- Haskell-Cafe - Haskell.org

wxHaskell: getting a checkbox state
by Mark Carter 14 Sep '05

14 Sep '05

I'm a complete n00b to Haskell, and I am trying to write an experimental app using wxHaskell. I'm getting on suprisingly well, given that I have practially no idea what I'm doing. In my main loop I have cbEdit <- checkBox p1 [text := "Edit Mode", on command := onCbEdit textlog ] where p1 is a panel, and textlog is a textCtrl that I want to write output to. In my where clause I have defined: onCbEdit textlog = do appendText textlog "onCbEdit\n" So far, this works. The thing that's got me totally stumped is this: suppose I want onCbEdit to do something dependent on the state of the checkbox (i.e. whether it is checked or unchecked). I've tried a variety of things, but I can't get anything to work. To put things into a wider context, here's the message I posted to fa.haskell yesterday: I'm the rawest of n00bs to Haskell. I was interested in evaluating it, and I've seen some GUI stuff that looked pretty impressive. I wanted to write a protoype for a commercial application to do hydrocarbon accounting. Which is to say, you've a number of so-called "streams" - think of them as graph nodes - feeding into each other in a directed fashion (assume the graph is acyclic). Then there's a bunch of business logic which determines how the values of one stream are affected by the values in another. An example of a stream might be a platform. Another one might be a well. Each stream would display on the window as some funky symbol, with its name underneath. It would be nice if the application had an edit mode, where you could add, move, delete, and join streams in a window; and a data mode, where you could click on the stream you were interested in and examine its properties. Is there a Haskell GUI toolkit which is particularly suitable for this? Other requirements of the toolkit would be: * Works on Windows (XP) * commercial-friendly licence: GPL is out, and even LGPL is a bit fiddly * free * bonus points for being easy to use, and well-documented ___________________________________________________________ To help you stay safe and secure online, we've developed the all new Yahoo! Security Centre. http://uk.security.yahoo.com

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RE: [Haskell-cafe] static typing and interactivity
by Simon Marlow 14 Sep '05

14 Sep '05

On 18 August 2005 16:11, Jake Luck wrote: >> One slight annoyance using Haskell is the inability to load modules >> with type problems in the interactive environment (i.e. GHCi). When >> I have a type error, it would be nice to have an interactive way to >> explore what the compiler thinks about the types involved -- as it >> is, I have to resort to adding type signatures more or less at >> random to narrow down the problem. >> >> I'm not sure if it is technically feasible to (partially) load a >> module with definitions that fail type checking, but if it were, I >> thing it would make developing Haskell programs even nicer :-) > > Along similiar lines, it would be quite nice if one can mark their > haskell code(working or not) with "breakpoints" and drop the > programmer into GHCi so they can poke around, especially inside a > do-construct. e.g. something the evalutation engine can check during > reduction maybe? I find myself writing a lot of testing frameworks, > maybe this is a good thing!, when I program. How do most of the folks > here debug their large code base? I would really like to do this, and I've put some thought into how to do it. Basically you need 2 things: - A generic way to display data found in the heap. Heap data doesn't carry around it's type, but we need a way to display the arguments to polymorphic functions (for example). Fortunately GHCi has all the information about the layout of known data types, all that's needed is a way to map a heap closure to GHCi's representation of constructor (a DataCon), which is quite possible - some support from the dynamic linker is required, though. - A way to set breakpoints. This is far easier for interpreted code than for compiled code: for interpreted code we can just replace the appropriate BCO (byte code object) with a special breakpoint object. A breakpoint would contain information about its free variables, and a pointer to the original BCO. The harder part is to map source locations to byte code objects, retaining the information through the various compiler passes (HsDebug did some work in this direction, I believe). Anyone interested in working on this, please get in touch. Cheers, Simon

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RE: [Haskell-cafe] Re: Thread pool in GHC
by Simon Marlow 14 Sep '05

14 Sep '05

On 07 September 2005 16:04, Dinh Tien Tuan Anh wrote: > So is there a way to reuse thread ? > I think eventhough threads in GHC are extremely lighweight, it still > make a different if u keep launching and killing thread in so many > times. My advice would be to try without a thread pool first, and see if you have any performance problems. My guess is that you won't - threads are so lightweight that you might end up with *worse* performance if you use a thread pool. Cheers, Simon

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some algorithm help with jhc
by John Meacham 14 Sep '05

14 Sep '05

I have started working on jhc more recently and have come across some places where I think my algorithms could be improved but was not sure exactly where to start so thought I would ask the list since perhaps someone here has some insight. After a long time of trying various methods of speeding up the fixpoint iteration of my points-to analysis (the current main bottleneck) I decided to step back and look at the basic problem again. It turns out I can express the problem as one of constraint satisfaction resulting in much smaller code (600 lines vs 2000) and 10fold speedups with my unoptimized first draft solver. It is much faster but still not as fast as I'd like. I don't know a lot about constraint problems, but my intuiton says this particular problem is of a type that should be particularly easy to solve but am uncertain where to start in my searching to find a fast algorithm. My constraints come in two types of rules. the rules are of the form (where a and b are variables to be solved for and x and y are values in my domain) * a >= f(b) where x >= y implies f(x) >= f(y) * enforce g(a) as a new set of rules where if x >= y then g(x) subsumes g(y) f and g are typed thusly. f :: domain -> domain g :: domain -> set of rules the following useful derived rules can be expressed by the above. a >= b a == b if z(a) then b >= c now, the reason my intuiton says there should be a fast solution is that there is no way for the variables to decrease. every added rule can only add to the least solution and not shrink any set. so, my basic question is, is this a known form of constraint satisfaction problem? does it admit a particulary fast solution as my intuition tells me? does it have a name I can search for on scholar.google.com? my current first draft implementation represents each rule as an IO action taking a difference set that propegates said difference set and then adds it to the current set for each var. The second problem I am facing is one of debugging. my code dealing with jhc core and all the optimizations that are performed on it has gone through a lot of evolution. over time, bugs have been introduced that are very hard to track down, the symptom would be suddenly having core that doesn't typecheck or has an unknown identifier or worse a segfault in the generated program. backtracking to find the error is quite tedious, mainly involving commenting out transformations until I find the offending one, then rederiving the correct code and comparing it to what I have written. I have decided to remedy the situation and start using QuickCheck to verify all my transformations are meaning and type preserving. however, the problem arrises in how to come up with a meaningful instance of Arbitrary for expressions in jhc core. It is not clear at all how to come up with code that generates random yet "interesting", well typed, convergent, terms in the extended lambda calculus with things like recursive definitons and primitives. even if I solved that, the problem of deciding whether two expressions (which might be functions) do the 'same thing' is undecidable, so how do I even test if the transformations are meaning preserving? Ideally, someone would have written a paper on this. I have seen several papers on generating suitable random graphs for testing graph algorithms, but have not come across one on creating typed lambda calculus terms. perhaps someone else has come across this same problem and has some insights? I am interested in ideas, brainstorming and pointers to papers or terms to search for as much as ready made solutions. In any case, I think they are interesting problems to begin with... hopefully someone out there thinks so too :) John -- John Meacham - ⑆repetae.net⑆john⑈

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RE: [Haskell-cafe] Haskell poker server
by Simon Marlow 13 Sep '05

13 Sep '05

On 28 August 2005 20:00, Joel Reymont wrote: > I get a message from Erlang once data arrives over TCP and the > message is a {tcp, Socket, Bin} tuple where Bin is binary data. I can > easily extract what I need using Erlang binary pattern matching: > > read(<<24, GID:32, Seq:16>>) -> > {24, GID, Seq}. I didn't see anyone mention this (apologies if I missed it), but the House project does parsing of network packets and has some useful combinators for parsing binary data: http://www.cse.ogi.edu/~hallgren/House/ Cheers, Simon

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RE: [Haskell] mailing list headaches
by Simon Marlow 13 Sep '05

13 Sep '05

On 08 September 2005 17:53, Glynn Clements wrote: > Frederik Eaton wrote: > >> However, threading by "References", which RFC 2822 says >> SHOULD be possible, and which works on my other folders, doesn't work >> well on Haskell mailing lists. Presumably the issue is that there are >> a large number of Windows users with strange mail clients which don't >> insert "References" headers. > > It isn't so much that there are a large number of such users, but that > two of the core developers are among them (and are both employed by > Microsoft, so RFC-conformance probably isn't an option). Yes, this is partially our fault. When using Outlook through Exchange, the In-Reply-To header isn't generated at all. Outlook via SMTP works fine, though. I reported the bug several years ago, let's see if Office 12 fixes it :) Cheers, Simon

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Language Workbenches - the Haskell solution?
by Yoel Jacobsen 13 Sep '05

13 Sep '05

It seems that Martin Fowler's article "Language Workbenches: The killer-App for Domain Specific Languages?" - http://www.martinfowler.com/articles/languageWorkbench.html - has generated some nice dynamic solution where a configuration file is written in the same language as the program. Notable examples are lisp - http://lispm.dyndns.org/news?ID=NEWS-2005-07-08-1 and python - http://billionairebusinessman.blogspot.com/2005/09/drop-that-schema-and-put… I'm trying to create an _elegant_ solution in Haskell. But I'm stuck. Since the native record-like access in Haskell syntax is using labelled fields in datatype decleration but the later are strictly compile-time. Therefore, if I compile my program and add a field in the configuration file (written in Haskell, using, for instance hs-plugins), I'll need to recompile the data declaration as well. Further, what is the type of the parser? Consider the following implementation: source = "#123456789012345678901234567890123456789012345678901234567890\n\ \SVCLFOWLER 10101MS0120050313.........................\n\ \SVCLHOHPE 10201DX0320050315........................\n\ \SVCLTWO x10301MRP220050329..............................\n\ \USGE10301TWO x50214..7050329..............................." data Configuration = Config String [(String, Int, Int)] config = [ Config "SVCL" [("CustomerName", 4, 18), ("CustomerID", 19, 23), ("CallTypeCode", 24, 27), ("DateOfCallString", 28, 35)], Config "USGE" [("CustomerID", 4, 8), ("CustomerName", 9, 22), ("Cycle", 30, 30), ("ReadDAte", 31, 36)]] -- parse takes the configuration, a line from the source string and generate a record parse :: Configuration -> String -> Record What is the type of Record? Anyway, any elegant solution or a hint towards one are most welcome. Thanks, Yoel

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Template Haskell and Types
by Gracjan Polak 13 Sep '05

13 Sep '05

Hi, Probably very simple question about template haskell: How do I make a type for an argument to splice? Example: data MyData = MyData1 | MyData2 mysplice mytype = [| litE $ stringL $ show mytype |] main = do putStrLn $(mysplice MyData) The above is not accepted, error: Compiling Main ( thtest.hs, thtest.o ) thtest.hs:51:34: Not in scope: data constructor `MyData' So how do I provide type as an argument? Besides: documentation that I found for th is very dated. Could somebody point me to something more up to date about th? Thanks! -- Gracjan

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RE: [Haskell-cafe] Language Workbenches - the Haskell solution?
by Ralf Lammel 13 Sep '05

13 Sep '05

Hi Yoel, This looks like fun. Just some general comments: - One should try to *also* provide a simple solution. Here I would pick a solution that doesn't need a lot of explanation (of Haskell and the solution) and a lot of defense to score with it. - As a second step, one should identify a sweet spot upfront. The sweet spot could indeed be to get some extra static typing; some extra DSL checks; some increased declarativeness. (Or all of it.) To this end, I would study the DSL problem at hand, and not so much yet think of Haskell encoding. You can only score with Haskell, if there is something to be scored with in the problem at hand. - Now one can explore the identified sweet spot by trying to put strong Haskell features such as hs-plugins or Data.Generics or Data.Dynamic or phantoms or all of them or something else to work. - I haven't looked at the discussion, but a sophisticated Java solution could also try to score with dynamic class loading, reflection, perhaps even with compile-time reflection a la OpenJava. One would need to be prepared to cope with that ;-) - XML wasn't mentioned in your message. I wonder whether they discuss it in the actual thread. (Yes, you might be saying XML is not the same language as you are programming in, but it so easy to process XML in many languages, and one uses XSD for the DSL syntax). In fact, in Haskell I would strongly consider using HaXML or similar technology for non-trivial configuration problems. XML makes configuration also more portable. In reality, I don't see much value in using the programming language syntax for representing the configuration information. This makes it only harder to process those configurations with other tools. Ralf > -----Original Message----- > From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe- > bounces(a)haskell.org] On Behalf Of Yoel Jacobsen > Sent: Thursday, September 08, 2005 11:41 AM > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] Language Workbenches - the Haskell solution? > > I'm trying to create an _elegant_ solution in Haskell. > ... > Further, what is the type of the parser?

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Where to start about writing web/HTTP apps ?
by gary ng 12 Sep '05

12 Sep '05

Hi, I just start learning haskell and have to say that it is stunning in how precise it can be(coming from a background of C then python/perl/js). I want to write apps for WEB and have briefly read WASH. However, that seems to be a CGI based solution. What I want is a native HTTP server(written in haskell), like Twisted/Cherrypy in Python. Are there any boilerplate(I know I should scrap the boilerplate but I need to have something to get start) for reference ? In addition, during my learning process, I keep on using my old experience as reference such as writing simple programs that needs functions like ltrim/rtrim/substr etc. that is in almost any language I have used. But it seems that haskell doesn't have it. I know that a haskell expert can write them in no time with things like dropWhile and reverse, it is a bit frustrating for new comers from a imperative background. Oh, while I am still here, I am reading "The Evolution of a Haskell Programmer" http://www.willamette.edu/~fruehr/haskell/evolution.html and learning the various way to tackle the same problem. Obviously, there are lots of things I don't know what it is about and I would tackle them as time go by. But I have problem even with the seems to be simple one like this : fac n = foldr (\x g n -> g (x*n)) id [1..n] 1 I can understand a foldl or foldr version but have problem with this, especially the "id" and the trailing "1" and the function being folded takes 3 parameters instead of 2(as in standard foldr/foldl solution). Would appreciate if someone can knock on my head and tell me what is going on in it. thanks for help in advance. regards, gary ______________________________________________________ Click here to donate to the Hurricane Katrina relief effort. http://store.yahoo.com/redcross-donate3/

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