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Why is this type ambiguous?
by Maurício CA 26 Oct '09

26 Oct '09

import Foreign import Foreign.C genericCast :: (Storable a, Storable b) => a -> IO b genericCast v = let dummy = undefined size = max (sizeOf v) (sizeOf dummy) in if False then return dummy else allocaBytes size $ \p -> poke p v >> peek (castPtr p) ---- Code above gives me this: Ambiguous type variable `a' in the constraint: `Storable a' arising from a use of `sizeOf' at src/Bindings/C.hs:28:27-38 ---- It seems to refer to '(sizeOf dummy)'. But isn't the type of 'dummy' defined by 'return dummy' beeing a possible return value (and, so, dummy :: b)? Thanks, Maurício

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Re: Built-in/more elegant version of this function?
by Jordan Cooper 26 Oct '09

26 Oct '09

Am Montag 26 Oktober 2009 18:49:51 schrieb Jordan Cooper: >> I was wondering if there is a built-in way to do what this function I >> wrote does (or at least a more elegant way, preferably without >> explicit recursion). >> >> f xss = map safeHead xss : f (map safeTail xss) >> >> (where safeHead/Tail are versions of head/tail that don't crash on [] >> input) > >Not quite the same, but > >Prelude> :m +Data.List >Prelude Data.List> transpose [[1,2],[],[3,4,5]] >[[1,3],[2,4],[5]] > >could be more or less what you want. Thank you, I think that will work nicely!

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Built-in/more elegant version of this function?
by Jordan Cooper 26 Oct '09

26 Oct '09

I was wondering if there is a built-in way to do what this function I wrote does (or at least a more elegant way, preferably without explicit recursion). f xss = map safeHead xss : f (map safeTail xss) (where safeHead/Tail are versions of head/tail that don't crash on [] input)

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nim
by John Moore 26 Oct '09

26 Oct '09

Hi All, In the game of nim I want to make a move such as take x from pile A. This will then be subtracted from the starting list.Is this correct in as much as I want to have a move Take 3 from pile A: input would be A3 I need it to return a list as I wish to take this result and use zipwith (-) [starting list][result]. How do I use the result, do I store in a variable? typeOfMove :: (a,b) -> [Int] typeOfMove ax |ax = [x,0,0] |bx = [0,x,0] |cx = [0,0,x] John

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nim programme
by John Moore 25 Oct '09

25 Oct '09

Hi All, I'm attempting to write a program for the game nim.(The game of Nim is played with two players and several piles of stones. On each move a player removes as many stones as they would like but form only one pile. The player who takes the last stone wins) It not as simple as I first thought. Here is my basic starting points. Any comments would be greatly appreciated. I not well versed in Haskell yet so simple(basic) Haskell rather than well written haskell if you understand what I mean. Complicated monads are way out of my league. 1) I first get the program to give me three random piles by doing nim = do x <- getStdRandom $ randomR (1,10) y <- getStdRandom $ randomR (1,10) z <- getStdRandom $ randomR (1,10) return [x,y,z] Cant get this to work! 2) Now I need to get the program to ask for a number and which pile to remove the number from. This is tricky I thought about asking to find the elementAt elementAt :: [a] -> Int -> a elementAt list i = list !! (i-1) put this in a variable then asking the palyer how many to take away. and then subtracting the number from and then putting it back into the list but this seem impossible. Then the second player would do the same. 3) Finally we would end up with a case statement like f x = in case of x [0,0,1]-> You win [0,1,0]-> You win [0,0,1]-> You win [_,_,_]-> keep playing. Lets know what you think please, getting confused. John

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random
by John Moore 24 Oct '09

24 Oct '09

Hi All, Can anyone help me I want to produce a list of three random numbers for e.g. [7,8,1] I tried using x <- getStdRandom $ randomR (1,10) but don't really understand this and it only generates one number. Any help greatly appreciated. Regards John

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using quickcheck for blackbox testing for 3rd party apps.
by Srikanth K 24 Oct '09

24 Oct '09

Hi All, I am exploring use of quickCheck to define the test-specification for a 3rd party component. The component is basically an Command/Response type of application. Hence to validate it, I thought that the best way might be to take the following approach(based on the quickCheckM paper) a. describe the component in a purely functional way. The functions would take the "AppState", "Command" as parameters and would return a Response. Let me call this the "Abstract Application" The signature for this would therefore be of the form abstract_App :: AppState -> Command -> Response b. define an agent (Let me call it as Real_App) that that would (i) take a initial State, "Command" and send it to the "Application Under test" using some IPC mechanisms(probably socket or FIFO files) (ii) parse the received response and send it back as "Response". As a result the the API for the agent would be of the form real_APP :: AppState -> Command -> IO Response c. write a quick-check specification that would send a bunch of commands to the "Abstract Application" and check for equivalence with the value returned by the "real_App" However, being a newbie, I am not able to understand how I can extract the "Response" part from the IO Response. I want something as follows prop_App state_x command_y = (abstract_App state_x command_y ) == (real_App state_x command_y) The issue here is that since the reall_App would return a "IO Response" and not "Response" I cannot really compare the abstract_App with the real_App. Any thought on how I can extract the value from the "IO Response" so that I can compare the the two implementations for equvivalence. FYI, I have gone through the various quickcheck related docs like http://www.cs.chalmers.se/~rjmh/QuickCheck/ http://www.haskell.org/haskellwiki/QuickCheck_as_a_test_set_generator Thanks - Srikanth

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Caching evaluation of lazy lists
by Philip Scott 23 Oct '09

23 Oct '09

Hi folks, Quick question for you. Suppose I have some lazy list, x, which is very expensive to evaluate, and I want to do something with it more than once; for example: f x = g (take 5 x) (take 6 x) Is Haskell clever enough to only evaluate the list as much as is needed, or will it evaluate it once to get the first five values, and again to get the first 6 (when really it only needed to get one more). Or is it really really clever, and works out it needs to take 6 to begin with, then just give you the first 5 for the first call? More tricky - is there a way to make that cache (if it exsists) persist in an interactive session? Eventually I am intending to write my own application with a little console at the bottom which does some ghci-esque magic, but for now lets say I am in ghci and I want to call f again with the same list. Is there a way to avoid it from forcing a recomputation of my lazy list? Any advice greatly appreciated, Philip

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\x -> x < 0.5 && x > -0.5
by Michael Mossey 23 Oct '09

23 Oct '09

Is there a nifty way to write filter (\x -> x < 0.5 && x > -0.5) xs without explicitly using x? Maybe arrows? I have a vague understanding that arrows can "send" an argument to more than one computation. -Mike

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Haskell Output Help
by Chandni Navani 23 Oct '09

23 Oct '09

I have a list of lists which all contain strings. [[String]]. I need to figure out how to print them so that after each individual string, there is a new line. If this is the initial list [["abc", "cde"] ["fgh", "ghi"]] [["abc" "cde"] ["fgh", "ghi"]] Can anyone help me figure this out? Thanks.

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