Immutability In Swift

Gustavo Barcena

JibJab Bros. Studios

"In functional programming, you think of immutable data structures and functions that convert them. In object-oriented programming, you think about objects that send messages to each other."

-Chris Eidhof [1]

Why use Immutability?

• Isolate variables that can change
• Prevent other actors from changing your data
• Immutable data is typically thread safe by default

let

Value and Reference Types

• We typically first learn these as pass by value and pass by reference
• Think about the difference in C between
  • void functionA(int a) 
  • void functionA(int *a) 
• Value types always create copy on assignment
• Reference types share data on assignment

"Use a value type when:

• Comparing instance data with == makes sense
• You want copies to have independent state
• The data will be used in code across multiple thread

Use a reference type (e.g. use a class) when:

• Comparing instance identity with === makes sense
• You want to create shared, mutable state" [2]

Value Types

Primitives

Tuples

Enumerations

Structs

Reference Types

Classes

Enums are powerful

• In a switch statement, Swift forces you to use all of possible values
• Enum are no longer just glorified ints.
  • They can hold information as well
  • Optionals are defined as an enum
• You can add functions, computed properties, initializers to enums
• Enums can conform to protocols

How NSString is similar

• NSString by itself is an immutable class
• However NSMutableString is a subclass of NSString
• • polymorphisms allows use anywhere we use NSString
  •  (this is what copy is the recommended type for a string)
• Polymorphism can get in the way of immutablilty
  • Swift doesn't support class immutability

What should I do?

Immutability as the default

• Prefer let instead of var everywhere
• Use structs before classes
• Use enums before structs

Questions?

Sources

•  [1] http://www.objc.io/issue-16/power-of-swift.html
•  [2] https://developer.apple.com/swift/blog/?id=10
•  [3] http://nomothetis.svbtle.com/immutable-swift