June 2012 - Beginners

Re: [Haskell-beginners] wxHaskell install errors
by Gregory Guthrie 06 Jun '12

06 Jun '12

I already did this, this is the instruction given in: http://www.haskell.org/haskellwiki/WxHaskell/Windows I copied a summary of this issue(s) to the wxHaskell mailing list, hopefully they can give some assistance. ------------------------------------------- > To install wxWidgets you can find how in the 'Readme' as I did, or you can use what I have > used on windows 7: > > cd c:\wx\build\msw > mingw32-make -f makefile.gcc BUILD=debug > > I'm not sure if the 'BUILD=debug' is really needed, but it worked for me. > > Hope it helps, > Carlos Gomez >

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Re: [Haskell-beginners] graphics.gloss errors
by Gregory Guthrie 06 Jun '12

06 Jun '12

Apparently I needed to upgrade my examples, the API seems to have changed across versions. ------------------------------------------- > Subject: graphics.gloss errors > > Why do I get this error, when trying the first line example from: > http://hackage.haskell.org/packages/archive/gloss/1.0.0.0/doc/html/Graphics… > > >ghci > GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... > linking ... done. > Loading package integer-gmp ... linking ... done. > Loading package base ... linking ... done. > Prelude> :m Graphics.Gloss > Prelude Graphics.Gloss> let main = displayInWindow "My Window" (200, 200) (10, 10) white > (Circle 80) > <interactive>:4:12: Not in scope: `displayInWindow' > > Also I notice that cabal shows the current version of GLFW as GLFW-0.5.0.1, But their > web-site shows it as 2.7.5 - big difference?! > > I have: > Gloss 1.7.4.1 > GLUT 2.3.0.0 > GLFW 0.5.0.1 > Opengl 2.5.0.0

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wxHaskell install errors
by Gregory Guthrie 06 Jun '12

06 Jun '12

I had all of the wxHaskell programs installed and working fine, but now with an update to Haskell Platform and thus also ghci, I need to upgrade all of them (the previously working programs fail) requiring an update for the underlying wxWidgets from 2.8 to 2.9 versions. I downloaded and built wxWidgets 2.9, since the current wxHaskell libraries require it (I couldn't find any existing binaries online). The compile seemed to be successful, but when I then try to use it from the wxHaskell programs I get an error that I don't know how to fix: C: >cabal install wx Resolving dependencies... [1 of 1] Compiling Main ( C:\Users\guthrie\AppData\Local\Temp\wxc-0. 90.0.3-7324\wxc-0.90.0.3\Setup.hs, C:\Users\guthrie\AppData\Local\Temp\wxc-0.90. 0.3-7324\wxc-0.90.0.3\dist\setup\Main.o ) Linking C:\Users\guthrie\AppData\Local\Temp\wxc-0.90.0.3-7324\wxc-0.90.0.3\dist\ setup\setup.exe ... Configuring wxc-0.90.0.3... Configuring wxc to build against wxWidgets 2.9 setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: wxmsw29ud_all, wxtiffd, wxjpegd, wxpngd, wxzlibd,wxregexud, wxexpatd, wxregexud This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. cabal: Error: some packages failed to install: wx-0.90.0.1 depends on wxc-0.90.0.3 which failed to install. wxc-0.90.0.3 failed during the configure step. The exception was: ExitFailure 1 wxcore-0.90.0.1 depends on wxc-0.90.0.3 which failed to install.

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histogram over large data
by Ian Knopke 06 Jun '12

06 Jun '12

Hi, I'd like to build a histogram, or even just a frequency count of some categorical data from a large database. In a language such as perl I'd do something like this: my %freq; # hash while (my $item = get_next_from_db()){ $freq{$item}++; } and then sum the hash values and divide the value of each key by the sum to get the histogram. Is there an easy way to do the same thing in Haskell? It looked like an easy task but I seem to be having a lot of trouble getting this to work properly, as it doesn't seem to be behaving very lazily. I'm guessing I should be doing something with the State monad, but I'm not very good at using that yet. Ian

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GHC7.4.1 -> LLVM ->W64
by Alexander.Vladislav.Popov 06 Jun '12

06 Jun '12

Hi Haskellers, I want to build 64-bit Windows application. Will 64-bit mingw and compilation via LLVM help me? Who will shed light on it and lead me through darkened forest of my search? Alexander

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graphics.gloss errors
by Gregory Guthrie 05 Jun '12

05 Jun '12

Why do I get this error, when trying the first line example from: http://hackage.haskell.org/packages/archive/gloss/1.0.0.0/doc/html/Graphics… >ghci GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m Graphics.Gloss Prelude Graphics.Gloss> let main = displayInWindow "My Window" (200, 200) (10, 10) white (Circle 80) <interactive>:4:12: Not in scope: `displayInWindow' Also I notice that cabal shows the current version of GLFW as GLFW-0.5.0.1, But their web-site shows it as 2.7.5 - big difference?! I have: Gloss 1.7.4.1 GLUT 2.3.0.0 GLFW 0.5.0.1 Opengl 2.5.0.0

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attoparsec and EOF
by Arthur Clune 05 Jun '12

05 Jun '12

I'm writing a simple attoparsec parser, which works fine except for handling end of file: value :: Parser T.Text value = takeWhile1 (/= ' ') accessLogLine::Parser AccessLogLine accessLogLine = do lts <- number lelapsed <- skipSpace *> number lclientIP <- space *> value -- [... more like this ...] lhierarchy <- space *> value lmimeType <- space *> takeWhile1 (not . isEndOfLine) <* endOfLine return $ AccessLogLine lts lelapsed lclientIP laction lsize lmethod lurl lident lhierarchy lmimeType accessLogParser::Parser [AccessLogLine] accessLogParser = do result <- many accessLogLine return result main = do contents <- (S.readFile "/home/arthur/Work/data/sample_acccess_log") print (A.maybeResult $ (A.parse accessLogParser $ decodeUtf8 contents)) This all works, but I need to cover the case where the final line in the file does not end with '\n' and I can't get that to work I found http://stackoverflow.com/questions/2988988/problem-with-incomplete-input-wh…, which hints at what my problem is, but I can't see how to apply that here. Grateful for some hints. Arthur -- Arthur Clune arthur(a)clune.org

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How to improve performace of a dfs program
by xiong.hua 05 Jun '12

05 Jun '12

hi ,all : I'm a learn haskell couple of days , and I want to solve some problem [http://codeforces.com/contest/194/problem/D] with haskell , I know the algorithm for the problem and implement it with haskell , but i jsut get TLE on the test case . Here is my code , how i can implement it more Efficient ? code : import Data.Bits import Data.Int --import Debug.Trace calc :: [Int64] -> [Int64] -> Int64 calc a b = sum $ zipWith (*) a b -- a p r fun2op :: [Int64] -> [Int] -> Int64 -> [Int64] --fun2op a x r | -- trace ("fun2op " ++ show a ++ " " ++ show x ++ " " ++ show r ) False = undefined fun2op _ [] _ = [] fun2op a (x:xs) r = (( a !! (x-1)) + r) : (fun2op a xs r ) gor :: [Int64] -> [Int64] -> Int -> Int -> Int64 -> Int64 gor a k dep u ans | ( ( u - dep ) .&. 1 :: Int) == 0 = max ans $ calc a k | otherwise = ans -- a , b , p , k , r , u , last , dep , ans dfs :: [Int64] -> [Int64] -> [Int] -> [Int64] -> Int64 -> Int -> Int -> Int -> Int64 -> Int64 dfs a b p k r u l dep ans -- | trace ("dfs " ++ show a ++ " " ++ show l ++ " " ++ show dep ++ " " ++ show ans) False = undefined | dep == u = max ans $ calc a k | l == 1 = df1 | otherwise = max (df1) ( dfs (zipWith ( xor ) a b ) b p k r u 1 (dep+1) tmpans) where pa = fun2op a p r tmpans = gor a k dep u ans df1 = dfs pa b p k r u 2 (dep + 1) tmpans rInt64 :: String -> Int64 rInt64 = read rInt :: String -> Int rInt = read main = do ss <- getLine let tmp = words ss let u = rInt (tmp !! 1) let r = rInt64 ( tmp !! 2) let x = map (rInt) $ words ss let x = map (rInt) $ words ss aa <- getLine let a = map (rInt64) $ words aa bb <- getLine let b = map (rInt64) $ words bb kk <- getLine let k = map (rInt64) $ words kk pp <- getLine let p = map (rInt) $ words pp print ( dfs a b p k r u 0 0 (rInt64 "-1000000000000000000")) -------------- Best Regards GTALK: ivoryxiong AT gmail.com

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Looking for the paper that defines Arrows.
by Michael Litchard 05 Jun '12

05 Jun '12

I'm looking for this. Is it necessary to pay someone for the privilege (the only option I've discovered) ? John Hughes, Generalising Monads to Arrows, in Science of Computer Programming 37, pp67-111, May 2000

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How to solve this using State Monad?
by kak dod 02 Jun '12

02 Jun '12

Hello, A very good morning to all. I am a Haskell beginner. And although I have written fairly complicated programs and have understood to some extent the concepts like pattern matching, folds, scans, list comprehensions, but I have not satisfactorily understood the concept of Monads yet. I have partially understood and used the Writer, List and Maybe monads but the State monad completely baffles me. I wanted to write a program for the following problem: A DFA simulator. This I guess is a right candidate for State monad as it mainly deals with state changes. What the program is supposed to do is: ====================== It should read a description of a DFA given as a 5 tuple (q, sigma, delta, q0, finals) where - q: a finite set of states - sigma: a finite set of input symbols called the alphabet - delta: a transition function (delta : Q × S -> Q) - q0: start state (q0 belongs-to Q) - finals: a set of accept states (F belongs-to Q) def taken from wikipedia<http://en.wikipedia.org/wiki/Deterministic_finite_automaton#Formal_definiti…> and it should also read an input string (over alphabet sigma) and then it should run the input string on the DFA which it should build from the given description and should output (produce) a list of states through which the DFA has passed as it consumed the input string. You can assume that 'q' the set of state is of integers only. Also you can assume that sigma consist only of single character English alphabets ( ['A'..'Z']). The delta will be given as a list of 3-tuple. You don't need to do file IO. Sample input is following 2-tuple: input = (dfa, input-string) where dfa = ([0,1], ['A','B'], [(0,'A',0), (0,'B',1), (1,'A',1), (1,'B',0) ], 0, [1]) input-string = "AAABBABBAABBBABAAAB" Expected output: output = runDFA input -- output = [0,0,0,1,0,0,1,0,0,0,1,0,1,1,0,0,0,0,1] ====================== I wrote a recursive program to do this without using any monads. I simply send the entire dfa, the input string and its partial result in the recursive calls. How to do this using State Monad? Sorry, I may be a block-head but even after reading many tutorials on monads, I am still confused about the state monad. Either the tutorials give a very complicated example or they don't give a complete example that loads and runs properly in the GHCi . Again sorry to say, but the Random number generation example given in RealWorldHaskell didn't help me either. And yes, I have read Brent Yorgey's article on Monad tutorial fallacy too. So, you Monad gurus over here can consider me a block-head, but let me assure you people that I am a sincere person trying to learn this beautiful but difficult concept. I sincerely hope that a monadic solution (that uses Control.Monad.State ) to the DFA problem I gave, will help me understand the working of State Monad. Please note that I wish your solution to use the Control.Monad.State. Thanks in advance. kak

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