Closure of operations illustrates yet another example of coercing composition and structure upon object-oriented designs.

Essentially, closure simplifies reasoning about a problem by restricting the domain of the discourse. The example of a functional implementation of a domain exhibits this characteristic at a fundamental level. The operation of applying a domain event is closed under the set of domain states. In terms of persistence, this naturally translates to event- sourcing, but also supports persistence in a key-value store or ORM with no required modification.