write-functional

To write pure functional code, implement business logic as pure functions that return new values without mutating inputs, and surface side effects only at the I/O boundary. This approach preserves referential transparency and ensures a testable core.

The best way to build transformation pipelines without global mutation is using composition over mutation. Create modular functions that compose cleanly, ensuring data flows through the pipeline by returning new values rather than altering existing state.

Handle stateful parsing by isolating side effects at the edge interfaces. Keep the parsing core pure by passing state through the transformation pipeline explicitly, returning new values without mutating the original inputs.

You need strong types to ensure functional code quality because they drive safety and explicit error handling. Leveraging strong types with a pure core and clearly defined edge interfaces improves reliability and makes the business logic highly testable.

Avoid a pure functional approach when your task requires heavy in-place mutation for performance or when isolating side effects at the boundary is impractical. If referential transparency cannot be preserved, the functional style will add unnecessary complexity.