Mithun Sasidharan
65 lines
2.2 KiB
Markdown
65 lines
2.2 KiB
Markdown
# Enums
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Goals:
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* Better syntax for passing constructor parameters when defining enum constants
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* Resolve issues with annotation syntax for enum constants
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## Example
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Simple enum:
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``` kotlin
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enum class Foo {
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A
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B
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C {
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override fun foo() { ... }
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}
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open fun foo() {}
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}
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```
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Enum with constructor:
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``` kotlin
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enum class Foo(val s: String) {
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A("a")
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B("b")
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C("c") {
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override fun foo() { ... }
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}
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open fun foo() {}
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}
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```
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Issues
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* Enum literals syntax clash with annotation syntax
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* Option 1.1: Forbid short annotation syntax in enums. **downside**: cannot annotate functions/properties/classes in this enum
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* Option 1.2: Add a separator between enum constants and members, and forbid short annotation syntax only on enum entries 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)
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* Option 1.3: prefix each entry with a soft-keyword, e.g. `entry`. **downside**: verbosity
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* Option 1.4 **chosen**: Add a semicolon separator after the last enum constant, **and** a comma separator between different enum constants.
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* How do we specify other supertypes for a constant (if any)
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* Option 2.1 **chosen**: Leave unsupported, use cases are very few, and Java does not support it
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* Option 2.2: `A("s"): OtherType`
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Example for option 1.4:
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``` kotlin
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enum class Foo(val s: String) {
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A("a"), // semicolon CAN NOT be used here!
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B("b"), // comma is MANDATORY after each enum constant except the last one
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C("c") {
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override fun foo() { ... }
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}; // semicolon is MANDATORY here, even if no member follows
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open fun foo() {}
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}
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```
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Notes:
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* Almost no overhead in the most common case of no members at all: `enum class E {A, B, C; }
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* Clear error message: if the parser sees no semicolon after the last constant:
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* it reports an error saying "There must be a semicolon after the last enum entry", which is rather instructive
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* a quick fix can even guess the right position for the semicolon most of the time
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* Today, there's no way of naming an enum entry `public` (or any other soft-keyword used as a modifier), which is unfortunate
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