January 2011 - Beginners

scanl and infinite sequences [scanl for Fibonacci sequences]
by Seung Soo, Ha 25 Jan '11

25 Jan '11

I was playing with the project Euler problems and found heaps of Fibonacci generating codes in Haskell. One caught my eye: fib = 1 : scanl (+) 1 fib ....(1) when my understanding was limited to "folds just replace : with the function you provide scans show all the intermediate states of your fold" this line of code was very easy to understand. Efficient too(according to the wiki article on Haskell) But now that I'm trying to grasp foldl and foldr, I no longer understand that line of code!!! This is especially confusing, as afaik, (1) is generating an infinite list but I thought you can't use foldl for infinite lists! Tried replacing scanl with scanr (foldr is supposed to work with infinite lists right?) fib = 1 : scanr (+) 1 fib .....(2) somewhat surprisingly, (2) crashes. I've read numerous articles on folds on the haskell wiki, and also this stack overflow post (http://stackoverflow.com/questions/3082324) Just as I thought I was getting a hold on the concept of folds... This.. What am I missing here?

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Bug or Intended behavior? ( Exception: stack overflow )
by Seung Soo, Ha 24 Jan '11

24 Jan '11

my setup: phenom II quadcore, 4GB mem Ubuntu Maverick While implementing my own reverse in "99 haskell questions", I made the following mistake: (Caution: stack over flow causing code below) reverse'::[a] -> [a] reverse' [] = [] reverse' [x] = [x] reverse' xs = last xs : init (reverse' xs) The last line is the wrong part. It should be reverse' xs = last xs : reverse' (init xs) anyway, the original code seems to cause an infinite recursion by repeating the last part forever. When I try this out on ghci: *Main> reverse' [1,2,3,4,5] [5*** Exception: stack overflow *Main> The problem is, unless I manually exit ghci, the "stack" does not seem to be freed (garbage collected?) and memory remains fully occupied by ghci. I tried waiting a bit, but it stayed the same. Is this expected behavior? Shouldn't the GC kick in? Is this orthogonal of the GC? I think this would help me understand haskell and GHC much better.

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Processing individual csv line
by Shakthi Kannan 24 Jan '11

24 Jan '11

Hi, I am trying to parse a .csv file and process the individual lines. Here is some code snippet (based on RWH Chapter 16 [1]): === test.hs === import Text.ParserCombinators.Parsec import System.Environment (getArgs) parseCSV :: String -> Either ParseError [[String]] parseCSV input = parse csvFile "(unknown)" input main = do do c <- getContents case parse csvFile "(stdin)" c of Left e -> do putStrLn "Error parsing input:" print e Right r -> mapM_ print r === END === Given an input file foo.csv: a,b,c d,e,f g,h,i and running the above test.hs using: $ runghc test.hs < foo.csv I get the following output: ["a,b,c"] ["d,e,f"] ["g,h,i"] Instead of using mapM_ print r, how can I obtain and pass each of the above lists (for example, ["a,b,c"]) to another function for processing? Appreciate any inputs, Thanks! SK [1] RWH. Using Parsec. http://book.realworldhaskell.org/read/using-parsec.html -- Shakthi Kannan http://www.shakthimaan.com

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Re: [Haskell-beginners] Looking for some guidance to installing GHCI on MAC
by Wes Morgan 23 Jan '11

23 Jan '11

I just used Homebrew (http://mxcl.github.com/homebrew/) Once that's installed, 'brew install ghc' gets you GHC, GHCi, cabal, etc. Wes On Jan 23, 2011, at 6:00 AM, "beginners-request(a)haskell.org" <beginners-request(a)haskell.org> wrote: > Re: [Haskell-beginners] Looking for some guidance to > installing GHCI on MAC

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Question about List Type Constraint and Null
by aditya siram 22 Jan '11

22 Jan '11

Hi all, The following function gives me an "Ambiguous type variable `a' in the constraint: `Read a' arising from a use of `res'" error: test :: Read a => String -> Maybe [(a,String)] test s = if null res then Nothing else Just $ fst $ head res where res = reads s The reason as 'jmcarthur' so patiently explained on IRC is that 'res' is used twice and the type constraint 'a' is different for each use, hence the ambiguity. I get that. But I have a further question why should 'null ...' care about the type of its list argument? Isn't it polymorphic? So it shouldn't make a difference that the 'a' inside res is ambiguous because we know for sure that it always returns a list of some kind. Thanks, -deech

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Improve my FFI code please...
by Sean Charles 21 Jan '11

21 Jan '11

Hi list, I am *still* cursing the fact that I like where Haskell is taking me but that I am too dumb to get it into my head as quickly as I'd like! I will not give up and in that vein I have been beating myself up trying to run before I can walk as usual by attempting to write an FFI wrapper around the Ubuntu libcwiimote library! Having been inspired by johnnylee.net and having found his code is for Windows and C# I have taken it on myself to reproduce what he's done but using Haskell and OpenGL. Laugh, I nearly cried because it's taken hours of frustration just to get this far! I've built the hardware and now I need the software as usual. So, here we go... what I post here is the beginnings of a Wiimote wrapper. I *think* I understand Monads properly now, Brian Beck video on C9 (MSDN) really really helped a lot, I'd recommend that to anybody still struggling with the concept. What I want to know is can my code be improved by using monadic chaining and if so, with what and how (LMAO) and also have I overlooked or otherwise not used the correct forms, idioms etc. to achieve what I have so far ? I am really keen to know how the real gurus out there could reduce this code down or make it 'more haskell like'. Basically, I want you to correct it, improve it, show me and others how to do it right! I want it de-constructed, ripped to pieces, laughed at, openly mocked and I will learn from it and enjoy it because I am a developer! ;) LOL It compiles with ... ghc --make -lcwiimote -o wmscan WiiMote.hs and runs like so ... sean@sean-desktop:~/Documents/bluetooth$ ./wmscan 00:21:BD:A3:97:C7 Connecting to 00:21:BD:A3:97:C7 00:21:BD:A3:97:C7connected OK, disconnecting ...OK sean@sean-desktop:~/Documents/bluetooth$ ...and I was proud as hell, almost as proud as watching my son born but without the goo. Many thanks. Sean ---- {-# LANGUAGE ForeignFunctionInterface #-} import Foreign import Foreign.C.Types import Foreign.Ptr import Foreign.C.String import System #include "wiimote_api.h" foreign import ccall unsafe "wiimote_api.h wiimote_open" c_wiimote_open :: CString -> IO (Ptr Word8) foreign import ccall unsafe "wiimote_api.h wiimote_close" c_wiimote_close :: (Ptr Word8) -> IO CInt wiiOpen :: String -> IO (Maybe (Ptr Word8)) wiiOpen wid = do handle <- withCString wid c_wiimote_open case handle of nullPtr -> return Nothing handle -> return (Just handle) wiiClose :: Ptr Word8 -> IO Bool wiiClose wptr = do response <- c_wiimote_close wptr case response of 0 -> return True _ -> return False -- Open the wiiremote, print the handle and close it test :: String -> IO () test wiiHID = do wiimote <- wiiOpen wiiHID case wiimote of Just handle -> do putStr (wiiHID ++ " connected OK, disconnecting ... ") status <- wiiClose handle case status of True -> putStrLn "OK" False -> putStrLn "FAIL" Nothing -> putStrLn "FAIL" return () -- expects a single HID string on the command line ... main :: IO () main = do args <- getArgs case args of [hid] -> do putStrLn ("Connecting to " ++ hid) test hid _ -> putStrLn "Supply ONE Wiimote HID address!"

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Re: [Haskell-beginners] Improve my FFI code please...
by sean＠objitsu.com 21 Jan '11

21 Jan '11

Edward, Daniel, thanks. > On Friday 21 January 2011 00:35:02, Sean Charles wrote: > wiiOpen :: String -> IO (Maybe (Ptr Word8)) > wiiOpen wid = do > ? ?handle <- withCString wid c_wiimote_open > ? ?case handle of > ? ? ?nullPtr -> return Nothing > ? ? ?handle -> return (Just handle) > Unless nullPtr is a macro that gets replaced with a literal by the > preprocessor, that won't do what you want. "nullPtr" is a generic name and > matches everything, so you'll always take the first branch (with -Wall, ghc > should warn about overlapping patterns in that case). Daniel, you have confused me. In my RWH book there is a test from the PCRE example of nullPtr == x, so I just did my code using a case instead, I am sure it's correct!?!?!? If I run my program with the Wiimote off, it prints FAIL and if I press the red sync. button it then prints out OK and the memory address of the allocated structure so it surely must be workng as I expected? Anybody? Edward, I am going to apply your comments over the weekend and see if I can truly understand them. I get the rough idea. Thanks guys. Sean.

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trouble installing glib on windows 7
by Michael Litchard 20 Jan '11

20 Jan '11

I have Haskell Platform installed, trying to install glib. Here's what happens. $ cabal install glib Resolving dependencies... C:\Users\PERFOR~1\AppData\Local\Temp\glib-0.12.02360\glib-0.12.0\Gtk2HsSetup.hs: 25: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when build ing (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\Users\PERFOR~1\AppData\Local\Temp\glib- 0.12.02360\glib-0.12.0\Gtk2HsSetup.hs, C:\Users\PERFOR~1\AppData\Local\Temp\glib -0.12.02360\glib-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\Users\PERFOR~1\AppData\Local\Temp\glib- 0.12.02360\glib-0.12.0\Setup.hs, C:\Users\PERFOR~1\AppData\Local\Temp\glib-0.12. 02360\glib-0.12.0\dist\setup\Main.o ) Linking C:\Users\PERFOR~1\AppData\Local\Temp\glib-0.12.02360\glib-0.12.0\dist\se tup\setup.exe ... Configuring glib-0.12.0... setup.exe: pkg-config version >=0.9.0 is required but it could not be found. cabal.exe: Error: some packages failed to install: glib-0.12.0 failed during the configure step. The exception was: ExitFailure 1 Okay, so all I need to do is install pkg-config $ cabal install pkg-config cabal.exe: There is no package named pkg-config. Perhaps you need to run 'cabal update' first? hmm, so how do I install pkg-config? cabal update didn't help

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Editor to use for Hugs
by patricklynch 20 Jan '11

20 Jan '11

Good afternoon, I am currently using VIM as an editor with Hugs on a Mac. I tried TextEdit but I am looking for something more powerful. Is there an editor that is designed to work with Hugs on the Mac? Thank you

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GUI (Glade installation)
by Stephanie Wong 19 Jan '11

19 Jan '11

Hi, I am new to GUIs and for my project I need to develop a GUI. I found a demo that is very useful but unfortunately, I am unable to compile it and get this error message: 'Could not find module 'Graphics.UI.Gtk.Glade': Use -v to see the a list of the files searched for.' I have installed Gtk2hs and able to use other demos without Glade. I have searched Glade and found many different versions and the installation seems a bit complicated. I'm working on Windows 7. Would you be able to recommend a version of Glade that is compatible for Windows 7 and perhaps give me some instructions on how to install it please. Many Thanks, StephThis message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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