Contracts over inheritance.
WHAT A PROTOCOL IS
A set of requirements a type can declare it satisfies.
WHAT THAT PROVIDES
A contract without an implementation.
WHY IT MATTERS
Code depends on capability rather than a concrete type, which makes substitution and testing possible.
WHAT PROTOCOL EXTENSIONS ADD
Default implementations, shared by every conforming type.
WHY THAT IS SIGNIFICANT
It provides code sharing without inheritance, and works for value types.
WHAT COMPOSITION MEANS HERE
A type conforming to several protocols, each small and focused.
WHAT TO PREFER
Small protocols describing one capability.
WHY
Large protocols force conforming types to implement things they do not need.
WHAT ASSOCIATED TYPES PROVIDE
Protocols parameterised by a type the conforming type chooses.
WHAT THEY COMPLICATE
Using the protocol as a plain type, which requires additional handling.
WHAT THE MODERN KEYWORDS DISTINGUISH
A specific concrete type the caller need not know Any conforming type, boxed at runtime
WHAT TO PREFER
The first, where the type is fixed, since it avoids runtime cost.
