(load "util.scm")
(load "misc/evaluator.scm")

(display "\nex-4.31 - explicit-laziness\n")

; Exercise 4.31.  The approach taken in this section is somewhat unpleasant,
; because it makes an incompatible change to Scheme. It might be nicer to
; implement lazy evaluation as an upward-compatible extension, that is, so that
; ordinary Scheme programs will work as before. We can do this by extending the
; syntax of procedure declarations to let the user control whether or not
; arguments are to be delayed. While we're at it, we may as well also give the
; user the choice between delaying with and without memoization. For example,
; the definition

;(define (f a (b lazy) c (d lazy-memo))
;  ...)

; would define f to be a procedure of four arguments, where the first and third
; arguments are evaluated when the procedure is called, the second argument is
; delayed, and the fourth argument is both delayed and memoized. Thus, ordinary
; procedure definitions will produce the same behavior as ordinary Scheme,
; while adding the lazy-memo declaration to each parameter of every compound
; procedure will produce the behavior of the lazy evaluator defined in this
; section. Design and implement the changes required to produce such an
; extension to Scheme. You will have to implement new syntax procedures to
; handle the new syntax for define. You must also arrange for eval or apply to
; determine when arguments are to be delayed, and to force or delay arguments
; accordingly, and you must arrange for forcing to memoize or not, as
; appropriate.

; (display "\nex-4.32\n")
; (display "\nex-4.33\n")
; (display "\nex-4.34\n")