September 2013 - Beginners

Help needed in understanding MonadWriter syntax.
by Vinay Emani 16 Sep '13

16 Sep '13

Hi, I'm trying to understand the MonadWriter class definition syntax. Now, I'm using GHC 7.6 and the class is defined as class (Monoid w, Monad m) => MonadWriter w m | m -> w where .... I've never seen '|' in the middle of a class definition, could some one explain this? It's used in the guard syntax as well as to separate data constructors in a type definition. Regards Vinay

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Haskell for server?
by Miro Karpis 16 Sep '13

16 Sep '13

Please can you help me with following? I would like to try Haskell in our company. We need to make a simulation data server that would do read/write requests into a DB (didn't decide on a DB yet). Read/write will be done by several clients - so far I think that we might have around 1000 clients. Size of the transferred data will be quite small (maximum 10 MB). Every client can have around 10 requests per second. All the network will be in-house (local). It needs to be stable and fast, since our clients will be a modules that do physics calculations. It should also support parallelism, concurrency and in the future maybe cloud computing. Snap framework looks very promising, but am not 100% sure. Please do you have any suggestions to what framework and DB would be appropriate to this set-up? Cheers, m.

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Plotting interactive charts in haskell - maybe with wxHaskell?
by Nathan Hüsken 13 Sep '13

13 Sep '13

Hey, Iwant to plot finanzial charts in haskell. They should be interactive in the sense, that I want to be able to soom into a specific time period and scroll through the data. I looked through haskell plotting libraries, but did not find anything that seemed to provide this interactivity? Did I miss something? So I was thinking I could use wxHaskell for this. Now I am wondering, can one of the plotting library plot into a wxHaskell "area" (by which I mean into some widget, I do not know which, provided by wxHaskell)? Thanks Nathan

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Strange Failure in extern code via FFI
by Nathan Hüsken 13 Sep '13

13 Sep '13

Hey, I have C++ code which works perfectly when compiled in C++ program. Now I want to call this C++ code from haskell via FFI. When doing so, I get this error in a HUnit test program: test-rf: malloc.c:2369: sysmalloc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed. Something wrong with memory allocation I guess. The code I am running is complex and not mine. But I ran gdb over the problem and the error occurs at this code: data = alloc_.allocate(capacity); gdb tells me, that capacity==2, so it does not seem to be an out of memory error. Also about alloc_, gdb tells me: (gdb) p alloc_ $3 = {<__gnu_cxx::new_allocator<unsigned int>> = {<No data fields>}, <No data fields>} The stack frames above the shown code are: #0 0x00007ffff69a8037 in raise () from /lib/x86_64-linux-gnu/libc.so.6 #1 0x00007ffff69ab698 in abort () from /lib/x86_64-linux-gnu/libc.so.6 #2 0x00007ffff69efb7a in ?? () from /lib/x86_64-linux-gnu/libc.so.6 #3 0x00007ffff69f306c in ?? () from /lib/x86_64-linux-gnu/libc.so.6 #4 0x00007ffff69f4580 in malloc () from /lib/x86_64-linux-gnu/libc.so.6 #5 0x00007ffff7b3666d in operator new(unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6 #6 0x000000000040dbd0 in allocate (__n=2, this=0x7fffffff9a48) at /usr/include/c++/4.7/ext/new_allocator.h:94 So ... it seems to be something with the new_allocator ... Maybe I am doing something different than in the C++ code, but I do not know what so I am suspecting it has something to do with the haskell FFI. Any Ideas? Thanks! Nathan

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2D rectanlge array type
by Nathan Hüsken 12 Sep '13

12 Sep '13

Hey, I am experimenting with machine learning in haskell. For training a model, I need to input a list of features. A features itself is a set of floating point numbers. So my training data has the type: [Feature] What I am wondering is which type I should use for Feature. As I said, it is a set of floats. But every feature must have exactly the same number of floats. I could use type Feature = [Float] but that would not ensure that every feature has the same number of flaots. I could use tuples type Feature = (Float,Float,Float,Float,Float ...) The number of features varies from application to application. And I do not know how to encode that with tuples. Also the number of features can get very big (in extreme cases up to ~1000, in normal cases ~100). How would you do that? Thanks! Nathan

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truncate results depend on strict/lazy
by Bryan Vicknair 11 Sep '13

11 Sep '13

Deep in a WAI web app, I have a function that converts a String from a web form like "0.12" to an Int 12. Converting "0.12" to the Float 0.12 is working. However, converting the Float 0.12 to the Int 12 does not work as expected unless I use the trace function. In the following, f = 0.12::Float, gotten from a function that parses "0.12" into 0.12. In the following expression, the result is: Success (Just 11). > Success $ Just $ truncate (f * 100) In the following expression, the result is: Success (Just 12) > let expanded = f * 100 > ans = truncate expanded > in trace (show expanded) $ Success $ Just $ ans That made me think that "f * 100" had to be strictly evaluated before given to truncate for some reason, so I tried using seq to get the same effect, but that didn't work. Am I correct in assuming that laziness has something to do with this problem?

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newbie's perplexities about instance of "Info a" of Chapter 13 (Thompson's Haskell book)
by Carmine Moleti 11 Sep '13

11 Sep '13

Hi to everyone, I'm learning Haskell going through Thompson's craft of functional programming (3rd ed). As of now I've reached Chapter 13 where classes definition and instances are discussed. At some point the "Info a" class is introduced which has the following interface: class Info a where examples :: [a] size :: a -> Int then it goes on explaining that every instance of that class must comply to the interface shown. So far so good. My perplexities start when actual instances are implemented. Take, for example the following one: instance Info Int where examples = [-100..100] size _ = 1 I promptly tried to fiddle with this via ghci and while I was able to access the "size" function of the interface I have no clue about how to reach the "examples" part and that prevents me from understanding the next step when an instance of "Info [a]" is discussed. About that I don't quite get the definition of its "examples", and the reasons it is defined taking into accounts "all the one and two element lists that can be built up from exmaples of type a" (cit.) Can anyone shed some light about this, please? Thanks in advance for your help. Best regards, Carmine

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wxHaskell programm segfaults when copying shared object files into same directory
by Nathan Hüsken 09 Sep '13

09 Sep '13

Hey, I have a small wxHaskell program, compiled under ubuntu linux. I want to give it (in a compiled version) to a friend. Since wxHaskell seems to work only with shared linking, the program depends on a lot of *.so files. Trying to send the so files with the program, I copied all of them (all which are wxHaskell related) into the same directory as the program. Interestingly when I do this, and try to run the program it segfaults. Anyone has an Idea why? Thanks! Nathan

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Maybe monad and computations
by Emmanuel Touzery 07 Sep '13

07 Sep '13

Hello, I'm often using the Maybe monad to combine Maybe computations one after the other without conditionals. But I'm not sure how to do it, when I have several operations which return Maybe and I want all the indiviual values in the end, not a combination of the values. Currently I do: let allTogether = do ma <- doA mb <- doB return (ma, mb) case allTogether of Nothing -> ... Just (a, b) -> ... This way in this case I have one case instead of two, however I'm sure there must be a nicer way to achieve this result? Thank you! Emmanuel

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8 queens problem using multithreading
by Igor Pinheiro Leão 06 Sep '13

06 Sep '13

Hi, can someone help me to understand how to implement a multithreading 8 queens problem using Haskell. If it's not asking to much I also need a good Haskell thread related source. It would be great. Kind Regards, Igor -- Igor Vinícius Graduando em Ciência da Computação

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