Knowledge · Anti-Patterns

Smell Mutable Data

Code smell indicating overuse of mutable state that makes reasoning about code difficult

Impact

Changes to data can often lead to unexpected consequences and tricky bugs. Functional programming is based on the notion that data should never change and updating a data structure should always return a new copy with the change, leaving the old unchanged. Even when not going fully immutable, limiting where data changes occur makes code easier to understand and reduces the opportunities for bugs.

Symptoms

Changes to data can often lead to unexpected consequences and tricky bugs. Functional programming is based on the notion that data should never change and updating a data structure should always return a new copy with the change, leaving the old unchanged. Even when not going fully immutable, limiting where data changes occur makes code easier to understand and reduces the opportunities for bugs.

Source

Martin Fowler — Refactoring: Improving the Design of Existing Code, 2nd