Optionals no longer implicitly evaluate to true when they have a value and false when they do not, to avoid confusion when working with optional Bool values. Instead, make an explicit check against nil with the == or != operators to find out if an optional contains a value.
Swift now has a Nil Coalescing Operator (a ?? b), which unwraps an optional’s value if it exists, or returns a default value if the optional is nil.
Updated and expanded the Comparing Strings section to reflect and demonstrate that string and character comparison and prefix / suffix comparison are now based on Unicode canonical equivalence of extended grapheme clusters.
You can now try to set a property’s value, assign to a subscript, or call a mutating method or operator through Optional Chaining. The information about Accessing Properties Through Optional Chaining has been updated accordingly, and the examples of checking for method call success in Calling Methods Through Optional Chaining have been expanded to show how to check for property setting success.
Added a new section about Accessing Subscripts of Optional Type through optional chaining.
Updated the Accessing and Modifying an Array section to note that you can no longer append a single item to an array with the += operator. Instead, use the append method, or append a single-item array with the += operator.
Added a note that the start value a for the Range Operators a...b and a..<b must not be greater than the end value b.
Rewrote the Inheritance chapter to remove its introductory coverage of initializer overrides. This chapter now focuses more on the addition of new functionality in a subclass, and the modification of existing functionality with overrides. The chapter’s example of Overriding Property Getters and Setters has been rewritten to show how to override a description property. (The examples of modifying an inherited property’s default value in a subclass initializer have been moved to the Initialization chapter.)
Updated the Initializer Inheritance and Overriding section to note that overrides of a designated initializer must now be marked with the override modifier.
Updated the Required Initializers section to note that the required modifier is now written before every subclass implementation of a required initializer, and that the requirements for required initializers can now be satisfied by automatically inherited initializers.
Infix Operator Functions no longer require the @infix attribute.
The @prefix and @postfix attributes for Prefix and Postfix Operators have been replaced by prefix and postfix declaration modifiers.
Added a note about the order in which Prefix and Postfix Operators are applied when both a prefix and a postfix operator are applied to the same operand.
Operator functions for Compound Assignment Operators no longer use the @assignment attribute when defining the function.
The order in which modifiers are specified when defining Custom Operators has changed. You now write prefix operator rather than operator prefix, for example.
Added information about the dynamic declaration modifier in Declaration Modifiers.
Added information about how type inference works with Literals.
Added more information about Curried Functions.
