Constructor Interfaces

[Remi Forax]

We may need something like this for Valhalla, when we will revisit how to constraint the type arguments of universal generics. 

The kind of constraints you describe on type parameters already exist in C# or TypeScript and was more recently introduced in Go, and there is the type class of Haskell too. 

regards, 
Rémi 

> From: "Red IO" <redio.development at gmail.com>
> To: "amber-dev" <amber-dev at openjdk.org>
> Sent: Wednesday, January 25, 2023 8:03:14 AM
> Subject: Constructor Interfaces

> 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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