Why is this not possible: ```fun interface A { fun foo(b: Int = 42) // Single abstract member ca...

# getting-started

[hfhbd]

Why is this not possible:

fun interface A {
    fun foo(b: Int = 42) // Single abstract member cannot declare default values
}

[Umar Ata]

Screenshot 2022-02-22 at 1.01.49 AM.png

[Umar Ata]

since you written fun in the prefix of interface therefore it is causing issues

[hfhbd]

Yes, I want to keep the fun modifier to use it as a FunctionalInterface

A { }.foo()

[Umar Ata]

I am not sure if it is allowed for interface

[Joffrey]

@Umar Ata fun interface is valid and defines functional interfaces

[Joffrey]

@hfhbd to be honest I don't know, but I guess it may have to do with the fact that it wouldn't know how to convert the interface into a function: it could be either a () -> Unit or a (Int) -> Unit, and I guess that ambiguity could cause problems during SAM conversions

[hfhbd]

@Joffrey What do you mean? The parameter of the body?

A { i -> // this?

}

Even with default values in the definition (the interface), you still would need to declare the variables in the body, implementation.
If you mean calling the (single) member foo, the compiler is able to infer the correct overload, like its normal behavior with default values.
Or do you mean this: val a: (Int) -> Unit = A { }? This does not compile, because even a fun interface does not inherit from KFuction (and its invoke) by default.

[Joffrey]

I meant the very last part: using an instance of the fun interface where a function is expected. I thought SAM conversions were supposed to work in that case, but I guess I was wrong