Immutabilitet

Immutabilitet, the Danish term for immutability, is the property of an object whose state cannot be changed after creation. In programming, immutable values or data structures cannot be modified in place; any operation that would seem to modify them instead yields a new object with the updated state. This contrasts with mutable objects, which can be altered after creation.

Immutability is a central concept in functional programming and in many modern languages that favor predictable, side-effect-free code. It enables referential transparency, meaning an expression can be replaced by its value without changing program behavior. Implementations use persistent (or structural-sharing) data structures, copy-on-write techniques, and controlled mutation through new instances rather than in-place edits. Languages such as Haskell, Erlang, and Clojure embrace immutability by default or offer immutable collections; other languages provide explicit immutable types or modifiers like const or final.

Practically, immutability supports safer concurrent execution, easier reasoning about code, and robust auditing. It underpins patterns such as event sourcing and append-only logs, where history is preserved by storing immutable records of changes. In databases and distributed systems, immutability helps ensure consistency, tamper-evidence, and reproducible state across replicas.

Trade-offs include potential increases in memory use and CPU time, since edits may require creating new objects. The cost can be mitigated with sharing, structural sharing, and efficient persistent structures. The choice between mutable and immutable design depends on performance, resource constraints, and the need for correctness and auditability.