6.001 Recitation #7                                        Fall 1997
Prof. Albert R. Meyer


          5-minute-problem #15: Controlling Order of Evaluation
                      in The Metacircular Evaluator
          (posed in email, 11/14/97 for presentation Wed, 11/19)

We might expect that with the precise code for a Scheme evaluator
in front of us, we could figure out exactly the order in which it
evaluates the subexpressions in a combination.  The LIST-OF-VALUES
procedure on page 367 of Chapter 4, Section 4.1 in SICP evaluates those
subexpressions:

(define (list-of-values exps env)
  (if (no-operands? exps)
      '()
      (cons (eval (first-operand exps) env)
            (list-of-values (rest-operands exps) env))))

But examining LIST-OF-VALUES, we realize that our metacircular
evaluator inherits the order of evaluation that the underlying Scheme
happens to use, so order of evaluation remains undetermined.

Explain this observation to the class.  Then show how to modify
LIST-OF-VALUES so that it always evaluates expressions left to right --
likewise for right to left.

SOLUTION

LEFT TO RIGHT:
(define (list-of-values exps env)
  (if (no-operands? exps)
      '()
       (let ((first-val (eval (first-operand exps) env)))
         (cons first-operand       
               (list-of-values (rest-operands exps) env)))))

RIGHT TO LEFT:
(define (list-of-values exps env)
  (if (no-operands? exps)
      '()
       (let ((rest-vals
              (list-of-values (rest-operands exps) env)))
         (cons (eval (first-operand exps) env)
               rest-vals))))