April 2005 - Haskell-Cafe

GHC Survey source?
by Alson Kemp 15 Apr '05

15 Apr '05

Simons, Would y'all hand out the source code for the survey @ http://www.haskell.org/ghc/survey/start.cgi? I looked through the CVS repo but couldn't find it. Aside from generalities (GHC, WASH, etc), I wanted to see some actual web programming with Haskell. Thanks, Alson

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Do you use GHC?
by Simon Peyton-Jones 15 Apr '05

15 Apr '05

Dear Haskell folk, PLEASE READ THIS IF YOU USE THE Glasgow Haskell Compiler (GHC). I virtually never send messages to overlapping sets of lists, but I'm doing so this time, because I'd really like to get to everyone who uses GHC. Here at GHC HQ, we are often asked how many people use GHC, and what kind of folk they are. Since GHC is free software, we have no clue. Download stats tell us a bit, but not much, because GHC gets out through a variety of other routes too (e.g. Debian). We also have little idea about which aspects of GHC are most valued, nor of what enhancements would be most sought after, or which platforms are most important to you. We hear quite a bit from the bleeding-edge folk who push the limits, but much less from people who just want to get the job done. Thus motivated, Tomasz Zielonka kindly put together a (WASH-powered) survey for us, which asks you to tell us a bit yourself and how you use GHC. We've had about 230 responses so far; thank you to those 230 folk. But I'm pretty certain there are more than 230 people in the world who use GHC! THIS MESSAGE IS TO ASK YOU TO TAKE 5 MINS TO COMPLETE THE SURVEY http://www.haskell.org/ghc/survey/start.cgi We'd like to hear from *absolutely everyone* who uses GHC: whether you are a first-year undergraduate or a famous professor; whether you use GHC for industrial applications or recreation; whether you use higher-rank polymorphism or have only just learned what functional programming is. Everyone! We'll use the information to guide our future priorities, and we'll publish some kind of summary of what we learn in due course. It's all anonymous, of course, unless you choose to say who you are. If you think it's appropriate, please circulate this announcement locally, e.g. to your students who may not be on the Haskell mailing list Many thanks Simon PJ and Simon Marlow

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Haskell with Apache?
by Graham Klyne 14 Apr '05

14 Apr '05

I'm interested in running Haskell code invoked from an Apache web server request. I discovered the mod_haskell project [1] which looks prima facie v. promising. Does anyone have any recent experience of using this (there apparently being no recent activity on this project)? I wonder if there's any other project or activity in this area I should be aware of? #g -- [1] http://losser.st-lab.cs.uu.nl/mod_haskell/ ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Call for Contributions -- HC&A Report (May 2005 edition)
by Andres Loeh 14 Apr '05

14 Apr '05

Dear Haskellers, it is nearly time for the eighth edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 03 May 2005 (please send your contributions to hcar at haskell.org, in plain ASCII or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, write a short entry and submit it to the me. * If you are interested any project related to Haskell that has not previously been mentioned in the HC&A Report, tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming 6 months. If you have only recently been exposed to Haskell, it might be a good idea to browse the November 2004 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the beginning of May. They will be compiled into a coherent report which will appear sometime during May. As always, this is a great opportunity to update your webpages, make new releases, announce of even start new projects, or to point at some developments you want every Haskeller to see! As the purpose of the report is to collect recent or current activities, we encourage you to update all existing summaries and reports. We will probably drop any topics that have not had any activity for the past year, i.e., since May 2004, but we would very much prefer you to present an updated description of the topic. Of course, new entries are more than welcome. Reports should generally be kept brief and informative, ranging from a few sentences to a few hundred words, to keep the whole report reasonably sized. Looking forward to your contributions, Andres (current editor) ----------------------------------- topics New suggestions for current hot topics, activities, projects, etc. are welcome - especially with names and addresses of potential contacts, but here is a non-exclusive list of likely topics (see also http://www.haskell.org/communities/topics.html ): General Haskell developments; Haskell implementations; Haskell extensions; Standardization and documentation; Haskell tutorials, howtos and wikis; Organisation of Haskell tool and library development; Haskell-related projects and publications; new research, fancy tools, long-awaited libraries, cool applications; Feedback from specialist mailing lists to the Haskell community as a whole; Haskell announcements; all (recent) things Haskell Announcements: if you've announced anything new on the Haskell list over the last six months, you'll want to make sure that is reflected in this edition! Project pings: if you're maintaining a Haskell tool or library or somesuch, you'll want to let everyone know that it is still alive and actively maintained, even if there have been no new additions, but all the more if there have been new developments. Tutorials: if you've fought with some previously undocumented corner of Haskell, and have been kind enough to write down how you did manage to build that graphical user interface, or if you've written a tutorial about some useful programming techniques, this is your opportunity to spread the word (short, topic-specific, and hands-on tutorials that only show how to achieve a certain practical task would do a lot to make things easier for new Haskellers - please write some!) Applications: if you've been working quietly, using Haskell for some interesting project or application (commercial or otherwise), you might want to let others know about what you're using Haskell for, and about your experiences using the existing tools and libraries; are you using Haskell on your job? An interesting thread about using Haskell and more generally functional programming for non-Haskell things seems to recur with reasonable frequency - why not write a sentence or two about your use of Haskell for our report? Feedback: if you're on one of the many specialist Haskell mailing lists, you'll want to report on whatever progress has been made there (GUI API discussions, library organisation, etc.) If you're unsure whether a contact for your area of work has come forward yet, have a look at the report's potential topics page, or get in touch with me. I have contacted last time's contributors, hoping they will volunteer to provide updates of their reports, and will update the contacts on the topics page fairly regularly. But where you don't yet see contacts listed for your own subject of interest, you are very welcome to volunteer, or to remind your local community/project team/mailing list/research group/etc. that they really ought to get their act together and let the Haskell community as a whole know about what they've been doing! :-) -------------------------- what should I write? That depends on your topic, but as a general rule, it shouldn't take you long. A simple sentence or two about your use of Haskell could go into the "Individual Haskellers and their projects" section. If you're a company, or if you're working on a project using Haskell as the implementation language, a paragraph on that could go into the "Commercial Applications" and "Non-Commercial Applications" sections. A typical summary report about a tool/library/project/application/... would be between 1 and 3 paragraphs of ASCII text (what's it about? major topics and results since the last report? current hot topics? major goals for the next six months?) plus pointers to material for further reading (typically to a home page, or to mailing list archives, specifications and drafts, implementations, meetings, minutes, ...). For those who prefer templates to fill in, the report is edited in LaTeX, and an entry template might look something like this: \begin{hcarentry}{<MYSTUFF>} \report{<MY NAME>} \status{<PROJECT STATUS IN ONE LINE>} \makeheader <WHAT IS IT?> <WHAT IS ITS STATUS? / WHAT HAS HAPPENED SINCE LAST TIME?> <CAN OTHERS GET IT?> <WHAT ARE THE IMMEDIATE PLANS?> \FurtherReading \url{<PROJECT URL>} \end{hcarentry} Browsing through previous editions should give you a good idea of the variety of possibilities, ranging from very brief to extensive.

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Closest analog to popen()
by Dimitry Golubovsky 14 Apr '05

14 Apr '05

Hi, Does there exist any analog of popen in the standard Haskell libraries? I need to execute an external program (not a Haskell funciton) - maybe in a shell, being able to feed something to its stdin, and finally catch the exit code. Does there exist a way to do this without dealing with multithreading, writing my own ffi and other stuff? Something like rawSystem, but acting asynchronously (returning a writeable pipe handle, and a process ID), and later I would use getProcessStatus to catch the exit code. PS I have found http://community.moertel.com/ss/space/Tom's+Haskell+code/POpen.hs so far, but it does not seem to be part of the standard library. -- Dimitry Golubovsky Anywhere on the Web

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predicates in XML toolboxes
by Henning Thielemann 14 Apr '05

14 Apr '05

The XML toolboxes HaXml, HXML and the XML toolbox uses one function type (called filter) for different purposes. The functions of types predicates a -> Bool selectors, transformators a -> a list-valued functions a -> [a] are all implemented with the one type (a -> [a]). In this design predicates don't return False or True, but an empty list or a one-element list containing the input value. Transformators return single element lists. In my opinion this means a significant loss of type safety and quality of documentation. Are there more type safe XML/HTML processor libraries in Haskell?

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Passing Constructors as arguments
by Ralph Hodgson 13 Apr '05

13 Apr '05

I am learning Haskell and have set a small exercise for myself on a frames and slots program. Would appreciate some help on how to pass constructors of data structures as arguments to a function. Thanks, -- Ralph __________ A Frames test > module Frames > where Define frame slots: > type FirstName = String -- first name > type LastName = String -- last name > type Organization = String > type Email = String > type WorkPhone = String > type CellPhone = String > type TelephoneNumber = String Define slots for a contact > data ContactProperty = FN FirstName > | LN LastName > | OR Organization > | EM Email > | WP TelephoneNumber > | MP TelephoneNumber > deriving (Show, Eq) > data Contact = Contact [ContactProperty] > deriving (Show, Eq) > type Contacts = [ Contact] Now I need a way to extract properties from the frame. Start by testing pattern matching without using parameters. Then I need to find out how to pass a constructor as a parameter. > getProperty:: [ContactProperty] -> FirstName > getProperty ((FN fn):_) = fn > getProperty (_:xs) = getProperty xs > getProperty [] = "unknown" > firstName:: Contact -> FirstName > firstName (Contact cpl) = getProperty cpl Define Contacts > c1::Contacts > c1 = > [ ( Contact [(FN "Ralph"),(LN "Hodgson"),(OR "TopQuadrant"),(EM "rhodgson(a)topquadrant.com")]), > ( Contact [(FN "Mills"),(LN "Davis"),(EM "mdavis(a)project10x.com")])] Tests > t1=firstName $ head c1 -- should be "Ralph" > t2=firstName $ last c1 -- should be "Mills" ___________________________

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foldr1 min [(maxBound::Int)%1,1 % 2]
by Bo Herlin 13 Apr '05

13 Apr '05

Hi How come > foldr1 min [(maxBound::Int) % 1,1 % 2] 2147483647 % 1 but > foldr1 min [2147483647 % 1,1 % 2] 1 % 2 Why??? /Bo Herlin

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[Q] A typing problem
by Didier Verna 13 Apr '05

13 Apr '05

Hi ! I'm trying to write a function that combines folding and mapping. This function would take two arguments (a folding function and a mapping function), and would return a function of one argument (a list of 'a''s) returning a 'a'. The idea is to write something like sumsquare = foldmap (+) square Here's what I write: foldmap :: (c -> c -> c) -> (c -> c) -> (c -> c -> c) foldmap f m = foldr1 f (map m) But this gives a typing error: ERROR "src/ssq4.hs":2 - Type error in application *** Expression : foldr1 f (map m) *** Term : map m *** Type : [b] -> [b] *** Does not match : [a] What can I do to get what I want ? Thanks. -- Didier Verna, didier(a)lrde.epita.fr, http://www.lrde.epita.fr/~didier EPITA / LRDE, 14-16 rue Voltaire Tel.+33 (1) 44 08 01 85 94276 Le Kremlin-Bicêtre, France Fax.+33 (1) 53 14 59 22 didier(a)xemacs.org

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Re: [Haskell-cafe] Binary file r/w example
by Bo Herlin 13 Apr '05

13 Apr '05

Yes, I am, and the IOExtension module IS one of my problems. The other problem is that I am interested in this Ptr thing, and I havent seen any examples in using it, and is seems like Ptr is not in any of my Haskell-books or in any of the tutorials that I have found. Henning Thielemann wrote: > > On Wed, 13 Apr 2005, Bo Herlin wrote: > >> Hi, does anyone have a small but complete example on how to read a >> (portion of) a binary file, like r/w the header of a Midi-file? >> >> It seems like readBinaryFile (ans writeBinaryFile) is deprecated and I >> should use hGetBuf (and hPutBuf), but what is this Ptr thing? >> >> How would I implement readBinaryFile using hGetBuf? > > > > Are you aware of Haskore (www.haskell.org/haskore and > cvs.haskell.org/darcs/haskore) which provides a MidiFile reader and > writer? It has no good solution for reading and writing binary files, > but uses a platform dependent IOExtension module. :-( >

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