January 2009 - Beginners

problem 8 / 9 chapter 4, RWH
by Michael Litchard 23 Jan '09

23 Jan '09

I noticed that in problem 4, the use of Either is needed, which hasn't been introduced yet. I've gotten stuck on 8/9, and am wondering if there is something needed that hasn't been introduced yet as well? Put another way, can problem 8/9 be done given the parts of the language introduced thus far? Regards, Michael Litchard

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The case expression
by Erik de Castro Lopo 23 Jan '09

23 Jan '09

Hi all, Ocaml's match .. with expression (very similar to Haskell's case) allows multiple matches for a single result (naive example): let f x = match x with | 0 -> "zero" | 1 | 3 | 5 | 7 -> "odd" | 2 | 4 | 6 | 8 -> "even" _ -> "bad number" Is there a similar thing in Haskell? At the moment I have to do something like : f x = case x of 0 -> "zero" 1 -> "odd" 3 -> "odd" 5 -> "odd" 7 -> "odd" 2 -> "even" 4 -> "even" 6 -> "even" 8 -> "even" _ -> "bad number" Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "One of our programming maxims is "make illegal states unrepresentable" by which we mean that if a given collection of values constitute an error, then it is better to arrange for that collection of values to be impossible to represent within the constraints of the type system." -- Yaron Minsky http://www.haskell.org/sitewiki/images/0/03/TMR-Issue7.pdf

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Is ghc 6.8.2 useful?
by Erik de Castro Lopo 22 Jan '09

22 Jan '09

Hi all, I'm using hoogle (http://www.haskell.org/hoogle/) as my main Haskell documentations source, but I'm stuck on using ghc 6.8.2 and I keep finding stuff listed in hoogle that isn't available in 6.8.2. Is there any solution other than installing a more recent version of ghc? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "Education is an admirable thing, but it is well to remember from time to time that nothing that is worth knowing can be taught." -- Oscar Wilde

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System.Process and ghc 6.8.2
by Erik de Castro Lopo 22 Jan '09

22 Jan '09

Hi all, I'm using ghc/ghci 6.8.2 and looking at the System.Process module. Hoogle lists functions like readProcess and readProcessWithExitCode but they don't seem to exist. Is 6.8.2 too early? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "Christianity has a nasty habit of ignoring the major problems of our time, including overpopulation and exhaustion of resources, because they aren't mentioned in the Bible." -- Paula L. Craig

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Type Operator
by Kellen J. McClain 21 Jan '09

21 Jan '09

I have a quick question. Recall that: class Monad m where (>>=) :: m a -> (a -> m b) -> m b ... and suppose I have a data type Sample: data Sample a b = ... how could I define Sample to be an instance of Monad such that: (>>=) :: Sample a c -> (a -> Sample b c) -> Sample b c ? I would like to use a (\a -> ...)-like operator, but for types. So, something like this: instance Monad (\a -> Sample a c) where (>>=) :: Sample a c -> (a -> Sample b c) -> Sample b c a >>= f = ... but that obviously doesn't work. Alternatively I would like to use a type declaration and partially apply it: type SampleFlip b a = Sample a b instance Monad (SampleFlip c) where (>>=) :: SampleFlip c a -> (a -> SampleFlip c b) -> SampleFlip c b which translates to: (>>=) :: Sample a c -> (a -> Sample b c) -> Sample b c But this doesn't work either, and ghc extensions don't add this functionality. Can I do this in Haskell?

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This is problem 2, chapter 4 RWH
by Michael Litchard 21 Jan '09

21 Jan '09

splitWith' :: (a -> Bool) -> [a] -> [[a]] splitWith' p x = case takeUntil p x of [] -> [] x' -> subset : splitWith' p x'' where (subset, x'') = break p x' where takeUntil :: (a -> Bool) -> [a] -> [a] takeUntil p' = takeWhile q where q x = not (p' x) It doesn't give me the right behavior though. It should work like this: splitWith' odd [2,2,1,4,4,3,6,6,5] [[2,2],[4,4],[6,6]] but it works like this instead [[2,2]] I'm pretty sure I modeled splitWith' on words, which is what the exercise indirectly suggests. Can someone help me step through this code to figure out what's wrong. It looks right to me. Michael Litchard

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System.IO.withFile
by Rafael Gustavo da Cunha Pereira Pinto 20 Jan '09

20 Jan '09

Hi folks, I am trying to use the withFile function of System.IO, but it always return an empty string when testing. Could someone explain why: main= do h<-openFile "test.cir" ReadMode c<-hGetContents h print c > runhaskell test1.hs > "* Teste\n\nR1 1 0 10\nC1 1 0 10uF\nI1 1 0 1mA\n\n.DC \n.PRINT\n" works and main= (withFile "test.cir" ReadMode hGetContents) >>= print > runhaskell test1.hs > "" don't? Thanks! -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Printf and bool
by Aditya Mahajan 19 Jan '09

19 Jan '09

Hi, I like the printf function from Text.Printf to display results on the terminal. I noticed that there is no format specifier for Bool. Can someone suggest how to create an instance of PrintfArg for Bool? Aditya

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MULTICONF-09 final call for papers
by John Edward 16 Jan '09

16 Jan '09

MULTICONF-09 final call for papers The 2009 Multi Conference in Computer Science, Information Technology and Control systems and Computational Science and Computer Engineering (MULTICONF-09) (website: http://www.PromoteResearch.org) will be held during July 13-16 2009 in Orlando, FL, USA. We invite draft paper submissions. The event consists of the following conferences: · International Conference on Artificial Intelligence and Pattern Recognition (AIPR-09) · International Conference on Automation, Robotics and Control Systems (ARCS-09) · International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-09) · International Conference on Enterprise Information Systems and Web Technologies (EISWT-09) · International Conference on High Performance Computing, Networking and Communication Systems (HPCNCS-09) · International Conference on Information Security and Privacy (ISP-09) · International Conference on Recent Advances in Information Technology and Applications (RAITA-09) · International Conference on Software Engineering Theory and Practice (SETP-09) · International Conference on Theory and Applications of Computational Science (TACS-09) · International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-09) The website http://www.PromoteResearch.org contains more details. Sincerely John Edward Publicity committee

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Is complete decoupling and abstraction possible in Haskell?
by Tomer Libal 16 Jan '09

16 Jan '09

Hi, I must add that although in general Theorem provers are very performance oriented, we are not interested in performance in our prover as it is used mainly for testing different refinements and extensions. Therefore, we consider haskell as well (as c++). Also, is there a big reference project, implemented in Haskell, that exhibits some of the OO concepts like decoupling, modulation and abstraction? I understand functional programming has other core values like ease of programming and easy validation of correctness but is it also suitable for large structured projects worked on by many programmers (using decoupling for example)? I have read the previous thread about Monads with great interest. The abstration ideas contained in Wadler's papers gave me hope that there is a way to do a full decoupling and abstraction in Haskell, but I have no idea how. I am interested in building a completely generic resolution theorem prover that all of its parts can be implemented separately. In fact, we already have this prover programmed in c++ but I would be glad if I can reprogram it in Haskell for further extensions of the different parts. The idea of the resolution prover is to get a set of clauses in clasual logic and to try and refute them by using resolution. Some of the parts that need to be decoupled are: - Refinements which are datastructures that return the pair (or more) of clauses each time that need to be processed. They are chosen according to some heuristic, i.e. smallest clauses, etc. Each Refinement instance should be able to return the next pair to be processed according to its inner state and heuristics. - Resolvers that process two clauses and tries to unify a specific literal in each. The abstraction here deals with each resolver processing in a different way. Some do factorization and some modulation and each one of this processes have several kinds that should be decoupled as well (i.e. modulator that works by a brute force BFS or

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