ForEach

This is the first terminator we'll see. Terminator functions return something other than a new collection, so you can't chain the result of this call to other calls.

In the case of forEach(), it returns Unit. So it's like the plain, old for loop:

val numbers = (0..5)

numbers.map { it * it}          // Can continue
.filter { it < 20 }      // Can continue
.sortedDescending()      // Still can
       .forEach { println(it) } // Cannot continue

Do note that forEach() has some minor performance impacts compared to the traditional for loop. 

There's also forEachIndexed(), which provides an index in the collection alongside the actual value:

numbers.map { it * it }
.forEachIndexed { index, value ->
    print("$index:$value, ")
}

The output for the preceding code will be as follows:

0:1, 1:4, 2:9, 3:16, 4:25,

Since Kotlin 1.1, there's also the onEach() function, which is a bit more useful, since it returns the collection again:

numbers.map { it * it}         
.filter { it < 20 }     ...