September 2013 - Beginners

Thrashed again by cabal and libraries...
by emacstheviking 20 Sep '13

20 Sep '13

I am wanting to use GLFW-b with Haskell on Mountain Lion. However, I think I have done something wrong along the way. I end up with "GLFW.initialize" not found when cabal tries to link and I suspect it is some kind of version conflict going on. Question: when I "cabal-dev install GLFW-b", how does the build process know where to find the headers to build with? I installed the latest GLFW (3.0.2) from the site and built them on the mac and I was not aware of telling cabal where to look but yet it still built and installed GLFW-b somehow. Presumably it looked in /usr/include i.e. all the usual places and was satisfied enough to have built something. When I "cabal install" my project however it just keeps saying it can't find "GLFW.initialize" which means either it can't find any library to link against or it can't find that function in a library it is finding. I completely uninstalled GHC/Platform from my Mac to prepare for this project and so far I have got nowhere! So, any suggestions on how to make cabal look in a specific folder to resolve header files and the library I built would be most welcome as I don't know that much about passing low level compiler and linker flags to it....I am about to learn. All the best. Sean.

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Inverse trig functions
by Adrian May 20 Sep '13

20 Sep '13

Hi All, When Haskell says "inverse trig function", it seems to mean the inverse of the return value of a trig function. I want those functions that return an angle whose sine, cosine or whatever equals the parameter I pass. Where are those? TIA, Adrian.

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How to convert a float or double number into a string?
by yi lu 19 Sep '13

19 Sep '13

If I use `show`, show 123.45, it will return "123.45", a desired answer. However, for show 123.45678901234567890, it will return "123.45678901234568". I want to save all digits into a string. I suppose I use a wrong type of number, which is Float. But what can I do to work right? Yi PS: If you receive this mail twice, please ignore one of them.

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How long is the pointer from a Vector valid (after it has been extracted by unsafeWidth)
by Nathan Hüsken 19 Sep '13

19 Sep '13

Hey, Background: I am making a wxhaskell backend for diagrams. Currently I am attempting to implement dashing. The function penSetDashes :: Pen a -> Int -> Ptr c -> IO () takes a bit muster, represented by an array of chars (second parameter is the number of the used bits). The problem is, that the underlying C++ function expects the pointer to be valid even after the function has been executed (until the pen is deleted or another dashing is set). So I am creating a Vector Word8 with the desired bitmuster and I am using Data.Vector.Storable.withUnsafe to pass its internal pointer to penSetDashes. But the pointer must point to something valid even after that! So I was thinking, what if I define data MyPen a = MyPen (Pen a) (Vector Word8) and always keep the vector of dashes with the pen. Will the pointer stay valid? If there are other Ideas on how to this, I am open! Regards, Nathan

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using the joker "_" on a constructor
by TP 19 Sep '13

19 Sep '13

Hi, I have a question about pattern matching. Consider the following code: ------------------ data Foo = Bar Int | Foo Int f :: Foo -> Bool f (Foo n) | even n = True | odd n = False f (Bar n) | even n = True | odd n = False main = do print $ f $ Bar 1 print $ f $ Bar 2 print $ f $ Foo 1 print $ f $ Foo 2 ------------------ Why is it not possible to simply write for f: f v = case v of _ n | even n -> True _ n | odd n -> False or f (_ n) | even n = True | odd n = False (in both cases we get a parse error)? Thanks, TP

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Help getting glib installed
by Britt Anderson 18 Sep '13

18 Sep '13

I sent this yesterday, but it never showed up on the list. I apologize in advance if a repeated post appears, but I am rather stuck. I am looking for help to figure out my problem with cabal install glib I am starting fresh with ghc 7.6.3 built from source and cabal-install 1.18 downloaded from hackage and installed using the sh bootstrap.sh command. All works fine, as far as I can tell (e.g ghci runs and cabal install gtk2hs-buildtools worked). However in trying to cabal-install packages that depend on glib I am having problems. I think I fixed the first problem which was a conflict for the function name moreRecentFile. This exists in the SetupWrapper.hs file, but is also now a part of Distribution.Simple.Utils. I edited the SetupWrapper.hs and renamed its version of the function. The next problem to appear was with the processing of Gtk2HsSetup.hs file. The error is that constructor libraryConfig is not visible. I think that this is because as of Cabal-1.18 this constructor is not a part of Distribution.Simple.LocalBuildInfo. Is there some new accessor function or other work around that is known, or is the problems something else entirely? Thank you, /Britt

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import literal haskell file
by Nathan Hüsken 18 Sep '13

18 Sep '13

Hey, When I do import ModuleName and have a file ModuleName.lhs (in literal haskell), ghc complains that it cannot find the module. Is it somehow possible to import the lhs file without rewriting it? Thanks! Nathan

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Exception: bind: resource busy (Address already in use)
by Miro Karpis 16 Sep '13

16 Sep '13

Hi, I'm trying to work on following code below (code is a copy from here<http://cogsandlevers.blogspot.no/2013/05/a-tcp-server-haskell-example.html>). Problem is that when I close the server with ctrl+c and try to run it again I get: *** Exception: bind: resource busy (Address already in use). In documentation of listenOn is written: NOTE: To avoid the "Address already in use" problems popped up several times on the GHC-Users mailing list we set the ReuseAddr<http://hackage.haskell.org/packages/archive/network/2.3.0.14/doc/html/Netwo…>socket option on the listening socket. If you don't want this behaviour, please use the lower level listen<http://hackage.haskell.org/packages/archive/network/2.3.0.14/doc/html/Netwo…> instead. Please how can I fix this? (ghci version 7.6.3) cheers m. import Network (listenOn, accept, PortID(..), Socket) import System.IO (hSetBuffering, hGetLine, hPutStrLn, BufferMode(..), Handle) import Control.Concurrent (forkIO) echoImpl :: Handle -> IO () echoImpl client = do line <- hGetLine client hPutStrLn client line echoImpl client clientHandler :: Socket -> IO () clientHandler sock = do (client, _, _) <- accept sock hSetBuffering client NoBuffering forkIO $ echoImpl client clientHandler sock felix :: IO () felix = do sock <- listenOn $ PortNumber 5002 putStrLn $ "Echo server started .." clientHandler sock

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Merge Sort Stack Overflow
by Florian Lammel 16 Sep '13

16 Sep '13

Hi, I've just started learning Haskell and am trying to implement some basic algorithms. My shot at merge sort looks like this: mergeSort :: Ord a => [a] -> [a] mergeSort [] = [] mergeSort [x] = [x] mergeSort xs = merge (mergeSort as) (mergeSort bs) where (as, bs) = splitInHalf xs splitInHalf :: [a] -> ([a], [a]) splitInHalf [] = ([], []) splitInHalf [x] = ([x], []) splitInHalf (x:y:xys) = (x:xs, y:ys) where (xs, ys) = splitInHalf xys merge :: Ord a => [a] -> [a] -> [a] merge xs [] = xs merge [] ys = ys merge (x:xs) (y:ys) = if x < y then x:(merge xs (y:ys)) else y:(merge ys (x:xs)) As far as I can tell, my implementation is more or less the same as on rosetta code [0], literate programs [1] and several stack overflow questions [2][3]. The algorithm works, but for large lists (for example, 500k random numbers), I get a stack overflow. So my question is: How can I change my code so that it works on larger inputs? Or, more generally, how can I write recursive algorithms in functional languages that require more nested function calls than fit in the stack? Regards Florian [0] http://rosettacode.org/wiki/Sorting_algorithms/Merge_sort#Haskell [1] http://en.literateprograms.org/Merge_sort_(Haskell) [2] http://stackoverflow.com/questions/7554226/haskell-merge-sort-sorting-words… [3] http://stackoverflow.com/questions/1215432/merge-sort-in-haskell

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import module with (..)
by Miro Karpis 16 Sep '13

16 Sep '13

Please, what does mean the (..) brackets when we are importing a package? For example: import Network (listenOn, accept, *PortID(..)*, Socket) cheers, m.

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