- Haskell-Cafe - Haskell.org

Rewriting a Python application to Haskell
by basvandijk＠home.nl 02 Sep '06

02 Sep '06

Hello, The company I work for [1] develops highly sensitive trace gas detectors. We also develop software that is used to control these detectors from a PC or laptop. With this software a user can setup an experiment, execute it and analyse and visualise the data in real-time. While executing, the application gives commands to the detector and gets concentrations from the detector which it plots. See [5] for a screenshot. The application is written in Python and uses wxPython for its GUI. The software is highly threaded; The experiment executor, the analyzer, the communication system and the GUI all have their own threads. In order to synchronise these threads we make extensive use of locks [2]. This thread synchronisation mechanism worked fine in the beginning. However, as the application becomes more complex, it is increasingly difficult not to introduce bugs that are caused by forgetting to release or acquire locks. Also the use of locks makes the code hard to understand. Finally it's hard to compose functions that use locks. I've long been interested in Haskell and played around with it for some time. Recently I became aware of the Haskell STM implementation [3]. I think STM can solve my thread synchronisation problems. Because of this and because I would like to know how suitable Haskell is for developing real-world GUI applications, I would like to rewrite our application in Haskell (in my free time of course ;-) ). I think I need to use wxHaskell for the GUI because it's runs on a lot of platforms and we are currently using wxPython which should make the transition easier. 1) I would like to know if its difficult to use wxHaskell and STM side by side and if there are any issues I need to know about. 2) Also, we use pySerial [4] for serial communication with the detector. Is there something like that for Haskell or is it easy to make it yourself? 3) How portable is Haskell? We like to run our application on Mac, Windows and Linux. Greetings, Bas van Dijk [1] http://www.sensor-sense.nl [2] http://docs.python.org/lib/lock-objects.html [3] http://haskell.org/ghc/docs/latest/html/libraries/stm/Control-Concurrent-ST… [4] http://pyserial.sourceforge.net [5] http://members.home.nl/basvandijk/vc-screenshot.png

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Re: [Haskell-cafe] getContents and lazy evaluation
by Donn Cave 02 Sep '06

02 Sep '06

Quoth Julien Oster <haskell(a)lists.julien-oster.de>: ... | But what happens when two processes use the same file and one process is | writing into it using lazy IO which didn't happen yet? The other process | wouldn't see its changes yet. That's actually a much more general problem, one that I imagine applies to hPutStr et al. too. Application level writes are ordinarily buffered in process space by the I/O library, so output from an ordinary C program may not appear on disk (or in kernel space disk I/O buffer) until just before the program exits. | As for two processes writing to the same file at the same time, very bad | things may happen anyway. Sure, lazy IO prevents doing communication | between running processes using plain files, but why would you do | something like that? Quite a few reasons, depending on how you define communication. You might even be tempted to use hGetContents in such cases. For example, one common way to share a file is to interlock around some resource, and when you acquire the lock, you read the file (get its contents) and release the lock. Donn Cave, donn(a)drizzle.com

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Re: Exercise in point free-style
by John Hughes 02 Sep '06

02 Sep '06

> From: Julien Oster <haskell(a)lists.julien-oster.de> > Subject: [Haskell-cafe] Exercise in point free-style > > I was just doing Exercise 7.1 of Hal Daumé's very good "Yet Another > Haskell Tutorial". It consists of 5 short functions which are to be > converted into point-free style (if possible). > > It's insightful and after some thinking I've been able to come up with > solutions that make me understand things better. > > But I'm having problems with one of the functions: > > func3 f l = l ++ map f l > > Looks pretty clear and simple. However, I can't come up with a solution. > Is it even possible to remove one of the variables, f or l? If so, how? > > Thanks, > Julien Oh, YES!! Two ways to remove l: func3a f = uncurry ((.map f).(++)) . pair func3b f = uncurry (flip (++).map f) . pair And just to make sure they're right: propab new f l = func3 f l == new f l where types = f :: Int->Int quickCheck (propab func3a) quickCheck (propab func3b) If you don't mind swapping the arguments, then propc f l = func3 f l == func3c l f where types = f :: Int->Int func3c l = (l++) . (`map` l) With the arguments swapped, you can even remove both! propd f l = func3 f l == func3d l f where types = f :: Int -> Int func3d = uncurry ((.(flip map)) . (.) . (++)) . pair MUCH clearer! The trick is to observe that l is duplicated, so you need to use a combinator that duplicates something. The only one available here is pair, which you then have to combine with uncurry. It would be nicer to have (f &&& g) x = (f x,g x) available. (&&& is one of the arrow combinators). Then you could remove l by func3e f = uncurry (++) . (id &&& map f) which is sort of readable, and remove both by func3f = (uncurry (++).) . (id &&&) . map John

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data structures question
by Tamas K Papp 01 Sep '06

01 Sep '06

Hi, Having read some tutorials, I would like to start using Haskell "for real", but I have some questions about data structures. The mathematical description of the problem is the following: assume there is a function V(a,b,theta), where a and b can have two values, High or Low, and theta is a number between zero and n (n is given). The range of V is the real numbers. Then there is an algorithm (called value iteration, but that's not important) that takes V and produces a function of the same type, called V'. The algorithm uses a mapping that is not elementwise, ie more than the corresponding values of V are needed to compute a particular V'(a,b,theta) -- things like V(other a,b,theta) and V(a,b,theta+1), where data State = Low | High other :: State -> State other High = Low other Low = High Question 1: V can be represented as a 3-dimensional array, where the first two indices are of type State, the third is Int (<= n). What data structure do you suggest in Haskell to store V? Is there a multidimensional array or something like this? Let's call this structure TypeV. Question 2: I would like to write valueit :: TypeV -> TypeV valueit V = mapondescartesproduct [Low,High] [Low,High] [0..n] mapV where -- mapV would calculate the new V' using V -- mapV :: State -> State -> Int -> Double to fill the new data structure. How to do this sensibly? Thanks, Tamas

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HaXml question
by Tim Newsham 01 Sep '06

01 Sep '06

I thought this one would be easy but I'm starting to think its not. I am playing with HaXml and I want to transform an XML tree into another tree. The transforms are simple enough, but the kicker is that I want them to be "stateful." In this example, the state is a random number generator. So the transformation depends on the current node and the random number generator state. I want to transform every node in the tree. My feeble attempt at this is given in: http://www.thenewsh.com/~newsham/x/tweaker.hs What I see when I run it is that the value of "p" in the "tweak" function is identical each time. Indeed, it seems "g" itself is the same each time "tweak" is invoked (I wrote a variant of the program that used sequential integers instead of generators to verify this). So here's what I think is going on. You can probably skip this section because I am probably off base anyway... I guess "keep" is being applied to one node, and returning a list of 1 node, and this is zipped with my infinite list of generators and as a result I get the same generator each time. If this is the case I guess foldXml is not the right tool for this job. Perhaps I want a newly-written foldXml that works with LabelFilters rather than CFilters? Ok.. so what's really going on here. Can I do what I want to do? Whats the right tool to transform every node in the tree? Is there a more general approach to doing stateful transformations? Thanks in advance for letting me waste your time. P.S. any stylistic advice, or alternate approaches also welcome. The objective here is to learn... Tim Newsham http://www.thenewsh.com/~newsham/

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[Parsec] A combinator to match between M and N times?
by Stephane Bortzmeyer 31 Aug '06

31 Aug '06

Parsec provides "count n p" to run the parser p exactly n times. I'm looking for a combinator "countBetween m n p" which will run the parser between m and n times. It does not exist in Parsec. Much to my surprise, it seems quite difficult to write it myself and, until now, I failed (the best result I had was with the "option" combinator, which unfortunately requires a dummy value, returned when the parser fails). Does anyone has a solution? Preferrably one I can understand, which means not yet with liftM :-)

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Re: [Haskell-cafe] monads once again: a newbie perspective
by Andrea Rossato 31 Aug '06

31 Aug '06

Il Thu, Aug 31, 2006 at 02:39:59PM +0100, Simon Peyton-Jones ebbe a scrivere: > Andrea > > Don't forget to link to it from here! > http://haskell.org/haskellwiki/Books_and_tutorials#Using_monads Simon, I'll do. But now the text is far from being complete: there's only the code... (the most difficult part, for me at least!). Thanks for your kind attention. Andrea

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Haskell for hp-ux (ia64)
by Green Bryan - bgreen 31 Aug '06

31 Aug '06

Does anyone know of a binary for hp-ux (ia64)? I have permission from my company to try and go forward with using Haskell for development with my team. But, we are running hp-ux on a 16 processor(ia64) box and I can't find a build for it. B Green ************************************************************************* The information contained in this communication is confidential, is intended only for the use of the recipient named above, and may be legally privileged. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please resend this communication to the sender and delete the original message or any copy of it from your computer system. Thank you. *************************************************************************

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Galois Connections seeking a developer
by Isaac Jones 31 Aug '06

31 Aug '06

Galois is seeking a full-time candidate for software development and systems integration in the field of high assurance computing. A successful candidate should have a good understanding of the inner workings of databases, good development skills in a number of languages, including at least one functional language (preferably Haskell), and web development. The candidate should have excellent Linux and Unix skills. If the candidate does not know Haskell, they should be good at learning new programming languages, and can reasonably expect to be fluent in Haskell within a few months. Tasks: * Database analysis * Python and PHP web development * Learning and adapting Linux-related technologies including Xen, SELinux, and Knoppix * Creating or modifying Debian packages * Haskell development Knowledge: * Databases implementation internals * Web development, Services-Oriented Architectures * Fluent in Haskell or other functional languages * Grounding in computer security * XML * Linux and Unix Nice-to-have: * Extreme Programming (XP) development experience * Experience deploying software products * Open source software development experience * Ability to get US security clearance Education: * Masters degree or equivilent experience Please respond with a resume and a cover letter explaining your fit to jobs(a)galois.com. Feel free to forward to interested parties. peace, isaac

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haskell -> fpga
by Matthew Pocock 30 Aug '06

30 Aug '06

Hi, Does anybody know of any compilers that can compile portions of a haskell program to an FPGA? I have found a few domain-specific languages implemented in haskell for FPGAs, but so far no back-ends for compilers like ghc. Thanks, Matthew

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