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Ocaml for Haskellers tutorial
by José Iborra 17 Apr '10

17 Apr '10

As a Haskeller who is forced to switch to Ocaml by the job market (or at least by the job interview market), I could really use a "Ocaml for Haskellers" tutorial. I was wondering if there are others who have been in this situation recently and can share a few pointers, or better yet, write a blog entry pointing to good sources of information and highlighting the pitfalls. For instance, I have heard that Ocaml is only fast when one uses loops instead of folds, but I wonder if this is an overstatement. Of course due to the strict semantics of Ocaml many of the assumptions that come from laziness will no longer be valid, but I wonder how different is idiomatic Ocaml from idiomatic Haskell in real world. Thanks, pepe

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ANN: HaskellTorrent/Combinatorrent v0.2.0
by Jesper Louis Andersen 17 Apr '10

17 Apr '10

Goodbye HaskellTorrent, hello Combinatorrent! Due to the internationally acclaimed Boing!'er Shae Erisson, HaskellTorrent has been renamed into Combinatorrent. We thus present Combinatorrent v0.2, Easter eggs release. This release marks yet another milestone in getting a decent bittorrent client for Haskell. There are still some performance regressions to be eliminated, but overall it looks far better than in the v0.1.1 release. Also, the system leaks memory, albeit very slowly (or so sayeth the great heap profiler). This is the focus for the next version. We might also look into integrating the FAST extension, since it plugs a series of problematic spots in the original bittorrent protocol specification. If you want to hack on Combinatorrent, there are a lot of interesting things to do. I try, deliberately, to keep tasks around which can be used to gain foothold in the source code. Any level of skill is needed and I am happy to help a newbie if he or she gets stuck on something. The source code is at http://github.com/jlouis/combinatorrent should you want to tinker with it. Any kind of patch is welcome, be it fixes or improvements to the goat teleporter. So come help us avoiding success! Apart from numerous small optimizations and bug fixes, we can present the following big whats-new thingies (with an imaginary neon-red blinking what's new sign): NEW STUFF: * Directory Watching: Every 5 seconds, we wake up to check a directory for new torrents. A .torrent file dropped into the directory will subsequently be downloaded automatically. * Support multiple torrents at once: If given multiple torrents on the command-line the client will now download all of them. * Download rarest piece first: Use the psqueue package for getting access to a priority search queue and use it to maintain a histogram over rare pieces. Always try to get the rarest piece first. * Change all use of CML to STM: Overall, it looks like this change improved the code. There are still some need for cleanup after the bomb was thrown, but it does look like it will turn out positively. If CML put you off the track for hacking on Combinatorrent, you can now do it with STM! * Introduce RTS benchmarks: See http://jlouis.github.com/combinatorrent where we plot SVG-based sparklines for various key RTS parameters over time (works best in Opera or Chrome). A slow-changing regression has a much harder time hiding now. * Fix issue #3: A grave bug in the Piece Manager made the assertions fail when endgaming. Fix. -- J.

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ghc package problem
by Phyx 17 Apr '10

17 Apr '10

Hi all, I'm having a rather weird problem, Ghc-pkg list shows a package installed but doing a ghc -make will give an error saying that It can't find a module that's supposed to be in the installed package. And cabal install always reinstall all the dependencies on every change. It doesn't seem to see that they're already installed.

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GHC Api typechecking
by Phyx 17 Apr '10

17 Apr '10

Hi all, I was wondering if someone knows how to do the following: Im looking to typecheck a string using the GHC Api, where I run into problems is that I need to construct a Target, but the TargetId only seem to reference physical files. Ofcourse I can write the string to a file and typecheck that file, but I would like to do it all in memory and avoid IO if possible. Does anyone know if this is possible? For the record Im trying to create the target as follows createTarget :: String -> IO Target createTarget content = do clock <- getClockTime buffer <- stringToStringBuffer content return $ Target { targetId = TargetModule (mkModuleName "string:internal") ß problem , targetAllowObjCode = True , targetContents = Just (buffer,clock) } typeCheckStringOnly :: String -> IO (ApiResults Bool) typeCheckStringOnly contents = handleSourceError processErrors $ runGhc (Just libdir) $ do dflags <- getSessionDynFlags setSessionDynFlags dflags target <- liftIO $ createTarget contents addTarget target load LoadAllTargets let modName = mkModuleName "string:internal" ß problem again, dont know how to create the dependency graph then. graph <- depanal [modName] True (\a->setSession $ a { hsc_mod_graph = graph }) =<< getSession value <- fmap typecheckedSource (typeCheck modName) return $ ApiOk True Cheers, Phyx

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Re: [Haskell] Monads Terminology Question
by Conor McBride 17 Apr '10

17 Apr '10

Hi (Redirecting to cafe, for general chat.) On 12 Apr 2010, at 01:39, Mark Snyder wrote: > Hello, > > I'm wondering what the correct terminology is for the extra > functions that we define with monads. For instance, State has get > and put, Reader has ask and local, etc. Is there a good name for > these? Yes. Indeed, quite a lot of energy has been expended on the matter. It's worth checking out work by Plotkin and others on "Algebraic Effects" (often transmitted around Haskell-land by helpful citizens like Dan Piponi and Heinrich Apfelmus). This work distinguishes two kinds of "extra function": operations (e.g. get, put, ask, throwError, etc) and control operators (local, catchError, etc). *Operations* have types like s1 -> ... sn -> M t where the s's and t are thought of as "value" types, and M is yer monad. You can think of M as describing an "impure capability", permitting impure functions on values. You might even imagine specifying M's collection of operations by a signature, with this made up notation. sig M where f s1 ... sn :: t Note that I'm careful to mark with :: where the inputs stop and the outputs start, as higher-order functions make this ambiguous. For example sig State s where get :: s put s :: () sig Reader r where ask :: r sig Maybe where throw :: a Many popular monads can be characterized exactly by the signature of their operations and the equational theory those operations must obey (e.g. laws like put s >> get >>= f == put s >> f s). The point of these monads is to deliver the capability specified by the operations and equations. The similiarity between the signatures above and the typeclasses often declared to support monadic functionality is no coincidence. Note that every (set of) signature(s) induces a datatype of command-response trees whose nodes are labelled with a choice of operation and inputs, whose edges are labelled with outputs, and whose leaves carry return values. Such a tree represents a "client strategy" for interacting with a server which offers the capability, at each step selecting an operation to perform and explaining how to continue as a function of the value returned. The equational theory of the operations induces an equivalence on strategies. Command-response trees up to operational equivalence give the most general implementation of the specified monad: return makes leaves, >>= pastes trees together, and each operation creates a node. The monad comes from its operations! But what of local, catchError, and other such things? These are *control operators*, and they operate on "computations", with types often involving resembling a -> (b -> M c) -> M d Typically, the job of a control operator is to make local changes to the meaning of the operations in M's signature. A case in point is "local", whose job is to change the meaning of "ask". It's really shadowing one reader capability with another. Similarly, catchError can be thought of as offering a local exception. Old LISPheads (like me!) might think of operations as EXPRs and control operators as FEXPRs. Haskell does a neat job of hiding the distinction between the two, but it may be conceptually helpful to dig it out a bit. Control operators don't give rise to nodes in command-response trees; rather, they act as tree transformers, building new strategies from old. I could start a pantomime about why operations are heroes and control operators are villains, but I won't. But I will suggest that characterising monads in terms of the operations and/or control operators they support is a useful (and increasingly modular) way to manage effects in programming. After all, most end-user applications effectively equip a bunch of user-operations with a semantics in terms of system-operations. All the best Conor

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Weird behaviour with positional parameters in HDBC-mysql
by Martijn van Steenbergen 17 Apr '10

17 Apr '10

Dear café (CC John and Chris), I'm having some trouble with getting positional parameters in HDBC-mysql to work. Most of the time they work fine, but sometimes (and often enough that it's a serious bother) the parameters don't reach the server correctly. Let me first describe my setup: * Ubuntu 8.10 intrepid (inside VMWare Fusion on Mac OS X Snow Leopard) * mysql Ver 14.12 Distrib 5.0.67, for debian-linux-gnu (i486) using readline 5.2 * ghc 6.10.3 * HDBC-2.2.4 * HDBC-mysql-0.6.1 I have a database with a single, empty table: > CREATE TABLE `foo` ( > `id` int(11) NOT NULL auto_increment, > PRIMARY KEY (`id`) > ) ENGINE=MyISAM AUTO_INCREMENT=1031 DEFAULT CHARSET=latin1 And I have the following Haskell program: > import Database.HDBC > import Database.HDBC.MySQL > import Data.List > import Data.Traversable > import Control.Monad > > main :: IO () > main = do > conn <- connectMySQL defaultMySQLConnectInfo > { mysqlUnixSocket = "/var/run/mysqld/mysqld.sock" > , mysqlUser = "root" > , mysqlDatabase = "db" > } > for [1..1000] $ \n -> do > let sql = "select id from foo where id in (" > ++ intercalate "," (replicate (fromInteger n) "?") ++ ")" > stmt <- prepare conn sql > execute stmt (map SqlInteger [1..n]) > finish stmt > return () The program produces no output. All 1000 queries that are sent to the database are expected to return 0 results, since table foo is empty. This is all fine. The problem is in the queries that the server receives. If I turn on full logging, I expect all queries to have a "where id in (...)" where there is an increasing list of integers within the parentheses. But when examining the logs, some are small negative numbers, NULLs or just other numbers: > 506 Execute [980] select id from foo where id in > (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26, > 27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,824,825,826,827,828,829, > 830,831,832,833,834,835,836,837,838,839,840,841,842,843,844,845,846,847, > 848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865, > 866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883, > 884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901, > 902,903,904,905,906,907,908,0,0,34359738368,-5210566582833489576,NULL, > NULL,0,34359738368,-5210565758199768552,NULL,NULL,0,34359738368, > -5210564933566047528,NULL,NULL,0,34359738368,-5210564108932326504,NULL, > NULL,0,34359738368,-5210563284298605480,NULL,NULL,0,34359738368, > -5210562459664884456,NULL,NULL,158,159,160,161,162,(snip) The server then faithfully executes the statement and returns the results expected for the query it reports to have received, which (in a table with actual data) is not what the Haskell program was expecting. If I compile the program once, the results vary per run. If I don't use positional parameters but instead render the full SQL query myself, there is no problem. Is this a problem in HDBC or in HDBC-mysql? What's going on here? How do I fix or work around this? Many thanks, Martijn.

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Simple binary-protocol through network test
by Yves Parès 16 Apr '10

16 Apr '10

Hello, I'm trying to use the packages Network and Control.Monad.BinaryProtocol together, with a very simple program in which a client sends an operation to the server, which computes the result and sends it back. But the server holds when trying to receive (Server.hs, line 22), whereas the client has actually sent the data (Client.hs, line 17). The server stops to hold only when the client is killed with a Ctrl-C. This should be rather simple, and I can't get to know why it doesn't work. The files are here: http://old.nabble.com/file/p28157883/Client.hs Client.hs http://old.nabble.com/file/p28157883/Server.hs Server.hs http://old.nabble.com/file/p28157883/SharedData.hs SharedData.hs ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Simple-binary-protocol-through-network-test-tp2815788… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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statistics build error
by Keith Sheppard 16 Apr '10

16 Apr '10

Hello, I ran into this error while trying to install statistics. Does this indicate that I need to upgrade my GHC before I can install? kss@CATSKILL:~/projects/> cabal update Downloading the latest package list from hackage.haskell.org kss@CATSKILL:~/projects/> cabal install statistics Resolving dependencies... Downloading erf-1.0.0.0... Configuring erf-1.0.0.0... Preprocessing library erf-1.0.0.0... Building erf-1.0.0.0... [1 of 1] Compiling Data.Number.Erf ( src/Data/Number/Erf.hs, dist/build/Data/Number/Erf.o ) ar: creating archive dist/build/libHSerf-1.0.0.0.a Installing library in /Users/kss/.cabal/lib/erf-1.0.0.0/ghc-6.10.1 Registering erf-1.0.0.0... Reading package info from "dist/installed-pkg-config" ... done. Writing new package config file... done. Downloading primitive-0.2.1... Configuring primitive-0.2.1... Preprocessing library primitive-0.2.1... Building primitive-0.2.1... [1 of 7] Compiling Data.Primitive.MachDeps ( Data/Primitive/MachDeps.hs, dist/build/Data/Primitive/MachDeps.o ) [2 of 7] Compiling Control.Monad.Primitive ( Control/Monad/Primitive.hs, dist/build/Control/Monad/Primitive.o ) [3 of 7] Compiling Data.Primitive.Types ( Data/Primitive/Types.hs, dist/build/Data/Primitive/Types.o ) [4 of 7] Compiling Data.Primitive.Array ( Data/Primitive/Array.hs, dist/build/Data/Primitive/Array.o ) [5 of 7] Compiling Data.Primitive.ByteArray ( Data/Primitive/ByteArray.hs, dist/build/Data/Primitive/ByteArray.o ) Data/Primitive/ByteArray.hs:57:27: Not in scope: `newAlignedPinnedByteArray#' cabal: Error: some packages failed to install: mwc-random-0.5.0.0 depends on primitive-0.2.1 which failed to install. primitive-0.2.1 failed during the building phase. The exception was: exit: ExitFailure 1 statistics-0.5.1.0 depends on primitive-0.2.1 which failed to install. vector-0.5 depends on primitive-0.2.1 which failed to install. vector-algorithms-0.3.1 depends on primitive-0.2.1 which failed to install. Thanks, Keith -- keithsheppard.name

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Re: [Haskell-cafe] Floyd Warshall performance (again)
by John Lato 16 Apr '10

16 Apr '10

> From: Mathieu Boespflug <mboes(a)tweag.net> > > Dear haskell-cafe, > > I implemented the Floyd Warshall algorithm for finding the shortest > path in a dense graph in Haskell, but noted the performance was > extremely poor compared to C. Even using mutable unboxed arrays it was > running about 30 times slower. I rewrote the program several times, to > arrive at the following definition: > My results are basically the same as Max's, but if I change > #define SIZE 1500 to > sIZE = 1500 and all references from "SIZE" to "sIZE", something ... changes. A lot. MacBook-1:Desktop johnlato$ time ./FW Allocating ... Allocating ... done real 0m0.027s user 0m0.013s sys 0m0.014s I can't figure this out at all. Can anyone else confirm, or offer a good explanation? John

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Floyd Warshall performance (again)
by Mathieu Boespflug 16 Apr '10

16 Apr '10

Dear haskell-cafe, I implemented the Floyd Warshall algorithm for finding the shortest path in a dense graph in Haskell, but noted the performance was extremely poor compared to C. Even using mutable unboxed arrays it was running about 30 times slower. I rewrote the program several times, to arrive at the following definition: module Main where import Data.Array.Base import Data.Array.ST import Data.Array.Unboxed import Control.Monad.ST import Control.Monad import System.IO import GHC.Exts #define SIZE 1500 -- Actual graph is immaterial for this benchmark. graph = [] shortestPath :: [(Int, Int, Int)] -> UArray Int Int shortestPath g = runSTUArray $ do let mnew = newArray (0, SIZE * SIZE) 1 mread arr i j = unsafeRead arr (i * SIZE + j) mwrite arr i j x = unsafeWrite arr (i * SIZE + j) x unsafeIOToST $ hSetBuffering stdout LineBuffering unsafeIOToST $ putStrLn "Allocating ..." pm <- mnew unsafeIOToST $ putStrLn "Allocating ... done" let loop1 SIZE = return () loop1 k = let loop2 SIZE = return () loop2 i = let loop3 SIZE = return () loop3 j = do xij <- mread pm i j xik <- mread pm i k xkj <- mread pm k j mwrite pm i j (min xij (xik + xkj)) loop3 (j + 1) in loop3 0 >> loop2 (i + 1) in loop2 0 >> loop1 (k + 1) loop1 0 return pm main = shortestPath graph `seq` return () These 3 nested loops run a lot faster than generating a list of IO actions and then running them using sequence_. But still this program runs 3 times slower than it's C counterpart: $ time ./FloydWarshall Allocating ... Allocating ... done ./FloydWarshall 45,75s user 0,09s system 97% cpu 46,815 total $ time ./a.out ./a.out 14,92s user 0,03s system 98% cpu 15,230 total The source code for the C program is given below. Using the vector package instead of GHC's inbuilt ST arrays doesn't improve the situation any, neither does compiling via C. Any idea why the rather unidiomatic Haskell program is still 3 times slower, despite using unchecked indexing and the fact that the three nested tail recursive functions above shoud behave exactly as the 3 nested for loops in the C program? Best regards, Mathieu

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True Random Numbers
by Alex Rozenshteyn 16 Apr '10

16 Apr '10

Does haskell have a way of using /dev/random to generate random *things*? Currently I'm just reading the data into a byte string, converting it into bits, and keeping track of it in the state monad. -- Alex R

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Nomic game in Haskell
by Dupont Corentin 16 Apr '10

16 Apr '10

Hello Café, do you know Nomic? It's a fabulous and strange game where you have the right to change the rules in the middle of the game! In fact, changing the rules is the goal of the game. Changing a rule is considered as a move. Of course even that could be changed! www.nomic.net I'm wondering if it could be possible to implement a Nomic (or an helper for the game) in Haskell. Haskell seems appropriate for that, since functions are first order objects, and Haskell is good at DSLs. What I think, is creating a DSL where a rule could be expressed as a function, or a monad. Many kinds of rules could be expressed: 1. - a rule that says that player #1 must be named Grand Counselor. 2. - a rule that says that the following rules must be voted at the majority, and how. 3. - a rule that says that the money is the ECU. 4. - a rule that says that if a rule is accepted, the author is awarded 1 ECU. 5. - a rule that says that all rules must have a name. 6. - a rule that says that democracy is abolished. God save the King, Player #1! 7. - a rule that says that a rule mustn't suppress another. As you can see, the rules are highly self-referencing! some rules, like #1, describes a protocol, involving IO (players voting). some rules, like #7, could be implemented with a test over new rules (that rule could test proposed rules against rule suppression). some rules, like #5, must inspect the code of other rules. A proposed rule must pass all active rule's criterias to become active. Once active, a rule should be applyed immediatly to all active rules, and to all futur rules, until it is repealed. Any comments? Cheers, Corentin

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Haskell web dev - ASP .NET analogue?
by Christopher Done 16 Apr '10

16 Apr '10

The state of Haskell web development is exciting and messy. We don't have a de facto way to do it like Rails or Django or ASP .NET, but we do have many imitations and ideas popping up. You can easily judge whether the community is happy with the state of web development by the number of new web frameworks coming out all the time. I'm wondering if there is a ASP .NET analogue for Haskell. ASP .NET manages compiled languages like C# well by compiling a given page when you run it, and also allowing you to build pages into DLLs and then link them in your project. I have used this development process for a year in a commercial business and quite like it. It's just really easy to develop. Often you make little mistakes on a given page, so you just edit one bit of code -- you don't want to have to run a command to reload this code yourself. I think that, with Hint, one could write a Haskell equivalent. The nice thing is you can test your Haskell code in GHCi and then save the page and load it in the browser and have it auto-recompile. As to how pages are generated I guess is up to whoever is using it. You have a kind of standard separation in ASP .NET: backend.vb: Sub submit(sender As Object, e As EventArgs) lbl1.Text="Your name is " & txt1.Text End Sub frontend.asp: <form runat="server"> Enter your name: <asp:TextBox id="txt1" runat="server" /> <asp:Button OnClick="submit" Text="Submit" runat="server" /> <p><asp:Label id="lbl1" runat="server" /></p> </form> One could blatantly copy this model. And it's not a bad model. To start with anyway. It has the benefit that it's familiar to many web programmers, and, importantly, business practice/industry. My coworkers who only deal with HTML/CSS/JavaScript wouldn't care if backend.vb became backend.hs. I wrote an ecommerce site (http://productsforhair.co.uk/) a year ago in Haskell and used the Text.XHtml.Strict and hated it to be honest. It looks like a complete mess. Look at these two examples of using EDSLs to produce content. Messy and unmanageable. http://paste.lisp.org/display/97840 http://paste.lisp.org/system-server/show/lisppaste/web-server As much as I love EDSLs like html combinators, as soon as you get a nontrivial project with big complex elements it soon gets out of hand. BlazeHtml makes this a little nicer with its `do' syntax but it's still a problem.

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Automating injections: adding a type parameter
by Romulus 15 Apr '10

15 Apr '10

Hello everyone, It's my very first post to this mailing list, though i read it for several months. I'm very interested in Swiertra's automatic injection technique [1]. http://www.cse.chalmers.se/~wouter//Publications/DataTypesALaCarte.pdf<http://www.cse.chalmers.se/%7Ewouter//Publications/DataTypesALaCarte.pdf> http://www.cse.chalmers.se/~wouter//Misc/LaCarte.hs<http://www.cse.chalmers.se/%7Ewouter//Misc/LaCarte.hs> As a first exercise, i've tried to write an interpreter/transformer for propositional logic, in the spirit of [2]. http://www.haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf data LProp r = LProp String instance Functor LProp where fmap _ (LProp x) = LProp x data LAnd r = LAnd r r instance Functor LAnd where fmap f (LAnd x y) = LAnd (f x) (f y) Automatic injections are cool things to write helpers that work with any type for formulae extending LProp with coproducts: -- not very expressive, but it's not the point type ConjLogicF = LProp :+: LAnd type ConjLogic = Mu ConjLogicF lprop :: (LProp :<: f) => String -> Mu f lprop = \s -> inject (LProp s) land :: (LAnd :<: f) => Mu f -> Mu f -> Mu f land = \x y -> inject (LAnd x y) However, i'd like to parameterize the type of propositionnal variables for the logic, that is, not only strings, but a given type parameter p. For instance (that should make LProp' and LAnd' bifunctors i guess): data LProp' p r = LProp' p instance Functor (LProp' p) where fmap _ (LProp' x) = LProp' x data LAnd' p r = LAnd' r r instance Functor (LAnd' p) where fmap f (LAnd' x y) = LAnd' (f x) (f y) type ConjLogicF' p = (LProp' p) :+: (LAnd' p) type ConjLogic' p = Mu (ConjLogicF' p) lprop' :: ((LProp' p) :<: (f p)) => p -> Mu (f p) lprop' = \x -> inject (LProp' x) I'm stuck with the definition of the helper for LAnd'. I expect : land' :: ((LAnd' p) :<: (f p)) => Mu (f p) -> Mu (f p) -> Mu (f p) land' = \x y -> inject (LAnd' x y) ... but ghci 6.10.4 does not really like this definition... Could not deduce (LAnd' p :<: f p1) from the context (LAnd' p1 :<: f p1) arising from a use of `inject' at PropSample.hs:128:16-33 Possible fix: add (LAnd' p :<: f p1) to the context of the type signature for `land'' or add an instance declaration for (LAnd' p :<: f p1) In the expression: inject (LAnd' x y) In the expression: \ x y -> inject (LAnd' x y) In the definition of `land'': land' = \ x y -> inject (LAnd' x y) Failed, modules loaded: none. Does anybody have a clue for this problem ? I don't really understand where is the trouble actually. Cheers, PS: haskellers rulez ;) [1] Swierstra, W. Data types à la carte J. Funct. Program. Cambridge University Press, 2008, 18, 423-436 [2] Knowles, K. First-Order Logic à la Carte The Monad.Reader, 2008, issue 11 -- Romuald THION Docteur level 1.0 - Great master access control, +3 against half-dead "la vie, c'est comme un jeu mal foutu dont on ne connait pas les règles et où il n'y a pas de sauvegardes"

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ANN: haskell-src-exts 1.9.0
by Niklas Broberg 15 Apr '10

15 Apr '10

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.9.0! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts 1.8.2 --> 1.9.0 =============== * OptionPragma is renamed to the more descriptive ModulePragma, and adds a constructor AnnModulePragma for handling ANN pragmas preceding module header. * Add instances for Eq/Ord/Data/Typeable for Fixity. * Add 'parseFileWithComments' and 'parseFileContentsWithComments' to L.H.Exts . * More informative error messages when HSX tags are mismatched (thanks Gracjan Polak for the patch!) Cheers, /Niklas

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What is the consensus about -fwarn-unused-do-bind ?
by Ivan Lazar Miljenovic 15 Apr '10

15 Apr '10

As of 6.12.1, the new -fwarn-unused-do-bind warning is activated with -Wall. This is based off a bug report by Neil Mitchell: http://hackage.haskell.org/trac/ghc/ticket/3263 . However, does it make sense for this to be turned on with -Wall? For starters, why should this warning apply only to do blocks and not to explicit usage of >>, etc.? That is, the following code (as specified in the above bug report) generates an error: do doesFileExist "foo" return 1 yet this doesn't: doesFileExist "foo" >> return 1 If monadic code is going to return an error, then shouldn't _all_ monadic code do so, and not just those in do blocks? Secondly, a fair number of packages seem to be disabling this globally; the packages I know of (from mentions on mailing lists and grepping on /srv/code/*/*.cabal on code.haskell.org) that have the -fno-warn-unused-do-bind option being passed to GHC in their .cabal file include: * HsOpenCL * leksah-server * xmonad (including xmonad-contrib) * xmobar * pandoc My reason for bringing this up is that I'm soon about to release a new version of my graphviz library, and am debating what to do. Note that most of these errors are being caused by usage of a monadic-style of parsing (before anyone tells me I should be using an Applicative style instead, polyparse doesn't support Applicative, so I can't) and as such the "return value" is being evaluated anyway. The way I see it, I have 4 options: 1. Do as the error suggests and preface usage of these parser combinators with "_ <-". 2. Use some function of type "(Monad m) => m a -> m ()" instead of doing "_ <-". 3. Duplicate the parser combinators in question so that I have one version that returns a value and another that does the main parser and then returns (); then use this second combinator in do blocks where I don't care about the returned value. 4. Put "-fno-warn-unused-do-bind" in the .cabal file. The first two options don't appeal to me as being excessive usage of boilerplate; the third involves too much code duplication. However, I am loath to just go and disable a warning globally. What does the Haskell community think? Is -fwarn-unused-do-bind a worthwhile warning (and code should be updated so as not to cause it to find anything to warn about)? Or is it more of a hindrance to be disabled? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Can't login to GHC trac
by Erik de Castro Lopo 15 Apr '10

15 Apr '10

Hi all, I'm having trouble logging into GHC's Trac bug tracker at: http://hackage.haskell.org/trac/ghc/ I'm using Iceweasel 3.5.8 (Debian's rebranded Firefox) and I've also tried Galeon. The message I get is: Redirect Loop Redirection limit for this URL exceeded. Unable to load the requested page. This may be caused by cookies that are blocked. The browser has stopped trying to retrieve the requested item. The site is redirecting the request in a way that will never complete. * Have you disabled or blocked cookies required by this site? * NOTE: If accepting the site's cookies does not resolve the problem, it is likely a server configuration issue and not your computer. I do not have cookies disabled. I have hwoever tried deleting the cookies for hackage.haskell.org and trying again, but that didn't help either. Anyone else have the same problem? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

3 4

Can't install Criterion package on ghc ..
by Mozhgan kabiri 15 Apr '10

15 Apr '10

Hi, I am trying to install Criterion package, but I keep getting an error and I can't figure it out why it is like this !! mozhgan@Mozhgan-KCh:~$ cabal install Criterion Resolving dependencies... Configuring vector-algorithms-0.3... Preprocessing library vector-algorithms-0.3... Building vector-algorithms-0.3... [1 of 9] Compiling Data.Vector.Algorithms.Common ( Data/Vector/Algorithms/Common.hs, dist/build/Data/Vector/Algorithms/Common.o ) [2 of 9] Compiling Data.Vector.Algorithms.Search ( Data/Vector/Algorithms/Search.hs, dist/build/Data/Vector/Algorithms/Search.o ) [3 of 9] Compiling Data.Vector.Algorithms.Radix ( Data/Vector/Algorithms/Radix.hs, dist/build/Data/Vector/Algorithms/Radix.o ) [4 of 9] Compiling Data.Vector.Algorithms.Optimal ( Data/Vector/Algorithms/Optimal.hs, dist/build/Data/Vector/Algorithms/Optimal.o ) [5 of 9] Compiling Data.Vector.Algorithms.Insertion ( Data/Vector/Algorithms/Insertion.hs, dist/build/Data/Vector/Algorithms/Insertion.o ) [6 of 9] Compiling Data.Vector.Algorithms.Merge ( Data/Vector/Algorithms/Merge.hs, dist/build/Data/Vector/Algorithms/Merge.o ) [7 of 9] Compiling Data.Vector.Algorithms.TriHeap ( Data/Vector/Algorithms/TriHeap.hs, dist/build/Data/Vector/Algorithms/TriHeap.o ) [8 of 9] Compiling Data.Vector.Algorithms.Intro ( Data/Vector/Algorithms/Intro.hs, dist/build/Data/Vector/Algorithms/Intro.o ) ghc: panic! (the 'impossible' happened) (GHC version 6.10.4 for i386-unknown-linux): idInfo co{v a9WB} [tv] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug cabal: Error: some packages failed to install: criterion-0.5.0.0 depends on vector-algorithms-0.3 which failed to install. statistics-0.5.1.0 depends on vector-algorithms-0.3 which failed to install. vector-algorithms-0.3 failed during the building phase. The exception was: exit: ExitFailure 1 I tried to install Statistics and vector-algorithms package separately before installing the Criterion package itself .. but it wasn't successful. Thanks, Mozhgan

4 3

Simple game: a monad for each player
by Yves Parès 14 Apr '10

14 Apr '10

Hello Cafe, I have a question about program design. Let's say I have a simple sequential game (a TicTacToe for instance, but with more than 2 players). I have a Player datatype which is like: data Player m = Player { plName :: String, -- unique for each player plTurn :: GameGrid -> m Move -- called whenever the player must play } As you may guess, the 'm' type variable is intended to be a Monad. The goal is that every player has his own monad. For instance : - a human player needs to interact with the program, so its monad would be IO, or an instance of MonadIO. - a network player, which transparently sends the game state and receives moves through network, must also have access to IO to play. - an AI doesn't need to interact with the outside of the program, so its monad can be the one we want (e.g. Identity). First, do you think it is a good way to design the program? I want the game to be totally independent of the players who are currently playing. They can be humans, AIs, AIs and network players and humans, and so on. But when running the game, the program cannot "switch" from a player's monad to another. If we want every player to run in his own monad, I think we have to stack every players' monad, and lift their actions each time they have to play. This is not a problem, the sole condition now is that every player has type: (Monad m, MonadTrans m) => Player m. But I think it's a little bit of overkill, and it may be a little bit complicated to implement such a thing. What I try to avoid is having every player running in IO monad. Do you have any suggestion? ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Simple-game%3A-a-monad-for-each-player-tp28183930p281… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

8 13

problem getting cabal-install to work on CentOS 5.2
by Chris Dornan 14 Apr '10

14 Apr '10

Hi, I am trying to install cabal-install 0.8.2 with GHC 6.12.1 on a CentOS 5.2 system and am running up against a 'ExitFailure 127' error. I can download the package and run 'cabal install' inside the package directory and all will be fine, but if I let cabal download and install the package cabal runs into problems as soon as it tries to configure. For example, if I try 'cabal install permutation --verbose=3' it finishes like this: link: done *** Deleting temp files: Deleting: /tmp/ghc1324_0/ghc1324_0.lpp *** Deleting temp dirs: Deleting: /tmp/ghc1324_0 /tmp/permutation-0.4.132758/permutation-0.4.1/dist/setup/setup configure --verbose=3 --ghc --prefix=/home/chris/.cabal --user --constraint=base ==3.0.3.2 --constraint=ghc-prim ==0.2.0.0 cabal: Error: some packages failed to install: permutation-0.4.1 failed during the configure step. The exception was: ExitFailure 127 I have tried the same exercise with GHC 6.10.4 with cabal-install 0.8.0 the same results. I have downloaded the Cabal sources to try and add some instrumentation to find out what is going wrong I have yet to succeed. Does anybody have any suggestions? Is there an easier way of getting cabal-install working on CentOS? Any help much appreciated! Chris

4 5

ghc #defines
by gladstein＠gladstein.com 14 Apr '10

14 Apr '10

I may need to put some #ifdef conditionalizations into some cross-platform code. I'm wondering where I can find the definitions of symbols like __NHC__. I tried googling but I'm swamped by uses, no definitions that I can see.

3 2

a way to convert partial functions to functions with Maybe's
by Ozgur Akgun 14 Apr '10

14 Apr '10

Cafe, Is there a way (without going into the IO lands) to achieve this: data T = A | B | C func1 :: T -> T func1 A = B func1Fixed :: T -> Maybe T func1Fixed A = Just B func1Fixed _ = Nothing I want a function to generate func1Fixed, given func1. I guess this is doable with exception handling (catching the error from func1)? Is it the only way? Best, -- Ozgur Akgun

5 5

unboxed types in classes
by Phyx 14 Apr '10

14 Apr '10

Hi all, I was wondering if it's possible to have a class declaration that would except both unlifted types and lifted types as instances. For instance, I'm looking for something like class Foo a b where bar :: a -> b instance Foo Int Int where bar = id instance Foo Int# Int where bar = iBox Regards, Phyx

2 1

Cabal dependency hell
by Maciej Piechotka 14 Apr '10

14 Apr '10

While I love Haskell it's packaging system have some problems - especially with parsec. Currently I'm not able to install an practically anything using cabal due to version mismatches (or at least packages linking to both version of parsec). I found the following problems in various cabal packages (the examples are only examples - please do not be offended): - The constraints are too loose. It is written that package works with '>0.6 && <1' but in 0.8 the API has been changed (example of tagsoup & hxt. Fixed hxt versions depends on ghc 6.12... and ghc 6.10 if you change 2 lines in cabal file) - The constraints are too tight. It is written that package works with 'parsec <3' but it can run with 3.0 and 3.1 (a lot of packages) Additionally: - Monomorphism restriction & autodetection of types may cause that extending interface (such as changing signature from a -> IO a to MonadIO m => a -> m a) may break some programs - You cannot change the restrictions on already submitted programs. If I figure out that due to minor change my package does not work with parsec 3.1 I can push the new version with fix but there is no way to mark package on hackage as 'this version is not compatible with parsec >=3.1' I understand that it is not problem only with haskell/cabal but also with many other packaging systems - especially those language-specific. I understand the need of ability of changing API (and haskell-specific problem what constitutes as API change). However I hope that some improvements are possible. Best regards PS. I understand that content may be flame-gen. I am sorry in advance if such circumstances happen. However I believe that possible improvements in process are worth the risk.

13 21

GSoC: Hackage 2.0
by Matthew Gruen 13 Apr '10

13 Apr '10

Hi Haskellers, I'm Matt Gruen (Gracenotes in #haskell), and the Hackage 2.0 SoC project at <http://hackage.haskell.org/trac/summer-of-code/ticket/1587> really piqued my interest. It seems doable, in a summer, to make the new hackage-server more-than-deployment-ready as well as clearing out some items in the hackage bug tracker[0]; so, I've been working on a proposal. In this email I'd like to consolidate my mental notes for haskell-cafe and formulate a roadmap towards a more social Hackage. The most vital part is getting hackage-server <http://code.haskell.org/hackage-server/> to a state where it can be switched in place of hackage-scripts <http://darcs.haskell.org/hackage-scripts/>, and doing it properly, organizing the code so it can be extended painlessly in the future. Duncan Coutts, Antoine Latter, and others have done some great work on it in the past few years. I've been using Haskell for 1.33 of those years. I think I could become fluent in the codebases after a week of dedicated study, although even the hackage-server and hackage-scripts repositories don't include some components such as the current build report script. For putting the 2.0 in Hackage 2.0, any interface changes should help the library users and the library writers/uploaders without hurting either of them. Hackage should contain more of the right kind of information. Statistics help everyone, and they're a pretty good gauge on the direction of Hackage as a whole. Package popularity contents are one form of this. Reverse dependencies and even dependency graphs[1] are great, if I can integrate and expand Roel van Dijk's work[2]. There should also be some space on package pages, or on pages a link away from them, for users to contribute information and suggestions. Coders can explain why or why not the package met their needs, as a sort of informal bug/enhancement tracking service. Another helpful flavor of information is package relationships beyond dependencies: 'Deprecated in favor of Foo', 'a fork of Foo', or 'does blah instead of Foo's plugh functionality'. This will help people find the 'right' package among look-alikes. There could also be links to illustrative usage examples for getting started. Happstack's state management will be a good match for updating and serving this information in real time. There's also a need for a more interactive form of package documentation, but this should strengthen relationships with existing tools like Haddock and Cabal, not bypass the tools. For example, adding a changelog[3] or making Haddock's declaration-by-declaration commentary more wiki-like[4]. Changelogs seem to be within the scope of Hackage 2.0, integrating with Cabal; Haddock wikification might not be, perhaps deserving a separate student-summer session of its own. These can improve the package page and documentation subtrees. More generally, how can library users find the package they want? Categories themselves are great, but a tag system could identify and group specific package functionality. There could be sorting by ratings and reviews (4/5 lambdas!). Metadata searches, like those Sascha Böhme implemented in SoC 2007[5], could be integrated. It's not always obvious which ideas will help and which won't see good returns, which makes it all the more important to bring hackage-server to a state where future extensions can be easily written, submitted and deployed. That's the goal here. On the technical side, I realize I'd need to spend a not-insignificant amount of time on a user account system, dealing with authentication and related issues. One additional bit of functionality to manage is the hackage build system, which is used to ensure that packages build and to generate documentation. When building depends on FFI or OS-specific bindings, specific versions of other packages, compiler choice or compiler version choice, including language extensions, this is not trivial. One of two good routes is running cabal server-side to generate build reports and alleviate the miscompiling a smidge. Given that cabal install dependency calculation seems to be up in the air still, the current Setup-running script might end up staying for the moment, with some basic integrity checking (no cycles or other impossible dependency scenarios). The other not-mutually-exclusive approach is accepting failed build reports from users as a web service[6] to generate a matrix of the platforms that seem to encounter the most trouble. Ultimately, it's important that I, or whoever ends up doing the project, plans for it to benefit you guys. What do you think? —Matt Gruen PS: how do you feel about augmenting the .cabal format to provide more information for Hackage? [0] Active Tickets by Component, scroll down to HackageDB. http://hackage.haskell.org/trac/hackage/report/9?USER=anonymous&page=3 [1] Re: ANN: Reverse Dependencies in Hackage (demo). http://www.haskell.org/pipermail/haskell-cafe/2009-October/067847.html [2] Show reverse dependencies. Includes a patch for hackage-scripts. http://hackage.haskell.org/trac/hackage/ticket/576 [3] Add changelog feature to hackagedb. http://hackage.haskell.org/trac/hackage/ticket/299 [4] Reddit comment. The other comments are good too. http://www.reddit.com/r/haskell/comments/8bylw/ask_haskell_reddit_how_can_w… [5] Hackage Web Interface, Doc-Browser. http://code.google.com/soc/2007/haskell/appinfo.html?csaid=D505A03B8B64C3BF [6] cabal-install should report build results to hackage server. http://hackage.haskell.org/trac/hackage/ticket/184

12 20

ANNOUNCE: darcs 2.4.1
by Reinier Lamers 12 Apr '10

12 Apr '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4.1. This release fixes a number of issues that were found in darcs 2.4. The most important of these issues were that darcs did not work on Windows shares, and that darcs would not work if it was built with a wrong version of the mmap package. The easiest way to install darcs is using the Haskell Platform [1]. If you have installed the Haskell Platform or cabal-install, you can install this release by doing: $ cabal update $ cabal install --reinstall darcs Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.1.tar.gz and build it by hand as explained in the README file. What's New ---------- * Important changes in Darcs 2.4.1 * darcs works again on Windows network shares * missing documentation and test files have been added to the tarball * darcs will no longer give errors about a nonexistent file when compiled with the wrong mmap * moving files between directories can no longer break the directory index * darcs handles the case that someone uses 'remove -r' on an untracked directory * darcs ignores symlinks again, like all pre-2.4 releases * Bugs Fixed in Darcs 2.4.1 * 1645: ignore symlinks * 1750: uncover help text for 'darcs show pristine' * 1753: restrict mmap to version used by hashed-storage * 1754: fix issue458.sh on systems with xattrs * 1756: moving files between directories breaks index * 1757: add test files to distribution tarball * 1765: refuse to remove non-tracked directories recursively * 1767: send CRC erros to standard error Reporting bugs -------------- If you have an issue with darcs 2.4.1, you can report it via the web on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net. Kind Regards, the darcs release manager, Reinier Lamers [1]: You can download the Haskell platform from http://hackage.haskell.org/platform/

1 0

dependent types
by Andrew U. Frank 12 Apr '10

12 Apr '10

in modeling real application we have often the case that the type of some object depends on a value. e.g. small_boat is a vessel with size less than a constant. big_boat is a vessel with a larger size. for example, many legal classifications are expressed in this form (e.g. car vs. truck) depending on the type, operations are applicable or not (e.g. road with restriction 'no trucks'). how could one model such situations in haskell? i assume there are several possibilities...

6 8

ANN: vcd-0.0.0
by Tom Hawkins 12 Apr '10

12 Apr '10

I uploaded a small library [1] for generating VCD files [2], which can be viewed with waveform tools like GTKWave [3]. Though VCD is commonly associated with Verilog simulation, at Eaton we use it to visualize vehicle data in realtime: data is pulled off the CAN bus, formated to VCD, then piped into GTKWave. Most operations are in the IO monad, but if there's a better way to do it, I'm open to suggestions. I hope to add a simple VCD parser to the library soon; again suggestions welcome. -Tom [1] http://hackage.haskell.org/package/vcd [2] http://en.wikipedia.org/wiki/Value_change_dump [3] http://en.wikipedia.org/wiki/GTKWave

1 0

mingw32_HOST_OS and friends
by gladstein＠gladstein.com 12 Apr '10

12 Apr '10

1 0

Cabal update problem
by Maciej Podgurski 12 Apr '10

12 Apr '10

Hi, after updating to cabal-install-0.8.0/Cabal-1.8.0.2 with GHC 6.10.4, I always get an error when updating the package list: > cabal update Downloading the latest package list from hackage.haskell.org cabal: Codec.Compression.Zlib: premature end of compressed stream Is there a way to fix this without rebuilding from the latest darcs repo? Best wishes, Maciej

6 10

Re: [Haskell-cafe] Cabal, GHC and Preprocessors
by Ashley Yakeley 12 Apr '10

12 Apr '10

On Mon, 2010-04-12 at 06:51 +0100, Malcolm Wallace wrote: > Even without that, it may be possible to get what you want, using > -pgmPcpphs -optP-cpp -optP-ansi > that is, to override ghc's addition of -traditional with -ansi. > However I'm not sure exactly what order the preprocessor arguments > will reach cpphs - in a choice between -traditional and -ansi, it is > the last one on the cpphs commandline that will take effect. This worked in my .cabal file: cpp-options: --cpp -ansi ghc-options: -pgmPcpphs Thanks! -- Ashley Yakeley

1 0

Cabal, GHC and Preprocessors
by Ashley Yakeley 12 Apr '10

12 Apr '10

I want identifier concatenation in Haskell. For instance: #define CDecl(n) = class C_##n a where { f_##n :: a }; CDecl(1) CDecl(2) CDecl(3) (Actual motivator involves generating by type kind.) I have no trouble switching on CPP, but this doesn't work. The trouble is, GHC uses gcc for preprocessing, and it passes "-traditional" which switches this off. There doesn't seem to be a flag to override this. I've tried replacing the GHC preprocessor with cpphs using -pgmP, but GHC passes include files using "-include", while cpphs only accepts "--include". I've tried telling Cabal to use cpphs, but even if you rename the source file to .cpphs, it will still use GHC's gcc preprocessor rather than cpphs. In any case, it's not clear how to tell Cabal to pass --hashes to cpphs. I've tried using Template Haskell instead, but you can't easily splice identifiers, only expressions, types and top-level declarations. -- Ashley Yakeley

1 0

haskell gsoc proposal for richer numerical type classes and supporting algorithms
by Carter Schonwald 11 Apr '10

11 Apr '10

Hello All, I would like to know if there is enough community interest in following gsoc project proposal of mine for me to write up a proper haskell gsoc app for it . (and accordingly if there is a person who'd be up for having the mentoring role) Project: Alternate numerical prelude with a typeclass hierarchy that follows that found in abstract algebra, along with associated generic algorithms for various computations can be done on datastructures satisfying these type clases. In particular, the motivating idea is the following: yes it'd be useful (for more mathematically inclined haskellers) to have a numerical prelude whose hierarchy means something, but for even that subset of the haskell user population, such an alternate prelude is only useful if users get extra functionality out of that more detailed infrastructure (and I believe that some of the infrastructure that becomes feasible would be worthwhile to the larger haskell community). the basic plan is as follows 1) define a typeclass hierarchy that covers everything such as monoids - groups - rings - fields, and various points in between. After some experimenting, it has become pretty clear to me that all of these need to be defined indirectly with the help of template haskell so that the names of the various operators can be suitably parameterized (so that * and + etc aren't the only choices people have). This part itself isn't terribly difficult, though theres a lot of important intermediate algebraic structures that also need to be defined, and as I currently plan it, i'm very much inclined towards specifying all the required properties of each algebraic structure as testable properties, so that in a certain sense all these type clases could be interpreted as "adding facts" to an inference engine . This part is easy, just a lot of support type classes which a user wouldn't often encounter or deal with unless they're doing real math, in which case understanding them is probably key for correctly writing the desired code anyways. For those reading this who don't know the abstract algebra terminology: *a monoid* is a set with an operation "+" which has an identity element, an example would be lists and the append operation. *a group * is a monoid where the operation is invertible, it could be something as simple as positive rational numbers under multiplication or the various translations and rotations that one does to 3d objects such as when doing open gl programming where in this latter case the "*" or "+" operation is composition of these (invertible) transformations *a ring *will have both a "+", and a "*", and the "+" will be a group over the set of objects in the ring and the "*" a monoid, over the objects in the ring. a simple example would just be the integers with + and * as they usually are. Plus some rules about 0 and 1. *a field * would be a ring where all the nonzero elements are invertible with respect to multiplication (ie nonzero elements form a multiplicative group). A standard example would be rational numbers with the standard + and * and so forth for many other algebraic objects. 2) providing algorithmic machinery that makes this all worthwhile: in particular there are two veins of algorithmic machinery that would be goals of the gsoc project, one which is certainly feasible, and the other which i'm still working out the design details for but feel is very likely. respectively: a) generic algorithms for various standard algebraic computational problems, for example a generic euclidean algorithm that will work on any sort of data structure/object/algebraic thingy that satisfies all the necessary algebraic properties (such as multivariate polynomials!), with specialized versions for various cases where better algorithms are known and the nature of the representation can be exploited (eg binary gcd on the integers). This part of the project would essentially be me going through some computational number theory /algebra texts such as http://www.shoup.net/ntb/ (A Computational Introduction to Number Theory and Algebra) and implementing every interesting/useful algorithm both generic and specialized variants. As haskell's numerical typeclass hierarchy currently stands, it is impossible to implement generic versions of these algorithms in haskell, and I don't think any widely used language aside from haskell (except perhaps scala?) even has the right abstraction facilities to make it feasible. Of course certain specialized cases would even benefit perhaps by actually having them be in turn implemented in a more low level fashion (eg via some external c/c++ library), but that itself is not really important for the core project and would be beyond the scope of a single summers work. b) because the type classes would directly encode what manipulations and equational reasoning steps are valid, the same information could be used to safely do computer assisted algebra/mathematics. In particular, the various types which are instances of algebraic type classes (say Field Rationals) could also be indexed by a sort whose members are CAS and Value, so (Field (Rationals Value)), and depending on what the context expects, we either get a computation, or a lightweight way of lifting mathematical expressions into a higher order abstract syntax! Then it would be possible to do interesting things like symbolic differentiation of functions, eg polynomial expressions etc. For this latter deliverable, theres still a few questions and problems I'd need to work out (such as how names are handled and what sort of primitive operations should be used/exposed), but nothing that seems insurmountable to making it work, and in a fairly light weight way for the end user at that! any constructive feedback would be very appreciated, please note that while irrespective of gsoc summer funding considerations I'm planning on putting this together... the funding would mean that I wouldn't have a day job taking up a good chunk of my time/energy for this coming summer. Also, I do in fact have enough of an algorithms, pl, and math background to undertake this project, though I'll leave that info for the gsoc app if there proves to be an interest in the proposal. again, thanks for any feedback! -Carter Schonwald

9 13

libraries and executables in cabal
by Paul Brauner 11 Apr '10

11 Apr '10

Hi, i'm working on a project made of - lots of modules - one excutable importing these modules - another excutable importing these same modules I don't especially want to expose those modules as libraries, especially on hackage, since they are meaningless without the executables. But, if I declare two executables in my .cabal file, cabal will compile all those modules two times each time I want to compile the executables. Is there a way to tell cabal that those modules should be considered part of a "private" library ? Paul

3 2

developing against privately patched libraries, and cabal
by Dougal Stanton 11 Apr '10

11 Apr '10

If you're making local changes against a library you don't own (with the ultimate intention of sending those changes back upstream to the library maintainer) it makes sense change the version number to avoid clashes with the canonical version of the library. Of course, it's easy to lose track and end up publishing your own program against a non-existent (outside your hard disk) version of the library. I'd like to make it very obvious, both in mypogram.cabal and library.cabal that one is a patched copy and the other has to be compiled against a patched copy. Does cabal provide any way of marking a version private? I thought initially to just mark the version field in the patched library as X.y-dougal and enforce my program to compile against that, but it doesn't seem to recognise the -dougal suffix there. Thoughts? D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

5 6

Alternative dependencies in Cabal file
by Matthias Reisner 11 Apr '10

11 Apr '10

Hi, for a package I need to ensure the user uses a certain package configuration. So how would I rewrite the following pseudo-cabal description? Build-Depends: packageA < X, packageB < Y or packageA >= X && < X', packageB >= Y && < Y' or packageA >= X', packageB >= Y' Build-Depends: ... common dependencies ... where neither A nor B is the base package. Maybe I have to use if/else blocks, but I don't know what conditions to use then. Regards, Matthias

4 4

passing cpp options to c2hs from cabal
by Chris Casinghino 11 Apr '10

11 Apr '10

Hi all, I have a question about cpp, c2hs and cabal. The short version: What can I put in my cabal file to get "-cppopts=-U__BLOCKS__" passed as an argument in calls to c2hs? Longer story: I need to set up my cabal file so that c2hs gets this extra option to make things build smoothly on some macs. If I run cabal from the command line, I can pass it: cabal install --c2hs-options='--cppopts=-U__BLOCKS__' And this works great. I need to make my .cabal file do this, but c2hs-options doesn't seem to be accepted there. I tried: cpp-options: "-U__BLOCKS__" But if I run cabal install -v I can see this isn't being passed on to c2hs: ... /home/ccasin/.cabal/bin/c2hs --include=dist/build --cppopts=-D__GLASGOW_HASKELL__=610 --cppopts=-Icontrib/libpuz/include --output-dir=dist/build --output=Codec/Game/Puz/Internal.hs ./Codec/Game/Puz/Internal.chs ... Thanks! --Chris Casinghino

3 3

Difficulties installing LLVM bindings
by Aran Donohue 11 Apr '10

11 Apr '10

Hi Haskell-Cafe, I can't get the LLVM bindings for Haskell to install. Does anyone know what I might need to do? Has anyone seen this error before? Here's the problem: (Installing from latest darcs source) llvm-haskell aran$ cabal install Resolving dependencies... ...snip... checking for unistd.h... yes checking llvm-c/Core.h usability... yes checking llvm-c/Core.h presence... yes checking for llvm-c/Core.h... yes checking for LLVMModuleCreateWithName in -lLLVMCore... no configure: error: could not find LLVM C bindings cabal: Error: some packages failed to install: llvm-0.7.0.1 failed during the configure step. The exception was: exit: ExitFailure 1 I've got the latest LLVM (from source) installed using the default "sudo make install", i.e. into /usr/local. Using cabal install --configure-option --with-llvm-prefix=/usr/local doesn't change the result. LLVM is indeed based in /usr/local. The binaries are in /usr/local/bin/, libLLVM*.a is in /usr/local/lib, etc. Google just shows that this error has cropped up a couple times before. I tried manually disabling the check in the configure script but then generating code fails with massive run-time errors that look like link problems. I'm on an up-to-date Snow Leopard, using the latest llvm bindings from darcs, cabal-install 0.8.0, cabal library 1.8.0.2, ghc 6.12.1, and LLVM from svn. LLVM works fine from C++, at least. I worked through the Kaleidoscope tutorial and a few hand-coded .ll files without a hitch. Have you seen anything like this before? Any tips or things to try? I can't think of what magic setting I'm missing. Thanks, Aran

4 7

Multiple versions of a cabal package Or what's the right way to update?
by david fries 11 Apr '10

11 Apr '10

Dear Caféistas I desperately need your collective knowledge I have been working on porting the haskell-platform to the FreeBSD operating system for a while now and some versioning issues have come up. FreeBSD is still running on GHC-6.10.4 (6.12 is in the works). The ghc package contains a couple of libraries that ghc is built against and requires to run. For instance, network-2.2.1.2 is one of them. The problem is that the Haskell Platform requires version 2.2.1.4. Under normal circumstances we would port network-2.2.1.4 to FreeBSD and then specify it as a dependency of the Platform port. But then we end up with two (exposed) versions of network installed on the system. For example, let's assume that I have the following Haskell source (Networking.hs): module Networking where import Data.ByteString.Lazy import Data.Maybe import Network.HTTP import Network.Stream import Network.URI getBinary :: String -> IO ByteString getBinary url = do uri <- return $ fromJust $ parseURI url rsp <- simpleHTTP $ request uri getResponseBody rsp where request uri = Request { rqURI = uri , rqMethod = GET , rqHeaders = [] , rqBody = empty } When I try to compile it (`ghc --make Networking.hs`) I get the following: Networking.hs:12:30: Couldn't match expected type `network-2.2.1.2:Network.URI.URI' against inferred type `URI' In the first argument of `request', namely `uri' In the second argument of `($)', namely `request uri' In a stmt of a 'do' expression: rsp <- simpleHTTP $ request uri However, the following thing could help: $ ghc --make Networking.hs -package network-2.2.1.2 The workaround ain't acceptable to the FreeBSD committers, so I have to come up with something better... In a perfect world, I'd be able to install the required HP dependencies without tripping ghc up. Then we could switch over to the platform as a foundation of GHC-6.12. I have lots of questions. Is it smart to have multiple installed versions of the same library? Up until now, we could avoid such situations. What would be the correct approach to updating network? I guess it would involve rebuilding all installed packages that had previously been compiled with ghc? Though I hope not - our package management system could pull that off if necessary. If it is perhaps a bad idea to update certain "core" libraries, is there a way to declare them as such in the cabal file? regards and TIA, dave

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Cabal-install
by Andrew Coppin 11 Apr '10

11 Apr '10

OK, so apparently my Google skills are lacking. I can't find anything but the most terse documentation of the cabal-install tool online. Somewhere there surely must be something which explains how to control this thing. Anyway, can anybody tell me how I can change the default settings so that I get profiling libraries built by default, and Haddock documentation built by default? (I'm on Windows, in case that makes a difference...)

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GSoC proposal: Extend Cabal Preprocessors.
by Diego Echeverri 11 Apr '10

11 Apr '10

Hi! I finish writing my proposal (maybe a bit too late). I would be glad to read any feedback. http://socghop.appspot.com/gsoc/student_proposal/show/google/gsoc2010/diego… Best Regards, Diego Echeverri

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Dutch HUG Day: Program and Call for Participation
by Sean Leather 11 Apr '10

11 Apr '10

[Apologies for multiple copies.] ------------------------------------------- Dutch HUG Day 24 April 2010 Ordina office Niewegein, The Netherlands ------------------------------------------- *** *** Please register! *** *** Email Sean Leather <leather(a)cs.uu.nl> by 20 April! *** INTRODUCTION To celebrate our first anniversary, the Dutch Haskell Users Group (HUG) invites Haskell and functional programming enthusiasts in the Netherlands and surrounding area to an afternoon symposium. The Dutch HUG is an informal group with monthly meetings including talks on novel applications of Haskell and socializing in the neighborhood pub. The group was founded on Sunday, April 19, 2009 at the Haskell Hackathon hosted in Utrecht. After a successful year, we renew the trend with the Dutch HUG Day. INVITATION *** Please send an email to Sean Leather <leather(a)cs.uu.nl> by Tuesday, 20 April! *** Join us for a half day of fun and functional programming! We have some great "praatjes" (the Dutch word for talks) lined up. (Just in case you were confused by my "Call for Praatjes," all talks will be in English.) Along with talks, you can join us afterwards for a buffet dinner, graciously provided by our sponsor Ordina. The registration will allow Ordina to know how many people to expect and to satisfy their security concerns. They are also looking for recruiting options and will send attendees an email describing potential opportunities. PROGRAM We are happy to announce a preliminary program. Note that the particular titles or order of talks may change, but the general schedule should remain the same. 13:00 Reception 13:30 Welcome 13:45 Lightweight Program Inversion -- Janis Voigtländer (University of Bonn) 14:00 Functional Programming in the Industry? -- Stef Joosten (Ordina, Open University of the Netherlands) 14:30 BlazeHtml: Design of a Blazingly Fast HTML Combinator Library -- Jasper van der Jeugt (Ghent University) 15:00 Functional Programming at typLAB -- Erik Hesselink (typLAB) 15:30 Break 16:00 Cλash: Haskell-ish Hardware Descriptions -- Matthijs Kooijman (formerly University of Twente) 16:30 Why Haskell Does Not Matter -- Stefan Holdermans (Vector Fabrics) 17:00 Lightweight Monadic Regions -- Bas van Dijk (Radboud University Nijmegen, Sensor Sense) 17:30 Sirenial: Type-safe SQL Wrapper -- Martijn van Steenbergen (Utrecht University) 17:40 Discussion 18:00 Dinner 19:00 Depart 19:30 Drinks LOCATION The event will be held at the Ordina office in Niewegein, a town not far south of the city of Utrecht. You can get there by train to Utrecht Centraal and tram (the Utrecht "sneltram") to P+R Westraven. Details on the location can be found at the following link, and we will put more information on our website soon. * http://www.ordina.com/Vestigingen/Nieuwegein.aspx SPONSOR We are grateful to Ordina for allowing us to host this event on their premises. They are also kindly providing drinks during the breaks and dinner afterwards for all participants. You should come and hear from our host how they make use of functional programming. * http://www.ordina.com/ in English * http://www.ordina.nl/ in het Nederlands ORGANIZERS * Sean Leather * Chris Eidhof MORE INFORMATION * Dutch HUG: http://dutchhug.nl/ * Dutch HUG Day: http://dutchhug.nl/DutchHugDay/

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Re: [Haskell-cafe] haskell gsoc proposal for richer numerical type classes and supporting algorithms
by Paul McJones 11 Apr '10

11 Apr '10

Dear Carter, Although I'm not an active Haskell programmer, I'd like to add my support for you to write up your GSOC application. In the first five chapters of the book /Elements of Programming/ (Addison-Wesley, 2009), my coauthor Alex Stepanov and I undertook a somewhat similar effort, only using C++ instead of Haskell. We started with semigroups and monoids, and ended with rings and modules and a careful treatment of GCD. C++ does not have a direct analog of the Haskell type class, so we developed our own informal conventions for documenting the definitions of concepts and the corresponding type requirements in function templates and class templates. The concept definitions extracted from the book are available here: http://www.elementsofprogramming.com/eop-concepts.pdf and the source code here: http://www.elementsofprogramming.com/code.html I subsequently translated the concepts and functions from the first (functional) half of our book into Haskell. It was a useful learning exercise for me (giving me an appreciation for Edward Kmett's comments on your proposal), and has led to fruitful discussions with Henning Thielemann (see the Haskell Numerical Prelude and http://www.haskell.org/haskellwiki/Mathematical_prelude_discussion) and Jacques Carette (MathScheme, etc.). Paul McJones

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Length of lists within list of lists?
by boblettoj 10 Apr '10

10 Apr '10

Hi i want to know if there is any method i can use to count the number of items in each list in a list of lists, eg. someMethod [[a, b, c], [a], [b, d, e, f]] would return [3, 1, 4] is there anything that can do this? thanks -- View this message in context: http://old.nabble.com/Length-of-lists-within-list-of-lists--tp28203363p2820… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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persist and retrieve of IO type?
by Daryoush Mehrtash 10 Apr '10

10 Apr '10

Is there a way to persist a [IO ()] to say a file then retrieve it later and execute it using a sequence function? Thanks, Daryoush

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unlifted types in classes
by Phyx 10 Apr '10

10 Apr '10

Hi all, I was wondering if it's possible to have a class declaration that would except both unlifted types and lifted types as instances. For instance, I'm looking for something like class Foo a b where bar :: a -> b instance Foo Int Int where bar = id instance Foo Int# Int where bar = iBox Regards, Phyx

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Pictures from Tucson
by Tom Hawkins 10 Apr '10

10 Apr '10

http://tomahawkins.org/Pictures/2010

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Hackage accounts and real names
by David House 10 Apr '10

10 Apr '10

Hi, An issue came up on #haskell recently with Hackage accounts requiring real names. The person in question (who didn't send this email as he's wishing to remain anonymous) applied for a Hackage account and was turned down, as he refused to offer his real name for the username. Those of us in the conversation thought this a bit of an odd policy, and were wondering where this came from. It also emerged that a couple of other people had been held back from getting Hackage accounts because of this reason. Seeing as it's also trivially easy to fake a name, what's the purpose of this restriction? Thanks, -David

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ANN: hledger 0.9 released
by Simon Michael 10 Apr '10

10 Apr '10

I'm pleased to announce a new hledger release, with many bugfixes and small improvements, GHC 6.12 support, and a separate library package to make building (h)ledger-compatible tools easier. Thanks to Oliver Braun and Gwern Branwen for code contributions this release. Just in time for tax filing! :) Patches, feedback, contributions welcome. Best, -Simon home: http://hledger.org Release notes: 2010/04/10 hledger 0.9 ...................... * ghc 6.12 support * split off hledger-lib package, containing core types & utils * parsing: ignore D, C, N, tag, end tag directives; we should now accept any ledger 2.6 file * parsing: allow numbers in commodities if double-quoted, like ledger * parsing: allow transactions with empty descriptions * parsing: show a better error for illegal month/day numbers in dates * parsing: don't ignore trailing junk in a smart date, eg in web add form * parsing: don't ignore unparsed text following an amount * parsing: @ was being treated as a currency symbol * add: fix precision handling in default amounts (#19) * add: elide last amount in added transactions * convert: keep original description by default, allow backreferences in replace pattern * convert: basic csv file checking, warn instead of dying when it looks wrong * convert: allow blank/comment lines at end of rules file * print: always show zero amounts as 0, hiding any commodity/ decimal places/price, like ledger * register: fix bad layout with years < 1000 * register: fix a Prelude.head error with reporting interval, -- empty, and --depth * register: fix a regression, register should not show posting comments * register: with --empty, intervals should continue to ends of the specified period * stats: better output when last transaction is in the future * stats: show commodity symbols, account tree depth, reorder slightly * web: -fweb now builds with simpleserver; to get happstack, use - fwebhappstack instead * web: pre-fill the add form with today's date * web: help links, better search form wording * web: show a proper error for a bad date in add form (#17) * web: fix for unicode search form values * web: fix stack overflow caused by regexpr, and handle requests faster (#14) * web: look for more-generic browser executables * web: more robust browser starting (#6) * error message cleanups * more tests, refactoring, docs Stats: 58 days, 2 contributors, 102 commits since last release. Now at 3983 lines of non-test code, 139 tests, 53% coverage.

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ANN: darcsum.el 1.2 released
by Simon Michael 10 Apr '10

10 Apr '10

With Christian's blessing, I have taken over maintenance of darcsum and would like to announce the 1.2 release: darcs get http://joyful.com/repos/darcsum -t 1.2 darcsum is an occasionally fragile but tremendously useful emacs ui for darcs. There is also vc-darcs.el, but I am quite productive with this one. For a long time it has been hanging with certain of my repos, due to unrecognised output from darcs; this release fixes all hangs I know about. There's plenty more in TODO so please send patches. Or, if there is a more serious emacs hacker out there willing to be maintainer for at least the next release, that would be great. Thanks to Christian and the other committers for keeping this useful tool alive. -Simon

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