If a Java functional interface has its type parame...

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[Dico]

If a Java functional interface has its type parameter used invariantly, but it can't declare it as intravariant,
how can I make a class that implements it with the type parameter in invariant position? Expanded in a thread with code.

[Dico]

given this JAVA functional interface, which cannot declare its type parameter T as invariant -

public interface SingleResultCallback<T> {
    /**
     * Called when the operation completes.
     * @param result the result, which may be null.  Always null if e is not null.
     * @param t      the throwable, or null if the operation completed normally
     */
    void onResult(T result, Throwable t);
}

I want to implement this by wrapping a Continuation<T> - and I want the type parameter to be in invariant position.

class CallbackImpl<in T>(val cont: Continuation<T>) : SingleResultCallback<T> {
    override fun onResult(result: T, error: Throwable?) {
        if (error != null) {
            cont.resumeWithException(error)
        } else {
            cont.resume(result)
        }
    }
}

However, the in produces an error when I try to implement SingleResultCallback

[Dico]

Can I make the compiler trust me?

[Dico]

😂

[Dico]

Btw, SingleResultCallback is from com.mongodb.async.client

[Dico]

Not sure this is the right approach..

[Pavlo Liapota]

typealias can be an option
https://pl.kotl.in/H1zL1Ebt4