November 2005 - Haskell-Cafe

RE: [Haskell-cafe] Spurious program crashes
by Simon Marlow 23 Nov '05

23 Nov '05

On 23 November 2005 13:29, Joel Reymont wrote: > On Nov 23, 2005, at 1:18 PM, Simon Marlow wrote: > >> After subsequent dicsussion, do you still think something strange was >> going on here? > > Yes, but in a different thread. The "Postmortem" one. > >> so this basically loops until there are no messages in the channel, >> and then exits. Is that what you wanted, or did you want it to keep >> reading from the channel until told to die? > > I probably made a mistake someplace as I wanted to read until told to > die but ONLY if the channel was empty. I replaced that code with > Tomasz's elegant solution so now I read until Nothing is read from > the channel. > > logger :: Handle -> IO () > logger h = > do ss <- getChanContents parent > logger' ss > where logger' [] = return () > logger' (Nothing:_) = return () > logger' ((Just x):xs) = > do putStrLn x > hPutStrLn h x > logger' xs > yield The yield is unnecessary. Also, getChanContents is considered by some (me included) to be poor style, because it relies on lazy I/O. This should work just as well: logger h = do m <- readChan parent case m of Nothing -> return () Just x -> do hPutStrLn h x; logger h Cheers, Simon

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RE: [Haskell-cafe] Spurious program crashes
by Simon Marlow 23 Nov '05

23 Nov '05

On 21 November 2005 16:43, Joel Reymont wrote: > I'm being quite careful with resources these days. The outstanding > issues are > > 1) Crashes on Mac OSX that are not reproduced on Linux, Windows, etc. > > 2) Some kind of a problem with Chan. getChanContents retrieves things > smoothly, readChan only does it for the first few lines. Simon? > Anyone? After subsequent dicsussion, do you still think something strange was going on here? The code does look strange: logger :: MVar () -> IO () logger die = do empty <- isEmptyChan parent unless empty $ do x <- readChan parent putStrLn x alive <- isEmptyMVar die when (alive || not empty) $ logger die so this basically loops until there are no messages in the channel, and then exits. Is that what you wanted, or did you want it to keep reading from the channel until told to die? STM is a better solution, as already suggested. Without STM, the best way to do this is to multiplex everything into a single channel (i.e. send the die message down the channel). > 3) Different performance of the logger thread on Mac OSX and Windows. > > I'm having thousands of threads write their trace messages to a Chan. > The logger On Windows I only see the first few lines of output when > using isEmptyChan/readChan to retrieve values in a loop. On Mac OSX I > do see smooth output. Context switch behaviour might be different between MacOS X and Windows. With the above code, it might be that the logger thread found an empty channel at sp,e point and exited. Does that make sense? > On Windows I run out of memory because all the output sent to the > chan builds up and is never processed. I can process it by replacing > isEmptyChan/readChan with getChanContents but then my logger thread > hangs forever (right semantics) and hangs everything else that waits > for the logger thread to check an MVar and exit. yes, because the logger thread has exited. Cheers, SImon

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

23 Nov '05

"Albert Lai" <trebla(a)vex.net> writes: > I offer a simpler, more direct, and pre-existing correspondence between a functional > programming construct and unix pipes: Maybe my point wasn't clear. Of course this idea of comparing lazy evaluation to Unix pipes is very old (long before July 2004, I'm sure). The point I'm making is that there is an old idea that may be underused. We use ($) all over the place, but if there are a lot of them (and especially if they are spread over several lines) it becomes awkward to read the whole thing backward to trace through the function from beginning to end. In these cases, it's much simpler to use (\|) = flip ($) -- (#) seems to me too pretty for other purposes to use it here. infixl 0 \| -- Again, why can't this be negative or Fractional?? What I'm asking is really a question of pedagogy and style. This style seems reasonable to me. OTOH, there are some reasons not to do things in this way. Maybe any function big enough to benefit from writing it this way should be broken up anyway. Or maybe getting used to this style where the laziness is right in your face could make it more difficult for people to learn to reason through less obvious laziness. I'm really trying to figure out whether this approach is worth pursuing, rather than imply that this is a completely original idea. 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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Records (was Re: [Haskell] Improvements to GHC)
by Joel Reymont 23 Nov '05

23 Nov '05

I second this motion! I rather like Simon's proposal. On Nov 17, 2005, at 5:00 PM, Fraser Wilson wrote: > Yeah, I thought you might have tried that at some point :-) > > I like http://research.microsoft.com/~simonpj/Haskell/records.html > > cheers, > Fraser. > > On 11/17/05, Joel Reymont <joelr1(a)gmail.com > wrote: Don't get me > started, please :-). I tried making each field a > separate class but then needed to compose records of difference field > instances which led to HList which led to GHC eating up all my memory > and crashing, etc. > > I can see where you are going but if I have 250 records with shared > fields then that's a whole lot of extra boiler plate code to marshall > between the functions with prefixes to the class method > implementations. The road to hell is paved with good intentions ;-). > > Thanks for the tip, though. > > On Nov 17, 2005, at 2:12 PM, Fraser Wilson wrote: > > > To solve this problem I just made them all instances of a class > > with a gameId function. Still, not ideal. -- http://wagerlabs.com/

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RE: Re[2]: [Haskell-cafe] using of data types as kinds
by Simon Peyton-Jones 23 Nov '05

23 Nov '05

Sorry should be fixed now (or shortly when the mirror does its stuff) Simon | -----Original Message----- | From: Bulat Ziganshin [mailto:bulatz@HotPOP.com] | Sent: 22 November 2005 19:28 | To: Simon Peyton-Jones | Cc: haskell-cafe(a)haskell.org | Subject: Re[2]: [Haskell-cafe] using of data types as kinds | | Hello Simon, | | Tuesday, November 22, 2005, 8:17:38 PM, you wrote: | | SPJ> I have not written anything about what I plan to do in GHC, but | SPJ> basically it amounts to allowing you to use a data type as a | SPJ> kind. Busy doing GADTs and impredicativity at the moment though | | Simon, i can't download file | | http://research.microsoft.com/~simonpj/papers/boxy/boxy-pldi.ps.gz | | reffered at http://research.microsoft.com/~simonpj/papers/boxy/ | | | -- | Best regards, | Bulat mailto:bulatz@HotPOP.com | |

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Shortening if-then-else
by Joel Reymont 23 Nov '05

23 Nov '05

Is there a shorter way to write the if-then-else part below? -- tryTakeSeat :: [Word8] -> Word8 -> ScriptState (Maybe Word8) tryTakeSeat _ _ = do ... if (cmdType cmd) /= (CmdSitError Server) then return $ Just seat_num else return Nothing -- Thanks, Joel -- http://wagerlabs.com/

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Re: Frag: a First Person Shooting game
by Joel Reymont 22 Nov '05

22 Nov '05

Mun, I hope this ushers the era of Haskell games! I, for one, was certainly looking for something like this. Did you encounter any difficulties in your development because of Haskell? Was lazy evaluation ever a problem? Why did you choose Yampa to program the game entities? Did this give you a particular advantage? Isn't your frame rate a bit low? Thanks, Joel On Nov 22, 2005, at 6:05 AM, Mun Hon Cheong wrote: > Frag is a 3D First Person Shooting game. > > Features: > > *Yampa, a domain-specific embedded language > for the programming of hybrid systems that > using the concepts of Functional Reactive > Programming (FRP) was used to program the > game entities. -- http://wagerlabs.com/

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re-definition of '.'
by Bulat Ziganshin 22 Nov '05

22 Nov '05

Hello haskell-cafe, if we will define "." as: a.b = b a then we can use "." for 1) field selection 2) sequential functions application in OOP style: [1..100] .map (2*) .sum 3) qualified identifiers - if we treat module name as the record containing all its exported functions: "import M (f,g,h)" defines record M with fields f, g and h The only problem is priority of this operation - for field selection it need to be greater than for application: infixl 11 . while for OOP-styled operations with arguments - its priority must be minimal. one possible solution is rather revolitionary: raise priority of "." if there is no spaces around it, so the following will be interpreted precisely as we want: record.field .List.map (2*) .sum -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Monad Transformer question
by Fan Wu 22 Nov '05

22 Nov '05

Hi Haskell gurus, I'm learning Haskell now and here I'm quite puzzled over some code about Monad Transformers. The code is like: type NDS a = StateT ProblemState [] a getVar :: Var -> NDS (Maybe Value) getVar v = do vs <- gets vars return $ lookup v vs What puzzles me is that, I think the Monad of the do block shall be the "NDS (Maybe Value)" in declaration, but the type of gets is gets :: (MonadState s m) => (s -> a) -> m a So "gets" returns a Monad of type m ([] is this case), which seems to be different from "NDS (Maybe Value)", but GHC does not complain about it. If I comment out the type declaration of "getVar :: Var -> NDS (Maybe Value)" and let GHC interpret the type, then the type of getVar is like: getVar :: (MonadState ProblemState m) => Var -> m (Maybe Value) So does it mean "StateT ProblemState m" and the "m" as in "MonadState ProblemState m" is the same thing? I guess I must missed something when trying to understand the Monad Transformers. Please give me some insights. Thanks, Fan

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RE: [Haskell-cafe] using of data types as kinds
by Simon Peyton-Jones 22 Nov '05

22 Nov '05

Read about Omega! I have not written anything about what I plan to do in GHC, but basically it amounts to allowing you to use a data type as a kind. Busy doing GADTs and impredicativity at the moment though Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Bulat Ziganshin | Sent: 22 November 2005 15:41 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] using of data types as kinds | | Hello , | | in the HCAR GHC team wrote "On the type system front, we hope to ... | Allow you to use data types as kinds, in a manner similar to Tim | Sheard▓s Omega language". can someone point me where i can read about | this? | | -- | Best regards, | Bulat mailto:bulatz@HotPOP.com | | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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