- Haskell-Cafe - Haskell.org

Haskell and concurrency
by Graham Klyne 25 Jun '04

25 Jun '04

[Reminiscing mode] Recent discussions about Haskell concurrency remind me of a design I attempted some years ago for a real-time web content analyzer proxy (the sort of thing that companies would deploy to prevent their employees from downloading viruses from the web, etc.). The design was based on my (imperfect) understanding of lazy functional languages and their implementation (with some help from, SPJ's book), in particular using a graph-reduction model to evaluate a function like isSafe :: [octet] -> Bool The intent was that there could be an arbitrary and potentially very large number of concurrent web accesses passing through the proxy, so a thread-per-access could be unacceptably expensive, especially when each thread would most of the time be waiting for data. The advantage that I saw in the graph-reduction approach was that the current state of each evaluation would be entirely in the current expression graph, and there would be no per-transaction overhead other than the essential state information. The graph-reduction engine itself would have a number of worker threads, each of which would perform any graph reductions for which available incoming data was available -- a kind of demand-and-availability driven evaluation model. (The design was too ambitious to be implemented in the actual product, though it did spawn a company joke about Schonfinkel's device, and I understand that a number of ideas did eventually find their way into the implementation over time, but that's another story.) Anyway, I was wondering if any of the work on concurrent Haskell or concurrent functional programming addresses similar systems or ideas, in which implementation/hardware concurrency is decoupled from application concurrency requirements, and particularly with a view to applications requiring very low-overhead fine-grained concurrency? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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how to get started: a text application
by Max Ischenko 25 Jun '04

25 Jun '04

Hi all, I'm going to try to implement a version of Markdown tool[1] in Haskell. The application is rather simple: take a text file with some (simple) mark-up embedded in it and turn it into another text file, this time with XHTML markup. I need some guidelines on how to get started. I'll have to struggle with both the language itself (as I am a newcomer) plus with libraries that I may need. If I to do this in some convenient language, like Python, I'd use a lexer to parse the input, a state machine to build some internal representation and some serializer to write this into XHTML. Which approaches could you suggest for that kind of application in Haskell? I'd especially glad to hear about some "idiomatic" or "native" ways to solve the problem. I've made a quick search and found tools like Parsec, HaXML, Happy and WASH/HTML. Not sure which ones I'll need. [1] - http://daringfireball.net/projects/markdown/ TIA, max.

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RE: [Haskell-cafe] readEitherChan
by Simon Marlow 24 Jun '04

24 Jun '04

On 19 June 2004 03:06, Thomas Hallgren wrote: > Tomasz Zielonka wrote: > >> On Tue, Jun 15, 2004 at 06:29:59PM -0400, S. Alexander Jacobson >> wrote: >> >> >>> readEitherChan::Chan a -> Chan b -> Either a b >>> >>> eitherChan a b left right = readEitherChan a b >>= either left >>> right >>> >>> >> ... I think it would be safer to create once new Chan (Either a b), >> and then read from it. >> >> createEitherChan :: Chan a -> Chan b -> IO (Chan (Either a b)) >> >> > Although createEitherChan might be the best solution for some > application, it would be interesting to know what the recommended way > to implement the function eitherChan above is, if that is what you > really need? > > The eitherChan function corresponds to guarded nondeterministic > choice, usually written (a?P)+(b?Q) process calculi, i.e., a rather > fundamental concurrency operator. > > Section 5 of [1] enumerates some reasons why guarded nondeterministic > choice is not available as a primitive in Concurrent Haskell. Section > 5.2 describes a function select for nondeterministic choice, but I > don't see how to use that to implement readEitherChan (or > takeEitherMVar)... It's a hard problem. Trying to code up takeEitherMVar by hand always seems to lead to solutions with race conditions - I can't see a way to do it. Perhaps takeEitherMVar should be provided as a primitive, but I'm concerned that the same problems will crop up if we tried to implement takeEitherMVar in a multi-threaded runtime (GHC doesn't currently have a true multi-threaded runtime, but we'd like to do it someday). So I think in order to do this you have to build another abstraction around MVars. I've attached some code I just hacked up to do just that; it's untested (but compiles), and has a couple of known deficiencies, but it might be along the right lines. Cheers, Simon

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 24 Jun '04

24 Jun '04

What you are suggesting - whilst it seems reasonable requires a fundamental change to the type system. At the moment [] signifies an empty list of type 'a' ... what you are suggesting requires [] to have a different type from [a]... This means a simple case statement : case a of (a:as) -> [] -> would now not type because everything on the LHS of the cases must be the same type. Infact I don't see how it could be any other way... 'a' has a type call it [a]... how could [] be any other type? Keean.

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RE: atomic MVar overwrite (was RE: [Haskell-cafe] How to use QSem?)
by Simon Marlow 24 Jun '04

24 Jun '04

On 23 June 2004 17:46, S. Alexander Jacobson wrote: > Basically, what I want is a tryReadSampleVar > function. > > Or, if we stick with MVars, I don't want another > thread to operate between the tryTakeMVar and the > putMVar. In particular, I don't want another > thread to believe (mistakenly) that the mvar is > empty when it is simply being updated. > > I suppose I could define a new abstraction where I > have a second mvar that acts as an exclusive lock > on the the first MVar. e.g. > > data XMVar a = XMVar (MVar ()) MVar a > > overWriteXMVar (XMVar lock mvar) val = > withMVar lock (\_->tryTakeMVar mvar >> putMVar mvar val) > tryTakeXMVar (XMVar lock mvar) val = > withMVar lock (\_->tryTakeMVar mvar) > > But, I think it would be more sane simply to > have a tryReadSampleVar somewhere. If you send me code for tryReadSampleVar, I'll incorporate it. Unfortunately SampleVar is abstract, so you'll need to take the source for that library (libraries/base/Control/Concurrent/SampleVar.hs in a GHC distribution) and modify it to create your own version. Cheers, Simon

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Polymorphic algebraic type constructors
by Graham Klyne 23 Jun '04

23 Jun '04

If I have a polymorphic algebraic type (T a) with several type constructors, only one of which actually references the type parameter, is there any way to express type conversion for the type-parameter-independent constructors without actually mentioning all the constructors? Here's a simple example based on Either: [[ data A = A String deriving (Show, Eq) data B = B String deriving (Show, Eq) f :: (a->b) -> Either String a -> Either String b f g (Right a) = (Right $ g a) f g (Left s) = (Left s) -- f g (s) = (s) -- doesn't work a2b (A s) = (B s) t1 = f a2b (Left "x") t2 = f a2b (Right (A "y")) ]] The second case for 'f' throws a type error when the constructor 'Left' is omitted, because the type of 's' is fixed to be Either String A when the required result (in this case, because of a2b) is Either String B. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 23 Jun '04

23 Jun '04

Adrian Hey wrote: I think all occurences of [] have type forall a. [a] The case expression.. case a of (a':as) -> <expr1> [] -> <expr2> Should be typed as if written.. case a of (a':as) -> let a=(a':as) in <expr1> [] -> let a=[] in <expr2> ---------------------------------------------------- Well, we will have to agree to disagree, I find the current system makes perfect sense, and is easy to reason with. If I want a let binding I can write one. The difference is really this: case a of (a:as) -> <expr1> e -> e In this case e represent 'the same []' as on the left hand side. in the let binding this is a new and fresh '[]'... When I pattern match a variable in a case extression I really expect it to get the type of the case... There are circunstances when I go on to use e, and if it is not grounded by the case expression I will get hit by the monomorphoism restriction, and have to add non haskell98 type annotation to ground 'e'... for example If I pass e to a polymorphic function, it won't know what type e has... case a of (a':as) -> let a=(a':as) in <expr1> [] -> let a=[] in (polyfn a) where polyfn a has a type like polyfna :: Ord a -> [a] -> String Now the above won't work and you will have to do: [] -> let a=[] in (polyfn (a :: [SomeListType])) and of course if you are using generics then how can polyfn get the original type of 'a' in the case expression... for example: polyfn :: Typable a -> [a] -> ShowS polyfn a = shows (typeOf a) then: case a of (a:as) -> <expr1> e -> polyfn e is correct, but: case a of (a':as) -> let a=(a':as) in <expr1> [] -> let a=[] in (polyfn a) does what? Keean.

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RE: atomic MVar overwrite (was RE: [Haskell-cafe] How to use QSem?)
by Simon Marlow 23 Jun '04

23 Jun '04

On 23 June 2004 17:13, S. Alexander Jacobson wrote: > Ah, that worked. Thank you. The MVar > documentation is also very brief. > > I would also like to overwrite the contents of an > MVar atomically regardless of whether it is full > or empty. I am current using. > > overWrite mvar val = tryTakeMVar mvar >> putMVar mvar val > > But it is not atomic :-( The lib functions > (modifyMVar and tryPutMVar) are atomic but seem to > require that you know in advance the MVar's state. The question to ask here is "atomic with respect to what?". modifyMVar is atomic only with respect to other threads performing modifyMVar-type operations on the MVar (i.e. take followed by put). If another thread is doing repeated puts into the MVar, then modifyMVar's atomicity is defeated. There isn't really a problem here, you just need to be sure that you use your MVars in a consistent way - you can always define a new abstraction that doesn't permit operations to be performed in the wrong order. So in order to tell you how to write your overWrite function, we need to know what other operations are being concurrently performed on the MVar, and what invariants you expect to hold. Cheers, Simon

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RE: [Haskell-cafe] How to use QSem?
by Simon Marlow 23 Jun '04

23 Jun '04

On 22 June 2004 21:44, S. Alexander Jacobson wrote: > The GHC documentation on QSem is very sparse. I > would like to give a thread exclusive access to a > resource. > > My *guess* based on the documentation is that I > can create an exclusive lock using: > > logSem <- newQSem 1 If the maximum value of your QSem is 1, then an MVar will do. That is, an MVar can be used as a simple semaphore, and a QSem is necessary if you need a quantity semaphore (there are > 1 units of the resource). > And then any thread that wants to lock the > resource uses: > > withLogSem x = do waitQSem logSem; y <- x; signalQSem logSem; > return y > > as follows: > > withLogSem $ rotate curLogPos import Control.Concurrent, create the semaphore like this: m <- newMVar () and use it like this: withMVar m $ \_ -> ... critical section ... If you are using the semaphore to protect a mutable variable, then you can handily merge the two ideas into an MVar, which behaves like a thread-safe mutable variable. Cheers, Simon

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online judge with Haskell
by Tomek Noiński 23 Jun '04

23 Jun '04

I have read a post to this list from about a year ago, considering Haskell support on online judges, like acm.uva.es. I think our online judge (a system for the verification of the correctness of submitted programs, which solve problems selected from a repository) might interest you. The Sphere Online Judge (SPOJ) is available at http://spoj.sphere.pl, and supports solutions written in 20 different programming languages including Haskell. We would like to invite you to solve problems at our server. At present, nearly half of the rapidly expanding problemset (currently more than 60 tasks) can be solved using functional languages easily within the time-limit. It is possible to add tasks dedicated to functional programming and the development team will be glad to cooperate if you are willing to create some new interesting problems. With best wishes, Adrian Kosowski, Michal Malafiejski, Tomasz Noiński, Gdansk University of Technology, Poland

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 23 Jun '04

23 Jun '04

>f (i:is) = even i : f is >f e = e This still makes perfect sense to me... you see the let binding: let e@[] = e has no additional information about the type of [] so it must be fully polymorphic. if the function binding we know typeOf e == typeOf (i:is) and that i is a member of the class Integral (from the application of even) As we always take the most precise available type we end up with forall a. Integral a => [a] as the type for 'e'... Now Bool is not a member of Integral so it is a type error! Keean.

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RE: [Haskell-cafe] Ord (IORef a)?
by Simon Marlow 23 Jun '04

23 Jun '04

On 23 June 2004 03:53, John Meacham wrote: > Yeah, i forgot about the GC moving things around. actually, if the GC > could be coaxed to move things around, but never change the relative > order of IORefs (like a compacting collector), then the address could > still be used for Ord and Eq. This is a rather restrictive global invariant, I don't think it's practical. > how is Eq implemented such that the GC doesn't move one side after you > read the address of the other? The sameMutVar# primitive is atomic with respect to the GC. It ought to be possible to implement Ord on IORef using StablePtr, but for some reason StablePtr isn't an instance of Ord. Cheers, Simon

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RE: [Haskell-cafe] Ord (IORef a)?
by Simon Marlow 23 Jun '04

23 Jun '04

On 22 June 2004 10:38, Simon Peyton-Jones wrote: >> From: John Meacham >> would it be possible to provide an Ord instance for (IORef a)? For >> things like loop detection, one may need to make many IORef >> comparasions and being able to use an efficient set would be a >> really big win. >> >> Since IORefs are created in the IO monad, the actual order can be >> arbitrary without breaking referential transparency (as long as it >> doesn't change over the lifetime of the IORef.) > > I think that'd be entirely reasonable. Two runs of the same program > might give observably different behaviour wrt Ord, but that's ok > because the refs are allocates in the monad, as you say. > > There is an efficiency cost though. Each IORef would need to have an > extra field, to record its allocation ID. (Address is not enough -- > the garbage collector can mangle them.) > > My own view is that this is fine -- IORefs shouldn't be in your inner > loop, so an extra word in each is no big deal. Unless there are > views to the contrary, I'd be happy to see this in GHC. I'm torn. If it were free, it would be a no-brainer. But I know of several cases where IORefs are performance-critical (or at least performance-important). We should implement & measure. Cheers, Simon

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RE: [Haskell-cafe] Ord (IORef a)?
by Simon Peyton-Jones 23 Jun '04

23 Jun '04

| From: John Meacham | would it be possible to provide an Ord instance for (IORef a)? For | things like loop detection, one may need to make many IORef comparasions | and being able to use an efficient set would be a really big win. | | Since IORefs are created in the IO monad, the actual order can be | arbitrary without breaking referential transparency (as long as it | doesn't change over the lifetime of the IORef.) I think that'd be entirely reasonable. Two runs of the same program might give observably different behaviour wrt Ord, but that's ok because the refs are allocates in the monad, as you say. There is an efficiency cost though. Each IORef would need to have an extra field, to record its allocation ID. (Address is not enough -- the garbage collector can mangle them.) My own view is that this is fine -- IORefs shouldn't be in your inner loop, so an extra word in each is no big deal. Unless there are views to the contrary, I'd be happy to see this in GHC. Simon

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 22 Jun '04

22 Jun '04

>to be [] in each case. ahh but in this example: f :: [Int] -> [Bool] f (i:is) = even i : f is f e@[] = e e is an empty list of Ints not an empty list of Bools! you mean: f :: [Int] -> [Bool] f (i:is) = even i : f is f _ = [] Keean.

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How to use QSem?
by S. Alexander Jacobson 22 Jun '04

22 Jun '04

The GHC documentation on QSem is very sparse. I would like to give a thread exclusive access to a resource. My *guess* based on the documentation is that I can create an exclusive lock using: logSem <- newQSem 1 And then any thread that wants to lock the resource uses: withLogSem x = do waitQSem logSem; y <- x; signalQSem logSem; return y as follows: withLogSem $ rotate curLogPos Am I misunderstanding QSem? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 22 Jun '04

22 Jun '04

>(I still think it's a bug though:-) It is definitely not a bug... you cannot assert that the types: Either String a Either String b are both equal ant not equal at the same time. You either mean: f :: (a->a) -> Either String a -> Either String a Or you mean f :: (a->b) -> Either String a -> Either String b actually the second form a and b can be the same but they cannot be unified inside the closure. One way of doing this is to group all the string types on one side: so instead of: data A a = A a | B String | C String | D String you do: data A a = A a | B Strings data Strings = S1 String | S2 String | S3 String you can then write: f :: (a->b) -> A a -> A b f g (A a) = (A $ g a) f g (B a) = (B a) and use (S1 . B), (S2 . B), (S3 . B) to access each string. Keean.

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 22 Jun '04

22 Jun '04

>data E a b > = L1 { r :: a } > | L2 { r :: a } > | L3 { r :: a } > | L4 { r :: a } > | R b > deriving Show How is this different from: data E a b = L1 a | L2 a | R b f g (R a) = R (g a) f _ other = other Isn't this more or less what was in the original... The problem is the type: f :: (a->b) -> Either String a -> Either String b this line: f _ other = other says: Either String a == Either String b -- which it doesn't Keean.

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 22 Jun '04

22 Jun '04

>If I have a polymorphic algebraic type (T a) with several type data MyType a = A a | B String | C Int So how do you expect to get the contents of B and C outm when they are differenf Types - surely you mean: data MyType a = A a | B MyStrings Keean.

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Ord (IORef a)?
by John Meacham 22 Jun '04

22 Jun '04

would it be possible to provide an Ord instance for (IORef a)? For things like loop detection, one may need to make many IORef comparasions and being able to use an efficient set would be a really big win. Since IORefs are created in the IO monad, the actual order can be arbitrary without breaking referential transparency (as long as it doesn't change over the lifetime of the IORef.) John -- John Meacham - ⑆repetae.net⑆john⑈

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Using HaXml filters for XML namespaces
by Graham Klyne 21 Jun '04

21 Jun '04

I am (at last!) looking to implement an XML namespace processing filter for HaXml, having completed the various structural changes needed to carry namespace information in the content model. The basic idea is to process a document such that: (a) all xmlns:xx='xxuri' attributes are removed (b) all attribute and element names of the form xx:foo are replaced by a URI xxuri and local name foo, corresponding to a currently in-scope namespace declaration. This raises some questions, which may simply be that I'm overlooking some obvious way of using the CFilter combinator library: (1) is there a combinator for updating attributes, in a fashion similar to the way 'chip' updates elements? (I wrote my own "atip" for general entity substitution in attributes, but it seems that there should be a way to do this in the library.) (2) it appears that I need the XML processing filter to carry a symbol table of in-scope namespaces, which will be updated through the traversal as new xmlnx:xx declarations come into scope. I cannot see any way to do this with the existing combinators; I think this would need a form of recursive combinator, along the lines of 'foldXml' that uses a newly computed CFilter function for each successive recursion. I imagine something like: mangleXml f mkf c = (f' `o` (chip (mangleXml f` mkf))) c where f' = mkf f c (where I would use mkf to scan the attributes and create an augmented version of f that takes into account new xmlns declarations, and f would be a function that replaces names with qnames on an element and its attributes.) Am I missing a trick with the existing combinators? ... (An alternative approach I considered was to use a monadic form of CFilter function which could carry and update the namespace table as the traversal progresses, but that appears to require that (a) many of the existing combinators would have be re-written, and (b) it doesn't cleanly deal with issues of scoping xmlns declarations.) ... #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: (to haskell-cafe) comment on language shootout
by Iavor S. Diatchki 21 Jun '04

21 Jun '04

hello, Greg Morrisett wrote: > Iavor S. Diatchki wrote: > >> what would be cool is to have a place where >> one has many versions of the same program, but each written in the >> "most natural" >> way for the particular language --- a kind of "rosetta stone" for >> programming languages. > > > The ICFP programming contest (and other contests) serve this > purpose pretty well. I've found it very instructive to stare > at solutions in languages with which I'm not familiar. And the > tasks at hand tend to be more realistic (e.g., rendering) > than micro-benchmarks. that's a good point! one thing i don't like about programming contests, is that usually time is very limited, so often one ends up with terrible hacks that kind of work... one might argue though, that "real programmers" are often faced with such situations. -iavor

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Laws in category theory..
by Ron de Bruijn 21 Jun '04

21 Jun '04

I have been thinking about the following laws of a monad. o is composition. # associative law of a monad mu o (mu o T) == mu o (T o mu) The notation: Tmu, that's the same as T o mu, right? How do I relate this to x*(y*z) == (x*y)*z ? # the identity law of a monad mu o (T o eta) =={id}_\mathcal{C}= mu o (eta o T) I also don't understand why I can't create the following law out of the assocaitive law: (mu o T) == (T o mu) , bacause the first part of the expression is the same. Please, enlighten me :) Regards, Ron __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - 100MB free storage! http://promotions.yahoo.com/new_mail

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readEitherChan
by S. Alexander Jacobson 19 Jun '04

19 Jun '04

I'd like a function that blocks until data is available on one of two channels and then returns that data wrapped in an Either. Here is my naive implementation: readEitherChan::Chan a -> Chan b -> Either a b readEitherChan a b = do var <- newEmptyMVar forkIO (readChan a >>= putMVar var . Left) forkIO (readChan b >>= putMVar var . Right) val <- readMVar return val eitherChan a b left right = readEitherChan a b >>= either left right But creating an MVar and starting two threads feels like a lot of overhead for a simple operation. Is there a better way or is forkIO so efficient that I shouldn't care? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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x11 libs
by Matthew Roberts 17 Jun '04

17 Jun '04

I have ghc installed on OSX 10.3 via darwin ports. I have no X11 libs and I can't find anywhere on the web to get them. Can anyone help me out? Cheers, Matt

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Modules and files
by Andrei de A. Formiga 16 Jun '04

16 Jun '04

Hi, Is it possible to have more than one module defined in a single file ? As far as I can tell, the Haskell Report doesn't prohibit this, but so far my tests with hugs and ghc indicate they don't accept multi-module files. Is this standard ? Thanks. --- []s, Andrei de A. Formiga __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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EuroHaskell 2004 pictures
by Shae Matijs Erisson 15 Jun '04

15 Jun '04

The EuroHaskell 2004 Conference was great fun. Shae Erisson gave a Haskell tutorial, Jeremy Bobbio gave a talk about hOp, and John Hughes gave a talk/tutorial about Arrows. Code was written, pizza was purchased, most attendees claimed to enjoy themselves. A social event for programmers may be an oxymoron, but it was nice to get so many Haskellers in the same room. Until you've experienced the range of jokes and puns that can come from a CS Student Pub named "The Monad" you haven't plumbed the depths of FP humor. Anders Carlsson has pictures on his website: http://ridley.csbnet.se/gallery/eurohaskell Some of my favorites are the signs: http://ridley.csbnet.se/gallery/eurohaskell/p1010059 - (this is a bathroom / watercloset / toilet, insert your local term) http://ridley.csbnet.se/gallery/eurohaskell/p1010052 and of course the unicycle pictures: http://ridley.csbnet.se/gallery/eurohaskell/p1010026 http://ridley.csbnet.se/gallery/eurohaskell/p1010047 Tune in next year for EuroHaskell 2005! -- Shae Matijs Erisson - Programmer - http://www.ScannedInAvian.org/ "I will, as we say in rock 'n' roll, run until the wheels come off, because I love what I do." -- David Crosby

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RE: [Haskell-cafe] forkIO preemption
by Simon Marlow 15 Jun '04

15 Jun '04

On 07 June 2004 22:59, S. Alexander Jacobson wrote: > When you forkIO and one of the threads blocks > (e.g. on a socket or a readchan), do execution > automatically resume on other threads and does the > blocked thread consume timeslices while it is > blocked? In GHC: yes, no. > In other words, is there any performance > penalty in setting timeslices very large in a > server app (where all I care about is throughput)? No. But there probably isn't much to gain either. Cheers, Simon

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Type conversion problems
by Christian Hofer 13 Jun '04

13 Jun '04

Hi, after some playing around with monad transformers and starting to like those specialities of Haskell, I am currently really feeling thrown back by a simple problem: trying to write a sine-function... Here is the part of my code that is troubling me: fac :: Integer -> Integer fac n = product [1..n] term :: Double -> Integer -> Double term x n = (-1.0::Double)**(fromInteger n) * (x**(fromInteger (2*n + 1))) / (fromInteger (fac (2*n + 1))) The term function is supposed to be the direct translation of the formula of a term within the sum. But it isn't: it is actually cluttered with lots of type conversions, and I had a hard time figuring out how to make it work at all. I hope that this is not the easiest way to do that and I would appreciate any help on how to handle those type conversion issues... Thanks, Chris

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Calling Haskell from Java
by Andrei de A. Formiga 11 Jun '04

11 Jun '04

Greetings, I need to call Haskell functions from a Java program. In the Java side, I can use JNI, which expects C-call functions exported from dynamically linked libraries. So, a primitive way to get what I want is to generate a dynamic library from Haskell code with exported ccall functions. This brings me to my first question: 1) is ghc able to create dynamic libraries in all platforms ? I searched the documentation and found only mention of a currently unsupported way to create DLLs in Win32, and nothing about other platforms. In case this is not possible, one solution might be to statically link ghc-generated code with a C dynamic library which calls the exported Haskell functions. Going further from the primitive, low-level JNI x FFI way, it would be great to automate the wrapping and marshalling necessary, so: 2) is there any tool to help with Haskell function exports and data marshalling to other languages ? I looked at HDirect but it predates the standard FFI definition and it doesn't seem to be supported. I am aware of tools to help calling Java from Haskell, but none to go the other way around. Thanks in advance for any help. --- []s, Andrei Formiga __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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Re: Modelling Java Interfaces with Existential data types
by Ralf Laemmel 11 Jun '04

11 Jun '04

Hi Mike, Let's redirect to Haskell cafe. http://www.mail-archive.com/glasgow-haskell-users%40haskell.org/msg06288.ht… http://www.mail-archive.com/glasgow-haskell-users%40haskell.org/msg06289.ht… thanks for your time to look into the HList paper. >1. It looks like your HList is basically a "sugarized" version of > >data AnyMyInterface = Impl1 MyImplementation1 | Impl2 MyImplementation2 > >with the same drawback: you require me to explicitly list all the possible >implementations of my interface I don't think that anything was required to be explicit: If you mean this here: type MyList = MyImplementation1 :*: MyImplementation2 :*: HNil ... then note that this type is inferred! So with HLists, there is nothing like your Impl1 and Impl2, and no new datatype like your AnyMyInterface. Just for clarity: (i) I like comparing Haskell datatypes with Java classes. (ii) In Haskell, when you want to say that you have different implementations for the same interface, or when you want to say that different datatypes provide implementations of a certain interface, then you use Haskell's type class system. (iii) When you define an interface in Java, you rather define a type class in Haskell. Type class are of course more powerful, but we don't mind. (iv) When you say "implements" in Java, you rather say "instance" in Haskell. (v) When you define the methods of the interface to be implemented in Java, you define the methods of the instance. I would claim the Haskell code ends up being more modular :-) You immediately get a simple form of Java-like interface polymorphism in Haskell, say when you got a constraint, then you are polymorphic over implementations (instances) of that constraint. The crux of your concern seems to be that you want to package values of different types (Java: classes). Indeed you want to have heterogeneous containers modulo subtyping. And our lovely HLists are just good for that, but they come with the added value that they don't make the elements opaque as it is the case with the sometimes troublesome existentials. > It >might be possible to load them via hs-plugins or to obtain using something >similar to Clean Dynamics (btw, is there anything similar available for >Haskell?). Data.Dynamics Not needed here. >2. There will also be problems in gluing the utility/legacy code. E. g. if I >want to call any list function on HLists (e.g. reverse). The idea of having >heterogeneous FiniteMap can be implemented by storing the single-element >lists, but it will complicate the code, which will work with the map. I concur. Using HList for everything is perhaps too invasive. The HList paper makes clear that there are certain cases where HLists are really needed, and they are complementary what generics provide you with in Java. I was just able to use them for something were a Generic Java programmer uses Generics and subtyping for. So existentials are perhaps Ok. However, directly comparing the opaque values is untypeable, >I'm completely confident with applying (==) to different type. It's quite a >common operation in OO-world. Yes, what you need is type-safe cast. See paper 1 here: http://www.cs.vu.nl/boilerplate/ >I would like to have the following behaviour: > >instance Eq AnyMyInterface where > (==) (AnyMyInterface a1) (AnyMyInterface a2) = > if (typeOf a1) == (typeOf a2) then false else a1 == a2 > Java is just a weakly typed subset of Haskell :-) Here we go ... -- Yet another heterogeneous equality yaHEq :: (Typeable a, Typeable b, Eq a) => a -> b -> Bool yaHEq a b = case cast b of Just a' -> a == a' Nothing -> False *Main> yaHEq True True True *Main> yaHEq True "True" False *Main> As you said yesterday that the equality issue is your main problem, let's wipe this out for existentials as well. I revise your existential wrapper type to include all goodies that are needed, say Eq and Typeable: data AnyMyInterface = forall a. ( Eq a , Typeable a , MyInterface a ) => AnyMyInterface a This is just your list type: type MyList' = [AnyMyInterface] Here are two lists: list4 = [ AnyMyInterface $ MyImplementation1 10 , AnyMyInterface $ MyImplementation1 10 ] list5 = [ AnyMyInterface $ MyImplementation1 10 , AnyMyInterface $ MyImplementation2 10 ] Here is a demo: *Main> list4!!0 == list4!!1 True *Main> list5!!0 == list5!!1 False Cheers, Ralf PS1: All code is here: http://homepages.cwi.nl/~ralf/HList/code.html PS2: I guess Sheard's work on parameterised modules might also interest you. http://portal.acm.org/citation.cfm?id=507648&dl=ACM&coll=portal And John Huges Restricted Data types as it shows how to reify dictionaries:

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Re: Haskell-Cafe Digest, Vol 10, Issue 3
by Ron de Bruijn 11 Jun '04

11 Jun '04

>G'day all. Good day to you too. >I don't know what you mean by "more complex". A dot >is just a dot, and >it has no internal structure that we can get at using >category theory >alone. Some dots may play specific roles in relation >to other dots and >arrows, but no dot is any more complex than any other, >really. Well, intuitvively seen, I meant. In some categories, the dot might stand for a functor, and in some other as a simple object. >For a counter-example, think of the dual category >Set^{op}. A morphism >f : a -> b in that category means that there is a >function f^{op} : b -> a > >where a and b are sets, however f probably isn't a >function at all. Well, what is it then? I see a function as something where you put something in and you get a result. In this case, you already say, there is a morphism b->a, wel than the following of b to a and return a then is a function? A couple of days ago, I thought of the distinction between a function and a morphism, as in that a function operates on a hole set of objects, a.k.a. domain. And a morphism only on one. Is that the distinction you mean? If all of the above is false, then probably I don't know what a function is. (It looks like the more you are busy with things, the less you seem to know of it). >In a category which is a partial order, there is a >morphism f : a -> b >if and only if a <= b. (Or is it a => b? Can never >remember.) Here, >the morphisms really have no internal structure at >all. If the >category >has a finite number of objects, you can represent the >whole thing using >a bit matrix, and each morphism can be identified with >a bit set to >"true". Preserving internal structure is not much more than preservation of composition, right? >I think the problem here is that you have the idea >that a morphism is >a process that turns one object into another. In many >(probably most) >interesting, practically significant cases, that's >true, but it need >not be. Well, I think I don't know what a function is, because the following of an arrow, represents at least for me a clear mapping of turning one object in to another. Or do you mean things like in logic, that you can see the morphism, as relations (instances of axioms) between logic objects. So the morphism a->b would mean that a implicates b. This way, the arrow is not a process of turning an a into a b. Hmm, that seems logical. >I think it really helps to try to understand category >theory mostly as >a language for talking about things, and not >necessarily "things" in >and of themselves. Using this understanding, a >morphism is a noun, not >a verb. It's a concrete thing describing a >relationship between >objects, >not necessarily an action that you perform on objects. I think you have already answered my question this way. But a confirmation would be nice. It seems I had to read your explanation >1 time. >I don't know if any of this helps or not. Well, it certainly helps. Then the multiplication issue: Is the following a good summary? A multiplication is just a name for an operation that is defined or not defined for each mathematical construction in terms of to which laws the operation should comply. The laws are then things like communativity and so on. Regards, Ron __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Re: [Haskell-cafe] Join and it's relation to >>= and return
by Ron de Bruijn 11 Jun '04

11 Jun '04

I am pretty sure, that >>= is to monads what * is to for example natural numbers, but I don't know what the inverse of >>= is. And I can't really find it anywhere on the web(papers, websites, not a single sole does mention it. It should have type, at least that's what I think: inv::M a->M b I say this, because I find this definition of a multiplication operation: 1. There exists a unique special element called neutral such that the operation on any element and the neutral does not change the element. 2. For every element there exists an inverse such that the operation on an element and its inverse is always neutral. 3. The operation is associative: it does not matter how you apply the operation to three elements. You may apply it to the first two and then to the result and the third element. Or you may first apply the operation to the last two and then to the first and the result of the previous operation. An operation may also be commutative 4. The order of two elements in operation is not important. According to 2 there should be an inverse. For join such an inverse is simple: to apply the type constructor on the monad. But I tried to somehow link it with bind, but than the types don't seem to match. So to be concrete: what's the inverse of bind? If I did make some errors, please tell me so. __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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Re: [Haskell-cafe] RE: Modelling Java Interfaces with Existential data types
by MR K P SCHUPKE 10 Jun '04

10 Jun '04

>interested in how to (ab?)use type classes Surely there's no such thing as abuse of a language feature in a language with formal semantics! Keean.

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Re: [Haskell-cafe] Join and it's relation to >>= and return
by Ron de Bruijn 10 Jun '04

10 Jun '04

Hello again, I have thought a while about morphisms and although I had written down in my paper that a functor and also a natural transformation are also morphisms, but in a different category, I now am not sure anymore of this. If you see everything(objects and morphisms) as dots and arrows, and some arrows and some dots are just some more complex than others, then this holds, but is that legal? Intuitively seen, it is. According to some paper, morphisms are not like functions, in that you can apply some morphism to an object, but that you can only compose them. But I would say that if you have the morphism f:a->b, that is a arrow from dot a to dot b. That there clearly is a notion of following that arrow, in effect applying a function. And suppose there is the following path of morphisms: a---->b---->c---->d, with a..d are dots. Then I would say there are three functions(constructed by composition)(in fact more, because of identity mapping) from a that when followed give new objects. This following of arrows, looks a lot like general function application, as in f(x) = 2x for example. It's btw quite hard to write the essence of monads down in a clear and precise way. I hope you can give some feedback on the above. Ragards, Ron P.S. The question about multiplication still stands. Probably multiplication is a set of laws defined on a mathematical object that must hold. And for each mathematical object there is such definition. Is this correct? __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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Re: how to write a list builder? fixpoint?
by Graham Klyne 09 Jun '04

09 Jun '04

[moved to Haskell-cafe] This makes my head hurt! Let me see if I get this right. When the type checker sees: build' as a a2 ... then it invokes the second instance of BuildList (i.e. BuildList a (a->r)) and returns the value: build'(a:as) a2 ... But when it sees just: build' as a (as a value, not used as a function application), then the type checker invokes the first instance and returns: reverse$ a:as So they key here appears to be that the type-checker can distinguish between function application and other uses of an expression? #g -- At 18:18 01/06/04 -0700, oleg(a)pobox.com wrote: > > Is it possible to write a function to build a list [a]? > > so that I can write [a,b,c,d] as "getBuilt $ build a b c d"? > >Yes, in the format very close to desired. > > > {-# OPTIONS -fglasgow-exts #-} > > {-# OPTIONS -fallow-undecidable-instances #-} > > > > module Foo where > > > > class BuildList a r | r-> a where > > build' :: [a] -> a -> r > > > > instance BuildList a [a] where > > build' l x = reverse$ x:l > > > > instance BuildList a r => BuildList a (a->r) where > > build' l x y = build'(x:l) y > >That's it. It works both on GHC and Hugs. > >*Foo> build' [] True :: [Bool] >[True] >*Foo> build' [] True False :: [Bool] >[True,False] >*Foo> build' [] True False False :: [Bool] >[True,False,False] >*Foo> build' [] 'a' 'b' 'c' 'd' 'e' :: [Char] >"abcde" >*Foo> build' [] (1::Int) :: [Int] >[1] >*Foo> build' [] (1::Int) (2::Int) :: [Int] >[1,2] >*Foo> build' [] (1::Int) (2::Int) (3::Int) :: [Int] >[1,2,3] > >Note that the type annotation [Bool] etc. at the end is required: it >is the delimiter of the list. Who would have thought that the type >annotation can play the role of Nil... > >_______________________________________________ >Haskell mailing list >Haskell(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Modelling Java Interfaces with Existential data types
by Tom Pledger 09 Jun '04

09 Jun '04

Hi. Another way is to take advantage of Haskell's laziness: > class Foo a where > foo :: a -> Int > > data Oof1 = Oof1 Int > instance Foo Oof1 where foo (Oof1 i) = i > > data Oof2 = Oof2 Int > instance Foo Oof2 where foo (Oof2 i) = i > > list1 = [foo (Oof1 10), foo (Oof2 20)] That's all it takes. The applications of foo in list1 don't get evaluated until list1 is used, e.g. by sum. Regards, Tom

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forkIO preemption
by S. Alexander Jacobson 07 Jun '04

07 Jun '04

When you forkIO and one of the threads blocks (e.g. on a socket or a readchan), do execution automatically resume on other threads and does the blocked thread consume timeslices while it is blocked? In other words, is there any performance penalty in setting timeslices very large in a server app (where all I care about is throughput)? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Join and it's relation to >>= and return
by Ron de Bruijn 07 Jun '04

07 Jun '04

Hello, The last 3 or 4 days I have been studying a lot of Category Theory so that I would be able to explain the concept of a monad to some people at the university in a "learn to present something"-course. newtype S a = State -> (a,State) -- functor T to map objects mapS::(a-> b) -> (S a -> S b) -- functor T to map morphisms unitS :: a -> S a --\eta joinS::S(S a)-> S a -- \mu This is a complete monad using a direct mapping from Category Theory. I really like it, because it's mathematically grounded. But I don't know how to map this to Haskell monads using the standard "bind" and "return", as I explain below. Although, I do believe I understand most parts of it(as in Functor, Natural Transformation and so on), I don't really understand how the join function (conventional monadic multiplication as it's called in the documentation) relates to the >>= and return functions that are mostly used. I read somewhere at Google that join can be expresed in terms of >>= and return, but I couldn't find it anymore. I also read that the Kleisli triple has more resemblance to how monads are used in practice, but it has some additional constraints. I don't understand why there are these additional constraints and I don't understand it's relevance, because according to some book there's a one-one mapping from monads to Kleisli triples and reverse. I also don't understand why join is a "multiplication", because a multiplication takes two operands (as in 5*4), at least all multiplication operators I know do so. I think what's happening is that one structure is taken out of the other. And then, the two are "merged" in some way. So this way, there are two operands. Is this correct? (I really could use a function definition in Haskell of join in terms of bind and return). Can somebody explain me (with a lot of detail) how this works? Regards, Ron __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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emacs haskell-mode indentation
by Nathan Weston 01 Jun '04

01 Jun '04

I am learning haskell from "The Haskell School of Expression", and finding the indentation in the emacs mode rather unhelpful. When I try to type in the following example from the book: main = runGraphics ( do w <- openWindow "Graphics" (300,300) drawInWindow w (text(100,200) "Hello, world!") k <- getKey w closeWindow w ) emacs wants to indent it like so: main = runGraphics ( do w <- openWindow "Graphics" (300,300) drawInWindow w (text(100,200) "Hello, world!") k <- getKey w closeWindow w ) (For those not using a fixed-width font, in the first example, everything lines up under the 'w <-', in the second, everything lines up under the 'do') If I let emacs have its way, I get a syntax error. Am I doing something wrong here? Is the emacs mode broken? Thanks, Nathan

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new Library Infrastructure spec.
by Isaac Jones 01 Jun '04

01 Jun '04

(followups to libraries(a)haskell.org) Hail Haskellers! A commonly-identified obstacle to the spread of Haskell libraries is that there is no standard way a) for authors to package a library or other tool for distribution to others b) for customers to install such a package Platforms vary. Haskell compilers vary. Packages have dependencies. Etc. Isaac Jones has coordinated an effort to address this problem. We've had a lot of discussion between him and (at least some of) the folk involved in the GHC, Hugs, and nhc implementations. This discussion has led to a new concrete proposed specification[1]. Here it is: http://www.haskell.org/libraryInfrastructure/proposal See also the project home page: http://www.haskell.org/libraryInfrastructure/ We would welcome your feedback, either as a potential library or tool author, or as a potential consumer, or both. The specification isn't complete in every detail, but it seems better to post it before investing in details that may be rendered void by subsequent discussion. We hope this will be an important spec, and will be with us for a long time. So now's the time to look at it. Send feedback to libraries(a)haskell.org. (You can subscribe to this list at: http://www.haskell.org/mailman/listinfo/libraries .) Sincerely, Isaac Jones, Simon Marlow, Ross Paterson, Malcolm Wallace, Simon Peyton Jones [1] This specification differs from the previous proposal both in technical details, and in that it is the combined efforts of the undersigned. Those familiar with the previous proposal will not find large high-level differences, but some details have been made more concrete and some details have changed.

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[Haskell-cafe] ANNOUNCE: Haskell Communities & Activities Report (6th ed., May 2004)
by Arthur van Leeuwen 25 May '04

25 May '04

Just a bit more than half a year after the previous edition, I am pleased to announce on behalf of the many contributers that the Haskell Communities and Activities Report (6th edition, May 2004) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in several formats: PDF, for screenreading as well as printing, HTML, for those of you who prefer not to deal with plugins or external viewers, and PostScript, for those of you who have nice quick printers that do not grok PDF. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as we did. If you haven't encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and well in time for the actual deadline ;) ) to the call. The editor collects all the contributions into a single report and feeds that back to the community When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of October 2004: target deadline for contributions to the October 2004 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to `register' with the editor for a simple e-mail reminder in the middle of October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Ofcourse, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Enjoy, communicate, and program! Arthur van Leeuwen -- /\ / | arthurvl+hcar(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

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[Haskell-cafe] Solving differential equations in terms of power series
by Henning Thielemann 24 May '04

24 May '04

I've attached a Haskell module demonstrating the beauty of solving ordinary differential equations in terms of power series. A power series is expressed by the (infinite) list of its coefficients. What do you think about that?

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Re: [Haskell-cafe] Re: Lazy XML handling
by MR K P SCHUPKE 24 May '04

24 May '04

>The standard representation should be a tree datatype Why not separate the parser and tree builder, have the parser stream the tags out, and define the tag stream as the interface to a DOM builder... parse :: String -> [(XmlTreeDepth,XmlElement)] buildDOM :: [(XmlTreeDepth,XmlElement)] -> DOM Thats how I would do it anyway. I appreciate the comments about efficency, I was really talking about simplicity. The algorithms on the steam can be quite simple, and hence avoid bugs. I would still want to dispense with the DOM, which if you think about the system thus: Parser -> [stream] -> DomBuilder -> DOM - operations on DOM You can push the operations on the dom via a structural transformation back through the DomBuilder, such that they operate directly on the stream... In other words the user composes the functions as if they are operating on a real DOM... only the DOM is never constructed and the operations are applied direct to the stream output of the parser. make sense? Keean.

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RE: [Haskell-cafe] Working with Network
by Niklas Broberg 23 May '04

23 May '04

Marco Righele wrote: >Ideally each session would work more or less as follows: > >do (hdl,hn,pn) <- accept socket > text <- hGetContents hdl > inputs <- parse text > replies <- sequence $ map reply inputs > sequence $ map (hPrint hdl) replies > ... > >Where parse and reply are IO functions that produce and consume some kind >of higher level representation of the text that the client sent. >Of course this won't work because the code will first try to read all the >input and then to write back all the replies. I think that if I had two >different handles for input and output I wouldn't have this problem, am I >right ? Is there a way to handle such situation without having to read the >input text in chunks ? This for example would allow me to use a library >like Parsec to parse the input (that looks like works on string and not on >handles) It looks like the lazy read problem rears its ugly head again. =) Try the following: do (hdl,hn,pn) <- accept socket text <- hGetContents hdl if text == text then return () else return () inputs <- parse text replies <- sequence $ map reply inputs sequence $ map (hPrint hdl) replies ... The conditional will force 'text' to be read in full, which will free the handle for writing. /Niklas _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963

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Working with Network
by Marco Righele 22 May '04

22 May '04

Hello everybody. I'm trying to learn to use Sockets with Haskell, so I started playing with a small app that waits for a client and start an interactive session, i.e. waits for some input, answers to it, wait for some more etc. Ideally each session would work more or less as follows: do (hdl,hn,pn) <- accept socket text <- hGetContents hdl inputs <- parse text replies <- sequence $ map reply inputs sequence $ map (hPrint hdl) replies ... Where parse and reply are IO functions that produce and consume some kind of higher level representation of the text that the client sent. Of course this won't work because the code will first try to read all the input and then to write back all the replies. I think that if I had two different handles for input and output I wouldn't have this problem, am I right ? Is there a way to handle such situation without having to read the input text in chunks ? This for example would allow me to use a library like Parsec to parse the input (that looks like works on string and not on handles) Thanks, Marco

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Neural Nets
by schrum2＠southwestern.edu 22 May '04

22 May '04

Hello all. I was wondering if anyone knows of ways to implement neural nets in Haskell, and if there are any sites or projects that you could direct me to. I'm specifically interested in how to represent the neural net in Haskell, and then how to represent the neural net as genes in a genetic algorithm.

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Re: [Haskell-cafe] Re: Lazy XML handling
by Niklas Broberg 21 May '04

21 May '04

MR K P Shupke wrote: >Can you give me an example where processing HAS to be in a different order? No, but I can give an example where a list based encoding is less sufficient. I'm coming from a completely different view here, namely that of using Haskell not to query or transform but to create XML. I'm currently in a project[1] trying to fix a working implementation of Haskell Server Pages[2], i.e. using Haskell as a server-side scripting language for dynamic webpages. Clearly the most used mode of operation in this setting is constructing an XML tree rather than querying or transforming an existing one. With this in mind, the list based encoding would be less efficient than a treelike datatype. Building the XML tree is likely to be done bottom-up, and adding a parent to a subtree would be O(n) with a list, but O(1) with a tree datatype. Building a whole tree from the bottom up is O(n^2) for a list, but O(n) for the tree datatype. MR K P Shupke wrote: >The nested lists does not allow any extra flexibility in traversal, at the >cost of >greater complexity. The one case it is simpler is if you want to look up a >single >node by its 'tree address' - This is more complex with the list-rep, as you >have to count nodes at a given level, skipping irrelevent nodes... The fact that the list based representation is indeed more complex is in my opinion a clear indication that it should not be the standard. Compare this to Haskell strings where the standard is the simple but ineffective [Char], but where there is another more efficient representation, PackedString, if you really need it. Simplicity and ease of understanding is not to be taken lightly. By using a tree datatype representation you also get pattern matching directly on the structure of the tree, a very useful feature to have. MR K P Shupke wrote: >The list rep is very good at data extraction, if you want to extract a >sub-tree >of a given type you just ignore all tags until you find one of the correct >type, then output all tags until the depth returns to the level of the >initial >tag. That might possibly be a programmatic advantage, though I don't see how it would be more difficult with a tree datatype. In any case it is no more efficient, both are O(n). As far as I can tell there is no single area where the list representation would outperform a tree datatype, other than the special case of lazy streaming. My suggestions are thus: The standard representation should be a tree datatype, both because it is more general and efficiently supports a wide range of operation modes (construction, querying, transformation, rendering), and also because that is what lies closest to the intuition of XML. However, the ability to stream large XML structures lazily is clearly useful and is difficult to achieve using a tree datatype. Why not add it as a specialisation? The standard representation would be a datatype XML living in Data.XML (or similar), and the specialized would be a type XMLStream living in Data.XML.Stream. There should of course also be functions to convert between the types. Comment away, /Niklas [1] http://www.dtek.chalmers.se/~d00nibro/hsp/ [2] http://www.research.microsoft.com/~emeijer/Papers/HSP.pdf _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963

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GHC and libc
by Per Larsson 21 May '04

21 May '04

When I compile my haskell program (on a linux machine) with GHC and send it to a colleague he can't run it because he has a somewhat older version of libc. Is there a GHC switch that I have missed that enables you to statically link the parts of libc that is used by the haskell program? Alternatively have someone other tips for solving this problem? (I'm reluctant to force all my colleagues to install GHC, because of the size and long installation time, and I can't let them use Hugs because I've used some GHC-only features in my program.) Per Larsson

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c2hs and gtk+hs problem
by scott west 18 May '04

18 May '04

Hello all, I've recently attempted to get the gtk+hs bindings operational, with evidently no success. They both compile fine, but when trying to "make all" the examples in the gtk+hs tree, it gives up with: /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(C2HS.o)(.text+0x32): In function `__stginit_C2HS_': : undefined reference to `__stginit_CForeign_' /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(C2HSBase.o)(.text+0x28): In function `__stginit_C2HSBase_': : undefined reference to `__stginit_MarshalUtils_' /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(C2HSMarsh.o)(.text+0x32): In function `__stginit_C2HSMarsh_': : undefined reference to `__stginit_CString_' /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(C2HSMarsh.o)(.text+0x3c): In function `__stginit_C2HSMarsh_': : undefined reference to `__stginit_MarshalUtils_' /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(C2HSMarsh.o)(.text+0x50): In function `__stginit_C2HSMarsh_': : undefined reference to `__stginit_MarshalAlloc_' /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(C2HSMarsh.o)(.text+0x5a): In function `__stginit_C2HSMarsh_': : undefined reference to `__stginit_Ptr_' /usr/local/lib/c2hs-0.12.0/ghc6/libc2hs.a(NewStorable.o)(.text+0x28): In function `__stginit_NewStorable_': : undefined reference to `__stginit_Storable_' collect2: ld returned 1 exit status Does anyone have any possible ways to resolve this? Thanks for any help!! Regards, Scott

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