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Please, help me for myFirstCode
by Sok H. Chang 26 Oct '10

26 Oct '10

Hello, everyone. I need help for my first code. I write this code for extracting one sentence from text file. The code is ----------------------------------------------------------------------------------------------------------------------- import System import System.Random main = do myHeader <- readFile "C:\\Documents and Settings\\myHeader.txt" putStrLn $ myExtractSentence myHeader myExtractSentence file = ary !! ranNoInt where ranNoInt =<< ranNo -- *mark ranNo = randomRIO (0, n-1) n = length ary ary = lines file ---------------------------------------------------------------------------------------------------------------------- There are two questions. 1) I've got following error message. Haskell>ghc --make Test01.hs -o Test [1 of 1] Compiling Main ( Test01.hs, Test01.o ) Test01.hs:10:9: parse error (possibly incorrect indentation) How can fix it? 2) Is it OK at "*mark" line? Thank you. S. CHANG

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Directory statistics
by Alexander.Vladislav.Popov 26 Oct '10

26 Oct '10

Hi, haskellers. I’ve written a program to assess directory utilization in the selected path. It displays on the console a list indicating directory size and path. The list is sorted by descending size. I need some criticism and some models I could follow to write a more concise and expressive program. Thanks in advance. Alexander. > {-# LANGUAGE ScopedTypeVariables #-} > module Main (main) where > import Control.Exception (SomeException, finally, bracket, handle) > import Control.Monad > import Data.List > import System.Directory > import System.Environment > import System.IO > import Text.Printf > main :: IO () > main = do > args <- getArgs > if null args > then > putStr "Displays directory utilization in the selected path.\nUsage: dirstat <path>\n" > else > mapM_ ds3 args It safely returns a file size. If an error occurs during file opening, it will return 0. > filesize :: FilePath -> IO Integer > filesize path = (withFile path ReadMode hFileSize) `catch` const (return 0) Get size of a directory. > ds :: FilePath -> IO Integer > ds path = do > contents <- getDirectoryContents path `catch` const (return []) > let visibles = getVisible contents > let path' = clrSlash path > a <- (liftM sum) $ sequence $ map (\p -> filesize (path' `mkpath` p)) visibles -- size of a current dir > (liftM ((+a) . sum)) $ mapM (\p -> ds (path' `mkpath` p)) visibles -- current + children Returns a list of pairs: (file size, path) > ds2 :: FilePath -> IO [(Integer, FilePath)] > ds2 path = do > contents <- getDirectoryContents path > let visibles = getVisible contents > let path' = clrSlash path > let paths = map (\p -> path' `mkpath` p) visibles > let pairs = map (\p -> (ds p, p)) paths > a <- sequenceFst pairs > return $ (reverse . sort . filter (\e -> fst e > 0)) a Compare it to the function *sequence :: (Monad m) => [m a] -> m [a]* > sequenceFst :: (Monad m) => [(m t, t1)] -> m [(t, t1)] > {-# INLINE sequenceFst #-} > sequenceFst ms = foldr k (return []) ms > where > k (ms, p) m' = do { s <- ms; xs <- m'; return ((s,p):xs) } Driver > ds3 :: FilePath -> IO () > ds3 path = do > s <- ds2 path > prn s > where > prn [] = return () > prn (s:ss) = (putStr . showDir) s >> prn ss Auxiliary > skipDots = (`notElem` [".", ".."]) > getVisible = filter skipDots > mkpath p1 p2 = p1 ++ "/" ++ p2 > clrSlash = reverse . dropWhile (\c -> c =='/' || c == '\\') . reverse Displays information about a directory. > showDir :: (Integer, String) -> String > showDir (s,p) = printVolume s 3 ++ "\t" ++ show p ++ "\n" *Main> shred "1234567890" ["123","456","789","0"] > shred [] = [] > shred ss = (take 3 ss) : (shred (drop 3 ss)) *Main> prettyNum "1234567890" "1 234 567 890" > prettyNum = concat . intersperse " " . (map reverse) . reverse . shred . reverse *Main> units (1024*1024*1024) [1073741824,1048576,1024,1,0] > units :: Integer -> [Integer] > units 0 = [0] > units x = x : units' x > where > units' 0 = [] > units' x = y : units' y > where > y = round (fromIntegral x / 1024) Just SI prefixes > prefix = ["B", "K", "M", "G", "T", "P", "E", "Z", "Y"] *Main> tagged (1024*1024*1024) [("1 073 741 824","B"),("1 048 576","K"),("1 024","M"),("1","G"),("0","T")] > tagged x = (map (prettyNum . show) $ units x) `zip` prefix *Main> printVolume (1024*1024*1024) 5 1 024 M > printVolume x width = printf "%*s %s" width (fst one) (snd one) > where > one = head $ dropWhile (\p -> length (fst p) > width) $ tagged x

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Re: [Haskell-cafe] running and understanding a lifting program
by Patrick LeBoutillier 25 Oct '10

25 Oct '10

Patrick, I found this program interesting and decided to spend a bit of time on it, even though I'm still a newbie. I did a few things to simplify the code, here are some comments: 1) I chose to rename the arithmetic functions in the Number class instead of trying to overload the "real" ones, I'm not that good at Haskell yet... 2) The program had some errors, namely I think the definition of the Point type should be: data Point a = Point a a to allow for different types of Points. 3) The Points class seemed useless in the end, I simply defined the dist function at the top level. 4) If you import Control.Monad, it makes functions (and therefore "Changing v") into a Monad (maybe my terminology is off here...) and allows you to use the general liftM and liftM2 lifting functions instead of defining your own. Here's the complete program: {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} data Point a = Point { x ::a, y :: a } type Time = Float -- Functor Changing, which adds time parameter t to its input value. -- For example, Changing Float indicates a changing floating number -- (i.e. a function of time). type Changing v = Time -> v -- Lifting functions lift1 op a = \t -> op (a t) lift2 op a b = \t -> op (a t) (b t) class Number a where add, sub, mul :: a -> a -> a square, squareRoot :: a -> a square a = a `mul` a instance Number Float where add = (+) sub = (-) mul = (*) squareRoot = sqrt instance Number (Changing Float) where add = lift2 add sub = lift2 sub mul = lift2 mul squareRoot = lift1 squareRoot -- The distance operation is defined as follow dist :: Number a => Point a -> Point a -> a dist a b = squareRoot $ square((x a) `sub` (x b)) `add` square ((y a) `sub` (y b)) -- Running the code -- If p1 and p2 are two 2D static points, -- their distance d is calculated as follows: p1, p2 :: Point Float p1 = Point 3.4 5.5 p2 = Point 4.5 4.5 -- distance between p1 and p2 --> 1.55 d = dist p1 p2 -- For 2D moving points mp1 and mp2, their distance md, -- which is a function of time, is calculated as follows: mp1, mp2 :: Point (Changing Float) mp1 = Point (\t -> 4.0 + 0.5 * t) (\t -> 4.0 - 0.5 * t) mp2 = Point (\t -> 0.0 + 1.0 * t) (\t -> 0.0 - 1.0 * t) -- distance between mp1 and mp2 md = dist mp1 mp2 -- distance md for time 2 ----> 5.83 > > > This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Overlapping Instances
by John Smith 25 Oct '10

25 Oct '10

Following is a simplification of some code which I have written. I have an overlapping Show instance for A, which is more specific than the general instance for arrays, so I would expect it to be acceptable as an overlapping instance. Nevertheless, I get the following compiler error: Overlapping instances for Show A arising from a use of `show' at 13:17-22 Matching instances: instance (Ix a, Show a, Show b) => Show (Array a b) -- Defined in GHC.Arr instance [overlap ok] Show A -- Defined at 9:9-14 In the expression: show a In the definition of `show': show (B a) = show a In the instance declaration for `Show B' Compilation failed. I've tried UndecidableInstances and IncoherentInstances, but they don't seem to help. What am I doing wrong? Many thanks in advance for any assistance. -John {-# LANGUAGE TypeSynonymInstances, OverlappingInstances #-} import Data.Array type A = Array Int Bool data B = B A instance Show A where show a = "foo" instance Show B where show (B a) = show a

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Re: [Haskell-cafe] running and understanding a lifting program
by Bastian Erdnüß 25 Oct '10

25 Oct '10

On Oct 24, 2010, at 15:12, Patrick Browne wrote: > hi, > I am trying to run and understand a lifting program from [1]. > The program lifts points to moving points that vary their position over > time. > I made some effort to run the progrm but I do not know how to overide > the +,-,*,sqr, and sqrt from the Num class. Below is my current attempt. > > I do not wish to change or imporve the code, rather I wish to understand > it as it stands and find out what needs to be added to get the outputs > shown below i.e. distance between points p1 and p2 --> 1.55 and the > distance between moving points mp1 and mp2 for time 2 ----> 5.83. I got pretty close with {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} import Prelude hiding ((+), (-), (*), sqrt) import qualified Prelude data Point a = Point a a type Time = Float type Changing v = Time -> v lift0 a = \t -> a lift1 op a = \t -> op (a t) lift2 op a b = \t -> op (a t) (b t) class PlusMinus a where (+), (-) :: a -> a -> a instance PlusMinus Float where (+) = (Prelude.+) (-) = (Prelude.-) instance PlusMinus s => PlusMinus (Point s) where (+) (Point x y) (Point x' y') = Point (x+x') (y+y') (-) (Point x y) (Point x' y') = Point (x-x') (y-y') class PlusMinus a => Number a where (*) :: a -> a -> a sqr, sqrt :: a -> a sqr a = a * a instance Number Float where (*) = (Prelude.*) sqrt = Prelude.sqrt class Number s => Points s where x, y :: Point s -> s x (Point x' y') = x' y (Point x' y') = y' dist :: Point s -> Point s -> s dist (Point x y) (Point x' y') = sqrt $ sqr (x-x') + sqr (y-y') instance Points Float instance PlusMinus v => PlusMinus (Changing v) where (+) = lift2 (+) (-) = lift2 (-) instance Number v => Number (Changing v) where (*) = lift2 (*) sqrt = lift1 sqrt instance Points (Changing Float) > [1] A Mathematical Tool to Extend 2D Spatial Operations > to Higher Dimensions: by Farid Karimipour1,2, Mahmoud R. Delavar1, and > Andrew U. Frank2 > > http://books.google.ie/books?id=JUGpGN_jwf0C&pg=PA153&lpg=PA153&dq=Karimipo… Cheers, Bastian

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Hexpat: GeneriXMLString instances not recognized?
by Aleksandar Dimitrov 24 Oct '10

24 Oct '10

Hi list, I'm trying to use Hexpat to efficiently SAX-parse an XML document (of quite some size.) So I figured it'd be a good idea to use Data.Text as the string type for my parsed XML data. It seems Hexpat's sax-parse function only accepts files in ByteString format, which is fair enough, but here's the really weird thing: the following line yields a compilation error! > import Text.XML.Expat.SAX > import qualified Data.Text as T > import qualified Data.ByteString.Lazy as L >main = do f <- L.readFile someFile > let x = parse defaultParseOptions f :: [SAXEvent T.Text T.Text] > ... The error: No instance for (GenericXMLString T.Text) arising from a use of `parse' at Polyparse.hs:15:12-38 Possible fix: add an instance declaration for (GenericXMLString T.Text) In the expression: parse defaultParseOptions f :: [SAXEvent T.Text T.Text] That's mighty strange, seeing as there is an instance for GenericXMLString Data.Text.Text in hexpat's source code (indeed, it's one of its advertised features!) Here is the documentation (as well as the source: http://hackage.haskell.org/packages/archive/hexpat/0.19.4/doc/html/Text-XML… I went ahead and tried copying the instance declaration from the source to my own code. I noticed it didn't compile, since the type signature of peekByteStringLen (called from the Text instance declaration for GenericXMLString) clashed with the function's use of ByteString.Internal.ByteString (the type signature specified whatever ByteString Data.ByteString exports.) Now my question is twofold: what am I overlooking? Why didn't it compile? I can install and compile the very same code easily via cabal, but it doesn't compile inside my program. Is there some magic at work that I missed? Secondly, why doesn't ghc recognize the instance declaration GenericXMLString Data.Text.Text, when it really should? (It does recognize the declaration for String!) Thanks for any pointers! Aleks

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network.curl wondering about the relationship between CurlCookieJar and CurlCookieFile
by Michael Litchard 23 Oct '10

23 Oct '10

The problem is, while doing an HTTP(S) conversation between my client, and the application server, there's a point where the server delivers a a few new cookies. Now, I am able to get the initial cookies for the jar, but I don't get the new cookies when I am supposed to. I started looking at the various constructors offered by Network.Curl and came across CurlCookieFile. I'm wondering if this is the missing piece. If I need it, how does it fit in with CurlCookieJar? I'm including my current code, and the headers I'm getting. I'll note where the cookies are supposed to come in but don't. Any clues on how to troubleshoot, and some info on CurlCookieFile would be greatly appreciated. > import Network.Curl > import System (getArgs) > import Text.Regex.Posix > -- | Standard options used for all requests. Uncomment the @CurlVerbose@ > -- option for lots of info on STDOUT. > opts = [CurlCookieJar "cookies" , CurlVerbose True, CurlFollowLocation True] > -- | Additional options to simulate submitting the login form. > loginOptions user pass = > method_POST ++ [CurlPostFields [ "login=" ++ user, "password=" ++ pass ]] > main = withCurlDo $ do > -- Get username and password from command line arguments (will cause > -- pattern match failure if incorrect number of args provided). > [user, pass] <- getArgs > -- Initialize curl instance. > curl <- initialize > setopts curl opts > -- GET initial request > r <- do_curl_ curl ("https://172.16.1.18/showLogon.do") method_GET > :: IO CurlResponse > -- POST request to login. > r <- do_curl_ curl "https://172.16.1.18/default/showLogon.do" (loginOptions user pass) > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed to log in: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else do > -- POST request showFlashCheck.do > r <- do_curl_ curl ("https://172.16.1.18/showFlashCheck.do") method_GET > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed at showFlashCheck.do: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else do > -- GET request > r <- do_curl_ curl ("https://172.16.1.18/logon.do?flashVersion=9.0.100") method_GET --- right here, the server should respond with more cookies > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed at login.do?flashVersion=10.0" > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else do > -- go go gadget webForwards! > r <- do_curl_ curl ("https://172.16.1.18/launchWebForward.do?resourceId=4&policy=0&returnTo=%2Fs…") method_GET > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed at Web Foward" > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else putStrLn $ respBody r Headers ....> GET /showLogon.do HTTP/1.1 Host: 172.16.1.18 Accept: */* < HTTP/1.1 200 OK < Date: Sat, 23 Oct 2010 21:32:09 GMT < Expires: Thu, 01 Jan 1970 00:00:00 GMT * Added cookie SSLX_SSESHID="5oskk7cts6epk" for domain 172.16.1.18, path /, expire 0 < Set-Cookie: SSLX_SSESHID=5oskk7cts6epk;Path=/ * Added cookie lbTrack="BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------" for domain 172.16.1.18, path /, exp ire 1287870429 < Set-Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------;Path=/;Expires=Sat, 23-Oct-10 21:47:09 GMT;Secure < Content-Type: text/html;charset=UTF-8 < Pragma: no-cache < Cache-Control: no-cache < Transfer-Encoding: chunked < * Connection #0 to host 172.16.1.18 left intact * Re-using existing connection! (#0) with host 172.16.1.18 * Connected to 172.16.1.18 (172.16.1.18) port 443 (#0) > POST /default/showLogon.do HTTP/1.1 Host: 172.16.1.18 Accept: */* Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------; SSLX_SSESHID=5oskk7cts6epk Content-Length: 32 Content-Type: application/x-www-form-urlencoded < HTTP/1.1 200 OK < Date: Sat, 23 Oct 2010 21:32:09 GMT < Content-Type: text/html;charset=UTF-8 < Pragma: no-cache < Cache-Control: no-cache < Transfer-Encoding: chunked < * Connection #0 to host 172.16.1.18 left intact * Re-using existing connection! (#0) with host 172.16.1.18 * Connected to 172.16.1.18 (172.16.1.18) port 443 (#0) > GET /showFlashCheck.do HTTP/1.1 Host: 172.16.1.18 Accept: */* Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------; SSLX_SSESHID=5oskk7cts6epk < HTTP/1.1 200 OK < Date: Sat, 23 Oct 2010 21:32:09 GMT < Content-Type: text/html;charset=UTF-8 < Transfer-Encoding: chunked < * Connection #0 to host 172.16.1.18 left intact * Re-using existing connection! (#0) with host 172.16.1.18 * Connected to 172.16.1.18 (172.16.1.18) port 443 (#0) > GET /logon.do?flashVersion=9.0.100 HTTP/1.1 <------ right here is where it falls down. No cookies for me. Host: 172.16.1.18 Accept: */* Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------; SSLX_SSESHID=5oskk7cts6epk < HTTP/1.1 302 Moved Temporarily < Date: Sat, 23 Oct 2010 21:32:09 GMT < Content-Type: text/html < Location: https://172.16.1.18/default/showLogon.do?msgId=0 < Content-Length: 0 < * Connection #0 to host 172.16.1.18 left intact * Issue another request to this URL: 'https://172.16.1.18/default/showLogon.do?msgId=0' * Re-using existing connection! (#0) with host 172.16.1.18 * Connected to 172.16.1.18 (172.16.1.18) port 443 (#0) > GET /default/showLogon.do?msgId=0 HTTP/1.1 Host: 172.16.1.18 Accept: */* Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------; SSLX_SSESHID=5oskk7cts6epk < HTTP/1.1 200 OK < Date: Sat, 23 Oct 2010 21:32:09 GMT < Content-Type: text/html;charset=UTF-8 < Pragma: no-cache < Cache-Control: no-cache < Transfer-Encoding: chunked < * Connection #0 to host 172.16.1.18 left intact * Re-using existing connection! (#0) with host 172.16.1.18 * Connected to 172.16.1.18 (172.16.1.18) port 443 (#0) > GET /launchWebForward.do?resourceId=4&policy=0&returnTo=%2FshowWebForwards.do HTTP/1.1 Host: 172.16.1.18 Accept: */* Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------; SSLX_SSESHID=5oskk7cts6epk < HTTP/1.1 302 Moved Temporarily < Date: Sat, 23 Oct 2010 21:32:09 GMT < Content-Type: text/html < Location: https://172.16.1.18/default/showLogon.do < Content-Length: 0 < * Connection #0 to host 172.16.1.18 left intact * Issue another request to this URL: 'https://172.16.1.18/default/showLogon.do' * Re-using existing connection! (#0) with host 172.16.1.18 * Connected to 172.16.1.18 (172.16.1.18) port 443 (#0) > GET /default/showLogon.do HTTP/1.1 Host: 172.16.1.18 Accept: */* Cookie: lbTrack=BFQZHBGQBMURVJWIMWXTPIGSYYPCRCRTLSJUZBMJYZCQTTXNSVPH--------; SSLX_SSESHID=5oskk7cts6epk < HTTP/1.1 200 OK < Date: Sat, 23 Oct 2010 21:32:09 GMT < Content-Type: text/html;charset=UTF-8 < Pragma: no-cache < Cache-Control: no-cache < Transfer-Encoding: chunked < * Connection #0 to host 172.16.1.18 left intact * Closing connection #0

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A problem with network.curl and a possible solution
by Michael Litchard 22 Oct '10

22 Oct '10

I'm using the code from the following blog as a template to create a session on a work server using ssl. To begin with, I am sticking with the example site github.com. I have noticed that the code that does a POST, does a GET request instead. I suspect this is a problem with the underlying libcurl. I have decided to test out another library, network.browser. If I get the same behavior, then I know I'm the one screwing up. If I get the right behavior, I will move over to network.browser. The problem is, I am having difficulty writing the same functionality with network.browser. I don't know how to "activate", or manage the cookie jar. And I don't know how to send data with a POST request. The url, and sample code are below. I would like a demonstration of how to accomplish the same this using Network.Browser. With the help of the blog example, Network.Curl is accessible enough , but I just can't get the POST request to work. http://flygdynamikern.blogspot.com/2009/03/extended-sessions-with-haskell-c… > import Network.Curl > import System (getArgs) > import Text.Regex.Posix > -- | Standard options used for all requests. Uncomment the @CurlVerbose@ > -- option for lots of info on STDOUT. I like the verbose output, and the way the cookie jar is automagically made. I will manage the cookie jar if I have to, I just need to know how. > opts = [ CurlCookieJar "cookies" {- , CurlVerbose True -} ] > -- | Additional options to simulate submitting the login form. > loginOptions user pass = > CurlPostFields [ "login=" ++ user, "password=" ++ pass ] : method_POST > main = withCurlDo $ do > -- Get username and password from command line arguments (will cause > -- pattern match failure if incorrect number of args provided). > [user, pass] <- getArgs > -- Initialize curl instance. > curl <- initialize > setopts curl opts > -- POST request to login. > r <- do_curl_ curl "https://github.com/session" (loginOptions user pass) > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 302 > then error $ "Failed to log in: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else do > -- GET request to fetch account page. > r <- do_curl_ curl ("https://github.com/account") method_GET > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed to retrieve account page: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else putStrLn $ extractToken $ respBody r > -- | Extracts the token from GitHub account HTML page. The code below is not important for my purposes, > extractToken body = head' "GitHub token not found" xs > where > head' msg l = if null l then error msg else head l > (_,_,_,xs) = body =~ "github\\.token (.+)" > :: (String, String, String,[String])

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Creating lists of random numbers
by Tom Hobbs 21 Oct '10

21 Oct '10

Hi all, I'm trying to create a list of random integers (either 0s or 1s). I've got one way to work, but I'm trying to use replicate to replace this implementation but it's not going well. Can anyone help, please? Here's the version I want to replace; ugly :: Int -> [Int] ugly 0 = [] ugly s = ugly (s-1) ++ [ran] where ran = unsafePerformIO (randomRIO (0,1)) :: Int This one works as expected. Here's the broken version; broken :: Int -> [Int] broken s = replicate s ran where ran = unsafePerformIO (randomRIO (0,1)) :: Int The problem with broken is that "ran" seems to be evaluated once and then replicated, so my list contains the same random number s times. I'd like it to be re-evaluated with each replication. Also, I've heard of the dangers of using unsafePerformIO. Can anyone suggest a different way to generate random numbers? (Finally, a question that doesn't involve serialisation!) Cheers, Tom

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pattern match vs. pure function
by Bastian Erdnüß 21 Oct '10

21 Oct '10

Suppose I would want to write a function rmap that acts like rmap [f,g,h] x == [f x, g x, h x] Do I gain any advantage or disadvantage (beside readability) from using rmap = flip $ map . flip id over rmap [] _ = [] rmap (f:fs) x = f x : rmap fs x ? I could imagine Haskell can optimize the first version better since it refers to the built in map function. But beside that, does Haskell struggle with the combinatory stuff in the first version? Or does it get optimized away? And how "expensive" are pattern matches on the other side, anyway? Thanks for reading, Bastian

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