Is there a way to define singleton alt class case

[dadhi]

I have a Messages and probably many more abstractions which a best represented as ADT here.

 public abstract alt class Msg : IMsg<Counter>
    {
        alt Increment(); // here I want a single instance, and better of base type Msg
        alt Decrement();
    }

Btw, kudos for such an interesting project!

[qwertie]

Wow, I'm not sure how I didn't notice this message for two days. Sorry.

alt class doesn't support singletons or even non-public constructors, but here's what else you can do. I take it you want C# code that looks like this?

	public abstract class Msg : IMsg<Counter> {
		public Msg() { }
	}
	class Increment : Msg {
		private static Increment _value = new Increment();
		public static Increment Value { get { return _value; } }
		private Increment() { }
		...
	}
	class Decrement : Msg {
		private static Decrement _value = new Decrement();
		public static Decrement Value { get { return _value; } }
		public Decrement() { }
		...
	}

I typically write repetitive code using the "define" macro:

public abstract class Msg : IMsg<Counter> {
	public Msg() { }
}

define MakeSingletonClass($ClassName, $BaseClass, {$(..members);}) {
	public class $ClassName : $BaseClass {
		private static $ClassName _value = new $ClassName();
		public static $ClassName Value { get { return _value; } }
		private this() { } // constructor
		$members;
	}
}
MakeSingletonClass(Increment, Msg)
{
    public void MethodOfIMsg() => Console.WriteLine("Incrementor!");
}
MakeSingletonClass(Decrement, Msg)
{
    public void MethodOfIMsg() => Console.WriteLine("Decrementor!");
}

A couple of notes:

• In EC#, f(a) {b;} is a syntactic alias for f(a, {b;}), so MakeSingletonClass(Decrement, Msg) {...} calls the MakeSingletonClass($ClassName, $BaseClass, {$(..members);}) macro.
• The semicolons in {$(..members);} and $members; are important and required.

[dadhi]

Thanks for the answer, it helps.

Macros are powerful, and easy to forget about it in C# world :)

Btw, I more more see an examples of moving last argument out of a list to the make a kind of scope.. Kotlin recently. Good for DSLs.