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  • Accidentally uninstalled Ubuntu 14.04 system settings. How to get them back?

    - by Dan
    Somehow, cleaning up useless software (using software center), I uninstalled Ubuntu "system settings". I did this by mistake. Now I fail to find system settings application using software center (and there are so many items in history...). It seems strange to me because usually when I try to uninstall something critical (system testing for example), the dependencies manager tells me It will uninstall the whole desktop system then. I am sure I did not have that warning. So I need the name of the software to install or a command line command rather than a system restore to get it back. Very interesting thing. If you ever want to play with this and reproduce it, you will be confused to see Ubuntu Mobile system settings instead! Yes, mobile network and touchscreen settings! Happy pre-release viewing!

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  • Is Haskell's type system an obstacle to understanding functional programming?

    - by FarmBoy
    I'm studying Haskell for the purpose of understanding functional programming, with the expectation that I'll apply the insight that I gain in other languages (Groovy, Python, JavaScript mainly.) I choose Haskell because I had the impression that it is very purely functional, and wouldn't allow for any reliance on state. I did not choose to learn Haskell because I was interested in navigating an extremely rigid type system. My question is this: Is a strong type system a necessary by-product of an extremely pure functional language, or is this an unrelated design choice particular to Haskell? If it is the latter, I'm curious what would be the most purely functional language that is dynamically typed. I'm not particularly opposed to strong typing, it has its place, but I'm having a hard time seeing how it benefits me in this educational endeavor.

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  • Is Haskell's type system an obstacle to understanding functional programming?

    - by Eric Wilson
    I'm studying Haskell for the purpose of understanding functional programming, with the expectation that I'll apply the insight that I gain in other languages (Groovy, Python, JavaScript mainly.) I choose Haskell because I had the impression that it is very purely functional, and wouldn't allow for any reliance on state. I did not choose to learn Haskell because I was interested in navigating an extremely rigid type system. My question is this: Is a strong type system a necessary by-product of an extremely pure functional language, or is this an unrelated design choice particular to Haskell? If it is the latter, I'm curious what would be the most purely functional language that is dynamically typed. I'm not particularly opposed to strong typing, it has its place, but I'm having a hard time seeing how it benefits me in this educational endeavor.

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  • How to verify system using right GPU, after system reset [duplicate]

    - by Antoros
    This question already has an answer here: Is my mobile AMD card being used? 2 answers OS: Windows 8 CPU: Intel® Core™ i7 Processor 3635QM GPU 1 : Intel HD Graphics 4000 GPU 2 : AMD Radeon™ HD 8870M other info: System Spects Problem: im unsure that CCC is using AMD card instead of Intel's, i have encountered several issues since updating to 8.1 and i don't know what to do What happened: Installed 8.1 patch first day After 1 minute of use, BBSOD, windows never loaded again System restore wouldnt recognize 8.0 restore points i did a system reset to windows 8 since the laptop was only 3 weeks old System Broke, it did restore to factory BUT kept the registry almost intact, i had to install almost everything again, since the factory drivers where working with the updated one's registry and several problems CCC Broke too <- What i've already done Installing new drivers on top of old ones didnt work, so i used AMD uninstaller first Uninstalled and Re-installed Intel's HD Graphics Driver Tried to install mobile center, but AMD told me that it wasnt compatible (even if thats the only driver that they provide via their page as seen Here) Tried to use Auto-Detect, couldnt install driver because card was disabled because it didnt have the drivers... (see what they did here?) Had to use a workaround with Samsung Update, the driver didnt appear as download so had to use search and downloaded the driver manually. Now the graphic card appears on device manager and catalyst but as 8800 series (not exact model), and cant check the card with neither dxdiag/GPU-z/HWMonitor when right-clicking on CCC only Intel card appears launching a game and using as "high performance" would speed it up a little but i cant be sure How to verify its working properly? HWMonitor wont show AMD card even when set to high performance Latest GPU-Z wont work because a problem with Intel's, and legacy ones wont either what can I do now? I don't even know if I fixed my problem or not, and i also want to to use Adobe Premier with it, and its locked (the option to run it with the amd card not intels) Edit: now it seems to work, but cant change the setting for adobe Premiere and other programs that i Need to

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  • Windows Server 2008 R2 stuck at System Recovery Options

    - by Magnus
    One of my Windows Server 2008 R2 server has started to go into System Recovery Options at reboot. I have tried to let it go through a System Image Recovery, which it says completes successfully. But after reboot it again wants to do a system recovery. Safe mode doesn't work; it still wants to do a system recovery. Putting in the install DVD trying to do a repair actually brings up the very same System Revoery dialog again. Nothing has changed configuration wise. Any ideas?

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  • Windows Server 2008 R2 stuck at System Recovery Options

    - by Magnus
    My Windows Server 2008 R2 has started to go into System Recovery Options at reboot. I have tried to let it go through a System Image Recovery, which it says completes successfully. But after reboot it again wants to do a system recovery. Safe mode doesn't work; it still wants to do a system recovery. Putting in the install DVD trying to do a repair actually brings up the very same System Revoery dialog again. Nothing has changed configuration wise. Any ideas?

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  • Identity Claims Encoding for SharePoint

    - by Shawn Cicoria
    Just to remind myself, the list of claim types and their encodings are listed here at the bottom. http://msdn.microsoft.com/en-us/library/gg481769.aspx Where for example: i:0#.w|contoso\scicoria ‘i’ = identity, could be ‘c’ for others # == SPClaimTypes.UserLogonName . == Microsoft.IdentityModel.Claims.ClaimValueTypes.String Table for reference: Table 1. Claim types encoding Character Claim Type ! SPClaimTypes.IdentityProvider ” SPClaimTypes.UserIdentifier # SPClaimTypes.UserLogonName $ SPClaimTypes.DistributionListClaimType % SPClaimTypes.FarmId & SPClaimTypes.ProcessIdentitySID ‘ SPClaimTypes.ProcessIdentityLogonName ( SPClaimTypes.IsAuthenticated ) Microsoft.IdentityModel.Claims.ClaimTypes.PrimarySid * Microsoft.IdentityModel.Claims.ClaimTypes.PrimaryGroupSid + Microsoft.IdentityModel.Claims.ClaimTypes.GroupSid - Microsoft.IdentityModel.Claims.ClaimTypes.Role . System.IdentityModel.Claims.ClaimTypes.Anonymous / System.IdentityModel.Claims.ClaimTypes.Authentication 0 System.IdentityModel.Claims.ClaimTypes.AuthorizationDecision 1 System.IdentityModel.Claims.ClaimTypes.Country 2 System.IdentityModel.Claims.ClaimTypes.DateOfBirth 3 System.IdentityModel.Claims.ClaimTypes.DenyOnlySid 4 System.IdentityModel.Claims.ClaimTypes.Dns 5 System.IdentityModel.Claims.ClaimTypes.Email 6 System.IdentityModel.Claims.ClaimTypes.Gender 7 System.IdentityModel.Claims.ClaimTypes.GivenName 8 System.IdentityModel.Claims.ClaimTypes.Hash 9 System.IdentityModel.Claims.ClaimTypes.HomePhone < System.IdentityModel.Claims.ClaimTypes.Locality = System.IdentityModel.Claims.ClaimTypes.MobilePhone > System.IdentityModel.Claims.ClaimTypes.Name ? System.IdentityModel.Claims.ClaimTypes.NameIdentifier @ System.IdentityModel.Claims.ClaimTypes.OtherPhone [ System.IdentityModel.Claims.ClaimTypes.PostalCode \ System.IdentityModel.Claims.ClaimTypes.PPID ] System.IdentityModel.Claims.ClaimTypes.Rsa ^ System.IdentityModel.Claims.ClaimTypes.Sid _ System.IdentityModel.Claims.ClaimTypes.Spn ` System.IdentityModel.Claims.ClaimTypes.StateOrProvince a System.IdentityModel.Claims.ClaimTypes.StreetAddress b System.IdentityModel.Claims.ClaimTypes.Surname c System.IdentityModel.Claims.ClaimTypes.System d System.IdentityModel.Claims.ClaimTypes.Thumbprint e System.IdentityModel.Claims.ClaimTypes.Upn f System.IdentityModel.Claims.ClaimTypes.Uri g System.IdentityModel.Claims.ClaimTypes.Webpage Table 2. Claim value types encoding Character Claim Type ! Microsoft.IdentityModel.Claims.ClaimValueTypes.Base64Binary “ Microsoft.IdentityModel.Claims.ClaimValueTypes.Boolean # Microsoft.IdentityModel.Claims.ClaimValueTypes.Date $ Microsoft.IdentityModel.Claims.ClaimValueTypes.Datetime % Microsoft.IdentityModel.Claims.ClaimValueTypes.DaytimeDuration & Microsoft.IdentityModel.Claims.ClaimValueTypes.Double ‘ Microsoft.IdentityModel.Claims.ClaimValueTypes.DsaKeyValue ( Microsoft.IdentityModel.Claims.ClaimValueTypes.HexBinary ) Microsoft.IdentityModel.Claims.ClaimValueTypes.Integer * Microsoft.IdentityModel.Claims.ClaimValueTypes.KeyInfo + Microsoft.IdentityModel.Claims.ClaimValueTypes.Rfc822Name - Microsoft.IdentityModel.Claims.ClaimValueTypes.RsaKeyValue . Microsoft.IdentityModel.Claims.ClaimValueTypes.String / Microsoft.IdentityModel.Claims.ClaimValueTypes.Time 0 Microsoft.IdentityModel.Claims.ClaimValueTypes.X500Name 1 Microsoft.IdentityModel.Claims.ClaimValueTypes.YearMonthDuration

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  • Thoughts on type aliases/synonyms?

    - by Rei Miyasaka
    I'm going to try my best to frame this question in a way that doesn't result in a language war or list, because I think there could be a good, technical answer to this question. Different languages support type aliases to varying degrees. C# allows type aliases to be declared at the beginning of each code file, and they're valid only throughout that file. Languages like ML/Haskell use type aliases probably as much as they use type definitions. C/C++ are sort of a Wild West, with typedef and #define often being used seemingly interchangeably to alias types. The upsides of type aliasing don't invoke too much dispute: It makes it convenient to define composite types that are described naturally by the language, e.g. type Coordinate = float * float or type String = [Char]. Long names can be shortened: using DSBA = System.Diagnostics.DebuggerStepBoundaryAttribute. In languages like ML or Haskell, where function parameters often don't have names, type aliases provide a semblance of self-documentation. The downside is a bit more iffy: aliases can proliferate, making it difficult to read and understand code or to learn a platform. The Win32 API is a good example, with its DWORD = int and its HINSTANCE = HANDLE = void* and its LPHANDLE = HANDLE FAR* and such. In all of these cases it hardly makes any sense to distinguish between a HANDLE and a void pointer or a DWORD and an integer etc.. Setting aside the philosophical debate of whether a king should give complete freedom to their subjects and let them be responsible for themselves or whether they should have all of their questionable actions intervened, could there be a happy medium that would allow the benefits of type aliasing while mitigating the risk of its abuse? As an example, the issue of long names can be solved by good autocomplete features. Visual Studio 2010 for instance will alllow you to type DSBA in order to refer Intellisense to System.Diagnostics.DebuggerStepBoundaryAttribute. Could there be other features that would provide the other benefits of type aliasing more safely?

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  • Whys is System process listening on Port 80?

    - by Seth Spearman
    I am running Windows 7 RC1. I have multiple issues getting IIS to work on my system and today when I installed a new application and I tried to load it using http:\localhost\MyApplication I get absolutely no errors and I get no page load. Just a pretty, white blank page. I did some digging and I found something about some other process listening on port 80 so I did a scan using netstat -aon | findstr 0.0:80 and discovered that PID 4 was listening on that port. PID 4 does not show in task manager so I fired up Process Explorer and it showed me that PID 4 is the System process. (Multiple google searches seems to indicate that System always uses PID 4). Since then I am basically stuck. I have no idea why System needs port 80 and what to do about it. If you google the following strings you will find two helpful Experts-Exchange articles at the top of the search results and you can read them for some helpful information. (If I gave the direct URL to the pages then Experts-Exchange would ask you to pay...but when you click on the results from a google search you can scroll all of the way to the bottom to read the exchanges.) Here are the google searches... "System Process is listening on port 80 (Vista)" "SYSTEM Process is listening on Port 80 and Preventing IIS Default Website from Running" The last entry from the first result showed how to do a trace of http.sys at the following URL: http://blogs.msdn.com/wndp/archive/2007/01/18/event-tracing-in-http-sys-part-1-capturing-a-trace.aspx Trace showed nothing useful. Any thoughts?

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  • How to remove NTFS system files from a previous Vista installation

    - by Boldewyn
    I'm trying to shrink my system partition under Win Vista. It's all fine, except that in front of the last 300MB of the volume sits a single file, that cannot be moved by defrag or other means from its position. It's called C:\$Extend\$UsnJrnl:$J, and my assumtion is, that it is left from a previous installation of Vista, when I re-set up the system. Now, googling for this kind of files brings interesting results, but no solution to my problem: Files left on the disk can become ownerless in a new setup of Windows and inaccessible (even for administrators). To be able to access them again, I found the tip to use takeown to re-assign them to the Admin group (or anyone else). Works like a charm for normal files, but not for the C:\$Extend stuff. The C:\$Extend folder is a system folder of the NTFS file system, where the journal is stored (especially in a file called $UsnJrnl:$Data, whose name is surprisingly close to mine). You can delete the journal with fsutil usn /delete C:, however, this doesn't work from within the booted system (as I found out trying). Also, I'm not quite sure of the side effects. You can't move the NTFS own files with standard defrag tools. The same holds, by the way, for not accessible files. Every bit of knowledge out there is targeted to either not accessible files or the $Extend NTFS stuff, but noone addresses my problem involving both, an inaccessible system file. Question: How can I remove this file, or at least how can I move it on the disk?

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  • Efficiently separating Read/Compute/Write steps for concurrent processing of entities in Entity/Component systems

    - by TravisG
    Setup I have an entity-component architecture where Entities can have a set of attributes (which are pure data with no behavior) and there exist systems that run the entity logic which act on that data. Essentially, in somewhat pseudo-code: Entity { id; map<id_type, Attribute> attributes; } System { update(); vector<Entity> entities; } A system that just moves along all entities at a constant rate might be MovementSystem extends System { update() { for each entity in entities position = entity.attributes["position"]; position += vec3(1,1,1); } } Essentially, I'm trying to parallelise update() as efficiently as possible. This can be done by running entire systems in parallel, or by giving each update() of one system a couple of components so different threads can execute the update of the same system, but for a different subset of entities registered with that system. Problem In reality, these systems sometimes require that entities interact(/read/write data from/to) each other, sometimes within the same system (e.g. an AI system that reads state from other entities surrounding the current processed entity), but sometimes between different systems that depend on each other (i.e. a movement system that requires data from a system that processes user input). Now, when trying to parallelize the update phases of entity/component systems, the phases in which data (components/attributes) from Entities are read and used to compute something, and the phase where the modified data is written back to entities need to be separated in order to avoid data races. Otherwise the only way (not taking into account just "critical section"ing everything) to avoid them is to serialize parts of the update process that depend on other parts. This seems ugly. To me it would seem more elegant to be able to (ideally) have all processing running in parallel, where a system may read data from all entities as it wishes, but doesn't write modifications to that data back until some later point. The fact that this is even possible is based on the assumption that modification write-backs are usually very small in complexity, and don't require much performance, whereas computations are very expensive (relatively). So the overhead added by a delayed-write phase might be evened out by more efficient updating of entities (by having threads work more % of the time instead of waiting). A concrete example of this might be a system that updates physics. The system needs to both read and write a lot of data to and from entities. Optimally, there would be a system in place where all available threads update a subset of all entities registered with the physics system. In the case of the physics system this isn't trivially possible because of race conditions. So without a workaround, we would have to find other systems to run in parallel (which don't modify the same data as the physics system), other wise the remaining threads are waiting and wasting time. However, that has disadvantages Practically, the L3 cache is pretty much always better utilized when updating a large system with multiple threads, as opposed to multiple systems at once, which all act on different sets of data. Finding and assembling other systems to run in parallel can be extremely time consuming to design well enough to optimize performance. Sometimes, it might even not be possible at all because a system just depends on data that is touched by all other systems. Solution? In my thinking, a possible solution would be a system where reading/updating and writing of data is separated, so that in one expensive phase, systems only read data and compute what they need to compute, and then in a separate, performance-wise cheap, write phase, attributes of entities that needed to be modified are finally written back to the entities. The Question How might such a system be implemented to achieve optimal performance, as well as making programmer life easier? What are the implementation details of such a system and what might have to be changed in the existing EC-architecture to accommodate this solution?

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  • Mixing Silverlight-Specific System.Xml.Linq dll with Non-Silverlight System.Xml.Linq dll

    - by programatique
    I have a Logic layer that references Silverlight's System.Xml.Linq dll and a GUI that is in WPF (hence using the non-Silverlight System.Xml.Linq dll). When I attempt to pass an XElement from GUI project to a method in the Logic project, I am getting (basically) "XElement is not of type XElement" errors. To complicate matter, I am unable to edit the Logic layer project. The Non-Silverlight DLL is at: C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Xml.Linq.dll THe Silverlight DLL is at: C:\Program Files (x86)\Microsoft SDKs\Silverlight\v3.0\Libraries\Client\System.Xml.Linq.dll I am new to C# but I'm fairly sure my issue is that I am referencing different DLL's to access the System.Xml.Linq namespace. I attempted to replace my non-Silverlight System.Xml.Linq.dll with the Silverlight's System.Xml.Linq.dll, but received assembly errors. Is there any way to resolve this short of scrapping my WPF GUI project and creating a Silverlight project?

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  • HTTP Content-Type in ASP.Net SoapHttpClientProtocol

    - by Daniel Fone
    Hi there, I have a problem with a Web Service Consumer written in ASP.NET. The error message is: System.InvalidOperationException: Client found response content type of 'application/xml; charset=utf-8', but expected 'text/xml'. The client is based on System.Web.Services.Protocols.SoapHttpClientProtocol. We can't change the Content-Type given by the provider, this has to be 'application/xml; charset=utf-8'. Is there any way to change what Content-Type the SoapHttpClientProtocol expects? Unfortunately, we are probably limited to .NET 1.1. Thanks! Update: We found a way to change the Content-Type sent by the provider, and this solved the problem. I'd still be curious to know how to change the expectations of the consumer though.

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  • Getting ActiveRecord (Rails) to_xml to use xsi:nil and xsi:type instead of nil and type

    - by nbeyer
    The default behavior of XML serialization (to_xml) for ActiveRecord objects will emit 'type' and 'nil' attributes that are similar to XML Schema Instance attributes, but aren't set in a XML Namespace. For example, a model might produce an output like this: <user> <username nil="true" /> <first-name type="string">Name</first-name> </user> Is there anyway to get to_xml to utilize the XML Schema Instance namespace and prefix the attributes and the values? Using the above example, I'd like to produce the following: <user xmlns:xsi="http://www.w3.org/1999/XMLSchema-instance" xmlns:xs="http://www.w3.org/1999/XMLSchema"> <username xsi:nil="true" /> <first-name xsi:type="xs:string">Name</first-name> </user>

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  • Unable to cast object of type 'System.Object[]' to type 'System.String[]'

    - by salvationishere
    I am developing a C# VS 2008 / SQL Server website application. I am a newbie to ASP.NET. I am getting the above error, however, on the last line of the following code. Can you give me advice on how to fix this? This compiles correctly, but I encounter this error after running it. DataTable dt; Hashtable ht; string[] SingleRow; ... SqlConnection conn2 = new SqlConnection(connString); SqlCommand cmd = conn2.CreateCommand(); cmd.CommandText = "dbo.AppendDataCT"; cmd.Connection = conn2; SingleRow = (string[])dt.Rows[1].ItemArray; My error: System.InvalidCastException was caught Message="Unable to cast object of type 'System.Object[]' to type 'System.String[]'." Source="App_Code.g68pyuml" StackTrace: at ADONET_namespace.ADONET_methods.AppendDataCT(DataTable dt, Hashtable ht) in c:\Documents and Settings\Admin\My Documents\Visual Studio 2008\WebSites\Jerry\App_Code\ADONET methods.cs:line 88 InnerException:

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  • Type attribute for text_field

    - by Koning Baard XIV
    I have this code: <%= f.text_field :email, :type => "email", :placeholder => "[email protected]" %> So people can enter their email on an iPhone with the email keyboard instead of the ASCII keyboard. However, the output is: <input id="user_email" name="user[email]" placeholder="[email protected]" size="30" type="text" /> which should be: <input id="user_email" name="user[email]" placeholder="[email protected]" size="30" type="email" /> Is there a way to force Rails to use the email type instead of text, or must I use HTML directly? Thanks

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  • postgres - sharing same pg_hba.conf between IPV4 system and IPV6 system

    - by StackTrace
    I am trying to package postgres from one machine to another. The source is windows 7 with IP V6 and target is windows XP with IPv4. Starting postgres on windows XP gives error 2010-11-01 12:01:07 IST LOG: invalid IP address "::1": Unknown host 2010-11-01 12:01:07 IST CONTEXT: line 76 of configuration file "C:/postgres/data/pg_hba.conf" 2010-11-01 12:01:07 IST FATAL: could not load pg_hba.conf -- postgres - sharing same pg_hba.conf between IpV4 system and IpV6 system Here is how my pg_hba.conf looks like # TYPE DATABASE USER CIDR-ADDRESS METHOD # IPv4 local connections: host all all 127.0.0.1/32 trust # IPv6 local connections: host all all ::1/128 trust

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  • wpf: design time error while writing Nested type in xaml

    - by viky
    I have created a usercontrol which accept type of enum and assign the values of that enum to a ComboBox control in that usercontrol. Very Simple. I am using this user control in DataTemplates. Problem comes when there comes nested type. I assign that using this notation EnumType="{x:Type myNamespace:ParentType + NestedType}" It works fine at runtime. but at design time it throws error saying Could not create an instance of type 'TypeExtension' Why? Due to this I am not able to see my window at design time. Any help?

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  • Java class object from type variable

    - by Alexander Temerev
    Is there a way to get Class object from the type variable in Java generic class? Something like that: public class Bar extends Foo<T> { public Class getParameterClass() { return T.class; // doesn't compile } } This type information is available at compile time and therefore should not be affected by type erasure, so, theoretically, there should be a way to accomplish this. Does it exist?

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  • Unboxing to unknown type

    - by Robert
    I'm trying to figure out syntax that supports unboxing an integral type (short/int/long) to its intrinsic type, when the type itself is unknown. Here is a completely contrived example that demonstrates the concept: // Just a simple container that returns values as objects struct DataStruct { public short ShortVale; public int IntValue; public long LongValue; public object GetBoxedShortValue() { return LongValue; } public object GetBoxedIntValue() { return LongValue; } public object GetBoxedLongValue() { return LongValue; } } static void Main( string[] args ) { DataStruct data; // Initialize data - any value will do data.LongValue = data.IntValue = data.ShortVale = 42; DataStruct newData; // This works if you know the type you are expecting! newData.ShortVale = (short)data.GetBoxedShortValue(); newData.IntValue = (int)data.GetBoxedIntValue(); newData.LongValue = (long)data.GetBoxedLongValue(); // But what about when you don't know? newData.ShortVale = data.GetBoxedShortValue(); // error newData.IntValue = data.GetBoxedIntValue(); // error newData.LongValue = data.GetBoxedLongValue(); // error } In each case, the integral types are consistent, so there should be some form of syntax that says "the object contains a simple type of X, return that as X (even though I don't know what X is)". Because the objects ultimately come from the same source, there really can't be a mismatch (short != long). I apologize for the contrived example, it seemed like the best way to demonstrate the syntax. Thanks.

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  • Explain ML type inference to a C++ programmer

    - by Tsubasa Gomamoto
    How does ML perform the type inference in the following function definition: let add a b = a + b Is it like C++ templates where no type-checking is performed until the point of template instantiation after which if the type supports the necessary operations, the function works or else a compilation error is thrown ? i.e. for example, the following function template template <typename NumType> NumType add(NumType a, NumType b) { return a + b; } will work for add<int>(23, 11); but won't work for add<ostream>(cout, fout); Is what I am guessing is correct or ML type inference works differently? PS: Sorry for my poor English; it's not my native language.

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  • functional dependencies vs type families

    - by mhwombat
    I'm developing a framework for running experiments with artificial life, and I'm trying to use type families instead of functional dependencies. Type families seems to be the preferred approach among Haskellers, but I've run into a situation where functional dependencies seem like a better fit. Am I missing a trick? Here's the design using type families. (This code compiles OK.) {-# LANGUAGE TypeFamilies, FlexibleContexts #-} import Control.Monad.State (StateT) class Agent a where agentId :: a -> String liveALittle :: Universe u => a -> StateT u IO a -- plus other functions class Universe u where type MyAgent u :: * withAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String -> StateT u IO () -- plus other functions data SimpleUniverse = SimpleUniverse { mainDir :: FilePath -- plus other fields } defaultWithAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String -> StateT u IO () defaultWithAgent = undefined -- stub -- plus default implementations for other functions -- -- In order to use my framework, the user will need to create a typeclass -- that implements the Agent class... -- data Bug = Bug String deriving (Show, Eq) instance Agent Bug where agentId (Bug s) = s liveALittle bug = return bug -- stub -- -- .. and they'll also need to make SimpleUniverse an instance of Universe -- for their agent type. -- instance Universe SimpleUniverse where type MyAgent SimpleUniverse = Bug withAgent = defaultWithAgent -- boilerplate -- plus similar boilerplate for other functions Is there a way to avoid forcing my users to write those last two lines of boilerplate? Compare with the version using fundeps, below, which seems to make things simpler for my users. (The use of UndecideableInstances may be a red flag.) (This code also compiles OK.) {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} import Control.Monad.State (StateT) class Agent a where agentId :: a -> String liveALittle :: Universe u a => a -> StateT u IO a -- plus other functions class Universe u a | u -> a where withAgent :: Agent a => (a -> StateT u IO a) -> String -> StateT u IO () -- plus other functions data SimpleUniverse = SimpleUniverse { mainDir :: FilePath -- plus other fields } instance Universe SimpleUniverse a where withAgent = undefined -- stub -- plus implementations for other functions -- -- In order to use my framework, the user will need to create a typeclass -- that implements the Agent class... -- data Bug = Bug String deriving (Show, Eq) instance Agent Bug where agentId (Bug s) = s liveALittle bug = return bug -- stub -- -- And now my users only have to write stuff like... -- u :: SimpleUniverse u = SimpleUniverse "mydir"

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  • Cannot initialize non-const reference from convertible type

    - by Julien L.
    Hi, I cannot initialize a non-const reference to type T1 from a convertible type T2. However, I can with a const reference. long l; const long long &const_ref = l; // fine long long &ref = l; // error: invalid initialization of reference of // type 'long long int&' from expression of type // 'long int' Most problems I encountered were related to r-values that cannot be assigned to a non-const reference. This is not the case here -- can someone explain? Thanks.

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