Enums

Enums Are Disjoint Unions

• AKA "sum types". Idea is to have a single value represent one of several alternatives

• Alternatives can be names, or value-carrying names

Using enum

• Alternatives normally carry type-qualified names

  enum E { A, B }
  
  ...
  
  let x = E::A;
  

  but there is a trick:

  use E::*;
  
  let x = A;
  

• Can impl an enum just as with a struct

• Enum values come in tuple or structured form: tuple is more "normal", because…

• Pattern matching is the only sane way to extract carried values from an enum

• Compiler checks that all alternatives are matched, as with other types

  https://play.rust-lang.org/?gist=fbde8cb698a32c782fab7cbfcd92e169

• Generics and lifetimes work the same as with structs

Non-Value-Carrying enums Are "Special"

• Alternatives have a corresponding usize value

• Values start at 0, increase by 1

• Can set values for elements (unspecified elements autoincrement, be careful)

• Can cast alternative to usize, but no built-in way to convert usize to alternative

• https://play.rust-lang.org/?gist=550961904c20aac0d49a78fe48bd4e0f

The "Void" enum

• What should "enum Void;" mean?

• Apparently no inhabitants of this type

• Thus, can't really pass it around or whatever

• Rust prefers the type "!" for an uninhabited type, but this isn't fully consistent in the language yet; sometimes it means "bottom"

Why Are Sum Types A Big Deal?

• Avoid redundant storage, as with C union

• "Tags" are managed by the language, so no tag mismatch issues

• Combine (limited) convenience with type and memory safety

• "And-or trees" are a thing

Examples

http://github.com/pdx-cs-rust/playing-cards

http://github.com/PoHuit/plan-b