- Haskell-Cafe - Haskell.org

Need help - my haskell code is over 50 times slower than equivalent perl implementation
by C K Kashyap 01 Jul '14

01 Jul '14

Dear cafe, I have created a reproducible app here - https://github.com/ckkashyap/haskell-perf-repro Essentially I am trying to open a number of files and printing out their sized by reading them in and computing it's length. I have the equivalent perl program also there. I'd appreciate it very much if someone could take a look at my Haskell implementation and point out what I am doing wrong. Since it's over 50 times slower than the perl code, I assume I am doing something obviously incorrect. Regards, Kashyap

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Algorithm for labeling same-colored regions in a 2D array?
by Steve Trout 01 Jul '14

01 Jul '14

(Apparently joining the list through the Google Groups interface doesn't work, but the post still shows up there. Sorry if you see this twice!) For fun, I'm writing a rules engine for the board game Go (in Haskell obviously). I have a board, which is a grid of spaces that can contain a black piece, a white piece, or nothing. Part of what i need to do is to detect regions of the same "color" (black, white, or empty) and get information about them like their size, the color of the bordering spaces, etc. Regions are bounded by other colors or the board edges in cardinal directions. I'm just starting on this so I haven't picked my data structures yet, but Vector (Vector _) or Map (Int, Int) _ are what I've toyed with. I have a vague idea of how I would go about this in an imperative language with mutation. I'd iterate over the board, keeping lists of points contained in the same region, combining (relabeling) regions if they happen to connect later on. (I realize there are probably better algorithms in those languages, but that would be my first attempt.) I know i could cobble something like this together in the State monad, but I have a feeling that there ought to be a better (more "Haskellish"/pure-functional) way. What would a Haskellish algorithm for this kind of thing look like? I wouldn't mind a complete answer or just a nudge in the right direction. Thanks, -Steve

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Making new Unboxed Algebraic Types? (A Repa Problem)
by Hans Georg Schaathun 30 Jun '14

30 Jun '14

Hi, could someone offer som advice on handling composite types with the Repa library? I can see a /long/ way around my problem, but I wonder what is the /best/ way ... The problem is that I am trying to parallellise a probabilistic algorithm using Repa. To handle pseudo-randomness in parallel, I couple each data element with a generator state, in a tuple. The generator state is of type TFGen (tf-random package) or StdGen¹ (random package), neither of which is an Unbox instance. Hence, I have been using boxed repa arrays. The code does not execute in parallel, and if I understand correctly this is because of the Boxed type. There is a workaround², but that requires an NFData instance. To make a NFData instance and work through the code to support it should be as simple as it is tedious, but it would still leave the other drawbacks of a boxed type. (In particular, I do sequential fold because the Repa fold function did not support boxed types.) I could of course rewrite the published libraries to use tuples instead of algebraic types, and benefit from the pre-made Unbox instances, but sounds messy and error-prone (as well as boring). If I have to I have to. Is there a quick and robust way to make Unbox instances from composite datatypes? Or any other advice on how to approach the matter? Both random ideas, references, and concrete solutions are welcome. [The other element of the tuple is an vector of unboxed constituent type. I am not sure how that's going to work, but I'll cross that bridge when I get there. For the time being it is the generator that stops type checking.] TIA ¹ Yes, I know that StdGen is statistically unsound. ² http://stackoverflow.com/questions/16097418/parallel-repa-code-doesnt-creat… -- :-- Hans Georg

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Monad laws
by Rafael Almeida 30 Jun '14

30 Jun '14

Hello, You guys have been great taking my questions. Thank you. Now I have another one :) What can go wrong if I make an Monad instance but don't follow Monad rules (identity and associativity)? Sure it would be misleading for someone using the non-conforming class. They may make code that assume those laws, although they don't hold. However, could it make the compiler generate bad code or some standard functions to behave badly or even break? []'s Rafael

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Re: [Haskell-cafe] Making new Unboxed Algebraic Types? (A Repa Problem)
by Hans Georg Schaathun 30 Jun '14

30 Jun '14

On Tue, Jul 01, 2014 at 01:43:32AM +0400, Dmitry Dzhus wrote: > This implies that the amount of generator states used in your parallel > computation is equal to data array size N. This is not the case in > general (unless the computation is split into N processes running in > parallel by the scheduler). On the contrary, commonly you would even > want to minimize the amount of generator states used (for resource > efficiency reasons). Fair point. The large number of states may be a disadvantage. > Generator states may be implicitly passed into sequential > subcomputations (scheduled by the parallel planner). This of course > calls for some hacking on Repa harness that runs the computations in > parallel (computeP). Such a solution would probably not be determinstic, and it would definitely make the output depend on the run-time environment (such as the number of threads). I am interested in the particular problem of pseudo-random, /deterministic/, parallel programs. I think the large number of generator states is necessary to achieve deterministic, parallel behaviour. If someone knows otherwise, I would be curious to learn. > Some time ago I was also struggling find a way to parallelize a > probabilistic algorithm (Monte-Carlo molecular simulation) and > eventually I settled with even simpler solution based on strategies > instead of Repa: > https://github.com/dzhus/dsmc/blob/master/src/Control/Parallel/Stochastic.hs. > It can be easily ported to any stateful mapping computation that needs > parallelization (including your PRNGs of choice). See the whole package > for usage examples. Thanks a lot for the pointer. I shall have a look. In fact I am curious to learn about strategies as well. (Contrary to low-level hacking of Repa features which I am not at all curious about :-) -- :-- Hans Georg

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ANNOUNCE: stm-containers
by Nikita Volkov 30 Jun '14

30 Jun '14

A post with introduction to the library, motivation behind it and its performance analysis: http://nikita-volkov.github.io/stm-containers/ Hackage: http://hackage.haskell.org/package/stm-containers GitHub: https://github.com/nikita-volkov/stm-containers

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Re: [Haskell-cafe] [Haskell] I've just published an introductory book on Haskell Data Analysis
by Henning Thielemann 30 Jun '14

30 Jun '14

Am 30.06.2014 19:24, schrieb Heinrich Apfelmus: > The word "blog" was hyperlinked to a blog post of mine [1] which has > absolutely nothing to do with finance or data analysis. I don't know > how, on the basis of this evidence, Packt came to the conclusion that > I would have any expertise in "Haskell financial data modelling and > predictive analysis". The blog post contains a screenshot showing Dollar and Euro amounts. FRP might be "Financial Rewarding Programming". It matches the scope of the book pretty much, doesn't it? :-)

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ANN: rest
by Adam Bergmark 30 Jun '14

30 Jun '14

We at Silk are proud to announce the release of our Haskell REST framework `rest'. Please see the related blog post for more information: http://engineering.silk.co/post/90354057868/announcing-rest-a-haskell-rest-… Cheers, Adam

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Testing web-interfacing applications
by Mateusz Kowalczyk 30 Jun '14

30 Jun '14

Hi, Whenever I write a program which has to interface with the web (scraping, POSTing, whatever), I never know how to properly test it. What I have been doing up to date is fetching some pages ahead of time, saving locally and running my parsers or whatever it is I'm coding at the moment against that. The problem with this approach is that we can't test a whole lot: if we have a crawler, how do we test it it goes to the next page properly? Testing things like logging in and such seems close to impossible, we can only test if we are making a good POST. Let's stick to a crawler example. How would you test that it follows links? Do people set up local webservers with few dummy pages they download? Do you just inspect that GET and POST ‘look’ correct? Assume that we don't own the sites so we can't let the program run tests in the wild: page content might change (parser tests fail), APIs might change (unexpected stuff back), our account might be locked (guess they didn't like us logging in 20 times in last hour during tests) &c. Of course there is nothing which can prevent upstream changes but I'm wondering how we can test the more-or-less static stuff without calling out into the world. -- Mateusz K.

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Munich Haskell Meeting
by Heinrich Hördegen 30 Jun '14

30 Jun '14

Dear all, today again is out monthly Haskell Meeting in Munich. I completly forgot inviting you! Sorry! Today at 19h30, we will meet here: http://www.max-emanuel-brauerei.de/de/Kontakt.html I will try to reserve a table for 10 to 12 persons. If the weather is fine, it will be outside. Please be early, because I cannot defend the table very long if nobody shows up! Cheers, Heinrich

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