constructing procedures, we find that CONS can be viewed as a constructor of modules, just as MAPGEN was. CONS constructs modules ("cons cells") which use SETQ to maintain a local state.
Side Effects and Equality
"Things are seldom what they seem,
Skim milk masquerades as cream..."
— Gilbert and Sullivan
(H.M.S. Pinafore)
"Plus ça change, plus c'est la même chose."
— Alphonse Karr
Our descriptions of SETQ and RPLACA, both informal and meta-circular, are imprecise. They admit a number of drastically different interpretations of the behavior of the system. We would all agree that for RPLACA to mean anything at all like what we want, the expression:
((LAMBDA (X)
(PROGN (RPLACA X 'Z)
(CAR X)))
(CONS 'A '(B C)))
Puzzle #1
should evaluate to Z. But what about this case:
((LAMBDA (X Y)
(PROGN (RPLACA X 'Z)
(CAR Y)))
(CONS 'A '(B C))
(CONS 'A '(B C)))
Puzzle #2
Should this evaluate to A or Z? Nearly all LISP systems would produce A, but there are arguments for both possibilities. Similarly, should this:
