compile.sh

valac -o test first.vala second.vala main.vala

# OR

# ???

first.vala

class First {
  public int a() {
    return 4;
  }

  public Second get_second() {
    return new Second(this);
  }
}

main.vala

void main() {
  Second s = new Second(new First());
  s.p();

  First f = new First();
  f.get_second().p();
}

second.vala

class Second {
  private First f;

  public Second(First f) {
    this.f = f;
  }

  public void p() {
    print(@"$(f.a())\n");
  }
}