April 2012 - Haskell-Cafe

process-conduit appears to hang on windows
by grant 16 Apr '12

16 Apr '12

Hi, I am trying to use process-conduit on windows, but it appears to hang when using the conduitCmd. Is there a reason why this doesn't work? Thanks for any help, Grant {-# LANGUAGE QuasiQuotes #-} import System.Process.QQ import Data.Conduit import qualified Data.Conduit.Binary as CB import qualified Data.Conduit.List as CL import Data.Conduit.Process import qualified Data.ByteString as B import System.IO main :: IO () main = runResourceT $ [scmd|dir *.txt|] $$ [ccmd|find /i "help"|] =$ CB.sinkHandle stderr main1::IO() main1 = runResourceT $ sourceCmd "dir" $= conduitCmd "find /i \"help\"" $$ CB.sinkHandle stdout

2 3

Connection to database
by yrazes 16 Apr '12

16 Apr '12

Hi guys, I was trying to get a connection MySql Haskell and I followed this steps: 1. install cabal-install install libghc-haskelldb-hdbc-dev (headers del haskell y hdbc) install libmysqlclient-dev (headers del mysqlclient) 2. git clone git://github.com/bos/hdbc-mysql.git 3. julita@yulys:~/hdbc-mysql$ ls ChangeLog Database README.markdown Test.hs COPYING HDBC-mysql.cabal Setup.lhs testsrc julita@yulys:~/hdbc-mysql$ runhaskell Test.hs but I got this error message: Database/HDBC/MySQL.hs:65:8: Could not find module `Database.HDBC.MySQL.RTS': Use -v to see a list of the files searched for. * I tried to do something about the Setup.lhs, but I could not do anything at all. If somebody can help me, I will appreciate it. Best regards, Julita

2 2

CFP SCAM 2012 - submissions due May 4 (abstracts: April 29)
by Sonia Haiduc 16 Apr '12

16 Apr '12

********************************************************************** * 12th IEEE International Working Conference on Source * * Code Analysis and Manipulation * * SCAM 2012 * * http://scam2012.cs.usask.ca/ * * * * September 23 - 24, 2012 * * Riva Del Garda, Trento, Italy * ********************************************************************** * *** CALL FOR PAPERS *** * ********************************************************************** OVERVIEW/SCOPE ************** The aim of SCAM is to bring together researchers and practitioners working on theory, techniques and applications, which concern analysis and/or manipulation of the source code of computer systems. While much attention in the wider software engineering community is properly directed towards other aspects of systems development and evolution, such as specification, design and requirements engineering, it is the source code that contains the only precise description of the behavior of the system. The analysis and manipulation of source code thus remains a pressing concern. COVERED TOPICS AND PAPER FORMATS ******************************** We welcome submission of *full papers* that describe original and significant work in the field of source code analysis and manipulation. SCAM explicitly solicits results from any theoretical or technological domain that can be applied to the following or similar topics: - program transformation - abstract interpretation - program slicing - source level software metrics - decompilation - source level testing and verification - source level optimization - program comprehension SCAM also welcomes *short tool papers* that introduce tools for source code analysis and manipulation. Tool papers should describe a concrete source code analysis or manipulation tool that is available for download and general use and can be run and demonstrated at the conference. Submitted papers should not be longer than 10 pages (6 pages for tool papers). All papers submitted should follow IEEE Computer Society Press Proceedings Author Guidelines. The papers should be submitted electronically in PDF form via the conference web site.Submitted papers must not have been previously published or concurrently submitted for consideration elsewhere. In addition to the main and tool tracks, SCAM 2012 solicits 2-page *Frontiers position papers* describing a significantly new idea in the theory, technology, application or engineering of source code analysis and manipulation systems. Frontiers authors will join in an open panel discussion of the frontiers of the area at the conference. IMPORTANT DATES *************** Abstract submission (full and frontiers papers): April 29, 2012 Papers submission (full and frontiers papers): May 04, 2012 Tool papers submission: May 31, 2012 Notification full and frontier papers: June 16, 2012 Notification tool papers: June 21, 2012 Camera ready submission: July 06, 2012 Working Conference: September 23 - 24, 2012 PROCEEDINGS *********** All accepted papers will appear in the proceedings which will be published by the IEEE Computer Society Press. Best papers from SCAM 2012 will be considered for revision, extension, and publication in a special issue of the Journal of Software: Evolution and Process. ORGANIZING COMMITTEE ******************** *General Chair* James R. Cordy, Queen's University, Canada *Program Co-Chairs* Mariano Ceccato, Fondazione Bruno Kessler, Italy Zheng Li, Beijing University of Chemical Technology, China *Tool Chair* Chanchal K. Roy, University of Saskatchewan, Canada *Local Arangements Chair* Alessandro Marchetto, Fondazione Bruno Kessler, Italy *Finance Chair* Dave Binkley, Loyola University Maryland, USA *Publications Chair* Dawn Lawrie, Loyola University Maryland, USA *Publicity Chair* Sonia Haiduc, Wayne State University, Detroit, USA *Web chair* Minhaz Fahim Zibran, University of Saskatchewan, Canada

1 0

Announce: keera-three-balance-checker-0.0.1
by Ivan Perez 16 Apr '12

16 Apr '12

Hi everyone, I wrote a small app to keep my Three Mobile PayG balance always under control. I've been using it for a while and it seems to work fine. It's just an icon docked in the system bar that shows the current balance when you move the mouse over it. If you don't know what Three Mobile is, then you don't need this. If you use a different operator and would find this kind of app handy, let me know and we can probably find a way to make it work for you. Code, instructions and screenshots: https://github.com/ivanperez-keera/keera-three-balance-checker My cabal repo: remote-repo: ivanperez-keera:http://ivanperez-keera.github.com/packages/archive If any of you find it useful, I'd be glad to know. If you find any bugs, drop me a line and we'll find out how to fix them together. Cheers, Ivan. PS. Why Announce instead of ANNOUNCE? Because this is not a big deal, it's just a toy.

2 1

Building cabal-install for new ghc build
by Lyndon Maydwell 16 Apr '12

16 Apr '12

Hi Café. I'm building GHC from package ghc-7.4.1-src.tar.bz2 as the binary download was throwing segfaults for me (and apparently a few others). This has worked well and my issues with GHC and GHCi are now resolved. However I have needed to build cabal-install. This can't be done using cabal-install because it isn't installed :) I've found that I had to make several modifications to the bootstrap.sh and cabal-install.cabal files in order to get this to work. I am wondering how the maintainers of the Haskell-platform, etc are dealing with these issues. There seems to be a cabal-install-7.4.1 package on Hackage to address these issues, but I couldn't get it to work for me. Is this actually being used by the community? I realize that I am making trouble for myself by not just using the Haskell-Platform, but there are some packages I want to use that depend on recent versions of base. Thanks!

2 2

ANNOUNCE : Juicy.Pixels 1.2.1
by Vincent Berthoux 16 Apr '12

16 Apr '12

Hello everyone, I'm pleased to announce you the new version of Juicy.Pixels, the 1.2.1, no new major features, here's the changelog : - Removed dependency on the array package. - Updated dependency list to handle latest mtl version. - Helper functions to save files without thinking about it. - Some code snippets in the API documentations GitHub : https://github.com/Twinside/Juicy.Pixels Hackage : http://hackage.haskell.org/package/JuicyPixels Regards Vincent Berthoux

1 0

Conduit Best Practices for leftover data
by Myles C. Maxfield 16 Apr '12

16 Apr '12

Hello, I am interested in the argument to Done, namely, leftover data. More specifically, when implementing a conduit/sink, what should the conduit specify for the (Maybe i) argument to Done in the following scenarios (Please note that these scenarios only make sense if the type of 'i' is something in Monoid): 1) The conduit outputted the last thing that it felt like outputting, and exited willfully. There seem to be two options here - a) the conduit/sink should greedily gather up all the remaining input in the stream and mconcat them, or b) Return the part of the last thing that never got represented in any part of anything outputted. Option b seems to make the most sense here. 2) Something upstream produced Done, so the second argument to NeedInput gets run. This is guaranteed to be run at the boundary of an item, so should it always return Nothing? Instead, should it remember all the input it has consumed for the current (yet-to-be-outputted) element, so it can let Data.Conduit know that, even though the conduit appeared to consume the past few items, it actually didn't (because it needs more input items to make an output)? Remembering this sequence could potentially have disastrous memory usage. On the other hand, It could also greedily gather everything remaining in the stream. 3) The conduit/sink encountered an error mid-item. In general, is there a commonly-accepted way to deal with this? If a conduit fails in the middle of an item, it might not be clear where it should pick up processing, so the conduit probably shouldn't even attempt to continue. It would probably be good to return some notion of where it was in the input when it failed. It could return (Done (???) (Left errcode)) but this requires that everything downstream in the pipeline be aware of Errcode, which is not ideal.I could use MonadError along with PipeM, but this approach completely abandons the part of the stream that has been processed successfully. I'd like to avoid using Exceptions if at all possible. It doesn't seem that a user application even has any way to access leftover data anyway, so perhaps this discussion will only be relevant in a future version of Conduit. At any rate, any feedback you could give me on this issue would be greatly appreciated. Thanks, Myles

2 6

strange GHCi type inference behavior involving map and partially applied functions
by Ting Lei 15 Apr '12

15 Apr '12

Hi All, I found a really strange case where GHC and GHCi behave differently in inferring types. It took me hours to figure this out. The following program {-# LANGUAGE NoMonomorphismRestriction #-} g x i = x ++ show i [a,b] = map g ["X","Y"] will not load without "NoMonomorphismRestriction". With that option, it will load and return the correct types (as expected): *Main> :t [a,b] [a,b] :: Show a => [a -> [Char]] *Main> a 1 "X1" However, if I do the same thing GHCi, the type inferencing seems to have been messed up: *Main> let g x i = x ++ show i *Main> let [a,b] = map g ["X","Y"] *Main> :t [a,b] [a,b] :: [() -> [Char]] *Main> :t map g ["X","Y"] map g ["X","Y"] :: Show a => [a -> [Char]] Note how in the last two instances the terms on the left and right-hand sides of the definition return different types. Also, the correct return type should be a list of unary functions taking an (Show a) as the parameter. Now the result is unusable: *Main> a 1 <interactive>:52:3: No instance for (Num ()) arising from the literal `1' Possible fix: add an instance declaration for (Num ()) In the first argument of `a', namely `1' In the expression: a 1 In an equation for `it': it = a 1 I am using GHCi 7.4.1 under windows. I also tried this under GHC 7.0x Is this a GHCi bug or could anyone please explain why this can of strange behavior happens? If this is a bug, could anyone with an account help file a bug for this? Thanks in advance, Ting

4 4

Re: [Haskell-cafe] [Haskell] ANNOUNCE: notcpp-0.0.1
by Steffen Schuldenzucker 15 Apr '12

15 Apr '12

On 04/13/2012 10:49 PM, Ben Millwood wrote: > I'm pleased to announce my first genuinely original Hackage package: > notcpp-0.0.1! > > http://hackage.haskell.org/package/notcpp > > [...] Why is it > scopeLookup :: String -> Q Exp with n bound to x :: T => @scopeLookup n@ evaluates to an Exp containing @Just x@ , not > scopeLookup :: String -> Q (Maybe Exp) with n bound to x :: T => @scopeLookup n@ evaluates to Just (an Exp containing @x@) ? Shouldn't n's being in scope be a compile time decision? That would also make the "openState: runtime name resolution has its drawbacks :/"[1] a compile time error. -- Steffen [1] http://hackage.haskell.org/packages/archive/notcpp/0.0.1/doc/html/NotCPP-Sc…

2 1

Re: [Haskell-cafe] How helpful is h-99 (and should we complete the missing ones)?
by Daniel Hlynskyi 15 Apr '12

15 Apr '12

> > So here's my question: how useful is h-99 (are they overrated as learning > tools)? I find myself solve most of them in a "from the scratch" fashion > (e.g., no Monad, no Applicative, no Functor aside from List and a few > Maybe). Some of them are paper-worthy, for example the prime problems. I > hope some guru-level Haskeller could do away the missing few, and maybe > dive deeper into the surface to produce more insights (like the knights > travel page or the sieve paper, which are both beautiful). As another new-haskeller I say: Yes & No. "Yes" for necessity of several described algorithms and gained intuition for real programming. "No" for needed ammount of work to understand pitfalls of Haskell when real programming. And of-course, problems and solutions are not annotated with there typical real world aplications, they are not obvious for average beginners. Why would I make Binary tree balanced, when I don't know, what I'll gain (except balanced binary tree)?

2 1