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The Berkeley Logo Object System (BLOS) -- implemented without CLOS

Prototype OOP becomes problematic only when it hides its own lookup graph — BLOS avoids this by making the graph explicit, shallow, and inspectable.

Kind	Structure	Access
Word	scalar	identity
List	ordered n-tuple	position
Array	indexed n-tuple	index
Object	named n-tuple	name

Objects are Logo's structs -- but upgraded into environments. They are

• not just data
• not just methods
• but
  • a self-contained workspace,
  • workspace with anchestry,
  • participants in name resolution.

Lists answer where, objects answer who. Objects abridge data and context.

Objects add one more place where Logo variables may live

• an object-local variable table
• accessed only when that object is current.

Conceptually:

object = {
  variables: {name -> value},
  procedures: {name -> procedure},
  parents: [object...]
}

with dynamic lookup, not lexical. -> In Logo, procedures serve also as BLOS methods.

The Common Lisp implementation artefact (not the semantic center):

(defstruct blos-object
  variables   ; alist or hash-table: symbol -> value
  procedures  ; alist or hash-table: symbol -> procedure
  parents)    ; list of blos-object

The defstruct supports storage, but meaning emerges from the evaluator, not from the data structure.

Crucial distinction: In CLOS: Objects respond to messages. In BLOS: Procedures are looked up relative to an object.

Logo concept	CL implementation role
Object	blos-object struct
Object variable	lookup entry in variables slot
Object procedure (method)	lookup entry in procedures slot
Inheritance	parent chain lookup
Current object	dynamic variable *current-object*
ask/talkto	dynamic rebinding of context
usual.foo	explicit parent traversal

talkto TALKTO object

changes the current object to object. (Note that the input is an object, not the name of an object.) Talkto can be used only at toplevel or within a pause (when typing into a Logo prompt, not inside a procedure).