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• Can Haskell's monads be thought of as using and returning a hidden state parameter?

  - by AJM

  I don't understand the exact algebra and theory behind Haskell's monads. However, when I think about functional programming in general I get the impression that state would be modelled by taking an initial state and generating a copy of it to represent the next state. This is like when one list is appended to another; neither list gets modified, but a third list is created and returned. Is it therefore valid to think of monadic operations as implicitly taking an initial state object as a parameter and implicitly returning a final state object? These state objects would be hidden so that the programmer doesn't have to worry about them and to control how they gets accessed. So, the programmer would not try to copy the object representing the IO stream as it was ten minutes ago. In other words, if we have this code: main = do putStrLn "Enter your name:" name <- getLine putStrLn ( "Hello " ++ name ) ...is it OK to think of the IO monad and the "do" syntax as representing this style of code? putStrLn :: IOState -> String -> IOState getLine :: IOState -> (IOState, String) main :: IOState -> IOState -- main returns an IOState we can call "state3" main state0 = putStrLn state2 ("Hello " ++ name) where (state2, name) = getLine state1 state1 = putStrLn state0 "Enter your name:"

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• Using items in a list as arguments

  - by Travis Brown

  Suppose I have a function with the following type signature: g :: a -> a -> a -> b I also have a list of as—let's call it xs—that I know will contain at least three items. I'd like to apply g to the first three items of xs. I know I could define a combinator like the following: ($$$) :: (a -> a -> a -> b) -> [a] -> b f $$$ (x:y:z:_) = f x y z Then I could just use g $$$ xs. This makes $$$ a bit like uncurry, but for a function with three arguments of the same type and a list instead of a tuple. Is there a way to do this idiomatically using standard combinators? Or rather, what's the most idiomatic way to do this in Haskell? I thought trying pointfree on a non-infix version of $$$ might give me some idea of where to start, but the output was an abomination with 10 flips, a handful of heads and tails and aps, and 28 parentheses. (NB: I know this isn't a terribly Haskelly thing to do in the first place, but I've come across a couple of situations where it seems like a reasonable solution, especially when using Parsec. I'll certainly accept "don't ever do this in real code" if that's the best answer, but I'd prefer to see some clever trick involving the ((->) r) monad or whatever.)

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• What is a good method to solve cabal install problems?

  - by sp3ctum

  I've used the cabal package manager for Haskell programs to install libraries and new projects that I've cloned from some repositories. More often than not, I keep running into problems. Most projects make installing them seem super easy, but in my case that's not always true - sometimes they are very hard to get running. Some are so hard, in fact, that I've lost interest in the project solely because of not being able to install it. So instead of complaining, I'd like to ask what I should do to better this situation. I'd like to use my most recent problem as an example. I'm interested in trying out the Gitit project. It's a promising looking personal wiki that runs on various version control systems. So here's what I've done: Clone from Github run cabal install in the project directory like I'm told on the project install page: mika@eka:~/git/gitit$ ls BLUETRIP-LICENSE CHANGES HCAR-gitit.tex LICENSE Network README.markdown RELANN-0.6.1 Setup.lhs TANGOICONS YUI-LICENSE data expireGititCache.hs gitit.cabal gitit.hs plugins mika@eka:~/git/gitit$ cabal install Resolving dependencies... cabal: cannot configure happstack-server-7.0.7. It requires base64-bytestring ==1.0.* For the dependency on base64-bytestring ==1.0.* there are these packages: base64-bytestring-1.0.0.0. However none of them are available. base64-bytestring-1.0.0.0 was excluded because gitit-0.10 requires base64-bytestring ==0.1.* mika@eka:~/git/gitit$ So now I'm thinking: well, I'll install happstack-server on its own, maybe that will work: mika@eka:~/git/gitit$ cabal install happstack-server Resolving dependencies... Warning: happstack-server.cabal: Ignoring unknown section type: test-suite Configuring happstack-server-7.0.7... cabal: At least the following dependencies are missing: blaze-html ==0.5.*, hslogger >=1.0.2, monad-control ==0.3.*, network >=2.2.3, sendfile >=0.7.1 && <0.8, system-filepath >=0.3.1, text >=0.10 && <0.12, threads >=0.5, transformers-base ==0.4.* cabal: Error: some packages failed to install: happstack-server-7.0.7 failed during the configure step. The exception was: ExitFailure 1 So looks like there are some dependencies missing. But isn't installing these dependencies the whole point of using cabal in the first place? What should I do? File bug reports (to which project?), install the dependencies manually or something else? Bonus points for explaining what causes these kinds of problems.

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• Moving data files failing

  - by Miles Hayler

  Trying to migrate data from C: to D: via the SBS console is failing. The wizard starts running but drops out in the first few seconds. I'll post the full logs, but the important lines appear to be as follows: An exception of type 'Type: System.IO.FileNotFoundException, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' has occurred. Message: The system cannot find the file specified. (Exception from HRESULT: 0x80070002) Stack: at TaskScheduler.TaskSchedulerClass.GetFolder(String Path) at Microsoft.WindowsServerSolutions.Common.WindowsTaskScheduler..ctor(String taskPath, String taskName) BaseException: Microsoft.WindowsServerSolutions.Storage.Common.StorageException: GetServerBackupTaskStatus: fail to find the task --- ErrorCode:0 I've been googling for days with no luck. I have found that mscorlib is a component of .net, and I've discovered multiple instances of the file in %windir%, %windir%\winsxs, %windir%\Microsoft.net Anyone come across and fixed this one before? --------------------------------------------------------- [1516] 110315.190856.1105: Storage: Initializing...C:\Program Files\Windows Small Business Server\Bin\MoveData.exe [1516] 110315.190856.2875: Storage: Data Store to be moved: Exchange [1516] 110315.190856.5305: TaskScheduler: Exception System.IO.FileNotFoundException: [1516] 110315.190856.5605: Exception: --------------------------------------- An exception of type 'Type: System.IO.FileNotFoundException, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' has occurred. Timestamp: 03/15/2011 19:08:56 Message: The system cannot find the file specified. (Exception from HRESULT: 0x80070002) Stack: at TaskScheduler.TaskSchedulerClass.GetFolder(String Path) at Microsoft.WindowsServerSolutions.Common.WindowsTaskScheduler..ctor(String taskPath, String taskName) [1516] 110315.190856.5625: Storage: Exception Microsoft.WindowsServerSolutions.Common.WindowsTaskSchedulerException: [1516] 110315.190856.5635: Exception: --------------------------------------- [b]An exception of type 'Type: Microsoft.WindowsServerSolutions.Common.WindowsTaskSchedulerException, Common, Version=6.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has occurred.[/b] Timestamp: 03/15/2011 19:08:56 Message: Failed to find the task path Stack: at Microsoft.WindowsServerSolutions.Common.WindowsTaskScheduler..ctor(String taskPath, String taskName) at Microsoft.WindowsServerSolutions.Storage.Common.ServerBackupUtility.GetServerBackupTaskStatus() --------------------------------------- An exception of type 'Type: System.IO.FileNotFoundException, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' has occurred. Timestamp: 03/15/2011 19:08:56 Message: The system cannot find the file specified. (Exception from HRESULT: 0x80070002) Stack: at TaskScheduler.TaskSchedulerClass.GetFolder(String Path) at Microsoft.WindowsServerSolutions.Common.WindowsTaskScheduler..ctor(String taskPath, String taskName) [1516] 110315.190856.5665: Storage: Error Retrieving Server Backup Task Status: ErrorCode:0 BaseException: Microsoft.WindowsServerSolutions.Storage.Common.StorageException: GetServerBackupTaskStatus: fail to find the task ---> ErrorCode:0 BaseException: Microsoft.WindowsServerSolutions.Common.WindowsTaskSchedulerException: Failed to find the task path ---> System.IO.FileNotFoundException: The system cannot find the file specified. (Exception from HRESULT: 0x80070002) at TaskScheduler.TaskSchedulerClass.GetFolder(String Path) at Microsoft.WindowsServerSolutions.Common.WindowsTaskScheduler..ctor(String taskPath, String taskName) --- End of inner exception stack trace --- at Microsoft.WindowsServerSolutions.Common.WindowsTaskScheduler..ctor(String taskPath, String taskName) at Microsoft.WindowsServerSolutions.Storage.Common.ServerBackupUtility.GetServerBackupTaskStatus() --- End of inner exception stack trace --- at Microsoft.WindowsServerSolutions.Storage.Common.ServerBackupUtility.GetServerBackupTaskStatus() at Microsoft.WindowsServerSolutions.Storage.MoveData.Helper.get_ServerBackupTaskState() [1516] 110315.190857.6216: Storage: Backup Task State: Unknown [1516] 110315.190857.9347: Storage: Launching the Move Data Wizard! [1516] 110315.190857.9397: Wizard: Admin:QueryNextPage(null) = Storage.MoveDataWizard.GettingStartedPage [1516] 110315.190857.9417: Wizard: TOC Storage.MoveDataWizard.GettingStartedPage is on ExpectedPath [1516] 110315.190857.9577: Wizard: Storage.MoveDataWizard.GettingStartedPage entered [1516] 110315.190857.9657: Wizard: Admin:QueryNextPage(Storage.MoveDataWizard.GettingStartedPage) = Storage.MoveDataWizard.DiagnoseDataStorePage [1516] 110315.190857.9657: Wizard: TOC Storage.MoveDataWizard.DiagnoseDataStorePage is on ExpectedPath [1516] 110315.190857.9657: Wizard: Admin:QueryNextPage(Storage.MoveDataWizard.DiagnoseDataStorePage) = Storage.MoveDataWizard.NewDataStoreLocationPage [1516] 110315.190857.9657: Wizard: TOC Storage.MoveDataWizard.NewDataStoreLocationPage is on ExpectedPath [1516] 110315.190857.9657: Wizard: Admin:QueryNextPage(Storage.MoveDataWizard.NewDataStoreLocationPage) = null [1516] 110315.190857.9697: Wizard: ---------------------------------- [1516] 110315.190857.9697: Wizard: The pages visted: [1516] 110315.190857.9697: Wizard: Current Page := [TOC Storage.MoveDataWizard.GettingStartedPage] [1516] 110315.190857.9697: Wizard: [TOC] : TOC Storage.MoveDataWizard.DiagnoseDataStorePage [1516] 110315.190857.9697: Wizard: [TOC] : TOC Storage.MoveDataWizard.NewDataStoreLocationPage [1516] 110315.190857.9697: Wizard: Step 1 of 3 [1516] 110315.190907.0406: Wizard: Admin:QueryNextPage(Storage.MoveDataWizard.GettingStartedPage) = Storage.MoveDataWizard.DiagnoseDataStorePage [1516] 110315.190907.0416: Wizard: Storage.MoveDataWizard.GettingStartedPage exited with the button: Next [1516] 110315.190907.0416: WizardChainEngine Next Clicked: Going to page {0}.: Storage.MoveDataWizard.DiagnoseDataStorePage [1516] 110315.190907.0496: Wizard: Storage.MoveDataWizard.DiagnoseDataStorePage entered [1516] 110315.190907.0606: Wizard: Admin:QueryNextPage(Storage.MoveDataWizard.DiagnoseDataStorePage) = Storage.MoveDataWizard.NewDataStoreLocationPage [1516] 110315.190907.0606: Wizard: Admin:QueryNextPage(Storage.MoveDataWizard.NewDataStoreLocationPage) = null [1516] 110315.190907.0606: Wizard: ---------------------------------- [1516] 110315.190907.0606: Wizard: The pages visted: [1516] 110315.190907.0606: Wizard: [TOC] visited: TOC Storage.MoveDataWizard.GettingStartedPage [1516] 110315.190907.0606: Wizard: Current Page := [TOC Storage.MoveDataWizard.DiagnoseDataStorePage] [1516] 110315.190907.0616: Wizard: [TOC] : TOC Storage.MoveDataWizard.NewDataStoreLocationPage [1516] 110315.190907.0616: Wizard: Step 2 of 3 [19772] 110315.190907.0656: Storage: Starting System Diagnosis [19772] 110315.190907.0656: Storage: Getting Data Store Information [19772] 110315.190907.1086: Storage: Create the list of storage and DB directory path [19772] 110315.190907.1246: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingTasks..ctor [19772] 110315.190907.1546: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingTasks.Initialize [19772] 110315.190907.1596: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.Initialize [19772] 110315.190907.1606: Messaging: Exchange install path: C:\Program Files\Microsoft\Exchange Server\bin [19772] 110315.190908.4157: Messaging: E12 Monad runspace created ID: Microsoft.PowerShell [19772] 110315.190908.4237: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190908.4287: Messaging: Executed management shell command: get-exchangeserver [19772] 110315.190910.2369: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190910.2369: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.Initialize [19772] 110315.190910.5699: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingTasks.GatherAdminInfo [19772] 110315.190910.5699: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190910.5719: Messaging: Executed management shell command: get-user -Identity "dmagroup.local\Administrator" [19772] 110315.190911.0870: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.0880: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.0880: Messaging: Executed management shell command: get-mailbox -Identity "d2ae2bf0-48a7-4ce9-9e72-bb3c765454ac" [19772] 110315.190911.1300: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.1310: Messaging: User Administrator is mail enabled and can use MessagingManagement to send mail. [19772] 110315.190911.1310: Messaging: Email address used for user: [email protected] [19772] 110315.190911.1440: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.1440: Messaging: Executed management shell command: get-group -Identity "Domain Admins" [19772] 110315.190911.1630: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.1640: Messaging: User Administrator is a member of Domain Admins and can use MessagingManagement to manage Exchange. [19772] 110315.190911.1640: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingTasks.GatherAdminInfo [19772] 110315.190911.1640: Messaging: MessagingManagement enabled for Exchange management: True [19772] 110315.190911.1640: Messaging: MessagingManagement enabled for mail submission: True [19772] 110315.190911.1640: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingTasks.Initialize [19772] 110315.190911.1640: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Tasks.TaskMoveExchangeData.CreateDataStoreDriveList [19772] 110315.190911.1670: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.Initialize [19772] 110315.190911.1670: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.1670: Messaging: Executed management shell command: get-storagegroup -Server "SERVER01" [19772] 110315.190911.2990: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.3070: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.Initialize [19772] 110315.190911.3070: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.3070: Messaging: Executed management shell command: get-mailboxdatabase -Server "SERVER01" [19772] 110315.190911.4440: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.4520: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.Initialize [19772] 110315.190911.4520: Messaging: Begin Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.4520: Messaging: Executed management shell command: get-publicfolderdatabase -Server "SERVER01" [19772] 110315.190911.5240: Messaging: End Microsoft.WindowsServerSolutions.Messaging.Management.MessagingRunspace.StaticExecute [19772] 110315.190911.5510: Storage: Data Store Drive/s Details:Name=C:\,Size=12675712420 [19772] 110315.190911.5510: Storage: Data Store Size Details: Current Total Size=12675712420 Required Size=12675712420 [19772] 110315.190911.5510: Storage: MoveData Task can move the Data Store=True [19772] 110315.190911.8401: Storage: An error was encountered when performing system diagnosis : ErrorCode:0 BaseException: Microsoft.WindowsServerSolutions.Storage.Common.StorageException: WMI error occurred while accessing drive ---> System.Management.ManagementException: Not found at System.Management.ManagementException.ThrowWithExtendedInfo(ManagementStatus errorCode) at System.Management.ManagementObjectCollection.ManagementObjectEnumerator.MoveNext() at Microsoft.WindowsServerSolutions.Storage.Common.DriveUtil.IsDriveRemovable(String drive) --- End of inner exception stack trace --- at Microsoft.WindowsServerSolutions.Storage.Common.DriveUtil.IsDriveRemovable(String drive) at Microsoft.WindowsServerSolutions.Storage.Common.DataStoreInfo.LoadAvailableDrives() at Microsoft.WindowsServerSolutions.Storage.Common.MoveDataUtil.CanMoveData(DataStoreInfo storeInfo, MoveDataError& error) at Microsoft.WindowsServerSolutions.Storage.MoveData.DiagnoseDataStorePagePresenter.DiagnoseDataStore(Object sender, DoWorkEventArgs args) [1516] 110315.190912.0331: Storage: An error occured during the execution: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> ErrorCode:0 BaseException: Microsoft.WindowsServerSolutions.Storage.Common.StorageException: Diagnosing the Data Store failed (see the inner exception) ---> ErrorCode:0 BaseException: Microsoft.WindowsServerSolutions.Storage.Common.StorageException: WMI error occurred while accessing drive ---> System.Management.ManagementException: Not found at System.Management.ManagementException.ThrowWithExtendedInfo(ManagementStatus errorCode) at System.Management.ManagementObjectCollection.ManagementObjectEnumerator.MoveNext() at Microsoft.WindowsServerSolutions.Storage.Common.DriveUtil.IsDriveRemovable(String drive) --- End of inner exception stack trace --- at Microsoft.WindowsServerSolutions.Storage.Common.DriveUtil.IsDriveRemovable(String drive) at Microsoft.WindowsServerSolutions.Storage.Common.DataStoreInfo.LoadAvailableDrives() at Microsoft.WindowsServerSolutions.Storage.Common.MoveDataUtil.CanMoveData(DataStoreInfo storeInfo, MoveDataError& error) at Microsoft.WindowsServerSolutions.Storage.MoveData.DiagnoseDataStorePagePresenter.DiagnoseDataStore(Object sender, DoWorkEventArgs args) at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument) --- End of inner exception stack trace --- at Microsoft.WindowsServerSolutions.Storage.MoveData.DiagnoseDataStorePagePresenter.backgroundWorker_RunWorkerCompleted(Object sender, RunWorkerCompletedEventArgs e) --- End of inner exception stack trace --- at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) at System.Delegate.DynamicInvokeImpl(Object[] args) at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme) at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj) at System.Threading.ExecutionContext.runTryCode(Object userData) at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme) at System.Windows.Forms.Control.InvokeMarshaledCallbacks() at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData) at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context) at Microsoft.WindowsServerSolutions.Common.Wizards.Framework.WizardFrameView.Create() at Microsoft.WindowsServerSolutions.Common.Wizards.Framework.WizardChainEngine.Launch() at Microsoft.WindowsServerSolutions.Storage.MoveData.MainClass.LaunchMoveDataWizard() at Microsoft.WindowsServerSolutions.Storage.MoveData.MainClass.Main(String[] args)

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