July 2009 - Haskell-Cafe

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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Jane Street
by Matthias Görgens 17 Jul '09

17 Jul '09

If one of you knows something about working at Jane Street, I'd be happy to have exchange some mails. I am considering applying there. Thanks!

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Alternative IO
by Cristiano Paris 17 Jul '09

17 Jul '09

As a joke, I wrote an instance of Alternative for IO actions: {-# LANGUAGE ScopedTypeVariables #-} module Main where import Control.Applicative import Control.Exception instance Alternative IO where empty = undefined x <|> y = handle (\ (_ :: SomeException) -> y) x This would allow to write IO code which failsafes to a value if the previous computation failed, i.e.: *Main Control.Applicative> undefined <|> print "Hello" "Hello" *Main Control.Applicative> print "Hello" <|> undefined "Hello" It seems a neat way to catch exception in some scenarios. What do you think? Why is not Alternative IO defined in Control.Applicative? Thanks, Cristiano

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Python vs Haskell in tying the knot
by Cristiano Paris 17 Jul '09

17 Jul '09

On Fri, Jul 17, 2009 at 12:41 PM, Cristiano Paris<frodo(a)theshire.org> wrote: > ... > Now, to confirm my hypothesis, I wrote a slight different version of > fib, like follows: > > fib' n = 1:1:(fib' n) `plus` (tail $ fib' n) where plus = zipWith (+) > > i.e. I inserted a fictious argument n in the definition of fib'. > > Now, if I try "take 30 $ fib' 100", it takes significntly longer than > "take 30 fib": specifically, the latter is instantaneous, while the > former takes about 5 seconds to complete on my MacBook Pro. Is this an > evidence that the "tying the knot" process is going on in the first > version? BTW, after a -O2 compilation, fib' is apparently as fast a fib. Cristiano

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Line drawing algorithm
by CK Kashyap 17 Jul '09

17 Jul '09

Hi All, I am working on a diagraming utility in Haskell. I started with line drawing. I am doing the basic stuff using the y = mx + c formula to draw a line between (x1,y1) and (x2,y2) Here's what I need to do - if dx > dy where dx = (x2 - x1) and dy = (y2 - y1) then I need to vary x between x1 and x2 and find the various y's however if dy > dx then I need to vary y beteen y1 and y2 and get various x's In the code below, I've only taken care of the situation where dx > dy - I was thinking if there was a better way to do it that takes care of the other condition as well without repeating the code. type Point = (Integer,Integer) line :: Point -> Point -> [Point] -- get all the points in the line line p1@(x1,y1) p2@(x2,y2) = line' start end start slope where (start,end) = reorderPoints p1 p2 slope = ((fromIntegral (y2-y1)) / (fromIntegral (x2-x1))) reorderPoints (px1,py1) (px2,py2) | px1 < px2 = (p1,p2) | otherwise = (p2,p1) line' :: Point -> Point -> Point -> Double -> [Point] line' start@(x1,y1) end@(x2,y2) point@(x3,y3) slope | x3 == x2 = [end] | otherwise = [point] ++ line' start end (newX,newY) slope where newX = x3 + 1 newY = y1 + round (slope * (fromIntegral (newX - x1))) hello = line (1,1) (10,10) Regards, Kashyap

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