How to declare an immutable struct with public fields?

Question

I want to write the equivalent of this C# in F#:

struct Vector2 {
    public readonly int X;
    public readonly int Y;
    public Vector2(int x, int y) {
        X = x;
        Y = y;
    }
}

This forces the user to supply arguments to create an instance [EDIT: this is wrong for value types - all value types have a default constructor]. A default Vector2 could be provided with a static readonly field as well, i.e. Vector2.Zero.

It looks like the only way to get public fields is via the "val" keyword, but doesn't seem to let me initialize them with the default constructor, and I don't want to have two constructors:

  [<Struct>]
  type MyInt(value) =
        val public Value : int = value;;

          val public Value : int = value;;
  -------------------------------^

stdin(7,32): error FS0010: Unexpected symbol '=' in member definition

I know this can be done with member bindings but this creates properties, not fields, if I understand well.

Answer 1

According to this http://msdn.microsoft.com/en-us/library/vstudio/dd233233.aspx, that could be done as

type Vector2 =
   struct 
      val public X: int
      val public Y: int
      new(x: int, y: int) = { X = x; Y = y }
   end