October 2009 - 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 Casey Hawthorne 01 Oct '09

01 Oct '09

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. If this is true, it needs to be pushed. And if by changing a few lines of source code one can develop a whole family of similar applications, that needs to be pushed, also. :) -- Regards, Casey

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DSL and GUI Toolkits
by Günther Schmidt 01 Oct '09

01 Oct '09

Hi, I managed to abstract parts of the business logic of my application by employing a very naive DSL and it actually worked quite well. I'll probably replace that with a "Finally Tagless" version, just for the sake of it. But now I'd really try to abstract the hard part, the GUI logic. I had been switching back and forth between wxHaskell and Gtk and eventually settled for wxHaskell. And my gui code is a huge mess, it works, for now, but it'd be hell if I even had to change the tiniest bit. So I'm wondering if I could go the same way as with (parts) of the business logic, using a DSL here too. Now I could do with some input on the merits of this approach. Günther

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Can this be done?
by Cristiano Paris 01 Oct '09

01 Oct '09

I wonder whether this can be done in Haskell (see muleherd's comment): http://www.reddit.com/r/programming/comments/7wi7s/how_continuationbased_we… Cristiano

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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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