recently I came across this article -&gt; <https://medium.com/google-developer-experts/companion-obj...

# announcements

[Marko Novakovic]

recently I came across this article -> https://medium.com/google-developer-experts/companion-objects-kotlins-most-unassuming-power-feature-fb5c0451fbd0
It showcases some great use-cases but it made me a bit confused
For example, what really is the difference between

fun interface Validation : (String) -> Boolean {
    companion object
}

val Validation.Companion.validateEmail
    get() = Validation { it.isNotEmpty() }

val Validation.Companion.validatePassword
    get() = Validation { it.isNotEmpty() }

val isFormValid = Validation.validateEmail("email") && Validation.validatePassword("password")

and

typealias Validation = (String) -> Boolean

val validateEmail: Validation = { email: String -> email.isNotEmpty() }
val validatePassword: Validation = { password: String -> password.isNotEmpty() }

val isFormValid = validateEmail("email") && validatePassword("password")

What are use-cases where first one would add value over second one? Or is this just semantic difference?

[Marko Novakovic]

you can add extensions to functions and to typealias shown above like the “concrete” type, type is type and functions are types too

[Tomasz Krakowiak]

@Marko Novakovic I don't think you can add extension to typealias in kotlin. Maybe you can add extension to target type referencing it via alias in extension declaration - https://pl.kotl.in/U2JIFiO6j

[Marko Novakovic]

@Tomasz Krakowiak
Run this:

typealias Validation = (String) -> Boolean

val validateEmail: Validation = { email: String -> email.isNotEmpty() }
val validatePassword: Validation = { password: String -> password.isNotEmpty() }

val isFormValid = validateEmail("email") && validatePassword("password")

val <http://Validation.int|Validation.int>: Int
    get() = 1

fun Validation.test(): Unit = println("Test")

fun main() {
    validateEmail.test()
    val int = <http://validateEmail.int|validateEmail.int>
    println(int)
}

[Marko Novakovic]

@Tomasz Krakowiak didn’t try that, interesting indeed

[Tomasz Krakowiak]

@Marko Novakovic The example was incorrect.

[Tomasz Krakowiak]

Should be:

typealias Validation = (String) -> Boolean
typealias SomethingUrelated = (String) -> Boolean

fun Validation.test(): Unit {
    println("Test")
}

fun main() {
    val somethingUrelated : SomethingUrelated = TODO()
    somethingUrelated.test()
}

And this one does compile.

[Tomasz Krakowiak]

Also:

typealias Validation = (String) -> Boolean
typealias SomethingUrelated = (String) -> Boolean

fun Validation.Companion.test(): Unit { // Unresolved reference: Companion
    println("Test")
}

[Tomasz Krakowiak]

The things are different if you use SAM of course : )

[Marko Novakovic]

true 😄