- Haskell-Cafe - Haskell.org

HaXml 1.13 -> 1.22 upgrade
by Michael Orlitzky 12 Dec '11

12 Dec '11

I'm trying to migrate one my programs from the old HaXml API to the new. Please, someone save me. I'm currently stuck with this, which works in 1.13. All of the filters work on Content, so I make some from the root element with the "root_elem = CElem root" line. -- |Takes an XML String as an argument, and returns the -- status that was parsed from it. Should only be used -- on XML string where a <status> is a top-level element. parse_status :: String -> [Status] parse_status xml_data = catMaybes maybe_status where (Document _ _ root _) = xmlParse xml_file_name xml_data root_elem = CElem root status_element = (single_status root_elem) maybe_status = map status_from_content status_element In the new API, xmlParse returns the root element with type (Element i) rather than just Element. And the Content constructor I have to use is (CElem (Element i) i), but I have no way to pass the correct 'i' to it. I just want to parse a few elements from an XML file.

3 4

Re: [Haskell-cafe] Module name space question
by John Lato 12 Dec '11

12 Dec '11

> From: Christoph Breitkopf <chbreitkopf(a)googlemail.com> > > Hi, > > I recently asked about what interfaces to implement for a new data type. > Following the rule that the last 10% of work take the second 90% of time, > some other questions have come up. > > If anyone wants to look at the code in question: > http://www.chr-breitkopf.de/comp/IntervalMap > > Some time ago, I was looking for a data structure to search in sets of > possibly > overlapping intervals, and found only Data.SegmentTree, which did not fit > my needs > (nice term for "I did not understand the type signatures"). I can't answer any of your specific questions, although I've published a related structure in the splaytree package (http://hackage.haskell.org/package/splaytree) The interface is minimal, although it suits my needs and I'd be happy to extend it if someone else found it worthwhile. The difference between my data structure (which I've called a RangeSet) and a standard IntervalSet is that overlapping ranges are combined, and if part of a range is deleted, the affected range is modified and possibly split e.g. > let set1 = singleton $ range 0 5 > set2 = insert (range 2 5) set1 after this, set2 has one node representing the range 0-7. Staying on topic, my package puts everything under `Data.SplayTree.xxx`, which is different from the fingertree package organization. AFAICT the selection of one convention over the other is fairly arbitrary, although more packages seem to use the "one second-level hierarchy" structure (e.g. bytestring, text, containers). John L.

1 0

No instance for (Read POSIXTime)
by dokondr 12 Dec '11

12 Dec '11

Hi, I need to read / write epoch seconds from / to plain text files. When I try to read POSIXTime that I use in my own data type: data TimedClassRecT = TCR {timeStamp :: POSIXTime, classCosMap :: Map.Map String Float} deriving (Eq, Read, Show) I get the following error: No instance for (Read POSIXTime) arising from the 'deriving' clause of a data type declaration Possible fix: add an instance declaration for (Read POSIXTime) or use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Read TimedClassRecT) What should I do to provide Read instance for POSIXTime? I would rather not implement it myself. Thanks!

2 3

Tracing Prelude.read exceptions
by dokondr 12 Dec '11

12 Dec '11

Hi, I got quite used to a sequence providing simple data persistence : 1) Store my data to a file: writeFile fileName (show someData) 2) Some time later read this data back: line <- readFile fileName let someData = read line :: SomeDataType Having this done hundreds of times I now got stuck with step 2) trying to read moderately complex structure back. I get read exception in run-time: fromList *** Exception: Prelude.read: no parse I have checked and rechecked my types, data files, etc. - and still no idea. So my question: Is there any way to trace Prelude.read exceptions to see exactly on what data element read fails in run-time? Thanks!

3 2

Recommended class instances for container type
by Christoph Breitkopf 12 Dec '11

12 Dec '11

Hello, I'm in the process of implementing a container data type, and wonder what class instances are generally considered necessary. E.g. is it ok to start out with a Show that's adequate for debugging, or is it a 'must' to include instances of everything possible (Eq, Ord if possible, Read, Show, Functor, ...). And what about the more experimental things? Say, DeepSeq, Typeable, Data? I'd like to keep this simple at start, and I've admittedly not followed recent developments in Haskell-land (recent meaning the last 10 years or so. I _do_ know about hierachical modules ;-) ). OTOH, if not having such instances makes it impossible to do things the modern way, I'd probably take the time to implement (and maybe understand) them. Thanks, Chris

6 8

How did iteratees get their names?
by Douglas McClean 12 Dec '11

12 Dec '11

I love iteratees as a paradigm for IO, but I am having trouble developing a relationship with the names. Could someone explain their origin? It seems like if iteratees consume things, enumerators produce them, andenumeratees do both that names like Sink, Source, and Transformer or Consumer, Producer, and Transformer might be more relatable choices? Is there some reason apart from convention why these names wouldn't fit the concepts well? -- J. Douglas McClean

5 6

Why doesn't this work? (palindrome :: IO)
by Alexej Segeda 11 Dec '11

11 Dec '11

Hi! A couple of months ago, I wrote an exam in an introductory Haskell course and failed, all because of an assignment that I was convinced would work, but for some reason, it didn't. The assignment was to write a function that would take a line, then determine whether it's a palindrome or not. My code follows: palindrome :: IO () palindrome = do putStr "Type in a word" s <- getLine case s of (s == reverse s) -> putStrLn (s ++ " is a palindrome") otherwise -> putStrLn (s ++ " is not a palindrome") The interesting thing is, that if I change the "case ... of" statement to an "if ... then ... else" statement, this magically starts to work. Since I no longer am enrolled (I have to take the course next year), I can't ask a teacher, but my curiosity still bugs me. Why doesn't this work? And why does it work with a "if ... then ...else" statement?

6 10

Generating Code
by L Corbijn 11 Dec '11

11 Dec '11

Hello, In an attempt to reduce the amount of boring repetitive work to update the OpenGLRaw package I've created a generator to do it partially for me. It currently uses haskell-src-exts for representing the haskell source of the modules. Though haskell-src-exts does an excellent job for representing haskell source, it seems to be more aimed at parsing haskell source and doing something with it, rather than generating it. For cpp macros, say for the use of different calling conventions used, can't be used directly, nor is there a really good way to use comments (at least so it seems to me). So I'm interested if there are other libraries that are more suitable to the task of generating haskell code for library use, and thus generate 'human readable' exported code (so no TH). I'm also interested in how other projects generate code for their packages. Greetings, Lars Corbijn

10 15

ANNOUNCE: smallcheck-0.6; test-framework-smallcheck-0.1
by Roman Cheplyaka 11 Dec '11

11 Dec '11

I'm pleased to announce a new release of SmallCheck. The highlights for this release are: * Default Generic implementation of Serial instance, contributed by Bas van Dijk This means that you don't need to write instances by hand for your types to generate test values for them. See [1] for the details. [1]: http://hackage.haskell.org/packages/archive/smallcheck/0.6/doc/html/Test-Sm… * Revised, organised into modules and documented API * Integration with test-framework, through test-framework-smallcheck package It allows to organize SmallCheck properties into a test suite (possibly together with HUnit or QuickCheck tests), apply timeouts, get nice statistics etc. Thanks to Max Bolingbroke for cooperation on this front. Please refer to the README for more information about SmallCheck: https://github.com/feuerbach/smallcheck/blob/master/README.md Hackage links: http://hackage.haskell.org/package/smallcheck-0.6 http://hackage.haskell.org/package/test-framework-smallcheck-0.1 -- Roman I. Cheplyaka :: http://ro-che.info/

1 0

Sharing a room for the CRA-W/CDC/SIGPLAN Mentoring Workshop at POPL?
by Joachim Breitner 11 Dec '11

11 Dec '11

Hi, I will attend the CRA-W/CDC/SIGPLAN Mentoring Workshop at POPL next year, and will have to share a room with another attendee. As we can give preferences, I was wondering if another Haskeller might be in the same position and interested in sharing a room. Greetings, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

1 0