Having the ability to create structures in Dart

[qardgroupadmin]

Greetings, I was recently reading the comments on a proposal I made, which discusses implicit constructors, and the idea came to me because the Dart language doesn't have the typical structures that use the reserved word struct and implicit constructors. What better way to explain something than by showing a code example to provide better context:

This is an example of what a basic structure would look like.

struct User {
    active: bool,
    username: String,
    email: String,
}

void main() {
  final myUser = User(true, "maria", "mariafx@mail.com");

  print(myUser.username); //print: maria
}

This is an example of how to instantiate a structure using dot shorthands.

struct User {
    active: bool,
    username: String,
    email: String,
}

void main() {
  final User myUser = .new(true, "maria", "mariafx@mail.com");

  print(myUser.username); //print: maria
}

The purpose of having structs in the Dart language is to give meaning to classes that only carry data, acting as a specific abstraction for that particular use case. You can read more about what structs are in the official Rust documentation.

Rust structures

The structures give meaning to the code and allow for simplified interoperability with low-level languages ​​such as Rust and C++.

[mmcdon20]

You can do the following in current dart:

typedef User = ({bool active, String username, String email});

void main() {
  final User myUser = (active: true, username: "maria", email: "mariafx@mail.com");

  print(myUser.username); //print: maria
}

[qardgroupadmin]

It's an interesting solution, but if I create data that way, I can't use it to inherit and add behavior to that data. However, with a struct, I could inherit the data to encapsulate the operations that modify it, for example:

typedef User = ({bool active, String username, String email});

class AuthManager extend User { //sintax error
  
}

void main() {
  final User myUser = (active: true, username: "maria", email: "mariafx@mail.com");

  print(myUser.username); //print: maria
}

The goal is to be able to separate the data from the operations; see this example:

struct User {
    active: bool,
    username: String,
    email: String,
}

final class AuthManager extend User {
  
}

void main() {
  final myUser = AuthManager(true, "maria", "mariafx@mail.com");

  print(myUser.username); //print: maria
}

The idea is to create a structure that allows grouping data under a name, making it easily transferable within the application. This is very useful for passing parameters to methods. If a structure requires specific behavior, this can be achieved by using it as a base class for inheritance, allowing the derived class to add further functionality, just like in classic object-oriented programming. This differentiates it from the example just shown, which uses record types, which are also useful but cannot be inherited from. An interesting feature is the use of implicit constructors in the derived class.

An additional benefit is that having structures in the Dart language makes interoperability with low-level languages ​​easier because the syntax is quite similar, which simplifies its use.

[mmcdon20]

Would primary constructors solve your problem?

https://github.com/dart-lang/language/blob/main/accepted/future-releases/primary-constructors/feature-specification.md

The language designers are currently working on primary constructors, so I think it's likely to be added to the language sometime this year.

With the addition of primary constructors, you could write

class User(
  final bool active, 
  final String username, 
  final String email,
);

final class AuthManager(
  super.active, 
  super.username, 
  super.email,
) extends User;

[qardgroupadmin]

I have carefully read the information you shared, and now I understand better. After reading it, I can tell you that I'm not talking about primary constructors, but I do have several things to say about them. I'm specifically referring to this:

proposed structures

If you'll allow me to comment on primary constructors for classes, from my point of view, I don't see any point in them because they break with the existing object-oriented programming paradigm. The struct keyword solves the problem because it's a new way to encapsulate data within Dart.

Furthermore, the Dart language has records, which was the first example you showed me, and which is another way to solve the same problem. The difference between a record and a struct is the limited inheritance capability to add behavior to the already encapsulated data.

I really like the progress the Dart language is making. Structs could arrive soon, all in due time. I see that someone already thought of this before me. Structs are necessary within the language; I would use them a lot. Thank you for your attention. I think this is already solved with structs written in the following way:

struct Component(int x, int y, int z, int w = 1);

[lrhn]

I'm not sure I understand the distinction between the.structs your asking for, and classes. You say

classes that only carry data,

but also

but if I create data that way, I can't use it to inherit and add behavior to that data.

so you want the structs to be usable as superclasses of classes with behavior, without having behavior themselves.

You can't use records, which is Dart's existing structured data that otherwise fit the bill perfectly, because you want to subclass the type.
There is extension types in on records, if you want to add behavior to pure data.

Or, if you remember the proverb "favor composition over inheritance", you can use a record for the data, and have a record field instead of trying to extend the record type.

Primary constructors also seen to fit the requirements perfectly;

class User({final String username, final String email, final bool active});

is so close to the syntax you propose, and it will create a class with only data which you can extend. Not having behavior doesn't require anything special, it just means not declaring any.
(I have no idea how primary constructors "break with the existing object-oriented programming paradigm", they're just syntactic sugar. Is the problem that they don't break with it?)

What better way to explain something than by showing a code example to provide better context

Can you perhaps give a real use-case which is not covered by any of the existing options, and with potentially better syntax come Primary Constructors.

[qardgroupadmin]

Greetings, this is a difficult question to answer. To do so, it's necessary to know what a struct is and what problem you're trying to solve.

A struct is a way to group different variables in a immutable way, where it has a primary constructor, and the getter, hashCode, toString, and equals methods are provided by default, with a syntax like this:

Example of an immutable struct

struct Component(int x, int y, int z, int w = 1);
void main() {
    final comp = Component(5, 6, 5);
}

Another variation of structs would be in the way we pass their parameters, using {} to have named parameters in the following way:

struct Component({int x, int y, int z, int w = 1});

void main() {
    final comp = Component(x: 5, y: 6, z: 5);
}

This is how I imagine structs; the problems arise when we think about inheritance. Now, I believe that inheritance between structs is perfectly fine, but inheritance between a struct and a class is only necessary to add behavior. Let's look at the following examples:

Example of inheriting a structure and a class to add behavior.

struct User(bool active, String  username, String email);

final class AuthManager extend User {
   String greet() => "hello $username";
}

void main() {
  final myUser = AuthManager(true, "maria", "mariafx@mail.com");

  print(myUser.username); //print: maria
  print(myUser.greet());       //print: hello maria
}

Another example of the inheritance of structures and pattern sequences:

abstract struct Operand;

struct ConstantOperand(Value c) extends Operand;

struct IdentifierOperand(Identifier i) extends Operand;

Operand replaceIn(Operand oper, Identifier t, Value c) {
  return switch (oper) {
    case ConstantOperand(_) : oper;
    case IdentifierOperand(var i) where i == t: ConstantOperand(c);
    case IdentifierOperand(_) : oper;
  }
}

I hope to answer all your questions. Something else I've noticed that seems inconsistent is the reserved word final, but I'll address that at another time.

[lrhn]

Apart from the automatically provided equals, hash code and toString, I see nothing new, which cannot be handled adequately by classes with primary constructors.

This is basically a request for a way to create classes with a default equality and toString.
Which might be reasonable, at least for immutable classes. (Shouldn't have equality based on mutable fields.)
Say declaring a
data class Foo({final String name, final String address}); which gets fired-based equality.
(And maybe toString, but that's really only for debugging, so I'd prefer to not specify it.).

[qardgroupadmin]

It's a matter of making the code easier to read; it's easier to differentiate a struct from a class than one class from another. Besides, we already have a series of class modifiers to enable quite interesting object-oriented programming. Primary constructors seem to contradict these rules, whereas structs, being something new, don't modify object-oriented programming; they simply add to it without breaking existing rules. For now, it's just a proposal, and you, as the administrator, have the final say. I think the debate is getting too long for something so obvious. I'm going to relax with 🟢🔵🟣🌈🌧️🌨️🪴🌳🌲. I'll be attentive to your questions and I'm here to contribute to improving the Dart language ecosystem 🤝

[mateusfccp]

You can do the following in current dart:

typedef User = ({bool active, String username, String email});

void main() {
final User myUser = (active: true, username: "maria", email: "mariafx@mail.com");

print(myUser.username); //print: maria
}

This is not good, as User is not a new type.

Primary constructors is probably the best we can get for him + code generation for equals, etc. Unless we get native data structures in Dart, but there's already a issue for it.