Mutables

Mutables are data values whose internal state can be changed after they are created. In programming languages, mutability describes whether a variable, reference, or the data structure it points to can be altered. Some languages treat mutation as a core, unrestricted feature, while others emphasize immutability, requiring explicit permissions or patterns to modify state. The concept of mutability influences how programs are reasoned about, particularly with regard to side effects and state changes.

Mutable entities commonly include arrays, lists, maps, and objects. These structures can be updated in place by procedures or methods, such as changing a field, adding an element to a collection, or reassigning a reference to a different object. Many languages also offer both mutable and immutable variants of a structure, or provide mechanisms that isolate mutation to restricted regions of code. Even in languages with immutable data, variables can reference objects whose contents may change, depending on the object's design.

The use of mutables offers performance advantages for frequent updates and can simplify imperative programming models. However, mutability increases the potential for subtle bugs arising from unintended state changes, aliasing, and difficulties in reasoning about program flow. In concurrent or multi-threaded contexts, mutable state often requires synchronization to prevent data races. As a result, language designers frequently provide controlled mutation constructs, encourage immutable data where possible, or employ patterns such as copy-on-write or persistent data structures to balance performance with correctness.