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A Scala trait can intercept a method, call the next implementation through super, and add behavior before or after it. abstract override marks such an implementation as requiring an appropriate concrete method later in the class linearization. A character-counting plugin can update its cached counts only after a successful underlying document edit, preserving its additional representation invariant without changing the edit's abstract result.

Recursion in which the recursive call is the last operation required to obtain the current call's result. An accumulator can encode the state that would otherwise be updated by a loop. A suitable compiler can replace the call by a jump, avoiding growing call stack storage. In Scala, a tail-recursion annotation requests a check that the recursive method is eligible for this transformation.

A reusable type and collection of members that can be composed into a class in object-oriented programming. In Scala, a trait can declare required members and provide concrete implementations. Several traits can contribute mixin behavior to one class, subject to linearization and member-conflict rules.