- Haskell-Cafe - Haskell.org

Constructor constraints
by Rich Neswold 21 Sep '05

21 Sep '05

Hello, I've looked through the two tutorials and the Report, but couldn't find help on this topic. My question is whether you can place constraints on new data types. For instance, I want to make a new type that is a 4 element tuple where each element is greater than or equal to the previous entry. Is this possible? i.e. data Category = Membership a a a a I'd like to be able to prevent invalid Category data from being created. Any information would be appreciated. -- Rich AIM : rnezzy ICQ : 174908475

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case of (was: [Haskell] Mixing monadic and non-monadic functions)
by Andrew Pimlott 21 Sep '05

21 Sep '05

On Sat, Sep 17, 2005 at 06:56:36PM +0100, Ben Rudiak-Gould wrote: > * The new syntax is really nice as a replacement for the annoyingly > common "x <- foo ; case x of..." idiom that I've always disliked. I might wish for "case of" to mean "\x -> case x of": foo >>= case of ... Useful outside of monads, eg to write anonymous functions for map. Andrew

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Trapped by the Monads
by Mark Carter 20 Sep '05

20 Sep '05

I'm puzzling out how to get a Bool from am IO Bool. I know I'm not supposed to, but I don't see any way around my predicament. The basic setup is: I have an edit box, and a panel. If you click the LMB on the panel when the edit box is checked, this means you want to move a graphical object around the panel. If it is unchecked, then clicking the LMB means you want to add a graphical object. The relevant bits I've managed to put together so far are: mainFrame = do -- main application frame streams <- varCreate [] ... cbEdit <- checkBox p1 [text := "Edit Mode", on command ::= onCbEdit textlog] -- p1 is the panel, ignore textlog let isEditing = get cbEdit checked -- returns type IO Bool windowOnMouse p False {- no motion events -} (onMouse p streams isEditing) ... where onMouse w streams isEditChecked mouse = case mouse of MouseLeftDown pt mods -> if isEditChecked then findStream w streams pt else addStream w streams pt other -> skipCurrentEvent -- unprocessed event: send up the window chain where -- define findStream and addStream The problem is that isEditChecked is of type IO Bool, not Bool. I presume that I should actually be taking a different (non-imperative) approach, and I'm wondering if anyone could suggest what that approach should be? Many apologies for being a clueless n00b. ___________________________________________________________ Yahoo! Messenger - NEW crystal clear PC to PC calling worldwide with voicemail http://uk.messenger.yahoo.com

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How to locate a <<loop>>?
by Jørgen Hermanrud Fjeld 20 Sep '05

20 Sep '05

Hi. I have a generated Haskell program that emits a "Fail: <<loop>>" at runtime. Are there some tools or standard manner for identifying the lines that participate in the loop? The use of "-debug" and "-prof -auto-all" only provides a call stack, and the loop is, is I understand it, due to a circular definition, so the call stack is of little help. -- Sincerely | Homepage: Jørgen | http://www.hex.no/jhf | Public GPG key: | http://www.hex.no/jhf/key.txt

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Re: [Haskell-cafe] Use Haskell to extract GXL representation
by Santoemma Enrico 20 Sep '05

20 Sep '05

Sara, I've used extensively HaXml and the tool DtdToHaskell to do xml processing. HaXml marshalls/demarshalls xml data and DtdToHaskell creates the "data" statements to handle xml in haskell fashion. I learnt how to write a palatable DTD for the delicate DtdToHaskell, but in your case I guess you're not the dtd's author. It's very simple, once everything is setup: import MYDTD myXml :: String -> IO MYDTD myXml fileName = return $ fReadXml fileName where MYDTD.hs is the output of: $ DtdToHaskell MYDTD.dtd MYDTD.hs Feel free to ask if you'll have troubles with DtdToHaskell. I remember I fought a lot :) Salus, Enrico > -----Messaggio originale----- > Da: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org]Per conto di Sara Kenedy > Inviato: sabato 17 settembre 2005 3.11 > A: Haskell-Cafe(a)haskell.org > Oggetto: [Haskell-cafe] Use Haskell to extract GXL representation > > > Dear you all, > > > Currently, I am working with Haskell and GXL file (Graph > eXchange Language). GXL > is a sublanguage of XML and its syntax is based on XML DTD. > > 1) In my work, I use GXL representation to represent a quantification > (e.g. forall(x:Z|x = 3 and x^2 - 3x + 2 =0)) > > 2) My objective is to write a Haskell module to extract the content of > the GXL file such that a prover theorem (e.g, ICS) and a computer > algebra system (e.g, Matlab) can read the content of GXL file. > > If any of you have experiences or any ideas about the problem, please > share with me. > > I really appreciate for that.Thanks a lot. > > Sara. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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a new Monad
by Malcolm Wallace 20 Sep '05

20 Sep '05

Microsoft has announced the following: Developers can also expect a new scripting language for management applications, called Monad. Monad is an object-oriented language based on .NET, and provides command-line based management while enabling management services to be passed between different commands. http://www.theregister.co.uk/2005/09/16/microsoft_longhorn_server/ What a strange choice of name for a language...! :-) Malcolm

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RE: [Haskell-cafe] Template Haskell and Types
by Simon Peyton-Jones 20 Sep '05

20 Sep '05

| > putStrLn $(mysplice ''MyData) | > | | Thanks for responses. Is there any up-to-date documentation avaliable? Template Haskell is, alas, poorly documented. I would really welcome someone to volunteer to help write better documentation. Meanwhile, as the user manual says, the stuff about quoting names is described in a design note http://research.microsoft.com/~simonpj/tmp/notes2.ps Simon

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TH Q Monad and fail
by Gracjan Polak 20 Sep '05

20 Sep '05

Hi all, The Q Monad in template haskell has fail method. As I understand it, it throws some kind of exception. How do I catch this exception? Some code I'm trying to create: infoToCode :: Info -> Q Exp infoToCode (ClassI dec) = -- ClassI Dec fail "ClassI not supported" -- this will be implemented infoToCode (ClassOpI name xtype name2 fixity) = -- ClassOpI Name Type Name Fixity fail "ClassOpI not supported" infoToCode (TyConI dec) = -- TyConI Dec fail "TyConI not supported" infoToCode (PrimTyConI name int bool) = -- PrimTyConI Name Int Bool fail "PrimTyConI not supported" infoToCode (DataConI name xtype name2 fixity) = -- DataConI Name Type Name Fixity fail "DataConI not supported" infoToCode (VarI name xtype maybedec fixity) = -- VarI Name Type (Maybe Dec) Fixity fail "TVarI not supported" infoToCode (TyVarI name xtype) = -- TyVarI Name Type fail "TyVarI not supported" nameToCode1 :: Name -> Q Exp nameToCode1 name = do info <- reify name Code <- infoToCode info runIO $ putStrLn $ pprint Code return Code -- Here be dragons... nameToCode name = nameToCode1 name `catch` c where c e = do runIO $ putStrLn $ show e fail "refailed" -- Gracjan

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Dread __DISCARD__
by Steven Elkins 20 Sep '05

20 Sep '05

Hello everyone, I'm a Haskell newbie trying out various programs from the web. I'm trying to compile one called bjpop-ray (from Bernie Pope, I think) and I hit this at link-time: ghc --make -O0 -package wx Main.hs -o bjpop-ray Chasing modules from: Main.hs Skipping Data ( ./Data.hs, ./Data.o ) Skipping Sphere ( ./Sphere.hs, ./Sphere.o ) Skipping Parser ( ./Parser.hs, ./Parser.o ) Skipping Vector ( ./Vector.hs, ./Vector.o ) Skipping Polygon ( ./Polygon.hs, ./Polygon.o ) Skipping RayTrace ( ./RayTrace.hs, ./RayTrace.o ) Skipping Main ( Main.hs, Main.o ) Linking ... /usr/bin/ld: Undefined symbols: ___DISCARD__ collect2: ld returned 1 exit status I'm on Mac OS 10.4.2, using ghc 6.4 (from the haskell.org .dmg) and gcc 4.0.0. Other wxHaskell programs (the samples and my own experiments) compile without tripping over this. Googling __DISCARD__ led me to "...the binary on the ghc web site has the evil __DISCARD__ mangler problem..." on the darwinports bug mailing list, so I guess I'm not the first person to see this. Is there a workaround? Does the Darwin port have the same problem? Should I just work on the many other things I've yet to grasp? Usually ghc works fine. Thanks, Steve -- How wonderful it is that nobody need wait a single moment before starting to improve the world. -- Anne Frank Paradise is exactly where you are right now...only much, much better. -- Laurie Anderson

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RE: [Haskell-cafe] How to debug GHC
by Simon Marlow 20 Sep '05

20 Sep '05

On 10 September 2005 21:15, Frederik Eaton wrote: > On Fri, Sep 02, 2005 at 04:40:05PM +0400, Bulat Ziganshin wrote: >> Hello Nils, >> >> Friday, September 02, 2005, 10:47:05 AM, you wrote: >> >>> Compile your program with -prof -auto-all (make sure you have >>> the >> >>> I tried this out under GHC 6.4/Linux and got a segmentation fault >>> instead of a stack trace. Under GHC 6.2.2 it seemed to work, though. >> >> this error is already fixed in current pre-6.4.1 version > > I'm using a 2005/9/3 version of 6.4.1 and running into situations > where the "stack trace" has function A calling function B, where when > I look at the code, A never calls B. Is this normal? Is it some > side-effect of laziness? It sure makes the traces a lot less useful. It could be a bug - can you reduce the example and report it? GHC's profiler tries to overlay a lexical call graph on to the dynamic execution of the program. It does this more or less in the way you described before: every function gets an extra argument describing the call context. However, there are some tricky areas: notably CAFs. We don't as yet have a principled description of the mechanism, and I know of various cases where odd results are obtained with the current system. Also, the optimiser has to be careful not to change the shape of the call graph, and I suspect there are cases where it goes wrong. Cheers, Simon

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