January 2013 - Beginners

Natural functions versus existential types
by Darren Grant 07 Jan '13

07 Jan '13

I've seen some cases where Haskell examples dive into existential types instead of using natural higher order functions. For instance, the "Existential type" wiki page [1] section 2.2 proposes existentials as the solution to a heterogeneous list problem where a ray-tracer must evaluate a trace function for different types of objects. Is there a good reason for this, or is it just a case of prior language assumptions leading to unnatural code? Why could I not just make the list homogeneous by supplying a list of partially evaluated trace functions instead? Sometimes I read an argument that existentials are required due to unknown constraints at compile time, but do not have an intuition for this situation yet. For instance, I read that IO requires existentials. Still working on that one. :) Cheers, Darren [1] http://www.haskell.org/haskellwiki/Existential_type

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Confused by the qualified imports
by Libor Wagner 06 Jan '13

06 Jan '13

Hi, I was looking into Gloss examples for usage of Repa arrays. For example the Snow example (http://code.ouroborus.net/gloss/gloss-stable/gloss-examples/raster/Snow/Mai…) What I do not get is that there is qualified import of the Repa package, but the "Z" and ":." are used without the prefix. When I some times accidentally omit the prefix the ghc always yell at me. What em I missing? Thanks, Libor

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Functional analysis and design
by Martin Drautzburg 06 Jan '13

06 Jan '13

Hello all, often, when I read tutorials or lectures about haskell, I am absolutely intrigued by the solutions presented there. It often creates this "aha" effect and I think "yes, this perfectly describes the problem to solve, this is what the problem IS". But alas, I have difficulties to come up with equally brilliant solutions for my own problems. As for learning haskell, I am now pretty comfortable with it, but I fail to apply it to real world problems. I am pretty good at semantic data modelling, but this technique gives me nothing but trouble, when I try to apply it in the functional world (while it works well in the OO world). What I am trying now it asking "what do I want the system to compute in the first place" and then think about how to implement these top-level functions. Do you think that this is a good way to start? Other than that I was trying to find some information about haskell as a specification language, but could not find anything. Is this a sensible idea at all? If not, how would you write a specification if not in haskell itself? So if you have any pointers on how to address a non-trivial problem in haskell, this would by much appreciated. -- Martin

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Haskell connectTo problem
by Alexander 0xAX 05 Jan '13

05 Jan '13

Hello, I try to connect to host: >>> connectTo "httpbin.org" (PortNumber 80) {handle: <socket: 8>} It's ok. But if i try to connect: >>>connectTo "httpbin.org/ip" (PortNumber 80) *** Exception: getAddrInfo: does not exist (Name or service not known) Why i got this error? Thank you.

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Re: [Haskell-beginners] Game of Life try
by Mateusz Kowalczyk 04 Jan '13

04 Jan '13

I recommend that you read a guide/tutorial/book on Haskell first, before you try to implement this. Having a very limited knowledge about the language will only provide frustration when your solution doesn't work when you try to approach it naively, trying to use methods from a very different language. A widely recommended book is Learn You A Haskell [1]. After reading it, it should become fairly obvious how to approach your problem as well as how to solve it. You'll soon realise that there's simply no need for things such as `while' or a mutable array. To just hint towards what you might want is a function that takes a list (or list of lists, whatever your representation may be)) and returns a list with the next generation. You can then use any of the graphics libraries (or just print ASCII) to visualise the transformation. Again, this will be a lot easier when you actually study the basic language concepts first, instead of diving head-in and trying to hammer C++ into Haskell. [1] http://learnyouahaskell.com/ Mateusz Kowalczyk Oops, forgot to CC the mailing list. On 04/01/13 20:41, Емануела Моллова wrote: > Hello all! :) I'm new to Haskell and I would like to try to implement > a simple version of Game of Life. I can imagine it in C++ for example, > but I have a little difficulties in Haskell. All I can think for is a > mutable array with all elements - 0 in the beginning and the user to > be able to write in the coordinates of the cells, which are alive and > this cells to become 1. Then to iterate through the array (but I'm not > sure if I can iterate, there is no 'while' here) and to make the > changes (but maybe I will need a new array for the changes, because > when the first change happens, it will affect the result). Also I was > thinking of how to make it visible, so I tried to make all 0-s red and > all 1-s green, and after each iteration to clear the screen with ANSI > so that it looks a bit like animation, but none of these ideas work... > Here is an orientation in my ideas: > > import Data.Array.IO <http://Data.Array.IO> > import System.Console.ANSI > > main :: IO () > main = do > arr <- newArray ((1,1), (10,10)) 0 :: IO (IOArray (Int, Int) Int) > writeAraay arr (1,1) 1 > a <- readArray (1,1) > setSGR [SetColor Foreground Dull Red] > putStr [a] > > Could you please suggest me what is a good place to store the > information about my cells (mutable, unmutable array, lists, > tuples...) and any ideas and tutorials at all would be really > appreciated! Thank you very much in advance! :) > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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Machine Learning/ANN's in Haskell.
by Jack Hunt 03 Jan '13

03 Jan '13

Hi all, I have been interested in Haskell for a long time now, and am definitely going to invest a considerable amount of time into learning it. Next year is the final year of my CS degree, and I am set on having a final year project orientated around ML. Now, my question is this:- What are the advantages/disadvantages of using Haskell for such a project? Bearing in mind of course that I am an absolute beginner. Regards, Jack Hunt

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string -> bytestring
by Christopher Howard 01 Jan '13

01 Jan '13

Hi. I'm trying to use pcre-light in a small text-manip app. However, the module seems to want the incoming text data as a ByteString, whereas I in my child-like naivety am taking the data from stdin as Strings. After searching around, I tried using the fromString function from Data.ByteString.UTF8 to convert from String to ByteString; but all I get is: code: -------- Couldn't match expected type `ByteString' with actual type `bytestring-0.9.2.1:Data.ByteString.Internal.ByteString' -------- So I'm not sure what to do. -- frigidcode.com

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