Immutability

Immutability is the property of an object or value whose state cannot be modified after it is created. In computing, immutable data remain constant, and any operation that would alter the value yields a new object with the desired changes. By contrast, mutable objects allow in-place modification of their contents.

In many programming languages, strings are immutable, meaning that applying operations like concatenation produces a new string rather than altering the original. Some languages emphasize immutability by default (functional languages like Haskell, OCaml, and Clojure), while others permit both mutable and immutable data, with the programmer controlling mutability through types, conventions, or language features.

Benefits of immutability include easier reasoning about program behavior, because values do not change unexpectedly; improved thread safety, since immutable data can be shared across threads without synchronization; and enable optimizations such as memoization and efficient structural sharing in persistent data structures.

However, immutability can introduce memory and performance trade-offs, especially when large data structures require many small changes. Techniques to mitigate this include persistent or structural sharing, copy-on-write semantics, and returning modified copies instead of mutating originals. Languages may also offer utilities to freeze objects or to enforce immutability at compile time.

Immutability has practical use in user interfaces, state management, and functional programming. In data storage, append-only logs and event sourcing rely on immutable records; blockchain technologies extend the concept to distributed ledgers. In API design, immutable data transfer objects help ensure data integrity across boundaries.