Higher order functions

A higher-order function is a function that takes functions as parameters, or returns a function

Ex1:

fun higherOrderFunction(block: () -> Unit) {
    block()
}

fun foo() {
    higherOrderFunction {
        println("do something here")
    }
}

Ex2:

fun <T> retry(retryLimit: Int, fn: () -> T): T {
    var retryCount = 0
    while (retryCount < retryLimit) {
        try {
            return fn()
        } catch (e: Exception) {
            retryCount++
        }
    }
    throw RuntimeException("All retries are done..")
}

retry(TOTAL_RETRIES) {
    println("I will be printed n times...")
}

Lambdas can access its closure, i.e. variable declared in outer scope. sum is not part of lambda stackframe hence needs to be copied and passed around.

var sum = 0
ints.filter { it > 0 }.forEach {
    sum += it
}
println(sum)

Exercise

Filter list with custom filters

The custom filter operation should be passed on a param to a higher order function to perform filtering.