SELF — one factory to rule them all

Another promise fulfilled — having pinned constructors in hand I could add “SELF” which behaves like implicit generics. Or you can think of it as virtual static feature.

Probably the best way to present it is using factory pattern:

struct Foo
  // constructor
  def pinned init() Foo 
  do
    return self // "this" in C++ family
  end

  def static Create() SELF
  do
    return new SELF();
  end
end

struct Bar : Foo
  def pinned init() Bar
  do
    return self 
  end
end  

The “SELF” which interests us sits in “Create” method. When you call it as “Foo.Create” you will get instance of “Foo”, but when you call it as “Bar.Create” you will get “Bar” object. Please note that such call does not cause an error:

var obj Bar = Bar.Create();

The above snippet would be erroneous if “Create” was defined differently, namely:

  ...
  def static Create() Foo
  do
    return new SELF();
  end

forcing you to write:

var obj Foo = Bar.Create(); 
// Bar is created nevertheless

But since it was defined with result type as “SELF”, the actual result type changes from call to call and compiler knows about it.

To summarize — “self” is a polymorphic reference to an object, “SELF” is a polymorphic reference to a type. You can create new instances using “SELF” as long as the used constructor is marked as pinned.

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