- Haskell-Cafe - Haskell.org

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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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

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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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