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ANN (2 Libs) -- hvac 0.1b, a lightweight web framework and HStringTemplate 0.3
by Sterling Clover 04 Jun '08

04 Jun '08

1) hvac 0.1b: transactional, declarative framework for lightweight web applications. 2) HStringTemplate 0.3 1) hvac 0.1b hvac (short for http view and controller) has been my project for the last little while, and is finally in a fairly usable state, so I'm opening up the repo (darcs get http://community.haskell.org/~sclv/ hvac/) for folks to play with and to get some feedback. While not quite yet ready for hackage, the package as provided should be fully cabal installable. Documentation is available at http:// community.haskell.org/~sclv/hvac/html_docs/hvac/ The aim of hvac is to provide an environment that makes the creation of lightweight fastcgi based web applications as simple as possible, with an emphasis on concise, declarative style code, correct concurrent transactional logic, and transparency in adding caching combinators. There are two included example programs, naturally neither of which is feature complete. They share a common login module of about 50 lines of code, excluding imports, templates, and database schema. The first program is a classic, greenspun-style message board with basic login functionality. It totals roughly 40 lines and tends to use just under 4mb of resident memory on my system. The second is a wiki based on Pandoc and the PandocWiki code. The code totals roughly 30 lines (rendering borrowed from PandocWiki aside) and uses about 5mb of memory. hvac processes all requests in the STM monad, with some bells attached to properly interleave STM with session, database and filesystem operations such that they all conceptually occur together in a single transaction per request. Currently it is only fully tested with sqlite, but it should operate, modulo a few tweaks, with any database accessible via HDBC. hvac is particularly designed to use the HStringTemplate library as an output layer, in a simple declarative fashion. As the StringTemplate grammar is explicitly sub-turing, this ensures a clean separation of program logic from presentation, while providing a nonetheless fairly powerful language to express typical display tasks. The included cache combinators, still experimental, should allow a simple and fine-grained control over the level of caching of various disk-bound operations. Phantom types are used to ensure that no functions that modify state may be cached. To give a flavor of hvac code, the following is the complete (twenty lines!) source of the wiki controller (due to sql statements, some lines are rather long): wikiController tmpl = h |/ "login" *> login_plug tmpl <|> (h |/ "wiki" |\\ \pageName -> h |// "POST" *> withValidation [ ("contents", return) ] (\ [contents] -> do pageId <- selectVal "id from pages where name=?" [toSql pageName] maybe (addErrors [("Login","must be logged in.")] >> continue) (\user -> case fromSql pageId of Just (_::Int) -> execStat "insert into page_hist (pageId,contents,author) values(?,?,?)" [pageId, toSql contents, toSql . userName $ user] Nothing -> do execStat "insert into pages (name,locked) values(?,?)" [toSql pageName, toSql (0::Int)] pid <- selectVal "max(id) from pages" [] execStat "insert into page_hist (pageId,contents,author) values(?,?,?)" [pid, toSql contents, toSql . userName $ user]) =<< getSes continue) <|> do pageId <- selectVal "id from pages where name=?" [toSql pageName] (join $ renderf (tmpl "showPage") ("pageName", pageName) <$> "pageContents" |= selectRow "* from page_hist where pageId=? order by time desc limit 1" [pageId] )) <|> (redirect . ( ++ "/wiki/Index") =<< scriptName) Future directions for work on hvac include: Stress testing for correctness of transactional logic and benchmarks. Exploration of various efficiency tweaks. Unit tests. Further development of the cache combinator API. Improvement of the example apps and addition of a few others (a blog maybe). Expansion of the library of validator functions. Exploration of transferring hvac to the standard FastCGI bindings (currently it uses a custom modified version to work properly with STM). Improvement of the database layer, particularly with regards to common paging functions. Creation of a set of simple combinators for generating CRUD (create read update delete) pages. Creation of a minimal set of standard templates (maybe). 2) HStringTemplate 0.3.1 This release of HStringTemplate (up now at Hackage) fixes a number of bugs pointed out to me by its small but growing user base (thanks, cinema, elliottt!) ranging from the minor (a particular two-level iteration pattern wasn't working properly) to the truly irritating (poor handling of file groups). It's still unfortunately skimpy on the docs, outside of the haddocks and the main StringTemplate grammar documentation at http://www.stringtemplate.org (although the examples from hvac should also prove helpful). However, it does have a set of very nice and handy new features for development. * renderf, a function similar in spirit to printf, that takes an arbitrary number of heterogeneous (String, value) tuples as arguments. This should cut down considerably on long setAttribute chains. Additionally, with custom instances (not, I'll grant, trivial to write) it can be used to declaratively chain together strings of attribute retrieval functions in arbitrary monads, as in the above code example from hvac. * dumpAttribs, a function/template that prints out the tree of the entire attribute environment a template is operating in -- extremely handy for development. * nullGroup, also for use in development, a simple way to display more information about templates that can't be found. Error messages in usafeVolatileDirectoryGroup have also been significantly improved. * getStringTemplate', a version of getStringTemplate guaranteed not to be inlined. While the optimizer will still sometimes rearrange code such that a volatile group is not updated properly, this at least helps remedy the situation (I think). * Some minor changes: For grammar reasons, dots have been removed from template names -- however, underscores and slashes are now available. Additionally, there's a much improved logic for which aspects of a local environment are overridden and preserved when a template is called from another. For both of these libraries, patches, comments, bug reports, requests, and of course contributions more than welcome! Regards, Sterl.

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type-level integers using type families
by Peter Gavin 04 Jun '08

04 Jun '08

Has anyone else tried implementing type-level integers using type families? I tried using a couple of other type level arithmetic libraries (including type-level on Hackage) and they felt a bit clumsy to use. I started looking at type families and realized I could pretty much build an entire Scheme-like language based on them. In short, I've got addition, subtraction, & multiplication working after just a days worth of hacking. I'm going to post the darcs archive sometime, sooner if anyone's interested. I really like the type-families based approach to this, it's a lot easier to understand, and you can think about things functionally instead of relationally. (Switching back and forth between Prolog-ish thinking and Haskell gets old quick.) Plus you can do type arithmetic directly in place, instead of using type classes everywhere. One thing that I'd like to be able to do is lazy unification on type instances, so that things like data True data False type family Cond x y z type instance Cond True y z = y type instance Cond False y z = z will work if the non-taken branch can't be unified with anything. Is this planned? Is it even feasible? I'm pretty sure it would be possible to implement a Lambda like this, but I'm not seeing it yet. Any ideas? Pete

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Implementing ParseChart with Data.Map
by Krasimir Angelov 03 Jun '08

03 Jun '08

Hi, I have to write ParseChart implementation with Data.Map/Set. The chart is type like this: type Chart k v = Map k (Set v) now I need operation like: insert :: k -> v -> Chart k v -> Maybe (Chart k v) where the result is (Just _) if the (k,v) is actually added to the chart or Nothing if it was already there and nothing have to be done. The straight forward implementation is: case Map.lookup k chart of Nothing -> Just (Map.insert k (Set.singleton v) chart) Just set | Set.member v set -> Nothing | otherwise -> Just (Map.insert k (Set.insert v set) chart) The problem with this is that both the Map and the Set are traversed twice. The first time from lookup/member and the second time from insert. Does someone have an idea how to do this with the current libraries? There are the Map.updateLookupWithKey and the Map.alter functions: updateLookupWithKey :: Ord k => (k -> a -> Maybe a) -> k -> Map k a -> (Maybe a,Map k a) alter :: Ord k => (Maybe a -> Maybe a) -> k -> Map k a -> Map k a which are the closest that I need. The problem is that the first doesn't allow the client function to be called if there isn't matching key in the map and the second is doesn't allow to return value from the client function. What I really need is an alterLookúp function: alterLookúp :: Ord k => (Maybe a -> (b,Maybe a)) -> k -> Map k a -> (b,Map k a) The chart manipulation is in the tight loop of my application so I need fast code. Any other ideas? Regards, Krasimir

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Rotating backdrop (aka learning Haskell)
by Yann Golanski 03 Jun '08

03 Jun '08

To help me learn Haskell, I decided on a simple (AH!) problem: given a list of images, display a random one as my desktop backdrop. After some time, change the image. Simple? What I actually want to do is a little more specific: Read a list of images (one per line) from a file. Given that a working day is about 8 hours, I want to see all the images during the day. So, the time between changes should be (nbr_of_images) / (8 * 60 * 60) seconds. Of course, if the file changes (I add or remove any number of images) this need to change and be recalculated. Clearly, I want some interaction with a pseudo-random number generator. Because this is a learning exercise, I want to have a pretty GUI for this. Three buttons: Exit (which quits the application), Reset (re-reads the file whether it changed or not) and Next (display the next image). Then I want a counter and a progress bar telling me when the next change will occur. 1- Get a list out of a file: I managed to do that using the following: parseImageFile :: FilePath -> IO [String] parseImageFile file = do inpStr <- readFile file return $ filter (/="") (breaks (=='\n') inpStr) Nice, simple and I understand what it is doing. 2- Get a random element from a list and remove it: Okay, this I understand less well. I looked at the solutions of problems 23 and 20 in http://www.haskell.org/haskellwiki/99_questions so there is a skeleton there. However, my list is IO [String].... Hum, monads. Any pointers as to how to do that? 3- Wait and do something later.... How, I have no idea how to do that! Help? 4- I guess that progress bars and updating text will be somewhere in the GUI (I chose wxHaskell)... Again, no idea where. 5- How do you call an external program in Haskell? Either xv or Esetroot will do the job of displaying the image. Is there a Haskell native way to do that? Once this is done and I have commented to the code, I will be happy to put it onto the wiki as a teaching aid. Thanks. -- yann(a)kierun.org -= H+ =- www.kierun.org PGP: 009D 7287 C4A7 FD4F 1680 06E4 F751 7006 9DE2 6318

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Mutually Recursive Modules
by Richard Giraud 03 Jun '08

03 Jun '08

Hello I'm using GHC 6.8.2 with mutally recursive modules. I'm familiar with how to do simple cases in GHC ({-# SOURCE #-} and .hs-boot files) but I can't figure out how to get it to work for a particular set of modules. Is it known (i.e., proven) that GHC 6.8.2 can compile any set of mutually recursive modules without refactoring? Are there known limitations? Thanks, Richard G.

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[Newbie question] -- Looping stdin until condition is met
by Martin Blais 03 Jun '08

03 Jun '08

Allright, this is a definitely a newbie question. I'm learning Haskell and going through the exercises in the beautiful Hutton book, and one of them requires for me to write a loop that queries a line from the user (stdin), looping until the user enters a valid integer (at least that's how I want to implement the interface to the exercise). I have tried tons of variants and modifications of code, and I can't find the way to implement this. Here is what my emacs buffer is at now:: import Text.Read import System.IO import qualified Control.Exception as C getNum :: IO Int getNum = do line <- (hGetLine stdin) x <- (C.catch (do return (read line :: Int)) (\e -> getNum)) return x main = do x <- getNum putStr ((show x) ++ "\n") Now, I've tried the Prelude's catch, the Control.Exception catch, I've tried moving it at the top of getnum, I've tried without catch, I've tried a ton of other shtuff, so much that I'm starting to think that Emacs is going to run out of electrons soon. I asked some half-newbie friends who are insanely enthousiastic about Haskell and they can't do it either (I'm starting to think that those enthousiastic friends are dating a beautiful girl with 3 PhDs, but she has a 2-inch thick green and gooey wart smack on her nose and they're so blindly in love that they can't admit that she does). I've asked some university profs and they sidetrack my question by saying I shouldn't do I/O so early. Can anyone here restore my faith in the Haskell section of humanity? 1. How do I catch the exception that is raised from "read"? 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) 3. Please do not tell me I should solve the problem differently. Here is the problem I'm trying to solve, and nothing else: "Write a program that reads a line from the user, looping the query until the line contains a valid integer." It shouldn't be too hard i think. The best answer would be a two-liner code example that'll make me feel even more stupid than I already do. Thanks in advance.

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cabal install fail for happs-hsp-template
by Thomas Hartman 03 Jun '08

03 Jun '08

Can cabal install be made to work for this package? ***** thartman@thartman-laptop:~>sudo cabal install happs-hsp-template [sudo] password for thartman: cabal: internal error: could not construct a valid install plan. The proposed (invalid) plan contained the following problems: Package Cabal is required by several packages, but they require inconsistent versions: package ghc-6.8.2 requires Cabal-1.2.3.0 package plugins-1.2 requires Cabal-1.5.2 thartman@thartman-laptop:~>cabal --version cabal-install version 0.4.7 using version 1.3.11 of the Cabal library thartman@thartman-laptop:~>ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 thartman@thartman-laptop:~>ghc-pkg list | grep -i ghc /usr/local/lib/ghc-6.8.2/package.conf: (ghc-6.8.2), haskell-src-1.0.1.1, haskell98-1.0.1.0, hpc-0.5.0.0,

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Vancouver Haskell users meeting
by Jon Strait 03 Jun '08

03 Jun '08

Anyone else here from Vancouver (Canada)? I thought it would be great to have a little informal get-together at a local cafe and share how we're currently using Haskell, or really anything (problems, comparisons, useful software tools, etc.) in relation to Haskell. I'm scheduling a meeting for this Thursday, June 5th. for 7PM at <http://www.waazubee.com/content/directions.php> Waazubee Cafe - http://www.waazubee.com/content/directions.php (at Commercial Dr. and 1st Ave.) They have wireless internet access. I'll get a table near the back, bring my laptop, and will have a copy of Hudak's SOE book (the front cover is impossible to miss) out on the table. If anyone wants to meet, but this Thursday is not a good day for you, let me know what days are better and we'll move the meeting. If anyone is sure that they will come this Thursday, you might let me know, so I can have an idea about the resistance in changing the day, if needed. Thanks, Jon

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modelling C in Haskell ..
by Galchin, Vasili 03 Jun '08

03 Jun '08

Hello, I don't want to write kludgy Haskell code! typedef struct blah { int val1; union { int val2; struct { int val3; int val4; } } } C_type; question: in Haskell, can I embed definition of the "union" inside of the C typedef, i.e. recursion definition? Or must I have a separate definition for the "union" which I "instantiate" inside the Haskell "typedef", i.e. Haskell "data"? Kind regards, Vasili

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Damnit, we need a CPAN.
by Achim Schneider 03 Jun '08

03 Jun '08

No, wait, there's cabal-install. And it doesn't even depend on 20+ packages I'd have to install manually. My day, for now, is saved, at least if portage installs it without hesitation. Ah, there it is: >>> Compiling source >>> in /var/tmp/portage/dev-haskell/cabal-darcs-0/work/cabal-darcs-0 ... * Bootstrapping Cabal... <no location info>: can't find file: Setup.lhs * * ERROR: dev-haskell/cabal-darcs-0 failed. Ok, let's try it the other way 'round...: % wget \ http://hackage.haskell.org/packages/archive/cabal-install/0.4.0/cabal-insta… % tar xvzf cabal-install-0.4.0.tar.gz % cd cabal-install-0.4.0 % runhaskell Setup.lhs configure --prefix=/home/ksf/cabal-inst --user Configuring cabal-install-0.4.0... % runhaskell Setup.lhs install Installing: /home/ksf/cabal-inst/bin Setup.lhs: dist/build/cabal/cabal: copyFile: does not exist (No such file or directory) % runhaskell Setup.lhs build Preprocessing executables for cabal-install-0.4.0... Building cabal-install-0.4.0... [...] % runhaskell Setup.lhs install Installing: /home/ksf/cabal-inst/bin % cabal update % cabal install phooey [...] type 'make' to build wxhaskell. type 'make install' to install wxhaskell. type 'make help' to receive help on all other make targets Configure Succeeded. Indeed. It succeeded. Now what? Building and installing by hand, it seems. Building works, installing too, but registering fails: the Makefile does not seem to like --user: % runhaskell Setup.lhs register --user -=-= Cabal executing: make register=-=- cat config/wxcore.pkg | sed -e "s|\${wxhlibdir}|/home/ksf/cabal-inst/lib|" | ghc-pkg update - Reading package info from stdin ... done. WARNING: unversioned dependencies are deprecated, and will NOT be accepted by GHC 6.10: base haskell98 Unable to rename "/usr/lib64/ghc-6.8.2/package.conf" to "/usr/lib64/ghc-6.8.2/package.conf.old" Saving old package config file... ghc-pkg.bin: /usr/lib64/ghc-6.8.2/package.conf: renameFile: permission denied (Permission denied) Just a second... # emerge wxhaskell -pv These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild UD] dev-lang/ghc-6.4.2 [6.8.2] USE="X%* doc opengl%* -binary -ghcbootstrap -openal% -test%(-bash-completion%)" 0 kB [0=>1] [ebuild N ] dev-haskell/wxhaskell-0.9.4 USE="doc" 0 kB In other words: No way. The only program that touches anything in /usr/ is portage. Well, at least ghc-pkg update works on a mangled wxcore.pkg. Rationale: We need a CPAN, a cabal that is smart enough to know what to to, even if building depends on make, grapefruit authors that commit to hackage, a cabal-inst that can install from darcs (or at least from a local directory), or maybe just a make-replacement for haskell, like java has with ant (which is really cool if you successfully do not notice the xml-syntax). I would already shut up if there were an ebuild generator, but then I'm not that egoistic ;) Anyway, three hours after wanting to try it out, grapefruit is up and running, and I did not even have to install its dependencies by hand. It seems like grapefruit could benefit from nestable cabal packages, though: It has one big script that installs many packages. You could also call them dependencies. I'm going to shut up now. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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Default definitions for associated type synonyms
by Dreixel 02 Jun '08

02 Jun '08

Hello, Does anyone know if it is possible to specify a default definition for an associated type synonym? When I tried: class A a where > type B a = a > GHC (version 6.9.20080309) told me: "Type declaration in a class must be a kind signature or synonym default". However, when I look at the parser I see no way to specify such synonym default. Is this possible? Thanks, Zé Pedro

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GHCi panic
by Andrew Coppin 02 Jun '08

02 Jun '08

I don't suppose this will surprise anybody greatly, but... Apparently if you write a Haskell module that is 400 KB in size and defines a single CAF consisting of a 45,000-element [String], GHCi panics when attempting to load it interpretted, and hits a stack overflow attempting to load it compiled. GHC also takes forever to compile it in the first place, and eventually spits out a 5 MB interface file later followed by a 16 MB object file. And attempting to compile a trivial module against it again causes a stack overflow. Presumably the designers of GHC just didn't expect anybody to try to do anything this weird? ;-) I was hoping that doing things this way round would be *more efficient*. But this is apparently not the case at all, so I'll just go back to reading the file at runtime instead... [Presumably if I was desparate I could convert the data into some kind of preinitialised C structure and manually link it in - if I was that determined.]

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Images and GUIs in Haskell
by Ronald Guida 02 Jun '08

02 Jun '08

Two questions: 1. In a Haskell program, if all I want to do is output an image, like a graph or chart, what is the simplest library to use? N.B. Simpler := easier to get minimal functionality. I really don't want to wade through a bunch of boilerplate or climb a steep learning curve just to be able to plot a few lines or circles. 2. Suppose I want interactivity. For example, I want to plot a line graph, and then let the user click and drag the data points. From what I understand about GUIs, I would need to track mouse buttons (up and down), mouse movements, and possibly keystrokes. (I know this is the complete opposite extreme from my first question) In this case, what would be the best (not necessarily simplest) library to use? What would you recommend?

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haskell compiler on NetBSD amd64
by Donn Cave 02 Jun '08

02 Jun '08

So here I am with 64 bit Athlon hardware, running amd64 NetBSD (a.k.a. x86_64), reasonably motivated to compile Haskell. From what I make of the porting instructions, my only hope is to install FreeBSD-amd64 on another partition and download the 6.6.1 ghc for that platform, use it to generate .hc files to build 6.6.1 on NetBSD, and then use 6.6.1 to build 6.8 or whatever the current version may be by the time I get there? -- Donn Cave <donn(a)avvanta.com>

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Ghc / cgi static linking
by Pieter Laeremans 02 Jun '08

02 Jun '08

Hello, I'm researching the use of Haskell to replace some perl scripts (in a web app). The app is deployed with a webhosting provider. CGI scripts can be executed => I can use Haskell. I've tried some hello world cgi scripts, compiled them on the same linux the hosting company uses and deployed them. It worked! But I would like to implement a search feature for the website. For Java/php/perl there 's lucene. For haskell there 's holumbus. Unfortunately, sqlite is a requirement for holumbus. It is not installed at the server of the hosting company. Is it possible to staticly link the sqlite3 library using ghc ? And are there other lucene like libs available like holumbus for haskell ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org>

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Re: [Haskell-cafe] hmatrix
by Thomas Hartman 01 Jun '08

01 Jun '08

what package do you install/import to get at Vector? 2008/5/31 Thomas Hartman <tphyahoo(a)gmail.com>: > what package do you install/import to get at Vector? > > 2008/5/31 Anatoly Yakovenko <aeyakovenko(a)gmail.com>: >> What is the most efficient way to update a position in a matrix or a >> vector? I came up with this: >> >> updateVector :: Vector Double -> Int -> Double -> Vector Double >> updateVector vec pos val = vec `add` v2 >> where >> v2 = fromList $ (replicate (pos) 0.0) ++ ((val - (vec @> >> pos)):(replicate ((dim vec)- pos - 1) 0.0)) >> >> but this seems pretty inefficient to me. >> >> thanks, >> Anatoly >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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hint / ghc api and reloading modules
by Evan Laforge 01 Jun '08

01 Jun '08

I'm using "hint", but since it's basically a thin wrapper around the GHC API, this is probably a GHC api question too. Maybe this should go to cvs-ghc? Let me know and I'll go subscribe over there. It's my impression from the documentation that I should be able to load a module interpreted, make changes to it, and then reload it. This is, after all what ghci does. It's also my impression that the other imported modules should be loaded as object files, if the .hi and .o exist, since this is also what ghci does. However, if I load a module and run code like so (using hint): GHC.loadModules ["Cmd.LanguageEnviron"] GHC.setTopLevelModules ["Cmd.LanguageEnviron"] GHC.setImports ["Prelude"] cmd_func <- GHC.interpret (mangle_code text) (GHC.as :: LangType) It works fine until I change LanguageEnviron. If I make a change to a function, I don't see my changes in the output, as if the session is only getting partially reset. If I insert a syntax error, then I do see it, so it is recompiling the file in some way. However, if I *rename* the function and call it with the new name, I get a GhcException: During interactive linking, GHCi couldn't find the following symbol: ... etc. So I examined the code in hint for loadModules and the code in ghci/InteractiveUI.hs:/loadModule, and they do look like they're doing basically the same things, except a call to rts_revertCAFs, which I called too just for good measure but it didn't help (I can't find its source anywhere, but the ghci docs imply it's optional, so I suspect it's a red herring). Here's a condensed summary of what hint is doing: -- reset GHC.setContext session [] [] GHC.setTargets session [] GHC.load session GHC.LoadAllTargets -- rts_revertCAFs -- load targets <- mapM (\f -> GHC.guessTarget f Nothing) fs GHC.setTargets session targets GHC.load session GHC.LoadAllTargets -- interpret let expr_typesig = "($expr) :: xyz" expr_val <- GHC.compileExpr session expr_typesig return (GHC.Exts.unsafeCorce# expr_val :: a) -- GHC.compileExpr maybe_stuff <- hscStmt hsc_env ("let __cmCompileExpr = "++expr) ([n],[hv]) <- (unsafeCoerce# hval) :: IO [HValue] return (Just hv) and then ghci does: -- load GHC.setTargets session [] GHC.load session LoadAllTargets targets <- io (mapM (uncurry GHC.guessTarget) files') GHC.setTargets session targets GHC.load session LoadAllTargets rts_revertCAFs putStrLn "Ok, modules loaded: $modules" -- interpret GHC.runStmt session stmt step -- GHC.runStmt Just (ids, hval) <- hscStmt hsc_env' expr coerce hval to (IO [HValue]) and run it carefully So it *looks* like I'm doing basically the same thing as ghci... except obviously I'm not because ghci reloads modules without any trouble. Before I go start trying to make hint even more identical to ghci, is there anything obviously wrong here that I'm doing?

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Parsec, updateState, and failure
by Dimitry Golubovsky 01 Jun '08

01 Jun '08

Hi, If a parser which updated user state fails, will such update be reverted? Suppose we have two parsers combined with <|> p = p1 <|> p2 p1 has the following: p1 = try $ do ... -- getting something from the input stream updateState (\st -> ...) -- updated state based on what gotten from the input x <- p3 -- p3 should see updated state and return something updateState (\st -> ...) -- updated state again (if p3 succeeded) return x If p3 fails, p1 fails too (second updateState will not be reached). But what will p2 (tried next) see in the user state? Will it be state after the first updateState, or will failure of p1 roll the update back? Is there any bracket- or try-catch-finally-like mechanism for Parsec? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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Upd: Parsec, updateState, and failure
by Dimitry Golubovsky 31 May '08

31 May '08

Hi, So far, I figured it out like this: updateState ... mbx <- (p3 >>= return . Just) <|> (return Nothing) updateState ... case mbx of Just x -> return x Nothing -> pzero but this seems a bit clumsy - is there a more elegant solution? ---------- Forwarded message ---------- From: Dimitry Golubovsky <golubovsky(a)gmail.com> Date: Sat, May 31, 2008 at 10:12 AM Subject: Parsec, updateState, and failure To: haskell <haskell-cafe(a)haskell.org> Hi, If a parser which updated user state fails, will such update be reverted? Suppose we have two parsers combined with <|> p = p1 <|> p2 p1 has the following: p1 = try $ do ... -- getting something from the input stream updateState (\st -> ...) -- updated state based on what gotten from the input x <- p3 -- p3 should see updated state and return something updateState (\st -> ...) -- updated state again (if p3 succeeded) return x If p3 fails, p1 fails too (second updateState will not be reached). But what will p2 (tried next) see in the user state? Will it be state after the first updateState, or will failure of p1 roll the update back? Is there any bracket- or try-catch-finally-like mechanism for Parsec? Thanks. -- Dimitry Golubovsky Anywhere on the Web -- Dimitry Golubovsky Anywhere on the Web

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is there some book about haskell and data struct and alg?
by smellcode 31 May '08

31 May '08

is there some book about haskell and data struct and alg? i mean data struct and algorithm in haskell i am freshman i want to study haskell with data struct and alg -- Hu Jinpu (nickname: smellcode) Web developer email =~ /(^hujinpu)\@(gmail)\.(com)$/ or email.gsub!(/hujinpu/, 'smellcode') http://hujinpu.net

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Re: [Haskell] Programme terminates silently
by Bertram Felgenhauer 31 May '08

31 May '08

(redirecting to haskell-cafe. the haskell mailing list is primarily for announcements) (sorry, Thomas, if you get this message twice) Thomas Bevan wrote: > I've written the programme below. > > The lircLoop should never terminate. Unfortunately it does. Worse, no error > messages are generated. > Not even the final line "Closing down" is printed. > > How is this possible? I couldn't find the Hmpf.Fork module you're using, so I couldn't test this. However, one possibility is that program receives a signal that is neither handled or ignored. This is the case for SIGPIPE [1]. You can strace your program to find out whether that's the case. System.Posix.Process provides facilities for installing your own signal handlers. HTH, Bertram [1] http://hackage.haskell.org/trac/ghc/ticket/1619

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Re: [Haskell-cafe] Data.Tree.Zipper in the standard libraries
by Krasimir Angelov 31 May '08

31 May '08

An updated Zipper version is uploaded here: http://www.haskell.org/haskellwiki/User_talk:Kr.angelov It doesn't use State monad anymore and it returns Maybe. This seems to be the common preference, is it? Feel free to vote against. Should we change Data.Map also? There is another proposal for changes in findMin/findMax so it is better to make this two breaking changes together rather than in a later release. It also allows to pass whole new subtree in insert{Left/Right/Down}. On Sat, May 24, 2008 at 3:41 AM, <ajb(a)spamcop.net> wrote: > G'day all. > > Quoting Don Stewart <dons(a)galois.com>: > >> This is Haskell, we should use Maybe. > > This is Haskell, more abstract is good. > > I do agree, though, that Monad is arguably the wrong abstraction. Something > like this would arguably be better: > > class (Functor f) => Fail f where > return :: a -> f a > fail :: String -> f a > > (And yes, the String is arguably important; Maybe doesn't give you that.) > > Cheers, > Andrew Bromage > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Fwd: Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
by Martin Blais 31 May '08

31 May '08

FYI. On Sat, 31 May 2008 02:11:13 +0200, "Daniel Fischer" <daniel.is.fischer(a)web.de> said: > Am Samstag, 31. Mai 2008 02:28 schrieb Martin Blais: > > Allright, this is a definitely a newbie question. > > > > I'm learning Haskell and going through the exercises in the > > beautiful Hutton book, and one of them requires for me to > > write a loop that queries a line from the user (stdin), > > looping until the user enters a valid integer (at least > > that's how I want to implement the interface to the > > exercise). I have tried tons of variants and modifications > > of code, and I can't find the way to implement this. Here is > > what my emacs buffer is at now:: > > > > import Text.Read > > import System.IO > > import qualified Control.Exception as C > > > > getNum :: IO Int > > getNum = do line <- (hGetLine stdin) > > x <- (C.catch (do return (read line :: Int)) (\e -> getNum)) > > return x > > All you need is a little strictness, > x <- (C.catch (return $! read line :: Int) (\e -> getNum)) > works. Another option is using evaluate instead of return. > The problem is that (read line :: Int) is not evaluated until it is > needed, > that is when it's going to be printed, but then it's too late to catch > the > exception. > > Some general remarks: > hGetLine stdin === getLine > do x <- action > return x > is the same as > action > > > > > main = do x <- getNum > > putStr ((show x) ++ "\n") > > print x > > > > Now, I've tried the Prelude's catch, the Control.Exception > > catch, I've tried moving it at the top of getnum, I've tried > > without catch, I've tried a ton of other shtuff, so much > > that I'm starting to think that Emacs is going to run out of > > electrons soon. I asked some half-newbie friends who are > > insanely enthousiastic about Haskell and they can't do it > > either (I'm starting to think that those enthousiastic > > friends are dating a beautiful girl with 3 PhDs, but she has > > a 2-inch thick green and gooey wart smack on her nose and > > they're so blindly in love that they can't admit that she > > does). I've asked some university profs and they sidetrack > > my question by saying I shouldn't do I/O so early. Can > > anyone here restore my faith in the Haskell section of > > humanity? > > > > 1. How do I catch the exception that is raised from "read"? > > By forcing the evaluation. > > > > 2. Where do I find the appropriate information I need in > > order to fix this? I'm probably just not searching in the > > right place. (Yes, I've seen the GHC docs, and it doesn't > > help, maybe I'm missing some background info.) > > Get used to lazy evaluation, the Hutton book should contain a chapter > about > that. > > > > > 3. Please do not tell me I should solve the problem > > differently. Here is the problem I'm trying to solve, and > > nothing else: > > > > "Write a program that reads a line from the user, > > looping the query until the line contains a valid > > integer." > > > > It shouldn't be too hard i think. The best answer would be a > > two-liner code example that'll make me feel even more stupid > > than I already do. > > > > Thanks in advance. > > >

1 0

I/O without monads, using an event loop
by Robin Green 31 May '08

31 May '08

I have been thinking about to what extent you could cleanly do I/O without explicit use of the I/O monad, and without uniqueness types (which are the main alternative to monads in pure functional programming, and are used in the Concurrent Clean programming language). Suppose you have a main event handler function, like this: eventMain :: (Event, SystemState AppState) -> (Command, SystemState AppState) This function could be called over and over in an event loop, until an EndProgram command was received, and the event loop would itself do all the actual I/O (the SystemStates are only in-memory representations of some part of the system state, plus the application's own state). Things like disk I/O could be done with commands which generate events when complete. Interprocess communication could be done in the same way. Then eventMain, and everything called by it, would be referentially-transparent, and yet non-monadic. You could of course build higher-level stuff on top of that. On the other hand, it's quite stateful, because anything you need to remember between events need to be recorded, either in the SystemState or externally (e.g. in a file). I suppose this is the most important disadvantage? Is there any published work or code using this approach, or something like it, in a pure functional language? I'm primarily interested in embedded system and desktop UIs, rather than say web-based systems, although both would be interesting. -- Robin

7 7

Lazy lists simulated by unboxed mutable arrays
by Henning Thielemann 31 May '08

31 May '08

I wonder whether the following idea has been investigated or implemented somewhere: We could simulate a list with strict elements, i.e. data StrictList a = Elem !a (StrictList a) | End by an unboxed array with a cursor to the next element to be evaluated and a function that generates the next element. Whenever an element with an index beyond the cursor is requested, sufficiently many new elements are written to the array and the cursor is advanced. This would still allow the nice tricks for recursive Fibonacci sequence definition. This will obviously save memory, but can we also expect that it is noticeably faster than (StrictList a) ?

6 18

test driving cabal install... cabal install and normal install (runghc Setup) don't mix... two package.conf files...
by Thomas Hartman 31 May '08

31 May '08

After a little drama with zlib, I managed to get cabal-install installed. I then attempted to do cabal install HAppS-Server since this is a module with a lot of dependencies, and in rapid development flux, so perenially painful for me to install. The result is that I managed to install everything up to HAppS-State, which I think is the last dependency, but then seemed to hang indefinitely in the middle of installing HAppS-Server at the end. OK, I thought, then perhaps I can do normal runghc Setup.hs after downloading and unzipping the tar from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Server-0.9… However, this resulted in error thartman@thartman-laptop:~/haskellInstalls/smallInstalls/HAppS-Server-0.9.2.1>runghc Setup.hs configure ... Setup.hs: At least the following dependencies are missing: HAppS-Data >=0.9.2... Strange, because I had just installed that module via cabal-install, and I could load it in ghci with :m +HappS.Data. I then ran ghc-pkg and got this strange result that my packages were broken into two different files. Is this by design? ghc-pkg list /usr/local/lib/ghc-6.8.2/package.conf: Cabal-1.2.3.0, Cabal-1.3.11, Cabal-1.5.2, DeepArrow-0.2, .... /home/thartman/.ghc/i386-linux-6.8.2/package.conf: HAppS-Data-0.9.2.1, HAppS-IxSet-0.9.2.1, HAppS-State-0.9.2.1, I am curious if anybody else is able to install HAppS-Server using cabal install, and whether they can shed any light on the other isuses I raised. Thomas.

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Re: [Haskell] Announce: hback - dual n-back memory game (0.0.2)
by Don Stewart 31 May '08

31 May '08

wojtowicz.norbert: > I'm pleased to publically announce a little project I've worked on. > Based on a recent research paper [0] that claims fluid intelligence > could be improved by training working memory, I wrote up a dual n-back > test memory game in Haskell and gtk2hs. This is still essentially an > alpha release: all comments are most welcome. > > Original post (includes gameplay instructions): > http://pithyless.com/blog/2008/05/18/hback-haskell-n-back-memory-game/ > > Updated version (overhaul of game with some feature updates): > http://pithyless.com/blog/2008/05/30/hback-0_0_2/ > > Cabal file can be found on hackage: > http://hackage.haskell.org/packages/archive/hback/0.0.2/hback-0.0.2.tar.gz > > [0] http://www.pnas.org/cgi/content/abstract/0801268105v1 Awesome! Is this the first real implementation of the game described in the paper? If so, that's a bit of a coup. Good work by the GTK team that has made these gui things in Haskell so easy. -- Don

1 0

Seen on HCAR: CoreErlang library
by Dimitry Golubovsky 31 May '08

31 May '08

Hi, I have just spotted the item 5.4.3 on the recently issued HCAR: CoreErlang is a Haskell library which consists of a parser and a pretty-printer for the intermediate language used by Erlang. There is no e-mail of the developer (Henrique Ferreiro Garcia) shown on the report, so I am posting this on the mailing list. What is the current status of this Core Erlang library? I might be interested to use some parts of it in the Haskell to Erlang converter that I am currently experimenting with. Please contact me at golubovsky at gmail dot com. Thank you. -- Dimitry Golubovsky Anywhere on the Web

1 0

Copying Arrays
by Tom Harper 30 May '08

30 May '08

I'm trying to implement some file I/O where I can read in a file to an array, but do so without having to know how much space I will need. (Before you suggest it, lists are too slow/space consuming.) I was thinking that one possible solution is to use a strategy used in dynamic arrays, where everytime you run out of space you double the capacity of the array. Trouble is, most arrays in Haskell would require you to traverse the entire array and copy each individual cell, which would make it worthless. Are there any low-level array types (MutableByteArray#, for example) that support a memcpy-like function where I could copy the entire array into the new one instead of copying it one value at a time? Is there another solution that I'm missing? -- Tom Harper MSc Computer Science '08 University of Oxford Mobile: +44 (0)7533 998 591 Skype: +1 949 273 4627 (harpertom)

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So how do people pronounce 'cabal' around here?
by Dan Piponi 30 May '08

30 May '08

In particular, which syllable gets the stress, and what are the lengths of the two vowels? Couldn't find anything in the FAQ (http://www.haskell.org/haskellwiki/Cabal/FAQ) -- Dan

6 9

zlib, missing zlib.h
by Thomas Hartman 30 May '08

30 May '08

Tried to install http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib which is required for http://darcs.haskell.org/cabal-branches/cabal-1.4 which is required for cabal-install. Got the following error. apt-get installed zlibc on a stab in the dark, but same result. advice? thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>runghc Setup.hs configure Configuring zlib-0.4.0.4... thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>runghc Setup.hs build &>out thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>head out Preprocessing library zlib-0.4.0.4... Stream.hsc:74:18: error: zlib.h: No such file or directory Stream.hsc: In function 'main': Stream.hsc:254:0: error: 'z_stream' undeclared (first use in this function) Stream.hsc:254:0: error: (Each undeclared identifier is reported only once thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>

5 4

ANNOUNCE: Haskell Communities and Activities Report (14th ed., May 2008)
by Janis Voigtlaender 30 May '08

30 May '08

On behalf of the many, many contributors, we are pleased to announce that the Haskell Communities and Activities Report (14th edition, May 2008) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. 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. We 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 2008: target deadline for contributions to the November 2008 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 November (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). Of course, 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. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Andres Loeh and Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

1 0

HDBC with SQL Server / OBDC
by Morten Holm Pedersen 30 May '08

30 May '08

am trying to do a simple DB connection from Haskell to a SQL Server 2005 (on Windows obviously). The DSN name ("Nylon") works from C++ but when running the below example (or any other I can think of) ghci crashes. Does anyone know a resolution for this or where the problem can possible be ? GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m Database.HDBC Prelude Database.HDBC> :m + Database.HDBC.ODBC Prelude Database.HDBC Database.HDBC.ODBC> do { conn <- connectODBC "DSN=Nylon"; xs <- getTables conn; putStr $ head xs; } Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package mtl-1.1.0.0 ... linking ... done. Loading package HDBC-1.1.4 ... linking ... done. Loading package HDBC-odbc-1.1.4.3 ... linking ... done. CRASH !!! Thanks Morten

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happs install seems to have become very slow since upgrading cabal Re: [Haskell-cafe] test driving cabal install...
by Thomas Hartman 30 May '08

30 May '08

sudo cabal install --global HAppS-State went fine, and ghc-pkg had output on just the one (global) file. but sudo cabal install --global HAppS-Server is slooooooow. I finally canceled this and did sudo runghc Setup.hs build -v3 with the same result: extremely, extremely slow compile. It seems to get hung up one one line: src/HAppS/Server/Facebook.hs:416:2: Warning: Defined but not used: `a' *** Desugar: Result size = 52971 *** Simplify: Result size = 34350 Result size = 33980 Result size = 33972 Result size = 33972 *** Specialise: Result size = 52989 *** Float out (not lambdas, not constants): Result size = 55269 ... and more and more of same. for 15 minutes now. With verbose output on at least I can see there is something going on behind the scenes, but still. I don't remember things taking this long to compile before I upgraded to cabal v1.4... cabal bug? cabal --version cabal-install version 0.4.7 using version 1.3.11 of the Cabal library thomas. 2008/5/29 Thomas Hartman <tphyahoo(a)gmail.com>: > I see. I had done > > sudo cabal install HAppS-Server > > but this just installed with root ownership in my home directory. > > I'm now trying with > > sudo cabal install --global HAppS-Server > > > 2008/5/29 Daniel Patterson <daniel(a)dbpatterson.com>: >> I think you need to install HAppS-Data etc globally... I'm not sure why >> locally installed (ie, only your user) packages wouldn't be picked up by runghc >> Setup.hs'ing, but if they are install system wide it should definitely work. >> >> I think it is cabal --global install foo >> >> On Thu, 29 May 2008 18:11:04 -0700 >> "Thomas Hartman" <tphyahoo(a)gmail.com> wrote: >> >>> After a little drama with zlib, I managed to get cabal-install installed. >>> >>> I then attempted to do >>> >>> cabal install HAppS-Server >>> >>> since this is a module with a lot of dependencies, and in rapid >>> development flux, so perenially painful for me to install. >>> >>> The result is that I managed to install everything up to HAppS-State, >>> which I think is the last dependency, but then seemed to hang >>> indefinitely in the middle of installing HAppS-Server at the end. >>> >>> OK, I thought, then perhaps I can do normal runghc Setup.hs after >>> downloading and unzipping the tar from >>> >>> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Server-0.9… >>> >>> However, this resulted in error >>> >>> thartman@thartman-laptop:~/haskellInstalls/smallInstalls/HAppS-Server-0.9.2.1>runghc >>> Setup.hs configure >>> ... >>> Setup.hs: At least the following dependencies are missing: >>> HAppS-Data >=0.9.2... >>> >>> Strange, because I had just installed that module via cabal-install, >>> and I could load it in ghci with :m +HappS.Data. >>> >>> I then ran ghc-pkg and got this strange result that my packages were >>> broken into two different files. Is this by design? >>> >>> ghc-pkg list >>> /usr/local/lib/ghc-6.8.2/package.conf: >>> Cabal-1.2.3.0, Cabal-1.3.11, Cabal-1.5.2, DeepArrow-0.2, >>> .... >>> /home/thartman/.ghc/i386-linux-6.8.2/package.conf: >>> HAppS-Data-0.9.2.1, HAppS-IxSet-0.9.2.1, HAppS-State-0.9.2.1, >>> >>> >>> I am curious if anybody else is able to install HAppS-Server using >>> cabal install, and whether they can shed any light on the other isuses >>> I raised. >>> >>> Thomas. >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

2 2

WebSense doesn't like darcs.haskell.org
by Chad Scherrer 30 May '08

30 May '08

My work uses WebSense to filter viewable pages - I don't have an option. I used to be able to get to darcs.haskell.org just fine, but apparently a /hacking directory was added somewhere, so WebSense put it on the naughty list. I put in a request for WebSense to review the site; hopefully it will soon be allowed again. But if not, I'll have to go through lots of red tape, which I'd really like to avoid. I assume/hope this "hacking" is only in the sense of making adjustments to code. Is this right? If there's anything involving DoS attacks, etc, I might never be able to get to it from work again. I get the impression WebSense is relatively widely used. Is anyone else here having similar trouble? Chad

5 7

reordering pure bindings in monads
by Tim Newsham 30 May '08

30 May '08

Intuitively it seems like the applicative expression: (++) <$> getLine <*> getLine should represent the same thing as the more traditional liftM2 expressions: do { x <- getLine; y <- getLine; return ((++) x y) } but it seems to me that you cant directly manipulate the first into the second. I get: do x2' <- getLine x1 <- return ((++) x2') x2 <- getLine return (x1 x2) the only way I can get from here to the liftM2 definition is if I treat "x1 <- return ((++) x2')" as "let x1 = (++) x2", and then allow it to be reordered after the second getLine. Then it is straightforward to reduce to the liftM2 expression above. It seems to me that this is a valid transformation if: - no side effects, including pattern match errors, can occur in the let (or x1 <- return ...). - movement doesnt change the dependencies - x1 isnt used between the original line and its new position - there are no new bindings for x2' introduced between the original line and the new line. Did I overlook anything? Do any haskell implementations allow rewrites like these to occur? Tim Newsham http://www.thenewsh.com/~newsham/

2 1

ambiguous constraint errors
by Evan Laforge 30 May '08

30 May '08

I have two related questions: #1 I'm getting some annoying type errors that I don't fully understand, and wind up having to do a workaround that I don't totally like. Here's a simplified version of my situation: data Ambi m = Ambi { ambi_monad :: m Int , ambi_int :: Int } some_ambi :: Monad m => Ambi m some_ambi = Ambi (return 5) 10 ambi_table :: Monad m => [(String, Ambi m)] ambi_table = [("default", some_ambi)] get_int :: String -> Maybe Int get_int sym = fmap ambi_int (lookup sym ambi_table) ----------- get_int produces: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `ambi_table' at ambi.hs:13:40-49 So I guess this means I'm not telling it which 'm', so it doesn't know how to resolve the 'return'... but the thing is, I'm not even using that value, so it doesn't matter what it resolves to. So it works if I pick some random monad: get_int sym = fmap ambi_int (lookup sym ambi_table :: Maybe (Ambi Maybe)) Note that I can't leave it as 'Monad m => Ambi m' because I still get an ambiguous type variable complaint. I'm a little disconcerted by having to pick some random dummy monad. Even worse, everything this type touches starts requiring explicit type declarations everywhere. Is there some easier way to do this? #2 This is somewhat related to another issue I've been having, which is that I have some kind of complicated type, e.g. '(SomeMonad some, Monad m) => some (SomethingM m Status)' that I use in a lot of places. It would be a lot less typing and easier to modify later if I wrote a type alias: type Command = (Monad some, Monad m) => some (State.StateT () m Status) but of course, this isn't allowed, since the type variables don't appear on the lhs, and if I put a context there, it's a syntax error. While I can write it with data: data (Monad some, Monad m) => Command some m = Command (some (State.StateT () m Status)) I've been told this doesn't mean what I expect it to, which is that the context constraints propagate up to and unify with the containing type (out of curiosity, since it's accepted, what *does* this do? I think I read it somewhere once, but now I forget and can't find it). And sure enough, using this type doesn't make my type declarations have the right contexts. So the first problem means that I have to declare types in various inconvenient places, and the second one means that I have to type out all the various class constraints (I can still alias away the non-polymorphic bits), and all my type declarations start looking much more complicated than they are. The "solution" I've been using for some of this is just to remove the polymorphism, so I can write a simple alias like type Command = SomethingM (State.StateT () Identity Status) and now I can think of "a command" and have various functions that take and return Commands, without caring that it's some kind of monad with context constraints. But of course, this isn't always possible since sometimes I need the type to remain polymorphic (i.e. while most of these I don't *think* will run in some other monad, some of them definitely get called in multiple contexts). Is there any nicer way around this? And what's the underlying issue that makes this necessary? I can live with all the context hair everywhere, but it sure would be nicer to be able to define it once and for all in one place.

4 5

Question on type synonym definition and language extensions
by Olivier Boudry 30 May '08

30 May '08

Hi all, I'm trying to define a type synonym for 2D MArray instances. The goal is to keep the function signature simple and readable using type `Matrix` instead of something like `(Ix i, MArray a e m) => m (a i e)`. After some "read, guess, try, error" cycles I came up with this: type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n) it requires option -XRank2Types to work. But then I can write my function as: test :: Matrix test = do { a <- newArray ((0,0),(5,8)) 0; writeArray a (0,0) 1; return a } Then I wanted to be able to give the Index and Value types in the type synonym but keep it flexible in terms of which MArray instance is used. I changed it to: type Matrix i n = forall m. forall a. (MArray a n m) => m (a (i,i) n) and the type signature of the test function becomes: test :: Matrix Int Double For this one I had to add an extra -XFlexibleContexts option to build it. It works and I'm quite happy with the look of my new function type signatures, but I'm just wondering if I'm doing something bad or if there is a cleaner/simpler way to define the same type synonym without requiring language extensions. What are the risks associated with using these two language extensions, non compatibility with other compilers or more? Thanks, Olivier.

3 4

Bug in parallel GHC runtime?
by Dusan Kolar 29 May '08

29 May '08

Hello all, The attached file was compiled by the following command: ghc -O2 --make -threaded ltest1pl.hs -o alall When run in a sequential mode, I get this result: ./alall Starting ... Lst1: 41666666650000 Lst2: 41669166700000 T1: 0m 1.0e-6s 36 End! On the other hand, when run in a threaded mode, I get the following error: ./alall +RTS -N2 Starting ... Lst1: 41666666650000 Lst2: 41669166700000 T1: 0m 0.0s Segmentation fault Is it fault of the GHC runtime, or is it something on my side? My machine: uname -a Linux pc 2.6.24-ARCH #1 SMP PREEMPT Sun Mar 30 10:50:22 CEST 2008 x86_64 Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz GenuineIntel GNU/Linux My ghc: ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 Thanks and regards Dusan --import Control.Concurrent --import Control.Concurrent.MVar import System.Time import Control.Parallel.Strategies --import Data.List (foldl') import qualified Data.ByteString as B sumAllSums [] = 0 sumAllSums l@(_:xs) = sumlist 0 l + sumAllSums xs where sumlist res [] = res sumlist sr (v:vs) = sumlist (sr+v) vs wlist2wbs [] = B.pack [] wlist2wbs l@(_:_) = B.pack $ encode l where encode :: Integral a => [Int] -> [a] encode [] = [] encode (x:xs) = if x==0 then 0:0:encode xs else fromIntegral (x `mod` 256) : fromIntegral (x `div` 256) : encode xs main = do putStrLn $ "Starting ..." let lst1 = [0..49999] let lst2 = [0..50000] let bs1 = wlist2wbs lst1 let bs2 = wlist2wbs lst2 tm1 <- getClockTime let (v1:v2:_) = parMap rnf sumAllSums [lst1,lst2] tm1' <- getClockTime putStrLn ("Lst1: " ++ show v1) putStrLn ("Lst2: " ++ show v2) let tdiff1 = diffClockTimes tm1' tm1 --let tdiff2 = diffClockTimes tm2' tm2 putStrLn $ "T1: " ++ show (tdMin tdiff1) ++ "m " ++ show (fromIntegral(tdSec tdiff1) + fromIntegral(tdPicosec tdiff1)/1e12) ++ "s" --putStrLn $ "T2: " ++ show (tdMin tdiff2) ++ "m " ++ show (fromIntegral(tdSec tdiff2) + fromIntegral(tdPicosec tdiff2)/1e12) ++ "s" putStrLn $ show $ {-ibs1 +-} B.index bs1 99999 + B.index bs2 49999 {-((bs1 + fromIntegral (B.index bs2 99999)) :: Integer)-} putStrLn $ "End!" {- main = do tm1 <- getClockTime putStrLn $ "Starting ... " mv1 <- newEmptyMVar mv2 <- newEmptyMVar t1 <- forkIO (putMVar mv1 $! sumAllSums [0..49999]) t2 <- forkIO (putMVar mv2 $! sumAllSums [1..50000]) v1 <- takeMVar mv1 v2 <- takeMVar mv2 killThread t1 killThread t2 putStrLn $ "Result: " ++ show (v1+v2) tm2 <- getClockTime let tdiff = diffClockTimes tm2 tm1 putStrLn $ "End! " ++ show (tdMin tdiff) ++ "m " ++ show (fromIntegral(tdSec tdiff) + fromIntegral(tdPicosec tdiff)/1e12) ++ "s" -}

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An alternative break
by Pieter Laeremans 29 May '08

29 May '08

Hello, I need a break function that splits the list one element further than the ordinary break. This is the simplest solution I could imagine: breakI :: (a -> Bool) -> [a] -> ([a], [a]) breakI p s = case break p s of ([], []) -> ([], []) (x, []) -> (x, []) (x, l) -> (x ++ [head l], tail l ) Is there a better way to write this ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org>

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Type Coercion
by PR Stanley 29 May '08

29 May '08

Hi (16 :: Float) is a perfectly legitimate statement although I'm surprised that it's allowed in a type strong language such as Haskell. It's a bit like casting in good old C. What's going on here? Paul

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Benchmarking Framework
by Tom Harper 29 May '08

29 May '08

I am in the process of writing a library for my MSc dissertation and would like to do some benchmarking. In doing so I need to compare the time and space of my library with some other code. Is there a framework for doing so in Haskell, aside from the Profiling tools in GHC? Basically I'm looking for something like QuickCheck, but that helps with generating repeatable tests to measure performance. Is there anything out there that anyone would recommend? -- Tom Harper MSc Computer Science '08 University of Oxford Mobile: +44 (0)7533 998 591 Skype: +1 949 273 4627 (harpertom)

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Examples of using Haskell for mathematics
by Brent Yorgey 28 May '08

28 May '08

Hi all! In a couple weeks I will be giving a short (15-min.) talk to an audience of mostly mathematicians, entitled "Executable Mathematics: A Whirlwind Introduction to Haskell". The idea will be to give a flavor of Haskell, its uniquenesses, and why it is a great language for playing around with mathematics, by way of some well-chosen examples. There are definitely plenty of such examples out there, and I've already found quite a few that I might use, but I thought I would send an email to the cafe to ask whether anyone has any code which you think particularly exemplifies some aspect of why Haskell is a great language for mathematics. I'm looking to include a wide range of examples, so any length (from a few to hundreds of lines of code) and any level (from simple number theory to things only a few people in the world understand) are fair game. Thanks in advance! And yes, I will definitely make the slides available after the talk. I don't know if I can promise a video, I kind of doubt the sessions will be videotaped. I guess we'll see. -Brent

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Re: static linking
by Peter Gammie 28 May '08

28 May '08

Hello, Further on my static linking woes: debian ~$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 -debian ~$ cat T.hs main :: IO () main = putStrLn "Hello world" -debian ~$ ghc -optl-static -static --make T.hs Linking T ... /usr/lib/gcc/i486-linux-gnu/4.2.3/../../../../lib/ librt.a(timer_create.o): In function `timer_create': (.text+0x107): undefined reference to `pthread_once' ... < references to missing pthread symbols > Does anyone have static linking working for GHC 6.8.2 and can inform me of the magic incantation? "Get the latest development version from darcs", perhaps? BTW when I said "move to the end", I meant "move to the end of -l flags". cheers peter On 27/05/2008, at 10:37 AM, Peter Gammie wrote: > Hello, > > I am having a bit of trouble static linking my program using GHC > 6.8.2. > > In brief: "rt" uses "pthread", but -lrt is the final thing on the > command line passed to the linker, and so the pthread symbols are > not resolved. Manually moving -lpthread to the end of the collect2 > command line does the trick. > > The question is how to do this using ghc. > > This is on Debian, using the 6.8.2-5 from unstable. > > I can provide the command lines if that is helpful. > > Note the same setup worked fine under GHC 6.6.1. > > cheers > peter

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Re: the Network.URI parser
by Neil Mitchell 28 May '08

28 May '08

Moving to haskell-cafe, as this almost certainly isn't a library issue. Hi > It most certainly is a security flaw. In the src of an img, yes, probably. In the href of a link, its a completely valid thing to do - and one that I've done loads of times. The URI is fine, its just the particular location that is dodgy. > whole pile of dodgy URIs. Most get culled (in my case) by the HaXml parser > and/or XHTML 1.0 Strict validation, and now I hope to eliminate the rest by > carefully handling the URIs. I don't think that's possible. A URI can validly have javascript, and can validly be a lot of things which are unsafe. > On that topic, does anyone have any good advice for handling these things? My advice is that you are targeting security at the wrong level. You shouldn't be cleaning the HTML to get a secure page, you should be having the level that interprets the HTML be secure regardless of the input. > If anyone knows of the state-of-the-art in this area, I'd appreciate a > pointer. > > http://htmlpurifier.org/live/smoketests/printDefinition.php > > doesn't seem to think the style attribute is unsafe. Have they not been > following the MySpace fiascos? Safety is a property of the HTML viewer, not of the HTML or CSS. Thanks Neil

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What's NaN?
by PR Stanley 28 May '08

28 May '08

Hello Why does sqrt (-16) return NaN? What is NaN? Thanks Paul

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re: a build problem
by Galchin, Vasili 28 May '08

28 May '08

Hi Duncan, In response to your below, I am running "runhaskell Setup.hs configure -v3 --prefix=$HOME". Is v3 the current highest Cabal highest verbosity level? FYI basically I am running ghc that I just built a couple of days ago from the ghc source distribution. Vasili (I. Galchin) ;^) On Tue, 2008-05-27 at 15:33 -0700, Thomas Hartman wrote: > I think dist gets populated when you do build. Actually we also stash the configuration in ./dist/setup-config so that gets created at configure time. Galchin's problem is that something goes wrong during the configure and so nothing gets created in dist. From the log it looks like ghc or ld are failing during one of the early tests we do when configuring ghc. To make debugging this kind of thing a bit less mysterious in future Cabal now logs in greater detail what happens when it calls external programs (when invoked at the highest verbosity level). Duncan

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Re: [Haskell-cafe] a build problem ....
by Thomas Hartman 28 May '08

28 May '08

I think dist gets populated when you do build. > > 2008/5/27 Galchin, Vasili <vigalchin(a)gmail.com>: >> when I do a "clean" followed by "configure" then dist is empty >> >> vasili >> >> On Tue, May 27, 2008 at 4:37 PM, Thomas Hartman <tphyahoo(a)gmail.com> wrote: >>> >>> maybe try >>> >>> runghc Setup.hs clean >>> >>> or fresh darcs checkout and try build again from scratch. it's helped >>> me in the past. >>> >>> thomas. >>> >>> >>> Am 26. Mai 2008 21:32 schrieb Galchin, Vasili <vigalchin(a)gmail.com>: >>> > Hello, >>> > >>> > One of my test cases will not build. I am running: >>> > >>> > vigalchin@ubuntu:~$ ghc -v >>> > Glasgow Haskell Compiler, Version 6.8.2, for Haskell 98, stage 2 booted >>> > by >>> > GHC version 6.8.2 >>> > Using package config file: /usr/local/lib/ghc-6.8.2/package.conf >>> > wired-in package base mapped to base-3.0.1.0 >>> > wired-in package rts mapped to rts-1.0 >>> > wired-in package haskell98 mapped to haskell98-1.0.1.0 >>> > wired-in package template-haskell mapped to template-haskell-2.2.0.0 >>> > wired-in package ndp not found. >>> > Hsc static flags: -static >>> > *** Deleting temp files: >>> > Deleting: >>> > *** Deleting temp dirs: >>> > Deleting: >>> > ghc-6.8.2: no input files >>> > Usage: For basic information, try the `--help' option. >>> > vigalchin@ubuntu:~$ >>> > >>> > Here is the build: >>> > >>> > vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell >>> > Setup.lhs configure --prefix=$HOME >>> > Configuring mlock-1.0... >>> > vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell >>> > Setup.lhs build >>> > Preprocessing executables for mlock-1.0... >>> > Building mlock-1.0... >>> > vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell >>> > Setup.lhs install >>> > Installing: /home/vigalchin/bin >>> > Setup.lhs: dist/build/mlock/mlock: copyFile: does not exist (No such >>> > file or >>> > directory) >>> > >>> > The install step fails. I guess "copyFile" is used to copy the >>> > executable to >>> > the install directory?? >>> > >>> > Regards, vasili >>> > >>> > >>> > >>> > >>> > _______________________________________________ >>> > Haskell-Cafe mailing list >>> > Haskell-Cafe(a)haskell.org >>> > http://www.haskell.org/mailman/listinfo/haskell-cafe >>> > >>> > >> >> >

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ANN: HSmugMug 0.1 - a haskell wrapper to the photo site smugmug's api
by Daniel Patterson 28 May '08

28 May '08

Released May 27th, 2008. I've been working for a little while on a haskell wrapper to the photo hosting site Smugmug's api. As of now, the library is pretty simple - it is 'read-only' - ie, you can't use it to upload to or create galleries, but I still think it is potentially useful to haskell/smugmug users. I created it primarily so I could integrate my photos at smugmug into my website, which you can see in the 'photos' section of my website - a very primitive but proof of concept-esque demonstration. I have deliberately not included much glue between the actual api and the use of it, because I think that is something that can more cleanly evolve out of use - if people find themselves repeating certain patterns when using the api, those would logically become part of the api wrapper. So, let me know what you think! Full release notes, brief tutorial, install instructions, etc, at http://dbpatterson.com/articles/10 . Feedback welcome! (this is the first thing longer than a 5 liner that I've written in haskell, so be gentle) Daniel

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a build problem ....
by Galchin, Vasili 28 May '08

28 May '08

Hello, One of my test cases will not build. I am running: vigalchin@ubuntu:~$ ghc -v Glasgow Haskell Compiler, Version 6.8.2, for Haskell 98, stage 2 booted by GHC version 6.8.2 Using package config file: /usr/local/lib/ghc-6.8.2/package.conf wired-in package base mapped to base-3.0.1.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0.1.0 wired-in package template-haskell mapped to template-haskell-2.2.0.0 wired-in package ndp not found. Hsc static flags: -static *** Deleting temp files: Deleting: *** Deleting temp dirs: Deleting: ghc-6.8.2: no input files Usage: For basic information, try the `--help' option. vigalchin@ubuntu:~$ Here is the build: vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs configure --prefix=$HOME Configuring mlock-1.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs build Preprocessing executables for mlock-1.0... Building mlock-1.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs install Installing: /home/vigalchin/bin Setup.lhs: dist/build/mlock/mlock: copyFile: does not exist (No such file or directory) The install step fails. I guess "copyFile" is used to copy the executable to the install directory?? Regards, vasili

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