October 2006 - Haskell-Cafe

Re: Re: Read a single char
by Hans van Thiel 25 Oct '06

25 Oct '06

On Tue, Oct 24, 2006 at 06:14:14PM +0200, Hans van Thiel wrote: > I have a similar question. When I use getChar with Hugs the newline > (caused by pressing 'Enter') seems to be carried over into the next > call of main, where it is treated as a single input character. > The workaround is getLine and head, but it would be nice to drop the > newline right away. I suppose the solution offered here: >>that is the correct behavior, input is never discarded, it is just >>saved >>up until you press enter for the first time. if you type >>foo<enter>, >>your next four getChars will get 'f' 'o' 'o' and '\n'. it is not >>haskell >>that is treating enter as specially, it is your terminal, getChar >>just returns exactly what was passed to it. Yes, but the problem is with two subsequent runs of main. I'd have thought the buffer would flush automatically and the second run would start with an empty buffer. Now I enter 'b', then press 'Enter' and it works as expected. Then I 'main' again, and it terminates right away with a blank line. If I enter 'a', 'b' and then Enter, the second run takes the second character. If I'm not doing anything wrong (always a possibility :-), then it seems more like a (small) bug in Hugs, as Neil suggested. I'll have to check what chgi does. John -- John Meacham - ⑆repetae.net⑆john⑈ Regards, Hans

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Fwd: [Haskell-cafe] Haskell custom search engine
by Diego Navarro 25 Oct '06

25 Oct '06

> Google now lets us create our own custom search engine pages, so I > whipped one up for Haskell, I volunteered. Are you planning to add just sites for Haskell-related software, or are research papers included in the scope of this? (Dude, where's my english grammar.) -- syntaxfree

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Re: Configurations proposal
by Brian Smith 24 Oct '06

24 Oct '06

On 10/24/06, Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> wrote: > > On the other hand, in Gtk2Hs I know one case where we do this. We have a > Graphics.UI.Gtk.Cairo api module that is only included if Gtk was built > against Cairo. In any case it could be faked by using cpp to just not > export anything rather than not having the module exposed at all. So > it's not clear that it's worth banning. Or maybe making it slightly > harder is worth it so that people don't get in the habit. Couldn't you split this into Gtk and Gtk-Cairo packages, where the latter is only built if Cairo is available? Similarly, in your GUI example, couldn't you have seperate foo and foo-gui packages, and only build the foo-gui package if the GUI libraries are available? Otherwise, how can you say "I depend on the Gtk package being built with Cairo support" and "I depend on the GUI portion of the foo package?" In general, optional groups of modules should be split off into separate packages, and there should be a way of building a bundle of related packages together (just like one can build a group of related executables together already). Regards, Brian

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Re: Read a single Char
by Hans van Thiel 24 Oct '06

24 Oct '06

Hello, I have a similar question. When I use getChar with Hugs the newline (caused by pressing 'Enter') seems to be carried over into the next call of main, where it is treated as a single input character. The workaround is getLine and head, but it would be nice to drop the newline right away. I suppose the solution offered here: hSetBuffering stdin NoBuffering will have the desired effect, but is there another way? Thanks, Hans van Thiel

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Guards with do notation?
by Misha Aizatulin 24 Oct '06

24 Oct '06

hello all, why is it not possible to use guards in do-expressions like do (a, b) | a == b <- getPair return "a and b are equal" Cheers, Misha

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Exception: Too many open files
by Bas van Dijk 24 Oct '06

24 Oct '06

Hello Haskellers, I'm wondering how to get the following to work: I need to parse about 18.000 files. I would like to have a function 'parseFiles' that parses all these files and returns the result in a list. When I execute 'test' from the simplified code below I get the error: *** Exception: file_0014994.txt: openFile: resource exhausted (Too many open files) So it seems that 'parseFiles' tries to open all the ~18.000 files and gets exhausted when opening the 14994 file. What I would like is 'take 3 =<< parseFiles' to read only the first 3 files. Is this possible, and if so, what is the best way to do this? Note that the code below is a bit simplified: ------------------------------------------------------------------------------ module Main where import Text.ParserCombinators.Parsec data T = ... test = print . take 3 =<< parseFiles parseFiles :: IO [T] parseFiles = mapM parseFile =<< getFileFPs getFileFPs :: IO [FilePath] getFileFPs = ... -- returns a large list of about 18.000 FilePaths parseFile :: FilePath -> IO T parseFile fp = liftM getRight $ parseFromFile someParser fp getRight (Right r) = r someParser :: Parser T someParser = ... ------------------------------------------------------------------------------ Greetings, Bas van Dijk

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split string into n parts
by Paul Brown 24 Oct '06

24 Oct '06

> I want to split a string into 5 parts of equal length, with the last fifth > padded if necessary, but can't get it right - here's what I've got - > > fifths :: String -> String > > fifths s = fifths' "" 0 s > > where l = (length s) `div` 5 > [... snip ...] > Any thoughts? Thanks! This isn't homework BTW, I'm having a go at the ruby > quiz puzzles in haskell, which seems to be a nice way to learn. Cool idea! Can you post a link for the puzzles? As for this one, don't you want the first multiple of five larger than the length of the string? You should be able to make things simpler if you auto-pad the string from the get-go (forgive any syntax errors, as I'm just composing in a browser here...): prepad :: Integer -> String -> String prepad n s | (length s) `mod` n == 0 = s prepad n s = prepad n (s ++ ' ') And now you can be sneaky: nths :: Integer -> String -> String nths n s = [first_nth n s] ++ nths (n-1) (after_first_nth n s) nths 1 s = s Where first takes the first n chars (e.g., take ((length s) `div` n)) and after_first_nth returns the tail of the list.. -- paulrbrown(a)gmail.com http://mult.ifario.us/ -- paulrbrown(a)gmail.com http://mult.ifario.us/

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Re: [Haskell-cafe] Haskell beginner questions
by jim burton 24 Oct '06

24 Oct '06

Cybertronic wrote: > > Hi all, I'm pretty much new to Haskell however I'm stuck on something > which is that I'm trying to create a function called display where I type > in a DVD name, e.g. dvd1, it returns d (String) and the multiplication of > q (Int) and i (Double) > > Here's what I've done so far: > > type Film = (Int,String,Int,Double) > > dvd1 :: Film > dvd1 = (1, "Space", 5, 9.99) > > display :: Product -> String > display (c,d,q,i) = d > > My display function only shows the string but unfortunately I'm stuck on > how to get the display function to multiply q (Int) and i (Double) > together and display it next to d. > > Can someone help me out please? :) > I'd recommend a tutorial like http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf > display :: Film -> String > display (c,d,q,i) = d ++ " " ++ show (fromIntegral q * i) q needs to be converted first and show will convert the result to a string. -- View this message in context: http://www.nabble.com/Haskell-beginner-questions-tf2500050.html#a6971248 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Exception: Too many open files
by Bas van Dijk 23 Oct '06

23 Oct '06

On Tuesday 24 October 2006 00:58, Greg Fitzgerald wrote: > > test = print . take 3 =<< parseFiles > > I haven't had time to double-check this code, but something like this ought > to work (no 'unsafe' operations!): > test = sequence . take 3 . map (print . parseFile) =<< getFileFPs > > Let me know how it goes. Indeed, the following works: test = sequence . take 3 . map (\fp -> print =<< parseFile fp) =<< getFileFPs Thanks, Bas.

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Segmentation fault with realdine library
by Maurício 23 Oct '06

23 Oct '06

Hi, This small program says "Segmentation fault": module Main (Main.main) where import Data.Char import System.Time import System.Console.Readline main :: IO () main = do readKey return () I don't understand anything about readline, so I probably should have to call some function before readKey (by they way, I don't know what readKey does). But I thought a segmentation fault is always something to be reported. Best, Maurício

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