lyng/docs/advanced_topics.md

Advanced topics

Closures/scopes isolation

Each block has own scope, in which it can safely uses closures and override outer vars:

blocks are no-yet-ready lambda declaration so this sample will soon be altered

var param = "global"
val prefix = "param in "
val scope1 = {
    var param = prefix + "scope1"
    param
}
val scope2 = {
    var param = prefix + "scope2"
    param
}
// note that block returns its last value
println(scope1)
println(scope2)
println(param)
>>> param in scope1
>>> param in scope2
>>> global
>>> void

One interesting way of using closure isolation is to keep state of the functions:

val getAndIncrement = {
    // will be updated by doIt()
    var counter = 0

    // we return callable fn from the block:
    fun doit() {
        val was = counter
        counter = counter + 1
        was
    }
}
println(getAndIncrement())
println(getAndIncrement())
println(getAndIncrement())
>>> 0
>>> 1
>>> 2
>>> void

Inner counter is not accessible from outside, no way; still it is kept between calls in the closure, as inner function doit, returned from the block, keeps reference to it and keeps it alive.