Companion code for https://medium.com/@aidan.vii/extension-classes-f9dcf5878b71

AppFeaturesExtension.kt

fun main() {
    FakeAppFeatures {
        if (SomeFeature.enabled) {
            println("some feature is enabled!")
        }
    }
}

sealed class Feature
object SomeFeature : Feature()

interface AppFeatures {
    fun isEnabled(feature: Feature): Boolean
}

object FakeAppFeatures : AppFeatures {
    override fun isEnabled(feature: Feature): Boolean = true
}

inline class AppFeaturesExtension(val appFeatures: AppFeatures) {
    val Feature.enabled: Boolean
        get() = appFeatures.isEnabled(this)

    val Feature.disabled: Boolean
        get() = enabled.not()
}

inline operator fun <T> AppFeatures.invoke(
    appFeaturesExtensionContext: AppFeaturesExtension.() -> T
): T = appFeaturesExtensionContext(AppFeaturesExtension(this))