When do you use fun interface instead of function type?

function type? You mean lambdas?

Lambdas can be used with both function type and function interface.

// fun type
val a: ()->Unit = { print("A") }

// fun interface
fun interface A {
   fun call()
}
val a: A = A { print("A") }

I personally prefer

fun interface

when it needs to be passed in for a specific purpose (like a specific type of event. Even though when implementing them the lambda looks the same more or less, but there's some restrictions as to where they can be passed. But recently I wanted to use it to implement a base UseCase interface, in the production code, I would implement it as a full class with deps on repos and such, whereas in my test code, I can provide a one-liner to Stub them. (The problem is that they don't work with

suspend fun

yet, so I had to try something else for that...)

@dave08 try this, this is the pattern I'm using for Usecases:

interface MyUseCase {
    suspend operator fun invoke(foo: Foo): Bar
}

...

val myUseCase: MyUseCase = ...
val bar = myUseCase(foo)

Yup, that's currently what I use, but for stubs to be one liners in our tests, I had to make a UseCase inline fun taking a lambda... instead of a simpler fun interface...