How about a “generic nullable class”. My idea is that instead of nullable and not null, there is a third option “parameterized null” (

<?>

). Than you could do something like this

data class Test<?>(
    val foo: Foo<?>,   // nullable depending on class parameter
    val bar: Bar,      // never null
    val baz: Baz:?     // allways nullable
)
val test1 = Test<?>(getFooOrNull(), Bar(), null)
val test2 = Test<!>(Foo(), Bar(), null)
test1.foo?.doSomething()
test2.foo.doSomething()

This could be used to easily create Builder classes for data classes with immutable values.

class<?> is already means wildcard in java right? wouldn't it be confusing coming from java background? data classes could be improved with builders, I agree on that! but they are super easy to create with compiler plugins. I can not see any other options where this would be useful 🤔

How would you restrict that type? Say I wanted

data class Test

's generic parameter restricted to implementers of

CharSequence

What exactly do you mean? Do you want

Test

to extend

CharSequence

? This special generic parameter can only be

!

or

?

. If you want to mix this with a normal generic type you could do use

class Foo<?, T: CharSequence>

made it super clear 😄 what you are looking for is a Higher Kinded type.

class Foo<?, T: CharSequence>

is the same saying class `Foo<`Option<>`, CharSequence>` and

class Foo<!, T: CharSequence>

is the same saying class `Foo<`Id<>`, CharSequence>`

yes, exactly.

Okay, one more thing I can't quite understand. Today, all generic type parameters have names (usually

T

or something). However, here you just use

?

. How do you reference this without confusing it with the nullability symbol? Can your

Test

have multiple

?

parameters? If so, how do you distinguish which is which?

I did not spend much time thinking about a proper syntax. I just wanted to get the idea out there. I agree, just using

?

would probably not work. This could lead to multiple issues. I guess they will have to be named and than they could maybe all start with a

?

. Not sure yet.