kotlin-fork/spec-docs/enums.md

Enums

Goals:

• Better syntax for passing constructor parameters when defining enum constants
• Resolve issues with annotation syntax for enum constants

Example

Simple enum:

enum class Foo {
    A
    B
    C {
        override fun foo() { ... }
    }
  
    open fun foo() {}
}

Enum with constructor:

enum class Foo(val s: String) {
    A("a")
    B("b")
    C("c") {
        override fun foo() { ... }
    }
  
    open fun foo() {}
}

Issues

• Enum literals syntax clash with annotation syntax
  • Option 1.1: Forbid short annotation syntax in enums. downside: cannot annotate functions/properties/classes in this enum
  • Option 1.2: Add a separator between enum constants and members, and forbid short annotation syntax only on enum entriesc themselves. downside: separator is not intuitive, hard to think of when doing this for the first time (the error message will be rather clear and instructive, though)
  • Option 1.3: prefix each entry with a soft-keyword, e.g. entry. downside: verbosity
• How do we specify other supertypes for a constant (if any)
  • Option 2.1: Leave unsupported, use cases are very few, and Java does not support it
  • Option 2.2: A("s"): OtherType

Example for option 1.2:

enum class Foo(val s: String) {
    A("a") // semicolon CAN NOT be used here!
    B("b")
    C("c") {
        override fun foo() { ... }
    }; // semicolon is MANDATORY here, if a member follows
  
    // if no semicolon was provided, `open` is another enum entry
    open fun foo() {}
}

Notes:

• No overhead in the most common case of no members at all: `enum class E {A B C}
• Clear error message: if the parser sees a member, but no semicolon before it:
  • it reports an error saying "There must be a semicolon separating enum entries from members", which is rather instructive
  • a quick fix can even guess the right position for the semicolon most of the time
• Today, there's no way of naming an enum entry public (or any other soft-keyword used as a modifier), which is unfortunate