September 2008 - Haskell-Cafe

ANN: vxml (validating xml lib) - proof of concept - need some guidance - bad type level performance (ghc)
by Marc Weber 13 Sep '08

13 Sep '08

Hi @ll. This shouldn't have been an announce yet.. it's more crying for help to get to know how / wether to continue :-) I had an idea: a xml generating library validating the result against a given dtd (to be used in web frameworks etc .. ) After one week of coding I got the result git clone git://mawercer.de/vxml (1) (see README) git hash as of writing : 4dc53 A minimal example looks like this (taken from test.hs): import Text.XML.Validated.TH $( dtdToTypes "dtds/xhtml1-20020801/DTD/xhtml1-strict_onefile.dtd" (XmlIds (Just "-//W3C//DTD XHTML 1.0 Strict//EN") (Just "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd") ) ) main = putStrLn $ xml $ ((html << (head << (title <<< "vxml hello world"))) << (body << (div <<< "PCDATA text" )) ) Which takes about 7 secs to compile on my (fast) 2.5 Ghz machine. Before Oleg telling me about how to use the TypeEq implementation found in HList it took several hours (switch the cabal configure flag to see experience the incredible slow TypeToNat implementation) However type checking the simple small document (2) already takes 35 seconds, duplicating the tags within the body raises compilation time to 74 seconds. (Have a look at test/XmlToQ.hs to automatically create the code generating such a given xml file (its used in run-testcases.hs) The dtd representation used is very easy to understand: (3a) shows allowed subelements of the head tag Seq = sequence Star = zero or more ... (3b) shows the list of allowed attributes for a more commonly used tag such as div the element list (3a) is 324 lines long and there are approximately 15 attributes. Use a line such as putStrLn $ xml $ debugEl div to get this view. All the work is done by class Consume st el r | st el -> r -- result is on of C,CS,R,F class Retry elType st el st' | st el -> st' st = state as given in (3a) el = child to be added (Elem Html_T) or PCDATA C = element consumed, end CS a = element consumed, continue with state a R a = retry with given state (can happen after removing a Star = ()* on a no match ) F a = no match, show failure a instance Retry elType C el C instance Retry elType (CS st) el st instance ( -- retry Consume st el r , Retry elType r el st' ) => Retry elType (R st) el st' Maybe you knowing much more about ghc internals have some more ideas how to optimize? I only came up with a) implement xml HTrue $ xmldoc -- type checking variant xml HFalse $ xmldoc -- unchecked but faster variant so you can run xml HTrue once a day only while having lunch.. ( doubt Maybe you have to take lunch two or more times on intermediate web projects .. ) b) change (A attrType) to attrType only and (Elem e) to e which does not work for the same reason as class TypeEq a b c | a b -> c instance TypeEq a a HTrue instance TypeEq a b HFalse doesn't. Of course a a will match a b as well, but the result is totally different.. So would it be possible to either tell ghc to ignore this happily taking the result of the better matching instance? Or to explicitely tell ghc when an instance matches the way it does in current implementations make it a no match if some constraints can or cannot be satisfied such as this: class TypeEq a b c | a b -> c instance TypeEq a a HTrue instance [ NoMatchOn a b -- if this can be satisfied ignore this instance ] => TypeEq a b HFalse class NoMatchOn a b instance NoMatchOn a a c) Having a reduction rule such as this: If the left side of (Or a b) returnns C (consumed) tell ghc to not evaluate the result of (Consume b element result) which must be done to see wether there is closer instance match (Is this correct ?) Of course not everything is implemented yet.. but except speed I consider it beeing quite usable. He, thanks for reading till the end. Any feedback and suggestions are appreciated. If you have any trouble contact me on irc. I've been using $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 Sincerly Marc Weber (1) If you are interested and either don't have git or are not familiar using it I can send you a tarball. (2) <!DOCTYPE html SYSTEM "/tmp/dtd.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en-US" xml:lang="en-US"> <head> <title>Example 6 - XHTML 1.0 Strict as application/xhtml+xml</title> <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" /> <link rel="stylesheet" type="text/css" href="style.css" /> </head> <body> <h1>Example 6 - XHTML 1.0 Strict as application/xhtml+xml</h1> <p> This document is valid XHTML 1.0 Strict served as <code>application/xhtml+xml</code>. </p> <p> This document references CSS rules contained in an external stylesheet via <code>link</code>. </p> <p> Note how the CSS rules for the background are applied in Netscape 7.x, Mozilla, Opera 7 but that Internet Explorer can not display the page at all. </p> <p> <a href="http://validator.w3.org/check/referer"><img src="http://www.w3.org/Icons/valid-xhtml10" alt="Valid XHTML 1.0!" height="31" width="88" /></a> </p> </body> </html> (3a) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Or (Seq (Elem Title_T) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Query (Seq (Elem Base_T) (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))))))) (Seq (Elem Base_T) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Seq (Elem Title_T) (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T))))))))))) (3b) (HCons (A Lang_A) (HCons (A Xml:lang_A) (HCons (A Dir_A) (HCons (A Id_A) (HCons (A Profile_A) HNil)))))

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Text.JSON idiomatic use
by ntupel 13 Sep '08

13 Sep '08

As a follow up to my previous JSON serialization post I came up with a first draft of some simple record type serialization/deserialization. What I would like to know is, whether this is the right approach or what better ways there are to make a custom data type an instance of class JSON. Any chance to reduce the amount of boilerplate required to do this? I would be grateful for any feedback (also general style comments are much appreciated). Many thanks! module Test where import Text.JSON data Message = Error { event :: String, channel :: String, id :: String, cause :: String, message :: String} | Join { event :: String, channel :: String, id :: String, name :: String} | Leave { event :: String, channel :: String, id :: String, really :: Bool} deriving (Eq, Show, Read) asJSString :: String -> JSValue asJSString = JSString . toJSString asString :: JSValue -> String asString (JSString s) = fromJSString s asBool :: JSValue -> Bool asBool (JSBool b) = b showErrorJSON, showJoinJSON, showLeaveJSON :: Message -> JSValue showErrorJSON (Test.Error evt cha id cau msg) = showJSON $ toJSObject [("event", evt), ("channel", cha), ("id", id), ("cause", cau), ("message", msg)] showJoinJSON (Join evt cha id nme) = showJSON $ toJSObject [("event", evt), ("channel", cha), ("id", id), ("name", nme)] showLeaveJSON (Leave evt cha id rly) = showJSON $ toJSObject [("event", asJSString evt), ("channel", asJSString cha), ("id", asJSString id), ("really", JSBool rly)] createMessage, readErrorJSON, readJoinJSON, readLeaveJSON :: [(String, JSValue)] -> Maybe Message readErrorJSON xs = do evt <- lookup "event" xs cha <- lookup "channel" xs id <- lookup "id" xs cau <- lookup "cause" xs msg <- lookup "message" xs Just (Test.Error (asString evt) (asString cha) (asString id) (asString cau) (asString msg)) readJoinJSON xs = do evt <- lookup "event" xs cha <- lookup "channel" xs id <- lookup "id" xs nme <- lookup "name" xs Just (Join (asString evt) (asString cha) (asString id) (asString nme)) readLeaveJSON xs = do evt <- lookup "event" xs cha <- lookup "channel" xs id <- lookup "id" xs rly <- lookup "really" xs Just (Leave (asString evt) (asString cha) (asString id) (asBool rly)) createMessage obj = do evt <- lookup "event" obj case asString evt of "/error" -> readErrorJSON obj "/me/add" -> readJoinJSON obj "/me/remove" -> readLeaveJSON obj _ -> Nothing instance JSON Message where showJSON x@(Test.Join _ _ _ _) = showJoinJSON x showJSON x@(Test.Leave _ _ _ _) = showLeaveJSON x showJSON x@(Test.Error _ _ _ _ _) = showErrorJSON x readJSON (JSObject o) = case createMessage . fromJSObject $ o of Just m -> Ok m Nothing -> Text.JSON.Error "Parsing failed." readJSON _ = Text.JSON.Error "Records must be JSObjects"

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template haskell -- include a file?
by Jason Dusek 13 Sep '08

13 Sep '08

I'd like to use template Haskell to include as a string in a Haskell file. How do I do it? Is there any lengthy documentation with examples for Template Haskell? The pages linked to from `haskell.org` are a little sparse. -- _jsn

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Haskell and Java
by Maurício 13 Sep '08

13 Sep '08

Hi, I use Haskell, and my friends at work use Java. Do you think it could be a good idea to use Haskell with Java, so I could understand and cooperate with them? Is there a a Haskell to Java compiler that's already ready to use? Thanks, Maurício

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JSON serialization/deserialization
by ntupel＠googlemail.com 13 Sep '08

13 Sep '08

Dear list members, I try to use Text.JSON (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/json) to serialize and deserialize record types. As I understand it, the data types need to be instances of class JSON. However I have difficulties to come up with a nice implementation of showJSON and readJSON. For example: module Test where import Text.JSON data Record = One { field1 :: String, field2 :: String } | Two { field :: String } deriving (Eq, Ord, Show, Read) showJSON (One x y) = toJSObject [("field1", x), ("field2", y)] showJSON (Two x) = toJSObject [("field", x)] readJSON x = -- ??? This lacks the instance declaration itself but my problem is more fundamental. How can I write the readJSON method? In general how would one approach the problem of serialization/deserialization? I am puzzled that the type classes Show and Read can be derived automatically. I can not think of any way to do this. Thanks for your help!

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Can you do everything without shared-memory concurrency?
by Bruce Eckel 13 Sep '08

13 Sep '08

As some of you on this list may know, I have struggled to understand concurrency, on and off for many years, but primarily in the C++ and Java domains. As time has passed and experience has stacked up, I have become more convinced that while the world runs in parallel, we think sequentially and so shared-memory concurrency is impossible for programmers to get right -- not only are we unable to think in such a way to solve the problem, the unnatural domain-cutting that happens in shared-memory concurrency always trips you up, especially when the scale increases. I think that the inclusion of threads and locks in Java was just a knee-jerk response to solving the concurrency problem. Indeed, there were subtle threading bugs in the system until Java 5. I personally find the Actor model to be most attractive when talking about threading and objects, but I don't yet know where the limitations of Actors are. However, I keep running across comments where people claim they "must" have shared memory concurrency. It's very hard for me to tell whether this is just because the person knows threads or if there is truth to it. The only semi-specific comment I've heard refers to data parallelism, which I assumed was something like matrix inversion, but when I checked this with an expert, he replied that matrix inversion decomposes very nicely to separate processes without shared memory, so now I'm not clear on what the "data parallelism requires threads" issue refers to. I know that both Haskell and Erlang only allow separated memory spaces with message passing between processes, and they seem to be able to solve a large range of problems -- but are there problems that they cannot solve? I recently listened to an interview with Simon Peyton-Jones where he seemed to suggest that this newsgroup might be a helpful place to answer such questions. Thanks for any insights -- it would be especially useful if I can point to some kind of proof one way or another. -- Bruce Eckel

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mailing list choices?
by Conal Elliott 12 Sep '08

12 Sep '08

I want to set up some kind of mailing list for reactive (which I plan to release soon). The most obvious thing is to set up a mailman-based list on haskell.org, but I wonder -- do people really want to keep using mailman technology? Or something more modern like Yahoo or Google groups. I use my email reader also with Yahoo & Google groups, so my day-to-day experience is the same as with mailman-based lists. For less regular operations like searching archives, I prefer the experience on Yahoo & Google group. And I like some of the other modern features, like personal profiles and polls. I've been wondering if haskellers really like mailman or are just used to it. Maybe people make new mailman lists simply because others did it before them. Any thoughts/preferences? Thanks, - Conal

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Workshop on Generic Programming: Call for Participation (co-located w/ ICFP08)
by Matthew Fluet (ICFP Publicity Chair) 12 Sep '08

12 Sep '08

Dear all, the Workshop on Generic Programming is only a few days away: 20th September 2008 (http://www.regmaster.com/conf/icfp2008.html) ==> Invited talk: The Generic Paradigm ==> Lambert Meertens (Utrecht University) ==> We have reserved 20 minutes for *lightning talks*. If you plan to ==> attend and if you would like to give a short talk (about half-baked, ==> exciting, new stuff) please drop me a short note. Slots will be ==> reserved on a first-come-first-serve basis. Looking forward to seeing you in Victoria, Ralf Hinze ============================================================================ CALL FOR PARTICIPATION Workshop on Generic Programming 2008 Victoria, Canada, 20th September 2008 http://www.comlab.ox.ac.uk/ralf.hinze/wgp2008/cfp.{html,pdf,ps,txt} The Workshop on Generic Programming is sponsored by ACM SIGPLAN and forms part of ICFP 2008. Previous Workshops on Generic Programming have been held in Marstrand (affiliated with MPC), Ponte de Lima (affiliated with MPC), Nottingham (informal workshop), Dagstuhl (IFIP WG2.1 Working Conference), Oxford (informal workshop), Utrecht (informal workshop), and Portland (affiliated with ICFP). ============================================================================ Preliminary program ------------------- 9:00 - 10:00, Session Chair: Ralf Hinze (University of Oxford) Welcome Invited talk: The Generic Paradigm Lambert Meertens (Utrecht University) 10:30 - 12:00, Session Chair: Jeremy Gibbons (University of Oxford) A Functional Model-View-Controller Software Architecture for Command-oriented Programs Alley Stoughton (Kansas State University) A Lightweight Approach to Datatype-Generic Rewriting Thomas van Noort (Radboud University Nijmegen), Alexey Rodriguez, Stefan Holdermans (Utrecht University), Johan Jeuring (Utrecht University and Open University of the Netherlands), Bastiaan Heeren (Open University of the Netherlands) Lightning talks 13:30 - 15:00, Session Chair: Ralf Hinze (University of Oxford) Report from the program chair Ralf Hinze (University of Oxford) Scala for Generic Programmers Bruno C. d. S. Oliveira, Jeremy Gibbons (University of Oxford) A Comparison of C++ Concepts and Haskell Type Classes Jean-Philippe Bernardy, Patrik Jansson, Marcin Zalewski, Sibylle Schupp, Andreas Priesnitz (Chalmers University of Technology and University of Gothenburg) Lightning talks 15:30 - 17:30, Session Chair: Patrik Jansson (Chalmers University of Technology and University of Gothenburg) Polytypic Programming in Coq Wendy Verbruggen, Edsko de Vries, Arthur Hughes (Trinity College Dublin) Bialgebra Views: A Way for Polytypic Programming to Cohabit with Data Abstraction Pablo Nogueira, Juan Jose Moreno-Navarro (Universidad Politecnica de Madrid) Discussion ============================================================================

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darcs hacking sprint, venues confirmed! (25-26 October)
by Eric Y. Kow 12 Sep '08

12 Sep '08

Dear darcs users and Haskell hackers, Earlier I wrote an announcement for the darcs hacking sprint on 25-26 October. Tonight, I am delighted to announce that we have two venues confirmed for the sprint and one serious offer. Venues ------ We plan to host the sprint across three sites: * CONFIRMED: Brighton, UK (University of Brighton) * CONFIRMED: Portland, USA (Galois Connections) * likely: Paris, France (Université Paris Diderot) So if you were waiting to book tickets, this is the time! For more details, please see http://wiki.darcs.net/index.html/Sprints Agenda ------ During this first sprint, we shall be focusing our attention on the day to day performance issues that darcs users commonly face. This is what we are reaching for: 1. Fast network operations. We want to make it very pleasant for users to darcs get a repository and pull some patches to it over http and ssh. Git does this very well, and we plan to learn from them. 2. Cutting memory consumption. We want to profile the heck out of operations like darcs record, darcs convert and darcs whatsnew. What's eating up all the memory? And how can we can cut it down to size? 3. Responsiveness. Sometimes basic darcs commands can take long enough for programmers to lose their train of thought. We want to track down these lost seconds and kill that dreaded context switch. Of course, if you are interested in other areas, then you can work on those instead. Note that if you are new to the darcs code or to Haskell, there will also be a lot interesting jobs for you to get started with. Everyone will have something to hack on, so come join us! Thanks very much to the University of Brighton, Galois connections and University of Paris VII for their generous offers. Hope to see you there, everyone! :-) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Connecting to wireless network
by Marco Túlio Gontijo e Silva 12 Sep '08

12 Sep '08

Hello, I've made a small program[0] to connect to a wireless network. Comments are welcome. Greetings. 0: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/n-m -- Marco Túlio Gontijo e Silva Página: http://marcotmarcot.iaaeee.org/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

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