How to limit the scope of extension functions in Kotlin.

limitedExtensions.kt

interface Extension

// Why is keyword `class` instead of `object`?
// Because we can use the `foo` function after importing `FooExtension.foo`.
class FooExtension : Extension {
    fun String.foo(): String = "Foo${this}"
}

class BarExtension : Extension {
    fun String.bar(): String = "Bar${this}"
}

fun <E : Extension> extendWith(extension: E, block: E.() -> Unit) = block.invoke(extension)

fun main(args: Array<String>) {
    val name = "Buz Bar"
//    println(name.foo())
//    println(name.bar())

    extendWith(FooExtension()) {
        println(name.foo())
//        println(name.bar())

        extendWith(BarExtension()) {
            println(name.foo())
            println(name.bar())
        }
    }
}

How about this solution:

interface Extension
operator fun <E: Extension> E.invoke(body: E.() -> Unit) = Unit

object FooExtension : Extension {
    fun String.foo(): String = "Foo${this}"
}

object BarExtension : Extension {
    fun String.bar(): String = "Bar${this}"
}

fun main(args: Array<String>) {
    val name = "Buz Bar"

//    println(name.foo())
//    println(name.bar())

    FooExtension {
        println(name.foo())
//        println(name.bar())

        BarExtension {
            println(name.foo())
            println(name.bar())
        }
    }
}

@Alex100 , good idea, but with a little bug

interface Extension
operator fun <E: Extension> E.invoke(block: E.() -> Unit) = block() // block won't be executed if we don't invoke it

object FooExtension : Extension {
    fun String.foo(): String = "Foo${this}"
}

object BarExtension : Extension {
    fun String.bar(): String = "Bar${this}"
}

fun main(args: Array<String>) {
    val name = "Buz Bar"

//    println(name.foo())
//    println(name.bar())

    FooExtension {
        println(name.foo())
//        println(name.bar())

        BarExtension {
            println(name.foo())
            println(name.bar())
        }
    }
}

Another way:

// Why is keyword `class` instead of `object`?
// Because we can use the `foo` function after importing `FooExtension.foo`.
class FooExtension(block: FooExtension.() -> Unit) {
    init { block() }

    fun String.foo(): String = "Foo${this}"
}

class BarExtension(block: BarExtension.() -> Unit) {
    init { block() }

    fun String.bar(): String = "Bar${this}"
}

fun main(args: Array<String>) {
    val name = "Buz Bar"

//    println(name.foo())
//    println(name.bar())

    FooExtension {

        println(name.foo())
//        println(name.bar())

        BarExtension {
            println(name.foo())
            println(name.bar())
        }
    }
}

object can't stop using the foo function after importing FooExtension.foo, class will generate duplicated instance.
Finally, I give up this way.