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Why is this not possible:
```fun interface A {
    fun foo(b: Int = 42) // Single abstract member cannot declare default values
}```

Screenshot 2022-02-22 at 1.01.49 AM.png

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

Yes, I want to keep the `fun` modifier to use it as a `FunctionalInterface`
```A { }.foo()```

I am not sure if it is allowed for interface

<@U01D43Q2PLZ> `fun interface` is valid and defines functional interfaces

<@U01JRLZH77T> 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 `() -&gt; Unit` or a `(Int) -&gt; Unit`, and I guess that ambiguity could cause problems during SAM conversions

<@UBXMWAA69> What do you mean? The parameter of the body?
```A { i -&gt; // 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) -&gt; Unit = A { }`? This does not compile, because even a `fun interface` does not inherit from `KFuction` (and its `invoke`) by default.

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