Any suggestions for the right approach to generics...

# announcements

[william]

Any suggestions for the right approach to generics here?

import androidx.lifecycle.MutableLiveData

class ClearableLiveData<T> : MutableLiveData<Clearable<T>>() {
    fun clear(t: T) {
        super.postValue(Clearable.Clear)
        super.postValue(Clearable.Value(t))
    }
}

sealed class Clearable<T> {
    object Clear: Clearable<Any>()
    data class Value<T>(val t: T): Clearable<T>()
}

The first call to super.postValue gives a type mismatch

[william]

I'm not entirely sure why not but these changes made the error go away

sealed class Clearable<out T> {
    object Clear: Clearable<Nothing>()
    data class Value<T>(val t: T): Clearable<T>()
}

[Michael de Kaste]

Clearable<Any> expects a type parameter; but since it's a singleton object you'll need Nothing because Nothing is a child of everything.

[Kroppeb]

the out is important too though. without it Clearable<Nothing> isn't a subtype of Clearable<T>

[william]

@Kroppeb could you explain that part any more? i don't quite understand it

[william]

it sounds like this:

Since we are using out T, any of the type parameters can be a supertype of T, which Nothing is a supertype of everything.

[Kroppeb]

You might wanna start reading here
https://kotlinlang.org/docs/reference/generics.html

[william]

i have - is my assumption above incorrect? @Kroppeb

[Kroppeb]

Since we are using out T, any of the type parameters can be a subtype of T, which Nothing is a subtype of everything.

[william]

ok so almost had that one then

[william]

thanks