November 2008 - Beginners

Re: [Haskell] Re: Help : A problem with IO
by abdullah abdul Khadir 15 Dec '08

15 Dec '08

Thanks all, I got it working finally. What did i learn ? a) I need to put a do after else for more than one instruction (?) b) All similar type of questions are to be redirected to haskell-beginner and haskell-cafe Points noted. Thank you once again, Abdullah Abdul Khadir On Wed, Nov 26, 2008 at 10:56 PM, wman <666wman(a)gmail.com> wrote: > if it should really read whole line, it must try to somehow chain the > values into the string / list of chars (using (:) ) > > getMyLine :: IO [Char] > getMyLine = do > c <- getChar > if (c == '\n') > then return "" -- imho [] looks nicer ;-) > else do > rest <- getMyLine > return $ c : rest > > -- or > > getMyLine :: IO [Char] > getMyLine = do > c <- getChar > if (c == '\n') > then return "" -- imho [] looks nicer ;-) > else getMyLine >>= return . (c:) > > -- or even shorter and still equivalent ;-) > > getMyLine :: IO [Char] > getMyLine = getChar >>= (\c -> if (c == '\n') then return [] else fmap > (c:) getMyLine) > > > > 2008/11/26 abdullah abdul Khadir <abdullah.ak2002(a)gmail.com> > >> Hi, >> >> The function getMyLine written by me is intended for getting a >> complete string from the standard input. >> >> import IO >> >> getMyLine :: IO [Char] >> getMyLine = do >> c <- getChar >> if(c == '\n') >> then return "" >> else cs <- getMyLine >> return [c] >> >> However I keep getting the following error: >> >> io.hs:14:2: Parse error in pattern >> Failed, modules loaded: none. >> >> I fail to understand what the error is. I tried out various things >> such as changing the alignment and so on to no avail. The following program, >> however, compiled successfully though it has the same structure as the >> previous program. >> >> checkd :: IO Bool >> checkd = do >> c <- getChar >> if(c=='d') >> then return True >> else return False >> >> Prelude> :load ./io.hs >> [1 of 1] Compiling Main ( io.hs, interpreted ) >> Ok, modules loaded: Main. >> *Main> checkd >> d >> True >> >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell >> >> >

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Wrapping random
by Torsten Otto 09 Dec '08

09 Dec '08

Hi all, I teach a high school class in Computer Science. The current programming goal is to implement chat-bots, and we're using Haskell of course. Now one of my students had the seemingly easy idea of having the bot answer with a random sentence if it doesn't have "good" answer. Random in Haskell has its problems. I understand why you can't just call a function as you would in Java. I'm not firm enough with monads myself (and certainly don't want to go there in the class beyond I/O) so I'm calling for help here: Is there a way to wrap the generation of random numbers so that for the students it works like a function? We have this working: > import System.Random > main = > do randomNumber <- randomRIO (1::Int,2) > print (randomAnswer randomNumber) > randomAnswer r > | (r == 1) = "Nope!" > | (r == 2) = "Absolutely!" > | otherwise = "Error!" Now, how can we use it for something like this: >findAnswer [] = "h" >findAnswer (x:xs) > | (z == "unknown") = findAnswer xs > | otherwise = z > where z = findWord x lexikon where instead of getting "h" we'd like to call a function that would give us one of the strings out of randomAnswer. (findAnswer looks through a list [(keyword,response)]. I've looked at realworldhaskell and the wikibook among other sources, but I can't manage to piece anything useful together. How do I manage to get something of type IO to represent itself as a String? Any help would be greatly appreciated. Regards, Torsten Otto

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ANNOUNCE: Haskell Communities and Activities Report (15th ed., November 2008)
by Janis Voigtlaender 03 Dec '08

03 Dec '08

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (15th edition, November 2008) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2009: target deadline for contributions to the May 2009 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Memoization
by abdullah abdul Khadir 01 Dec '08

01 Dec '08

Hi, I am a student and we had an assignment in Haskell. The question, was given a string of the form "1-2+3*5-7+4-6+3" i.e., any sequence of integers as well as some operators between them we had to find a maximum possible value for the expression as well as the expression itself . So for maxval "1-2+3*5-7+4-6+3" it is (76,"(1-((2+3)*((5-(7+4))-(6+3))))"). The function we had to write was maxval :: String -> (Int,String). For further details on the question, have a look at our sir's web page here<http://www.cmi.ac.in/%7Emadhavan/courses/programming08/assignment6.txt>. I solved the question, we had to use memoization, and submitted the solution. It is given below. Now the problem is I am just wondering if it can be solved in a better manner. Translation : Is there some way in Haskell to do it in a more simpler way and as well as to reduce the number of lines of the program. > {- > > --------------------------------------------------------------------------------------------------------------------------- > *********************** module Memo.hs > ****************************************************** > > --------------------------------------------------------------------------------------------------------------------------- > -} > module > Memo(Table,emptytable,memofind,memolookup,memoupdate,memoupdateArray) where > > data (Eq a) => Table a b c = T [(a,a,(b,c),(b,c))] > deriving (Eq,Show) > > emptytable :: (Eq a) => (Table a b c) > emptytable = T [] > > memofind :: (Eq a) => (Table a b c) ->(a,a)-> Bool > memofind (T []) _ = False > memofind (T ((y,z,(v1,s1),(v2,s2)):l)) x > | x == (y,z) = True > | otherwise = memofind (T l) x > > memolookup :: (Eq a) => (Table a b c) -> (a,a) -> ((b,c),(b,c)) > memolookup (T ((y,z,(v1,s1),(v2,s2)):l)) x > | x == (y,z) = ((v1,s1),(v2,s2)) > | otherwise = memolookup (T l) x > > memoupdate :: (Eq a) => (Table a b c) -> (a,a,(b,c),(b,c)) -> (Table a b c) > memoupdate (T l) x = T (x:l) > > memoupdateArray :: (Eq a) => (Table a b c) -> [(a,a,(b,c),(b,c))] -> (Table > a b c) > memoupdateArray t [] = t > memoupdateArray t (x:xs) = memoupdate (memoupdateArray t xs) x > > {- > > --------------------------------------------------------------------------------------------------------------------------- > ***********************End of module > Memo.hs*********************************************** > > --------------------------------------------------------------------------------------------------------------------------- > -} > > > {- > > --------------------------------------------------------------------------------------------------------------------------- > ******************The actual program , > assign-6.hs****************************************** > > --------------------------------------------------------------------------------------------------------------------------- > -} > > minArray :: [(Int,String)] -> (Int,String) > minArray ((x,expr):[]) = (x,expr) > minArray ((x,expr):l) |((min x (fst (minArray l))) ==x) = (x,expr) > |otherwise = (minArray l) > > maxArray :: [(Int,String)] -> (Int,String) > maxArray ((x,expr):[]) = (x,expr) > maxArray ((x,expr):l) |((max x (fst (minArray l))) ==x) = (x,expr) > |otherwise = (minArray l) > > import Memo > import Char > > type Tuple = (Int,Int,(Int,String),(Int,String)) > > maxval :: String ->(Int, String) > maxval expr = snd (memolookup (buildmemo expr 1 emptytable) (1,length > expr)) > > initmemo :: (String) -> (Table Int Int String) > initmemo expr = (memoupdateArray (emptytable) > [(i,i,(toInt(expr!!(i-1)),[expr!!(i-1)]), > (toInt(expr!!(i-1)),[expr!!(i-1)]))|i<-[1..length > expr],j<-[0,1],i `mod` 2 ==1]) > > buildmemo :: (String)->Int -> (Table Int Int String)-> (Table Int Int > String) > buildmemo expr col memo | (col > length expr) = memo > | (col == 1) = buildmemo expr 3 (memoupdateArray > (emptytable) > [(i,i,(toInt(expr!!(i-1)),[expr!!(i-1)]), > > (toInt(expr!!(i-1)),[expr!!(i-1)]))|i<-[1..length expr],i `mod` 2 ==1]) > | otherwise = buildmemo expr (col+2) (memoupdateArray > (memo) (createList expr memo (1,col))) > > createList :: String-> (Table Int Int String) -> (Int,Int) -> [Tuple] > createList expr memo (i,j) | j > (length expr) = [] > | otherwise = (i,j,min_expr,max_expr):(createList expr > memo (i+2,j+2)) > where > min_expr = minArray [x | (x,y) <- list] > max_expr = maxArray [y | (x,y) <- list] > list = [(compute memo (i,k) (k+2,j) > (expr!!k))|k<-[i..j-2],k `mod` 2 ==1] > > compute :: (Table Int Int > String)->(Int,Int)->(Int,Int)->Char->((Int,String),(Int,String)) > compute memo (x1,x2) (y1,y2) op |op == '+' = > ((min1+min2,"("++min1_expr++"+"++min2_expr++")"), > > (max1+max2,"("++max1_expr++"+"++max2_expr++")")) > |op == '-' = > ((min1-max2,"("++min1_expr++"-"++max2_expr++")"), > > (max1-min2,"("++max1_expr++"-"++min2_expr++")")) > |op == '*' = (minArray xs,maxArray xs) > where > xs = [(min1*min2,"("++min1_expr++"*"++min2_expr++")"), > (min1*max2,"("++min1_expr++"*"++max2_expr++")"), > (max1*min2,"("++max1_expr++"*"++min2_expr++")"), > > (max1*max2,"("++max1_expr++"*"++max2_expr++")")] > ((min1,min1_expr),(max1,max1_expr)) = (memolookup memo > (x1,x2)) > ((min2,min2_expr),(max2,max2_expr)) = (memolookup memo > (y1,y2)) > > minArray :: [(Int,String)] -> (Int,String) > minArray ((x,expr):[]) = (x,expr) > minArray ((x,expr):l) |((min x (fst (minArray l))) ==x) = (x,expr) > |otherwise = (minArray l) > > maxArray :: [(Int,String)] -> (Int,String) > maxArray ((x,expr):[]) = (x,expr) > maxArray ((x,expr):l) |((max x (fst (maxArray l))) ==x) = (x,expr) > |otherwise = (maxArray l) > > toInt :: Char -> Int > toInt x = ord x - ord '0' > > {- > > --------------------------------------------------------------------------------------------------------------------------- > ***********************End of program > assign-6.hs******************************************* > > --------------------------------------------------------------------------------------------------------------------------- > -}

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Installing Gtk2hs
by Colin Paul Adams 30 Nov '08

30 Nov '08

The install instructions seem very straight-forward, but they are not working for me. I did: ./configure --enable-docs --disable-deprecated make su -c'make install' but the demos won't compile (they can't find the libraries). checking the stderr from make shows: ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: mtl make: *** Deleting file `package.conf.inplace' package.conf.inplace: openBinaryFile: does not exist (No such file or directory) package.conf.inplace: openBinaryFile: does not exist (No such file or directory) ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: mtl make[1]: *** Deleting file `package.conf.inplace' package.conf.inplace: openBinaryFile: does not exist (No such file or directory) package.conf.inplace: openBinaryFile: does not exist (No such file or directory) on the commandline: Warning: -fffi is deprecated: use -XForeignFunctionInterface or pragma {-# LANGUAGE ForeignFunctionInterface#-} instead package.conf.inplace: openBinaryFile: does not exist (No such file or directory) make[1]: *** [glib/System/Glib.o] Error 1 make: *** [all] Error 2 My environment is Fedora 10 (64-bit). Ghc 6.10.1. Any suggestions? -- Colin Adams Preston Lancashire

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In-place lazy I/O
by Alexander Dunlap 30 Nov '08

30 Nov '08

Hi all, Suppose my program has one or more persistent files that it reads at or near the beginning of its execution and writes at or near the end. Thus, an extremely simplified model of my program could be > main = do > h <- openFile "some-file" ReadMode > c <- hGetContents h > w <- openFile "some-file" WriteMode > hPutStr w (f c) where f is some arbitrary function. My question is how to do this. It seems like the data from h won't necessarily be forced until f is called when it is written, and there is no guarantee that all of the data will even be used by f until it is written. Thus, I will get a file-locking error when trying to write the file. Do I have to rig things so that I know f will consume all of its input? Is it better to use strict I/O? Is there a better idiom for this entirely? Thanks for all of your help. Alex

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Help in Haskell
by Andre Paulo Machado 29 Nov '08

29 Nov '08

Hi folks..... Somebody could help me, please? Please, I would like a help in haskell programming, because I have with following question: I need to create a list of strings, like as a graph. See: initiaList = [ ("BSB,"SSA"), ("CNF","SSA"), ("CNF","GIG"), ("CNF","GRU"), ("GIG","CNF"), ("GIG","GRU"), ("GRU","BSB"), ("GRU","GIG"), ("GRU","CNF"), ("SSA","CNF") ] With this list above, I will create a function that returns a path with max number of links: example: maxLink = 4 (result until 4) command: myFunction "BSB" "GRU" 4 result: 1st Option: BSB -> SSA -> CNF -> GRU 2nd Option: BSB -> SSA -> CNF -> GIG -> GRU ---------------------------------------------------------------------------- command: myFunction "BSB" "GRU" 3 result: 1st Option: BSB -> SSA -> CNF -> GRU ---------------------------------------------------------------------------- How do this? I'm waiting from you, thank you so much. Andre

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Recommended GUI library?
by Colin Paul Adams 29 Nov '08

29 Nov '08

Looking at the wiki, it says that none of the high-level libraries are in a really good state for production usage (i.e. they are still all reasearch-level). What would people recommend? I'd really prefer to use a high-level library. -- Colin Adams Preston Lancashire

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Possible to update Haskell syntax
by cm 27 Nov '08

27 Nov '08

I'm very interested in learning Haskell, but one frustration is that lists always require commas. As an example [1,2,3,4] is a valid expression, but [1 2 3 4] is not. In this ideal world, another example would be that [(fun1 3 4) 7] would be a valid expression and be a two-element list equivalent to [fun1 3 4, 7]. If what I am suggesting is syntactically possible and doesn't break more advanced features of the language that I (as a beginner) am unaware of, I'm interested in patching the open source for my own use (as well as others who might want it).

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FFI, Foreign.Marshal.Array, etc.
by Alexey Beshenov 26 Nov '08

26 Nov '08

Hi! I'm playing with the FFI. Could anyone help me with a practical example? I have a function int solve_sys (double **a, int n, double b[]); where a is an array of size n*n and b is an array of size n (it contains the needed result if solve_sys returns 0). How could I wrap it in something like solveSys :: [[Double]] -> [Double] -> [Double] ? -- Setting Orange, Aftermath 38 YOLD 3174 Alexey Beshenov http://beshenov.ru/

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