== Why is this an issue?

If a mutable collection type is used but no mutating functions such as `add` or `remove` are ever called,
and the collection instance does not leave the scope of the function,
it can be replaced with the corresponding immutable collection type.

This is similar to why `val` should be used instead of `var` for local variables that are never re-assigned.

=== What is the potential impact?

==== Readability and Understanding

If an immutable collection type is used, it is evident to the readers that its content is never changed.
This makes it easier to understand the code because readers do not need to keep track of possible state changes of the collection.

==== Performance

In some cases, optimized implementation variants of collection classes can be used when the collection is immutable.

==== Wrong code

Developers might intend for a collection to remain unchanged and have their code relying on that constraint.
For example, a map could be expected to contain specific elements.
Changing the contents of a collection breaks that constraint.
Also, users of an API might otherwise downcast an immutable collection they got from a library
into a mutable collection, and so cause unforeseen side effects.

Declare collections that remain unchanged as immutable to avoid these mistakes.

== How to fix it

Replace mutable collection type names such as `MutableList` or `MutableMap`
with their immutable equivalents, such as `List` or `map`.

Replace builder functions that return mutable collection instances,
such as `mutableListOf` with their immutable counterparts, such as `listOf`.

=== Code examples

==== Noncompliant code example

[source,kotlin,diff-id=1,diff-type=noncompliant]
----
fun sum123(): Int {
    val list = mutableListOf(1,2,3) // Noncompliant, can be immutable
    return list.reduce { acc, it -> acc + it}
}
----

==== Compliant solution

[source,kotlin,diff-id=1,diff-type=compliant]
----
fun sum123(): Int {
    val list = listOf(1,2,3) // Compliant
    return list.reduce { acc, it -> acc + it}
}
----

==== Noncompliant code example

[source,kotlin,diff-id=2,diff-type=noncompliant]
----
fun sumList(list: MutableList<Int>): Int { // Noncompliant, can be immutable
    return list.reduce { acc, it -> acc + it}
}
----

==== Compliant solution

[source,kotlin,diff-id=2,diff-type=compliant]
----
fun sumList(list: List<Int>): Int { // Compliant
    return list.reduce { acc, it -> acc + it}
}
----

==== Noncompliant code example

[source,kotlin,diff-id=3,diff-type=noncompliant]
----
fun MutableList<Int>.sum(): Int { // Noncompliant, can be immutable
    return reduce { acc, it -> acc + it}
}
----

==== Compliant solution

[source,kotlin,diff-id=3,diff-type=compliant]
----
fun List<Int>.sum(): Int { // Compliant
    return reduce { acc, it -> acc + it}
}
----

== Resources

=== Articles & blog posts

* https://www.baeldung.com/kotlin/immutable-collections[Baeldung, Kotlin Immutable Collections]
