May 2008 - Haskell-Cafe

Re: [Haskell] [Haskell-cafe] Help with polymorphic functions
by Bulat Ziganshin 08 May '08

08 May '08

Hello Wei, Thursday, May 8, 2008, 11:10:08 PM, you wrote: > test :: a -> Int -> a > shift is defined as "a -> Int -> a" not exactly ;) this type signature is given inside class Bits, where 'a' isn't a free variable (as in standalone signature declaration), but means 'a' from type class header: class Num a => Bits a where shift :: a -> Int -> a so, this declaration is equivalent to the following standalone one: shift :: (Bits a) => a -> Int -> a from the common sense POV, you can't shift *ANY* type 'a', but only types that belong to the Bits class. so, shift cannot have signature w/o class, and the same remains true for `test` -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Workshop on Generic Programming: Call for Papers (co-located w/ ICFP08)
by Matthew Fluet (ICFP Publicity Chair) 08 May '08

08 May '08

CALL FOR PAPERS Workshop on Generic Programming 2008 Victoria, Canada, 20th September 2008 http://www.comlab.ox.ac.uk/ralf.hinze/wgp2008/cfp.{html,pdf,ps,txt} The Workshop on Generic Programming is sponsored by ACM SIGPLAN and forms part of ICFP 2008. Previous Workshops on Generic Programming have been held in Marstrand (affiliated with MPC), Ponte de Lima (affiliated with MPC), Nottingham (informal workshop), Dagstuhl (IFIP WG2.1 Working Conference), Oxford (informal workshop), Utrecht (informal workshop), and Portland (affiliated with ICFP). Scope ----- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, but only recently have generic programming techniques become a specific focus of research in the functional and object-oriented programming language communities. This workshop will bring together leading researchers in generic programming from around the world, and feature papers capturing the state of the art in this important emerging area. We welcome contributions on all aspects, theoretical as well as practical, of o adaptive object-oriented programming, o aspect-oriented programming, o component-based programming, o generic programming, o meta-programming, o polytypic programming, o programming with modules, and so on. Submission details ------------------ Deadline for submission: 30th June 2008 Notification of acceptance: 14th July 2008 Final submission due: 28th July 2008 Workshop: 20th September 2008 Authors should submit papers, in PostScript or PDF format, formatted for A4 paper, to Ralf Hinze (ralf.hinze(a)comlab.ox.ac.uk) or Don Syme (Don.Syme(a)microsoft.com) by 30th June 2008. The length should be restricted to 12 pages in standard (two-column, 9pt) ACM. Accepted papers are published by the ACM and will additionally appear in the ACM digital library. Programme committee ------------------- Ralf Hinze (co-chair) University of Oxford Patrik Jansson Chalmers University Andrew Lumsdaine Indiana University Conor McBride University of Nottingham Adriaan Moors Universiteit Leuven Fritz Ruehr Willamette University Tim Sheard Portland State University Don Syme (co-chair) Microsoft Research Todd Veldhuizen University of Waterloo

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Random numbers / monads - beginner question
by Madoc 08 May '08

08 May '08

Hello, I am just learning Haskell. Now, I encountered something that I cannot solve by myself. Your advice will be greatly appreciated. Given a list of numbers, I want to modify each of those numbers by adding a random offset. However, each such modified number shall stay within certain bounds, given by the integers minValue and maxValue. After that, I want to continue computation with the resulting list of type [Int]. But for demonstration, I made a program that just prints out the list: import IO; import Random minValue = 0::Int maxValue = 1000::Int normalize a | a < minValue = minValue | a > maxValue = maxValue | otherwise = a modify a = do offset <- randomRIO(-100::Int, 100) return(normalize(a + offset)) main = putStrLn $ show $ map (modify) [0, 200, 400, 600, 800, 1000] This program will not compile. GHC complains: test.hs:14:18: No instance for (Show (IO Int)) arising from a use of `show' at test.hs:14:18-21 Possible fix: add an instance declaration for (Show (IO Int)) In the first argument of `($)', namely `show' In the second argument of `($)', namely `show $ map (modify) [0, 200, 400, 600, ....]' In the expression: putStrLn $ show $ map (modify) [0, 200, 400, 600, ....] I understand that the result of the modify function is not an Int, as I would like to have it, but instead IO Int, and that cannot be applied to show. (I also did not quite understand why I need those brackets around the return value of the modify value. It won't compile if I leave them out, but I can accept that for now.) I also figured out how to generate a modified list of type [IO Int] and of type IO [Int]. However, I could not find out how to completely get rid of the IO monad and just get a mofied list of type [Int], which is what I really want. Please, do You have any advice for me? I tried for some hours, and now I am really angry at that IO monad that sticks to my pretty integers like glue! Also, any comment on the programming style and how I could achive my goals easier would be appreciated. (I left out comments and function types for the sake of brevity.) Thanks a lot in advance. Madoc. -- View this message in context: http://www.nabble.com/Random-numbers---monads---beginner-question-tp1712438… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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IO Help
by Mark Wallsgrove 08 May '08

08 May '08

Hey, I am studying Haskell as a unit at University. I find the concept and design idea's of Haskell interesting but I am finding my self struggling to understand the relationship between the normal state and IO state of Haskell. This is my situation: I have created 12 functions for a program which take in two types: type Catalogue = [Track] type Playlist = [Track] The definition for track is as follows: -- Track = ArtistDetails Title Length PCount data Track = Track ArtistType String Float Int deriving (Show,Eq,Read) -- Popular = Artist | Composor Performer data ArtistType = Popular String | Classical String String deriving (Show,Eq,Read) I have managed to save the data to a file using this code: --Saving Data saveData :: String -> Catalogue -> IO() saveData fileName catalogue = writeFile fileName (show catalogue) Problem now is reading the data back into the program. When I read the data back into the program it comes as IO [Track]. This is the code I have been using to load the data: loadData :: String -> Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue) I think I have missed a trick some where or I am using IO wrong, I really don't know. I believe it is the latter. I have been told that my definition for the function is wrong and that I should use the lazy approach to fix the problem. But this still leaves me with IO [Track]. Could someone inform me on what I am doing wrong? Thank you in advance Smoky

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(no subject)
by u4538637＠anu.edu.au 08 May '08

08 May '08

Hi I have a bit of a dilemma.I have a list of lists, eg, [[1,2,3],[4,5,6],[7,8,9]]. Imagine they represent a grid with 0-2 on the x axis and 0-2 on the y axis, eg, (0,0) is 1, (1,0) is 2, (2,1) is 6, etc and (2,3) is 9. I want to be able to put in the list of lists, and the (x,y) coordinate, and return the value. Also, I need to be able to replace a value in the list. Eg, if I wanted to replace (2,3) with 100, then the output of the expression would be [[1,2,3],[4,5,6],[7,8,100]]. Any help would be great!

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Re: [Haskell-cafe] Random numbers / monads - beginner question
by Thomas Dinsdale-Young 08 May '08

08 May '08

Madoc wrote: Given a list of numbers, I want to modify each of those numbers by adding a random offset. However, each such modified number shall stay within certain bounds, given by the integers minValue and maxValue. After that, I want to continue computation with the resulting list of type [Int]. Personally, I'd do something like this, isolate the IO code outside the algorithm to keep the algorithm pure: modify' :: Int -> Int -> Int modify' offset a = normalize (a + offset) generateInfiniteListOfRandomNumbers :: IO [Int] -- implementation left as an exercise main = do randomNumbers <- generateInfiniteListOfRandomNumbers print $ zipWith modify' randomNumbers [0, 200, 400, 600, 800, 1000] I may be wrong, but generateInfiniteListOfRandomNumbers won't terminate and I think it has to before the next IO action occurs. (Laziness is great, but I don't think you can really do lazy IO like that.) Instead of map :: (a -> b) -> [a] -> [b], I think you are looking for mapM :: Monad m => (a -> m b) -> [a] -> m [b]. * * * *Hope this helps, Thomas >

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Re: [Haskell-cafe] Induction (help!)
by PR Stanley 08 May '08

08 May '08

So, when you apply the function to the first element in the set - e.g. Zero or Nil in the case of lists - you're actually testing to see the function works. Then in the inductive step you base everything on the assumption that p holds for some n and of course if that's true then p must hold for Succ n but you have to prove this by taking Succ from n and thus going bakc to its predecessor which is also the hypothesis p(n). So, to reiterate assumption: if hypothesis then conclusion if p(n) then p(Succ n) proof of assumption if p(Succ n) = Succ(p(n)) then we've won. If pn+1) = p(n) + p(1) then we have liftoff! I'm not going to go any further in case I'm once again on the wrong track. Cheers Paul At 22:43 07/05/2008, you wrote: >On Wed, May 7, 2008 at 9:27 PM, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > One of you chaps mentioned the Nat data type > > > > data Nat = Zero | Succ Nat > > > > Let's have > > add :: Nat -> Nat -> Nat > > add Zero n = n > > add (Succ m)n = Succ (add m n) > > > > Prove > > add m Zero = m > >To prove this by induction on m, you would need to show: > >1) add Zero Zero = Zero >2) If "add m Zero = m", then "add (Succ m) Zero = Succ m" > >Number (1) is completely trivial, nothing more needs to be said. (2) >is easy, after expanding the definition. > >Here the P I used was P(x) := add m Zero = m, the thing we were trying >to prove. (1) is a base case, P(Zero). (2) is the inductive step, >"If P(m) then P(Succ m)". > >Hoping I don't sound patronizing: if you're still having trouble, then >I suspect you haven't heard what it means to prove an "if-then" >statement. Here's a silly example. > >We want to prove: If y = 10, then y - 10 = 0. > >First we *assume* the condition of the if. We can consider it true. > >Assume y = 10. >Show y - 10 = 0. >Well, y = 10, so that's equivalent to 10 - 10 = 0, which is true. > >Luke

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help in tree folding
by patrik osgnach 08 May '08

08 May '08

Hi. I'm learning haskell but i'm stuck on a generic tree folding exercise. i must write a function of this type treefoldr::(Eq a,Show a)=>(a->b->c)->c->(c->b->b)->b->Tree a->c Tree has type data (Eq a,Show a)=>Tree a=Void | Node a [Tree a] deriving (Eq,Show) as an example treefoldr (:) [] (++) [] (Node '+' [Node '*' [Node 'x' [], Node 'y' []], Node 'z' []]) must return "+∗xyz" any help? (sorry for my bad english)

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runInteractiveProcess and hGetLine on Windows
by Harri Kiiskinen 08 May '08

08 May '08

Hello all, I bumped into a "feature" which might be a bug, but to be certain, I'd like to hear your opinion. I'm running ghc 6.8.2 on Windows XP, and with ghci I do the following: Prelude System.Process System.IO> (inp,outp,err,ph) <- runInteractiveProcess "kpsewhich" ["testfile.txt"] Nothing Nothing ('kpsewhich' is a simple path searching utility used by web2c TeX system, returns the full path to the file if found in the system defined search path, but you all probably know that.) and then: Prelude System.Process System.IO> hGetLine outp which gives me: "./testfile.txt\r" as opposed to "./testfile.txt" which I get on my Linux box. Is the "\r" supposed to be at the end? I thought it is part of the line separator in Windows, and as such, should not be part of the line retrieved? Same thing happens when compiled with ghc. Best Wishes, Harri K.

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How can see old threads.
by Ambrish Bhargava 08 May '08

08 May '08

Hi, I want to know, how can see old threads (may be I can get my answers from there itself)? -- Regards, Ambrish Bhargava

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