Multi-valued enum

Crystal Contrib
1

Speaking of AI, I once asked ChatGPT to suggest the best way to code a variable or type that would meet the following conditions: 1) never occur, 2) occur once, 3) repeat times?
It suggested the following code:

struct Occurrence(T)
  enum Type
    Once
    Never
    Repeatable(Int32)
  end

  property value : T
  property type : Type

  def initialize(@value : T, @type : Type)
    raise "Invalid type" if @type == Type::Repeatable && @type.arg <= 0
  end

  def generate
    case @type
    when Type::Once
      [@value]
    when Type::Never
      []
    when Type::Repeatable
      Array.new(@type.arg, @value)
    end
  end
end

# Examples of use
once = Occurrence.new("Unique", Occurrence::Type::Once)
never = Occurrence.new("Never happens", Occurrence::Type::Never)
three_times = Occurrence.new("Three times", Occurrence::Type::Repeatable(3))

puts once.generate # Output: ["Unique"]
puts never.generate # Output: []
puts three_times.generate # Output: ["Three times", "Three times", "Three times"]

At the time, I think it was version 3.5 of ChatGPT—far from being as powerful as it is today—and the suggested syntax was, of course, incorrect.
Nevertheless, even though the enum here wasn’t directly similar to the concept of an “open enum” we’re discussing, I found the approach of making each item multi-valued to be interesting.
I didn’t look into it any further, though.

2

This concept is a tagged union. Rust for example allows this.

It’s actually not that far from enums in Crystal which can also be considered a tagged union of unit types.The extra step is just that the types can have more than a single value.

An equivalent in Crystal would be an actual union type:

record Once
record Never
record Repeatable(T), value : T

Type = Once | Never | Repeatable(Int32)