Implement till 4.30

ex-4_31-34.scm

@@ -0,0 +1,34 @@
+(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")
+