== Why is this an issue?

There are two types of sequence and stream operations:

* Intermediate operations, which return another sequence or stream
* Terminal operations, which return something else

Intermediate operations are lazy, meaning they aren't actually executed until and unless a terminal sequence operation
is performed on their results. Consequently, if the result of an intermediate sequence operation is not fed to a
terminal operation, it serves no purpose, which is almost certainly an error.

=== Noncompliant code example

[source,kotlin]
----
val sequence = sequenceOf("Hello", " ", "World")
var totalLength = 0
sequence.map { totalLength += it.length } // Noncompliant, do nothing, "map" is not a terminal operation
// here totalLength == 0
----

[source,kotlin]
----
val stream = listOf("Hello", " ", "World").stream()
stream.filter { it.isNotBlank() }  // Noncompliant, do nothing, "filter" is not a terminal operation
// the following "forEach" throws an exception, only one method can operate on a stream instance
// here "filter" has already operated on it.
stream.forEach { println(it) }
----

=== Compliant solution

[source,kotlin]
----
val sequence = listOf("Hello", " ", "World").asSequence()
var totalLength = sequence.map { it.length }.sum() // Compliant, the sequence ends with the terminal operation "sum"
// here totalLength == 11
----

[source,kotlin]
----
listOf("Hello", " ", "World").stream()
    .filter { it.isNotBlank() }
    .forEach { println(it) } // Compliant, the stream ends with the terminal operation "forEach"
----

== Resources

* https://kotlinlang.org/docs/sequences.html#sequence-operations[Sequence operations]