- Haskell-Cafe - Haskell.org

Several default implementations for instance methods
by Andreas Abel 04 Oct '14

04 Oct '14

Consider the following class for an overloaded pretty printer. For atomic data (numeric types, String etc.) one would implement prettyShow, for complex data either pretty or prettyPrec. > import Text.PrettyPrint > > class Pretty a where > pretty :: a -> Doc > prettyShow :: a -> String > prettyPrec :: Int -> a -> Doc Implementing one of these methods should be enough, giving default implementations for the other two. It is easy to get prettyShow and prettyPrec from pretty. > prettyShow = render . pretty > prettyPrec = const . pretty However, to define pretty from one of the others, I need two default implementations. > pretty = text . prettyShow > pretty = prettyPrec 0 Is there a way to get this to work? Workarounds (not entirely satisfactory): Technically, one could define a cycle of default implementations. Alternative 1: > pretty = prettyPrec 0 > prettyShow = render . pretty > prettyPrec _ = text . prettyShow Problem: Here, if pretty is defined, > prettyPrec _ = text . render . pretty instead of just > prettyPrec _ = pretty and (text . render) is not the identity (destroys inner document structure). Alternative 2: > pretty = text . prettyShow > prettyShow = render . prettyPrec 0 > prettyPrec _ = pretty Problem: Here, if prettyPrec is defined, > pretty = text . render . prettyPrec 0 instead of just > pretty = prettyPrec 0 I guess alternative 2 is worse than alternative 1, as one would usually define prettyPrec to get pretty, and not the otherway round. But none of these two alternatives really does the job. -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/

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Call for Contributions - Haskell Communities and Activities Report, November 2014 edition
by Mihai Maruseac 04 Oct '14

04 Oct '14

Dear all, We would like to collect contributions for the 27th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 Nov 2014 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac and Alejandro Serrano Mena FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2014/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2014/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

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ANN: hxt-css 0.1.0.0, a CSS selector engine for HXT
by Marios Titas 03 Oct '14

03 Oct '14

Hello everybody, I'd like to announce the first public release of hxt-css, a CSS selector engine for the Haskell XML Toolbox (HXT). Its main design goals are: * support as many CSS selectors as possible: it currently supports all CSS 3 selectors except the ones that do not make sense outside a web browser (e.g. such as :hover). For example, it supports weird things like div > span + p:not(:nth-of-type(3n-1)) * try to be 100% correct: in all tests I ran, the output of hxt-css was identical to that of firefox & chrome. * follow the conventions of other hxt packages: for example, error reporting is done the same way as hxt-xpath. Note, there is already a similar package in hackage called HandsomeSoup. - Marios

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u3502350@connect.hku.hk has indicated you're a friend. Accept?
by u3502350＠connect.hku.hk 03 Oct '14

03 Oct '14

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Need help with scrap-your-boilerplate
by Michael Sperber 02 Oct '14

02 Oct '14

I'm a casual Haskell user, and am trying to use scrap-your-boilerplate to write a transformation - and failing. The rub is that the base function is polymorphic, boiling down to this: data Foo a = Foo a bar :: Foo a -> Foo a bar x = x Now, I'm trying to use SYB like so: foo :: Typeable a => a -> a foo = mkT bar ... but I get: Could not deduce (Typeable a0) arising from a use of ‘mkT’ from the context (Typeable a) bound by the type signature for foo :: Typeable a => a -> a at foo.hs:... The type variable ‘a0’ is ambiguous Note: there are several potential instances: Is there any way I could make this work? Any help would be much appreciated! -- Regards, Mike

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Add linker flags to GHC 6.12.3 using extra-gcc-opts
by Shaun Jackman 02 Oct '14

02 Oct '14

Hi. I get the following error message when linking with GHC. ``` $ ghc -o hello-hs hello-hs.hs /projects/btl/linuxbrew/bin/ld: cannot find -lgmp collect2: ld returned 1 exit status ``` It's fixed by adding `-L/projects/btl/linuxbrew/lib` to the command line. ``` $ ghc -L/projects/btl/linuxbrew/lib -o hello-hs hello-hs.hs ``` I'd like to add the above `-L` option to the default linking arguments of GHC. I thought that I could do this by modifying the file `extra-gcc-opts`. I changed it from ` -fwrapv` to ` -fwrapv -L/projects/btl/linuxbrew/lib`, but it has no effect. I confirmed this by changing it to ` -fBROKEN`, and again no effect. The file location is `/projects/btl/linuxbrew/Cellar/ghc612/6.12.3/lib/ghc-6.12.3/extra-gcc-opts`. How do I change the default gcc link options of GHC 6.12.3? It would be difficult for me to upgrade to GHC 7.8.3 because the binary package does not run on CentOS 5.11. Yes, I know it's ancient. It's an HPC cluster, and I have no control over it. So, likewise, I'm not able to install `libgmp-devel` in `/usr`, which would also solve this issue. Thanks for your help. Cheers, Shaun http://sjackman.ca

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GHC 7.10 GHC-API Changes and proposed changes.
by Alan & Kim Zimmerman 02 Oct '14

02 Oct '14

There are a number of changes in GHC 7.10 that will make it easier for tool writers to use the GHC API. These include 1. More parser entrypoints, to explicitly parse fragments [Andrew Gibiansky] The following parsers are now provided parseModule, parseImport, parseStatement, parseDeclaration, parseExpression, parseTypeSignature, parseFullStmt, parseStmt, parseIdentifier, parseType, parseHeader 2. No more landmines in the AST [Alan Zimmerman] In the past it was difficult to work with the GHC AST as any generic traversals had to carefully tiptoe around an assortment of panic and undefined expressions. These have now been removed, allowing standard traversal libraries to be used. There is a third change currently being discussed, and I would like feedback on it's desirability. 3. Introduce an annotation structure to the ParsedSource to record the location of uncaptured keywords. At the moment the location of let / in / if / then / else / do etc is not captured in the AST. This makes it difficult to parse some source, transform the AST, and then output it again preserving the original layout. The current proposal, which can be seen at [1] and a proof of concept implementation at [2] returns a structure keyed to each AST element containing simply the specific SrcSpan's not already captured in the AST. This is the analogue of the Language.Haskell.Exts.Annotated.Syntax from haskell-src-exts, except a custom SrcSpanInfo structure is provided for each AST element constructor, and it is not embedded within the AST. [1] https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations [2] https://phabricator.haskell.org/D297 So, apart from HaRe, are there any other users who would be interested in using this? Regards Alan

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Haskell Weekly News: Issue 308
by Daniel Santa Cruz 02 Oct '14

02 Oct '14

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Reflection functional dependency as an associated type?
by Brandon Moore 01 Oct '14

01 Oct '14

I was trying to make a bridge like this class MonadStateTF m where type StateOf m :: *instance (MonadState s m) => MonadStateTF m where type StateOF m = s to allow writing constraints on state monads without a dangling type parameter type NumMonad m = (MonadStateTF m, Num (StateOf m)) Unfortunately, the declaration type StateOF m = s complains that "s" isn't mentioned on the left hand side rather than somehow making use of the fundep. Brandon

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Broken linkr in wiki for Ivo
by Evan Rowley 01 Oct '14

01 Oct '14

The theorem provers page has a link for Ivor at the bottom: http://www.haskell.org/haskellwiki/Libraries_and_tools/Theorem_provers This current link is dead: http://www.dcs.st-and.ac.uk/~eb/ivor.php This new link works: http://eb.host.cs.st-andrews.ac.uk/ivor.php -- -- - EJR

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