- Haskell-Cafe - Haskell.org

interaction between OS processes
by Andrea Rossato 21 Sep '07

21 Sep '07

Hi, there's something I don't get about interaction among OS processes and Haskell handles/channels. Suppose I have a very small program that takes a line and prints it till you write "quit": main = do s <- getLine case s of "quit" -> putStrLn "quitting" >> return () _ -> loop s where loop s = do putStrLn s main This is a small interactive process I would like to talk to from another Haskell program, like the following one which, indeed, is just a wrapper around the first. Now, if I write a single line with "quit", I get "quitting" back, otherwise the program doesn't work. I think I need some direction in order to understand how handles work. The same with channels, I'm afraid. Could you please point me in the right direction? Thanks for your kind attention. Andrea The not working code: import Control.Concurrent import System.Process import System.IO main = do c <- runInteractiveCommand "./main2" loop c loop c@(i,o,e,p) = do s <- getLine hPutStrLn i s hFlush i -- now "i" is closed, right? s' <- hGetLine o putStrLn s' loop c

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reverse with foldl
by PR Stanley 21 Sep '07

21 Sep '07

Hi reverse = foldl (xs x . x:xs) [] (xs ++ ) = foldl (\ys y -> ys ++ [y]) xs If I were to define reverse with foldl I would do it this way: reverse = foldl (\x -> \xs -> xs:x) [] Any idea what the first code frag is suposed to achieve? Thanks, Paul

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Re: [Haskell-cafe] What on earth
by PR Stanley 21 Sep '07

21 Sep '07

but ebook usually means a PDF which would be all right if the text didn't contain specialist symbols. The publishers weren't very helpful when I contacted them. According to the "lady" on the phone they didn't even have the latex source code. I'll probably try again. Hopefully this time I'll get someone more reasonable to deal with. Cheers, Paul At 00:51 21/09/2007, you wrote: >According to >http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780511292187, >it is available as an ebook. Perhaps if you contact them and explain >that you have the paper version, they will send you an electronic copy >of it? > >Rodrigo > >On 20/09/2007, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > tfoldr (-) 1 [4,3] > > = 4-3-(-1) > > = 2 > > A similar thing happens with foldr (/) 2 [8,2] = 8.0 > > I thought foldr \oplus v [] = v > > Any ideas? > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] What on earth
by Miguel Mitrofanov 20 Sep '07

20 Sep '07

> tfoldr (-) 1 [4,3] > = 4-3-(-1) > = 2 Erm? You mean "foldr"? foldr (-) a [x,y,z,t,w] == x - (y - (z - (t - (w - a)))) So, foldr (-) 1 [4,3] == 4 - (3 - 1). > A similar thing happens with foldr (/) 2 [8,2] = 8.0 8/(2/2) = 8/1 = 8 > I thought foldr \oplus v [] = v Yeah, that's right.

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Haskell guards
by Victor Nazarov 20 Sep '07

20 Sep '07

I still can't remember how guards are treated in Haskell. Here is the code snippet in question: foo a | a == 1 = 6 foo a | a == 2 = 7 foo a = 8 Would Haskell fall through to the third alternative if a is not equal to 1 or 2. I know that this can be rewritten more sanely, but I just consider guard behavior. What would be the value of (foo 3)? -- vir http://vir.comtv.ru/

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length defined with foldr
by PR Stanley 20 Sep '07

20 Sep '07

Hi length = foldr (. n . 1 + n) 0 Any idea how to define length with foldr. The above definition doesn't make much sense. Many thanks, Paul

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Spot the difference!
by PR Stanley 20 Sep '07

20 Sep '07

Hi \_ n -> 1 + n \_ -> (\n -> 1 + n) The outcome seems to be identical. is there a substantive difference between the two definitions? The second one seems to be more expressive of the currying principle. Any thoughts? Thanks, Paul

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Call for Abstracts for 3rd SAPLING Meeting, November 12, 2007
by Manuel M T Chakravarty 20 Sep '07

20 Sep '07

Call for Abstracts for 3rd SAPLING Meeting, November 12, 2007 Sydney Area Programming Language INterest Group http://www.comp.mq.edu.au/plrg/sapling.html 10am-4:30pm, November 12, 2007 Engineering Theatre, Building 32 (Engineering) Australian National University Invited Speaker --------------- Prof William M Waite, University of Colorado, Boulder, USA About SAPLING ------------- SAPLING aims to promote and develop academic research in the Programming Languages field within the Sydney area, including nearby regional centres such as Canberra, Newcastle and Wollongong. Important Dates: ---------------- - Submission deadline: Oct 15, 2007 - Notification: Oct 19, 2007 - Workshop: November 12, 2007 SAPLING Meetings ---------------- SAPLING meetings are held twice a year to allow programming language academics, researchers and students to present and discuss their research in an informal setting. The third meeting will be held at the Australian National University. SAPLING talks are generally of 20 minutes duration with substantial periods of time available for discussion. Talks can present completed work, outline work in progress, demonstrate a system, or provide a short tutorial on a topic of general interest. Submissions for talks should be in the form of an abstract no longer than one A4 page in plain text, PostScript or PDF format. The abstract should be emailed to Tony Sloane (asloane AT ics.mq.edu.au) by close of business on October 15, 2007. As far as possible, selection of presenters is non-competitive, with all interested qualified speakers given an opportunity to talk, within the time constraints of a one day meeting. Preference will be given to topics of general interest. No proceedings will be published but the talk abstracts and links to related material will be published on the SAPLING website. There is no registration fee to attend this meeting. Participants will be responsible for their own lunch costs. Accommodation ------------- For non-Canberra residents, some accommodation suggestions are available on the web site for this meeting: http://www.comp.mq.edu.au/~asloane/pmwiki.php/SAPLING/SAPLING072 Steering Committee: ------------------- Anthony Sloane, Macquarie (Chair), asloane AT ics.mq.edu.au Manuel Chakravarty, UNSW, chak AT cse.unsw.edu.au Clem Baker-Finch, ANU, Clem.Baker-Finch AT anu.edu.au

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wxhaskell package for ubuntu feisty, amd64
by Iván Pérez Domínguez 20 Sep '07

20 Sep '07

I'm trying to install wxhaskell in ubuntu feisty, with ghc-6.6. After several days of trial and error and a few hours trying to change all types in my program to gtk2hs types, I'm tired. Does anyone have a .deb for wxhaskell that works with ghc-6.6?

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What on earth
by PR Stanley 20 Sep '07

20 Sep '07

Hi tfoldr (-) 1 [4,3] = 4-3-(-1) = 2 A similar thing happens with foldr (/) 2 [8,2] = 8.0 I thought foldr \oplus v [] = v Any ideas? Thanks, Paul

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