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A small display problem using Helium
by Anonymous Anonymous 23 Mar '09

23 Mar '09

Hello, I've written something simple: main :: IO () main = do lijn <- getLine putStrLn lijn Now if I import it in Helium it will do the following: Test> main test -- (here I'm typing test) test it will be displayed two times, why? Because it displays my input and the output of the function. Now I want to write a function that does NOT display my input, any suggestions how I can achieve this? Thanks in advance!

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Hackage download and popularity statistics
by Don Stewart 23 Mar '09

23 Mar '09

For the first time, we've got download and popularity statistics from Hackage: http://www.galois.com/blog/2009/03/23/one-million-haskell-downloads/ Find out if your package made the top 100, and when we reach our 1 millionth hackage download! -- Don

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Hackage upload problems?
by Sebastiaan Visser 23 Mar '09

23 Mar '09

Hello, Currently I'm trying to upload a minor update of Salvia to Hackage to fix some dependency issues but Hackage times out all the time? Both the CLI tool and the web-interface do not react to my upload request. Any known problems here? Gr, Sebastiaan.

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Sometimes I wish there was a global variable
by Rafael Cunha de Almeida 23 Mar '09

23 Mar '09

Hello, I am writing a OpenGL program in haskell, it can be found in: http://github.com/aflag/galo/tree/master But I hope this e-mail will be self-contained :). My main function goes like this: (...) rotX <- newIORef (0.0::GLfloat) rotY <- newIORef (0.0::GLfloat) pos <- newIORef (0.0::GLfloat, 0.0, 0.0) displayCallback $= display (map f range) rotX rotY pos keyboardMouseCallback $= Just (keyboardMouse rotX rotY pos) (...) Notice that rotX, rotY and pos are meant to be used as comunication between the keyboardMouse and display functions. They need to be set as 0 first, so display won't do anything. Only when they user press a few buttons that those values change, so display behaves accordanly. In a state-based language I would place display and keyboardMouse in one module and let them communcate to each other like they want. In haskell, I'm not quite sure how to do it except by that parameter passing style. I thought about how state-monad may help with that. But I'm not sure how I'd make the state variable to be contained inside a display/keyboardMouse module. []'s Rafael

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Re: [Haskell-cafe] least fixed points above something
by Jens Blanck 23 Mar '09

23 Mar '09

> > > On Friday 20 March 2009 5:23:37 am Ryan Ingram wrote: > > On Fri, Mar 20, 2009 at 1:01 AM, Dan Doel <dan.doel(a)gmail.com> wrote: > > > However, to answer Luke's wonder, I don't think fixAbove always finds > > > fixed points, even when its preconditions are met. Consider: > > > > > > f [] = [] > > > f (x:xs) = x:x:xs > > > > > > twos = 2:twos > > > > How about > > > > > fixAbove f x = x `lub` fixAbove f (f x) > > > > Probably doesn't work (or give good performance) with the current > > implementation of "lub" :) > > > > But if "lub" did work properly, it should give the correct answer for > > fixAbove f (2:undefined). > > This looked good to me at first, too (assuming it works), and it handles my > first example. But alas, we can defeat it, too: > > f (x:y:zs) = x:y:x:y:zs > f zs = zs > > Now: > > f (2:_|_) = _|_ > f _|_ = _|_ > > fix f = _|_ > > fixAbove f (2:_|_) = 2:_|_ `lub` _|_ `lub` _|_ ... > = 2:_|_ > > Which is not a fixed point of f. But there are actually multiple choices of > fixed points above 2:_|_ > > f (2:[]) = 2:[] > forall n. f (cycle [2,n]) = cycle [2,n] > > I suppose the important distinction here is that we can't arrive at the > fixed > point simply by iterating our function on our initial value (possibly > infinitely many times). I suspect this example won't be doable without an > oracle of some kind. > > Ah well. > -- Dan > > Thanks for all comments on my question, especially those bashing my poor code. The above approach does not apply to my case. What I have is a monotone function f on a partial order satisfying f x >= x, for all x. Given that the partial order is in fact a cpo this is enough to guarantee that a least fixed point can be found above any point in the partial order simply by iterating f, although not necessarily in finite time. Taking the lub of x and the fixed point of f (over bottom) need not give a fixed point even if one exists. Think of reachability in a graph from a starting set. Let S be some fixed set, and let f return all points reachable in 0 or 1 step from the union of S and the argument to f. Then fix f is the set of points reachable from S, which is a fixed point. But adding some point x outside fix f will in general not give me a fixed point, even though a unique fixed point exists (the set reachable from the union of {x} and fix f). Jens

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SoC idea: interactive profiling
by Patai Gergely 23 Mar '09

23 Mar '09

Hello all, I entered a little proposal in the issue tracker: http://hackage.haskell.org/trac/summer-of-code/ticket/1570 As I see it, the graphical part could be a warm-up exercise with an already useful product, and interaction with the rts would be the actual challenge. Reasoning about laziness is far from trivial (at least I often feel lost when trying to grok all the interaction within Reactive ;), and being able to browse profiler output on the spot is something I felt the need for a few times already. Eventually, this could be a contribution to an IDE project, even though it could as well remain a standalone application for all the emacs/vi folk. Profiling tools for parallel programs are in the make already, but I haven't seen anything more convenient than hp2ps and little add hoc solutions for analysing resource usage patterns. How do you all go about profiling? Gergely -- http://www.fastmail.fm - Does exactly what it says on the tin

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MissingH bracketCD (aka bracketCWD) bug -- this is the infamous lazy io, right?
by Thomas Hartman 23 Mar '09

23 Mar '09

I got bitten by a bug (well, I call it bug) in bracketCD from HSH/MissingH demonstrated by the following code bracketCD is very useful for sysadminny one-offs, I use it all the time, but..... I suspect that unless people are very careful, this behavior will affect other users of bracketCD, in potentially very subtle and tricky ways. 1) -- is there a more elegant way to deal with lazy io than the hack below? (putStrLn the last character) 2) -- should MissingH function bracketCD be fixed, and if so how? import System.FilePath.Find (find) import System.Directory (getDirectoryContents, getCurrentDirectory, setCurrentDirectory) import System.Path (bracketCWD) -- from MissingH -- the fixed function, a more restrictive type than bracketCWD which lets you do any io, not just showables myBracketCWD :: Show a => FilePath -> IO a -> IO a myBracketCWD fp action = do oldcwd <- getCurrentDirectory setCurrentDirectory fp a <- action putStrLn $ show . last . show $ a -- force evaluation setCurrentDirectory oldcwd return a -- rather than listing "/", lists dir this module is in tWrong = bracketCWD "/" $ return . take 2 =<< listCurrentWithFind -- this lists filesystem root tRight = myBracketCWD "/" $ return . take 2 =<< listCurrentWithFind listCurrentWithFind = System.FilePath.Find.find (return True) (return True) "." -- for some reason, bracketCWD does the right thing with get tGetContents = myBracketCWD "/" $ return . take 2 =<< getDirectoryContents "." Actual code for bracketCWD in MissingH: -- this is from MissingH, seems there's a bug when attempt to wrap a find command bracketCWD :: FilePath -> IO a -> IO a bracketCWD fp action = do oldcwd <- getCurrentDirectory setCurrentDirectory fp finally action (setCurrentDiurectory oldcwd)

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How to send signal from Xmonad to my app?
by Magicloud Magiclouds 23 Mar '09

23 Mar '09

Hi, I have a little program, which shows a message box when gets 'USR1' signal. This works fine when I `kill -USR1 $PID`. But my code in xmonad.hs does not work. No error throws out. And as I inspected, the "pid" is right. Only nothing happened in my app. So I wonder what is wrong? 28 , ((mod4Mask, xK_x), sendSignal2PIDinFile sigUSR1 ".sidebar.pid") 31 sendSignal2PIDinFile signal pidfile = io $ do 32 pid <- readFile pidfile 33 signalProcess signal $ read pid -- 竹密岂妨流水过山高哪阻野云飞

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ANN: hranker: Basic utility for ranking a list of items (e.g. for the logo poll)
by Robin Green 23 Mar '09

23 Mar '09

hranker is a fun little command-line utility I have written to help a user rank a list of items (of any type implementing Show, Eq and Ord). I hope the code is sufficiently clear that it could also serve as an educational piece of code, especially for people wanting to learn how to use the HCL library. It is now available on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hranker Just typing hranker or hranker --help will yield a usage message. Here is an example usage of the program: In the unlikely event that anyone else wants to use it to help them prepare their votes in the Haskell logo poll (which is what I used it for just now), here is what to do: 1. cabal install hranker (or download it and runhaskell Setup.lhs etc.) 2. hranker 113 (because 113 is the number of items in the poll) 3. IMPORTANT: Read the warnings the program prints on startup! 4. To control the program, enter the first character of the command you wish to perform, and then press Enter. The command you have chosen may ask you for further information. Commands generally operate on the next unranked item, apart from annotate (which can operate on ranked or unranked items), remove (which always operates on ranked items), and quit. I recommend you use the annotate command a lot - it is really helpful for this kind of situation where you have a lot of numbered items to rank. If you use the remove command, don't remove *all* ranked items, because if you do you it may terminate. 5. If you rank *everything* somehow (remember, ties are allowed), the program will print an inverse mapping (items to ranks, instead of ranks to items) before terminating. If you choose to quit before that point, you won't get that inverse mapping, unless you explicitly invoke the mapping command. There is obvious room for improvement: apart from fixing the remove bug mentioned above, there is currently no simple way to remove or annotate just *one* item at a rank when multiple items are tied at that rank. Have fun! There are some hilarious entries in the logo poll, if you haven't yet voted. -- Robin

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Transforming XML using HXT, I'm confused.
by P Orrifolius 22 Mar '09

22 Mar '09

Hello, I'm trying to transform a fragment of XML using HXT, specifically I'm trying to strip out any disallowed content while retaining as much as possible. For example I might be given "<bar><evil>blah</evil></bar>". I want to get rid of the 'evil' element but retain it's content resulting in "<bar>blah</bar>", rather than just "<bar></bar>". My basic theory was to do a bottom up traversal replacing any invalid elements with their children. Unfortunately I've got myself thoroughly confused. The code below shows what I've tried so far (except the many random permutations of collapseInvalids I've tried!). I think one problem is that validElements is choosing based on the parent element but processing will just be given the child. Also I think some inputs will require multiple passes over the children during the traversal, eg given "...<baz>s6<bar>s7</bar></baz>..." I'd like "...<baz>s6s7</baz>..." but when processing the bar it is valid, it's only once you're processing the baz you see that you need to pull the s7 up. I'd really appreciate any advice, and I'm not wedded to the current code so feel free to throw it all out! :) Thanks P. ps: Is it possible to render an XmlTree as String without invoking IO? I've just been doing runLA (hread >>> collapseInvalids) frag1 and looking at the nodes. module Test where import Text.XML.HXT.Arrow --collapseInvalids = processBottomUp (processChildren ((getChildren `whenNot` validElements) `orElse` this)) -- <foo> can contain <foo>, <bar> or <baz>, not text validInFoo :: (ArrowXml a) => a XmlTree XmlTree validInFoo = foldr1 orElse ( map hasName ["foo", "bar", "baz"] ) -- <bar> can contain <foo>, <baz> or text, not <foo> validInBar :: (ArrowXml a) => a XmlTree XmlTree validInBar = foldr1 orElse ( isText : map hasName ["foo", "baz"] ) -- <baz> can only contain text validInBaz :: (ArrowXml a) => a XmlTree XmlTree validInBaz = foldr1 orElse ( isText : []) validElements :: (ArrowXml a) => a XmlTree XmlTree validElements = choiceA ( hasName "foo" :-> validInFoo : hasName "bar" :-> validInBar : hasName "baz" :-> validInBaz : isText :-> this : []) frag1 = concat ( "s1" : "<foo>" : "<bar>" : "s2" : "<evil>" : "s3" : "<baz>" : "s4" : "</baz>" : "s5" : "</evil>" : "s6" : "</bar>" : "</foo>" : "s7" : []) result1 = concat ( "s1" : "<foo>" : "<bar>" : "s2s3" : "<baz>" : "s4" : "</baz>" : "s5s6" : "</bar>" : "</foo>" : "s7" : []) frag2 = concat ( "s1" : "<foo>" : "s2" : "<evil>" : "s3" : "</evil>" : "<bar>" : "s4" : "<evil>" : "s5" : "<baz>" : "s6" : "<bar>" : "s7" : "</bar>" : "</baz>" : "s8" : "<bar>" : "s9" : "</bar>" : "</evil>" : "s10" : "</bar>" : "</foo>" : "s11" : []) result2 = concat ( "s1" : "<foo>" : "<bar>" : "s4" : "<evil>" : "s5" : "<baz>" : "s6" : "<bar>" : "s7" : "</bar>" : "</baz>" : "s8" : "<bar>" : "s9" : "</bar>" : "</evil>" : "s10" : "</bar>" : "</foo>" : "s11" : [])

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