Write an article about kotlin's type system, hope ...

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[Jintin]

Write an article about kotlin's type system, hope you'll like it.
https://medium.com/@jintin/the-beauty-of-kotlin-type-system-25d0a187b130

[Stephan Schröder]

so far, so good.
• in the section about Nothing you could add:
◦ that Notling is used for dead code analysis. The Java compiler has to hard-code all situations that can lead to dead code (e.g. after return or throw). In Kotlin every statement aften an expression
• How about adding a section how Unit is better than void , e.g. because in Java you need two interfaces Runnable and Callable while in Kotlin Runnaable is made redundant by Callable<Unit> (in Java Callable<void> is not a thing). Of course this is a specialisation about how how every functional interface that returns something can also be used in a way that returns nothing in

[Klitos Kyriacou]

@Stephan Schröder Well, there is Callable<Void>. Admittedly, Void is slightly different from void.

[Stephan Schröder]

I know that there is uppercase Void , I just never used it -or seen it used- in my close to quarter century of using Java.
How do you construct an instance of it? But as you said, it's not void , so Kotlin's type-sytem is indeed smoother in this respect.

[Klitos Kyriacou]

How do you construct an instance of it?

You don't. The Java API documentation says "The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void."

But as you said, it's not void , so Kotlin's type-sytem is indeed smoother in this respect.

Kotlin's Unit is not void either. Unit is an object. From Java's point of view, Unit is a class which has only one instance, Unit.INSTANCE. This is analogous to Void, which is a class that has no instances, so any Java method that returns a Void type can only return null, just as any Kotlin function that returns a Unit type can only return the instance Unit.

[Klitos Kyriacou]

Also, the Void type is used in a few Java API methods; for example, CompletableFuture.allOf returns a CompletableFuture<Void>.

[Stephan Schröder]

I understand that Unit isn't void, (though it "takes its place", there's probably a more correct formal way of stating this). I'm saying Unit allows a more ellegant typesystem than one that uses void .