Inherit function type in Kotlin

Question

Is there a way in Kotlin to inherit function types? What I want is to have a member like so: private lateinit var onPickedFunc: (Any) -> Task<Void>? so I can dynamically store the functions I need to run after a user has made it's choice. For example:

private fun showImagePicker(func: (Uri) -> Task<Void>?) {
        onPickedFunc = func
        val intent =
            Intent(
                Intent.ACTION_PICK,
                android.provider.MediaStore.Images.Media.INTERNAL_CONTENT_URI
            )
        startActivityForResult(intent, RC_PICK_IMAGE)
    }

private fun showMenu(func: (Context) -> Task<Void>?, v: View) {
        onPickedFunc = func
        PopupMenu(context, v).apply {
            // ProfileFragment implements OnMenuItemClickListener
            setOnMenuItemClickListener(this@ProfileFragment)
            inflate(R.menu.profile_settings_menu)
            show()
        }
    }

I did find a group of classes called Function, but none of them allow inheritance in the input parameter (one of them allows inheritance in the output parameter: Function<out R>). Is there any way to achieve that? Thanks!

Answer 1

I think the issue here is one of variance.

func is a function whose parameter is declared as a Uri.

But your onPickedFunc property would be a function whose parameter can be anything.  You should be able to call it with an Int, or a Uri, or a String, or any other non-null value.

So you can't assign func to it, because that (probably) wouldn't be able to handle an Int or a String or whatever.

In technical terms, the types aren't compatible.  Function parameters have in variance (contravariance); to be a subtype, it must accept a supertype of each parameter.  (This is the reverse of the function's return type, which has out variance AKA covariance.)  So the compiler would flag up a type mismatch.

If you want to store your (Uri) -> Task<Void> function in a property, it would have to be of type (Uri) -> Task<Void> or some supertype, which means the function would have to accept a Uri or some subtype.

(I don't know what type your Uri is, but if it were final, like java.net.URI, then the only subtype would be Uri itself.)

---

By the way, the kotlin.jvm.functions.Function… interfaces are an implementation detail; the compiler uses them to implement function types on the JVM, but they're not something you should refer to in code (except in special cases such as reflection, I think).