Semiimmutable

Semiimmutable is a term used in software design to describe data structures or objects that are partially immutable. In a semiimmutable object, certain attributes are fixed after construction and cannot be changed, while other attributes remain mutable and can be updated over the object's lifetime. This combination aims to provide some safety guarantees—such as invariants maintained by the immutable portion—while preserving flexibility where mutability is needed.

Implementation approaches vary by language. Common patterns include declaring some fields as final or readonly and others as mutable; exposing immutable views while offering controlled mutators; or using separate internal representations. Some languages support interior mutability, allowing mutation through safe wrappers or controlled references while presenting an immutable external interface.

Typical use cases include configuration or value objects where the identity and core properties should not change, while auxiliary data (such as caches, counters, or statistics) can be updated. Semiimmutable designs can improve thread-safety and reasoning about behavior without requiring full immutability and copying.

Trade-offs include API complexity and potential for subtle inconsistencies if mutable parts are not carefully managed. The benefit is a balance between safety and performance, as immutable portions can be reasoned about confidently while mutable parts handle dynamic updates.

Related concepts include immutability, mutability, partial immutability, copy-on-write, and interior mutability.