Friday, April 21, 2017

Quick Kotlin idiom: naming things

Kotlin is very much a better Java. A great example is this simple idiom for naming things:

open class Named<in T>(val name: String, check: (T) -> Boolean)
    : (T) -> Boolean by check {
    override fun toString() = name
}

So I can write something like this:

fun maybe_three(check: (Int) -> Boolean) {
    if (check(3)) do_the_three_thing()
    else println("\"$check says\", Do not pass go.")
}

maybe_three(Named("Is it three?") { i -> 3 == i })
maybe_three(Named("Is it four?") { i -> 4 == i })

The first call of maybe_three prints: Breakfast, lunch, dinner. The second call prints: "Is it four?" says, Do not pass go.

Many variations on this are possible, not just functions! What makes this example work nicely is delegation—the magical by keyword— for the general feature of naming things by overriding toString(); and for the function delegated to, the elegant lambda (anonymous function) syntax for the last parameter. You can delegate anything, not just functions, so you could make named maps, named business objects, et al, by using delegation on existing types without needing to change them.

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