May 2011 - Beginners - Haskell.org

Haskell on Arm
by Christopher Howard 18 May '11

18 May '11

(I hope two questions in one night is okay...) I was wondering how "doable" it would be to compile and run Haskell code on Arm. I was thinking about buying one of those tiny, inexpensive Arm PCs (there are quite a few models, apparently) and running Gentoo Linux on it. But it would be a lot cooler if I knew I would be able to run Haskell code on it as well. The GHC platforms page indicated only bare-minimum tier 2 support, and I couldn't seem to find any kind of "road map" tutorials to give me an idea what kind of challenges I would face. -- frigidcode.com theologia.indicium.us

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shortcut for printing to stderr
by Christopher Howard 18 May '11

18 May '11

I use a formula like this very often: hPutStrLn stderr "error message" Is there not some shortcut function in base that prints to stderr without having to pass in the handle? Obviously it is not hard to define one, but it would be neat to know if there was already one in base. (I couldn't seem to find one in the System.IO listing.) -- frigidcode.com theologia.indicium.us

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Reading JSON using Text.JSON
by Adrien Haxaire 17 May '11

17 May '11

Hello, I have this JSON string that I have put in a text file, test.json: {"coordinates": [0.0, 1.0]} using GHCi, I can read it like this: > s <- readFile "test.json" > let r = decode s :: Result JSValue > r Ok (JSObject (JSONObject {fromJSObject = [("coordinates",JSArray [JSRational False (0 % 1),JSRational False (1 % 1)])]})) I want to affect this array of two Double to the type Coordinates being defined like this: type Coordinates = (Double, Double) Using this type I can create a Node: data Node = Node Coordinates Number deriving (Eq, Ord, Show) The problem is that I have no idea what to do with decoded JSValue. It seems I have to define an instance of JSON to read it. Does this mean using pattern matching to extract the values ? What do I do with the Result type ? Should I get rid of it like this: fromResult :: Result JSValue -> JSValue fromResult (Ok js) = js fromResult _ = JSNull and work with the raw JValue ? These questions might be trivial, but I know only imperative languages, which is why I like so much to learn Haskell and functional programming, lots to learn and discover :) Many thanks in advance, Adrien

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A problem about Haskell 99 questions
by amazingjxq 17 May '11

17 May '11

In problem 7 <http://www.haskell.org/haskellwiki/99_questions/1_to_10>, the example in haskell is below: *Main> flatten (Elem 5)[5]*Main> flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]])[1,2,3,4,5]*Main> flatten (List [])[] > > What are the functions List and Elem? I cann't hoogle it.

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Pearls of Functional Algorithm Design - find the smallest free number
by Costello, Roger L. 16 May '11

16 May '11

Hi Folks, Here is a recurring problem: Find the difference between what you have versus what you need Here are some examples: 1. Cooking: a recipe calls for this list of ingredients: eggs, flour, milk, chocolate. In my kitchen I have some ingredients. Is there a difference between what the recipe requires versus what I have in my kitchen? 2. Product Inventory: the inventory sheet says one thing. The actual products on the shelf says another. Is there a difference between what the inventory sheet says versus what is actually on the shelves? This recurring problem calls for a nice algorithm to solve it. I am reading a fabulous book, Pearls of Algorithm Design [1]. It describes how to solve the problem. I created Powerpoint slides that expand on the book's description: http://www.xfront.com/Pearls-of-Functional-Algorithm-Design/Chapter1/Find-t… Comments welcome. /Roger [1] http://www.amazon.com/Pearls-Functional-Algorithm-Design-Richard/dp/0521513…

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TCP Server
by Raphael Päbst 16 May '11

16 May '11

Hi! I need a TCP server for my current project and am now wondering if I have to write it from scratch or if there is a package out there that lets me avoid this. I have seen examples of TCP servers for example in the Real World Haskell book, but they are a bit confusing to me at the moment, so I'd like to avoid tackling this one. Thanks Raf

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Genetic algorithm for knapsack problem
by Karol Samborski 16 May '11

16 May '11

Hi all, I'm a student and I got recently a task to do (and I want to do it in haskell). I have to implement "genetic algorithm for knapsack problem" and it would be nice if it will be fast. So my question is: what haskell datatypes, modules etc. should I use for this algorithm to achieve this goal? I mean fast as compared to implementation in imperative languages like C. Any advices ? Best regards, Karol Samborski

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Multiple versions
by Tom Murphy 15 May '11

15 May '11

Hi all, Despite my best efforts, there are multiple versions of the Haskell Platform on my system. I'm running Mac OSX, 10.6. I've installed the newest Haskell Platform, from its installer package. In /Library/Frameworks/GHC.framework/Versions, there is only one folder: the latest. Previously, though, I had to delete by hand an older version of GHC in the Versions folder. There are no Haskell or GHC folders in ~/Library. When I run the ghci that's in the /Library/Frameworks/GHC.framework/Versions folder, it's the right version. Yet when I type "ghci" in the prompt, it gives me "GHCi, version 6.10.4" Any help would be very appreciated! Tom

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Generic exception available?
by Christopher Howard 15 May '11

15 May '11

I have one spot in some code where I just want to raise an exception that contains the message "Database is formatted incorrectly." Is there some kind of generic way to throw that (like, e.g., in Ruby you just call 'raise "exception message"'. In this case, I don't particularly want to take the time to think about creating an Exception type or finding a suitable one. (BTW: Is there a hierarchical list of standard Haskell Exception instances somewhere?) -- frigidcode.com theologia.indicium.us

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Printing the entries of a Data.Map m
by Manfred Lotz 15 May '11

15 May '11

Hi all, I have a map with some two thousand entries. Now I want to print that map. I could do: print m but I like to use my own print function: mapM_ myprint (Map.toList m) I may be wrong but having to use Map.toList looks pretty inefficient. Question: I'd like to know if there is a more efficient way to do it? -- Manfred

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