🚀 go-pugleaf

A quick summary of the object mechanism in Object Logo:

There is a hierarchy of classes.  (We'll see later that the class/instance
distinction is not enforced, but for now let's think of it that way.)  To
create a new class, you say KINDOF <oldclass>, e.g.,

	MAKE "BACKWARDS.TURTLE KINDOF TURTLE

To make a particular turtle (an instance) you say

	MAKE "TORTUGA ONEOF TURTLE

or

	MAKE "TERRAPIN ONEOF :BACKWARDS.TURTLE

[Why the colon or not?  TURTLE is a primitive *procedure* that outputs
a class; BACKWARDS.TURTLE is a *variable* whose value is a class.  If
you don't like it you can say

	TO BACKWARDS.TURTLE
	OUTPUT :BACKWARDS.TURTLE
	END

but there's nothing new about that.]

Every instruction is carried out by some object.  (There is an object
called Logo at the root of the tree that's the default object if you
don't specify one.)  To direct an instruction to an object you say

	ASK :TORTUGA [FD 100]

More complicated instructions may involve more than one object:

	ASK :TORTUGA [SETHEADING TOWARDS ASK :TERRAPIN [POSITION]]

You can specialize methods by using TO in a class:

	? ASK :BACKWARDS.TURTLE [TO FORWARD :DIST]
	> USUAL.FORWARD (- :DIST)
	> END

That's basically it, except for the details:

1.  There is no real difference between a class and an instance; both are
just objects.  The difference between KINDOF and ONEOF is that the latter,
after creating the new object, does ASK <new-object> [EXIST]
The EXIST method generally creates instance variables, but it can do
anything you want.  So an instance is just a subclass with its own
instance variables.

2.  There's multiple inheritance.

3.  There are a number of predefined classes, not just turtles but
windows, text windows, graphics windows, menu bars, and so on.  But
the classes you create don't have to be about physical screen or
system-like things; you can specialize LOGO and get a bare-bones class.