May 2009 - Haskell-Cafe

Concurrent Haskell Actions with Timeout
by Cetin Sert 30 May '09

30 May '09

Hi how could one implement a function in concurrent haskell that either returns 'a' successfully or due timeout 'b'? timed :: Int → IO a → b → IO (Either a b) timed max act def = do Best Regards, Cetin Sert

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Missing a "Deriving"?
by michael rice 30 May '09

30 May '09

The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. It compiles fine, but upon trying it I get the following error message. It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. Michael ============ Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( graph4.hs, interpreted ) Ok, modules loaded: Main. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 <interactive>:1:0: No instance for (Show (c [Int])) arising from a use of `print' at <interactive>:1:0-14 Possible fix: add an instance declaration for (Show (c [Int])) In a stmt of a 'do' expression: print it ============================ data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es

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can someone point me to more help about Database.HDBC.ODBC?
by Michael P Mossey 30 May '09

30 May '09

I'm trying to use Database.HDBC.ODBC to connect to a MySQL server, and I cannot figure out the docs. I want to establish a connection, and I know the server url, username, and password. According to these docs... http://software.complete.org/static/hdbc-odbc/doc//HDBC-odbc/Database-HDBC-… ...I am supposed to pass a string to connectODBC. These docs do not explain the meaning of the passed string, but refer me to a microsoft document: http://msdn2.microsoft.com/en-us/library/ms715433(VS.85).aspx I cannot make any sense of this microsoft document. I guess I don't know enough about database interfaces in general. Can someone explain how to make a connect to a specific URL, or point me to some clearer docs? Thanks, Mike

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Re: [Haskell-cafe] Parsec float
by Tillmann Vogt 29 May '09

29 May '09

Bartosz Wójcik wrote: > Hi Everybody (especially Parsec Creator), > > is there any reason why float parses only positive numbers? > > I find following defition: > > float = lexeme floating <?> "float" > > floating = do{ n <- decimal > ; fractExponent n > } > > If floating was defined like > > floating = do{ n <- integer ... > > or > > floating = do{ n <- int ... > > instead then it would parse negative ones as well. > > Hi Bartek, I had the same problem. Daan Leijen gave me a similar answer than Malcom Wallace just gave you: "Usually the minus sign is treated as an operator in the language and treated as a separate token" He also gave me a workaround which finally resulted in this: myfloat = try (do{ symbol "-"; n <- float; return (negate n) }) <|> try float <|> do { i<-integer; return(fromIntegral i) } -- 0 is not recognized as a float, so recognize it as an integer and then convert it to float

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Hugs vs. GHCi
by Vladimir Reshetnikov 29 May '09

29 May '09

Hi, The following expression evaluates to 1 in GHCi, but results in an error in Hugs: let f x = let g y = [x,y] in (g 1, g []) in 1 What is the correct behavior? Thanks Vladimir

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Ensuring Type Class instances follow the 'rules'
by Hemanth Kapila 29 May '09

29 May '09

Hi all, Recently, I participated in a coding competition. As part of it, I had to write a program wherein I had to make my data-type an instance of Ord. An error in my implementation of compare resulted in me losing quite a bit of valuable time. As I wrote it, I had tested it out on the ghci and it seemed to work fine but I noticed that there was trouble when the List.sort started giving weird output. I then noticed that for certain instances (say t1,t2), both compare t1 t2 AND compare t2 t1 returned LT. Once I spotted it, it was easy enough to fix but I did lose an hour or so thinking something went wrong before I fed the list to the sort function. I was the only haskeller in the competition and I believe am among the first to finish a correct application (by a large margin, and) and I did manage to raise a bit of haskell-awareness, but if I had managed to spot the error earlier, the results would have been dazzling. Does any one here have any advice on dealing with such maladroitness on the part of the programmer, especially while creating instances to type-classes? Thanks and regards, Hemanth

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Floating instance and pi
by Paul Keir 29 May '09

29 May '09

Hi, I'd like to make my ADT an instance of the Floating class, but I'm not sure what to put for pi, and GHC gives a warning without it: Warning: No explicit method nor default method for `GHC.Float.pi' I tried setting it to undefined, but that gives an error: `pi' is not a (visible) method of class `Floating' Any idea? Paul

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IFL 2009: Second Call for Papers
by IFL 2009 29 May '09

29 May '09

Call for Papers IFL 2009 Seton Hall University SOUTH ORANGE, NJ, USA http://tltc.shu.edu/blogs/projects/IFL2009/ ** NEW ** Accomodations information available: http://tltc.shu.edu/blogs/projects/IFL2009/accommodations.html Jane Street Capital has joined IFL 2009 as a sponsor ********* The 21st International Symposium on Implementation and Application of Functional Languages, IFL 2009, will be held for the first time in the USA. The hosting institution is Seton Hall University in South Orange, NJ, USA and the symposium dates are September 23-25, 2009. It is our goal to make IFL a regular event held in the USA and in Europe. The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2009 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2009 will use a post-symposium review process to produce a formal proceedings which will be published by Springer in the Lecture Notes in Computer Science series. All participants in IFL 2009 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. These submissions will be screened by the program committee chair to make sure they are within the scope of IFL and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full arcticle for the formal review process. These revised submissions will be reviewed by the program committee using prevailing academic standards to select the best articles that will appear in the formal proceedings. TOPICS IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools. If you are not sure if your work is appropriate for IFL 2009, please contact the PC chair at ifl2009(a)shu.edu. Topics of interest include, but are not limited to: language concepts type checking contracts compilation techniques staged compilation runtime function specialization runtime code generation partial evaluation (abstract) interpretation generic programming techniques automatic program generation array processing concurrent/parallel programming concurrent/parallel program execution functional programming and embedded systems functional programming and web applications functional programming and security novel memory management techniques runtime profiling and performance measurements debugging and tracing virtual/abstract machine architectures validation and verification of functional programs tools and programming techniques FP in Education PAPER SUBMISSIONS Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English, conform to the Springer-Verlag LNCS series format and not exceed 16 pages. The draft proceedings will appear as a technical report of the Department of Mathematics and Computer Science of Seton Hall University. IMPORTANT DATES Registration deadline August 15, 2009 Presentation submission deadline August 15, 2009 IFL 2009 Symposium September 23-25, 2009 Submission for review process deadline November 1, 2009 Notification Accept/Reject December 22, 2009 Camera ready version February 1, 2010 PROGRAM COMMITTEE Peter Achten University of Nijmegen, The Netherlands Jost Berthold Philipps-Universität Marburg, Germany Andrew Butterfield University of Dublin, Ireland Robby Findler Northwestern University, USA Kathleen Fisher AT&T Research, USA Cormac Flanagan University of California at Santa Cruz, USA Matthew Flatt University of Utah, USA Matthew Fluet Toyota Technological Institute at Chicago, USA Daniel Friedman Indiana University, USA Andy Gill University of Kansas, USA Clemens Grelck University of Amsterdam/Hertfordshire, The Netherlands/UK Jurriaan Hage Utrecht University, The Netherlands Ralf Hinze Oxford University, UK Paul Hudak Yale University, USA John Hughes Chalmers University of Technology, Sweden Patricia Johann University of Strathclyde, UK Yukiyoshi Kameyama University of Tsukuba, Japan Marco T. Morazán (Chair) Seton Hall University, USA Rex Page University of Oklahoma, USA Fernando Rubio Universidad Complutense de Madrid, Spain Sven-Bodo Scholz University of Hertfordshire, UK Manuel Serrano INRIA Sophia-Antipolis, France Chung-chieh Shan Rutgers University, USA David Walker Princeton University, USA Viktória Zsók Eötvös Loránd University, Hungary PETER LANDIN PRIZE The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 euros.

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How to interprete profile output
by Sebastian Reese 29 May '09

29 May '09

Hi there, i am currently working on the mapreduce framework (http://darcs2.fh-wedel.de/repos/holumbus/) which can also be found in hackage. I was doing the simple WordFrequency example, which does nothing but counting word frequencies and runs a few seconds. When I run this program with ghc 6.10 and profiling option "+RTS -p -RTS" i get this (http://pastebin.com/f5163325) output. The interesting line is the one with "MAIN MAIN" in it. 1. Where does this MAIN come from? My main function is right under it. So is it some GHC profiling wrapper? 2. Why does it use over 90% of cpu time and 25% of allocated memory? So i know you can't tell me exactly why, but maybe someone can give me a hint. The mapreduce framework does a lot with network, IO, forkIO, MVars etc.. Is that maybe a reason for it? My guess is that there is some action, that can't be assinged to a specific cost centre and/or results from an empty MVar. thanks for your help Sebastian ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.

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ghc - force library search order
by John Lask 29 May '09

29 May '09

I need to force a library to be searched for unresolved symbols after all other libraries have been searched, and I would rather not resort to constructing the linker command line directly. Is there a way to do this? i.e. I want for example -lfoo to appear after all other haskell libraries and before system libraries (for example -lmsvcrt) once ghc has constructed the link command. i.e. how is it possible to coerce ghc into respecting dependencies between the libraries. GHC does a good job in general with native libraries, but there are allways corner cases. I have a feeling this is not possible, but it dosn't hurt to ask. jvl

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