How about a “generic nullable class”. My idea is t...

# language-evolution

[Burkhard]

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.

[Burkhard]

Maybe even allowing for multiple nullabllity parameters class Foo<?SomeName, ?SomeOhterName>

[kioba]

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 🤔

[Ky Leggiero]

How would you restrict that type? Say I wanted data class Test 's generic parameter restricted to implementers of CharSequence

[Burkhard]

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>

[kioba]

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>`

[Burkhard]

yes, exactly.

[Ky Leggiero]

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?

[Burkhard]

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.