July 2010 - Haskell-Cafe

A question about State Monad and Monad in general
by C K Kashyap 12 Aug '10

12 Aug '10

Hi, I looked at State Monad yesterday and this question popped into my mind. >From what I gather State Monad essentially allows the use of Haskell's do notation to "invisibly" pass around a state. So, does the use of Monadic style fetch us more than syntactic convenience? Again, if I understand correctly, in Mutable Arrays also, is anything getting modified in place really? If not, what is the real reason for better efficiency? -- Regards, Kashyap

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Can we come out of a monad?
by C K Kashyap 11 Aug '10

11 Aug '10

Hi, In the code here - http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28393#a28393 If I look at the type of modifiedImage, its simply ByteString - but isn't it actually getting into and back out of the state monad? I am of the understanding that once you into a monad, you cant get out of it? Is this breaking the "monad" scheme? -- Regards, Kashyap

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Re: [Haskell-cafe] iPhone/Android and Haskell [Was: Embedded funcional programming?]
by Liam O'Connor 10 Aug '10

10 Aug '10

Our best bet is to compile to ARM native code and then use the NDK to talk to the Java APIs. Cheers. ~Liam

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Re: [Haskell] ANNOUNCE: jhc 0.7.4
by Antoine Latter 09 Aug '10

09 Aug '10

On Sat, Jul 10, 2010 at 1:47 AM, John Meacham <john(a)repetae.net> wrote: > Announcing jhc 0.7.4! There have been a few major changes, the main one being > that there is now support for a garbage collector. This drastically increases > the number of programs which are feasable to compile with jhc. > > http://repetae.net/computer/jhc/ > Hi John, This looks fantastic and fun to play with. A few issues: * running DrIFT on src/E/TypeCheck.hs fails with an illegal bytesequence in hGetContents. I'm guessing that this is only an issue when building DrIFT with GHC 6.12+, and that the file contains bytes illegal in UTF8. I deleted everything funny looking in the file and then it went smooth * The way you use sed doesn't work with the BSD sed that ships with my Mac Book. Installing GNU sed and using it works. Similarly, BSD find doesn't know about '-name', so make hl-clean results in sadness. * jhci works great, but jhc crashes when I try to compile something: >>>>> jhc test1.hs jhc 0.7.4 (tokfekyuvi-27) Finding Dependencies... Using Ho Cache: '/Users/alatter/.jhc/cache' Main [test1.hs] Typechecking... [1 of 1] Main (.............................................) test1.hs:9 - Warning: defaulting: t93 => Jhc.Basics.Integer Compiling... [1 of 1] Main <..................................................> Collected Compilation... -- typeAnalyzeMethods -- BoxifyProgram -- Boxy WorkWrap -- LambdaLift E jhc: <stdout>: hPutChar: invalid argument (Illegal byte sequence) <<<<< Again, this seems like the handle is in UTF8 mode and we're trying to output something non-UTF8. * cabal install has a --jhc flag, but it doesn't seem to work: >>>>> cabal install byteorder --jhc Resolving dependencies... cabal: internal error: impossible <<<<< I have jhc installed in a non-standard location (under ${HOME}/usr) so I may need to have some environment variables set up. This is wil Cabal 1.8.0.4 and cabal-install 0.8.2 Keep up the good work! I have an implementation for STRefs I threw together this afternoon for jhc if you're interested. I can't test it properly, though, with the compiler crash above. Antoine

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Wildly off-topic: de Boor's algorithm
by Andrew Coppin 06 Aug '10

06 Aug '10

Given a suitable definition for Vector2 (i.e., a 2D vector with the appropriate classes), it is delightfully trivial to implement de Casteljau's algorithm: de_Casteljau :: Scalar -> [Vector2] -> [[Vector2]] de_Casteljau t [p] = [[p]] de_Casteljau t ps = ps : de_Casteljau t (zipWith (line t) ps (tail ps)) line :: Scalar -> Vector2 -> Vector2 -> Vector2 line t a b = (1-t) *| a + t *| b Now if you want to compute a given point along a Bezier spline, you can do bezier :: Scalar -> [Vector2] -> Vector2 bezier t ps = head $ last $ de_Casteljau t ps You can chop one in half with split :: Scalar -> [Vector2] -> ([Vector2], [Vector2]) split t ps = let pss = de_Casteljau t ps in (map head pss, map last pss) And any other beautiful incantations. Now, de Boor's algorithm is a generalisation of de Casteljau's algorithm. It draws B-splines instead of Bezier-splines (since B-splines generalise Bezier-splines). But I think I may have ACTUALLY MELTED MY BRAIN attempting to comprehend it. Can anybody supply a straightforward Haskell implementation?

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what's the best environment for haskell work?
by Rustom Mody 06 Aug '10

06 Aug '10

Do most people who work with haskell use emacs/vi/eclipse or something else?? Personal Note: I used gofer some 15 years ago. At that time I hacked up a emacs mode (I did not know of any then) along with some changes to gofer to have gofer inside emacs rather than vi inside gofer. Things have got more exciting now -- just trying to catch up!! [Note: My preferrred/default OS is debian-squeeze]

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hGetContents: resource exhausted
by Lally Singh 05 Aug '10

05 Aug '10

Hey all, This is on OpenSolaris. Simple attempts to build cabal packages give me this error, and I don't know what it means. Here's an example: [07/25 18:51::lally@sol type-level]$ runghc Setup.hs configure Configuring type-level-0.2.4... Setup.hs: fd:8: hGetContents: resource exhausted (Resource temporarily unavailable) Setup.hs: fd:8: resource exhausted Does anyone know what this means? I'm trying to just get the llvm bindings installed (requiring mtl & type-level). Cheers, -Lally

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design question: decision tree from "Programming Collective Intelligence"
by Daniel Lyons 04 Aug '10

04 Aug '10

Hi, I'm having a little trouble figuring out precisely how to port the decision tree code from the book "Programming Collective Intelligence." You can see the code here: http://code.google.com/p/memothing/source/browse/trunk/PCI/ch7/treepredict.… The design issue is that this code depends on dynamic typing pretty heavily. He has a table of heterogenous data. Each column has the same type, but that's implicit, not explicit. He depends on all of the values supporting "==", which he uses to get a list of distinct values from each column. These values are used by the divide set function which takes a column and a value as parameters. Based on the type of the value, he chooses a different partitioning function; for strings, it's just equality again, but for numeric types he uses >=. The decision tree nodes then collect not just the left and right branches but also the column number and value on which the split was performed. I have thought about this for several days and can't seem to come to a design that I like, so I'm wondering how others would approach the problem. I guess you could implement it as a list of lists of some data type that is either a string or numeric, but this feels a bit like a cop-out. How would you support creating a decision tree with different types of data? I imagine possibilities using existential quantification, SYB, Data.Data and other stuff, but I don't know how to make it work. I wonder if there is an obvious functional design hiding in here that doesn't depend on any fancy stuff, but I'm blinded to it by having read and understood the Python version of the code. Anybody have any suggestions? I'd greatly appreciate any help. Thanks, — Daniel Lyons

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Error Monad and strings
by Gerald Gutierrez 03 Aug '10

03 Aug '10

Reading the Control.Monad.Error documentation, I see that the Error class has noMsg and strMsg as its only two functions. Now, I understand that you can define your own Error instances such as in example 1 of the documentation, so why the need to always support strings via noMsg/strMsg ? What uses these? And if in my code, I will never throw an error with a string, am I supposed to implement these functions and then ignore them?

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ANN: scan-0.1.0.3, a Haskell style scanner
by Christian Maeder 03 Aug '10

03 Aug '10

Dear Haskell friends, I like to announce a Haskell style scanner at http://hackage.haskell.org/package/scan documented under http://projects.haskell.org/style-scanner/ It's best used in conjunction with hlint http://community.haskell.org/~ndm/hlint/ and gives many suggestions regarding the file format. Feedback is welcome. Thanks Christian (Discussions should only be posted to either haskell-cafe@ or beginners@)

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