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• How do I manipulate a tree of immutable objects?

  - by Frederik

  I'm building an entire application out of immutable objects so that multi-threading and undo become easier to implement. I'm using the Google Collections Library which provides immutable versions of Map, List, and Set. My application model looks like a tree: Scene is a top-level object that contains a reference to a root Node. Each Node can contain child Nodes and Ports. An object graph might look like this: Scene | +-- Node | +-- Node | +- Port +-- Node | +- Port +- Port If all of these objects are immutable, controlled by a top-level SceneController object: What is the best way to construct this hierarchy? How would I replace an object that is arbitrarily deep in the object tree? Is there a way to support back-links, e.g. a Node having a "parent" attribute?

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• ImmutableDictionary has no constructors defined

  - by lukasLansky

  So, I would like to write something like this: var d = new ImmutableDictionary<string, int> { { "a", 1 }, { "b", 2 } }; (using ImmutableDictionary from System.Collections.Immutable). It seems like a straightforward usage as I am declaring all the values upfront -- no mutation there. But this gives me error: The type 'System.Collections.Immutable.ImmutableDictionary<TKey,TValue>' has no constructors defined How I am supposed to create a new immutable dictionary with static content?

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• Advantages of compilers for functional languages over compilers for imperative languages

  - by Onorio Catenacci

  As a follow up to this question What are the advantages of built-in immutability of F# over C#?--am I correct in assuming that the F# compiler can make certain optimizations knowing that it's dealing with largely immutable code? I mean even if a developer writes "Functional C#" the compiler wouldn't know all of the immutability that the developer had tried to code in so that it couldn't make the same optimizations, right? In general would the compiler of a functional language be able to make optimizations that would not be possible with an imperative language--even one written with as much immutability as possible?

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• Value objects in DDD - Why immutable?

  - by Hobbes

  I don't get why value objects in DDD should be immutable, nor do I see how this is easily done. (I'm focusing on C# and Entity Framework, if that matters.) For example, let's consider the classic Address value object. If you needed to change "123 Main St" to "123 Main Street", why should I need to construct a whole new object instead of saying myCustomer.Address.AddressLine1 = "123 Main Street"? (Even if Entity Framework supported structs, this would still be a problem, wouldn't it?) I understand (I think) the idea that value objects don't have an identity and are part of a domain object, but can someone explain why immutability is a Good Thing? EDIT: My final question here really should be "Can someone explain why immutability is a Good Thing as applied to Value Objects?" Sorry for the confusion! EDIT: To clairfy, I am not asking about CLR value types (vs reference types). I'm asking about the higher level DDD concept of Value Objects. For example, here is a hack-ish way to implement immutable value types for Entity Framework: http://rogeralsing.com/2009/05/21/entity-framework-4-immutable-value-objects. Basically, he just makes all setters private. Why go through the trouble of doing this?

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• What Functional features are worth a little OOP confusion for the benefits they bring?

  - by bonomo

  After learning functional programming in Haskell and F#, the OOP paradigm seems ass-backwards with classes, interfaces, objects. Which aspects of FP can I bring to work that my co-workers can understand? Are any FP styles worth talking to my boss about retraining my team so that we can use them? Possible aspects of FP: Immutability Partial Application and Currying First Class Functions (function pointers / Functional Objects / Strategy Pattern) Lazy Evaluation (and Monads) Pure Functions (no side effects) Expressions (vs. Statements - each line of code produces a value instead of, or in addition to causing side effects) Recursion Pattern Matching Is it a free-for-all where we can do whatever the programming language supports to the limit that language supports it? Or is there a better guideline?

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• Should a primary key be immutable?

  - by Vincent Malgrat

  A recent question on stackoverflow provoked a discussion about the immutability of primary keys. I had thought that it was a kind of rule that primary keys should be immutable. If there is a chance that some day a primary key would be updated, I thought you should use a surrogate key. However it is not in the SQL standard and some RDBMS' "cascade update" feature allows a primary key to change. So my question is: is it still a bad practice to have a primary key that may change ? What are the cons, if any, of having a mutable primary key ?

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• Immutable design with an ORM: How are sessions managed?

  - by Programmin Tool

  If I were to make a site with a mutable language like C# and use NHibernate, I would normally approach sessions with the idea of making them as create only when needed and dispose at request end. This has helped with keeping a session for multiple transactions by a user but keep it from staying open too long where the state might be corrupted. In an immutable system, like F#, I would think I shouldn't do this because it supposes that a single session could be updated constantly by any number of inserts/updates/deletes/ect... I'm not against the "using" solution since I would think that connecting pooling will help cut down on the cost of connecting every time, but I don't know if all database systems do connection pooling. It just seems like there should be a better way that doesn't compromise the immutability goal. Should I just do a simple "using" block per transaction or is there a better pattern for this?

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• Impact of Multiple .ToUpper()'ing

  - by Kyle Rozendo

  Hi All, This is not a question of premature optimization per se. On the garbage collector and memory in general, what would hundreds of ToUpper() operations (many could be duplicated) do to a program, mainly in regard to the immutability of strings? Thanks, Kyle

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• Modern Java alternatives

  - by Ralph

  I'm not sure if stackoverflow is the best forum for this discussion. I have been a Java developer for 14 years and have written an enterprise-level (~500,000 line) Swing application that uses most of the standard library APIs. Recently, I have become disappointed with the progress that the language has made to "modernize" itself, and am looking for an alternative for ongoing development. I have considered moving to the .NET platform, but I have issues with using something the only runs well in Windows (I know about Mono, but that is still far behind Microsoft). I also plan on buying a new Macbook Pro as soon as Apple releases their new rumored Arrandale-based machines and want to develop in an environment that will feel "at home" in Unix/Linux. I have considered using Python or Ruby, but the standard Java library is arguably the largest of any modern language. In JVM-based languages, I looked at Groovy, but am disappointed with its performance. Rumor has it that with the soon-to-be released JDK7, with its InvokeDynamic instruction, this will improve, but I don't know how much. Groovy is also not truly a functional language, although it provides closures and some of the "functional" features on collections. It does not embrace immutability. I have narrowed my search down to two JVM-based alternatives: Scala and Clojure. Each has its strengths and weaknesses. I am looking for the stackoverflow readerships' opinions. I am not an expert at either of these languages; I have read 2 1/2 books on Scala and am currently reading Stu Halloway's book on Clojure. Scala is strongly statically typed. I know the dynamic language folks claim that static typing is a crutch for not doing unit testing, but it does provide a mechanism for compile-time location of a whole class of errors. Scala is more concise than Java, but not as much as Clojure. Scala's inter-operation with Java seems to be better than Clojure's, in that most Java operations are easier to do in Scala than in Clojure. For example, I can find no way in Clojure to create a non-static initialization block in a class derived from a Java superclass. For example, I like the Apache commons CLI library for command line argument parsing. In Java and Scala, I can create a new Options object and add Option items to it in an initialization block as follows (Java code): final Options options = new Options() { { addOption(new Option("?", "help", false, "Show this usage information"); // other options } }; I can't figure out how to the same thing in Clojure (except by using (doit...)), although that may reflect my lack of knowledge of the language. Clojure's collections are optimized for immutability. They rarely require copy-on-write semantics. I don't know if Scala's immutable collections are implemented using similar algorithms, but Rich Hickey (Clojure's inventor) goes out of his way to explain how that language's data structures are efficient. Clojure was designed from the beginning for concurrency (as was Scala) and with modern multi-core processors, concurrency takes on more importance, but I occasionally need to write simple non-concurrent utilities, and Scala code probably runs a little faster for these applications since it discourages, but does not prohibit, "simple" mutability. One could argue that one-off utilities do not have to be super-fast, but sometimes they do tasks that take hours or days to complete. I know that there is no right answer to this "question", but I thought I would open it up for discussion. If anyone has a suggestion for another JVM-based language that can be used for enterprise level development, please list it. Also, it is not my intent to start a flame war. Thanks, Ralph

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• Modern alternatives to Java

  - by Ralph

  I have been a Java developer for 14 years and have written an enterprise-level (~500 kloc) Swing application that uses most of the standard library APIs. Recently, I have become disappointed with the progress that the language has made to "modernize" itself, and am looking for an alternative for ongoing development. I have considered moving to the .NET platform, but I have issues with using something the only runs well in Windows (I know about Mono, but that is still far behind Microsoft). I also plan on buying a new Macbook Pro as soon as Apple releases their new rumored Arrandale-based machines and want to develop in an environment that will feel "at home" in Unix/Linux. I have considered using Python or Ruby, but the standard Java library is arguably the largest of any modern language. In JVM-based languages, I looked at Groovy, but am disappointed with its performance. Rumor has it that with the soon-to-be released JDK7, with its InvokeDynamic instruction, this will improve, but I don't know how much. Groovy is also not truly a functional language, although it provides closures and some of the "functional" features on collections. It does not embrace immutability. I have narrowed my search down to two JVM-based alternatives: Scala and Clojure. Each has its strengths and weaknesses. I am looking for opinions. I am not an expert at either of these languages; I have read 2 1/2 books on Scala and am currently reading Stu Halloway's book on Clojure. Scala is strongly statically typed. I know the dynamic language folks claim that static typing is a crutch for not doing unit testing, but it does provide a mechanism for compile-time location of a whole class of errors. Scala is more concise than Java, but not as much as Clojure. Scala's inter-operation with Java seems to be better than Clojure's, in that most Java operations are easier to do in Scala than in Clojure. For example, I can find no way in Clojure to create a non-static initialization block in a class derived from a Java superclass. For example, I like the Apache commons CLI library for command line argument parsing. In Java and Scala, I can create a new Options object and add Option items to it in an initialization block as follows (Java code): final Options options = new Options() { { addOption(new Option("?", "help", false, "Show this usage information"); // other options } }; I can't figure out how to the same thing in Clojure (except by using (doit...)), although that may reflect my lack of knowledge of the language. Clojure's collections are optimized for immutability. They rarely require copy-on-write semantics. I don't know if Scala's immutable collections are implemented using similar algorithms, but Rich Hickey (Clojure's inventor) goes out of his way to explain how that language's data structures are efficient. Clojure was designed from the beginning for concurrency (as was Scala) and with modern multi-core processors, concurrency takes on more importance, but I occasionally need to write simple non-concurrent utilities, and Scala code probably runs a little faster for these applications since it discourages, but does not prohibit, "simple" mutability. One could argue that one-off utilities do not have to be super-fast, but sometimes they do tasks that take hours or days to complete. I know that there is no right answer to this "question", but I thought I would open it up for discussion. Are there other JVM-based languages that can be used for enterprise level development?

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• Uses of persistent data structures in non-functional languages

  - by Ray Toal

  Languages that are purely functional or near-purely functional benefit from persistent data structures because they are immutable and fit well with the stateless style of functional programming. But from time to time we see libraries of persistent data structures for (state-based, OOP) languages like Java. A claim often heard in favor of persistent data structures is that because they are immutable, they are thread-safe. However, the reason that persistent data structures are thread-safe is that if one thread were to "add" an element to a persistent collection, the operation returns a new collection like the original but with the element added. Other threads therefore see the original collection. The two collections share a lot of internal state, of course -- that's why these persistent structures are efficient. But since different threads see different states of data, it would seem that persistent data structures are not in themselves sufficient to handle scenarios where one thread makes a change that is visible to other threads. For this, it seems we must use devices such as atoms, references, software transactional memory, or even classic locks and synchronization mechanisms. Why then, is the immutability of PDSs touted as something beneficial for "thread safety"? Are there any real examples where PDSs help in synchronization, or solving concurrency problems? Or are PDSs simply a way to provide a stateless interface to an object in support of a functional programming style?

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• Coding guidelines + Best Practices?

  - by Chathuranga Chandrasekara

  I couldn't find any question that directly applies to my query so I am posting this as a new question. If there is any existing discussion that may help me, please point it out and close the question. Question: I am going to do a presentation on C# coding guidelines but it is not supposed to limit to coding standards. So I have a rough idea but I think I need to address good programing practices. So the contents will be something like this. Basic coding standards - Casing, Formatting etc. Good practices - Usage of Hashset over other data structures, String vs String Builder, String's immutability and using them effectively etc Really I would like to add more good practices (Especially to improve the performance.) So like to hear some more good practices to be used with C#. Any suggestions??? (No need of large descriptions :) Just the idea is sufficient.)

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• Coding guidelines + Best Practises?

  - by Chathuranga Chandrasekara

  I couldn't find any question that directly applies to my query so I am posting this as a new question. If there is any existing discussion that may help me, please point it out and close the question. Question: I am going to do a presentation on C# coding guidelines but it is not supposed to limit to coding standards. So I have a rough idea but I think I need to address good programing practices. So the contents will be something like this. Basic coding standards - Casing, Formatting etc. Good practices - Usage of Hashset over other data structures, String vs String Builder, String's immutability and using them effectively etc Really I would like to add more good practices (Especially to improve the performance.) So like to hear some more good practices to be used with C#. Any suggestions??? (No need of large descriptions :) Just the idea is sufficient.)

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• Persistence provider for Java that supports final fields

  - by naeron84

  I'm very new to Java but I've been developing a habit to use final wherever possible declaring immutability which i think is a good thing. (Consider f#) I've read that JPA does not support final fields. Hibernate, TopLink? I'm not sure about these but i prefer JPA for now. Is that even possible theoretically - let's say through reflection - to modify final fields after creation? My guess would be... NO :) What would be certainly possible for a persistence solution is to support constructors with parameters. At least i see no reason that would make this impossible. Mapping would be a little tricky i guess. This is an alternative solution. Suggestions?

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• java serialization and final fields

  - by mdma

  I have an class defining an immutable value type that I now need to serialize. The immutability comes from the final fields which are set in the constructor. I've tried serializing, and it works (surprisingly?) - but I've no idea how. Here's an example of the class public class MyValueType implements Serializable { private final int value; private transient int derivedValue; public MyValueType(int value) { this.value = value; this.derivedValue = derivedValue(value); } // getters etc... } Given that the class doesn't have a no arg constructor, how can it be instantiated and the final field set? (An aside - I noticed this class particularly because IDEA wasn't generating a "no serialVersionUID" inspection warning for this class, yet successfully generated warnings for other classes that I've just made serializable.)

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• Method hiding with interfaces

  - by fearofawhackplanet

  interface IFoo { int MyReadOnlyVar { get; } } class Foo : IFoo { int MyReadOnlyVar { get; set; } } public IFoo GetFoo() { return new Foo { MyReadOnlyVar = 1 }; } Is the above an acceptable way of implementing a readonly/immutable object? The immutability of IFoo can be broken with a temporary cast to Foo. In general (non-critical) cases, is hiding functionality through interfaces a common pattern? Or is it considered lazy coding? Or even an anti-pattern?

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• Implementing simple business model in Haskell

  - by elmes

  Supose we have a very simple model: Station has at least one Train Train has at least two Stations The model has to allow to check what stations any given train visits and to check what trains visit a particular station. How to model it in Haskell? I am a Haskell newbie, so please correct me: once an object is created, you cannot modify it - you can only make a new object based on that one (~immutability). Am I right? If so, I'll have to create a lot of temporary variables with semi-initialized objects (during deserialization or even in unit tests). Basically what I need is an example of modeling domain classes in Haskell - after reading "Learn you a haskell.." I still have no idea how to use this language.

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• Are memory barriers necessary for atomic reference counting shared immutable data?

  - by Dietrich Epp

  I have some immutable data structures that I would like to manage using reference counts, sharing them across threads on an SMP system. Here's what the release code looks like: void avocado_release(struct avocado *p) { if (atomic_dec(p->refcount) == 0) { free(p->pit); free(p->juicy_innards); free(p); } } Does atomic_dec need a memory barrier in it? If so, what kind of memory barrier? Additional notes: The application must run on PowerPC and x86, so any processor-specific information is welcomed. I already know about the GCC atomic builtins. As for immutability, the refcount is the only field that changes over the duration of the object.

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• Do fields need to be explicitly final to have a "proper" immutable object?

  - by Yishai

  You often read about immutable objects requiring final fields to be immutable in Java. Is this in fact the case, or is it simply enough to have no public mutability and not actually mutate the state? For example, if you have an immutable object built by the builder pattern, you could do it by having the builder assign the individual fields as it builds, or having the builder hold the fields itself and ultimately return the immutable object by passing the values to its (private) constructor. Having the fields final has the obvious advantage of preventing implementation errors (such as allowing code to retain a reference to the builder and "building" the object multiple times while in fact mutating an existing object), but having the Builder store its data inside the object as it is built would seem to be DRYer. So the question is: Assuming the Builder does not leak the Object early and stops itself from modifying the object once built (say by setting its reference to the object as null) is there actually anything gained (such as improved thread safety) in the "immutability" of the object if the object's fields were made final instead?

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• "static" as a semantic clue about statelessness?

  - by leoger

  this might be a little philosophical but I hope someone can help me find a good way to think about this. I've recently undertaken a refactoring of a medium sized project in Java to go back and add unit tests. When I realized what a pain it was to mock singletons and statics, I finally "got" what I've been reading about them all this time. (I'm one of those people that needs to learn from experience. Oh well.) So, now that I'm using Spring to create the objects and wire them around, I'm getting rid of static keywords left and right. (If I could potentially want to mock it, it's not really static in the same sense that Math.abs() is, right?) The thing is, I had gotten into the habit of using static to denote that a method didn't rely on any object state. For example: //Before import com.thirdparty.ThirdPartyLibrary.Thingy; public class ThirdPartyLibraryWrapper { public static Thingy newThingy(InputType input) { new Thingy.Builder().withInput(input).alwaysFrobnicate().build(); } } //called as... ThirdPartyLibraryWrapper.newThingy(input); //After public class ThirdPartyFactory { public Thingy newThingy(InputType input) { new Thingy.Builder().withInput(input).alwaysFrobnicate().build(); } } //called as... thirdPartyFactoryInstance.newThingy(input); So, here's where it gets touchy-feely. I liked the old way because the capital letter told me that, just like Math.sin(x), ThirdPartyLibraryWrapper.newThingy(x) did the same thing the same way every time. There's no object state to change how the object does what I'm asking it to do. Here are some possible answers I'm considering. Nobody else feels this way so there's something wrong with me. Maybe I just haven't really internalized the OO way of doing things! Maybe I'm writing in Java but thinking in FORTRAN or somesuch. (Which would be impressive since I've never written FORTRAN.) Maybe I'm using staticness as a sort of proxy for immutability for the purposes of reasoning about code. That being said, what clues should I have in my code for someone coming along to maintain it to know what's stateful and what's not? Perhaps this should just come for free if I choose good object metaphors? e.g. thingyWrapper doesn't sound like it has state indepdent of the wrapped Thingy which may itself be mutable. Similarly, a thingyFactory sounds like it should be immutable but could have different strategies that are chosen among at creation. I hope I've been clear and thanks in advance for your advice!

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• Strings in .NET are Enumerable

  - by Scott Dorman

  It seems like there is always some confusion concerning strings in .NET. This is both from developers who are new to the Framework and those that have been working with it for quite some time. Strings in the .NET Framework are represented by the System.String class, which encapsulates the data manipulation, sorting, and searching methods you most commonly perform on string data. In the .NET Framework, you can use System.String (which is the actual type name or the language alias (for C#, string). They are equivalent so use whichever naming convention you prefer but be consistent. Common usage (and my preference) is to use the language alias (string) when referring to the data type and String (the actual type name) when accessing the static members of the class. Many mainstream programming languages (like C and C++) treat strings as a null terminated array of characters. The .NET Framework, however, treats strings as an immutable sequence of Unicode characters which cannot be modified after it has been created. Because strings are immutable, all operations which modify the string contents are actually creating new string instances and returning those. They never modify the original string data. There is one important word in the preceding paragraph which many people tend to miss: sequence. In .NET, strings are treated as a sequence…in fact, they are treated as an enumerable sequence. This can be verified if you look at the class declaration for System.String, as seen below: // Summary:// Represents text as a series of Unicode characters.public sealed class String : IEnumerable, IComparable, IComparable<string>, IEquatable<string> The first interface that String implements is IEnumerable, which has the following definition: // Summary:// Exposes the enumerator, which supports a simple iteration over a non-generic// collection.public interface IEnumerable{ // Summary: // Returns an enumerator that iterates through a collection. // // Returns: // An System.Collections.IEnumerator object that can be used to iterate through // the collection. IEnumerator GetEnumerator();} As a side note, System.Array also implements IEnumerable. Why is that important to know? Simply put, it means that any operation you can perform on an array can also be performed on a string. This allows you to write code such as the following: string s = "The quick brown fox";foreach (var c in s){ System.Diagnostics.Debug.WriteLine(c);}for (int i = 0; i < s.Length; i++){ System.Diagnostics.Debug.WriteLine(s[i]);} If you executed those lines of code in a running application, you would see the following output in the Visual Studio Output window: In the case of a string, these enumerable or array operations return a char (System.Char) rather than a string. That might lead you to believe that you can get around the string immutability restriction by simply treating strings as an array and assigning a new character to a specific index location inside the string, like this: string s = "The quick brown fox";s[2] = 'a';   However, if you were to write such code, the compiler will promptly tell you that you can’t do it: This preserves the notion that strings are immutable and cannot be changed once they are created. (Incidentally, there is no built in way to replace a single character like this. It can be done but it would require converting the string to a character array, changing the appropriate indexed location, and then creating a new string.)

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• Noob Objective-C/C++ - Linker Problem/Method Signature Problem

  - by Josh

  There is a static class Pipe, defined in C++ header that I'm including. The static method I'm interested in calling (from Objetive-c) is here: static ERC SendUserGet(const UserId &_idUser,const GUID &_idStyle,const ZoneId &_idZone,const char *_pszMsg); I have access to an objetive-c data structure that appears to store a copy of userID, and zoneID -- it looks like: @interface DataBlock : NSObject { GUID userID; GUID zoneID; } Looked up the GUID def, and its a struct with a bunch of overloaded operators for equality. UserId and ZoneId from the first function signature are #typedef GUID Now when I try to call the method, no matter how I cast it (const UserId), (UserId), etc, I get the following linker error: Ld build/Debug/Seeker.app/Contents/MacOS/Seeker normal i386 cd /Users/josh/Development/project/Mac/Seeker setenv MACOSX_DEPLOYMENT_TARGET 10.5 /Developer/usr/bin/g++-4.2 -arch i386 -isysroot /Developer/SDKs/MacOSX10.5.sdk -L/Users/josh/Development/TS/Mac/Seeker/build/Debug -L/Users/josh/Development/TS/Mac/Seeker/../../../debug -L/Developer/Platforms/iPhoneOS.platform/Developer/usr/lib/gcc/i686-apple-darwin10/4.2.1 -F/Users/josh/Development/TS/Mac/Seeker/build/Debug -filelist /Users/josh/Development/TS/Mac/Seeker/build/Seeker.build/Debug/Seeker.build/Objects-normal/i386/Seeker.LinkFileList -mmacosx-version-min=10.5 -framework Cocoa -framework WebKit -lSAPI -lSPL -o /Users/josh/Development/TS/Mac/Seeker/build/Debug/Seeker.app/Contents/MacOS/Seeker Undefined symbols: "SocPipe::SendUserGet(_GUID const&, _GUID const&, _GUID const&, char const*)", referenced from: -[PeoplePaneController clickGet:] in PeoplePaneController.o ld: symbol(s) not found collect2: ld returned 1 exit status Is this a type/function signature error, or truly some sort of linker error? I have the headers where all these types and static classes are defined #imported -- I tried #include too, just in case, since I'm already stumbling :P Forgive me, I come from a web tech background, so this c-style memory management and immutability stuff is super hazy. Edit: Added full linker error text. Changed "function" to "method" Thanks, Josh

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• Java: immutable Stack?

  - by HH

  I chose to use Stacks and Tables before knowing Collections has immutable empty things only for Set, Map and List. Because the size of table does not change after its init: Integer[] table = new Intger[0] I can use the zero-witdh table as an empty table. But I cannot use final or empty Stack to get immutable Stack: No immutability to Stack with Final import java.io.*; import java.util.*; public class TestStack{ public static void main(String[] args) { final Stack<Integer> test = new Stack<Integer>(); Stack<Integer> test2 = new Stack<Integer>(); test.push(37707); test2.push(80437707); //WHY is there not an error to remove an elment // from FINAL stack? System.out.println(test.pop()); System.out.println(test2.pop()); } } Java Api 5 for list interface shows that Stack is an implementing class for list and arraylist, here. The java.coccurrent pkg does not have any immutable Stack data structure. From Stack to some immutable data structure How to get immutable Stack data structure? Can I box it with list? Should I switch my current implementatios from stacks to Lists to get immutable? Which immutable data structure is Very fast with about similar exec time as Stack?

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• Write-only collections in MongoDB

  - by rcoder

  I'm currently using MongoDB to record application logs, and while I'm quite happy with both the performance and with being able to dump arbitrary structured data into log records, I'm troubled by the mutability of log records once stored. In a traditional database, I would structure the grants for my log tables such that the application user had INSERT and SELECT privileges, but not UPDATE or DELETE. Similarly, in CouchDB, I could write a update validator function that rejected all attempts to modify an existing document. However, I've been unable to find a way to restrict operations on a MongoDB database or collection beyond the three access levels (no access, read-only, "god mode") documented in the security topic on the MongoDB wiki. Has anyone else deployed MongoDB as a document store in a setting where immutability (or at least change tracking) for documents was a requirement? What tricks or techniques did you use to ensure that poorly-written or malicious application code could not modify or destroy existing log records? Do I need to wrap my MongoDB logging in a service layer that enforces the write-only policy, or can I use some combination of configuration, query hacking, and replication to ensure a consistent, audit-able record is maintained?

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• Noob Objective-C/C++ - Linker Problem/Function Def Problem

  - by Josh

  There is a static class Pipe, defined in C++ header that I'm including. The function I'm interested in calling (from Objetive-c) is here: static ERC SendUserGet(const UserId &_idUser,const GUID &_idStyle,const ZoneId &_idZone,const char *_pszMsg); I have access to an objetive-c data structure that appears to store a copy of userID, and zoneID -- it looks like: @interface DataBlock : NSObject { GUID userID; GUID zoneID; } Looked up the GUID def, and its a struct with a bunch of overloaded operators for equality. UserId and ZoneId from the first function signature are #typedef GUID Now when I try to call the function, no matter how I cast it (const UserId), (UserId), etc, I get the following linker error: "Pipe::SendUserGet(_GUID const&, _GUID const&, _GUID const&, char const*)", referenced from: -[PeoplePaneController clickGet:] in PeoplePaneController.o Is this a type/function signature error, or truly some sort of linker error? I have the headers where all these types and static classes are defined #imported -- I tried #include too, just in case, since I'm already stumbling :P Forgive me, I come from a web tech background, so this c-style memory management and immutability stuff is super hazy. Thanks, Josh

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