Quasiimmutability

Quasiimmutability refers to a state where an object or data structure can be considered immutable for practical purposes, even though it may not be strictly immutable in the absolute sense. This concept arises in programming and data management when direct modification is either impossible, prohibitively expensive, or undesirable, leading to behaviors that mimic immutability.

In many systems, true immutability means that once an object is created, its internal state can never be changed. Any operation that appears to modify an immutable object actually creates a new object with the desired changes. Quasiimmutability achieves a similar outcome through different mechanisms. One common scenario is when data is stored in a read-only medium, such as a CD-ROM or a blockchain. While the underlying storage might not be inherently immutable, the inability to alter the data once written makes it functionally quasiimmutable.

Another context for quasiimmutability is when dealing with large, complex data structures that are expensive to copy. Instead of creating a full copy for each apparent modification, systems might employ techniques like structural sharing or copy-on-write. In copy-on-write, a resource is shared until a modification is attempted. Only then is a copy made, and the modification is applied to the copy, leaving the original intact. This provides the illusion of immutability without the full overhead of creating new objects for every change.

The benefits of quasiimmutability are similar to those of true immutability, including simplified reasoning about program state, easier concurrency management due to the absence of race conditions on mutable data, and improved performance in certain caching scenarios. While not strictly immutable, quasiimmutable data offers a valuable trade-off between true immutability and the flexibility of mutable data.