprocessed by HSFFIG). As of now, I can open a display, build request packets manually and parse server responses (some of, but this is just a matter of writing more parsers), so this is more like a toolset for X11 protocol packets manipulations. I do not intend to implement a full analog of Xlib in Haskell. Instead, it might be interesting to create a "fully functional" GUI toolkit based on some already existing high-level interface. Any ideas will be appreciated. Dimitry Golubovsky Middletown, CT

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RE: [Haskell-cafe] STM, IO and event loops
by Simon Peyton-Jones 28 Nov '05

28 Nov '05

A good design pattern, I think, is to have a Haskell thread dedicated to each socket. It can suck on the socket and dump what it finds into an STM channel or buffer. Then other threads (the ones doing the work) can read it transactionally. It should be find to have lots of threads, esp if most of them are asleep. The only thing to watch out for is that GHC's runtime system will consume one *OS* thread for each *blocked* foreign call. So if you have 10k threads each making a separate call to the OS to read from 10k sockets, and they all block, you'll use 10k OS threads, and that will probably fail. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Joel | Reymont | Sent: 27 November 2005 11:12 | To: Haskell Cafe | Subject: [Haskell-cafe] STM, IO and event loops | | Folks, | | I'm trying to build an event-driven system that includes multiple | event channels. I would have one channel for network events and | another one for events sent by other threads. I would like to use STM | and `orElse` to poll the various event channels. | | Is there a reasonably safe way to do this with STM and IO? | | The network side of things currently times out after a given number | of seconds if no input is available but can also throw exceptions. | I'm having a hard time figuring out how to wrap this in STM. | | There's another alternative that I can think of... | | My poker bots are launched in separate threads but do not talk to | each other right now. They just receive events from the network. I | would like a poker bot to tell others to stop playing and exit, for | example. I guess I could build poker bots with an event loop that | reads from a TChan but... I would then need twice as many threads | since each bot has a socket associated with it and one thread per bot | would need to read from the socket and push events into the TChan. | | Are there any facilities in Haskell that can poll a set of 10k | descriptors and retrieve input from the first available? | | I don't know if I should be concerned with launching 20k threads vs. | 10k threads but it seems that disassociating bots from the network | has some positive effects. I could compose packets by hand in the | interpreter and feed them to the TChan by hand to test individual | bots, for example. | | Any advice is appreciated! | | Thanks, Joel | | -- | http://wagerlabs.com/ | | | | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Shortening if-then-else
by tpledger＠ihug.co.nz 27 Nov '05

27 Nov '05

Arjan van IJzendoorn wrote: | > Is there a shorter way to write the if-then-else part below? | > if (cmdType cmd) /= (CmdSitError Server) | > then return $ Just seat_num | > else return Nothing | | return $ if cmdType cmd /= CmdSitError Serv | then Just seat_num else Nothing There's a subtle change in semantics when we move the 'if' inside the 'return'. The original code requires the condition to be evaluated as part of the do-expression's monad's structure, but the translated code defers it. 'return $! if ...' would be closer to the original. Regards, Tom

1 0

STM, IO and event loops
by Joel Reymont 27 Nov '05

27 Nov '05

Folks, I'm trying to build an event-driven system that includes multiple event channels. I would have one channel for network events and another one for events sent by other threads. I would like to use STM and `orElse` to poll the various event channels. Is there a reasonably safe way to do this with STM and IO? The network side of things currently times out after a given number of seconds if no input is available but can also throw exceptions. I'm having a hard time figuring out how to wrap this in STM. There's another alternative that I can think of... My poker bots are launched in separate threads but do not talk to each other right now. They just receive events from the network. I would like a poker bot to tell others to stop playing and exit, for example. I guess I could build poker bots with an event loop that reads from a TChan but... I would then need twice as many threads since each bot has a socket associated with it and one thread per bot would need to read from the socket and push events into the TChan. Are there any facilities in Haskell that can poll a set of 10k descriptors and retrieve input from the first available? I don't know if I should be concerned with launching 20k threads vs. 10k threads but it seems that disassociating bots from the network has some positive effects. I could compose packets by hand in the interpreter and feed them to the TChan by hand to test individual bots, for example. Any advice is appreciated! Thanks, Joel -- http://wagerlabs.com/

2 1

Foreign function Intf./GHC newbie question
by John Zenger 26 Nov '05

26 Nov '05

Hours of poring over documentation has failed to provide me with the answer to what I am sure is a simple question for most of you. Your help is appreciated. I want to compile a Haskell program that calls a foreign C function. I'm using GHC. When GHC links, it says it can't find my C function. That is not surprising, because I can't figure out how I'm supposed to tell GHC where to find my C function. This is what I've got: ------ FILE: ffi.hs {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -#include "myc.h" #-} module Main where foreign import ccall "supernumber" supernumber :: Int bongo :: Int bongo = supernumber + 1 main = do putStrLn (show bongo) ------ FILE: myc.c static int supernumber() { return(72); } ------ FILE: myc.h static int supernumber(); ------- END OF FILES When I do "ghc ffi.hs," it comes back: "ffi.o(.text+0xa8):fake: undefined reference to `supernumber' " I am guessing that I need to tell it where myc.c is. Perhaps I have to compile myc.c first using gcc? That seems inconvenient. Any hints? I am using Windows XP and GHC 6.4, if that matters. _________________________________________________________________ Is your PC infected? Get a FREE online computer virus scan from McAfee® Security. http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963

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Can anyone help me with partition numbers?
by whoals (sent by Nabble.com) 25 Nov '05

25 Nov '05

A partition of a positive integer n is a representation of n as the sum of any number of positive integral parts. For example, there are 7 partitions of the number 5: 1+1+1+1+1, 1+1+1+2, 1+1+3, 1+2+2, 1+4, 2+3 and 5. Define a function parts which returns the list of distinct partitions of an integer n. For example, parts 4 = [[1,1,1,1],[1,1,2],[1,3],[2,2],[4]]. -- Sent from the Haskell - Haskell-Cafe forum at Nabble.com: http://www.nabble.com/Can-anyone-help-me-with-partition-numbers--t612336.ht…

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questions on lazy pattern, StateT monad
by Fan Wu 24 Nov '05

24 Nov '05

Hi Haskell gurus, I'm very puzzled on some code I saw in GHC Monad.StateT (which is about state monad transformers) source and hope you can kindly give me some insight into this. newtype StateT s m a = S (s -> m (a,s)) instance MonadPlus m => MonadPlus (StateT s m) where mzero = lift mzero mplus m1 m2 = do s <- peek let m1' = runState s m1 m2' = runState s m2 ???????----> ~(a,s') <- lift (mplus m1' m2') poke s' return a To illustrate my puzzle, say m is of List type and runState s m1 = m1' = [(a1, s1)] runState s m2 = m2' = [(a2, s2)] With the definition of lift (also in the StateT.hs file) as instance Trans (StateT s) where lift m = S (\s -> do x <- m return (x,s)) I got lift (mplus m1' m2') = lift ([(a1,s1), (a2,s2)]) = S (\s -> [ ((a1,s1),s), ((a2,s2),s)]) I'm puzzled over this line: ~(a,s') <- lift (mplus m1' m2') I think ~(a,s') shall be the value of the (StateT s m a) which is generated by "lift (mplus m1' m2')". My questions are - why use lazy pattern? - how is ~(a,s') extracted from (StateT s m a)? This looks like magic to me. In the example I have (a1,s1) and (a2,s2) in the lifted monad, but it seems (a,s') only represents one at a time. It looks like how data is pulled out of a List monad. Your help is highly appreciated. Thanks, Fan

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RE: [Haskell-cafe] GHCi always optimizes?
by Simon Marlow 24 Nov '05

24 Nov '05

On 22 November 2005 10:07, Dusan Kolar wrote: > My question is rather strange, maybe, even if simple: > Does ghci always translates with "-O" option set on? I didn't see anyone answer this specific question: the answer is "no", GHCi does not optimise. Optimisation cannot be used when interpreting, except that you're allowed to load in compiled optimised modules. Cheers, Simon

1 0

RE: [Haskell-cafe] Re: Re[2]: Uses forkOS all CPUS on-board?
by Simon Marlow 24 Nov '05

24 Nov '05

On 24 November 2005 01:35, Shae Matijs Erisson wrote: > Bulat Ziganshin <bulatz(a)HotPOP.com> writes: > >>>> They work, even in a context switching regime, but I can't >>>> exploit all the CPU's on my computer. Always is "active" >>>> just one thread >> this will not help. multi-processing support will be added in GHC >> 6.6, which must roll out due 6 months, as HCAR says > > In the meantime, you can try it yourself by grabbing ghc 6.5 from cvs > and reading http://www.haskell.org/~simonmar/papers/multiproc.pdf The code is still a bit green, but it's improving. I'm tracking down various crashes at the moment. By all means try it out though, and I'm interested in any test programs and benchmarks that folk have. To compile a program for SMP: - Add "GhcLibWays += s" to mk/build.mk when building GHC - Add -smp when compiling your code - +RTS -N2 -RTS to use 2 CPUs (for example) BTW you don't need to use forkOS to get parallelism, forkIO will do. Also we have now implemented par (as in Control.Parallel.par). Cheers, Simon

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Re: [Haskell-cafe] Records (was Re: [Haskell] Improvements to GHC)
by David Menendez 24 Nov '05

24 Nov '05

Chris Kuklewicz writes: > Would the record system describe at > http://lambda-the-ultimate.org/node/view/1119 > also be convertable "into System Fw, GHC's existing, strongly-typeed > intermediate language." ? Probably. Daan's current implementation uses MLF, which I believe is system F implemented for ML. (We're talking about the system in Daan Leijen's paper, "Extensible Records With Scoped Labels". Good stuff.) -- David Menendez <zednenem(a)psualum.com> | "In this house, we obey the laws <http://www.eyrie.org/~zednenem> | of thermodynamics!"

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Uses forkOS all CPUS on-board?
by Dusan Kolar 24 Nov '05

24 Nov '05

Hello haskellers, in past few days, a lot of stuff on concurrency went through the conference. I'm trying to use posted things and my own. They work, even in a context switching regime, but I can't exploit all the CPU's on my computer. Always is "active" just one thread and, thus, the computation is even slower than having a sequential version. Below, you can find my code - it computes nothing useful, it's been simplified to test parallelism, nothing else. Where's my error? Regards Dusan import Control.Concurrent -- computes nothing too much useful, but takes a long time ;-) sumAllSums [] = 0 sumAllSums l@(_:xs) = sumlist 0 l + sumAllSums xs where sumlist res [] = res sumlist sr (v:vs) = sumlist (sr+v) vs main = do putStrLn "Starting..." mv1 <- newEmptyMVar mv2 <- newEmptyMVar t1 <- forkOS $ mkSum1 mv1 t2 <- forkOS $ mkSum2 mv2 tt mv1 mv2 forkOS $ do killThread t1 killThread t2 putStrLn "Done!" where mkSum1 mv = do let res = sumAllSums [1..10000] let ms1 = "Sum1: " ++ show res seq (length ms1) (putMVar mv ms1) mkSum2 mv = do let res = sumAllSums [1..10001] let ms2 = "Sum2: " ++ show res seq (length ms2) (putMVar mv ms2) tt mv1 mv2 = do yield mr1 <- tryTakeMVar mv1 case mr1 of Just r1 -> do yield putStrLn r1 yield r2 <- takeMVar mv2 putStrLn r2 Nothing -> do mr2 <- tryTakeMVar mv2 case mr2 of Just r2 -> do yield putStrLn r2 yield r1 <- takeMVar mv1 putStrLn r1 Nothing -> tt mv1 mv2

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RE: [Haskell-cafe] Function application like a Unix pipe
by Scherrer, Chad 24 Nov '05

24 Nov '05

Bill Wood <william.wood3(a)comcast.net> writes: > Interesting note: in Richard Bird and Oege de Moor, _Algebra > of Programming_, pp. 2-3, the authors write > > As a departure from tradition, we write "f : A <- B" rather than > "f : B -> A" to indicate the source and target types associated > with a function "f". ... The reason for this choice has to do with > functional composition, whose definition now takes the smooth > form: if f : A <- B and g : B <- C, then f . g : A <- C is defined > by (f . g) x = f(g x). > > Further along the same paragraph they write: > > In the alternative, so-called diagrammatic forms, one writes > "x f" for application and "f ; g" for composition, where > x (f ; g) = (x f) g. > > I know I've read about the latter notation as one used by > some algebraists, but I can't put my hands on a source right now. > > I guess it's not even entirely clear what constitutes > "mathematical notation". :-) > > -- Bill Wood Good point. One of my undergrad algebra books ("Contemporary Abstract Algebra", by Gallian) actually used notation like this. Function application was written (x f). Some people even write the function as an exponential. But (f x) is still far more common. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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Problem over Context Free Grammar Transitive Closure Algorithm
by Victor Silva 24 Nov '05

24 Nov '05

Folks this is a parser for Context Free Grammars, for some reason when I go simplify the grammar trying to remove productions that substitute variables like in the case: S -> A, A -> B, B -> a, it works perfectly and returns S -> a, the problem is when the root symbol is on the listthat must be replaced as in: S -> aXa | bXb , X -> a | b | S | £(empty) instead of returning S -> aXa | bXb, X -> a | b | £ | aXa | bXb it ruturns X as X -> a | b | £ | S <- this S is wrong! it should return X -> a | b | £ | aXa | bXb! Im not sure about where is the mistake. If you can give me hints it would be grat. I will be checking the mail and also hanging over #haskell at freenode ifyou want to contact me directly. Thanks in advance for the help! Victor

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RE: [Haskell-cafe] Function application like a Unix pipe
by Scherrer, Chad 23 Nov '05

23 Nov '05

Henning Thielemann <lemming(a)henning-thielmann.de> writes: > Since > (a . b) x > a $ b x > a (b x) > > are equivalent, do you also want to reverse function and > argument in order to match argument order of . and $ ? > > That is > x (b . a) > x b $ a > (x b) a > ? I'm sorry, I'm not sure I understand your question. Are you asking whether one should be allowed to write x f instead of f x? I don't think anyone is advocating this, but is can be convenient to have an infix operator for this purpose. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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