MIT 6.001                                       Fall, 1997
Instructor: A. Meyer

                 Recitation #7, Notes for Fri., 11/14/97

T.A. Anna Chefter ran this recitation, covering the basic eval-apply cycle
of the meta-circular evaluator of Section 4.1.  The IN-CLASS EXERCISE was
to complete the definition of
                  (define (eval-sequence exps env) ...)
and many students were indeed able to derive the code used in Section 4.1
(without looking it up).

A couple of students used MAP:

(define (eval-sequence exps env)
 (let ((vals (map (lambda (exp) (meval exp env)) exps)))
   (list-ref vals (dec (length vals)))))  ;the last value

This is right PROVIDING the underlying Scheme evaluates the operands of a
combination from left to right, because MAP applies its procedure argument
to the elements of its list argument in the evaluation order of the
underlying Scheme.  So this definition with MAP will be INCORRECT in MIT
Scheme, for example, which goes right to left.

The definition of EVAL-SEQUENCE in Section 4.1 will correctly evaluate the
EXPS from left to right no matter what evaluation order the underlying
Scheme uses.  This observation provides nice motivation for next week's 5
min presentation.