- Beginners - Haskell.org

Fwd: Simple help with structuring a program
by Matt Williams 19 Aug '11

19 Aug '11

Dear All, Apologies for the very basic question. I have some familiarity with programming (Java/ Python), and have started learning Haskell (Real World Haskell, Haskell 98 Tutorial, YAHT). I think I know how to translate ideas in the real world into OOP approaches (Objects, Inheritance, Polymorphism, Patterns). I am having some problems trying to do this with Haskell. Should I think about types as being a useful way to structure things, with functions operating on them? Could anyone (very simply) explain the difference between a type and an object class (or point me to someone who has already done so). Is there an equivalent of the GoF design patterns for Haskell? Thanks and apologies for what are very basic questions, Matt

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Stockholm Haskell User Group?
by Obscaenvs 74 18 Aug '11

18 Aug '11

I am posting this again although it might be the wrong forum to do so: I feel there is definitely a need for a Haskell User Group in Stockholm. If you are interested, contact me: obscaenvs(a)gmail.com . My name is Fredrik. /F

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[m [a]] -> m [a]
by Guy 17 Aug '11

17 Aug '11

Is there a function which can do this, where m is not mplus? I would like all the lists to be joined inside the monad with ++.

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How to find a "best alternative"
by Martin Drautzburg 17 Aug '11

17 Aug '11

Hello all, I want to write transformations on musical motives and I came upon the following algorithmic question: For every note of the original motiv I can compute a sequence of transformend notes which are ordered by their "fitness" with respect of a certain criterion. If might for example compute the "fitness with respect to harmonic function". I have a whole set of such criteria. So in the end the questions are: what should the first transformend note be? Harmonic function tells me it should be a G, but could also be a C or a D ..., some other function tells me it should be a C, but could also be a C# or a Cb. It kind of reminds me of the Levenshtein Disance, but this gives me a "distance" from two samples. I need a function which returns a new sample when one sample is given, i.e. the one with the smallest distance. Any idea in which direction is shall focus my thoughts? -- Martin

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flatten
by Sunil S Nandihalli 16 Aug '11

16 Aug '11

Hello everybody, can somebody tell me as to what is wrong with the following code myflatten::[(a,a)]->[a] myflatten [] = [] myflatten (fx,sx):xs = fx:sx:(myflatten xs) It says that there is a parse error on the last line.. Thanks, Sunil.

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the first argument of take is an Int and not Integral..
by Sunil S Nandihalli 16 Aug '11

16 Aug '11

Hi everybody, I feel forcing the first argument of take to be an Int is unnecessarily restrictive .. Is there a rationale behind not making it just (Integral a) ? right now it is take::Int->[a]->[a] I was thinking may be take::(Integral b)=>b->[a]->[a] would be better... Thanks, Sunil.

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abstype?
by Christopher Howard 15 Aug '11

15 Aug '11

In my older (1992) intro programming book, the author uses the keyword "abstype" instead of "data" inside of module code. From what I can gather, abstype is supposed to hide constructors. But I've never seen this keyword used in any modern tutorials I've gone through. Is "abstype" still a valid keyword? Is is still relevant? -- frigidcode.com theologia.indicium.us

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a simple little problem
by Dennis Raddle 14 Aug '11

14 Aug '11

Can someone suggest an elegant way to write the following? fn :: [Maybe Float] -> Maybe Float in which, if the input list has all Nothing, then the result is Nothing if the input list has one or more Just x, then the result is Just x (in which the x is picked arbitrarily, could be the first one or last one) I have something like import Data.Maybe fn list = case catMaybes list of [] -> Nothing [x:_] -> fromJust x Next, an augmentation of this idea (or similar idea). fn2 :: [Maybe Float] -> Map Int Float When a Just x appears at position n in the list, then put (key=n, value=x) into the map. I have: import qualified Data.Map as M f2 list = M.fromList [(n,x) | (n,Just x) <- zip [0..] list]

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Invitation to connect on LinkedIn
by Andrew Smith B.Sc(Hons),MBA 14 Aug '11

14 Aug '11

LinkedIn ------------ I'd like to add you to my professional network on LinkedIn. - Andrew Andrew Smith B.Sc(Hons),MBA Founder and CEO at VTRL - Value Technology Research Ltd Edinburgh, United Kingdom Confirm that you know Andrew Smith B.Sc(Hons),MBA https://www.linkedin.com/e/-3dpk6t-grc8g5b9-3s/isd/3853823396/wkuHdNPh/ -- (c) 2011, LinkedIn Corporation

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QuickCheck with monadic IO
by Hartmut 13 Aug '11

13 Aug '11

Everyone, I want to run an QuickCheck on "a most simple" IO monad. When I compile and run :main, it says: ....................................................................................................+++ OK, passed 100 tests. Well, thats fine, since *1 mapped on any Int gives the same Int back. But it lasts 10 seconds (!) for doing those 100 Tests. Something must be wrong?! I'd appreciate your ideas. Hartmut module MonadicQuickCheck where import IO import Random import Test.QuickCheck import Test.QuickCheck.Monadic main = do quickCheck x x :: Property x = monadicIO $ do a <- pick arbitrary pre $ not(null a) b <- run $ testfunction a assert $ b == a testfunction :: [Int] -> IO [Int] testfunction x = do putStr "." return $ map (*1) x

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