August 2007 - Haskell-Cafe

interaction between OS processes
by Andrea Rossato 21 Sep '07

21 Sep '07

Hi, there's something I don't get about interaction among OS processes and Haskell handles/channels. Suppose I have a very small program that takes a line and prints it till you write "quit": main = do s <- getLine case s of "quit" -> putStrLn "quitting" >> return () _ -> loop s where loop s = do putStrLn s main This is a small interactive process I would like to talk to from another Haskell program, like the following one which, indeed, is just a wrapper around the first. Now, if I write a single line with "quit", I get "quitting" back, otherwise the program doesn't work. I think I need some direction in order to understand how handles work. The same with channels, I'm afraid. Could you please point me in the right direction? Thanks for your kind attention. Andrea The not working code: import Control.Concurrent import System.Process import System.IO main = do c <- runInteractiveCommand "./main2" loop c loop c@(i,o,e,p) = do s <- getLine hPutStrLn i s hFlush i -- now "i" is closed, right? s' <- hGetLine o putStrLn s' loop c

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Dynamic thread management?
by Hugh Perkins 17 Sep '07

17 Sep '07

Haskell/FP seems to have solved the hardest bit of threading, which is making it obvious which bits of a program are parallelizable, and which are not. Remains to actually parallelize out the programs. Am I being naive or is this trivial? There has been a lot of talk about parallelizing out a program statically, at compile time, but this would seem to suffer from the halting problem? The only way to know how long a function, or one of it's sub-functions, will take to run is to actually run it? Is there some reason why we cant just start a function running in a single thread, whilst running profiling, then after a while we check which bits of the function are taking time to run, and are parellizable, and we parallelize those out? This sounds reasonably trivial and workable? Basically, "parallizable" in a Haskell context means, as a first approximation, any map, foldr, foldl or derivatives, and any independent let assignments, then we can always add in extra parallizable cases later. Profiling is already built into Haskell AFAIK? so the profiling information is already available? Thoughts? Is there some reason why such an approach has been disregarded/is harder than it sounds? (By the way, can someone provide me a non-google link to the SPJ video talking about nested data parallism? google video is unavailable from my current location, and although I did watch this video once, it was on my second day of doing Haskell, a few weeks ago, so much of it was incomprehensible to me ;-) )

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Serial Communications in Haskell
by Ronald Guida 08 Sep '07

08 Sep '07

I'm on a Windows box and I'm looking for a way to talk to a serial port (for example, RS-232) from Haskell. I couldn't find a library to do this, so I am wondering how to create one. I have a fairly thorough understanding of how to open and use a serial port with the Windows API. In particular, to open a serial port, I have to use CreateFile, which is the same API call that opens files. In fact, if I call openFile from GHC, and pass "COM1:" as the filename, then I can get a writable serial port. > module Serial > where > import System.IO > > main = do > h <- openFile "COM1:" ReadWriteMode > hPutStrLn h "Hello World" I can't read from the port (I always get an immediate EOF), and I have no way of configuring things like the baud rate or the parity settings. Nevertheless, this demonstrates that openFile can at least open the serial port. What I would like to do is create some functions that would allow me to open and configure a serial port, and get a Handle back so that I can use the existing IO functions like hGetChar and hPutChar. I am assuming that hGetChar eventually calls win32::ReadFile and hPutChar eventually calls win32::WriteFile. These same two API calls would work for serial ports. In Windows, there are 23 API functions that apply specifically to serial ports. Out of these 23 functions, only a few of them are actually necessary if I just want to send and receive data. Of course, I don't know how to call Windows API functions from Haskell, and I have no idea how to hook things to the IO library so that I can use a Handle for a serial port. I'm looking for some advice on how to proceed. -- Ron

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Typeclasses and implicit parameters
by ajb＠spamcop.net 07 Sep '07

07 Sep '07

{-# OPTIONS -fglasgow-exts #-} -- G'day everyone. -- This is okay. f1 :: (?foo :: String) => String f1 = ?foo -- So is this. f2 :: (Show a, ?foo :: a) => a -> String f2 _ = show ?foo -- Hugs allows this. GHC rejects it on the grounds that "a" is unused -- on the right-hand side of the (=>). I think this is arguably a bug -- in GHC. f3 :: (Show a, ?foo :: a) => String f3 = show ?foo -- GHC rejects this. Hugs compiles it, but I can't call it as -- let ?foo = "Hello" in show Foo -- -- Is there a good reason to disallow this? data Foo = Foo instance (?foo :: String) => Show Foo where showsPrec _ Foo = showString ?foo . showString "Foo" -- Cheers, -- Andrew Bromage

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wanted: HAppS example combining state and io
by Thomas Hartman 06 Sep '07

06 Sep '07

In the latest happs (darcs pulled, updated head is 0.9.1 iirc), I am experimenting with the example file in src/HAppS/Examples/HTTP1.hs. I would like to combine state with io. Eventually io will mean stuff like reading from a database, but for now I'm just reading a file. The example file HTTP1.hs has an example that demonstrates state with macid. I added an example that allows you to execute arbitrary io. I tried, but was unable to, add a handler that combines state and io. , h ["iohandler"] GET $ ioReadFileHandler , h ["statehandler"] GET $ stateHandler --, h ["ioandstatehandler"] GET $ ioAndStateHandler ..... -- displays contents of HAPPS.hs in current directory ioReadFileHandler = iohandler $ readFile "./HAppS.hs" -- displays incremented state counter stateHandler = ok $ \() () -> modify (+(1::Int)) >> get >>= respond . show -- should combine effect of iohandler with statehandler -- specifically, should display contents of HAppS.hs, and under that an incremented state handler -- is this possible ioAndStateHandler = undefined undefined Is mixing state and io possible with HAppS? If so, an example showing how to do it would be extremely helpful. best, thomas. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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let and fixed point operator
by Peter Hercek 06 Sep '07

06 Sep '07

Hi, I find the feature that the construct "let x = f x in expr" assigns fixed point of f to x annoying. The reason is that I can not simply chain mofifications a variable like e.g. this: f x = let x = x * scale in let x = x + transform in g x When one is lucky then it results in a compile error; in worse cases it results in stack overflow in runtime. The annoying part is figuring out new and new variable names for essentially the same thing to avoid the search/evaluation of the fixed point. I suppose Haskell was designed so that it makes sense. The only usage I can see is like this: let fact = \x -> if x == 0 then 1 else x * fact (x-1) in ... but that is not any shorter than: let fact x = if x == 0 then 1 else x * fact (x-1) in So the question is what am I missing? Any nice use cases where fixed point search is so good that it is worth the trouble with figuring out new and new variable names for essentially the same stuff? Peter.

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FFI and DLLs
by Lewis-Sandy, Darrell 04 Sep '07

04 Sep '07

An early proposal for the FFI supported importing functions directly from dynamic link libraries: www. <http://www.haskell.org/hdirect/ffi-a4.ps.gz> haskell.org/hdirect/ffi-a4.ps.gz This looks like it was dropped from the final version of the addendum in favor of C header files as the sole form of import entities. Not being a C programmer, how would one go about importing a foreign function (e.g. void Foo(void) ) from a dynamic link library (e.g. foo.dll)?? Would someone be willing to provide an explicit example?

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Re: [Haskell-cafe] GHC 6.6.1 and SELinux issues
by Alexander Vodomerov 03 Sep '07

03 Sep '07

On Tue, Aug 28, 2007 at 08:53:05PM -0600, Stuart Jansen wrote: > On Wed, 2007-08-29 at 00:59 +0400, Alexander Vodomerov wrote: > > In what domain do you run GHC? > > Sorry about that, should've dug deeper. And here we have the difference: > > $ ls -Z /usr/lib/ghc-6.6.1/ghc-6.6.1 > -rwxr-xr-x root root > system_u:object_r:unconfined_execmem_exec_t /usr/lib/ghc-6.6.1/ghc-6.6.1 This explains that GHC works fine. unconfined_execmem_exec_t gives permission to map memory with PROT_EXEC and PROT_WRITE simultaneously. I've put GHC in unconfined_execmem_t and it started to work fine. But the problem is not in GHC -- it is in programs compiled by GHC. They also require exec/write memory. Only root can grant unconfined_execmem privileges, so simple user can not run binaries compiled by GHC. How do you solve this problem? Does Fedora GHC package has any additional patches? With best regards, Alexander.

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gtk library installation from ubuntu repository
by Vikrant 03 Sep '07

03 Sep '07

Hi, I am using ubuntu 7.04. If I try to install libghc6-gtk-dev package using apt-get (or aptitude) my installation hangs at following stage building GHCi library /usr/lib/haskell-packages/ghc6/lib/gtk- 0.9.10.5/HSgtk.o... it doesn;t move later and machine just slogs! Then I have no other way than killing the process which in turn does not allow me install anything later. In last three days I have installed ubuntu with haskell on three different machines and it seems I have always got this problem whenever I tried to install libghc6-soegtk-dev. does anybody know that the package @ ubuntu repository is faulty or something wrong here? any fix to this problem! (probably I should put this on ubuntu users list... but putting mail here because if it is problem with libghc6-soegtk-dev, then somebody from you must have experienced it!) Thanks N Regards, VIkrant

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Definition of the Haskell standard library
by Chris Smith 03 Sep '07

03 Sep '07

Can someone clarify what's going on with the standard library in Haskell? As of right now, I can download, say, GHC from haskell.org/ghc and get a set of libraries with it. I can visit http://haskell.org/ghc/docs/latest/html/libraries/, linked from the haskell.org home page, and see descriptions of all of those libraries. I can build with --make (or if I'm feeling masochistic, add several lines of -package options) and it works. That's all great. I've seen some stuff lately on -libraries and this list indicating that there's an effort to change this. People asking whether something should be included in the standard library are being told that there is no standard library really. I'm hearing that the only distinction that matters is "used by GHC" or "not used by GHC", and that being on hackage is as official as it gets. Am I misunderstanding? Is there something awesome about Hackage that I'm not seeing? I hope one of those two is the case. Otherwise, there's a serious mistake being made here. Having a web site where people can download any of hundreds of independent libraries is not the same thing as having a good standard library for the language. I don't want to see the day when setting up Haskell involves a day-long effort of figuring out what libraries to download and install from Hackage, and in what order to do it to satisfy all the dependencies, and new Haskellers poring over web sites for the thousandth time before realizing that so-and-so's GUI library hasn't actually been touched since they finished their class project in 1998 and doesn't build with the latest version of Qt or whatever. -- Chris Smith

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