- Haskell-Cafe - Haskell.org

Fwd: [Haskell-cafe] Can this be done?
by Alberto G. Corona 01 Oct '09

01 Oct '09

You can do it almost transparently with the Workflow package:: http://hackage.haskell.org/package/Workflow 2009/2/11 Cristiano Paris <cristiano.paris(a)gmail.com> On Wed, Feb 11, 2009 at 2:30 PM, Peter Verswyvelen <bugfact(a)gmail.com> > wrote: > > I haven't looked at the details, but I think this is what a library like > > Reactive from Conal Elliott could do, but as far as I understand it, it > is > > still work in progress. > > I'm interested in the possibility of > stopping/pickling/unpickling/resuming a computation. > > Cristiano > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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parsing error
by ShawnShawn 01 Oct '09

01 Oct '09

Hi there, I am trying to write some code for parsing string in "p==>q" or "p&&&q" or "p|||r", something like this.But I have trouble in reading operator like"==>""|||"... how can I fix it? I attached my code file with the email.Can some one give me some hints to fix the error and run the code?Thank you in advance!!! _________________________________________________________________ 上Windows Live 中国首页，下载最新版Messenger！ http://www.windowslive.cn

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ANNOUNCE: usb-0.1
by Bas van Dijk 01 Oct '09

01 Oct '09

Hello, I would like to announce the release of usb-0.1, a Haskell library for communicating with USB devices from userspace. usb is implemented as a high-level wrapper around Maurício C. Antunes' bindings-libusb which is a binding to the C library: libusb-1.* ( http://libusb.org ). All functionality from libusb is provided except for asynchronous IO. I'm still thinking about if and how I should add that. As usual, install usb using cabal: > cabal install usb Or download and install it manually from hackage: http://hackage.haskell.org/package/usb-0.1 Or get the development source using darcs: darcs get http://code.haskell.org/~basvandijk/code/usb Comments and patches are highly welcome. Note that this is the first release, so expect API changes in the future. Note that my brother uploaded 'usb-id-database' to hackage. A small library for associating usb identifiers like vendor, product, class, subclass or prototcol identifiers with their respective names and vice versa: http://hackage.haskell.org/package/usb-id-database This can be helpful when trying to find the right device to operate. In a short while, the package 'ls-usb' will be released. This is a simple program for showing all the usb devices currently attached to your system. It's a nice demonstration on how to use the library. regards, Bas

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New TMR editor
by Wouter Swierstra 01 Oct '09

01 Oct '09

Dear all, After several years at the helm, I've decided to step down as editor of the Monad.Reader. I am happy to announce that Brent Yorgey will take over my role as editor. I'd like to take this opportunity to thank Brent for helping to keep the Monad.Reader alive. I'm sure he'll do a fantastic job. Finally, I'd like to thank all the people who have contributed to the Monad.Reader over the last few years – you guys have made this publication, and my job as an editor, so much fun. All the best, Wouter

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c2hs, size_t, and CSize -- argh!
by John Van Enk 01 Oct '09

01 Oct '09

Hello List, I'm running into a problem with c2hs and how it parses the C typedef 'size_t'. On 32bit systems, this ends up being parsed as a CUInt. On 64bit systems, this ends up as a CULong. This gets especially sticky with function pointers. In order to make bindings with c2hs that work across the different word sizes, I have to write an indirection layer in C that defines wrappers for functions or re-defines typedef's for function pointers to include 'unsigned long' instead of 'size_t'. I see there is a ticket open for this: http://hackage.haskell.org/trac/c2hs/ticket/20 Has any one else run into this issue? Is there a good workaround that doesn't involve writing a C function/typedef for each collision? /jve

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Splitting data and function declarations over multiple files
by Peter Verswyvelen 01 Oct '09

01 Oct '09

I guess this is related to the expression problem. Suppose I have a datatype *data Actor = Ball ... | Paddle ... | Wall ...* and a function *move (Ball ...) = * *move (Paddle ...) = * *move (Wall ...) = * in Haskell one must put *Actor* and *move* into a single file. This is rather cumbersome if you work with multiple people or want to keep the files small and readable. Surely it is possible to use type classes, existentials, etc to split the data type into multiple ones, but that's already advanced stuff in a sense. But wouldn't it be possible to allow these to be put into multiple files, and let the compiler merge them back into one? A bit like C#'s partial keyword: in file Ball.hs: *partial data Actor = Ball ...* *move (Ball ...) =* in Paddle.hs *partial data Actor = Paddle ...* *move (Paddle ...) =* The compiler would then merge all partial data types and functions into one. As far as no overlap exists in the pattern matches in move, so that the order of the pattern matches does not matter at all, the partial trick should be possible no? Cheers, Peter

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ANNOUNCE: vty-ui 0.1
by Jonathan Daugherty 01 Oct '09

01 Oct '09

vty-ui is: An extensible library of user interface widgets for composing and laying out Vty user interfaces. This library provides a collection of widgets and a type class for rendering widgets to Vty Images. Get it from Hackage: http://hackage.haskell.org/package/vty-ui Or get the source with darcs: http://repos.codevine.org/vty-ui This package is motivated by the desire to create terminal user interfaces easily without having to worry about doing all of the structural book-keeping that so often comes with such an approach. Future releases will probably include more widget composition tools, more interesting widgets, and helper code for building interesting things. For now, the library contains some basic widgets (including horizontal and vertical box layout) and a List widget. See the vty-ui-demo program and source for an interactive demonstration of the available widgets. Future releases may also address performance, as I honestly don't have a very clear idea of how well my approach is going to scale for non-trivial interfaces. Please don't hesitate to send patches, feedback, and criticism directly to me at drcygnus AT gmail DOT com. Lastly, thanks a ton to the authors of Vty. It's a pleasure to use. Enjoy! -- Jonathan Daugherty

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Fwd: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Alberto G. Corona 01 Oct '09

01 Oct '09

;) Off topic: Maybe the entire space time, the universe and his history, is isomorphic to a mathematical structure. http://space.mit.edu/home/tegmark/toe_frames.html <http://space.mit.edu/home/tegmark/toe_frames.html> 2009/10/1 Peter Verswyvelen <bugfact(a)gmail.com> > > > On Thu, Oct 1, 2009 at 9:53 AM, Andrew Coppin <andrewcoppin(a)btinternet.com > > wrote: > >> Sure. But what is a computer program? It's a *list of instructions* that >> tells a computer *how to do something*. And yet, the Haskell definition of >> sum looks more like a definition of what a sum is rather than an actual, >> usable procedure for *computing* that sum. (Of course, we know that it /is/ >> in fact executable... it just doesn't look it at first sight.) >> > > Is it? The list of instruction is just an abstraction layer built on top of > purely physical process of electrons and transistors; I'm not sure how much > imperativeness remains at this level? Not to mention the quantum mechanical > processes that take place... And that are also just mathematical models... I > mean, it really depends from which angle and at which detail you look at it, > no? > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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river crossing puzzle
by pat browne 01 Oct '09

01 Oct '09

Hi, Does anyone know where there are any Haskell implementations of the the River Crossing puzzle (AKA Farmer/Fox/Goose/Grain). There are several variations but the general ideas are explained at: http://en.wikipedia.org/wiki/River_crossing_puzzle http://en.wikipedia.org/wiki/Fox,_goose_and_bag_of_beans_puzzle I have found one at: http://www.shido.info/hs/haskell9.html Thanks, Pat

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Re: [Haskell-cafe] Exceptions during exception unwinding
by Brian Bloniarz 01 Oct '09

01 Oct '09

Sorry for the garbled post, this should hopefully be plain text: > On Thu, 2009-10-01 at 03:29 +0000, Brian Bloniarz wrote: >> I.e. why does an exception raised during exception handling get >> propagated past the exception that triggered the handler? > > Because it's the obvious and sensible thing to do and it what every > other mainstream language with exceptions does. The behaviour you want, > to automatically discard any exceptions raised in the handler and to > always re-raise the original exception can be implemented using the > current semantics, but the reverse is not true. Good to know ... C++ (the mainstream language I'm most familiar with) treats throwing from a destructor during unwinding as a fatal error. I was suggesting having bracket ignore exceptions from closeSomeHandle when doAction threw, not that it'd ignore any exceptions from closeSomeHandle. It's only the case where bracket has 2 exceptions to choose from that's ambiguous, and I'm saying that there's a fair amount of code out there that'd prefer to see the first one. Your point about ^C gets us into an even grayer area -- maybe code would prefer to propagate whichever of the 2 exceptions is asynchronous, or define a criterion to choose which exception is more serious, who knows. All these behaviors are easy to implement using the building blocks that Control.Exception exports right now, you're right. So I'm free to build my own bracket variants with different behavior and that's what I'll do. I was just wondering if the default behavior that bracket/onException provide (which are themselves built from catch/block/unblock) is the one that most people want. Thanks, -Brian _________________________________________________________________ Microsoft brings you a new way to search the web. Try Bing™ now http://www.bing.com?form=MFEHPG&publ=WLHMTAG&crea=TEXT_MFEHPG_Core_tagline_… bing_1x1

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