Implementing Set<T>

Question

I'm trying to wrap my head around abstract by implementing a Set data-type, like so:

abstract Set<T>(Map<T, Bool>) {
  public inline function new() {
    this = new Map<T, Bool>();
  }

  public inline function has(item:T):Bool {
    return this.exists(item);
  }

  public inline function add(item:T):Set<T> {
    this.set(item, true);
    return null;
  }

  public inline function remove(item:T):Set<T> {
    this.remove(item);
    return null;
  }

  public inline function iterator():Iterator<T> {
    return this.keys();
  }
}

The compiler doesn't like this, though. It tells me Set.hx:8: characters 11-29 : Abstract Map has no @:to function that accepts IMap<util.Set.T, Bool>

I don't really understand this at all, since if I change the constructor to

public inline function new(val:Map<T, Bool>) {
  this = val;
}

and then instantiate with var set = new Set(new Map());, it works.

That's pretty gross, though. I'd like the ability to instantiate Sets without exposing the underlying implementation. Ultimately, I'd prefer a constructor with the signature new(?initial:Iterable<T>). Is this possible? Am I misunderstanding something?

Answer 1

The problem is that currently it's impossible to instantiate Map without they key type being known (and since Set.T is a free type parameter, this doesn't work). However since the constructor is inline, T may well be known at the call site. The problem is that the compiler still tries to generate Set.new. You can avoid this by prefixing it with @:extern. Working example: https://try.haxe.org/#1D06C