For lack of a better channel, I was trying to come...

# language-evolution

[Davio]

For lack of a better channel, I was trying to come up with a clever way to define a custom interface which extends both a function from T -> T and a predicate over T, I tried this, but it doesn't compile:

interface Processor<T> : (T) -> T, (T) -> Boolean

The error message is Type parameter R of 'Function' has inconsistent values: T, Boolean

Does anybody know if this is even possible or does anyone have an alternative other than reverting to Java's Predicate<T> for the second part?

[Alejandro Serrano.Mena]

what is your use case here? if a function extends from (T) -> T and (T) -> Boolean, it means that T must be equal to Boolean

[Alejandro Serrano.Mena]

if your use case is to have two functions, one of type (T) -> T, and another (T) -> Boolean, I think a regular class would work better

[Davio]

Yeah it's the latter, maybe it's because both lambdas internally only have a single method called invoke and you can't have 2 invokes with different output I guess

[Daniel Pitts]

Close, but it's not exactly that. It's actually the fact that you can't inherit from the same generic interface twice with different types:

interface  Foo<T>

class Bar : Foo<String>, Foo<Int>

This fails, no method signature at all to worry about.
My guess is that this is a consequence of type erasure.

[Youssef Shoaib [MOD]]

How do you intend to call this Processor? What should myProcessor(tValue) return? I think what you want is more like

interface Processor<T> {
  fun transform(t: T): T
  fun predicate(t: T): Boolean
}

And then use it as myProcessor::transform and myProcessor::predicate when need be