Explain Swift code

What this Swift sample shows

optionals

due: String? holds either a string or nil. due.map { ... } runs the closure only when there is a value, and ?? "" supplies the fallback when there isn't.

protocols

Describable requires a readable summary. Todo meets it with a computed property, so any code that takes a Describable can use a Todo.

closures

{ $0.minutes <= 30 } is a closure, and $0 is its first argument. map(\.title) passes a key path, a shorthand for { $0.title }.

Good to know

• Structs get a memberwise initializer for free. That is where Todo(title:due:minutes:) comes from.
• if let undated = ... unwraps the optional that first(where:) returns, so the body only runs when an undated to-do exists. Here it prints Water plants (5 min).
• quick is ["Water plants"], the only to-do that takes 30 minutes or less.

Watch out: first(where:) stops at the first match. Plan trip has no due date either but is never printed. Use filter to get all of them.