- Haskell-Cafe - Haskell.org

ANN: fixed-list -- A fixed length list library
by Job Vranish 21 Mar '10

21 Mar '10

I uploaded a new fixed length list library to hackage: http://hackage.haskell.org/package/fixed-list Its main advantages are: Very easy to use. Almost entirely Haskell98 (the non Haskell98 pieces are not critical, just nice) The datatype is a member of Foldable, Traverable, Applicative, Monad, etc... Then length of the list is encoded in the type in a natural way. Comments/critiques/suggestions welcome :) - Job

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installing Haskell
by love_pku 21 Mar '10

21 Mar '10

I am sorry to trouble you but I have problems to install Haskell. Things run smoothly when I do the command ‘./configure’, But some errors appeared when I run the command ‘make’. The details are as follows(in red font): Distribution/Client/HttpUtils.hs:127:49: Couldn't match expected type `network-2.2.1.2:Network.URI.URI' against inferred type `URI' In the fourth argument of `AuthBasic', namely `uri' In the first argument of `Just', namely `(AuthBasic "" usr pwd uri)' In the expression: Just (AuthBasic "" usr pwd uri) Distribution/Client/HttpUtils.hs:135:37: Couldn't match expected type `network-2.2.1.2:Network.URI.URI' against inferred type `URI' In the `rqURI' field of a record In the expression: Request {rqURI = uri, rqMethod = GET, rqHeaders = [Header HdrUserAgent userAgent], rqBody = ByteString.empty} In the definition of `mkRequest': mkRequest uri = Request {rqURI = uri, rqMethod = GET, rqHeaders = [Header HdrUserAgent userAgent], rqBody = ByteString.empty} where userAgent = "cabal-install/" ++ display Paths_cabal_install.version Error: Building the cabal-install-0.6.2 package failed make: *** [build.stamp] Error 2 If I install cabal-install manually, the errors will be: # /usr/bin/runhaskell Setup configure Configuring cabal-install-0.6.2... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package HTTP-4000.0.6 requires network-2.2.1.2 package cabal-install-0.6.2requires network-2.2.1.4

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Why does `flip` cause function type so different ?
by zaxis 21 Mar '10

21 Mar '10

>let f x xs = [x:xs,xs] > :t f f :: a -> [a] -> [[a]] >:t (>>=) .f (>>=) .f :: a -> ([[a]] -> [a] -> b) -> [a] -> b > :t (flip (>>=) .f) (flip (>>=) .f) :: a -> [[a]] -> [[a]] Why is the type of `(>>=) .f` and `flip (>>=) .f` so different ? Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/Why-does-%60flip%60-cause-function-type-so-different-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell hackathon in Philadelphia (Hac phi 2) in May?
by Brent Yorgey 20 Mar '10

20 Mar '10

We are gauging interest in another Haskell hackathon to be hosted at U Penn in May, the weekend of either May 15 or May 22. If you might be interested in attending such a thing, send me an email (please DON'T reply to all!) with your name and whether you have a strong preference for one or the other weekend. Feel free to respond even if you are not sure; will be a separate official registration procedure once details have been nailed down. -Brent

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Call for Praatjes: Dutch HUG Day
by Sean Leather 20 Mar '10

20 Mar '10

>From ZuriHac, we bring you... *Call for Praatjes: Dutch HUG Day* To celebrate our first anniversary, the Dutch Haskell Users Group (HUG) invites Haskell and functional programming enthusiasts in the Netherlands and surrounding area to a short 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 will continue the trend with the Dutch HUG Day. The Dutch HUG Day will be held in Utrecht on the afternoon of Saturday, April 24, 2010. The actual location is not confirmed; however, we will send out a notice as soon as it is known. *PRAATJE PROPOSALS* We seek people to give talks or "praatjes" in Dutch. The topic can cover anything in the functional programming or, more specifically, Haskell world. It may be your latest pet project, current research, or simply a demonstration of something cool that you found. The audience will be coming with a broad range of backgrounds, so praatjes should be oriented towards people having a general knowledge of Haskell programming. You can choose from one of the formats and topics below, or feel free to suggest something different. If you are interested in presenting, please send your proposal to Sean Leather by Saturday, April 3. More details are below. *FORMATS* - Talks (20 min) - Tutorials (25 min) - Lightning talks (5 min) - Demos (5 min) *TOPICS* - Development or design of a library or application - Comparison of libraries, applications, or languages - Lead a focused discussion (e.g. web programming options in Haskell) - Functional pearl (a "fun" talk) - Current research topic - Functional programming in industry - Experience report - Your favorite piece of software (and why) - Anything else (related to Haskell or functional programming) that you find interesting *IMPORTANT DATES* Praatje Proposal Deadline: Saturday, April 3 Dutch HUG Day: Saturday, April 24 *CONTACT* Send your proposal to Sean Leather: leather(a)cs.uu.nl Dutch HUG: http://dutchhug.nl/ Dutch HUG Day: http://dutchhug.nl/DutchHugDay/

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Takusen and iteratee problem
by Vasyl Pasternak 20 Mar '10

20 Mar '10

Hello Cafe. Today I stuck with the following problem: I want to read a file with iteratee package, and but it to database through Takusen package, but it doesn't work. The `Takusen` was built with `mtl` package, and `iteratee` - with `transformers`, so they are conflicting when used simultaneously. To be concrete, I want to run the code, that looks like: withSession (connect "dbname.db") (do execDDL (sql "create table x (value string)") fillFromFile "file.dat" ) fillFromFile fname = fileDriverFd iter fname iter = iterator that uses execDML (sql "Insert into ...." ).... I have no ideas how to solve the problem, so appreciate any recommendations, that will make it possible. Thanks, Vasyl

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re: [Haskell-cafe] HTTP package freezes on Windows 7
by Phil 20 Mar '10

20 Mar '10

Scrap my original query - the problem isn't as black & white as I thought. The below works fine - I've changed the response type from json to xml.... strange, but for some reason downloading json doesn't work.... it's fine on Linux. I'm guessing this is more likely to be a Windows issue rather than a Haskell issue - any ideas? import qualified Network.HTTP as HTTP main :: IO () main = do x <- getLocation print x getLocation = (HTTP.simpleHTTP $ HTTP.getRequest url) >>= HTTP.getResponseBody where url = "http://maps.google.com/maps/api/geocode/xml?address=London&sensor=false"

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hdbc-mysql on windows
by Roderick Ford 19 Mar '10

19 Mar '10

Hello, In order to build (cabal install HDBC-mysql) the HDBC-mysql package, I am having to make the following changes: (1) I had to change the mysql (5.1.45 community distrib msi) to use a wrapper exe on the mysql_config.pl and put this in c:\program files\mysql\bin since although the c:\program files\mysql\scripts\, where mysql_config.pl resides, was in the path, even the commandline call to it was not finding or using it...very strange. This of course doesn't directly have to do with the HDBC-mysql package. --The quick wrapper was: import System.Environment(getArgs) import System.Process(readProcessWithExitCode) import System.Exit(exitWith,ExitCode(ExitSuccess)) main = do args <- getArgs (ecode,out,err) <- readProcessWithExitCode "c:\\Perl64\\bin\\perl.exe" ("C:\\Program Files\\MySQL\\scripts\\mysql_config.pl" : args) "" if ecode == ExitSuccess then mapM_ putStrLn (lines out) else mapM_ putStrLn (lines err) exitWith ecode -- so now the output is: "C:\Program Files\MySQL\lib\mysqlclient.lib" "wsock32.lib" "advapi32.lib" "user3 2.lib" (2) Once that was done, the "cabal install HDBC-mysql" call was able to find the mysql_config. However, it was splitting the values returned by mysql_config on space, and thus failed to find mysql.h, as the resulting command was: C:\Program Files (x86)\Haskell Platform\2009.2.0.2\gcc.exe -c -BC:\ Program Files (x86)\Haskell Platform\2009.2.0.2\gcc-lib -IC:\Program Files (x86) \Haskell Platform\2009.2.0.2\include\mingw -D__GLASGOW_HASKELL__=610 -IIC:\Program -Iles\MySQL\include\mysql" ... -- then I tried taking the -I out of the mysql_config.pl script and got the following from the "cabal install HDBC-mysql" command was: C:\Program Files (x86)\Haskell Platform\2009.2.0.2\gcc.exe -c -BC:\ Program Files (x86)\Haskell Platform\2009.2.0.2\gcc-lib -IC:\Program Files (x86) \Haskell Platform\2009.2.0.2\include\mingw -D__GLASGOW_HASKELL__=610 -I:\Program -Iles\MySQL\include\mysql" ... blah blah blah cabal: Error: some packages failed to install: HDBC-mysql-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1 ... so finally I am downloading your package to see if I can figure out a fix for windows boxes. Just thought I would give you a heads up on this windows-related bug. Thanks, Roderick

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Re: explicit big lambdas
by oleg＠okmij.org 19 Mar '10

19 Mar '10

Paul Brauner wrote: > is there a way in some haskell extension to explicit (system F's) big > lambdas and (term Type) applications in order to help type inference? Actually, yes: newtype constructor introductions may act as a big lambda, with constructor elimination acting as type applications: http://okmij.org/ftp/Haskell/types.html#some-impredicativity Newtypes are also handy for turning type functions (defined by type families) into real lambdas. For example, given the following code > {-# LANGUAGE NoMonomorphismRestriction #-} > {-# LANGUAGE TypeFamilies, EmptyDataDecls #-} > {-# LANGUAGE ScopedTypeVariables #-} > > module S where > > data Z > data S a > > class Nat a where nat :: a -> Int > instance Nat Z where nat _ = 0 > instance Nat n => Nat (S n) where nat _ = succ (nat (undefined::n)) > > type family Add a b > type instance Add Z b = b > type instance Add (S a) b = S (Add a b) we would like to compose the type function Add (S (S Z)) twice. We may first try *> newtype F2 f x = F2{unF2 :: f (f x)} *> tf1 :: F2 (Add (S (S Z))) (S Z) *> tf1 = undefined alas, without success: Type synonym `Add' should have 2 arguments, but has been given 1 In the type signature for `tf1': tf1 :: F2 (Add (S (S Z))) (S Z) Indeed, type functions defined via type families can't be turned into closures. Newtypes help, introducing the needed Lambda behind the scenes: > newtype A2 a = A2{unA2 :: Add (S (S Z)) a} > > appA2 :: x -> A2 x > appA2 = undefined > > -- Now we can compose the type function, many times > tc = unA2 . appA2 . unA2 . appA2 > -- Inferred type: > -- tc :: a -> Add (S (S Z)) (Add (S (S Z)) a) > > tcrun = nat $ tc (undefined::S Z) > -- 5 One can see from the inferred type of tc that the type-checker has composed the partially applied Add function. I'll show a bit more elaborate example next week.

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Why no instance of Happstack.Data.Default.Default for Data.ByteString.Lazy
by Mads Lindstrøm 19 Mar '10

19 Mar '10

Hi Why is that Happstack has: instance Happstack.Data.Default.Default Data.ByteString But not one for: instance Happstack.Data.Default.Default Data.ByteString.Lazy I am trying to make a HTTP proxy using Happstack and the client part with the Network.HTTP (see Hackage). Network.HTTP.simpleHttp returns a lazy Bytestring when given a lazy Bytestring (which makes sense). Happstack on the other hand contains a lazy ByteString in its request type: data Request = Request { ... rqHeaders :: Headers, ... rqBody :: RqBody ... } newtype RqBody = Body Data.ByteString.Lazy.Internal.ByteString which Happstack.Server.SimpleHttp.simpleHTTP feeds into my application. However Happstack.Server.SimpleHttp.simpleHTTP expects something returned which is an instance of Happstack.Data.Default.Default which lazy ByteString is not. Confused? So am I. Who thought String handling could be so complex. Greetings, Mads Lindstrøm

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Trace
by Ben Derrett 19 Mar '10

19 Mar '10

Hi, I'm a new Haskell programmer and am trying to output the values of some of the variables (for debugging) as the program executes. At the moment to output the value of the function I use: f x y z = trace (show moves) moves where moves = [complicated expression of x y z] But really what I want is to be able to write: f x y z = trace2 [complicated expression of x y z] Is there a standard function to do this? I tried: trace2 a = trace (show a) a But I got the error: Couldn't match expected type `t1 -> t' against inferred type `String' In the expression: (show a) a In the definition of `trace2': trace2 a = (show a) a How do I fix it? Thanks, Ben

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Template Haskell
by Rafael Almeida 19 Mar '10

19 Mar '10

I was reading the good old template haskell paper by Sheard and Peyton Jones [1]. It looks like some API have changed, but things seems to be more or less the same. I got the printf example to run, and it is an alright example of something to do with template haskell. You lose the ability to use a dynamic generated string for the format, but that's not a practical drawback (I can't think of a practical reason for using anything other than a string literal for format anyway). Anyhow, what is gained by the template programming in that case is really the flexibility in the type system. You can generate the function with the correct type on the fly, during compiling. That's nice enough, but the article keeps saying that templates are nice for implementing your own custom compiler optimizations. I can see that you really can do so, but I fail to think of a good practical example where templates would be hands down the best approach. Perhaps haskellers with more experience have real life examples they've bumped into. So, please, share your experience with us. [1] http://research.microsoft.com/en-us/um/people/simonpj/papers/meta-haskell/m…

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Haskell (Byte)Strings - wrong to separate content from encoding?
by Mads Lindstrøm 19 Mar '10

19 Mar '10

Hi More and more libraries use ByteStrings these days. And it is great that we can get fast string handling in Haskell, but is ByteString the right level of abstractions for most uses? It seems to me that libraries, like the Happstack server, should use a string-type which contains both the content (what ByteString contains today) and the encoding. After all, data in a ByteString have no meaning if we do not know its encoding. An example will illustrate my point. If your web-app, implemented with Happstack, receives a request it looks like http://happstack.com/docs/0.4/happstack-server/Happstack-Server-HTTP-Types.… : data Request = Request { ... rqHeaders :: Headers, ... rqBody :: RqBody ... } newtype RqBody = Body ByteString To actually read the body, you need to find the content-type header, use some encoding-conversion package to actually know what the ByteString means. Furthermore, some other library may need to consume the ByteString. Now you need to know which encoding the consumer expects... But all this seems avoidable if Happstack returned a string-type which included both content and encoding. I could make a similar story about reading text-files. If some data structure contains a lot of small strings, having both encoding and content for each string is wasteful. Thus, I am not suggesting that ByteString should be scraped. Just that ordinarily programmers should not have to think about string encodings. An alternative to having a String type, which contains both content and encoding, would be standardizing on some encoding like UTF-8. I realize that we have the utf8-string package on Hackage, but people (at least Happstack and Network.HTTP) seem to prefer ByteString. I wonder why. Greetings, Mads Lindstrøm

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GLFW on OS X
by Carsten Schultz 19 Mar '10

19 Mar '10

Hi everyone, I am working with MacOS X 10.6.2 ghc 6.12.1 GLFW 0.4.2 OpenGL 2.4.0.1 mkbndl (freshly installed, should be 0.2.1) I have copied sample program from http://www.haskell.org/haskellwiki/GLFW#Sample_Program to a file named GLFWTest.hs. (I had to change some Floats to GLfloats.) No I do: > ghc --make GLFWTest.hs > mkbndl -f GLFWTest > open GLFWTest.app Is that what I am supposed to do? The program window remains white, if I move another window in front of it and away again, it becomes black, but if I draw lines on it (that is the function program) they do not appear. If I move or resize the window, it gets drawn correctly, and I can see that the lines that I had drawn had in fact been registered. From this point on the program works as expected. Is this an OS X specific issue? How do I solve it? Thanks Carsten

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Re: [Haskell-cafe] Iteratee performance
by John Lato 19 Mar '10

19 Mar '10

I think this is a bit easier to write with iteratee-HEAD. There are some significant changes from the 0.3 version, and not all the old functions are implemented yet, however the "cnt" iteratee can be written as: cnt :: Monad m => I.Iteratee S.ByteString m Int cnt = I.liftI (step 0) where step acc (I.Chunk bs) | S.null bs = I.icont (step acc) Nothing step acc (I.Chunk bs) = let acc' = acc + S.count '\n' bs in acc' `seq` I.icont (step acc') Nothing step acc str = I.idone acc str One significant change is that the kind of the first parameter to the Iteratee type has changed, so now it should be a fully-applied type instead of a type function. This means that ByteString can be used directly. "idone", "icont", and "liftI" simplify creation of iteratees, and it's usually not necessary to pattern match on the EOF constructor any more. The first "step" definition above can be left out for a small performance penalty. I wouldn't recommend basing any production code on iteratee-HEAD, as the interface isn't quite finalized yet. With this version of "cnt" and iteratee-HEAD, the iteratee version runs in about 2.2 seconds for the same tests as I used below. Changing the enumFd buffer size to 32K gives the following results: a 460MB input file (generated by cp Tiff.hs long.txt; cat long.txt >> long.txt): MusDept-MacBook-1:Examples johnlato$ time wc -l long.txt 12024249 long.txt real 0m0.997s MusDept-MacBook-1:Examples johnlato$ time ./test_bs < long.txt 12024249 real 0m1.161s MusDept-MacBook-1:Examples johnlato$ time ./test_iter long.txt 12024249 real 0m1.154s All time values are averages of 3 runs. The first run for the bytestring version was a bit long, otherwise run times were very consistent, within 0.004s for each executable. I don't see any reason the buffer size needs to be fixed at compile time. I'll make this change in the next major release. John > From: Vasyl Pasternak <vasyl.pasternak(a)gmail.com> > Subject: Re: [Haskell-cafe] Iteratee performance > To: Gregory Collins <greg(a)gregorycollins.net> > > Gregory, > > Thank you, your code helps, now my it runs in the speed of lazy > bytestring test but uses less memory with it. > > I've only added to your code more strictness in the recursion, my > version is below. > > BTW, I think it is more useful to let user set the chunk size for > reading, so I'd like to see this possibility in the iteratee package.

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Intermediate type variables
by Giuseppe Maggiore 19 Mar '10

19 Mar '10

Hi! How can I specify intermediate type variables to help the compiler do its job? The following, simple, code does not compile because the compiler cannot infer that the type variable a' is CInt; this makes sense, because finding that out would be quite hard. How can I tell the compiler that a' is CInt? {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} class C a a' where convert :: a -> a' class F a b where apply :: a -> b class S s a where select :: s -> a data CInt = CInt Int instance S (Int,String) Int where select (i,_) = i instance C Int CInt where convert = CInt instance F CInt Int where apply (CInt i) = i + 1 f :: forall s a a' b . (S s a, C a a', F a' b) => s -> b f s = let v = select s :: a v' = convert v :: a' y = apply v' :: b in y x :: Int x = f (10,"foo") Thanks

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stuck with a sample of "programming in haskell"
by 国平张 19 Mar '10

19 Mar '10

Hi, I am a beginner for haskell. I was stuck with a sample of "programming in haskell". Following is my code: --------------------------------------------------------------------- import Prelude hiding (return, fail) type Parser a = (String->[(a,String)]) return :: a -> Parser a return v = (\inp->[(v,inp)]) item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] failure :: Parser a failure = \inp -> [] parse :: Parser a->(String->[(a,String)]) parse p inp = p inp (>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = (\inp -> case parse p inp of [] -> [] [(v,out)]->parse (f v) out) p :: Parser (Char,Char) p = do x <- item item y <- item return (x,y) --------------------------------------------------------------------- But it cannot be loadded by Hug, saying: Couldn't match expected type `Char' against inferred type `[(Char, String)]' Expected type: [((Char, Char), String)] Inferred type: [(([(Char, String)], [(Char, String)]), String)] In the expression: return (x, y) In the expression: do x <- item item y <- item return (x, y) ------------------------------------------------------------------- I googled and tried a few days still cannot get it compiled, can someone do me a favor to point out what's wrong with it :-) ?

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ANNOUNCE: direct-fastcgi 1.0
by Dan Knapp 19 Mar '10

19 Mar '10

Yay! This was a week of hard work. I hope that somebody finds it valuable. This package is a native implementation of the FastCGI protocol, allowing Haskell to work with any webserver that supports it. It makes no attempt to imitate the interface of the cgi-3000 and fastcgi-3000 packages, because that API is tied too heavily to legacy interfaces, as with its handling of exceptions, logging, and time. Its advantage over those packages is primarily in the area of clean exception management and better control structure which allows more versatility in processing requests which are not simple GETs. In particular, POST of large files should be possible, as should server-push responses which return content a piece at a time. Also, of course, as a native implementation, there is no dependency on a C library, which simplifies the install process. http://hackage.haskell.org/package/direct-fastcgi -- Dan Knapp "An infallible method of conciliating a tiger is to allow oneself to be devoured." (Konrad Adenauer)

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little help please
by Roderick Ford 19 Mar '10

19 Mar '10

this will be EASY for you to fix, I am sure... what the heck am I doing wrong: import System.Environment(getArgs)import System.Process(readProcessWithExitCode)import System.Exit(exitWith) main = do args <- getArgs (ecode,out,err) <- readProcessWithExitCode "c:\\Perl64\\bin\\perl.exe" ("C:\\Program Files\\MySQL\\scripts\\mysql_config.pl" : args) "" if ecode == System.Exit.ExitCode.ExitSuccess then return out else return err exitWith ecode

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Abstraction in data types
by Darrin Chandler 19 Mar '10

19 Mar '10

Hi, Trying to get up to speed in Haskell, I'm playing with doing some abstraction in data types. Specifically, I have this: type Cartesian_coord = Float type Latitude = Float type Longitude = Float data Point = Cartesian (Cartesian_coord, Cartesian_coord) | Spherical (Latitude, Longitude) type Center = Point type Radius = Float data Shape = Circle Center Radius | Polygon [Point] This obviously stinks since a Polygon could contain mixed Cartesian and Spherical points. Polygon needs to be one or the other, but not mixed. I could define seperate types for Cartesian and Spherical and seperate CartesianPoly and SphericalPoly, but that doesn't seem very elegant and also increases as I add more coordinate systems and shapes. I read a little on GADTs, et al, but I'm not sure if that's what I want for this or not. Any help appreciated! -- Darrin Chandler | Phoenix BSD User Group | MetaBUG dwchandler(a)stilyagin.com | http://phxbug.org/ | http://metabug.org/ http://www.stilyagin.com/ | Daemons in the Desert | Global BUG Federation

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searching a function by providing examples of input/ouput pairs
by Paul Brauner 19 Mar '10

19 Mar '10

Hi, I was looking at hoogle documentation when I remembered that there is some nice, but quite unusable, feature of squeak (smalltalk) which allows you to search function in the library by giving a list of pairs of inputs/ouputs. When I'm saying that it is quite unusable, I mean that squeak has to try _every_ function, some of which may be very slow to deliver a result, or require some side effects. But, piggibacking such a feature on top of hoogle would surely be more efficient: 1. infer types for arguments and outout 2. look for matching functions using google 3. test them Has anyone tried that before? If not I would be glad to. Paul

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Parsec to parse tree structures?
by david fries 19 Mar '10

19 Mar '10

Hello Café Some time ago I wrote a parser for a project of one our customers. The format was proprietary and binary. The data was structured as a tree with tables pointing to sub tables farther in the file. (Well actually there was one or two cases where branches joined together, so I guess it was a directed graph.) Also it had no defined endianess, some tables were bigendian others were little endian and some were in their very own in-house-endianess. All in all, the typical binary data structure that has been in use and continuously extended for the last 15 years. You know the kind. Oddly enough, our customer never bothered to write a parser of their own. I wonder why. My parser was written in C# and wasn't particularly elegant, but it worked reliably. I was wondering how you would parse tree-like structures with Parsec (or other functional parsers)? Up to know, all examples I've seen were of sequential data. To parse trees, you'd essentially need to be able to follow pointers, parse whatever is there and then jump back. I guess I'd have to mess around with the internal state of the parser to do that, which is something I'd rather avoid. regards, dave

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How to improve its performance ?
by zaxis 19 Mar '10

19 Mar '10

import Data.List combination :: [a] -> [[a]] combination [] = [[]] combination (x:xs) = (map (x:) (combination xs) )++ (combination xs) samp = [1..100] allTwoGroup = [(x, samp\\x) | x <- combination samp] The above code is used to calculate all the two groups from sample data ? It is very slow ! Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/How-to-improve-its-performance---tp27940036p27940036.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Parsec monad transformer with IO?
by Stefan Klinger 18 Mar '10

18 Mar '10

Hello! Nice, Parsec 3 comes with a monad transformer [1]. So I thought I could use IO as inner monad, and perform IO operations during parsing. But I failed. Monad transformers still bend my mind. My problem: I don't see a function to actually lift the IO operation into the ParsecT. It should be something like lift :: IO a -> ParsecT s u IO a The following is a toy example, I just could not make something smaller: Let's parse command line arguments (tokens are Strings), and execute them while parsing. > import Text.Parsec.Prim > import Text.Parsec.Pos > import Text.Parsec > import System.Environment ( getArgs ) Command line interface parser (Clip) type: Inner monad IO, user state u, tokens are Strings, returns something typed a. > type Clip u a = ParsecT [String] u IO a Source code position for command line arguments: The line is always 1, column n represents the n-th command line argument. > nextPos p _ _ = incSourceColumn p 1 Two primitive parsers, one for flags (with a dash) and one for other arguments: clipFlag "x" accepts the command line flag "-x", and returns "x". > clipFlag :: String -> Clip u String > clipFlag x > = tokenPrim > id > nextPos > (\y -> if '-':x == y then Just x else Nothing) clipValue accepts any command line argument that does not tart with a dash '-'. > clipValue :: Clip u String > clipValue > = tokenPrim id nextPos test > where > test ('-':_) = Nothing > test other = Just other Now the test program: Load files given on the command line, and sum up their word count, until -p is given. -p prints the current word count and sets the counter to zero. Further files may be processed then. At the end, show the sum of all word counts. Example: foo has 12 words, bar has 34 words: main foo -p bar -p foo bar -p Counted 12 words, reset counter. Counted 34 words, reset counter. Counted 46 words, reset counter. Grand total: 92 > type CurrentCount = Int -- the user state used with Clip/ParsecT. root implements the command line grammar (<filename>+ "-p")* and returns the sum of all word counts. > root :: Clip CurrentCount Int > root > = do ns <- many (many1 loadFile >> printSize) > eof > return $ sum ns Interprets each non-flag on the command line as filename, loads it, counts its words, and adds the count to the state. > loadFile :: Clip CurrentCount () > loadFile > = do -- expect a filename > filename <- clipValue > -- load the file: IO > content <- lift $ readFile filename > -- add wordcount to state > modifyState ((+) (length $ words content)) If -p shows up on the command line, print accumulated count, reset counter to cero and return count for grand-total calculation. > printSize :: Clip CurrentCount Int > printSize > = do -- expect flag -p > clipFlag "p" > -- print current word count: IO > n <- getState > lift . putStrLn $ "Counted "++show n++" words, reset counter." > -- reset user state to zero, return word count for grand total > putState 0 > return n main just runs the root parser on the command line arguments and checks the result. > main > = do result <- getArgs >>= runParserT root 0 "command line" > case result of > Left err -> error $ show err > Right n -> putStrLn $ "Grand total: "++show n So where is the lift function? Does it exist? Here, I need your help. > lift :: IO a -> ParsecT s u IO a > lift = undefined Any comments are appreciated. Thank you! Stefan ____________________ [1] http://hackage.haskell.org/packages/archive/parsec/3.0.0/doc/html/Text-Pars… -- Stefan Klinger o/klettern /\/ bis zum send plaintext only - max size 32kB - no spam \ Abfallen http://stefan-klinger.de

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Happstack/Apache configuration problem
by Maciej Podgurski 18 Mar '10

18 Mar '10

Hi, I'm not sure if this is the right place to ask because the problem may lay in my Apache or system configuration. I use the Apache proxy module to forward a HTTP request from port 80 to a Happstack application listening on port 8000 via `ProxyPass / http://localhost:8000/' on Windows XP. When calling http://my_domain in the browser, I get `503 service temporarily unavailable' and in the apache logs `proxy: HTTP: attempt to connect to 127.0.0.1:8000 (localhost) failed'. So I tried to access http://127.0.0.1:8000 manually in the browser but got a connection error, while http://[::1]:8000/ works perfectly and displays the content returned from the Happstack application. Maybe I'm wrong, but isn't the problem here that Happstack only accepts IPv6 addresses, while Apache forwards an IPv4 request? Best wishes, Maciej

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Re: searching a function by providing examples of input/ouput pairs
by Janis Voigtländer 18 Mar '10

18 Mar '10

Paul Brauner schrieb: > Hi, > > I was looking at hoogle documentation when I remembered that there is > some nice, but quite unusable, feature of squeak (smalltalk) which > allows you to search function in the library by giving a list of pairs > of inputs/ouputs. > > When I'm saying that it is quite unusable, I mean that squeak has to try > _every_ function, some of which may be very slow to deliver a result, or > require some side effects. > > But, piggibacking such a feature on top of hoogle would surely be more > efficient: > > 1. infer types for arguments and outout > 2. look for matching functions using google > 3. test them > > Has anyone tried that before? If not I would be glad to. Sounds like something useful to have. And you could even have the system use a list of pairs of inputs/outputs to give you functions that are *not* yet in any library. :-) http://www.haskell.org/communities/11-2009/html/report.html#sect6.11.1 -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Call for community volunteers
by Malcolm Wallace 18 Mar '10

18 Mar '10

As you may know, community.haskell.org (also hosting code.h.o, trac.h.o, etc) exists as a resource to enable Haskellers to publish and collaborate on open source projects. The current admin team for this virtual host finds itself with a lack of time (and occasionally expertise) to maintain the services and respond to user requests. Hence, we are calling for new volunteers to join the c.h.o admin team. Ideally, you will have some skills in administering one or more of trac mail servers apache web server debian/linux systems and the willingness to learn the others, plus a hour or two a week to deal with requests, either coming in through the ticket system, or in the case of some failures, email or IRC. (There is a backlog of outstanding tickets to resolve too.) Apply to admin(a)community.haskell.org telling us how you think you can help. Regards, Malcolm

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[Announcement] Fedora 12 rpms for leksah 0.8.0.1 now available
by Lakshmi Narasimhan 18 Mar '10

18 Mar '10

Leksah - 0.8.0.1 rpms for Fedora 12 now available <goog_1268906153764>http://leksah.org/download.html About Leksah: *Leksah* is the Haskell IDE of choice. It is written in Haskell, uses Gtk, and runs on Linux, Windows and Mac OS X. Leksah is a practical tool to support the Haskell development process. It requires the Glasgow Haskell Compiler (GHC). *Installation Instructions * *Procedure for x86 platform* Before proceeding to install the leksah rpms, Uninstall leksah-0.6.1, ghc-binary-devel packages, incase you have installed them from the leksah download page. Use the following command yum remove leksah ghc-binary-devel Download this tar.gz package http://leksah.org/packages/dependencies-0.8.0.1.fc12.i386.tar.gz Extract the rpms from the tar.gz file. Download the following two rpms (links below) and put the rpms into the directory where the dependencies are located. http://leksah.org/packages/leksah-0.8.0.1-1.fc12.i386.rpm http://leksah.org/packages/leksah-server-0.8.0.1-1.fc12.i386.rpm Select the rpms, right click and select Install. To install from command line, change into the extracted directory, become superuser and type yum --nogpgcheck install ghc-binary-devel-0.5.0.2-1.fc12.i386.rpm ghc-deepseq-devel-1.1.0.0-1.fc12.i386.rpm ghc-ltk-devel-0.8-1.fc12.i386.rpm ghc-binary-shared-devel-0.8-1.fc12.i386.rpm ghc-hslogger-devel-1.0.10-1.fc12.i386.rpm haddock-leksah-2.5.0-1.fc12.i386.rpm leksah-0.8.0.1-1.fc12.i386.rpm leksah-server-0.8.0.1-1.fc12.i386.rpm *Procedure for x86_64 platform* Download this tar.gz package. http://leksah.org/packages/dependencies-0.8.0.1.fc12.x86_64.tar.gz Extract the rpms from the tar.gz file. Download the following two rpms (links below) and put the rpms into the directory where the dependencies are located. http://leksah.org/packages/leksah-0.8.0.1-1.fc12.x86_64.rpm http://leksah.org/packages/leksah-server-0.8.0.1-1.fc12.x86_64.rpm Select the rpms, right click and select Install. To install from command line, change into the extracted directory, become superuser and type yum --nogpgcheck install ghc-binary-devel-0.5.0.2-1.fc12.x86_64.rpm ghc-hslogger-devel-1.0.10-1.fc12.x86_64.rpm ghc-binary-shared-devel-0.8-1.fc12.x86_64.rpm ghc-ltk-devel-0.8-1.fc12.x86_64.rpm ghc-deepseq-devel-1.1.0.0-1.fc12.x86_64.rpm haddock-leksah-2.5.0-1.fc12.x86_64.rpm leksah-0.8.0.1-1.fc12.x86_64.rpm leksah-server-0.8.0.1-1.fc12.x86_64.rpm *Running leksah *Once installation is completed, leksah menu item appears under * Applications->Programming* (in GNOME) and *Development* menu of KDE. -- Regards Lakshmi Narasimhan T V

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STM Skip list implementation
by Peter Robinson 18 Mar '10

18 Mar '10

I've implemented a skip list that works in the STM monad. A skip list is a probabilistic data structure similar to a balanced tree. The main advantage of a skip list is that it doesn't require rebalancing, making it particularly suitable for concurrent programming. Here are the docs: http://darcs.monoid.at/tskiplist/dist/doc/html/tskiplist/Control-Concurrent… It's not yet on hackage as the implementation is very incomplete and mostly untested. However, you can download a cabalized package from here: http://darcs.monoid.at/tskiplist/dist/tskiplist-0.0.0.tar.gz Darcs repository: http://darcs.monoid.at/tskiplist/ Peter

3 3

Iteratee performance
by Vasyl Pasternak 17 Mar '10

17 Mar '10

Hi Cafe, Yesterday I played with iteratee package, and wanted to check its performance. I tried to count lines in a file, as Oleg in his famous lazy_vs_correct[1] article. The results somewhat disappointed me. The statistics and code follows, but shortly: lazy bytestring is the fastest, iteratee with bytestrings 10 times slower than lazy bytestring. When comparing lazy string and iteratee with [Char], than their results were close, but lazy string reading uses less memory and was a bit faster (20%). I performed test on 250Mb file with 5 millions lines. Now I am figuring out, is these tests are correct and this is ordinary behavior, so iteratee not so fast as I thought, or there is some mistake in my code. [1] http://okmij.org/ftp/Haskell/Iteratee/Lazy-vs-correct.txt --------------------- TIMING RESULTS ---------------------- $ time wc -l 5000000.txt 5000000 5000000.txt real 0m0.314s user 0m0.184s sys 0m0.124s $ time ./bytestring_test 5000000.txt 5000000 real 0m0.817s user 0m0.616s sys 0m0.200s $ time ./bytestring_iteratee_test 5000000.txt real 0m7.801s user 0m7.552s sys 0m0.252s $ time ./string_test 5000000.txt 5000000 real 0m47.427s user 0m46.675s sys 0m0.648s $ time ./string_iteratee_test 5000000.txt 5000000 real 0m59.225s user 0m57.680s sys 0m0.840s -------------------------- RTS INFO -------------------------------- ./bytestring_test 5000000.txt +RTS -sbs.out 807,225,096 bytes allocated in the heap 122,240 bytes copied during GC 59,496 bytes maximum residency (1 sample(s)) 22,424 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1540 collections, 0 parallel, 0.03s, 0.02s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.59s ( 0.79s elapsed) GC time 0.03s ( 0.02s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.62s ( 0.82s elapsed) %GC time 4.5% (2.8% elapsed) Alloc rate 1,372,743,081 bytes per MUT second Productivity 95.5% of total user, 72.1% of total elapsed ----- ./bytestring_iteratee_test 5000000.txt +RTS -siter.out 11,024,100,312 bytes allocated in the heap 893,436,512 bytes copied during GC 95,456 bytes maximum residency (1 sample(s)) 23,216 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 21030 collections, 0 parallel, 2.51s, 2.45s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.00s ( 0.02s elapsed) MUT time 6.37s ( 6.66s elapsed) GC time 2.52s ( 2.45s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 8.88s ( 9.12s elapsed) %GC time 28.3% (26.9% elapsed) Alloc rate 1,731,061,437 bytes per MUT second Productivity 71.7% of total user, 69.8% of total elapsed ----- ./string_test 5000000.txt +RTS -sstr.out 38,561,155,264 bytes allocated in the heap 9,862,623,816 bytes copied during GC 223,080 bytes maximum residency (5026 sample(s)) 47,264 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 68525 collections, 0 parallel, 22.50s, 22.51s elapsed Generation 1: 5026 collections, 0 parallel, 1.38s, 1.36s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 22.80s ( 23.55s elapsed) GC time 23.87s ( 23.87s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 46.67s ( 47.42s elapsed) %GC time 51.1% (50.3% elapsed) Alloc rate 1,691,170,222 bytes per MUT second Productivity 48.9% of total user, 48.1% of total elapsed ----- ./string_iteratee_test 5000000.txt +RTS -sstriter.out 40,164,683,672 bytes allocated in the heap 7,108,638,256 bytes copied during GC 212,624 bytes maximum residency (821 sample(s)) 50,264 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 75791 collections, 0 parallel, 33.14s, 33.75s elapsed Generation 1: 821 collections, 0 parallel, 0.56s, 0.63s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 23.99s ( 24.84s elapsed) GC time 33.69s ( 34.38s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 57.68s ( 59.22s elapsed) %GC time 58.4% (58.1% elapsed) Alloc rate 1,674,540,397 bytes per MUT second Productivity 41.6% of total user, 40.5% of total elapsed ------------------ SOURCECODE ----------------------------- $ cat bytestring_test.hs import System.Environment import Control.Monad import qualified Data.ByteString.Lazy.Char8 as B count s = liftM (length . B.lines) (B.readFile s) main = do [f] <- getArgs print =<< count f --------------- $ cat bytestring_iteratee_test.hs import qualified Data.Iteratee.IO as I import qualified Data.Iteratee as I import qualified Data.Iteratee.Char as I import qualified Data.Iteratee.WrappedByteString as I import System.Environment import Control.Monad count s = I.fileDriverRandom (cnt) s cnt :: (Monad m, Functor m) => I.IterateeG I.WrappedByteString Char m Int cnt = I.joinI $ I.enumLines I.length main = do [f] <- getArgs print =<< count f ------------------ $ cat string_test.hs import System.Environment import Control.Monad count s = liftM (length . lines) (readFile s) main = do [f] <- getArgs print =<< count f ---------------------- $ cat string_iteratee_test.hs import qualified Data.Iteratee.IO as I import qualified Data.Iteratee as I import qualified Data.Iteratee.Char as I import qualified Data.Iteratee.WrappedByteString as I import System.Environment import Control.Monad count s = I.fileDriverRandom (cnt) s cnt :: (Monad m, Functor m) => I.IterateeG [] Char m Int cnt = I.joinI $ I.enumLines I.length main = do [f] <- getArgs print =<< count f Best regards, Vasyl

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compiling 32 bits haskell program on 64 bits linux
by Vincent Hanquez 17 Mar '10

17 Mar '10

Hi Cafe, I was looking for a way to generate 32 bits haskell binary on a 64 bits linux. I've looked at the user guide and tried to use the -fvia-C with -optc -m32 but the generated C file doesn't compile with lots of error messages like the following: $ ghc -o hello hello.hs -fvia-C -optc "-m32" In file included from /usr/lib/ghc-6.10.4/include/Stg.h:206, from /tmp/ghc15772_0/ghc15772_0.hc:3:0: /usr/lib/ghc-6.10.4/include/Regs.h:235:0: error: invalid register name for ‘R4’ I can't find any others parameters that seems to do anythings about the code generation or around the C compiler. so, does anyone knows if it's possible ? or have some documentation/link about this kind of things ? Thanks, -- Vincent

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Re: [Haskell-cafe] Parsec to parse tree structures?
by John Lato 17 Mar '10

17 Mar '10

Hi Dave, > From: david fries <djf(a)gmx.ch> > > Hello Café > > Some time ago I wrote a parser for a project of one our customers. The > format was proprietary and binary. The data was structured as a tree > with tables pointing to sub tables farther in the file. (Well actually > there was one or two cases where branches joined together, so I guess it > was a directed graph.) Also it had no defined endianess, some tables > were bigendian others were little endian and some were in their very own > in-house-endianess. > All in all, the typical binary data structure that has been in use and > continuously extended for the last 15 years. You know the kind. > > My parser was written in C# and wasn't particularly elegant, but it > worked reliably. I was wondering how you would parse tree-like > structures with Parsec (or other functional parsers)? Up to know, all > examples I've seen were of sequential data. To parse trees, you'd > essentially need to be able to follow pointers, parse whatever is there > and then jump back. I guess I'd have to mess around with the internal > state of the parser to do that, which is something I'd rather avoid. Have you seen Edward Kmett's Parsing Trifecta talk/slides, available at http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ ? I think a similar approach would allow you to parse this data structure. The trick is to create a data source that can read from arbitrary locations. Also see http://okmij.org/ftp/Haskell/Iteratee/Tiff.hs for an iteratee-based TIFF reader. TIFF files have a similar structure, so this code could apply pretty directly. The parsing is quite low-level, but you could use iteratee-parsec [1] or attoparsec-iteratee [2] to use parser combinators with this technique. [1] http://hackage.haskell.org/package/iteratee-parsec [2] http://hackage.haskell.org/package/attoparsec-iteratee

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Mini-announce: AC-Color 1.1.3
by Andrew Coppin 17 Mar '10

17 Mar '10

I just uploaded a new version of the ever-unpopular AC-Colour package. (Yes, there's a country where it's spelled that way.) What it already did: Provides a simple strict unboxed RGB colour type, with arithmetic. The three channels can be either Double or Word8. (And there's some low-level hackery to convert between the two. For some reason just using "floor" is awesomely slow, but talking to GHC.Prim directly is much, much faster... which unfortunately makes this package far less portable than it should be!) What it does now: Colour maps. Give it some control points, and it'll (linearly) interpolate between them to give you a function Double -> Colour. (I haven't seen anything else on Hackage that will do this out-of-the-box. I've seen linear blends between two colours, but not fully automatic piece-wise linear maps like this.) Now obviously none of this is terribly earth-shattering. But I find myself needing this often enough that it's annoying to have to spend 10 minutes coding and debugging it every single time I want it. Maybe somebody else will find a use for it too. I paid special attention to trying to make the colour maps as general as possible. (E.g., the parameter range isn't limited to [0..1], it's arbitrary. Going off the end of the range produces sensible results. You can make the map loop. And so on.) Maybe if I'm feeling ambitious I'll try 2D colour maps. (Of course, the problem there is that your control points need not be coplanar - or even form a regular grid!)

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ANNOUNCE: Mac and Windows installers for Leksah 0.8 prerelease available
by Hamish Mackenzie 17 Mar '10

17 Mar '10

We have uploaded first pass at Windows and OS X binaries for 0.8. They are both built using GHC 6.12.1. Installation: Make sure ghc 6.12.1 is installed. Take care that wget and grep are on your PATH. Please try them out if you can and let us know if you find any issues. http://leksah.org/news.html

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compiler management for Haskell a la RVM?
by Mark Wotton 17 Mar '10

17 Mar '10

RVM (at http://rvm.beginrescueend.com) is a rather nice tool for managing multiple ruby installations - it gives support for switching between ruby environments (similar to gcc_select and friends), and also exporting lists of packages so that you can easily bring up a given set of gems in any of the interpreters. Do we have any similar system for ghc/cabal? I quite frequently find myself switching between 6.10 and 6.12 for various things, and it's always a bit painful to get your environment up to speed. Is there a golden road for this stuff, or do the compiler hackers here just munge the PATH? cheers Mark

4 3

th-kinds v0.0.0
by Louis Wasserman 17 Mar '10

17 Mar '10

I'm going to phrase this as a challenge, because I think that's likely to get the best response: I just released a package, th-kinds<http://hackage.haskell.org/package/th-kinds>, which attempts to automatically infer the kind of a specified type, type constructor, type family, type class, or pretty much anything else that has a kind. This package was developed in response to a sort-of challenge from Brent Yorgey on #haskell to create this functionality. So, uh, I stayed up last night until 5 am learning unification algorithms and implementing this...heh. It rolls its own kind inference, so I don't know if it's adequately powerful to handle all the wacky types you can construct. Things I have tested it on: - IO and ST - Many primitive types, including unboxed tuples and State# - mtl - newtype Fix f = Fix (f (Fix f)) - data Tree a = Leaf a | Bin (Tree a) (Tree a) - data Foo a b = a (tests proper defaulting behavior) - A type family from my old TrieMap package with kind -- I kid you not -- ((* -> *) -> ((* -> *) -> * -> *) -> (* -> *) -> * -> * -> *) Things that it will not work on, and that I don't think will change: - GADT types that cannot be reified by TH. Essentially, I think this is "the set of GADT data types that actually couldn't be implemented without GADTs." Not sure, though. In any event, at the moment, I don't think there's any hope of handling GADTs in TH at this point, so I don't really object to this problem. Anyway, y'all should attempt to break th-kinds. Tell me if you can construct a type for which my inference checker breaks, but not because you get an error message saying "Can't reify a GADT data constructor..." By the way, I'd like to bump these<http://hackage.haskell.org/trac/ghc/ticket/3916> tickets <http://hackage.haskell.org/trac/ghc/ticket/3920>. They're certainly related, but I'm not sure if they're genuinely equivalent. It's really irritating, though, that kind parsing is so broken in TH, and I think that a TH backend guru should be able to figure out where there's a foldr instead of a foldl... th-kinds includes a workaround to this bug, but if the bug is fixed, it'll break again. Yuck, bugs. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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ANNOUNCE: AusHac2010
by Alex Mason 17 Mar '10

17 Mar '10

G'day all, Over the last few days, Ivan Miljenovic and I have been gauging interest in having an Australian Haskell Hackathon. After some rather encouraging interest, we've decided to get the ball rolling by making a wiki page that we can use to organise the event: [http://www.haskell.org/haskellwiki/AusHac2010] If you're interested in coming, please put your name down on the list (or email me and I'll do it for you if you don't have an account). The more people we can get, the better it will be, and the more productive our work will be. So if you want to come along, play with some cool code, help out the community, and just meet an awesome bunch of like minded people, then we want to hear from you! Cheers, Alex Mason

1 0

Proposal: Australian Hackathon
by Ivan Miljenovic 17 Mar '10

17 Mar '10

Would other Australians be interested in having our own Hackathon (why should all those northerners have all the fun)? I'm thinking about organising it to be in the July break between university semesters. There was a previous consideration a few years back to have an OzHaskell group (http://www.haskell.org/haskellwiki/OzHaskell) but nothing seems to have eventuated out of it. In terms of projects, here are some ideas: * A plotting library using Ben's newly released Gloss library (for people who can't or won't install Gtk2Hs to get Chart working; Alex Mason is interested in this) * Various graph-related project (graphviz, generic graph class, etc.; this assumes someone else apart from me cares about this stuff) * Hubris if Mark Wotton comes along * LLVM if David Terei comes So, at least as an initial listing, we'd need to have a listing of: 1) Who's interested 2) What dates are good 3) What projects people want to work on 4) Where we can host this Let's get the ball rolling! -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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SoC Proposals?
by Jeff Wheeler 17 Mar '10

17 Mar '10

Is there any way to propose a SoC idea right now? My account doesn't seem to have been created correctly, so I can't login to the Trac. I think it'd be interesting for a student to abstract the layout model in xmonad, separating it into an independent library. Other applications could then use this library, like Yi, for other types of tiling. I'd love to see other ideas related to Yi, too. It's a great project but on the verge of death (although the maintainer, JPB, is interested in mentoring). -- Jeff Wheeler Undergraduate, Electrical Engineering University of Illinois at Urbana-Champaign

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Extending Cabal's Preprocessors for GSoC?
by Diego Echeverri 17 Mar '10

17 Mar '10

Hi guys! Thomas Schilling suggested me to cross-post this here. Basically I'm thinking about the possibility of extending Cabal's Preprocessor mechanism for the GSoC. I've been working to provide uuagc with a Custom build mechanism for Cabal and in this process I have stumbled upon some difficulties (Dependency resolution and configurability). I believe is possible to extend the current preprocessing mechanism to provide the following: * Independence from physical representation (Currently the module is assumed to be built from a single file with the same name that the module) * Dependency checks (precompile when one of the files on which it depends changed. I had to trick cabal and duplicate functionality to do this for uuagc. There should be a better way!) * Composability (Add the ability to chain existing preprocessors) * Configurabily (Make easier to add configuration sections to the cabal file. I've been using "x-" sections but I guess is possible to make it look like the already-supported preprocessors). Any feedback about this? Suggestions are greatly appreciated. Diego Echeverri On Mon, Mar 15, 2010 at 10:33 AM, Thomas Schilling <nominolo(a)googlemail.com> wrote: > Yes, you sent it to the right list. In fact for discussing summer of > code it might not be a bad idea to cross-post to haskell-cafe to get > some feedback from Cabal users. > > In my experience, it is most important for Summer of Code to have a > clearly defined set of "deliverables", i.e., have a good idea of what > features you want to have working at the end of the 10 weeks. (It's > actually 12 weeks, but the last 2 weeks are expected to be used for > documenting stuff.) > > Your problem description sounds plausable, but for a GSoC project it's > good to also have an idea how to solve the problem. E.g., do you plan > to build a custom framework or just extend Cabal to support more kinds > of preprocessors? > > Duncan and others have been working on a true dependency tracking > system similar to Make, but I think there are too many open questions > to turn that into a GSoC project just yet. It does raise the > question, however, how much overlap there would be with your idea. > Maybe you can find a well-defined subset? > > In any case, try to ask haskell-cafe for feedback and apply for both > ideas. I think you can apply for up to 10 projects, so it's no > problem to apply for more than one. But keep in mind that the quality > of the proposals matters. > > On 14 March 2010 01:40, Diego Echeverri <diegoeche(a)gmail.com> wrote: >> Hi guys! >> >> I was wondering about the possibility to work on Cabal for the >> summer of code. I've been helping one of my teachers to >> provide (improve) a "plugin" for the uuagc (I haven't spent too >> much time on it, but here's the fork I'm working on >> http://github.com/diegoeche/uuagc) I have found how flexible the >> cabal libraries are but also some things haven't been >> straightforward to do (I've been using Cabal 1.6.0.3). >> >> Basically there's seems to be an assumption of a one-to-one >> relation of modules and the sources of the files which is not >> followed by the uuagc pre-processor. I can use List.ag and inside >> of it use an include declaration to List-Data.ag. If I modify >> List-Data.ag cabal won't precompile it again because it will only >> check dates against List.ag (cabal doesn't know that the >> file that generates List.hs depends not only in List.ag). I had >> to override the build hook, duplicate the check (this time >> getting all the dependencies) and delete the generated file to >> trick cabal to precompile again... and well... that's seems quite >> a nasty hack to me. >> >> This assumption also affect the possibility to make the output of >> the pre-processor not necessarily an .hs file (and not necessarily >> carrying the same name). Checking the code for the >> pre-processors (Distribution.Simple.PreProcess) I noticed it would >> be good to have the possibility to chain pre-processors. I believe >> both issues are pretty much connected. >> >> I also found the following proposal here: >> http://hackage.haskell.org/trac/summer-of-code/ticket/1581 which >> sound also interesting to me. (and there's already a mentor >> available!) >> >> Do you think this could be a good project for the summer of code? >> Maybe any other ideas I should explore? Did I write to the right >> list? (please excuse me if I didn't) >> >> Thanks! >> Diego Echeverri >> >> _______________________________________________ >> cabal-devel mailing list >> cabal-devel(a)haskell.org >> http://www.haskell.org/mailman/listinfo/cabal-devel >>

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Re: [Haskell-cafe] How to define instance for type synonym with parameter.
by Vasyl Pasternak 17 Mar '10

17 Mar '10

> > Aren't ErrorT and ReaderT instances of Applicative already? (At least in > 'transformers' package?) > > what difference between 'mtl' and 'transformers' packages ? 'mtl' goes with Haskell Platform as standard package, so what is the benefit of 'transformers' library ? Thanks, Vasyl

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new Quake 3 BSP loader and viewer
by Csaba Hruska 16 Mar '10

16 Mar '10

Hi! I did a little experiment in last week with haskell. According this ( http://graphics.cs.brown.edu/games/quake/quake3.html) tutorial, I wrote a quake 3 bsp map loader and viewer from scratch. (It is independent from frag) The code is quiet small (~900 lines with comments) and it is based on binary and vector libraries. You can find the cabalized code here: http://code.google.com/p/lambdacube/source/browse/#svn/trunk/bsploader This was my first meet with vector library, so the code is probably not the most efficient one. Any improvement suggestions are welcome! :) Cheers Csaba PS: read the README file. navigation: arrows + mouse (holding right shift increase cam speed)

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HGL concurrency problems
by Carsten Schultz 16 Mar '10

16 Mar '10

[Cc to the HGL maintainer, I hope that is ok.] Hallo, I am trying to use HGL. My configuration is Mac OS X 10.6.2 (using X11) ghc 6.12.1 HGL 3.2.0.2 (via cabal-install) My program opens a window, draws points and lines, with a considerable amount of calculation between the lines, and then waits for a key stroke before exiting. What happens depends on whether I compile with -threaded or without. With -threaded: The programs runs ok, but after a short time stops updating the window, so that I never see when it has finished. Moving the mouse over the window can trigger updates. Without -threaded: Similar as in the other, but the program nor only stops updating the window, it also stops the calculation and sits idle. Playing with the mouse can again make it run for a short time. I attach the program in case that this might help. I did not try to extract a minimal example, but the program is short. Thanks for any help. I would also like to know if it works on other platforms/versions. Carsten module Main where {- Calculates the convex hull of a set of points in an inefficient way. O(n^3) For each `o' on stdout a red line segment should be drawn -} import qualified Graphics.HGL as HGL import System.Random import System.IO nrOfPoints = 10000 -- CONFIGURE HERE data Point = Point {xc, yc :: Float} randomPoints :: Int -> IO [Point] randomPoints n = sequence $ replicate n randomPoint where randomPoint = do phi <- randomRIO (0, 2*pi) r0 <- randomRIO (0,0.48 ** 2) let r = sqrt r0 return $ Point (r * cos phi + 0.5) (r * sin phi + 0.5) boundarySegments :: [Point] -> [(Point, Point)] boundarySegments ps = filter isBoundary (pairs' ps) where pairs' l = concat [[(a,b), (b,a)] | (a,b) <- pairs l] isBoundary s = all (`leftOf` s) ps leftOf :: Point -> (Point, Point) -> Bool q `leftOf` (p1,p2) = (xc p2-xc p1)*(yc q-yc p1) - (yc p2-yc p1)*(xc q-xc p1) >= 0 pairs :: [a] -> [(a,a)] pairs [] = [] pairs [x] = [] pairs (x:xs) = [(x,x') | x' <- xs] ++ pairs xs main :: IO () main = do HGL.runGraphics $ do points <- randomPoints nrOfPoints redPen <- HGL.createPen HGL.Solid 1 (HGL.RGB 255 0 0) w <- HGL.openWindowEx "hull" Nothing (ww,wh) HGL.Unbuffered Nothing drawInWindow w $ sequence_ $ map pt points drawInWindow w $ (HGL.selectPen redPen >> return ()) sequence_ $ map ((drawInWindow w) . seg) $ boundarySegments points drawInWindow w $ HGL.text (0,0) "done" putStr "\ndone\n" HGL.getKey w HGL.closeWindow w where drawInWindow w a = do putStr "o" >> hFlush stdout HGL.drawInWindow w a ww, wh :: Int ww = 400 * 2 wh = 300 * 2 wwf = fromIntegral ww whf = fromIntegral wh xf, yf :: Point -> Float xf p = xc p * wwf yf p = yc p * whf x, y :: Point -> Int x = round . xf y = round . yf pt :: Point -> HGL.Graphic pt (Point x y) = HGL.ellipse (round (x*wwf-2),(round(y*whf-2))) (round (x*wwf+2),(round(y*whf+2))) seg :: (Point, Point) -> HGL.Graphic seg (u,v) = HGL.line (x u, y u) (x v, y v)

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Second Call for Copy: Monad.Reader Issue 16
by Brent Yorgey 16 Mar '10

16 Mar '10

I already have one or two submissions, but a few more would be great. If you've been thinking about submitting something but weren't sure or haven't gotten around to it yet, now's the time! -------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 16 will be: **Friday, April 16, 2010** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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More strange errors when compiling code
by Giuseppe Maggiore 16 Mar '10

16 Mar '10

Hi! I am getting more problems with compilation. The two files where the problem occurs (a bit more of a general case of the sample of a few hours ago) are: ------------------------------------------------- --------------------Methods.hs------------------- ------------------------------------------------- {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} module Methods where import Records import References class Converter a rec_a where convert :: a -> rec_a data Method s a b = Method (s -> a -> (b,s)) (<<-) :: forall rec_a s a b c n . (CNum n, HasField n (b->(c,a)) (rec_a), Converter a rec_a) => (Reference s (rec_a)) -> n -> Method s b c (<<-) r n = Method(\s -> \(x :: b) -> let (v,s') = getter r s :: (rec_a,s) m = v .! n :: (b -> (c,a)) (y,v') = m x :: (c, a) v'' = convert v' :: rec_a ((),s'') = setter r s' v'' in (y,s'')) (.!!) :: Method s a b -> a -> Reference s b (Method m) .!! x = from_constant (Constant (\s -> m s x)) ------------------------------------------------- ---------------------Main.hs--------------------- ------------------------------------------------- {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} --module Main where import Records import References import ReferenceMonad import Methods val = firstLabel incr = nextLabel val instance Converter RecCounter (Counter RecCounter) where convert (RecCounter r) = r type Counter k = (Integer :* (() -> ((),k)) :* EmptyRecord) data RecCounter = RecCounter (Counter RecCounter) mk_test :: Integer -> RecCounter mk_test i = RecCounter( val .= i .* incr .= (\() -> ((), mk_test (i+1))) .* EmptyRecord) RecCounter test' = mk_test 0 res2 :: Reference (Counter RecCounter) Integer res2 = do (this <<- incr :: Method (Counter RecCounter) () ()) .!! () v <- (this <-- val) return v count = fst (getter res2 test') Whenever I try to compile I get the following error: *Methods> :load Main.hs [1 of 5] Compiling Records ( Records.hs, interpreted ) [2 of 5] Compiling References ( References.hs, interpreted ) [3 of 5] Compiling ReferenceMonad ( ReferenceMonad.hs, interpreted ) [4 of 5] Compiling Methods ( Methods.hs, interpreted ) [5 of 5] Compiling Main ( Main.hs, interpreted ) Main.hs:55:11: No instances for (HasField Z (() -> ((), a)) (AddField (() -> ((), RecCounter)) Emp tyRecord), Converter a (Counter RecCounter)) arising from a use of `<<-' at Main.hs:55:11-23 Possible fix: add an instance declaration for (HasField Z (() -> ((), a)) (AddField (() -> ((), RecCounter)) EmptyRecord), Converter a (Counter RecCounter)) In the first argument of `(.!!)', namely `(this <<- incr :: Method (Counter RecCounter) () ())' In a stmt of a 'do' expression: (this <<- incr :: Method (Counter RecCounter) () ()) .!! () In the expression: do (this <<- incr :: Method (Counter RecCounter) () ()) .!! () v <- (this <-- val) return v Failed, modules loaded: Methods, References, Records, ReferenceMonad. *Methods> Any hints? PS: just to give some perspective on the code: I am putting together a system for expressing mutable objects through monads. I could provide the rest of my code in case of need :) -- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444

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Re: [Haskell-cafe] ANNOUNCE: tupleinstances-0.0.1
by Diego Echeverri 16 Mar '10

16 Mar '10

On Tue, Mar 16, 2010 at 11:15 AM, Henning Thielemann <thunderbird(a)henning-thielemann.de> wrote: > > newtype T3 a = T3 (a,a,a) > > which could well be > > data T3 a = T3 a a a > I chose that specific representation just because at the end I want to use normal tuples. I thought this way was easier for "boxing" an "unboxing" > Are these types used for vector computations or what application do you have > in mind? I didn't have any specific application for the library. One day I just wanted to do an fmap with a tuple and somebody in #haskell suggested join (***) Of course this only works for binary tuples. I thought this would be a good excuse to play with template-haskell and give instances to some of the type classes I learnt about in the Typeclassopedia. I found it useful for vector computations. But aggregating with the Foldable instance is also handy. > Then again, is tuple-instance still an appropriate name? "There are only two hard things in Computer Science: cache invalidation and naming things" - Phil Karlton I'm open to suggestions :-) Diego Echeverri

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calling hledger zurihackers
by Simon Michael 16 Mar '10

16 Mar '10

Roman C. has cleverly added hledger to the list of topics for this weekend's ZuriHac. I will be working remotely at least on sunday (PDT), and available to support where possible. You are invited to join us! Some ideas - - add Chart or google charts to the web ui - smarter add/convert commands - separate library package - plugins - a how-to for tracking project finances; a tutorial; screencasts - portability, packaging, installability - code review/design review/planning Even one commit helps! Discussions too. Hope to see you "there". -Simon (sm) #ledger, #zurihac http://hledger.org http://www.haskell.org/haskellwiki/ZuriHac

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ANNOUNCE: direct-plugins-1.0
by Dan Knapp 16 Mar '10

16 Mar '10

For some time I have relied on Dons's Plugins package, which was perhaps something of a mistake as it's kind of messy inside and I don't think it's maintained. With the upgrade to GHC 6.12, it broke for me, and I wasn't able to get it fixed. But I was able to write my own subset of its functionality. I don't do most of the things that Plugins does - in fact I export only a single function, "load" - but I think it might be useful to somebody, so I've decided to release it. direct-plugins: Lightweight replacement for Plugins, specific to GHC http://hackage.haskell.org/package/direct-plugins This is my first post to this list; I hope it's appropriate. Please let me know if not. -- Dan Knapp "An infallible method of conciliating a tiger is to allow oneself to be devoured." (Konrad Adenauer)

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a poorly thought out suggestion for cabal and packages that have lots of instances
by Jeremy Shaw 16 Mar '10

16 Mar '10

Hello, There are a number of libraries like syb-with-class, happstack-data, etc, which provide a new class that potentially has a lot of new instances. For example, let's say I want a 'Data ctx Text' instance. Where should that go? To avoid orphan instances it would need to go in the 'text' package where Text is defined. But it seems unlikely that I am going to convince the authors of text to add additional dependencies on things like syb-with-class, happstack-data, etc, just to satisfy a small number of users. A second place to put the instance would be in the syb-with-class package. They will be orphan instances, but as long as you can't import the Data class without also getting those instances, it doesn't seem like much could go wrong. But now syb-with-class has to depend on all the libraries that it is going to provide instances for. So, I have just shifted the problem from one library to another. A third option is to just define the instance in the library where I want to use it. But now I have opened up the possibility of conflicting orphan instances for the type -- which is no good. A fourth option is to create a new package which contains just the instance. For example: syb-with-class-text This still opens the possibility of conflicting orphan instances. But at least there is a version that is supposedly the 'official' one. The problem is that it seems like this might really clutter up hackage? Also, it is annoying to have to create and maintain and entire new cabal package just for one single class instance. And if i want to install all the instances, it would be annoying to have to do 20 different cabal installs. What might be nicer is if I could have a single project directory that somehow generated all those instances. And if cabal new how to work with them as a family. Something like: cabal upload syb-with-class* cabal install syb-with-class* Obviously, I have not really thought this through all the way. I just know that I am continually wondering what I should do when I want to do something like add a Text instance for syb-with-class? If we were to get really experimental I might suggest that when you install syb-with-class, you don't automatically get all the extra instances by default. But the package system and compiler would know they exist. So if you tried to write code that required the missing instance, it would suggest that you need to install the extra package. This would help reduce instances of people accidentally creating a conflicting instance. But I am sure it makes something else horrible happen instead ;) - jeremy

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Code that doesn't compile - but should :)
by Giuseppe Maggiore 16 Mar '10

16 Mar '10

Hi! Can anyone tell me why this code does not work? I cannot seem to figure why it is broken... {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} data Data k = Pair Integer (() -> k) data RecData = RecData (Data RecData) mk_data x = RecData(Pair x (\() -> mk_data (x+1))) class Converter a f where convert :: a -> f a instance Converter RecData Data where convert (RecData r) = r class Selector s a where select :: s -> a f :: (Selector s (a->f a), Converter a f) => s -> (a->a) f s = let method = select s in (\x -> let res = method x in convert res) -- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444

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