October 2009 - Haskell-Cafe

Simple but interesting (for me) problem
by michael rice 22 Oct '09

22 Oct '09

There's a thread on the plt-scheme list about creating a function of NO arguments named NEXT that just returns the number of times it's been called, a piece of cake in Scheme, but how would one do this in Haskell? Would the best approach be to use a State monad? Michael

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How can i safely change the value of specified key ?
by zaxis 22 Oct '09

22 Oct '09

aaa <- newIORef ([]::[(Int,Int)]) writeIORef aaa [(1,1),(2,2),(3,3)] then if i want to change aaa to [(1,1),(2,222),(3,3)] , what's the best way ? re-write aaa is not permitted. Sincerely! -- View this message in context: http://www.nabble.com/How-can-i-safely-change-the-value-of-specified-key---… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANN: mecha-0.0.0
by Tom Hawkins 22 Oct '09

22 Oct '09

A few months ago, I started toying with a few alternative pump designs to power our hydraulic hybrids. After not being able to secure a ProE license, I searched for a free solid modeler to sketch out a few ideas. To my surprise, their are practically no open source 3D CAD packages available. So I created Mecha, DSL for constructive solid modeling. Mecha's geometry is based on octrees, which makes it easy to perform set operations on solids, as well as volumetric calculations such as center-of-mass, moments of inertia, and of course, total volume. Drawbacks of octrees include consuming a lot of memory and the loss of some surface information. To address the later, Mecha carries forward surface points and normals to the leaf nodes of the octree to assist in rendering, such that solids don't look like they're made from a bunch of LEGOS. Note this is a very early release. The only thing Mecha can do at the moment is draw a pretty blue ball -- well it can also zoom and pan on a pretty blue ball (mouse wheel). Establishing a primitive API and building a primitive library must be finished before Mecha an do anything useful. Any comments and suggestions are welcome. http://tomahawkins.org/

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pretty printing with comments
by Pasqualino "Titto" Assini 22 Oct '09

22 Oct '09

Hi, Is there any simple way of pretty printing haskell source code while preserving the comments? I am looking at the haskell-src-ext library. It can parse files with comments and it can pretty print but, for what I can see it cannot do both :-) (prettyPrint won't work on the structure returned by parseFileWithComments). Thanks, titto

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Real World Haskell Chapter 19 and GHC 6.10
by Michael Mossey 22 Oct '09

22 Oct '09

The examples in the "error handling" chapter (19) of RWH don't run under GHC 6.10. For instance, an example might be main = handle (\_ -> putStrLn "error") (print $ 5 `div` 0) Trying to load this results in "amigous type variable 'e' in the constraint: 'Exception e' arising from a use of 'handle' etc etc. I was able to fix this via the ludicrously complex: main2 = (handle :: (SomeException -> IO ()) -> IO () -> IO ()) (\_ -> putStrLn "Error calculating result") (print $ 5 `div` 0) Is there a more concise way to use "handle", or can someone point me to a tutorial that might explain the changes in 6.10 and in a general way how to get the RWH examples to run? Thanks, Mike

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Needing strictness?
by michael rice 21 Oct '09

21 Oct '09

Busy night. I wanted the question and response to be on the same line (see below), so I used putStr instead of putStrLn, but Haskell jumps the gun and goes to the second line to get the response before the question ever gets printed. Is this an example of needing strictness? How would I do that, or something else, to get the result I want? putStrLn "Some yes/no question (y/n)?" ans <- getLine Michael

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ANN: GPipe 1.02 and Vec-Transform 1.0.1
by Tobias Bexelius 21 Oct '09

21 Oct '09

I've uploaded new versions of these two packages to hackage: GPipe-1.0.2: * Removed rasterize and made rasterizeFront work for other primitives than triangles instead. * Fixed notB Vertex and Fragment instances Vec-Transform-1.0.1: * Added rotationLookAt Cheers Tobias Bexelius _________________________________________________________________ Windows Live: Dina vänner får dina uppdateringar från Flickr, Yelp och Digg när de skickar e-post till dig. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

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How to use "bracket" properly ?
by zaxis 21 Oct '09

21 Oct '09

winSSQ count noRed noBlue = do { yesRed <- [1..33] \\ noRed; yesBlue <- [1..16] \\ noBlue; bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ count yesRed yesBlue hd1); return () } will report: Couldn't match expected type `IO ()' against inferred type `[()]' In a stmt of a 'do' expression: bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\ hd1 -> pickSSQ count yesRed yesBlue hd1) However, the following works fine: winSSQ count noRed noBlue = do let yesRed = [1..33] \\ noRed let yesBlue = [1..16] \\ noBlue bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ count yesRed yesBlue hd1) Why ? -- View this message in context: http://www.nabble.com/How-to-use-%22bracket%22-properly---tp25953522p259535… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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list -> sublists
by satorisanitarium 21 Oct '09

21 Oct '09

I just started learning haskell and I just came to my first wtf moment. I'm trying to do something like this: calling: foo 3 [1,2,3,2,4,1,6,3,6,2,3,5,2,5,2,1,6,4] returns: [[1,2,3],[2,4,1,6,3],[2,3]] but i have no idea how to put something into a sublist. How to make a list of sublists out of a list, whether they be a list of numbers or a string. TX in advance. -- View this message in context: http://www.nabble.com/list--%3E-sublists-tp25975341p25975341.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Test cases for type inference
by Peter Verswyvelen 21 Oct '09

21 Oct '09

For learning, I would like to develop my own implementation of type inference, based on the paper "Typing Haskell in Haskell". At first sight, the source code of THIH contains a small number of tests, but I was wandering if a large test set exist? Thanks, Peter

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