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ANNOUNCE: The Haskell Platform 2009.2.0.2
by Don Stewart 04 Aug '09

04 Aug '09

We're pleased to announce the third release of the Haskell Platform: a single, standard Haskell distribution for everyone. The specification, along with installers (including Windows and Unix installers for a full Haskell environment) are available. Download the Haskell Platform 2009.2.0.2: http://hackage.haskell.org/platform/ The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. What you get: http://hackage.haskell.org/platform/contents.html With regular time-based releases, we expect the platform will grow into a rich, indispensable development environment for all Haskell projects. Since the first release, there have been over 100,000 downloads of the installers. Thanks! -- The Platform Infrastructure Team

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Re: Simple quirk in behavior of `mod`
by gladstein＠gladstein.com 04 Aug '09

04 Aug '09

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[Haskell Cafe] Troubles with StateT and Parsec
by Paul Sujkov 03 Aug '09

03 Aug '09

Hi haskellers, I have a few problems using monad transformers. I have such two functions: parseSyslog :: StateT Integer Parser TimeStamp parseString :: StateT Integer Parser LogString and the following code: parseString = do -- string parse here, all in the form of lift $ <parser> stamp <- lift $ lexeme parseTimestamp -- <?> "timestamp" message <- lift $ manyTill anyToken eof -- <?> "message" return (LogString <...parsed values here...> (check stamp console message) <...more parsed values here...>) where check :: (Maybe TimeStamp) -> Console -> String -> Maybe TimeStamp check Nothing Syslog message = case (lift parse $ parseSyslog "" message) of Left err -> Nothing Right res -> Just res <...other clauses here...> this code seems quite intuitive to me, however it doesn't compile with a king error: Couldn't match kind `(* -> *) -> * -> *' against `?? -> ? -> *' When matching the kinds of `t :: (* -> *) -> * -> *' and `(->) :: ?? -> ? -> *' Probable cause: `lift' is applied to too many arguments In the first argument of `($)', namely `lift parse' I'm not so familiar with monad transformers whatsoever, so I'll be very happy if someone can show me the right way. The code compile nicely if I use "parse" line in a such way: check Nothing Syslog message = case (parse (evalStateT parseSyslog 0) "" message) of but this is not what I really want. To be accurate, here is the sequence which I do want to have in the code: some user state is initialized; parseString gets called many times and changes the state via call to the parseSyslog (that is the only function that really uses/affects user state, everything else is pure Parsec code with it's own internal state). Two main problems that I have now is: 1) impossibility to use parse/parseTest functions with the (StateT <state type> Parser <parse type>) argument. I want it to be lifted somehow, but cannot see how 2) too many lifts in the code. I have only one function that really affects state, but code is filled with lifts from StateT to underlying Parser Sorry if the questions are silly; any help is appreciated -- Regards, Paul Sujkov

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Fwd: [Haskell-cafe] funct.prog. vs logic prog., practical Haskell
by Alberto G. Corona 03 Aug '09

03 Aug '09

The middle road could be Curry <http://en.wikipedia.org/wiki/Curry>, sorry, this Curry <http://www.informatik.uni-kiel.de/~curry/>, a functional-logic language. I know that curry has gained a lot of interest from prolog programmers. There are compilers from Curry to Prolog. It is a haskell 98 implementation (more or less) with all logic programming features. It can be seen also as a logic language with haskell syntax. Therefore, its syntax is more mathematical, rather that the ugly clause-based syntax of Prolog, that is at odds with anything except with pure aristothelian logic. 2009/8/2 Thomas ten Cate <ttencate(a)gmail.com> On Sun, Aug 2, 2009 at 12:25, Petr Pudlak<deb(a)pudlak.name> wrote: > > Hi all, > > > > I'd like to convince people at our university to pay more attention to > > functional languages, especially Haskell. Their arguments were that > > > > (1) Functional programming is more academic than practical. > > Which, even if it were true, is an argument *for* instead of *against* > teaching it at a university; that is what the word "academic" means, > after all... > > Thomas > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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ANN: yst 0.2.1
by John MacFarlane 03 Aug '09

03 Aug '09

I'm pleased to announce the release of yst, now available on HackageDB. yst generates static websites from YAML or CSV data files and StringTemplates. This approach combines the speed, security, and ease of deployment of a static website with the flexibility and maintainability of a dynamic site that separates presentation and data. The easiest way to get a feel for yst is to try it: cabal update cabal install yst yst create testsite cd testsite yst yst attempts to fill a niche between two kinds of site creation tools. On the one hand you have simple static site generators like webgen, webby, nanoc, and my old custom system using make and pandoc. On the other hand, you have dynamic web frameworks like rails and django. For my own smallish websites, I found that the dynamic frameworks were overkill. Nobody but me was going to edit the pages, and I didn't want the trouble of writing and deploying a dynamic site, setting up a web server, and administering a database. A static site would be faster, easier to deploy, and more secure. But the dynamic frameworks offered one thing that the static site generators did not: an easy way to separate data from presentation. This was becoming increasingly important to me as I found myself constantly updating the same information (say, publication data for a paper) in multiple places (say, a LaTeX CV and a differently formatted web listing of papers). What I wanted was a site generation tool that used YAML text files as a database and allowed different kinds of documents to be produced from the same data. I couldn't find anything that did just what I wanted, so I wrote yst. By way of illustration, here are the build instructions for HTML and LaTeX versions of a CV, plus a web page with a list of papers: - url: cv.html title: CV template: cv.st data_common: &cvdata contact: from contact.yaml jobsbyemployer: from jobs.yaml order by start group by employer degrees: from degrees.yaml order by year desc awards: from awards.yaml order by year desc group by title papers: from papers.yaml order by year desc where (not (type = 'review')) reviews: from papers.yaml order by year desc where type = 'review' talks: from talks.yaml where date < '2009-09-01' order by date desc group by title dissertations: from dissertations.yaml order by role then year group by role theses: from theses.yaml order by year then student courses: from courses.yaml order by number group by title data: <<: *cvdata html: yes - url: cv.tex title: CV inmenu: no template: cv.st layout: layout.tex.st data: <<: *cvdata html: yes - url: papers.html title: Papers template: papers.st data: papersbyyear: from papers.yaml order by year desc then title group by year yst's query language is limited, and there are lots of things you can do with a full-fledged database that you can't do with yst. But yst is ideal, I think, for small to medium data-driven sites that are maintained by a single person who likes working with plain text. It scratched my itch, anyway, and I release it in case anyone else has the same itch. Code, documentation, and bug reports: http://github.com/jgm/yst/tree/master John

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Writing a pnm file
by CK Kashyap 03 Aug '09

03 Aug '09

Hi, Now that I've understood how to generate raster points of a line in Haskell - the next thing I want to do is generate a pnm file with it. I've done it in perl as of now. In perl, I can have a scalar variable $x contain a string of 256*256*3 bytes (for 24-bit 256x256 image) and set pixels using substr on LHS. I was wondering how I could do something similar in Haskell? sub setPixel{ my($x,$y,$red,$green,$blue)=@_; my$pixel=pack "CCC",$red,$green,$blue; my$offset=$WIDTH*$y*3 + $x*3; substr($image,$offset,3) = $pixel; } Regards, Kashyap

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Hackage and version control
by Vasili I. Galchin 03 Aug '09

03 Aug '09

Hello, It seems to me that Hackage doesn't provide version control, e.g. check out and check in. Am I incorrect? Kind regards, Vasili

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FastCGI error handling
by Victor Nazarov 03 Aug '09

03 Aug '09

I've been trying to write some simple web application in haskell using FastCGI, HDBC and HStringTemplate. I've got stuck with the following problem. HDBC throws some exceptions and I wanted them to be caught and logged. The code was: -- Main.hs module Main (main) where ... import Control.Concurrent import Network.FastCGI driver :: CGI CGIResult driver = ... main :: IO () main = runFastCGIConcurrent' forkIO 10 (handleErrors test) But all I've got on the page and in the log is "SomeException" string. I've tried the workaround: -- Main.hs module Main (main) where import Network.FastCGI import Control.Exception import System.IO driver :: CGI CGIResult driver = ... ... main :: IO () main = runFastCGI driver `catch` \ex -> hPutStr stderr $ show (ex::SomeException) And this worked fine. I've got a detaied SqlException string in the log file. So I've decided to update exception handling in CGI package and written the following module: -- Network/CGI/Monad/NewException.hs module Network.CGI.Monad.NewException where import Prelude hiding (catch) import Control.Exception import Control.Monad import Control.Monad.Writer import Control.Monad.Reader import Network.CGI.Monad hiding (throwCGI, catchCGI, tryCGI) import Network.CGI (outputInternalServerError) -- | Throw an exception in a CGI monad. The monad is required to be -- a 'MonadIO', so that we can use 'throwIO' to guarantee ordering. throwCGI :: (MonadCGI m, MonadIO m, Exception e) => e -> m a throwCGI = liftIO . throwIO -- | Catches any expection thrown by a CGI action, and uses the given -- exception handler if an exception is thrown. catchCGI :: (Exception e) => CGI a -> (e -> CGI a) -> CGI a catchCGI c h = tryCGI c >>= either h return handleCGI :: (Exception e) => (e -> CGI a) -> CGI a -> CGI a handleCGI = flip catchCGI -- | Catches any exception thrown by an CGI action, and returns either -- the exception, or if no exception was raised, the result of the action. tryCGI :: (Exception e) => CGI a -> CGI (Either e a) tryCGI (CGIT c) = CGIT (ReaderT (\r -> WriterT (f (runWriterT (runReaderT c r))))) where f = liftM (either (\ex -> (Left ex,mempty)) (\(a,w) -> (Right a,w))) . try handleErrors = handleCGI outputException outputException e = outputInternalServerError [show (e :: SomeException)] Then I've updated Main.hs accordingly: module Main (main) where ... import Control.Concurrent import Network.FastCGI hiding (throwCGI, catchCGI, tryCGI, handleErrors, outputException) import Network.CGI.Monad.NewException driver :: CGI CGIResult driver = ... main :: IO () -- main = runFastCGIConcurrent' forkIO 10 (handleErrors test) -- This allways gives internal error without a chance to find out what happens main = runFastCGI $ handleErrors test -- This works fine So one variant of main function works, the other allways gives no output. What can be the case? -- Victor Nazarov

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haskell code pretty printer ?
by Paul.Brauner＠loria.fr 03 Aug '09

03 Aug '09

Hello, is there a pretty printer for haskell code somewhere ? I've googled and caballisted for this without success. I've written some small script using Language.Haskell.Pretty and Language.Haskell.Parser but the result isn't that 'pretty'. I mean, it outputs readable code but supercombinator's definitions aren't separated by a blank line for instance (despite having put 'spacing' to 'True'). Do you know of such a tool ? Regards, Paul

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ANN: atom 0.1.0
by Tom Hawkins 03 Aug '09

03 Aug '09

Atom is a Haskell DSL for hard realtime applications. This release includes support for assertions and functional coverage to aid simulation and testing. The rev of the minor version indicates a bit of library stability. This is the version we're using for our application, which officially went into production and hit the road last month -- literally. http://hackage.haskell.org/package/atom -Tom

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IFL 2009: Call for Papers and Participation
by IFL 2009 02 Aug '09

02 Aug '09

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Heap profiling project update
by Patai Gergely 02 Aug '09

02 Aug '09

Hi all, It's been a while since I announced anything about the project on the list, but I've been regularly posting about it on my blog [1]. Everyone should feel encouraged to check out the cabalised code [2] and play with it, stress test it. Adventurous ones can also look at the source. ;) Gergely [1] http://just-bottom.blogspot.com/ [2] http://code.google.com/p/hp2any/source/checkout -- http://www.fastmail.fm - Send your email first class

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Possible bug in Data.IP (network-data package)
by Mads Lindstrøm 02 Aug '09

02 Aug '09

Hi The network-data (version 0.0.2) [1] contains the module Data.IP. This modules encapsulates IPv4 headers. The Data.IP modules contains the type IPv4Header: data IPv4Header = IPv4Hdr { hdrLength :: Int , version :: Int , tos :: Int , payloadLength :: Int ... The code to turn IPv4 packages into IPv4Heder looks like [2]: instance Binary IPv4Header where put (IPv4Hdr ihl ver tos len id flags off ttl prot csum src dst) = do pW8 $ (ihl .&. 0xF) .|. (ver `shiftL` 4 .&. 0xF0) pW8 tos pW16 len pW16 id let offFlags = (off .&. 0x1FFF) .|. fromIntegral (fromEnum flags `shiftL` 13) pW16 offFlags pW8 ttl pW8 prot put csum put src put dst That is, the payload length is the 16-31'th bit of the IPv4 header. This field is according to [3] and [4] referred to as "total length" - not "payload length". The total length include both the length of the data and the header. When I read the term "payload length" I first thought it referred to the length of the package excluding the header. So I am right to see this as a bug in network-data ? Regards, Mads Lindstrøm [1] http://hackage.haskell.org/package/network-data [2] http://hackage.haskell.org/packages/archive/network-data/0.0.2/doc/html/src… [3] http://tools.ietf.org/html/rfc791 [4] http://en.wikipedia.org/wiki/IPv4#Header

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Retrieving inner state from outside the transformer
by Phil 01 Aug '09

01 Aug '09

Hi, I've hit a brick wall trying to work out, what should be (and probably is!) a simple problem. I have a StateT stack (1 State monad, 2 StateT transformers) which works fine and returns the result of the outer monad. I thought I understood this fine, but perhaps not. My understanding is that the result returned by the inner-most monad is always 'transformed' by the outer monads and thus the result you get is that computed in the outer transformer. The problem I have is now I'd like not only to get the final state of the outer most transformer, but I'd also like to know the final states of the the inner StateT and the inner State at the end of the computation (so that at a later point in time I can reinitialize a similar stack and continue with the same set of states I finished with). So I figured I could have a separate (parent) State Monad (not part of this stack) that would store the final state of the sequence below. I figured it couldn't be part of this stack, as one computation on the stack does not lead to one result in the parent State Monad; it is only the end states of the sequence I care about. Anyway, currently I just have the stack evaluated as below. Is there anyway from outside of the computation that I can interrogate the states of the inner layers? The only way I can see to do this is inside the outer monad itself. As I'm not using the result I could use 'lift get' and 'lift lift get' to make the outer transformer return the two inner states as it's result. I could ignore this result for the first (iterations-1) and bind a final iteration which uses replicateM instead of replicateM_. This strikes me as pretty horrible tho! So, in the example below if I want to modify the 'result' function so it returns no only the outer state, but also the two inners states as a tuple (Double,Double,Double) is there an easier way of doing this? result :: RngClass a => NormalClass b => a -> b -> MonteCarloUserData -> Double result initRngState initNormState userData = evalState a initRngState where a = evalStateT b initNormState b = execStateT ( do replicateM_ (iterations userData) (mc userData)) 0 Any advice greatly appreciated! Thanks, Phil.

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Re: [Haskell-cafe] ANN: atom 0.1.0
by Lee Pike 01 Aug '09

01 Aug '09

Note too that there's an example Atom program included with the source: http://hackage.haskell.org/packages/archive/atom/0.1.0/doc/html/Language-At… Lee > dons wrote: > We've had a few people playing with Atom to program the Arduino, and > John van Enk's been hacking too, > > Atom & Arduino :: Some Hacking (pt. 1) > http://blog.sw17ch.com/wordpress/?p=84 > > > An Atomic Fibonacci Server: Exploring the Atom (Haskell) DSL > http://leepike.wordpress.com/2009/05/05/an-atomic-fibonacci-server-explorin… > > Galois will prob. have a tech talk soon. > > -- Don > > thomas.dubuisson: >> Tom, >> I was asking earlier about any good sources of information for atom. >> It seems the once-good wiki is gone - are there tutorials for Atom >> hiding in forgotten corners? >> >> Thomas >> >> On Fri, Jul 31, 2009 at 1:49 PM, Tom Hawkins<tomahawkins(a)gmail.com> >> wrote: >>> Atom is a Haskell DSL for hard realtime applications. This release >>> includes support for assertions and functional coverage to aid >>> simulation and testing. The rev of the minor version indicates a >>> bit >>> of library stability. This is the version we're using for our >>> application, which officially went into production and hit the road >>> last month -- literally. >>> >>> http://hackage.haskell.org/package/atom >>> >>> -Tom >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>

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Re: Strange memory usage problem
by John Lato 01 Aug '09

01 Aug '09

> From: Ketil Malde <ketil(a)malde.org> > > Hi, > > I'm observing something I find quite strange. I have a program that > normally streams through a file, and it runs in the expected constant > space, consuming about 5MB, according to 'top'. When I uncomment a > certain block of code in the library it uses, space consumption > appears to grow linearly with the input size. > > This is a bit surprising to me, since the code in question isn't used > by the program at all, and yesterday, Cale confirmed this behavior on > a recent GHC (6.10.3, I think - mine is 6.8.2 as shipped with Ubuntu > 9.04). > I can confirm this behavior on GHC 6.10.4 on OSX 10.5.7. Try adding {-# INLINE getRB #-} above the getRB definition. That fixes it for me. I think that when the rest of the code is commented out, getRB is only called in one location, so GHC inlines it automatically. Since getRB is also called in the commented-out code, it probably isn't automatically inlined when that code is available. I suspect that the inlining allows GHC to infer some strictness property it otherwise can't. Cheers, John Lato

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Re: [Haskell] ANNOUNCE: OpenGL 2.3.0.0
by Felipe Lessa 01 Aug '09

01 Aug '09

On Wed, Jul 29, 2009 at 06:26:30PM +0200, Sven Panne wrote: > Apart from that, a bug in vertexAttribPointer has been fixed. ...and the new ObjectName, StateVar and Tensor packages are being used, and this is great! Thanks for the release. -- Felipe.

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ANN: tkhs-0.1.* Presentation Utility
by Yusaku Hashimoto 31 Jul '09

31 Jul '09

Hi, I'm pleased to announce the release of tkhs-0.1.*, Simple presentation utility. If you are thinking PowerPoint is overkill for your presentation, Tkhs may fit the purpose. See screenshot of running tkhs in my terminal: http://nonowarn.tumblr.com/post/152324109 When you invoke tkhs with slide file written in simple format text, tkhs displays the first slide. And you can operate a presentation by some keys. Details like slide file format and keybinds for operating slide are available at README[1] file included by the package. Project is hosted at patch-tag[2], Feel free to send me patch. Finally, I'm very grateful Evan Klizke and Brian Lewis. Evan is the author of hspresent[3] which inspired me to write tkhs. Brian sent me patch improving English. [1]: http://patch-tag.com/r/tkhs/snapshot/current/content/pretty/README [2]: http://patch-tag.com/r/tkhs/pullrepo [3]: http://hackage.haskell.org/package/hspresent Thanks, nwn

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Need feedback on my Haskell code
by CK Kashyap 31 Jul '09

31 Jul '09

Hi Everyone, I managed to write up the line drawing function using the following links - http://www.cs.helsinki.fi/group/goa/mallinnus/lines/bresenh.html http://rosettacode.org/wiki/Bresenham%27s_line_algorithm#Haskell line :: Point -> Point -> [Point] line (xa,ya) (xb,yb) = line' (x1,y1) (x2,y2) deltax deltay ystep isSteep 0 where isSteep = abs (yb - ya) > abs (xb - xa) (xa',ya',xb',yb') = if isSteep then (ya,xa,yb,xb) else (xa,ya,xb,yb) (x1,y1,x2,y2) = if xa' > xb' then (xb',yb',xa',ya') else (xa',ya',xb',yb') deltax = x2 - x1 deltay = abs (y2 - y1) ystep = if y1 < y2 then 1 else -1 line' :: Point -> Point -> Integer -> Integer -> Integer -> Bool -> Integer -> [Point] line' (x1,y1) (x2,y2) deltax deltay ystep isSteep error = if x1 == x2 then if isSteep then [(y1,x1)] else [(x1,y1)] else if isSteep then [(y1,x1)] ++ line' (newX,newY) (x2,y2) deltax deltay ystep isSteep newError else [(x1,y1)] ++ line' (newX,newY) (x2,y2) deltax deltay ystep isSteep newError where newX = x1 + 1 tempError = error + deltay (newY, newError) = if (2*tempError) >= deltax then (y1+ystep,tempError-deltax) else (y1,tempError) Can someone please provide feedback on this? In terms of, how do I get more Haskell'ism into it. Regards, Kashyap

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Strange memory usage problem
by Ketil Malde 31 Jul '09

31 Jul '09

Hi, I'm observing something I find quite strange. I have a program that normally streams through a file, and it runs in the expected constant space, consuming about 5MB, according to 'top'. When I uncomment a certain block of code in the library it uses, space consumption appears to grow linearly with the input size. This is a bit surprising to me, since the code in question isn't used by the program at all, and yesterday, Cale confirmed this behavior on a recent GHC (6.10.3, I think - mine is 6.8.2 as shipped with Ubuntu 9.04). Since this appears to be reproducible, I put together a test case. The steps are: darcs get http://malde.org/~ketil/biohaskell/biolib-unstable darcs get http://malde.org/~ketil/biohaskell/flower Build them using the usual incantations (./Setup.hs configure/build/install). Download the test input file: wget http://malde.org/~ketil/test.sff Run for instance this command line: fselect "And (Func GT len 100) (Func GT k2 0.5)" test.sff This should now run in constant space. To observe the linear growth space, unpull the last patch from biolib, i.e. this one: Fri Jul 31 00:14:09 CEST 2009 Ketil Malde <ketil(a)malde.org> * comment out recover functionality - otherwise FSelect leaks memory! M ./Bio/Sequence/SFF.hs -1 +7 This uncomments some otherwise unused code, which apparently triggers this behavior. I'm unable to see what causes this, so any suggestions welcome. -k -- If I haven't seen further, it is by standing in the footprints of giants

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gtksourceview link problem with leksah on windows
by Bart De Vylder 31 Jul '09

31 Jul '09

Dear Haskellers, i'm having troubles linking Leksah against gtksourceview on windows (XP). I've installed gtk2hs (using the windows installer) but am not sure whether/where to specify the library search path for ld. Here is the output I get: C:\Documents and Settings\bartdv>cabal install leksah Resolving dependencies... Configuring leksah-0.6.1... Preprocessing executables for leksah-0.6.1... Building leksah-0.6.1... [ 1 of 55] Compiling DeepSeq ( src\DeepSeq.hs, > dist\build\leksah\leksah-tmp\DeepSeq.o ) > ... > [55 of 55] Compiling Main ( src\Main.hs, > dist\build\leksah\leksah-tmp\Main.o ) Linking dist\build\leksah\leksah.exe ... C:\app\Haskell_Platform\2009.2.0.1\gcc-lib\ld.exe: cannot find > -lgtksourceview-2.0 collect2: ld returned 1 exit status cabal: Error: some packages failed to install: leksah-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1 any help/ tips are very welcome, I'm eager to leave emacs... Bart

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Deepest polymorphic functor
by Eduard Sergeev 31 Jul '09

31 Jul '09

I was wondering if it is possible to somehow change "deep" f_map from http://okmij.org/ftp/Haskell/deepest-functor.lhs article in a such a way that it would work not only for monotypes like in the provided example: test1 = f_map (+1) [[[1::Int,2,3]]] But for polymorphic types as well (e.g. behaves like simple map) so the following line would compile as well: test1 = f_map (+1) [[[1,2,3]]] ? -- View this message in context: http://www.nabble.com/Deepest-polymorphic-functor-tp24709303p24709303.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] [Haskell Cafe] Data construction: how to avoid boilerplate code?
by Luke Palmer 30 Jul '09

30 Jul '09

On Wed, Jul 29, 2009 at 10:15 AM, Paul Sujkov<psujkov(a)gmail.com> wrote: > Hi Luke, > > I'm not pretty sure. The thing I don't like is the need to copy-and-paste > all the code with enumeration constructors. So, now I have two types to make > Type data from, but they could be many, so I'll have many almost identical > makeTypeFromFoo-functions. The thing I need is something like (*): > > makeType :: ? -> Type > makeType c = case c of > ("-$-" or 240) -> Status > ("-M-" or 64) -> Message > ("-W-" or 32) -> Warning Well, you could write a helper like this: matchType :: (Eq a) => (a,a,a) -> a -> Type matchType (status,message,warning) x | x == status = Status | x == message = Message | x == warning = Warning To reduce the size of your specifications: makeTypeStr = matchType ("-$-", "-M-", "-W-") makeTypeInt = matchType (240, 64, 32) There are trade-offs to doing something like this. It's smaller, but harder to read as specification. But, because it uses a tuple, it will catch you if you add a new case but forget to add it to one of the makeType*s (providing you remember to change matchType). What you're asking for puts all the conversions in the same place, which forbids them from being split out, decoupled, and modularlized. What if, instead of simple values, you had a more involved parser for these things? Even though it's kind of verbose, I think what you already have is fine. You do have to repeat the names, but it is still content code, and the relationship of the content to the names is explicit. You might find tables like this in a user's manual for your software... Luke > > then I have all this fabric code in one place. I could use something like > Either for this example, but it will scale fine up to two types to build > value from; while I want to have arbitrary numbers of them > > And from another point, it would be great if I could avoid doubling the code > for datatype declaration with empty (no parameter) constructors, and actual > "constructing" code from the different types. So it could look somewhat > alike this (pseudo-code) (**): > > data Type = Status <-- (String "-$-", Integer 240) > | Message <-- (String "-M-", Integer 64) > | Warning <-- (String "-W-", Integer 32) > > however, I'm not sure this one is really needed. Something for the (*) is > much more interesting > > 2009/7/29 Luke Palmer <lrpalmer(a)gmail.com> >> >> On Wed, Jul 29, 2009 at 6:27 AM, Paul Sujkov<psujkov(a)gmail.com> wrote: >> > Hi haskellers, >> > >> > I have a datatype of this sort: >> > >> > data Type = Status >> > | Message >> > | Warning >> > | Error >> > | StepIn >> > | StepOut deriving (Eq, Show) >> > >> > and (at this moment) two fabric-like functions: >> > >> > makeType :: String -> Type >> > makeType c = case c of >> > "-$-" -> Status >> > "-M-" -> Message >> > "-W-" -> Warning >> > "-E-" -> Error >> > "->>" -> StepIn >> > "<<-" -> StepOut >> > otherwise -> error "Uknown Message Type" >> > >> > deduceType :: Integer -> Type >> > deduceType n = case n of >> > 240 -> Status >> > 64 -> Message >> > 32 -> Warning >> > 8 -> StepOut >> > 4 -> StepIn >> > 2 -> Error >> > otherwise -> error "Unknown Message Type" >> > >> > how can I avoid boilerplate code at this stage? The thing that I really >> > need >> > is some n-type constructor, kind of a fabric for a variety of types with >> > the >> > possibility to add them on the fly (I have simple Integer and String >> > here, >> > but they could be much more sophisticated). I don't need the possibility >> > to >> > unpack the original value (e.g. "-$-" or 240) once the Type is created - >> > in >> > this case I can always have some sort of mapping to deduce it at any >> > moment, >> > so it will be redundant to carry it through the code explicitly >> >> I am not sure what you're asking. Are you saying that what you have >> written is boilerplate? What code are you writing that could be >> automatically deduced with enough smartness? >> >> Give an example of what you might like the solution to look like. >> >> Luke >> >> > The example is rather short and simple, but I have some more places in >> > code, >> > where the same problem is observed. Is there any generic solution? >> > Thanks in >> > advance >> > >> > -- >> > Regards, Paul Sujkov >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> > > > > > -- > Regards, Paul Sujkov >

2 1

[Haskell Cafe] Data construction: how to avoid boilerplate code?
by Paul Sujkov 29 Jul '09

29 Jul '09

Hi haskellers, I have a datatype of this sort: data Type = Status | Message | Warning | Error | StepIn | StepOut deriving (Eq, Show) and (at this moment) two fabric-like functions: makeType :: String -> Type makeType c = case c of "-$-" -> Status "-M-" -> Message "-W-" -> Warning "-E-" -> Error "->>" -> StepIn "<<-" -> StepOut otherwise -> error "Uknown Message Type" deduceType :: Integer -> Type deduceType n = case n of 240 -> Status 64 -> Message 32 -> Warning 8 -> StepOut 4 -> StepIn 2 -> Error otherwise -> error "Unknown Message Type" how can I avoid boilerplate code at this stage? The thing that I really need is some n-type constructor, kind of a fabric for a variety of types with the possibility to add them on the fly (I have simple Integer and String here, but they could be much more sophisticated). I don't need the possibility to unpack the original value (e.g. "-$-" or 240) once the Type is created - in this case I can always have some sort of mapping to deduce it at any moment, so it will be redundant to carry it through the code explicitly The example is rather short and simple, but I have some more places in code, where the same problem is observed. Is there any generic solution? Thanks in advance -- Regards, Paul Sujkov

2 1

Re: Re: Proposal: TypeDirectedNameResolution
by Johannes Waldmann 29 Jul '09

29 Jul '09

While the discussion centers around overload resolution let me re-iterate a point that (e.g.,) Java does nicely: for their "x.f", if x :: T, then you write .f (unqualified) instead of .T.f (qualified), even if you needed qualification to declare the type of x, as in "foo.bar.T x;" This requires a mixture of type checking (for x) and name resolution (for f), which probably wouldn't fit with Haskell's current module and type system. Too bad ... Best - Johannes.

1 0

RFC: Unicode support in Alex
by Jean-Philippe Bernardy 29 Jul '09

29 Jul '09

Hello, I have modified the Alex lexer generator to support unicode. The general idea is that the state-machine works on the UTF8 representation of the text. I submit my work here for review in order to off-load the maintainer (Simon Marlow) as far as possible. The prototype is available on github: git://github.com/jyp/Alex.git Be sure to * checkout the "utf8" branch (so "git diff master" shows the changes) * Do a 2-stage bootstrapping before testing Caveats: * The generated code depends on some utf8 packages; * There is no attempt to fix the bytestring-based wrappers; * Left-context recognition is not table-based any more; * Presence of debug code. Bug reports, comments, and especially patches are welcome :) Thanks, -- JP

1 0

Adding a field to a data record
by Henry Laxen 29 Jul '09

29 Jul '09

Dear Group, It seems to me this should be easy, but I can't quite figure out how to do it without a lot of typing. Here is the question: Suppose you have a data type like: Data Foo = Foo { a :: Int, b :: Int, ... many other fields ... y :: Int } deriving (Eq, Read, Show, Typeable, Data) Now I would like to add a field z :: Int to the end of Foo. If I have a ton of data out on disk, which I wrote with, say writeFile "a.data" (show foo) -- where foo is a [Foo] say 1000 long, I would like to get a new "a.data" file which has a new z::Int field. So far the only way I can think of is to make a new Data Foo1, which includes the z::Int, read in a.data as a list of Foo, write a function like: fooTofoo1 :: Foo -> Foo1 fooTofoo1 xx = Foo1 {a = a xx, ... y = y xx, z = 1} then write the file back out, and perhaps use emacs to query-replace all the Foo1's back to Foo's, add the z::Int field back into Foo, and read it back. Please tell me there is a better way. Thanks in advance. Best wishes, Henry Laxen PS: I have read syb1, and syb2 a couple of times now, but so far haven't been able to connect it with this kind of problem.

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ANN: The Monad.Reader - Issue 14
by Wouter Swierstra 29 Jul '09

29 Jul '09

Dear all, I am pleased to announce that a new issue of The Monad.Reader is now available: http://themonadreader.wordpress.com/ Issue 14 consists of the following three articles: * Fun with Morse Code by Heinrich Apfelmus * Hieroglyph 2: Purely Functional Information Graphics Revisited by Jefferson Heard * Lloyd Allison’s Corecursive Queues: Why Continuations Matter by Leon P Smith Please note that I've moved the Monad.Reader to a new Wordpress blog. You may want to update your bookmarks. If you’d like to write something for the next issue of The Monad.Reader, please get in touch. I haven’t fixed the deadline for the next issue just yet, but expect a deadline late 2009. Wouter

1 0

runProcess does not care about hSetBuffering?
by Magicloud Magiclouds 29 Jul '09

29 Jul '09

Hi, Code like: (or_, ow_) <- createPipe or <- fdToHandle or_ ow <- fdToHandle ow_ hSetBuffering ow LineBuffering hSetBuffering or LineBuffering h <- runProcess cmd [] Nothing Nothing Nothing (Just ow) Nothing In the cmd process, the ow is not LineBuffering.... -- 竹密岂妨流水过山高哪阻野云飞

2 1

an instance in other than the last type parameters
by Petr Pudlak 28 Jul '09

28 Jul '09

Hi, I have probably a very simple question, but I wasn't able to figure it out myself. Consider a two-parameter data type: > data X a b = X a b If I want to make it a functor in the last type variable (b), I can just define > instance Functor (X a) where > fmap f (X a b) = X a (f b) But how do I write it if I want X to be a functor in its first type variable? Is that possible at all? Something like: > instance Functor ??? where > fmap f (X a b) = X (f a) b Thanks in advance, Petr

8 12

ghci identifier listing question
by Daniel van den Eijkel 28 Jul '09

28 Jul '09

Hi, in GHCi, after pressing the tab-key, all defined identifiers are listed. If an identifier was redefined, all old and shadowed versions are listed, too. Is it possible, to reach the (shadowed) values in any way? Another question: Is it possible to show only those identifiers that are defined during the interactive session? I only can list all identifiers which start with a given prefix, but I would like to know if GHCi can show me all identifiers defined in a given module or in the current session (by pressing Tab or alike). Best regards, Daniel

2 2

A Question of Restriction
by Brian Troutwine 28 Jul '09

28 Jul '09

Hello all. I would like to define a data type that is the super-set of several types and then each of the proper subset types. For example: data Foo = One | Two | Three | Four data Odd = One | Three data Even = Two | Four This, of course, does not work. It seems that such a thing should possible to express entirely in the type system, but I cannot think of how. Would someone be so kind as to explain how this sort of thing can be accomplished? Thanks, Brian

8 9

Importing Control.Arrow changes inferred type of (m >>= f) x in ghci
by Dan Weston 28 Jul '09

28 Jul '09

The following inferred type has a constraint that can be trivially satisfied, but isn't: Control.Monad> :t \ (m,f,x) -> (m >>= f) x \ (m,f,x) -> (m >>= f) x :: forall t a b. (Monad ((->) t)) => (t -> a, a -> t -> b, t) -> b -- In Control.Monad there is forall t. instance Monad ((->) t), -- so why is the vacuous Monad constraint still there? -- Nor can I remove it with a type annotation: Control.Monad> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b <interactive>:1:13: Inferred type is less polymorphic than expected [snip] -- Then if I just import a module: Control.Monad> :m + Control.Arrow -- Now, the Monad ((-> t) constraint disappears: Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b -- Although it still will not accept an explicit type annotation: Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b <interactive>:1:13: Inferred type is less polymorphic than expected [snip] Is there some explicit kind annotation I can/should give to get the most general type? Dan

2 1

Re: [Haskell] Re: 20 years ago
by Ketil Malde 28 Jul '09

28 Jul '09

[redirected from haskell@] Benjamin L.Russell <DekuDekuplex(a)Yahoo.com> writes: >> One often amusing outgrowth of this is that FP (OOP) fanatics anthropomorphize >> their functions (objects). Well, I don't think we do. Functions are just mappings of values to values, they may be opaque, but they're predictable, unchanging, and just...too boring to be antropomorphized. Objects contain all kinds of hidden state and dependencies, and the sheer unpredicatability of it all is the reason for the anthropomorphics - it a symptom of a disease, not a desirable quality. -k -- If I haven't seen further, it is by standing in the footprints of giants

5 9

Testing nested implication properties with QuickCheck?
by Ahn, Ki Yung 27 Jul '09

27 Jul '09

How should I you use QuickCheck for testing a property that is a nested implecation such as (A ==> B) ==> C ? The problem is (==>) in QuickCheck is that its type is Testable prop => Bool -> prop -> Property rather than Testable prop => prop -> prop -> Property. So, A ==> (B ==> C) would work but (A ==> B) ==> C won't work when A, B, C are boolean expressions. Is there some trick to handle the latter? Thanks, -- Ahn, Ki Yung

2 1

Testing nested implication properties with QuickCheck?
by Ahn, Ki Yung 27 Jul '09

27 Jul '09

How should I use QuickCheck for testing a property that is a nested implecation such as (A ==> B) ==> C ? The problem is (==>) in QuickCheck is that its type is Testable prop => Bool -> prop -> Property rather than Testable prop => prop -> prop -> Property. So, A ==> (B ==> C) would work but (A ==> B) ==> C won't work when A, B, C are boolean expressions. Is there some trick to handle the latter? Thanks, -- Ahn, Ki Yung

1 0

'cabal install lhs2tex' fails for me on os x -- help please
by Conal Elliott 27 Jul '09

27 Jul '09

Fails during configuration: bash-3.2$ cabal install lhs2tex Resolving dependencies... [1 of 1] Compiling Main ( /tmp/lhs2tex-1.1423397/lhs2tex-1.14/Setup.hs, /tmp/lhs2tex-1.1423397/lhs2tex-1.14/dist/setup/Main.o ) Linking /tmp/lhs2tex-1.1423397/lhs2tex-1.14/dist/setup/setup ... Configuring lhs2tex-1.14... cabal: Error: some packages failed to install: lhs2tex-1.14 failed during the configure step. The exception was: exit: ExitFailure 11 Note that the the first module to be compiled is Main. On my linux machine, Main is the *last* of several modules to be compiled. Here's my failure log: http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=3096#a3096 . If instead I use ./configure and make, i get errors due to use of the old Exception API: ... [17 of 19] Compiling FileNameUtils ( FileNameUtils.lhs, FileNameUtils.o ) FileNameUtils.lhs:81:35: Ambiguous type variable `b' in the constraint: `GHC.Exception.Exception b' arising from a use of `catch' at FileNameUtils.lhs:(81,35)-(89,60) Probable fix: add a type signature that fixes these type variable(s) ... Suggestions greatly appreciated! - Conal

3 4

Retiring forkIO
by Günther Schmidt 27 Jul '09

27 Jul '09

Hi, I'd like to retire forkIO and friends by using Delimited Continuations instead. Am I dead wrong here in my understanding of Delimited Continuations or can I pursue in this direction? The most immediate use for this is actully GUI problems where I'd use del-conts in place of forkIO for long-running computations because once I forkIO the computation so as not to block the UI thread I cannot call GUI code from the end of that thread. Well I can but then the app crashes because of single threading issues. Günther

2 2

Is there a sample code for hdbus?
by Magicloud Magiclouds 27 Jul '09

27 Jul '09

Hi, Although I have used dbus with ruby a little, but still, I do not understand hdbus's APIs.... Is there any sample that I could learn? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

1 0

Data.Binary.encode slower than show
by Grzegorz Chrupała 27 Jul '09

27 Jul '09

Hi all, I have a piece of code where I'm serializing a datastructure with the following type [(Int, (Map DType (IntMap Int)))], using Binary.encode The thing is it is very slow: actually quite a bit slower than just using show. This seems rather suspicious. Any idea what could be going on? import qualified Data.Map as Map import qualified Data.IntMap as IntMap import Data.IntMap ((!)) import Vector import Atom import Control.Monad.State import Indexing (indexBy) import Data.List (foldl') import System import System.IO import Debug.Trace import qualified Data.Binary as Binary import qualified Data.ByteString.Lazy as BS data DType = Fol | For | Sol | Sor deriving (Eq,Enum,Ord,Show,Read) instance Binary.Binary DType where get = do x <- Binary.get return (toEnum x) put x = Binary.put (fromEnum x) type Word = Int type Count = Int type WordMap = IntMap.IntMap find = IntMap.findWithDefault IntMap.empty distReps :: Map.Map (Word,Word) Count -> [(Int, (Map.Map DType (WordMap Count)))] distReps bidict = let bigrams = Map.toList bidict for = indexBy id bigrams fol = indexBy swap bigrams ws = map (fst . fst) bigrams r = flip map ws $ \w -> (w,Map.fromList [ (Fol , find w fol) , (For , find w for) , (Sol , sox for fol w ) , (Sor , sox fol for w ) ]) in trace (show $ r == r) r sox :: WordMap (WordMap Count) -> WordMap (WordMap Count) -> Word -> WordMap Count sox fox foy w = let xs = IntMap.keys (find w fox) f !z x = let xv = find x foy in xv == xv `seq` z `plus` xv in foldl' f IntMap.empty xs swap (!a,!b) = (b,a) readBigrams = fmap (map (\ [w,w',c] -> ((w,w'),read c)) . map words . lines ) getContents main = do [f] <- getArgs bigrams <- readBigrams let (bigrams',as) = flip runState empty (atomize bigrams) dr = distReps (Map.fromList bigrams') write f dr hPutStrLn stderr "Done this" write (f++".atom") as --write f d = writeFile f (show d) write f d = BS.writeFile f (Binary.encode d) atomize xs = mapM f xs where f ((w,w'),!i) = do !i_w <- toAtom w' !i_w' <- toAtom w let r = ((i_w,i_w'),i) r == r `seq` return r -- Grzegorz

2 1

TBC: Testing By Convention
by Peter Gammie 27 Jul '09

27 Jul '09

Hello, Mark and I would like to announce our test harness, which has features complementary to existing harnesses. TBC provides two main features: - It attempts to compile and run all tests, even if some do not compile or run. - Aspiring to the write-it-once principle, tests following conventions require a lot less boilerplate. It is at an embryonic stage of development, and would greatly benefit from feedback and/or patches. :-) Hackage: http://hackage.haskell.org/package/TBC git: http://github.com/peteg/TBC/ Enjoy! cheers peter -- http://peteg.org/

2 2

Is a bug?
by Linker 27 Jul '09

27 Jul '09

Hi,All! I find out that diff between GHC and Hugs: GHC: *Prelude> sqrt 3+4+9 14.732050807568877 Prelude> sqrt 16 4.0 Prelude> sqrt $3+4+9 14.732050807568877* Hugs:*Hugs> sqrt $ 3+4+9* *4.0* *Hugs> sqrt 3+4+9* *14.7320508075689* Which one is right? Thanks. -- Regards, Linker Lin linker.m.lin(a)gmail.com

2 2

ANN: Elerea version 1.x.x
by Patai Gergely 26 Jul '09

26 Jul '09

Hi all, Having taken a short break from my profiling project, I updated my little FRP library called Elerea [1] along with the runnable example programs [2]. The interface was changed into a monadic-applicative hybrid that distinguishes stateful and stateless combinators for safety reasons. Since this is a major change, this release bears a new major version number. Most importantly, the latcher was removed due to various practical issues, and it is replaced by much better behaved stateless higher-order constructs. The library is capable of handling arbitrary higher-order signals, e.g. dynamic collections of signals, as shown by the renewed breakout example (which used to cheat so far by treating the state of the brick collection as a single value). I intend to write some tutorials later, but feel free to experiment until then. Gergely [1] http://hackage.haskell.org/package/elerea [2] http://hackage.haskell.org/package/elerea-examples -- http://www.fastmail.fm - The professional email service

1 0

Recompiling Hackage to estimate the impact of a change [Was: StricterLabelledFieldSyntax]
by Johan Tibell 26 Jul '09

26 Jul '09

On Sun, Jul 26, 2009 at 3:34 AM, Ian Lynagh <igloo(a)earth.li> wrote: > Hi all, > > I've made a ticket and proposal page for making the labelled field > syntax stricter, e.g. making this illegal: > > data A = A {x :: Int} > > y :: Maybe A > y = Just A {x = 5} > > and requiring this instead: > > data A = A {x :: Int} > > y :: Maybe A > y = Just (A {x = 5}) > > http://hackage.haskell.org/trac/haskell-prime/ticket/132 > > http://hackage.haskell.org/trac/haskell-prime/wiki/StricterLabelledFieldSyn… > In general, I think it would be a good idea to provide some statistics of how many packages would break as the result of a backwards incompatible change. Without that data I find it hard to do a cost-benefit analysis. So I hereby suggest that we make a recompile of Hackage a requirement for any breaking language changes. I understand that it might not be that easy to recompile all of Hackage at the moment so we should try to come up with some step-by-step instructions on how to do it. In the future it would be nice to have a little script that does it and spits out some statistics. Cheers, Johan

4 5

ANN: haskell-src-exts-1.1.0
by Niklas Broberg 26 Jul '09

26 Jul '09

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.1.0, bringing you tuple sections, comments, and a few bug fixes. haskell-src-exts is a package for Haskell source code manipulation. In particular it defines an abstract syntax tree representation, and a parser and pretty-printer to convert between this representation and String. It handles (almost) all syntactic extensions to the Haskell 98 standard implemented by GHC, and the parsing can be parametrised on what extensions to recognise. haskell-src-exts 1.1.0: =================== * Adds rudimentary support for retaining comments while parsing a source document. The support currently comes in the following form: - The module Language.Haskell.Exts.Comments defines a datatype Comment with two constructors denoting single-line (--) and multi-line ({- -}) comments. - The Parseable class is extended with a function 'parseWithComments :: Parseable ast => ParseMode -> String -> ParseResult (ast, [Comment])' which simply returns all comments as a list alongside the AST. There are also specialised versions for the usual suspects. Example run: HSE> parseExpWithComments defaultParseMode "1 + {- comment -} 4 -- end\n" ParseOk (InfixApp (Lit (Int 1)) (QVarOp (UnQual (Symbol "+"))) (Lit (Int 4)),[Mu ltiLine (SrcLoc {srcFilename = "<unknown>.hs", srcLine = 1, srcColumn = 3}) " co mment ",SingleLine (SrcLoc {srcFilename = "<unknown>.hs", srcLine = 1, srcColumn = 19}) " end"]) This first release of comment support is as seen very rudimentary, but I'm releasing it early in the hope that people who need features tied to this will start looking at what's there and send me comments on what further support they would like to see and consider missing. * Support for TupleSections: <http://hackage.haskell.org/trac/ghc/ticket/3377> * Fixes a bug where the statements of a list comprehension were returned in the wrong order (oops!). * Fixes a bug where function definitions were printed incorrectly when using PPOffsideRule - thanks to Mathieu Boespflug for the patch. * Fixes the broken 'cabal test' - thanks to Robin Green for the entire test runner support! Cheers and Happy Haskelling, /Niklas

1 0

Pattern matching with where free variables can be used more than once
by Christopher Done 26 Jul '09

26 Jul '09

Wouldn't it be great if pattern variables could be used more than once in a pattern? Like so: foo [x,x,_,x] = "The values are the same!" foo _ = "They're not the same!" where this could be rewritten to: foo [x,y,_,z] | x == y && x == z = "The values are the same!" foo _ = "They're not the same!" It seems like a straight-forward translation and there wouldn't be a speed hit for normal patterns because it would only be triggered in compilation where the same free variable appears twice. Implications are: 1. in ``foo [x,y] = ...'', x has type a 1. in ``foo [x,x] = ...'', x has type Eq a => a Was this ever considered? Is it a bad idea for some reason I'm not aware of? On a mildly irrelevant note, I have observed newbies to the language wonder why on earth it's not already like this. Cheers

5 5

Can sub-classes inheriting instances of parent class?
by Szekeres István 25 Jul '09

25 Jul '09

Hi, I'm trying to do something like the following: class Foo a where doit :: (String -> IO ()) -> a -> IO () instance Foo Int where doit f = f . show instance Foo Char where doit f = f . (:[]) So now I have a "doit" function for Ints and Chars, that takes a function and does the IO with the given function. Now I could define function p as p :: (Foo a) => a -> IO () p = doit putStrLn so I could do main = do p 3 p 'X' in main, but I would like to compeltely hide the fact from p that it somehow belongs to the Foo class, so I would like to do something like this: class Foo a => Bar a where p2 :: a -> IO () p2 = doit putStrln but, though Foo Int and Foo Char are defined, Bar Int and Bar Char are not defined automatically, so given this class definition I will not be able to call p2 3 and p2 'X' Is there still some way to do something like this? Thanks, Istvan

1 0

Memoized game-tree evaluation
by Matthew 25 Jul '09

25 Jul '09

I'm working on defining a data type for representing game-trees for chess or other two-person board games. My plan is to memoize the evaluations of a game-state at each depth so that values don't need to be recomputed for future moves. Here's the definition I'm using so far: data GameState = GameState {color :: Color, board :: Board} data GameTree = GameTree {state :: GameState, values :: [Int], branches :: [GameTree]} genGameTree :: GameState -> GameTree genGameTree st | finalState st = GameTree st (repeat (evalState st)) [] | otherwise = GameTree st vs bs where bs = map genGameTree (nextStates st) vs = (evalState st) : map (\d -> (optimize (color st)) (map ((!!d) . values) bs)) [0..] optimize Black = minimum optimize White = maximum I'd like to rewrite the generation of "vs" to evaluate using alpha- beta cutoffs, but I'm having trouble coming up with a way to do so without losing memoization. I don't know if my failures so far are because ab-pruning is necessarily incompatible with memoization, or if I simply haven't found the right insight yet. Can anyone suggest a solution, or convince me there isn't one? Thanks in advance.

2 1

YAHT Question about Exercise 4.8
by Futari 25 Jul '09

25 Jul '09

Hi, I was trying to use the solution given, but I don't know how to use it... How do I create something that is of type List? data List a = Nil | Cons a (List a) >From the previous examples, I was thinking something like Cons 'a' 'b' 'c' would create a list ['a','b','c'] but it gave me errors... help please, thanks. -- View this message in context: http://www.nabble.com/YAHT-Question-about-Exercise-4.8-tp24655087p24655087.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

ANNOUNCE: graphviz-2999.1.0.2
by Ivan Lazar Miljenovic 25 Jul '09

25 Jul '09

This is a bug release that fixes a bug spotted by Srihari Ramanathan where the Dot representation of Color values were double-quoted when they shouldn't have been. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0