- Haskell-Cafe - Haskell.org

Plot data reconstruction reading pixel colors from image files
by Cetin Sert 18 Jul '09

18 Jul '09

Hi, How can I open and read colors of specific pixels of an image file in Haskell? Which packages, functions do you recommend? You can have a look at the quoted conversation for an idea on what I would like to automate. Best Regards, Cetin Sert > Hi dear *^o^*, > > Here are the points from Figure 37 > > [SEE ATTACHED PNG] > > Cisse > Goni > Kodougou > Nouna > normals > log10s > normals > log10s > normals > log10s > normals > log10s > 0.7999999999999998 > 0.5249999999999999 > 0.4249999999999998 > 0.375 > 0.41249999999999964 > 0.4249999999999998 > 0.8500000000000001 > 1.575 > 1.5999999999999999 > 1.325 > 1.0499999999999998 6.30957344480193 > 3.3496543915782757 > 2.6607250597988084 > 2.371373705661655 > 2.5852348395621885 > 2.6607250597988084 > 7.07945784384138 > 37.583740428844415 > 39.81071705534971 > 21.134890398366466 > 11.220184543019629 -0.5625 > -0.3250000000000002 > 0.16249999999999964 > 0.3374999999999999 > 0.5 > 0.8500000000000001 > 1.4874999999999998 > 2.1375 > 2.2625 > 1.875 > 1.0 0.27384196342643613 > 0.4731512589614803 > 1.4537843856076607 > 2.1752040340195222 > 3.1622776601683795 > 7.07945784384138 > 30.725573652674456 > 137.24609610075626 > 183.02061063110568 > 74.98942093324558 > 10.0 0.7250000000000001 > 0.34999999999999964 > 0.3374999999999999 > 0.5 > 0.625 > 0.6374999999999997 > 0.7624999999999997 > 1.2249999999999999 > 1.5125 > 1.2125 > 0.625 5.308844442309884 > 2.2387211385683377 > 2.1752040340195222 > 3.1622776601683795 > 4.216965034285822 > 4.340102636447436 > 5.787619883491203 > 16.788040181225597 > 32.546178349804585 > 16.31172909227838 > 4.216965034285822 0.3374999999999999 > 0.13749999999999973 > 0.375 > 0.6499999999999999 > 0.5749999999999997 > 0.47499999999999964 > 0.9874999999999998 > 1.825 > 2.025 > 1.4749999999999999 > 0.7624999999999997 2.1752040340195222 > 1.372460961007561 > 2.371373705661655 > 4.46683592150963 > 3.7583740428844394 > 2.9853826189179573 > 9.716279515771058 > 66.83439175686145 > 105.92537251772886 > 29.853826189179586 > 5.787619883491203 > Top to Bottom in the table is Left to Right in the figure. > > > Lots of love, > CS ;-) > > P/S: Sorry it took a bit longer than expected but it was lots of fun! > > ------------------------------------------------------------------- > > > > module Main where > > import Control.Monad > > f x = 3 - (x / 80) -- 80: number of pixels > d x = x - 2 -- pixel offset > > cisse, goni, kodou, nouna :: [Double] > cisse = [178,200,208,212,209,208,174,116,114,136,158] > goni = [287,268,229,215,202,174,123,71 ,61 ,92 ,162] > kodou = [184,214,215,202,192,191,181,144,121,145,192] > nouna = [215,231,212,190,196,204,163,96 ,80 ,124,181] > > disp :: (String, [Double]) → IO () > disp (town,pixels) = do > putStrLn $ town > putStrLn $ ">normals" > mapM_ print $ points > putStrLn $ ">log10s" > mapM_ print $ log10s > putStrLn $ "-------------------" > where > points = map (f . d) pixels > log10s = map (10 **) points > > main :: IO () > main = do > mapM_ disp [("Cisse", cisse),("Goni", goni),("Kodougou", kodou),("Nouna", > nouna)] > > > > -------------------- > > Cisse > >normals > 0.7999999999999998 > 0.5249999999999999 > 0.4249999999999998 > 0.375 > 0.41249999999999964 > 0.4249999999999998 > 0.8500000000000001 > 1.575 > 1.5999999999999999 > 1.325 > 1.0499999999999998 > >log10s > 6.30957344480193 > 3.3496543915782757 > 2.6607250597988084 > 2.371373705661655 > 2.5852348395621885 > 2.6607250597988084 > 7.07945784384138 > 37.583740428844415 > 39.81071705534971 > 21.134890398366466 > 11.220184543019629 > ------------------- > Goni > >normals > -0.5625 > -0.3250000000000002 > 0.16249999999999964 > 0.3374999999999999 > 0.5 > 0.8500000000000001 > 1.4874999999999998 > 2.1375 > 2.2625 > 1.875 > 1.0 > >log10s > 0.27384196342643613 > 0.4731512589614803 > 1.4537843856076607 > 2.1752040340195222 > 3.1622776601683795 > 7.07945784384138 > 30.725573652674456 > 137.24609610075626 > 183.02061063110568 > 74.98942093324558 > 10.0 > ------------------- > Kodougou > >normals > 0.7250000000000001 > 0.34999999999999964 > 0.3374999999999999 > 0.5 > 0.625 > 0.6374999999999997 > 0.7624999999999997 > 1.2249999999999999 > 1.5125 > 1.2125 > 0.625 > >log10s > 5.308844442309884 > 2.2387211385683377 > 2.1752040340195222 > 3.1622776601683795 > 4.216965034285822 > 4.340102636447436 > 5.787619883491203 > 16.788040181225597 > 32.546178349804585 > 16.31172909227838 > 4.216965034285822 > ------------------- > Nouna > >normals > 0.3374999999999999 > 0.13749999999999973 > 0.375 > 0.6499999999999999 > 0.5749999999999997 > 0.47499999999999964 > 0.9874999999999998 > 1.825 > 2.025 > 1.4749999999999999 > 0.7624999999999997 > >log10s > 2.1752040340195222 > 1.372460961007561 > 2.371373705661655 > 4.46683592150963 > 3.7583740428844394 > 2.9853826189179573 > 9.716279515771058 > 66.83439175686145 > 105.92537251772886 > 29.853826189179586 > 5.787619883491203 > ------------------- >

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Re: [Haskell-cafe] is closing a class this easy?
by Conor McBride 18 Jul '09

18 Jul '09

On 18 Jul 2009, at 01:43, Lennart Augustsson wrote: > As far as I know it works. It's an old Oleg trick. Then it probably does work. > The only drawback is that error messages may refer to Private. As I found out when probing its security. "No instance for Moo.Private" shows up. I guess that's what happens when you hide stuff: you get told what stuff's being hidden. In some situations, that's insecure, but here it's ok. Cheers (and have fun in China, Lennart!) Conor

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Re: [Haskell] ANN: data-ordlist-0.0.1 and NumberSieves-0.0
by Henning Thielemann 17 Jul '09

17 Jul '09

Leon Smith schrieb: > Two new packages have been uploaded to Hackage, one that implements bag > (multiset) and set operations on ordered lists, and another that offers > three different number theoretic sieves. > > http://hackage.haskell.org/package/data-ordlist > > Data.OrdList offers many of the same kinds of operations as Data.Set, > although Data.Set is likely to often be a better choice. However, > this library is not intended to be used as an abstract datatype for sets > and multisets, rather it is intended to be a convenient way for > efficiently dealing with lists that you happen to know are ordered. You could also implement that in terms of Map a Int.

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ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

Dear friends of Haskell and Functional Reactive Programming, its my pleasure to announce the first official release of Grapefruit, a library for Functional Reactive Programming (FRP) with a focus on user interfaces. With Grapefruit, you can implement reactive and interactive systems in a declarative style. User interfaces are described as networks of communicating widgets and windows. Communication is done via different kinds of signals which describe temporal behavior. Grapefruit consists of five packages with several interesting features: grapefruit-frp: Functional Reactive Programming core * The FRP implementation uses a hybrid push/pull-based approach. * Signals can be memoized by binding them to variables – like ordinary data structures. * Merging of event streams combines simultaneous events properly. * Signals cannot behave differently by starting them at different times. The type system is used to enforce proper aging of signals. grapefruit-records: A record system for Functional Reactive Programming * Input records can specify fields as being required or optional. * Uninteresting output fields can be ignored. The set of interesting fields is specified by pattern matching. grapefruit-ui: Declarative user interface programming * The interface for UI programming is platform-independent and can be implemented on top of different UI toolkits. * A concrete implementation can be selected at compile time or runtime. grapefruit-ui-gtk: GTK+-based implementation of the general UI interface * Gtk2Hs is used internally. grapefruit-examples: examples demonstrating features of Grapefruit * Circular communication and switching are demonstrated. Support for the following is planned for the future: * signals with incremental updates * user interfaces with changing structure * more kinds of UI components * graphical animations Further information about Grapefruit is available on its wiki page at <http://haskell.org/haskellwiki/Grapefruit>. If you have questions, applause or criticism, please get in touch with me. Wolfgang Jeltsch Principal Grapefruit developer

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Re: [Haskell] ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

[redirecting to haskell-cafe] Am Sonntag, 15. Februar 2009 00:25 schrieben Sie: > Hi Wolfgang, > > I was wondering if I can use FLTK as GUI backend for Grapefruit? This should be possible in principal. It just could be that my assumptions about how widgets are created and composed were too tight so that Grapefruit’s general interface doesn’t fit FLTK. In this case, please just tell me and I will try to make the interface more general. > I believe for this to make it happen, I would have to output FLTK's C++ > into C then create bindings for Haskell (via FFI). Is that doable or an > quite tall order? Recently, a student of mine has written a program which generates a Haskell Qt binding fully automatically from Qt header files. The generated binding consists of three layers. The first layer is C++ code which reexports Qt’s functionality as a pure C interface. The C interface is ugly for humans and not type safe (because C doesn’t know classes). The second layer consists of a couple of FFI declarations. The third layer is Haskell code which provides a nice interface similar to the original C++ interface. I still have to get the source code of the binding generator from that student but I hope this will happen soon. I want to publish it then on the web. It hope that it is possible to reuse this binding generator for other C++ libraries. > Jamie Clark Best wishes, Wolfgang

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What is the point of the 'What the bleep' names in haskell?
by Daryoush Mehrtash 17 Jul '09

17 Jul '09

Why do Haskell programmers (and libraries) name their function like "<@<" or "###"? Why not use a more descriptive label for functions? Daryoush

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excercise - a completely lazy sorting algorithm
by Petr Pudlak 17 Jul '09

17 Jul '09

Hi all, about a month ago, we were discussing sorting in Haskell with a friend. We realized a nice property of lazy merge-sort (which is AFAIK the implementation of Data.List.sort): Getting the first element of the sorted list actually requires O(n) time, not O(n * log n) as in imperative languages. And immediately we asked: Would it be possible to create a lazy selection/sorting algorithm so that getting any element of the sorted list/array by its index would require just O(n) time, and getting all the elements would still be in O(n * log n)? More precisely: The result of sorting an n-element list or array should be a structure that allows to ask i-th element of the result (for example, a lazy array). Asking k arbitrary elements should take no more than O(min(n * log n, k * n)) I believe that this could be done, but so far I wasn't able to implement and show it myself. I think the solution would be somewhat modified lazy quicksort (or "Median of Medians algorithm" if we want to be sure that we get good pivots). Perhaps somebody else would also like to give it a try? Or perhaps explain me why it's not possible? Best regards, Petr

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Oops in Haskell
by CK Kashyap 17 Jul '09

17 Jul '09

Hi, Can someone please send me a working example based on the contents posted in the URL below? http://yi-editor.blogspot.com/2008/12/prototypes-encoding-oo-style.html Thanks, Kashyap

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laziness blowup exercise
by Thomas Hartman 17 Jul '09

17 Jul '09

Challenge: change one function in the following pipeline so that t doesn't blow up when executed in ghci. import qualified Data.Set as S t = last . take (10^6) . iterate f $ S.empty f s = S.delete 1 . S.insert 1 $ s Please suggest more of these types of exercises if you have them and maybe we can collect the folk wisdom into a wiki page and/or exercise page for beginners. spoiler for the above problem below :) myiterate f x = let nxt = f x in nxt `seq` x : myiterate f nxt

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What's the status with unicode characters on haddock ?
by david48 17 Jul '09

17 Jul '09

Hello all, I made a small program for my factory and I wanted to try to document it using haddock. The thing is, the comments are in French and the resulting html pages are unreadable because the accentuated letters are mangled. It's not acceptable to use HTML entities, as I'd like the comments to remain readable when/if I edit the code. Anyone has had the same problem ? Found a workaround ? Thanks, David.

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