Mappers

A Mapper is an explicit transformation between two types. It makes type conversion visible and testable.

Protocol

Mapper[SourceType, TargetType] defines a single method: map(source) → target. It is a Protocol — any callable or object that satisfies the shape qualifies.

When to use

Use Mapper when data crosses a boundary: entity → DTO, DTO → domain object, persistence model → read model. It keeps transformation logic in one place rather than scattered across call sites.