Kotlin - Functional (SAM) interfaces VS Function types

Question

With Kotlin 1.4 we now have Functional Interfaces

fun interface Todo {
       fun run()
}

fun runBlock(todo: Todo){
   if(condition)
      todo.run()

}

fun runBlock{
   println("Hello world")
}

Before i was always using (T) -> T

inline fun runBlock(block: ()-> Unit){
   if(condition)
      block()
}

fun runBlock{
   println("Hello world")
}

So basically I can make the same task with both methods , there is any performance advantage by using Functional SAM() Interfaces over Function Type?.

Answer 1

It's a performance dis-advantage because the lambda is no longer inlined (unless the JIT decides to, but it won't be instant). Even if you mark runBlock as inline, the compiler will warn you that the argument won't be inlined.

There are only two reasons to use fun interfaces instead of function types:

1. Backwards compatibility when porting code using Java functional interfaces.
2. Not exposing Kotlin function types in API intended for use from Java.

To expand on point 1: before Kotlin 1.4 it was advised to keep functional interfaces as Java code, even if all your other code was Kotlin. This way you could use lambdas for parameters of those types both in Java and Kotlin code. But if you declared the interface in Kotlin, you could only use lambdas for them in Java.