November 2005 - Haskell-Cafe

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

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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

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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/

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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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