The summary.
VALUE TYPES ARE THE DEFAULT FOR A REASON
Copies cannot be changed unexpectedly by other code, and they cannot form reference cycles.
Use a class when identity matters or sharing is deliberate.
MODEL STATE SO INVALID STATES CANNOT BE EXPRESSED
Enumerations with associated values, with the compiler requiring every case to be handled.
ASK OF EVERY FORCE UNWRAP: WHAT IF THIS IS NOTHING?
Defensible only where absence indicates a programming error. Never for anything from input, the network or storage.
SMALL PROTOCOLS DESCRIBING ONE CAPABILITY
Large ones force conforming types to implement what they do not need.
ERRORS ARE VISIBLE IN THE SIGNATURE — USE THAT
Propagate what you cannot act on, add context, and never catch everything and continue silently.
ACTORS TURN DATA RACES INTO COMPILE-TIME PROBLEMS
Mark anything touching the interface as main-actor, and always support cancellation.
CAPTURE WEAKLY IN CLOSURES
Memory rising as screens open and close and never falling is the signature of a retain cycle. The memory graph shows what retains an object.
