SICP Solutions

Chapter 5, Computing with Register Machines

Exercise 5.24

Few things to note:

  • ev-sequence reads from unev. Thus before jumping to ev-sequence we need to set unev with the cond-actions to be executed.
  • eve-sequence itself pops from stack to read continue, thus we need to make sure that top of stack contains correct continue.
  • To iterate clauses, i used car and cdr. Ideally they should be abstracted with methods like first-clause and rest-clauses.
  • At present, error is logged in this case only: If else is not last clause, then error is reported only when the control reaches the else clause. Else if there is some expression that evaluated to true before else, then no error is logged.
  • If no clause, evaluates to true, false is returned.
  • Add the procedures like not, null?, car, cond-predicate, cond-else-clause? etc in the list of operations while creating eceval.
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ev-cond
  (assign exp (op cond-clauses) (reg exp))
  (save continue)
ev-cond-loop
  (test (op null?) (reg exp))
  (branch (label ev-cond-false))
  (assign unev (op car) (reg exp))
  (test (op cond-else-clause?) (reg unev))
  (branch (label ev-cond-else))
  (save exp)
  (save env)
  (assign exp (op cond-predicate) (reg unev))
  (assign continue (label ev-cond-decide))
  (goto (label eval-dispatch))
ev-cond-decide
  (restore env)
  (restore exp)
  (test (op true?) (reg val))
  (branch (label ev-cond-actions))
  (assign exp (op cdr) (reg exp))
  (goto (label ev-cond-loop))
ev-cond-else
  (assign unev (op cdr) (reg exp))
  (assign unev (op null?) (reg unev))
  (test (op not) (reg unev))
  (branch (label ev-cond-error))
ev-cond-actions
  ;(restore continue)
  (perform (op user-print) (const 'here))
  (perform (op user-print) (reg exp))
  (assign exp (op car) (reg exp))
  (assign unev (op cond-actions) (reg exp))
  (goto (label ev-sequence))
ev-cond-false
  (restore continue)
  (assign val (const false))
  (goto (reg continue))
ev-cond-error
  (assign val (const else-not-last-in-cond-error))
  (goto (label signal-error))

Example/Output:

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1 ]=> 
(define the-global-environment (setup-environment))
(start eceval)

;Value: the-global-environment

1 ]=> 

;;; EC-Eval input:

(define x -5)
(cond ((> x 0) x)
      ((= x 0) 0)
      (else (- x)))

(total-pushes = 3 maximum-depth = 3)
;;; EC-Eval value:
ok

;;; EC-Eval input:
(quote here)((else (- x)))
(total-pushes = 26 maximum-depth = 8)
;;; EC-Eval value:
5

;;; EC-Eval input:
(define x 5)

(total-pushes = 3 maximum-depth = 3)
;;; EC-Eval value:
ok

;;; EC-Eval input:

(cond ((> x 0) x)
      ((= x 0) 0)
      (else (- x)))
(quote here)(((> x 0) x) ((= x 0) 0) (else (- x)))
(total-pushes = 11 maximum-depth = 8)
;;; EC-Eval value:
5

;;; EC-Eval input:
(define x 0)

(total-pushes = 3 maximum-depth = 3)
;;; EC-Eval value:
ok

;;; EC-Eval input:

(cond ((> x 0) x)
      ((= x 0) 0)
      (else (- x)))
(quote here)(((= x 0) 0) (else (- x)))
(total-pushes = 21 maximum-depth = 8)
;;; EC-Eval value:
0

;;; EC-Eval input:

(cond ((> x 0) x)
      (else (- x))
      ((< x 0) (- x)))
else-not-last-in-cond-error

;;; EC-Eval input:
(cond (false 'abc) (false 'def))

(total-pushes = 5 maximum-depth = 3)
;;; EC-Eval value:
false

;;; EC-Eval input: