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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

2 1

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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Ensuring consistency in typeclass instances?
by Colin Paul Adams 26 Nov '08

26 Nov '08

Hello, I'm just reading through Real World Haskell - chapter 6 (typeclasses). The definition of Eq is shown, and it mentions that you can define either one or both of the two classes. What would happen if I were two define both == and /= for an instance, in such a way that they were not opposites? If I were doing this in Eiffel, the function definitions would have postconditions to state the relationships, and the first execution would trigger a violation, telling me what was wrong. Is there any similar facility in Haskell? -- Colin Adams Preston Lancashire

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Not quite another Haskell tutorial, but ...
by Janis Voigtlaender 26 Nov '08

26 Nov '08

... I submitted my Habilitation thesis last week. The first few chapters of it try to give an introduction to Haskell with emphasis on types and reasoning principles. That might be an interesting read for some, so I made it accessible at http://wwwtcs.inf.tu-dresden.de/~voigt/habil.pdf. And yes, the November HCAR will also be coming. Rather soon now that this cause for delay is out of the way. Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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