Tell about two semantics in the Swift language.

Swift has two main type semantics:

1. Value Semantics:

   • Types for which assigning or passing creates a new copy of data.
   • Changes made to one copy do not affect others.
   • Applies to struct, enum, and tuple.
   • Easier for understanding parallel execution and avoiding unintended side effects.

2. Reference Semantics:

   • Types for which assigning or passing passes a reference to the same data instance.
   • Changes through one reference are visible through all other references to that same instance.
   • Applies to class and actor.
   • Requires more careful handling, especially in multithreaded environments, due to potential state issues.

Sometimes semantics can be mixed or non-obvious, for example, when reference types are contained within a value type. Copy-on-write is a technique that can be used to emulate value semantics for types containing reference types, while optimizing performance.