- Haskell-Cafe - Haskell.org

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 16 Jun '04

16 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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