January 2012 - Haskell-Cafe

PhD studentship in Nottingham
by Graham Hutton 09 Jan '12

09 Jan '12

Dear all, I am currently advertising a PhD Studentship in Functional Programming at the University of Nottingham in the UK: http://www.nottingham.ac.uk/jobs/currentvacancies/ref/SCI1088 If you are interested in applying yourself, please drop me a note by email. If you know of any good candidates who may be interested in applying, or there is a local mailing list for advertising such things, I'd be much obliged if you could pass on the above link. Many thanks, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Are all monads functions?
by Yves Parès 09 Jan '12

09 Jan '12

Hello Café, One thought occur to me recently when explaining the concept of Monad to non-haskellers: internally, all standard Monads are newtypes wrapping functions: StateT is, WriterT is, ContT is. Even IO and ST are, both conceptually and in their implementation by GHC. ParsecT (not part of mtl, but still) is. And so on... So I'm saying that most of the time, we define a new monad: - either by building a monad stack of existing transformers (thus "melting" their internal functions), - or by creating a newtype containing a custom function. Thinking thusly, >>= would then just be a special function composition operator: in the spirit of '.' but for a specific type of functions. So my questions are: - Is it reasonable to think like that, or are there too many monads that cannot be defined like that, and then contradict me? - Is it reasonable to present monads to newcomers by saying : monads are basically always functions. 'return x' will then be a function that always return 'x' regardless of its input and >>= is a special composition for this occasion. -- The ⊥ is a lie.

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On stream processing, and a new release of timeplot coming
by Eugene Kirpichov 09 Jan '12

09 Jan '12

Hi Cafe, In the last couple of days I completed my quest of making my graphing utility timeplot ( http://jkff.info/software/timeplotters ) not load the whole input dataset into memory and consequently be able to deal with datasets of any size, provided however that the amount of data to *draw* is not so large. On the go it also got a huge speedup - previously visualizing a cluster activity dataset with a million events took around 15 minutes and a gig of memory, now it takes 20 seconds and 6 Mb max residence. (I haven't yet uploaded to hackage as I have to give it a bit more testing) The refactoring involved a number of interesting programming patterns that I'd like to share with you and ask for feedback - perhaps something can be simplified. The source is at http://github.com/jkff/timeplot The datatype of incremental computations is at https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Incremental.hs . Strictness is extremely important here - the last memory leak I eliminated was lack of bang patterns in teeSummary. There's an interesting function statefulSummary that shows how closures allow you to achieve encapsulation over an unknown piece of state - curiously enough, you can't define StreamSummary a r as StreamSummary { init :: s, insert :: a->s->s, finalize :: s->r } (existentially qualified over s), as then you can't define summaryByKey - you don't know what type to store in the states map. It is used to incrementally build all plots simultaneously, shown by the main loop in makeChart at https://github.com/jkff/timeplot/blob/master/Tools/TimePlot.hs Incremental building of plots of different types is at https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Plots.hs There are also a few interesting functions in that file - e.g. edges2eventsSummary, which applies a summary over a stream of "long" events to a stream of rise/fall edges. This means that you can define a "stream transformer" (Stream a -> Stream b) as a function (StreamSummary b -> StreamSummary a), which can be much easier. I have to think more about this idea. -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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"too many open files" using snap
by Eric Wong 09 Jan '12

09 Jan '12

Hi haskellers, I wrote a web server using snap framework. Basically, it's an API server using hdbc-mysql to connect to a mysql server. But it also serves some static web pages and images, using the serveDirectory function in snap-core. The program eats more and more memory slowly, but I couldn't spot the leak. It uses less than 10MB memory when starting. Sometimes, it can grow to 60-70MB and stop working correctly. Then I find that the log file is full of "too many open files" error. As I don't have any code open any files, I suspect it's the serveDirectory leaking. But I don't have any expertise on the core of snap, so, could some one give me some advice on this problem? Thanks. Eric

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How to use IndentParser. Any example/tutorial?
by Sukit Tretriluxana 09 Jan '12

09 Jan '12

Hi, I need to construct a parser that parse a language that use indentation to define blocks as Python does. I ran into Text.ParserCombinators.Parsec.IndentParser module but have no idea how to use it. Can anyone point me out to how to use this module? Any short example, tutorial, or whatever would be highly appreciated. Thanks, Ed

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numeric coercion in ghci vs hugs
by Doug McIlroy 09 Jan '12

09 Jan '12

After loading the one-line file default (Integer,Rational,Double) the input 1::Fractional t => t in ghci yields the output 1.0 and in hugs 1%1 The ghci answer appears not to respect the default declaration. What's going on? Doug McIlroy

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ANN: wxHaskell 0.13.2
by Jeremy O'Donoghue 08 Jan '12

08 Jan '12

Hi Lists, I am please to announce that wxHaskell 0.13.2 has just been uploaded to Hackage. This is mainly a bugfix release, although it brings a few useful changes: - Changes to support build under Haskell Platform 2011.4.0.0 - OpenGL support if your wxWidgets build is configured with it (reinstated functionality) - StyledTextControl support if your wxWidgets build is configured with it (reinstated functionality) This is intended to be the final wxHaskell release supporting wxWidgets 2.8.x. Provided that you have a suitable Unicode wxWidgets 2.8 install configured on your machine, you should be able to install with. cabal install wx In the near future, we will be committing an updated wxHaskell release supporting wxWidgets 2.9, but this implies a number of breaking API changes, and the size of team supporting wxHaskell is not large enough to cover both. As such, this release is recommended to those who will not be able to move to wxWidgets 2.9 and later. The next release has more significant improvements planned, including wrapping several additional wxWIdgets libraries and restoring correct operation in GHCi. We do not have a formal timeline as yet, but the code is working in test repositories, and mainly needs work to make it cross-platform clean. This release has been tested on the following platforms: - Windows 7 / Haskell Platform 2011.4.0.0 / wxPack 2.8.12 (Unicode, Release) - OpenGL is not enabled on the test platform, so OpenGL samples do not work. - StyledTextControl is not enabled on the test platform, so STC samples do not work. - All other sample code compiles, links and runs, but has only been tested for Unicode wxWidgets builds. - Ubuntu 10.0.4 LTS (32 bit) / GHC 6.12 / wxWidgets 2.8.10 - Repo packages: wx2.8-headers, libwxgtk2.8-0, libwxgtk2.8-dev, libglitz1, libglitz-gl1, libgl1-mesa-dri, libglu1-mesa, libglu1-mesa-dev, mesa-common-dev, libgl1-mesa-dev, libgl1-mesa-glx, ghc6 - Cabal packages: strict, stm, OpenGL, cabal-install - OpenGL is enabled on the test platform, and the samples compile and run. - StyledTextControl is not enabled on the test platform, so STC samples do not work. - All other sample code compiles, links and runs, but has only been tested for Unicode wxWidgets. I do not have access to an OS X platform for test, so would appreciate feedback on any issues found by OS X users. There are known to be issues on 64 bit OS X builds, which will be addressed in the next release. The source repository at code.haskell.org has not yet been updated with the patches. This will happen in the next day or so. Best regards Jeremy

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cabal and conditional build-depends
by Tristan Ravitch 08 Jan '12

08 Jan '12

Hi Cafe, I have a package that depends on several other Haskell libraries. One of these libraries depends on some native libraries (and associated headers) to build. I would like to make this dependency optional in my package if the libraries don't exist. To do this, I tried to switch my build-type from Simple to Configure in my .cabal file. Then I made a configure script that generates a <pkg>.buildinfo file as suggested in the cabal guide. The buildinfo file is generated and looks as I would expect. If the library is found on the system, it has a "build-info: ..." line. Unfortunately, when Cabal processes it, it says: Warning: archive-inspection.buildinfo: Unknown fields: build-depends (line 2) Fields allowed in this section: buildable, build-tools, cpp-options, cc-options, ld-options, pkgconfig-depends, frameworks, c-sources, default-language, other-languages, default-extensions, other-extensions, extensions, extra-libraries, extra-lib-dirs, includes, install-includes, include-dirs, hs-source-dirs, other-modules, ghc-prof-options, ghc-shared-options, ghc-options, hugs-options, nhc98-options, jhc-options It really doesn't seem to like conditional build-depends entries. Is this expected behavior? And is there a better way to go about having conditional build-time dependencies on Haskell libraries using Cabal? I guess I could start playing with a custom Setup.hs, but I always hate doing that unless absolutely necessary. Thanks

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Questions on the Haskell 2010 report
by Silvio Frischknecht 08 Jan '12

08 Jan '12

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I am trying to write a haskell compiler and i have stumbled upon 2 issues that i don't understand in the Haskell report 2010. Btw. these issues are also present in the 98 report. 1) first is this in section 5.1 Module Structure [1]. A module is defined like this. module → module modid [exports] where body | body body → { impdecls ; topdecls } | { impdecls } | { topdecls } You can't see it in this mail because there is no font but the curly braces { } are meant to match the tokens { } and are not meant as repetition. However, as I understand it, the layout parser would modify this code: module Foo where import Bar baz=baz to this code (which doesn't match): module Foo where { import Bar ; baz=baz } and not to this code (which would be required to match): module Foo where { { import Bar } ; { baz=baz } } So as i see the definition should state this instead: module → module modid [exports] where body | body body → { impdecls ; topdecls } | impdecls | topdecls 2) In the same section what is it supposed to mean if "module" consists of only a "body" and not "module modid [exports] where body". 3) In section 10.3 Layout [2] it is written. If a let, where, do, or of keyword is not followed by the lexeme {, the token {n} is inserted after the keyword, where n is the indentation of the next lexeme if there is one, or 0 if the end of file has been reached. Together with this rule L [] (m : ms) = } : L [] ms if m≠0 (Note 6) This results in an error if the program ends in a "where", ... Wouldn't it make more sense to put a {1} in case of EOF or just insert the tokens '{' and '}'. On the other hand if you really won't accept that why not simply say that it is an error. btw. GHC doesn't give an error. [1] http://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-990005.1 [2] http://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-178000… -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQIcBAEBAgAGBQJPCdgeAAoJEDLsP+zrbatWIjgP/RLISOIz6xZRDso/mVn+xbwh eMPdeqmzJJYpVm+sDdjmoEIAsrwXtYKc9r8LrlUOugbtXFhYYPcSox7Te4DO+Cm+ X+b8b80UQu1nKNoUofDqAXSQb/n7f5zgmaIoF2uvFieuJTE709oZCmBdTgupVdyJ D16VqmKmqpskNepnQuh9rV7vwSPs4PjnsUQDpxjXDvh0tSdp9byBZPcMwEgQb172 FJAAryCuKFBE8FRG+ANlSw2h8fH7rkMLmkjabje5NKtLa2UMKvBlXYa2p0EZwcd2 /bj/2GTksWdsDmqFp3lQJgPk2uxp5reosTiTk5JQz56GcPOTK3wmL62qDeH3+tdS pX5ZD8HeX8RdkLjuFi0fjiJq3L4HEy+OqHbLWoStPPqfY9clueSUJ0g0Dfc4OukU 0naJlASxKr4ovHBiTinrLI6bTTeeMj9QCDGh5hU0x1hJ5f80iQQaKYpI8cq1DxQv CICBFibFnbJ0Zfzfuk0ZJZ8Db1dJUdEGSYZo/QCtpxXwTyKr4zEU6FYHprScRRX8 H6ZDnVE6ZbzDbaUqeQVBXvUGHF+2vTmDo1EZ80BFhDVHjifvV9eHslbF0XD+NjYY iRPsgDzACYsMxvYaBsRlXWwmwWD+EfNerdHZL6h9dBZICwSr0pu4SCp616BR2BpK IuO4MG/B5vAoCs9cb0lG =O6Wd -----END PGP SIGNATURE-----

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Re: [Haskell-cafe] [web-devel] [ANNOUNCE] First release of crypto-conduit
by Aristid Breitkreuz 08 Jan '12

08 Jan '12

Yeah and that's annoying IMHO. :) It's not really important though. Aristid Am 07.01.2012 15:39 schrieb "Felipe Almeida Lessa" <felipe.lessa(a)gmail.com>:

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