Solutions to 4.27, 4.28 and 4.29.

diff --git a/src/sicp/ex4_27.rkt b/src/sicp/ex4_27.rkt
new file mode 100644 (file)
index 0000000..6013958
--- /dev/null
+++ b/src/sicp/ex4_27.rkt
@@ -0,0 +1,38 @@
+#lang racket
+
+(require "metacircular2-lazy.rkt")
+
+(define env1 (make-environment))
+(eval '(define count 0) env1)
+(eval '(define (id x) (set! count (+ count 1)) x) env1)
+
+#|
+
+> (eval '(define w (id (id 0))) env1)
+> (eval 'count env1)
+1
+> (eval 'w env1)
+#0='(thunk
+     (id 0)
+     #1=(#hash((< . (primitive #<procedure:<>))
+               (* . (primitive #<procedure:*>))
+               (> . (primitive #<procedure:>>))
+               (- . (primitive #<procedure:->))
+               (= . (primitive #<procedure:=>))
+               (true . #t)
+               (+ . (primitive #<procedure:+>))
+               (list . (primitive #<procedure:list>))
+               (null? . (primitive #<procedure:null?>))
+               (/ . (primitive #<procedure:/>))
+               (car . (primitive #<procedure:car>))
+               (false . #f)
+               (cdr . (primitive #<procedure:cdr>))
+               (cons . (primitive #<procedure:cons>))
+               (count . 1)
+               (id . (procedure (x) ((set! count (+ count 1)) x) #1#))
+               (w . #0#))))
+> (eval 'count env1)
+1
+> 
+
+|#
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diff --git a/src/sicp/ex4_28.rkt b/src/sicp/ex4_28.rkt
new file mode 100644 (file)
index 0000000..c241853
--- /dev/null
+++ b/src/sicp/ex4_28.rkt
@@ -0,0 +1,18 @@
+#lang racket
+
+(require "metacircular2-lazy.rkt")
+
+#|
+Forcing is needed for any higher order procedure. An example is map. If you evaluate the following 
+code with `actual-value' instead of `eval' of operator, it executes fine but if not, then eval gets
+a thunk object (the two operands) for evaluation and when it reaches `apply' inside the `cons' it 
+will fail as `apply' does not know about thunk objects.
+|#
+
+(define env1 (make-environment))
+(eval '(define (map f xs)
+         (if (null? xs)
+             '()
+             (cons (f (car xs)) (map f (cdr xs)))))
+      env1)
+(eval '(map (lambda(x) (* x x)) '(1 2 3)) env1)
\ No newline at end of file

diff --git a/src/sicp/ex4_29.rkt b/src/sicp/ex4_29.rkt
new file mode 100644 (file)
index 0000000..1c1a976
--- /dev/null
+++ b/src/sicp/ex4_29.rkt
@@ -0,0 +1,11 @@
+#lang racket
+
+#|
+
+Any program which will call itself or another function recursively or repeatedly (as another argument) 
+will benefit from memoization.
+
+non-memoized version will have count of 2 as x gets forced twice in the expression (* x x). In memoized
+version, it will be 1.
+
+|#
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