Constructor Interfaces

[Red IO]

I'm not sure how big of a closing factor this implementation would have.
Since the whole implementation is opaque to everyone but the compiler. If
implemented that way it would be vital to guard the information carrying
field from the user (against reflection for example). If done correctly
this would be so opaque that a full implementation change would be
possible. The sole requirement would be that you could use the type
variable the same way in the new implementation.
As I was exploring a possible implementation in my last mail I realized
that it (by design) is 90% preprocessor. I think I will continue exploring
it by trying to implement it using an annotation processor and explore
potential problems while using it.
Of cause the annotation processor will be ugly to use and for development
only.

Great regards
RedIODev

On Wed, Jan 25, 2023, 16:11 Brian Goetz <brian.goetz at oracle.com> wrote:

> This is a good example of a "you can, but you probably shouldn't" language
> feature.  The power-to-weight ratio isn't favorable; it is a lot of new
> machinery and concept to move the ball forward a small amount.  And as soon
> as the ball is moved forward by that amount, we will immediately be
> confronted by the next thing we can't do, and the solutions are likely to
> be an increasingly complex sequence of worse power-to-weight ratio ideas.
> (Careful observers of Java history might note that this phenomenon is
> especially evident in any proposal surrounding annotations.)
>
> As Michael K pointed out, other languages have explored more general, but
> more suitable, answers here; what you're looking for is a witness to
> conformance to a type class.  (Our friends in C# have pursued something
> similar through abstract statics; for various reasons, that's less of a
> good match for Java than for C#.)  This is not a small ask; its significant
> new complexity, but the power gained is much greater.  If we were to choose
> to invest in solving problems like this one, that would likely be the path,
> but this is a big lift and we have other big things on our plate right
> now.
>
> As a general note, while it is fun to imagine new language features,
> language design needs to be a holistic process.  If we did a hundred
> "point" features like this, what are the chances that the whole would hold
> together?  If we did this feature, what other potential feature directions
> are we implicitly foreclosing on?  These are the questions we address
> ourselves to when choosing what features to consider and not.
>
>
> On 1/25/2023 2:03 AM, Red IO wrote:
>
> Summary
> -------
>
> Enable a parameterized class to constrain the parameterized type to be
> constructible with a given list of parameters.
>
>
>
> Motivation
> ----------
>
> It is possible since JDK 8 to get a constructor (method) reference of an
> object. This allowed for the creation of an unknown class with a known
> constructor reference. But currently the only way to obtain such reference
> is at call site like this:
> Box<String> stringBox = new Box<>(String::new);
>
> It is inconvenient for the user to supply the the reference themselves and
> can confuse them as the type of the parameter is something like
> Supplier<String> which doesn't require the pased reference to be a
> constructor.
> It also clutters api's like "toArray" which requires an IntFunction to be
> type safe.
>
> Description
> -----------
>
> ConstructorInterface
> A ConstructorInterface is a special kind of interface similar to a
> FunctionalInterface. It also has similar constraints. It only allows
> abstract constructors and no other abstract methods. It can declare
> multiple constructors though. The definition of such interface would look
> similar to this:
>
> @ConstructorInterface //optional validation like FunctionalInterfaces
> public interface DefaultConstructible {
> new();
> new(char[] chars);
> }
>
> A parameterized type could declare this interface as a type bound for its
> parameter and therefore enabling it to be constructed safely. Like this:
> public class Box<E extends DefaultConstructible> {
> public Box() {
> E newElement = new E();
> }
> }
> The containing type is not forced to implement the ContructorInterface
> explicitly. It is implicitly implemented if the required constructor(s)
> is(are) present.
> public static void main(String[] args) {
> Box<String> stringBox = new Box<>(); //compiles because String has the
> required constructors.
> Box<java.sql.Date> dateBox new Box<>(); error: java.sql.Data does not
> satisfy the type bound DefaultConstructible
> }
> The interface might not be implemented by any class, since it doesn't
> follow the inheritance rule that extending classes of those who implement
> it also implement it. This requirement comes from the fact that extending
> classes do not necessarily need to have the same constructor signature and
> therefore don't qualify the requirements for the interface. Another option
> would be that extending classes of classes that implement a constructor
> interface explicitly are also required to supply the necessary constructors.
>
> class Foo implements DefaultConstructable {
> //both required by the interface
> public Foo() {}
> public Foo(char[] chars) {}
> }
>
> class Bar extends Foo {
> //the requirement for the required constructors is passed down.
> public Bar() {}
> public Bar(char[] chars) {}
> }
>
>
>
> public static <T extends Foo> T createT() {
> return new T();
> }
>
> public <T extends Foo> T wrapper() {
> return createT();
> }
> This would technically work but would require a lot of static analysis to
> find the real type of T to call its constructor.
> Restricting the use of "new T()" to type parameters that specify a
> constructor interface directly and only allow those to be resolved with a
> concrete type rather than another type parameter.
>
> Alternatives
> ------------
> An alternative would be to introduce new syntax to restrict the ability of
> certain constructors on a parameter type. Like c# does (but only for the
> default constructor) :
> public static T foo<T>() where T: new() {
> return new T();
> }
> In java:
> public static <T extends new()> T foo() {
> return new T();
> }
> The downside of this approach is obviously the introduction of new syntax
> rather than reusing the interface/inheritance syntax.
>
> Another alternative to this approach could be to implement static abstract
> methods. This would allow an interface to mandate a static Factory Method.
> The downside of this approach is that it requires the parameter class to
> actually implement the interface and the concept of type erasure would need
> to be addressed for static abstract methods to work. In contrast the
> ConstructorInterface enables every class that matches its contract to pass
> the type bound.
>
>
>
> Risks and Assumptions
> ---------------------
>
> As mentioned before the restriction the interface is giving on a type
> bound is different to normal interfaces, it restricts by its containing
> abstract constructors not by the type itself. It also makes use of the new
> operator on type variables.
>
>
>
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