May 2009 - Haskell-Cafe

traversing a tree using monad.cont
by Anatoly Yakovenko 07 May '09

07 May '09

So I am trying to traverse a tree in a specific order, but i have no idea where the things that i am looking for are located, and i want to avoid explicit backtracking. I was thinking i could do it with the continuation monad. Here is what i have module TestCont where import Control.Monad.Cont import Control.Monad.Identity import Control.Monad.State.Lazy --our stupid tree data Tree a = Tree [Tree a] | Leaf a --traverse all the branches search (Tree ts) next = do mapM_ (\ ti -> (callCC (search ti))) ts next $ () search tt@(Leaf a) next = do cur <- lift get case ((cur + 1) == a) of True -> do --the current leaf is what we want, update the state and return lift $ put a return $ () False -> do --the current leaf is not what we want, continue first, then try again next () search tt (\ _ -> error "fail") t1 = Leaf 1 t2 = Leaf 2 t3 = Tree [t1,t2] t4 = Leaf 3 t5::Tree Int = Tree [t4,t3] run = runIdentity (runStateT ((runContT $ callCC (search t5)) return) 0) it seems like next isn't quite doing what i want, because i don't think I ever try again after i call next $ () in the second clause. Any ideas? Thanks, Anatoly

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IO help
by applebiz89 07 May '09

07 May '09

I havent done much IO at all in haskell, only within the function itself. However I want to get the input from the interface for the function and havent done this before. In my main function, I want to ask the user what they would like to do 'become fan' for example, then with their choice initiate the function and ask for the appropriate input for that function. This is the code below: main :: IO() do putStr "Hi there! what is your name: " fanName = getLine do putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans of a film, 4 = Film released in a year: " input = getLine read input :: Int (if input == 1 then main x = insertFilm [] else if input == 2 then main x = becomeFan [] else if input == 3 then main x = numberOfFans []) Say they choose film in a given year function function: filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear films = [ title | (Film title director year fans) <- films, year == filmYear] I need to ask the user what filmYear they want to insert. But i need to do this from the main function...I chose to do an if statement to choose what function they want, but i dont know where to go from there? any help will be greatly appreciated. thanks apple -- View this message in context: http://www.nabble.com/IO-help-tp23423403p23423403.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Interesting Thread on OO Usefulness (scala mailing list)
by Paolo Losi 07 May '09

07 May '09

Hi all, I'm following an interesting thread on the scala mailing list: http://www.nabble.com/-scala--usefulness-of-OOP-td23268250.html Martin Odersky advocates the OO features of the scala language proposing an interesting problem where the OO approach seams valuable. I would be very much interested in seeing an Haskell solution to that problem. Any haskell guru want to take a stub at it or give an opinion from a pure FP point of view? Thanks Paolo

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calling a variable length parameter lambda expression
by Nico Rolle 07 May '09

07 May '09

Hi everyone. I have a problem. A function is recieving a lambda expression like this: (\ x y -> x > y) or like this (\ x y z a -> (x > y) && (z < a) my problem is now i know i have a list filled with the parameters for the lambda expression. but how can i call that expression? [parameters] is my list of parameters for the lambda expression. lambda_ex is my lambda expression is there a function wich can do smth like that? lambda _ex (unfold_parameters parameters) best regards

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WolframAlpha
by Vasili I. Galchin 07 May '09

07 May '09

Hello, http://blog.wolframalpha.com/2009/05/01/the-secret-behind-the-computational… <<< this is obviously a Wolfram Inc. blog so maybe not totally objective ... but here is a snippet that speaks in Haskell's favor: As a result, the five million lines of *Mathematica* code that make up Wolfram|Alpha are equivalent to many tens of millions of lines of code in a lower-level language like C, Java, or Python. * I am some what familiar with Mathematica and it's multi-paradigm nature (like F#, OCaml, etc.). In any case, I would like the Haskell community to view WolframAlpha as a challenge. For what is it worth I presently cabalizing Swish .... Based on my reading of WolframAlpha it is a semantic web ... i.e. formal knowledge representation! all Google results => http://www.google.com/search?q=WolframAlpha&ie=utf-8&oe=utf-8&aq=t&rls=com.… Kind regards, Vasili *

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cabal parse problems
by Vasili I. Galchin 06 May '09

06 May '09

Hello, I am trying to cabalize a package (swish .. semantic web) but am running into parse error: vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ runhaskell Setup.hs configure Setup.hs: swish.cabal:24: Parse of field 'exposed-modules' failed. vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ Below: is a fragment from my .cabal file. Line #24 starts with "Exposed-modules:" ..... ..... ..... Data-Files: README Exposed-modules: Swish.HaskellRDF.BuiltInDatatypes, Swish.HaskellRDF.BuiltInMap, Swish.HaskellRDF.BuiltInMapTest, Swish.HaskellRDF.BuiltInRules, Swish.HaskellRDF.ClassRestrictionRule, Swish.HaskellRDF.ClassRestrictionRuleTest, Swish.HaskellRDF.Datatype, Swish.HaskellRDF.GraphClass, Swish.HaskellRDF.GraphMatch, Any ideas why I am getting parse error? Pretty vague error message! Kind regards, Vasili

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Vector-like data structure
by Krzysztof Skrzętnicki 06 May '09

06 May '09

Hi I'm looking for a data structure with following characteristics: 1. O(1) lookup 2. O(1) modification 3. amortized O(1) append 4. O(1) size query This roughly characterizes C++ vector<> class. I'm ready to implement it myself, but first I would like to ask if anyone knows package with similar data structure. If there are many, which one would you choose and why? Best regards Christopher Skrzętnicki

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instance Monad (Except err)
by Martijn van Steenbergen 06 May '09

06 May '09

Hello, Mr. McBride and mr. Paterson define in their Applicative paper: > data Except e a = OK a | Failed e > instance Monoid e => Applicative (Except e) where ... Sometimes I'd still like to use >>= on Excepts but this "feels" wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Thanks, Martijn.

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Current status of Annotations
by Tom Lokhorst 06 May '09

06 May '09

Hello, I've been reading about Annotations [1] as implemented during last years Summer of Code, and about some earlier ideas [3] [2]. It seems though that these annotations have never been merged into HEAD [4]. Could anyone elaborate on the current status of this proposal? Has there been any further work or discussion about this subject that I haven't been able to dig up? - Tom Lokhorst [1] http://hackage.haskell.org/trac/ghc/wiki/Annotations [2] http://www.haskell.org/pipermail/haskell-prime/2006-November/001893.html [3] http://hackage.haskell.org/trac/haskell-prime/ticket/88 [4] http://www.haskell.org/pipermail/haskell-cafe/2008-September/047293.html

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Unfold fusion
by Adrian Neumann 06 May '09

06 May '09

Hello, I'm trying to prove the unfold fusion law, as given in the chapter "Origami Programming" in "The Fun of Programming". unfold is defined like this: unfold p f g b = if p b then [] else (f b):unfold p f g (g b) And the law states: unfold p f g . h = unfold p' f' g' with p' = p.h f' = f.h h.g' = g.h Foremost I don't really see why one would want to fuse h into the unfold. h is executed once, at the beginning and is never needed again. Can someone give me an example? So, this is what I got so far: unfold p f g.h = (\b -> if p b then [] else (f b): unfold p f g (g b).h = if p (h b) then [] else (f (h b)) : unfold p f g (g (h b)) = if p' b then [] else f' b: unfold p f g (h (g' b)) not very much. I kinda see why it works after I "unfold" some more, but I can't really prove it. I suspect I need some technique I haven't learned yet. I've heard about fixpoint induction, that looks promising, but Google knows very little about it. I hope somebody can give me some hints. Regards, Adrian

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