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code explanation
by moowoo9＠fastmail.fm 03 Jul '11

03 Jul '11

I'm a beginner, I'd like someone to help me understand a few lines of code My questions are: 1) Maybe String is an application of the constructor Maybe to the type String. What does it mean? Is there anything equivalent in the C language? 2) Is this a function with three arguments? type T = [Char] type P = (Char, Maybe String) type Delta = ((Maybe Char, Char), Maybe String) fromGtoM :: T -> [P] -> [Delta] 3) is this a usage of the above mentioned function? fromGtoM t p = terminalRules ++ varRules I dont'see why the function has three arguments but then only two are used t p. 4) what does ++ mean ? Sorry, for posting too many questions. Thanks in advance. -- moowoo9(a)fastmail.fm -- http://www.fastmail.fm - Access your email from home and the web

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(no subject)
by Jan Brosius 03 Jul '11

03 Jul '11

Hi, Is there in Haskell a function like unwindprotect in Lisp. Thanks Jan

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First haskell package..
by Mike Meyer 03 Jul '11

03 Jul '11

After much dithering, I'm finally ready to release a first haskell project, eddie. I figured I'd run it past the helpful people here for feedback before making a more official announcement. eddie is a command line tool that lets you use haskell functions to filter files. Functions is meant in the most general sense - any expression that can be evaluated to a type of "String -> String" can be used. The home page is http://eddie.googlecode.com/. There are a couple of pages of examples in the wiki. The 0.1 source tarball is available in downloads, and includes the .cabal file for building. It's not in hackage yet - I'm awaiting a username. <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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How to process a list that contains "undefined" values?
by Costello, Roger L. 03 Jul '11

03 Jul '11

Hi Folks, How would you find the maximum value in this list: [undefined, 10, undefined, 20] Is it bad practice to create lists that contain undefined values? If yes, what is a better way to express in a list a "no value present/available" value? For example would it be better to express the above list using Maybe: [Nothing, Just 10, Nothing, Just 20] Or perhaps something else? What do you recommend? /Roger

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error handling in libraries
by Michael Xavier 03 Jul '11

03 Jul '11

Hi Beginners, A little while ago I wrote my first library. Internally, it is pretty solid but it makes API calls out to the internet so network problems can be an issue. Additionally, if the API that I'm calling decides to change their format in the future, it is probably not a good idea that my library crash the program that uses it. I'm curious what the current best practice is for handling exceptions in library code. I realize that in self-contained projects, it is much easier to choose whatever you want because it will not directly effect the user, but in a library one must consider what exception handling system the end user is most likely to know. -- Michael Xavier http://www.michaelxavier.net

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mapM etc.
by Dennis Raddle 02 Jul '11

02 Jul '11

It's been about a year since I used Haskell and I'm rusty. I think I knew how to do this once, but I need a reminder. I've got some functions: getDeviceInfo :: Int -> IO DeviceInfo name :: DeviceInfo -> String I want to do something like func :: IO () func = do ds <- mapM getDeviceInfo [0..10] mapM_ (print . name) ds Is there a way to combine 'getDeviceInfo', 'name', and 'print' in one line? Dennis

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Problem with do statement
by Michael Litchard 02 Jul '11

02 Jul '11

I received an error message that might be due to indentation, and not what the error message claims. Here's the error, then the code. Below that I have some comments. ghc --make CreateSession.lhs [2 of 3] Compiling HtmlParsing ( HtmlParsing.lhs, HtmlParsing.o ) HtmlParsing.lhs:81:5: The last statement in a 'do' construct must be an expression: let makeIDPage = do { initial } > generateResourceHtml :: Curl -> String -> String -> FilePath -> IO (Either String String) > generateResourceHtml curl user pass ipFile = do > urlSequence <- popURLrec ipFile > let makeIDPage = do > initial = urlInitial urlSequence > login = urlLogin urlSequence > flash1 = urlFlash1 urlSequence > flash2 = urlFlash2 urlSequence > showWebForwards = urlShowWebForwards urlSequence > quickCreate = urlQuickCreate urlSequence > getResource = urlGetResource urlSequence > curlResp curl $ initial method_GET > curlResp curl $ urlLogin urlSequence $ loginOpts user pass > curlResp curl $ urlFlash1 urlSequence resourceOpts > curlResp curl $ urlFlash2 urlSequence resourceOpts > curlResp curl $ urlShowWebForwards urlSequence resourceOpts > curlResp curl $ urlQuickCreate urlSequence resourceOpts > curlResp curl $ urlGetResource urlSequence resourceOpts > runErrorT makeIDPage It seems to me the first do construct ends with a statement, runErrorT makeIDPage. Also the second do construct ends with a statement as well, > curlResp curl $ urlGetResource urlSequence resourceOpts So I'm either wrong about what the definition of a statement is, or this problem is really about indentation or similar. Clarification?

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OpenGL keyboardMouseCallback ...confused over type
by Sean Charles 02 Jul '11

02 Jul '11

I asked a question the other day about passing application state around an OpenGL program and I've consulted RWH and the inter-web extensively and I think I am on the right lines so far! In my main program, I use Data.IORef like this: gameState <- newIORef $ newGameState and GameState contains all the stuff to do with the game model. Next the confusing bit... to make myself learn things I typed out the full type of the keyboardMouseCallback function: myKeyMouse :: Key -> KeyState -> Modifiers -> Position -> IO () and slotted it into the code like so: keyboardMouseCallback $= Just myKeyMouse In order to be able to pass the "IORef GameState" into the keyboard handler I have to re-type my keyboardMouseCallback function like so: myKeyMouse :: IORef GameState -> Key -> KeyState -> Modifiers -> Position -> IO () and modify the assignment in main to this: keyboardMouseCallback $= Just (myKeyMouse gameState) But I haven't really understood *why* that is the case. The displayCallback is also similarly affected as it is now: myDisplay :: IORef GameState -> IO () ... displayCallback $= (myDisplay gameState) and again I am not sure what magic is happening here to have made it compile and work! My real source of confusion is over my apparent victory over the type system in that I appear to have extended the type signature (is that the correct term?) for both of the callbacks be prepending "IORef GameState" to them and yet it all compiles and works. Is "prepending" the key ? I feel a tingling of Eureka! organ but I am not sure yet. Explanations are welcome as I am sure there is something "important" here that will really advance my understanding of the type system. It smells like curry but I don't think... hang on... the keyboard one is wrapped in the Maybe monad... therefore(?) the function I am handing in can be *any* type so long as they actually match types! Is that it then? I can smell partial application and curry now. HELP! Thanks, Sean.

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version of base in Haskell Platform
by Dennis Raddle 01 Jul '11

01 Jul '11

I just installed the latest Haskell Platform on Windows XP. Then I tried to install the musicxml package, and it says it needs base 3.something. How do I find out what version of base comes with a version of the Haskell Platform? Thanks, Dennis

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Problem building MusicXML
by Dennis Raddle 01 Jul '11

01 Jul '11

My problem is that "cabal install" just hangs at the line "Building HaXml-1.22..." and I don't know how to figure out what it's trying to do. Background: My main goal is to use the 'musicxml' package. On Windows XP, I installed Haskell Package 2010.1.0.0. (I needed base 3 for compatibility with musicxml.) I did 'cabal install musicxml'. First I got warning: Warning: c:\Documents and Settings\Mike\Application Data\cabal\config: Unrecognized field world-file on line 15 Then it built text-0.11.1.3, polyparse-1.7, then it started to build HaXml-1.22 and got as far as printing "Building HaXml-1.22..." but hung. No CPU was being consumed. Dennis

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