- Haskell-Cafe - Haskell.org

cost of List.// for Ord types?
by David Roundy 08 Sep '04

08 Sep '04

Hello, I was wondering if the list diff operator \\ takes advantage of situations where the list data type is in class Ord, besides being in Eq. If it is only in Eq, then \\ must be O(n^2), while if it is in Ord, a O(nlogn) \\ can be written. Basically, I'm wondering if I should avoid using the standard library \\, and if so, whether there might be some better solution than writing my own (which would involve sorting the lists, and then the obvious picking off of the appropriate values). If there isn't a \\ that takes advantage of Ord, could there be? This being a more fundamental question: can you write a separate version of a function for the case where its argument is within a more specific class? -- David Roundy http://www.abridgegame.org

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Functional Reactive Programming
by Mark Carroll 07 Sep '04

07 Sep '04

I was wondering, how much active development is done on FRP frameworks these days. What direction is it going in, and who are the users? I haven't seen much new on Yampa lately so I wondered how that was doing, or if it was thought largely finished and maybe something else was going on. For instance, maybe more of the activity is in developing wxFruit or suchlike? -- Mark

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Installing HaXML,(using Hugs and WindowsXP)
by Tom Spencer 07 Sep '04

07 Sep '04

Has anyone successfully installed HaXML to be used with winhugs in XP. Using the build.bat included with HaXML it seems that it can only be installed for GHC. Any suggestions? Thanks Tom Spencer PS. Apologies that this is pretty vague

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QuickCheck - Extracting data values from tests
by Jorge Adriano Aires 04 Sep '04

04 Sep '04

Hello all, When using Quickcheck, is there some way to extract generated data values to the IO Monad? I know I can collect and print information about test cases, but that's not enough. Data may be pretty complex, and there may be no parsers for it. If a test suddenly goes wrong, just having it displayed doesn't seem that useful. I'd expect quickCheck to have type: quickCheck :: forall a. (Testable a) => a -> IO [a] Show I could just get the offending data with: please_be_empty <- quickCheck prop_foo Also, even when I'm implementing a generator, I want to see how it is working. Running a verboseCheck on some dummy property helps, but I may want to analyse the data, or some parts of it better - for instance, for many data structures I have alternative "show" functions that take parameters as arguments. Thanks in advance, J.A.

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Partially-applied type synonyms?
by Lyle Kopnicky 03 Sep '04

03 Sep '04

Hi all, I'm trying to write a monad transformer class called MonadPCont, for partial continuations, which fits in with the Control.Monad libraries. I'm having a typing problem. What I have so far looks like this: -------------------- module MonadPCont where import Control.Monad import Control.Monad.Cont import Control.Monad.Trans import Control.Monad.Reader import Control.Monad.Writer import Control.Monad.State import Control.Monad.RWS class (Monad (mc a), Monad (mc r)) => MonadPCont mc a r where shift :: ((forall b. Monad (mc b) => a -> mc b r) -> mc r r) -> mc r a reset :: mc a a -> mc r a instance MonadPCont Cont a r where shift f = Cont (\c -> runCont (f (\x -> Cont (\c' -> c' (c x)))) id) reset m = Cont (\c -> c (runCont m id)) type ContT' m r a = ContT r m a instance Monad m => MonadPCont (ContT' m) a r where shift f = ContT (\c -> runContT (f (\x -> ContT (\c' -> c x >>= c'))) return) reset m = ContT (\c -> runContT m return >>= c) -------------------- The error I get is: MonadPCont.hs:21: Type synonym `ContT'' should have 3 arguments, but has been given 1 In the instance declaration for `MonadPCont (ContT' m) i o' Failed, modules loaded: none. I guess it's not possible to partially apply a synonym for a type constructor. Essentially, I'm trying to do a 'flip', but at the type level. The underlying problem is that ContT is written to take the final result type (call it 'r') as the first parameter, and the underlying monad (call it 'm') as the second parameter, e.g. 'ContT r m a'. This is done so that 'ContT r' can be made an instance of the MonadTrans class. Unfortunately, I need 'PI r -> ContT r m', along with a and r, to be a member of the MonadPCont class (PI is the type binding operator). So I thought I'd define ContT' to take the arguments the other way around. Unfortunately, it can't be partially applied. Any ideas, or is it just not feasible to work this class into the library? Thanks, Lyle Kopnicky

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Stack overflow in ghci
by Ron de Bruijn 02 Sep '04

02 Sep '04

I have an expression that gives a stack overflow in ghci (official Debian unstable CVS version)) when I evaluate it. The expression doesn't use more than 150MB of memory (I have more). How can I make sure the stack overflow doesn't happen? There are no strictness flags in my program. But I use DData Maps(I don't know whether they are implemented strict). I also don't know whether strictness/lazyness has anything do to with it. *SomeModule> test *** Exception: stack overflow (148.67 secs, 250825284 bytes) Sometimes I get a negative number of bytes. I heard of the +RTS option. I used: ghci SomeModule.hs -someoptions +RTS -K1500000, but this doesn't seem to have any effect. I think there should be someother option to change it? Thanks in advance, Ron de Bruijn __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - Send 10MB messages! http://promotions.yahoo.com/new_mail

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Re: [Haskell-cafe] Implementing tryReadMVar
by MR K P SCHUPKE 02 Sep '04

02 Sep '04

>do e <- isEmptyChan ch -- is the channel empty? > case e of > True -> processFifo > False-> readChan ch >>= highPriorityOrPush >Now there is danger of blocking on the readChan. Erm, but it does not matter if the readChan blocks... This is specifically for the case where there is multiple threads writing and one reading... With multiple reading threads... well it depends on the plumbing, you could have one channel per reading thread, or one channel with a single thread looking ahead for high priority events, and then instead of a FIFO, another channel going to the worker threads that just read events: do e <- readChan ch if e highPriority then process e else writeChan ch2 e Keean.

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Implementing tryReadMVar
by Einar Karttunen 02 Sep '04

02 Sep '04

Hello Is it possible to implement an operation like tryReadMVar :: MVar a -> IO (Maybe a) in a good fashion? The semantics should be "Read the value of the MVar without taking it if it is filled, otherwise return Nothing". There are several easy and flawed implementations: tryReadMvar mv = do e <- isEmptyMVar mv case e of True -> return Nothing False-> readMVar mv >>= return . Just This does not work because there can be a thread switch between the isEmpty and readMVar. tryReadMVar mv = do mc <- tryTakeMVar mv case mc of Nothing -> return mc Just v -> putMVar mv v >> return mc Now this can block on the putMVar if there was a thread switch and someone filled the MVar behind our back. Using tryPutMVar does not help much as it just creates another race condition: tryReadMVar mv = do mc <- tryTakeMVar mv case mc of Nothing -> return mc Just c -> tryPutMVar mv v >> return mc Consider what happens if the tryPutMVar fails: -- read till we get the value with foobar in the middle loopTill mv = do foobar mc <- tryReadMVar mv case mc of Nothing -> loopTill mv Just v -> return v maybe (loopTill mv) process (tryReadMVar mv) error = do mv <- newEmptyMVar forkIO (mapM_ (\i -> putMVar mv i) [1..10]) mapM_ (\_ -> loopTill mv >>= print >> takeMVar mv >>= print) [1..10] If a tryPutMVar fails, then there will be less than ten values to read which will make the process block in takeMVar. This seems quite straightforward in C with GHC (might be wrong in the SMP case with locking?): tryReadMVarzh_fast { W_ mvar, info; /* args: R1 = MVar closure */ mvar = R1; info = GET_INFO(mvar); if (info == stg_EMPTY_MVAR_info) RET_NP(0, stg_NO_FINALIZER_closure); RET_NP(1, vStgMVar_value(mvar); } What is the best way to do this? - Einar Karttunen

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Re: [Haskell-cafe] Implementing tryReadMVar
by MR K P SCHUPKE 02 Sep '04

02 Sep '04

>might be a reason to want to prefer one event over another. You can still use a single channel... If you read all pending events on the channel into a FIFO (lazy list) then you can check for high priority events on read, and then deal with the next item on the top of the FIFO... something like the following (in pseudo code) while channel not empty read next event if event high priority process now else queue event in FIFO process first event in FIFO So inbetween processing low priority events we check ahead for any high priority ones... This could be extended with multiple FIFO's to deal with multiple priority levels... but this ensures all events are dealt with sequentially (if out of order)# Keean.

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Re: [Haskell-cafe] Implementing tryReadMVar
by MR K P SCHUPKE 01 Sep '04

01 Sep '04

>tryReadMVar mv = do mc <- tryTakeMVar mv The normal reason people want tryRead is to do something like unix's 'select' function, where you want to wait on one of several signals... In my opinion it is better to do this with a _single_ channel and have one thread taking from the channel, whilst all sources of the 'events' write to the same channel... so the refactoring would be like: data Event = Even1 | Event2 | Event3 ... c <- newChan forkIO (...) a <- readChan c case a of Event1 -> ... Event2 -> ... Keean.

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