June 2009 - Beginners

Re: [Haskell-beginners] if True than let...
by Bernhard Lehnert 25 Jun '09

25 Jun '09

> Since ifs are an expression in haskell, they return a value, All right, that explains it. Today I learned to think of that expression thing even in do blocks. Thank you to everyone! Cheers, Bernhard

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Re: [xmonad] xmonad vs. full screen
by Thomas Friedrich 25 Jun '09

25 Jun '09

Rickard Nilsson wrote: > Quoting Thomas Friedrich <info(a)suud.de>: >> 2) I would propose going back to the old behavior of handling >> full-screen-programs in xmonad. It really should be the *user* who >> decides how "full-screen" a full-screen-program shall be. Isn't this >> also more following the philosophy of xmonad? You as a user decide, >> what is happening on your screen? It might be not as intuitive, when >> switching to xmonad and you are used to other WMs, where full-screen >> means *full screen*. I really miss that functionality, now that its >> gone. What do you think? > > I think the old behaviour made much more sense. As it is now, it's not > possible to play a web flash movie in a separate, arbitrarily sized, > window. You either have to watch it fullscreen-floating, or integrated > in the web-page. > > @Thomas - Did you ever manage to revert Xmonad to the old behaviour? > No unfortunately I didn't. I changed my manageHook to myManageHookDef = scratchpadManageHook (W.RationalRect 0.25 0.375 0.5 0.35) <+> composeAll [ ... , isFullscreen --> doFullFloat , isDialog --> doCenterFloat ] This way, when I want to watch a Flash-Movie in full screen, the video window actually covers the full screen. However, the downside of it is that also xpdf and others start flouting when going to full screen. Which is stupid, as I always have to push them back into the tiling, when I want to have a "full screen" xpdf next to, say, emacs. Also you will have a little border around the flouting window, which doesn't look good in full screen. But I don't want to remove the borders from flouting windows, as I like to have a border, when a flouting window is not covering the whole screen. So as you see, the solution is far from optimal. Cheers, Thomas

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lhs2TeX
by Thomas Friedrich 24 Jun '09

24 Jun '09

Hi everyone, A Google-search wasn't successful, that's why I am asking here. I want to use the program lhs2TeX. In the manual it says there are different styles that you can use, and some examples how they look like. But they never say how to call them. If I like to see the verbatim-style or the tt-style. What do I include in my document? Cheers, Thomas

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HXT: Gathering text from Nodes
by aditya siram 23 Jun '09

23 Jun '09

Hi all, I'm having trouble using HXT to gather up text from all nodes with a certain name, and return it in a tuple with another node. So given 3 child nodes called "kid" containing "a", "b" and "c", and 1 child node called "name" containing "A", I want to return ("A", ["a","b","c"]). My current code at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=6187#a6187returns [("A","a")("B","b")("C","c")]. Any help is appreciated. -deech

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Re: [Haskell-beginners] Updating lists inside another list
by Lee Duhem 23 Jun '09

23 Jun '09

On Tue, Jun 23, 2009 at 11:29 AM, Joel Neely<joel.neely(a)gmail.com> wrote: > WRT the first case, I would have expected something like; > > addToInnerList [] a b = [(a,[b])] > > on the assumption that one "grows" a structure from an empty state. > Your thoughts? > I think it isn't what Aaron wants, that is "to add an element to the inner list of the pair with the head value of my choosing". Anyway, it is an option. lee

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Updating lists inside another list
by Aaron MacDonald 22 Jun '09

22 Jun '09

I have a list of pairs. Each pair has a value and a list. So, the structure looks like this: [ (a, [b, c]), (b, [a, c]) ] What I want to do is be able to add an element to the inner list of the pair with the head value of my choosing. So, for the above list: > addToInnerList [ (a, [b, c]), (b, [a, c]) ] a d [ (a, [b, c, d]), (b, [a, c]) ]

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FoldL/R Reducing List
by Waaaggh 22 Jun '09

22 Jun '09

Hi.how can i reduce list with foldl.r ? np (1,1,1,2,3,3,4,5,5) -> (1,2,3,4,5) ??? Azz.

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ghci reports "The last statement in a 'do' construct must be an expression" error
by Nico Rolle 21 Jun '09

21 Jun '09

Hi Heres my code snippet. It reports that my error is in line 9 right after the main definition. All functions that i call work under normal circumstances. Thanks module Benchmark where import ReadCSV import Operators import Data.Time.Clock (diffUTCTime, getCurrentTime) main = do xs <- readCSV "dataconvert/lineitem.tbl" '|' start <- getCurrentTime let pnp = projection [5] xs let snp = selection (\x -> (x!!0) > (Int 17000)) pnp end <- getCurrentTime putStrLn $ show (end `diffUTCTime` start) start2 <- getCurrentTime let pp = pProjection [5] xs let sp = pSelection (\x -> (x!!0) > (Int 17000)) pp end2 <- getCurrentTime putStrLn $ show (end2 `diffUTCTime` start2) return xs

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Modifications inside a Reader?
by Brian Troutwine 18 Jun '09

18 Jun '09

Hello all. I'm writing a UDP echo server, full source given below. The current implementation echoes back the "payload" of every incoming message but I would prefer that only unique payloads be echoed back. To that end I've started in with Data.BloomFilter but am not sure how to update it accordingly. I imagine that Reader is probably the wrong monad to be using though I'm unsure how I might modify my program to use State. Could someone lead me along a bit? Also, any general comments on the style of my program? Thanks, Brian -- import Prelude hiding (putStrLn, catch) import Network.Socket hiding (send, sendTo, recv, recvFrom) import Network.Socket.ByteString import Data.ByteString.Char8 hiding (head) import Control.Monad.Reader import Control.Monad (forever) import Control.Exception (bracket) import Data.BloomFilter import Data.BloomFilter.Easy import Data.BloomFilter.Hash (cheapHashes) data Globals = Globals { socketG :: Socket , bloomF :: Bloom ByteString } type Echo = ReaderT Globals IO run :: Echo () run = forever $ do s <- asks socketG (msg, addr) <- liftIO $ recvFrom s 1024 let [_, _, _, payload] = split ':' msg liftIO $ sendTo s payload addr main :: IO () main = bracket build disconnect loop where disconnect = sClose . socketG loop st = runReaderT run st build :: IO Globals build = do addrinfos <- getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just "1514") let serveraddr = head addrinfos sock <- socket (addrFamily serveraddr) Datagram defaultProtocol bindSocket sock (addrAddress serveraddr) return $ Globals sock $ emptyB (cheapHashes 3) 1024

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map increases length of list
by Aaron MacDonald 18 Jun '09

18 Jun '09

For some reason, the map function returns a list that has one more element than my input list. My input list is a range defined by [0, 60..359] (should translate into [0,60,120,180,240,300]). The function I'm giving to map is defined this way: ----- degreesToRadians :: Double -> Double degreesToRadians degrees = degrees * (pi / 180) ----- This is how I'm calling map overall: ----- > map degreesToRadians [0,60..359] [0.0,1.0471975511965976,2.0943951023931953,3.141592653589793,4.1887902047863905,5.235987755982989,6.283185307179586 ] ----- As you can hopefully see, there are seven elements instead of six. Getting the length confirms this: ----- > length [0,60..359] 6 > length $ map degreesToRadians [0,60..359] 7 ----- I do not seem to get this behaviour with the length if I either substitute the degreesToRadians function or substitute the [0,60..359] range. P.S. Is there a built-in function to convert degrees to radians and vice-versa?

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