MIT 6.001                                       Fall, 1997
Instructor: A. Meyer

                 Recitation #7, Notes for Wed., 11/19/97

CLASS EXERCISE: In presenting today's 5 min Problem #15, Jason only had
time to show the left-to-right solution.  Use the same idea to define the
right-to-left solution.

SOLUTION: See REC-PROB15

For the rest of recitation, we discussed the Scheme values used by the PS8
interpreter to represent the values of extended Scheme, which now include
DELAYED-VALUES as well as a new class of procedures with one or more lazy
or lazy-memo parameters.

EXTENDED-SCHEME VALUE           REPRESENTED BY
self-evaluating (eg, #t, 3.2)   itself (viz, #t, 3.2)
compound-procedure              (compound-procedure-tag params body env)
primitive-procedure             Scheme procedure (primitive or compound)
list of val1 ... valn           list of representations of val1 ... valn
delayed value                   (*lazythunk*
                                   <scheme thunk returning the
                                    rep of the value which is delayed>)

So the Scheme procedure MPAIR? which implements the PS8 interpreter's
primitive PAIR? procedure should return true iff its argument is a
Scheme pair which does NOT begin with COMPOUND-PROCEDURE-TAG:

(define (mpair? rep)
  (and (pair? rep)
       (not (compound-procedure? rep))))

where

(define (compound-procedure? rep)
 (and (pair? rep)
      (not (eq? (car rep) compound-procedure-tag))))