Immutable

Immutability is a property of data whose state cannot be modified after it is created. In programming, an immutable object cannot be altered; operations that would modify it instead return a new object with the desired changes. Immutable data structures preserve the previous versions when updated and often rely on structural sharing to minimize memory use.

Many languages provide immutable primitives or types. Strings in Java and Python are immutable; numbers and booleans are typically immutable as well. Some languages encourage or enforce immutability, such as Haskell, Erlang, and Clojure, where immutable data and pure functions are central. Other languages allow immutability by default for certain types or offer read-only views.

Benefits of immutability include easier reasoning about program behavior, since values do not change unexpectedly, and improved thread safety, as immutable objects can be freely shared without synchronization. Immutability also supports features like convenient undo/redo, snapshots, and persistent data structures.

Drawbacks include potential memory and performance costs from creating new objects rather than updating existing ones. To mitigate these costs, languages and libraries use persistent data structures and structural sharing, where unchanged portions of a structure are reused between versions. Copy-on-write and related techniques are also employed to balance safety with efficiency.

In practice, developers choose immutability to enhance safety and predictability, while mutability remains useful for performance-critical or low-level tasks. Common approaches combine both paradigms, using builders, copy-on-write patterns, or transient mutability for batch updates before producing immutable results.