Kotlin Return function as fun interface

Question

Functional interfaces work well when you want to inject a function as an interface, example:

fun interface MakeCoffee {
    operator fun invoke()
}

class CoffeeManager(private val makeCoffee: MakeCoffee) {...}

fun provideCoffeeManager(): CoffeeManager = CoffeeManager {  }

However if I try to return a function when the return type is a fun interface like this:

fun provideMakeCoffee(): MakeCoffee = {}

it will fail for a mismatch KFunction0<Unit> vs MakeCoffee.

Is there any workaround?

Answer 1

I found the solution to the problem in the end. @Tenfour04 answer works but it doesn't answer the question of "how to return a function as fun interface".

The example provided was very simple and maybe that's why the question was a bit misleading, but imagine you have the following case:

fun interface MakeCoffee {
    operator fun invoke(sugarAmount: Double, milkAmount: Double, coffeeAmount: Double)
}

//function you already have
fun doCoffee(sugarAmount: Double, milkAmount: Double, coffeeAmount: Double) { ... }

How do you return your doCoffee as MakeCoffee?

Solution

fun provideMakeCoffee(): MakeCoffee = MakeCoffee(::doCoffee)