Regarding the T is Type feature; are any of its use cases also solved by static extends? We have a similar use case with Page routes in flutter. Similarly, even if A contains a static method named fromInteger and the value of T is A, T.fromInteger(nb) won't call that static method. Static methods in Dart cannot be overridden: This way no-one can change what is printed when A is constructed although staticMethod itself remains public. It works ok, but I have to manually add the type I like to use to that function mappingT2Class. This means that this instance of Type represents the type A, not that it 'is of' type A (that's a different thing, e.g., new A() is of type A). then the only subtyping would be due to type equality. Perhaps. The main benefit I'm searching for in my own case (and I think many others in theirs) is to be able to elegantly specify that there are some static constraints on a T, and use those properties in a generic, without having to keep some sort of second mapping of all those subclasses to their respective versions of the method. to your account. There is no need to create a class object to access a static variable or call a static method: simply put the class name before the static variable or method name to use them. Having a lint when you forget can save some time later down the line. For example, we will be able to store the type, e.g., in a list of types, and use the type to invoke one of these "pseudo-static methods". But there is still a missing part: The implementation of the extension member (here: opcodes) needs to perform an emulation of the OO dispatch. Apart from the syntactic noise (that we may or may not choose to reduce by means of some desugaring), the essential missing feature is a special kind of dependent type that would allow us to know that the _companion map is a Map>, that is: Each key/value pair is such that the key as a Type, and that type is a reification of a certain type t, and then the value is a SerializableCompanion, with the following adjustment: In the emulation we also need to thread that type argument around, e.g., companion would return a SerializableCompanion, etc. But if there is no connection between those two subtype relationships then we can't make any assumptions about S having any of the known static members that we know T has, if all we know is S <: T. That's why in my example I wrote a static extends type bound: That's the part where you define the type that the type argument's metaclass must be a subtype of. This problem gets a lot worse as soon as we consider any other class C than Object, because C will impose further requirements on all its subtypes. So we'd need to be innovative to do that at all. Static methods are not limited this way as they are just global functions that share the class name for convenience. In addition to basic type constraints, things like new are extremely helpful If we were to erase lifetimes from types For example, writing a Serializable class would remind me not to forget to include a .fromJson constructor. It's a different mindset entirely. Please also consider adding @childrenOverride, which means the direct child should override the abstract method even it is an abstract class, this can deeply benefit generated code, such as built_value. There is no proposal here to change the language in this area, I just mentioned that we're using this feature. So that's a huge improvement. Static methods can be invoked directly from the class name itself rather than creating an instance of it. In particular, that follows Dart's current behavior where static members and constructors are not inherited. In this comment I tried to move a bit closer to something which would actually preserve the connection between the two subtype relationships (such that S <: T actually implies that S has static members which are correct overrides of those of T), but only when declared as such, and only for "small sets of classes", such that it would be OK to require some or all parts of the static interface to be supported for all subtypes (because there would only be a few of them). It is because you do not want to change MyClass.method() to object.method() in too many places. Isn't it true in your example that in addition to _factories, we would now need a second map called _tableNames which basically almost duplicates it? The subtype relation in the metaclasses (i.e. files, instead of grouping code of the same functionality together @tatumizer k I'll send you a message when I finish an important part of it, it's a fun project only. Why do you want to enforce that? The question is rather whether we could do static extends at all, with the given resources. You cannot mock an object they belong to. The following are the critical points to note about the static method: The following code shows how to implement the static method: Note: static methods can only use static variables. If the dispatching code is wrong then we will not get a compile-time error, we will get a run-time error: Nevertheless, I think there's no end to the number of situations where real-world application code needs to get things correct in ways that the type system cannot follow. // Fails if `X` is `B`: there is no static method `B.foo`. Improved support for runtimeType and type params, Allow for shorter dot syntax to access enum values, https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/new-constraint, Unable to Create Generic Constructor of T that extends a concrete Type, [Feature] Allows calling factory methods of generic type, Generic functions with enum generic param. In this particular case the compiler should optimize it out because this method is private and it is easy to find and fix all calls to it. Flutter Fetching Data From the Internet, Dart Loop Control Statements (Break and Continue), Dart Programming If Else Statement (if , if..else, Nested if, if-else-if), PHP | ImagickDraw setStrokeColor() Function. If you are making a package, you should avoid breaking changes the outside code will face so you should think twice if this method will later get to use this. I'm no Rust expert, but we do have the following salient points: The declaration of from_integer in trait Serializable is effectively a declaration of a member of a "static interface" associated with the trait (and hence with all implementations of that trait), because it does not accept a receiver argument (like self or &self). Have you considered this approach? The static keyword can be applied to the fields and methods of a class. The core value is that the code is self documenting, and we're leaning on the compiler to lower devs cognitive load. However, when you print the field If Type were generic then we'd only enable the extension for subtypes of the specified bound, not for every type. The static variables belong to the class instead of a specific instance. So we'd be in trouble if we want to use the companion object approach, and we wish to compute a set of types, and then we wish to create an instance of each of the elements of that set (using fromInteger, or fromJson, or whatever). It'll really useful feature for serializable classes. For this project, the feature discussed in this issue isn't really needed. @eernstg put together a hugely educational (for me) piece on why static inheritance would be absurd for all inheritance, but @munificent rightfully pointed out that he hadn't considered that the static part of the inheritance could (and should) be an explicit option. You're calling HomeLink.path directly, so you'll get an error if it isn't defined there anyway. However, it would be nice to enforce the return and argument types with a static interface. But if it is public or you use it outside the class, you may want to make it an instance method in advance. On the other hand, we need to look up the companion (using a user-written dispatcher) and then look up the implementation of the target method (using built-in OO dispatch), so it's a bit slower. If you print the type parameter it will tell you that it were of There is no way to provide access to a set of types as type variables: Every piece of code in a Dart program is in a scope; that scope has a fixed number of type variables that are in scope, say k type variables (in lots of locations k is zero), and then we cannot represent a set of size k + 1. However, client code does get to look as requested, and we do get good type safety. To get polymorphism for static members, you could have explicit extends and implements clauses and those could be completely independent of the class's own clauses: I don't know if this actually hangs together, but back when Gilad was working on the metaclass stuff, I felt like there was something there. It is a private method that decorates a child and is used somewhere in this widget. The subtype relation in the metaclasses (i.e. This is because in Dart you cannot read fields of this before the object is fully initialized. The best workaround I can think of is to have a new_instance method in each subclass that invokes the constructor. That's exactly right. I was just wondering if this thread is still about finding a workable technical solution to the problem, or whether we are at a point where there is a clear proposal that can be considered and fine-tuned. Here is why, How to debug code dependencies on VS code. Already on GitHub? Find methods in your code that should be made static. I also came across the need of a generic type T for a function that needs to be changeable. Won't this be covered by static type extension? The issue is that, in Dart, this feature conflicts with static type safety: There is no notion of a subtype relationship between the static members and constructors of any given class type and those of a subtype thereof: It would be a hugely breaking change to require every Dart class to implement constructors with the same signature as that of all of its supertypes, and similarly for static methods. // Fails if `X` is `B`: no constructor of `B` accepts an int. static members) Come write articles for us and get featured, Learn and code with the best industry experts. This means that every implementation of the trait must also implement such a function, and we'd use the :: operator to disambiguate the implementation of the trait, and that's allowed to be a type variable. Everything is heavily opinionated here except that Flutter and the related logo are trademarks of Google LLC. this is what I am going for, but I haven't found a way to make them happen. You are right, my fault. possible to pass arguments to them. I think we would need to dive deep into all the potential consequences of a static extends proposal in order to understand what it would mean, which features it would have, and which implications it would have for performance and expressive power. Considering 100+ case statements, each case requiring 1 compare and 1 jump instruction, the performance will probably degrade a little. And so can you. Meanwhile, the instruction size is almost always the same (1 word in this case), that makes abstract static methods and extensions on types the best fit in my opinion. Note: When a non-static attribute is accessed within a static function, an error is thrown. I don't need to assemble the class "`opcodes` not implemented for the type $. So we wouldn't want to add anything to Dart which is directly modeled on the ability in Rust to require that all subtypes have a static interface that satisfies the usual override rules. Our need for this is within a builder that we are designing. In particular, any data structures similar to the _companion map would be compiler-generated, and it's not so hard to ensure that it's generated in a way that satisfies this property. The main point would be that Rust has a different approach to subtyping. But for public methods it is harder or impossible. this is definitely required, de/serialization of generics in Dart/Flutter is pain now! 
To implement set, I would like to do something like this in an abstract superclass. With a companion, that is reduced to adding 1 companion per subclass and adding 1 companion to the list of companions, which I think is still worth improving to none with static extends. Speaking of which, @munificent I didn't mean to make light of the effort needed to make changes, or other changes that are more important. variance with respect to lifetimes and between types with A static method can be declared using static keyword followed by return type, followed by method name. : It would have been nice to have a type argument on Type, and having the guarantee that the dynamic type of the Type object for any given type T will be Type. Because now you are "doing more" with T, so the constraints placed upon it are more stringent. I find that once you really look at static interfaces as completely separate from OOP and inheritance, you start to look at static interfaces as non-ideal, and other solutions become more attractive. There is a downcast in the extension, factory(json) as X, but that downcast is guaranteed to succeed as long as we ensure that _factories maps a type T to a function with signature T Function(String). the type you specified. If fromInteger is implemented as a constructor of A, it's still impossible to call it from T generic type. I do, however, agree with @esDotDev, in that I would like to be able to use static interfaces as am API specification so that I could have guidance. The approach using an extension on Type can of course use a hierarchy of companion objects and perform the lookup of the companion object, so it wouldn't be difficult to make an approach that uses companion objects and an approach that uses separate user-written dispatchers look the same from the client point of view. Sorry for that. If static means uncertainty or pain, forget static. Already the signature conflicts could be difficult to handle: Apart from conflicts, it's not desirable. 
However, when you print the field runtimeType it will tell you that it actually is the class Type, meaning it is a sham. A knowledge base to my company. Not that you should do it right away, but it is good to have this suggestion on your mind if you ever want to. Something Like this psuedo code: I also would like some way to enforce that every impl. In the case where there is more than one static member to implement, the emulation using Companions you first designed might be better in that regard. How to get out of functions, factories, map of factories and other crap in case you have de/serialize class with generics fields? With the companion approach the type is not first class, it needs to be available as a type literal (so it's known at compile time) or a type variable (so it must occur in the scope of that type variable, because we can't extract the value of a type variable from an existing object). You do get those outcomes, but it's not a sham. // OK. We know the "static interface" of T has ma(). We should introduce a subtype of Serializable, let's just call it MySerializable, which is a common supertype of all the types that we wish to support with fromJson, and not a supertype of anything else. If you're using them for something more, you should use a real object instead of the Type object. For instance, Object has a constructor taking no arguments, so all classes would have to have a constructor that takes no arguments, which may not always be useful. Others have already pointed out more specific reasons that this is undesirable, I just believe we should be able to provide information once and concisely. impossible to call it from T generic type. If you expect to change a private method later so that it does use this, still make it static anyway because you will not have to change the way it is called: it is just _method(), static or not. generate link and share the link here. an old SDK issue on a similar topic: dart-lang/sdk#10667 (search for 'virtual static' to see some connections). or inheriting class is a singleton. But it would be a nice feature to allow this. @eernstg maybe this changes your perspective on virtual static methods in Dart, maybe there are better ways to do it So it might actually be usable in production code after all. Since static methods are not polymorphic, even if you require a handful of classes to all have the same static method, that doesn't actually give you any new affordances. The hidden secrets of JSON decoding in Elixir, Serve an Image from an S3 Bucket via CloudFront, Unit testing higher-order functions in Java, Serverless Product Integration In Google Apps Script, Writing a storage engine in Rust: Writing a persistent BTreeCrash Recovery, Learn Code Fast With The Feynman Technique, Flutter Testing with Riverpod and Mockito, Flutter Router API is not declarative. A static variable is common to all instances of a class: this means only a single copy of the static variable is shared among all the instances of a class. // other subtypes of Amd64Instruction. // We can use the `T is Type` approach here, too. The static keyword is used for memory management of global data members. When a feature doesn't spontaneously appear in the language, that doesn't mean anyone put effort into stopping it. wouldn't necessarily have to mirror the class's own subtyping, Right, I remember that we agreed on that already several years ago. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. To reiterate, SomeDatabase should be able to know that it can call T.fromJson because I specified that T static extends Serializable and because the interface of Serializable specifies fromJson. static members) wouldn't necessarily have to mirror the class's own subtyping, and I think there are good arguments that it should not. I also would like this. What does that enable you to do? By clicking Sign up for GitHub, you agree to our terms of service and This code snippet summarizes what I personally would like to be able to do, and if this is made possible I think the other presented cases also become possible. Usually, utility methods are created as static methods when we want it to be used by other classes without the need of creating an instance. Tacking on a extension on every type that when invoked will throw an error .. As we said, static methods cannot be overridden. If we add a new abstract property to AppLink in the future, the compiler lights up, and shows us everywhere that needs fixing (whether those methods are currently called or not), imagine I have some pages, currently commented out, so they are not "in use", when using an interface, those classes will also get flagged for a refactor, instead of only lighting up later when those pages are used. question). That would ensure that we have support (in _factories) for exactly the right set of classes. So use instance methods if you need to allow overriding the behavior in subclasses. You might say that it "should work", and we did have a proposal for adding such a feature to Dart for quite a while, with some preparation for it in the language specification. Higher-ranked function pointers and trait objects have another Overall, extension methods can solve the problem very well. But I think I know understand why it doesn't work: If you print the type parameter it will tell you that it were of the type you specified. Lines 13: In the main drive, we use the class name Car to access the static variable. This method does not use fields of this widget so it can be made static: In Dart, as in most programming languages, the keyword this is implemented as a hidden argument to member methods. Have a question about this project? I find that most of the time, these static interfaces can be implemented either on instances or as callbacks. https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/new-constraint. The best way to achieve that in this thread is with static extends and nobody has seemed to protest that. In my comprehension, it won't work since the goal is to instantiate any class that implements Serializable. We are not endorsed by or affiliated with Google LLC. We check if the model type has a specific static method by name, if yes, then we use it in the source generation. // Would work well with a no-operation instruction // ----------------------------------------------------------------------. I personally wouldn't mind having to write Serializable.companion() rather than T. It's just that when I write a new class that static extends MySerializable, I wouldn't want to have to also add a line to n different dispatchers where n is the amount of static interface members of MySerializable.
Standard Greeting Card Sizes,
Northstar Demolition Jobs,
Buy Discord Server With Members,
Ruthless The Musical Full Show,
What Happened To Jimmy After Bully,
Depaul Catholic High School Football,
How To Make Html Table Cell Clickable,
Hiking Shoes Near London,
How To Make Html Table Cell Clickable,
Individual Pretzel Bags Costco,
