; Explicit-control Evaluator with Harold (the magician (Y!))
;   Hal shortly emphasizes what has been done with Lisp until now.

; This section terminates in having the implicit and the explicit
; part of Lisp described. So to understand it from both perspectives.

; 9B:(7m44s) Constructing the evaluator machine
; REGISTERS:
;  EXP    expression to be evaluated
;  ENV    evaluation environment
;  FUN    procedure to be applied
;  ARGL   list of evaluated arguments
;
;  CONTINUE
;         place to go to next
;
;  VAL    result of evaluation
;  UNEV   temporary register for
;         expressions
; SAMPLE EVALUATOR-MACHINE OPERATIONS:
;  (assign val (fetch exp))
;  (branch
;   (conditional? (fetch exp))
;   ev-cond)
;  (assign exp (first-clause (fetch exp)))
;  (assign val
;   (lookup-variable-value (fetch exp)
;                          (fetch env)))

; 9B:(14m6s) Evaluator from the book
;   see also TEXINFO node: 4-1-1

; The contracts are as follows:
;;; EVAL-DISPATCH CONTRACT
;- The EXP register holds an expression to be evaluated.
;- The ENV register holds the environment in which the
;  expression is to be evaluated.
;- The CONTINUE register holds a place to go to next.
;
;- The result will be left in the VAL register. Contents
;  of all other registers may be destroyed.
;;; APPLY-DISPATCH CONTRACT
;- The ARGL register contains a list of arguments.
;- The FUN register contains a procedure to be applied.
;- The top of the STACK holds a place to go to next.
;
;- The result will be left in the VAL register. The
;  stack will be popped. Contents of all other registers
;  may be destroyed.
; 9B:(18m33s) BREAK

; 9B:(19m45s) Hal and Jay join to demonstrate a *running* program.
; 9B:(22m36s) Hal and Jay demonstrate EXAMPLE II (VARIABLE).
; 9B:(23m24s) Hal and Jay demonstrate EXAMPLE III (EXPRESSION).
; 9B:(33m32s) End of EXPERIMENT.
; 9B:(34m15s) Conclusion => QUESTIONS
;   -> Reduction-Process of Functions and Procedures
;      With reduction Hal means; there is nothing more on the stack.
;   -> Reduction in general means expansion into some other
;      form of underlying representation.
;   -> Lisp uses an inductive programming paradigm in which one uses
;      implicit recursive reduction to specify large parts of a program.
; BREAK

; 9B:(38m38s) Hal shows the last ingredient.
;   The EVAL/APPLY loop.
;   He shows the audience a recursively defined, iterative process.
; 9B:(51m41s) BREAK
;   Now we see a recursively defined, recursive process.

; 9B:(1h3m) Summarizing
; 9B:(1h5m40s) The Lisp Machine Lisp in Hal's hands.
; Relax (END)