November 2009 - Haskell-Cafe

Re: [Haskell-cafe] Haskell image libraries
by Pierre-Etienne Meunier 09 Nov '09

09 Nov '09

there is also a binding for libGD on hackage : http://hackage.haskell.org/package/gd And, of course, you can improve it, or write a binding to a more complete library. Or, even better, write a mix between http://www.libpng.org/pub/png/pngdocs.html and http://hackage.haskell.org/package/binary

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WinGHCi stuck in a loop
by Henk-Jan van Tuyl 09 Nov '09

09 Nov '09

L.S., I changed the options in WinGHCi and now WinGHCi is stuck in a loop each time I start it; how can I edit the options? I cannot find them in the registry. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Re: [Haskell] ANNOUNCE: control-monad-exception 0.5 with monadic call traces
by Henning Thielemann 09 Nov '09

09 Nov '09

Jose Iborra schrieb: > Folks, > > I'm happy to announce a new release of control-monad-exception with > monadic call traces, > available in Hackage. Grab it now while it is still online! > > Monadic stack traces are described in detail in a blog post [1]. > > In short, what this means for your code is the ability to generate > errors like this: > > 500 Internal Server Error > The CGI server failed with the following error: > DeleteException (BmPK 2009-10-26 19:39:51.031297 UTC "Testing RPO") > in deleteBenchmarkFromPK, > NarradarBenchmarkDB(src/NarradarBenchmarkDB.hs): (186, 44) > deleteBenchmarkFromPK, > NarradarBenchmarkDB(src/NarradarBenchmarkDB.hs): (186, 25) > deleteBenchmarkFromPK, > NarradarBenchmarkDB(src/NarradarBenchmarkDB.hs): (184, 17) > deleteBenchmarkFromPK, > NarradarBenchmarkDB(src/NarradarBenchmarkDB.hs): (180, 90) > deleteTests, NarradarBenchmarkCGI(src/NarradarBenchmarkCGI.hs): > (108, 3) > deleteTests, NarradarBenchmarkCGI(src/NarradarBenchmarkCGI.hs): > (106, 20) > cgiMain, NarradarBenchmarkCGI(src/NarradarBenchmarkCGI.hs): (52, 33) > cgiMain, NarradarBenchmarkCGI(src/NarradarBenchmarkCGI.hs): (52, 30) > cgiMain, NarradarBenchmarkCGI(src/NarradarBenchmarkCGI.hs): (50, 9) > cgiMain, NarradarBenchmarkCGI(src/NarradarBenchmarkCGI.hs): (46, 11) Sure, this is a nice functionality. But isn't it about debugging, not exception handling? Internal Server Error means to me, the server has a bug, thus we want to know, how to reproduce it, thus the stack trace. For handling expected irregularites, what exceptions are, you would not need that, right?

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Area from [(x,y)] using foldl
by michael rice 09 Nov '09

09 Nov '09

Here's an (Fortran) algorithm for calculating an area, given one dimensional arrays of Xs and Ys. I wrote a recursive Haskell function that works, and one using FOLDL that doesn't. Why would Haskell be "expecting" (t, t) out of ((*) (xold-x) (yold+y))? Michael ==================== AREA = 0.0 XOLD = XVERT(NVERT) YOLD = YVERT(NVERT) DO 10 N = 1, NVERT X = XVERT(N) Y = YVERT(N) AREA = AREA + (XOLD - X)*(YOLD + Y) XOLD = X YOLD = Y 10 CONTINUE AREA = 0.5*AREA ==================== area :: [(Double,Double)] -> Double area ps = abs $ (/2) $ area' (last ps) ps where area' _ [] = 0 area' (x0,y0) ((x,y):ps) = (x0-x)*(y0+y) + area' (x,y) ps *Main> let p = [(0.0,0.0),(1.0,0.0),(1.0,1.0),(0.0,1.0),(0.0,0.0)] *Main> area (last p) p 1.0 *Main> ==================== area :: [(Double,Double)] -> Double area p = foldl (\ (xold,yold) (x,y) -> ((*) (xold-x) (yold+y))) 0 ((last p):p) Prelude> :l area [1 of 1] Compiling Main ( area.hs, interpreted ) area.hs:29:40: Occurs check: cannot construct the infinite type: t = (t, t) Expected type: (t, t) Inferred type: t In the expression: ((*) (xold - x) (yold + y)) In the first argument of `foldl', namely `(\ (xold, yold) (x, y) -> ((*) (xold - x) (yold + y)))' Failed, modules loaded: none. Prelude>

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bit of help with n-ary please...
by spot135 09 Nov '09

09 Nov '09

Hi, Ok what im trying to do is replace an element in an n-ary and display the completed tree. So this is what ive got so far. data Tree a = Empty |Leaf a|Branch a [Tree a] deriving (Show) replace :: Eq a=>a->a->Tree a -> (Tree a) replace x y Empty = Element x replace x y (Leaf z) | x==z = (Leaf y) -- | otherwise = replace x y (Branch z lis) | x==z = (Branch y lis) |otherwise = replaceA x y clis replaceA :: Eq a=> a->a->[Tree a]->(Tree a) replaceA _ _ [] = Empty -- run out replaceA x y (h:r) |Q suc = suc |otherwise = replaceA x y r where suc=replace x y h Q :: a -> Bool Q a = True Q a = False Theres two main problems i am having. One is I'm not sure what should be the "otherwise" in the leaf function. Another problem is it doesn't show the whole tree just the part after the replacement. If you think this is complete bull I'll quite happily start over if somebody could give me some insight into what is the best way of tackling this problem is. Many thanks in advance spot -- View this message in context: http://old.nabble.com/bit-of-help-with-n-ary-please...-tp26258006p26258006.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Urgent: Please suggest me for Flow Caml Compiler
by Akshay Dave 08 Nov '09

08 Nov '09

Hello, I undertsand that this is Haskell cafe discussion forum. But I would highly appreciate if anyone can tell me where can I get Compiler and editor for FLow Caml ( it's a ML based language)) as I am not able to find it on net :(. Thanks in Adavnce! Akshay _________________________________________________________________ Hotmail: Trusted email with Microsoft's powerful SPAM protection. http://clk.atdmt.com/GBL/go/177141664/direct/01/ http://clk.atdmt.com/GBL/go/177141664/direct/01/

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getting ready to sprint! darcs: 14-15 Nov, Portland/Vienna
by Eric Kow 08 Nov '09

08 Nov '09

Hi everybody, We're less than one week away from the third Darcs Hacking Sprint, taking place in Portland and in Vienna on 14-15 November. Everybody is welcome (Haskell and Darcs newbies included), but please do let us know if you're thinking of attending! Preflight checklist ------------------- We've got our work cut out for us, so we want to make sure that we can hit the ground running. Here are three things each participant can do to help: 1. Request a code.haskell.org account http://community.haskell.org/admin/ 2. Build Darcs (install the Haskell Platform if needed) cabal install darcs darcs get --lazy http://darcs.net darcs cd darcs cabal install 3. Tell Eric your code.haskell.org login This is so that I can add you to the Darcs group Vienna Friday ------------- There's going to be quite a few of us arriving in Vienna on Friday. I propose we meet up at the Hostel Ruthensteiner, say at 11:00 for morning arrivals and 16:30 for those of us getting there in the afternoon. Perhaps we could walk around a bit, have a nice relaxed chat, find some food :-) http://www.hostelruthensteiner.com/ Meetings -------- There are few things we'll be talking about over the sprint (eg, darcs 2.4, SVN integration, filecache). If you're interested in joining in, please put your name down on the wiki so that we can work out a rough schedule. http://wiki.darcs.net/Sprints/2009-11 Nothing formal. Maybe just lunch and me scribbling in a notebook. Or maybe Darcs Hackers gesticulating wildly in front a white board. See you there -------------- That's all for now. Please shout if you have any questions or last minute comments. Otherwise, I'm really excited about this and looking forward to seeing you all this weekend! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Haskell image libraries
by Max Rabkin 08 Nov '09

08 Nov '09

Haskellers, To add image support to fdo-notify, I need an image type. Looking through Hackage, I didn't find any image library with the following features: * Load from a variety of formats (at least PNG and JPG, I'd say) * Efficient per-pixel access, or a way to dump the image into a ByteString as a bitmap (I need to serialise them into the protocol's bitmap format) Preferably, it should be possible to construct images programmatically too. Is there really no such library? It would be nice to have something like a Haskell equivalent of the Python Imaging Library, which is the de facto standard image library in Python and supports just about every type of operation on images you could ask for. Regards, Max

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Re: [Haskell-cafe] Is () a 0-length tuple?
by Matthew Gruen 08 Nov '09

08 Nov '09

Forgot to cc haskell-cafe. Trying again: ---------- Forwarded message ---------- From: Matthew Gruen <wikigracenotes(a)gmail.com> Date: Sat, Nov 7, 2009 at 2:16 PM Subject: Re: [Haskell-cafe] Is () a 0-length tuple? To: Pasqualino Titto Assini <tittoassini(a)gmail.com> On Sat, Nov 7, 2009 at 2:00 PM, Pasqualino "Titto" Assini <tittoassini(a)gmail.com> wrote: > The syntax is similar, but what else is? > > In JavaScript there is a "null" value, that is the only value of the null type. > > Isn't () the same thing? The only value of the unary type? > > Best, > > titto > > Pasqualino "Titto" Assini, Ph.D. > http://quicquid.org/ In JavaScript's case, there is not a null type. The null value belongs to the 'object' type, whereas the undefined value belongs to the 'undefined' type. This is all a lot less useful when you realize that JavaScript has a dynamic type system. But this is JSON, not JavaScript. In JSON, arrays, objects, strings, and numbers can be any number of values. Booleans can be two values. Null can only be one value. Personally, I think a better mapping for () would be JSNull, since both have only one value in normal form. However, there is not necessarily any natural mapping between Haskell values and JSON values. The library tries to provide as many as possible, including (), which it happens to map to JSArray [] instead of JSNull. As long as the library is internally consistent, though, it should be fine.

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call for help: FOSDEM devroom
by Ganesh Sittampalam 08 Nov '09

08 Nov '09

Hi, I'm thinking of trying to get a devroom for haskell.org at the next FOSDEM, which is Saturday-Sunday February 6th-7th 2010 in Brussels: http://www.fosdem.org/2010/call-developer-rooms The idea would be to try to introduce Haskell to people at FOSDEM who were interested, and thus help build our ties with We'd try to have a few introductory talks about Haskell, and some demos etc. It would probably also make sense to talk about/demo darcs at the same time. Apparently there's a lot of demand so it's a bit of a long shot, and I expect we'd only get one day, which is probably all we'd need. To make this work, we'll need several reasonably experienced Haskellers to turn up to help out with the talks, demos and talking to people in general. I've got a couple of people interested already but need more. So, please could you let me know, preferably within the next week, whether you would be interested in coming along and helping. You can email me directly or on-list as you prefer. The deadline for actually making the application for the devroom is 22nd November. Cheers, Ganesh

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