July 2005 - Haskell-Cafe

RE: [Haskell-cafe] FFI and callbacks
by Simon Marlow 25 Jul '05

25 Jul '05

On 23 July 2005 03:38, Duncan Coutts wrote: > The problem then as John noted is that the main loop of these toolkits > block and so the other Haskell threads would not get a chance to > schedule. So the challenge is to give the Haskell threads a chance to > schedule. [ good description of the multi-threaded GUI problem deleted ] Thanks for describing the problem in detail, I understand it better now. I think it comes down to this conflict: - you want to take advantage of the fact that GHC has lightweight "green" threads in order to do multithreading within a single OS thread, but - our "bound threads" design does not require the implementation to support lightweight threads, and hence doesn't let the programmer take advantage of them. When we were thinking about bound threads, the idea was to accommodate multiple implementations including the one-to-one and giant lock approaches. We consider(ed) GHC's mixture of lightweight and heavyweight threads to be an optimisation. However, the GUI library example is interesting, because if we could take advantage of lightweight threads it seems we could have multithreaded access to the GUI with better performance and no requirement to make all GUI calls from a single thread. But I'm not convinced. There are advantages to using the single GUI thread approach, and we don't have any measurements for whether the overhead is too much. Suppose you set up a channel containing requests of type (IO a, MVar a), where the MVar takes the result. You can build a combinator withGUI :: IO a -> IO a to perform a GUI operation (and this allows you to try multiple approaches without changing the code). Using withGUI you can send a whole batch of GUI operations in one go, it doesn't have to be one request per GUI call. You could also build asyncGUI :: IO a -> IO () which fires off a GUI request that doesn't require a result, and combines it with existing requests in a single batch if possible. Ordering would be retained, of course. The GUI thread will run multiple requests sequentially, it doesn't have to context switch between each request. And you get more parallelism, because the other threads continue to run while the GUI thread is in the main loop. Cheers, Simon

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Updating the Haskell Standard
by John Goerzen 25 Jul '05

25 Jul '05

There was a brief discussion on #haskell today about the Haskell standard. I'd like to get opinions from more people, and ask if there is any effort being done in this direction presently. I think an updated standard is overdue. I find it difficult anymore to write any but the most trivial of programs using pure Haskell 98. Some notable, and widely-used, features developed since then include: * Overlapping instances * FFI * Hierarchical namespace * Undecidable instances * All sorts of updates to the standard library It has been awhile since I wrote a Haskell program that can compile in pure Haskell 98 mode. I think it would benefit everyone if a more up-to-date standard were made available. I know that some people would like to hold off on such a process until their favorite feature (we'll call it feature X) is finished. I would argue that incremental addendums to the standard should be made more frequently, so that new features can be standardized more easily. Thoughts? Who organized the standardization effort last time?

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ForeignPtrs with liveness dependencies
by Einar Karttunen 25 Jul '05

25 Jul '05

Hello What is the correct way to express liveness dependencies for ForeignPtrs? I am wrapping a C library and need a way to keep ForeignPtrs alive until the finalizer for an another ForeignPtr has been executed. Basically I have two types, ForeignPtr A and ForeignPtr B and a function associate :: ForeignPtr A -> ForeignPtr B -> IO (). I want to keep all of the ForeignPtr Bs associated with a given ForeignPtr A alive until its finalizer has been run. The relationship is M:N - each ForeignPtr A may be associated with multiple ForeignPtr B and each ForeignPtr B may be associated with multiple ForeignPtr A. GHC documentation tells that touchForeignPtr is not enough as it makes no guarantees about when the finalizers are run. If it helps the finalizers are C functions which neither block nor perform callbacks into Haskell. - Einar Karttunen

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Newbie Question about Error Handling
by Gerd M 25 Jul '05

25 Jul '05

Hello! I'm confused by the error handling in Haskell. I've written a program that uses a combined monad of type: type MyMonad a = ErrorT MyErrorType (StateT MyStateType IO) a (MyErrorType is an instance of Error) Therefore, to handle IO Errors inside of MyMonad I need to write: foo :: MyMonad a foo = do ... inp <- liftIO (getChar `catchError` (\e -> if isEOFError e then return '\0' else return '?') ) case inp of '\?' -> throwError ... .... `catchError` myErrorTypeHandler This of course is cumbersome and ugly and I'm sure there is a way to do better. Helpful advice is welcome! _________________________________________________________________ FREE pop-up blocking with the new MSN Toolbar - get it now! http://toolbar.msn.click-url.com/go/onm00200415ave/direct/01/

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Re: [Haskell-cafe] Functional Programming lecture on video
by Carsten Otto 25 Jul '05

25 Jul '05

The lecture excercises can be downloaded from following homepage. The lecture notes are not available, but the difference between the written word in the notes and the spoken word in the lecture is minimal. http://www-i2.informatik.rwth-aachen.de/lufgi2/fp05/ The last two lectures will be digitalized in the next few days. -- Carsten Otto carsten.otto(a)gmail.com www.c-otto.de

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Best way to build strings?
by Andy Gimblett 24 Jul '05

24 Jul '05

A small stylistic question: what's the "best" way to build strings containing other values? For example, I have: data Process = Stop | Prefix String Process | External Process Process instance Show Process where show Stop = "Stop" show (Prefix l p) = "(" ++ l ++ "->" ++ show p ++ ")" show (External p q) = "(" ++ show p ++ " [] " ++ show q ++ ")" but to me the extensive use of ++ is not particularly readable. I'm very fond of Python's interpolation approach, where we'd have something like the following for the External case: def __str__(self): return "(%s [] %s)" % (self.p, self.q) which to me seems clearer, or at least easier to work out roughly what the string's going to look like. (The %s does an implicit "convert to string", btw). Is there a facility like this in Haskell? Or something else I should be using, other than lots of ++ ? Thanks, -Andy -- Andy Gimblett Computer Science Department University of Wales Swansea http://www.cs.swan.ac.uk/~csandy/

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Lists vs. Monads
by Greg Buchholz 24 Jul '05

24 Jul '05

Here's a question for the Haskell community that I've been wrestling with lately. When we say "lists are monads" what does that mean? I can see one of two things. First the slightly superficial... A.) Lists can be made members of the Monads class, and you can define a couple of functions, "bind" and "return" that obey the Three Laws. ...or the more fundamental... B.) Monads somehow define lists. If you had a version of Haskell without recursive data types you wouldn't be at a loss, because you could always use monads to recreate lists. Maybe "bind" would replace the job of "cons" and "return" would replace "head" or somesuch. I'm of the mind that A.) is the right interpretation, but I've seen the "lists are monads" meme mentioned enough that I sometimes question it. In fact, I had a hard time articulating B.) because I don't know what the other possibilies are. In case it isn't clear enough, here's another analogy... A.) Insects are cold blooded animals. ...while that is 100% true, many other animals are cold blooded. It doesn't get at the essence of what it means to be an insect. Compare with a statment like... B.) Insects are six-legged animals with an exoskeleton. ...which tells us a lot more about the fundamental nature of insects. Thanks, Greg Buchholz

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Arbitrary files in a Cabal package: how to install
by Dimitry Golubovsky 24 Jul '05

24 Jul '05

I am trying to use Cabal for buiding HSFFIG as a package. Among other files to install I have the special template for hsc2hs which is a C include file. If I specify it in the Includes: clause nothing happens: it is not copied anywhere during installation. I need this file to be placed somewhere in the package instalation location, so when programs are compiled with my package could include that file. Of course I may specify additional step in the Makefile, but it would be more consistent to do this in Cabal. How can arbitrary files be declared parts of a package, and destination directories for them be specified? I am using the simplified (?) version of Cabal that Ross suggested whrn I asked in this list whether it would be possible to use it with older GHC. Dimitry Golubovsky Middletown, CT

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building Ghc with msvc ?
by Jan Scott 23 Jul '05

23 Jul '05

Hi, Has anyone ever tried to build Ghc with Microsofts c compiler (msvc) ? Thanks Jan

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ANN: MissingH 0.11.3, now supporting Windows
by John Goerzen 22 Jul '05

22 Jul '05

Hello, I have just released MissingH 0.11.3. The big news about this version is that it now supports Windows, in addition to the continuing support for Posix platforms. Downloads and info available from: http://quux.org/devel/missingh or gopher://quux.org/1/devel/missingh -- John

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