add macros

10_mutation_assignment.scm

@@ -0,0 +1,34 @@
+;; -*- geiser-scheme-implementation: guile -*-
+
+(define chest 'sword)
+
+chest  ;  sword
+
+(set! chest 'gold) ;  #<unspecified>
+
+chest ;  gold
+
+
+(define (make-countdown n)
+  (lambda ()
+    (define last-n n)
+    (if (zero? n)
+        0
+        (begin
+          (set! n (- n 1))
+          last-n))))
+(define cdown (make-countdown 3))
+
+(cdown) ;  3
+(cdown) ;  2
+(cdown) ;  1
+(cdown) ;  0
+(cdown) ;  0
+
+(define vec (vector 'a 'b 'c))
+vec ;  #(a b c)
+(vector-ref vec 1) ;  b
+(vector-set! vec 1 'boop)
+(vector-ref vec 1) ;  boop
+vec ;  #(a boop c)
+

11_extensibility.scm

@@ -0,0 +1,40 @@
+;; -*- geiser-scheme-implementation: guile -*-
+
+(if (our-test)
+    (begin
+      (do-thing-1)
+      (do-thing-2)))
+
+(define (when test . body)
+  `(if ,test
+       ,(cons 'begin body)))
+
+(when '(our-test)
+  '(do-thing-1)
+  '(do-thing-2))
+                                        ;  (if (our-test) (begin (do-thing-1) (do-thing-2)))
+
+(define-macro (when test . body)
+  `(if ,test
+       ,(cons 'begin body)))
+
+(when (our-test)
+  (do-thing-1)
+  (do-thing-2))
+
+(define-syntax-rule (when test body ...)
+  (if test
+      (begin body ...)))
+
+(define-syntax-rule (for (item lst) body ...)
+  (for-each (lambda (item)
+              body ...)
+            lst))
+
+(for (str '("strawberries" "bananas" "grapes"))
+     (display
+      (string-append "I just love "
+                     (string-upcase str)
+                     "!!!\n")))
+
+;  #<unspecified>