- Haskell-Cafe - Haskell.org

New Hackage category: Error Handling
by Gregory Crosswhite 08 Dec '09

08 Dec '09

Hey everyone, When I uploaded my new package, "error-message", I also went ahead and created a new category: "Error Handling". I did this because there are a number of packages related to this issue, so it might be helpful to have them presented together in one place. Thus, if you have or are in the process of writing a package related to this, please consider adding it to this category sometime so that my package doesn't get lonely. :-) Cheers, Greg

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Hayoo and Hoogle (beginner question)
by drostin77 08 Dec '09

08 Dec '09

Hello Hopefully Helpful Haskell Community! (I really wanted that to be alliteration... couldn't come up with an h word for community) I've just started learning Haskell (working my way through "Real World Haskell" and really liking it)! I have started to appreciate that there are a lot of Haskell libraries I will want to search and access quite regularly when I work on Haskell projects, so I had the following questions: 1. Hoogle and Hayoo: I'm a bit confused by the difference here. From initial impressions it seems that Hayoo is linked to Hackage, but Hoogle is not? And it seems that Hayoo is, well, "Similar to Hoogle, but with less focus on type search." Should I check both of these when I want a library, or choose one and go with it...? Or is it indeed better to search Hayoo when I'm thinking of a named library and Hoogle when I think I know the type signature for just the function I need? 2. I much prefer a command line search interface to a browser, I see Hoogle offers such a tool, does Hayoo? P.S. If these questions are in the wrong place, or if they are already answered in detail somewhere that my Googling didn't find please feel free to paste a link and tell me to search better next time! -- View this message in context: http://old.nabble.com/Hayoo-and-Hoogle-%28beginner-question%29-tp26669924p2… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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OpenCL target for DPH?
by Marcus G. Daniels 07 Dec '09

07 Dec '09

Hi, I'm wondering if anyone has looked at OpenCL as target for Data Parallel Haskell? Specifically, having Haskell generate CL kernels, i.e. SIMD vector type aware C language backend, as opposed to just a Haskell language binding. Thanks, Marcus

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Is anyone reading Pattern Calculus by Barry Jay?
by Duane Johnson 07 Dec '09

07 Dec '09

I just bought a copy of Pattern Calculus [1] by Barry Jay and I would like to discuss the lambda- and pattern-calculus with anyone who is interested. Is there anyone else here who is reading the book and would like to discuss here (if it is appropriate) or take the discussion elsewhere? My knowledge of types has come primarily through reading this Haskell Cafe list, so I am by no means an expert. Just a tinkerer :) Regards, Duane Johnson [1] http://lambda-the-ultimate.org/node/3695

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Re: [Haskell-cafe] Re: [Haskell] ANNOUNCE: control-monad-exception 0.5 with monadic call traces
by klondike 07 Dec '09

07 Dec '09

Luke Palmer escribió: > On Fri, Nov 6, 2009 at 6:54 PM, klondike > <klondikehaskellcafe(a)xiscosoft.es> wrote: > >> Henning Thielemann escribió: >> >>> That's what I meant with my post: Programming errors (like "head []") >>> are not handled by control-monad-exception. As far as I understand, >>> control-monad-exception makes _exceptions_ explicit in the type >>> signatures, not programming errors. Why and how would you make >>> possible programming errors explicit in the type? But for exceptions >>> (e.g. "file could not be found") a detailed stack trace is not of much >>> use. >>> >> I think you have overlooked a few things. First, not every developer >> knows each one of the lines in the code well enough as to see where a >> exception comes from, specially when you are not the author of that code. >> >> Of course, that wouldn't mind so much unless you see another thing, if >> we don't know which exceptions can be launched by a operation then you >> will get it on the upper frame and rendered unable to solve it. Use >> typed exceptions (as this library intends to) you may say. Ok, now we >> have another problem, the strange habit of coders to keep the exceptions >> they don't know/can't treat going up and up and up, until then usually >> hit the top frame and you are screwed. You can check some Java code (to >> see an example on how this happens) as some of these exceptional >> conditions are put on the method's signature. >> > > You sound like you are expecting something that works exactly like the > imperative exception handling mechanisms you are used to, and are not > willing to accept anything else. > No, I sound like I am expecting programmers comming from the imperative world to do ugly things, maybe because I once was one. > This "strange habit" of programmers is simply bad practice, and leads > to just as brittle code as no exception handling, except that you get > inexplicable error message boxes with OK buttons instead of crashes. > You might as well just put the whole program in a catch block in IO > and bail with something noncomittal. > Now comes the time when I have to show you that not every exception could be handled, IE a file not found exception when looking for the config file can be fatal and force the program to stop. But what if this is on a library? How do you suggest that the programmer knows that the File Not Found exception is due to the lack of that config file, specially when the code is badly (or not) documented. > IMO, Haskell's typed exceptions (via eg. explicit-exception) *are* the > way to go. They keep the number of exceptional conditions that can > occur in a body of code small -- if some code has many exceptional > conditions, it forces you to *compose* them somehow, to come up with a > more precise idea of what your code is doing and how it can fail. > Yeah, the thing comes when you get the same kind of exceptions from different places, you need a way to tell them appart. > If you are just blindly passing exceptions up from the code below, the > interface to your code is getting more and more complex. With typed > exceptions, the types are reflecting the complexity of the interface > (which is precisely the purpose of types). > Well, it's not like blindly pass exceptions up but more like passing exceptions I don't know how to handle up.

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A bug of groupBy implement
by L.Guo 07 Dec '09

07 Dec '09

Hi there: My friend asked me a question, and i suppose he has found a bug of `groupBy'. Here is the code piece: > List.groupBy (\a b -> Foreign.unsafePerformIO (Text.Printf.printf "\t%d <= %d ?: %s\n" a b (show (a<=b)) >> return (a<=b))) [7,3,5,9,6,8,3,5,4] I have tested it in GHC 6.10.4 (Win XP) and GHC 6.8.3 (Linux), both give the wrong result (categaried): 7 <= 3 ?: False 3 <= 5 ?: True 3 <= 9 ?: True 3 <= 6 ?: True 3 <= 8 ?: True 3 <= 3 ?: True 3 <= 5 ?: True 3 <= 4 ?: True [[7],[3,5,9,6,8,3,5,4]] Regards -------------- L.Guo 2009-12-08

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The Haskell Web News: December 2009 Edition
by Don Stewart 07 Dec '09

07 Dec '09

The Haskell Web News is a monthly summary of the hottest news about the Haskell programming language, as found in our online communities. If you want to catch up with what’s been happening in Haskell, this might be the journal for you. http://haskellwebnews.wordpress.com/2009/12/05/whats-new-in-haskell-decembe… What’s been happening with the Haskell programming language for the past month, as voted by readers of The Haskell Reddit. This is the first edition of the Web News, so feedback on the content, schedule and goals is welcome. -- Don

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Is it possible: Haskell platform - portable edition
by Rafael Gustavo da Cunha Pereira Pinto 07 Dec '09

07 Dec '09

Hello, Has anyone considered the possibility of making a *Portable* Haskell Platform distribution (portable meaning on a USB stick) for Windows? This worked form me: I installed GHC on one machine, copied it to a USB stick and created a batch to set the environment path and open a command prompt. This gives me the freedom to use Haskell wherever I am! The only downside is that I have to edit packages.conf by hand everytime I install a package. I would like to improve this, by making it more automatic. Do you haskellers think it is possible? Regards, Rafael Gustavo da Cunha Pereira Pinto

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ANNOUNCE: PortAudio Windows Tutorial and Binaries
by M Xyz 07 Dec '09

07 Dec '09

I got a lot of great help this weekend from Haskell-Cafe, thanks. Now that I have portaudio up and running I put up a tutorial and a 103 kb download of all the windows binaries and files. I hope this helps people exploring digital audio with Haskell. http://www.subreddits.org/misc/haskell/portaudio.html

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computing lists of pairs
by Christian Maeder 07 Dec '09

07 Dec '09

Wizards, I've the following small piece of code \begin{code} pairs :: [a] -> [b] -> [[(a, b)]] pairs l1 = map (zip l1) . takeKFromN l1 takeKFromN :: [b] -> [a] -> [[a]] takeKFromN s l = case s of [] -> [[]] _ : r -> [ a : b | a <- l, b <- takeKFromN r l] \end{code} I have a predicate: p :: (a, b) -> Bool and I want to keep only those results of pairs which fulfill "all p". I do so currently by "filter (all p) (pairs l1 l2)", but I want to generate the beginning of this pair lists more efficiently, because the result list of pairs may become very large, soon: length (pairs l1 l2) == length l2 ^ length l1 Any ideas (or other hints) to improve the code? "pairs" computes all different mappings from all elements of l1 to some elements of l2. "takeKFromN" computes all possible sequences of length l1 with elements from l2. I somehow want to integrate the predicate into the generation. Cheers Christian

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