1 files changed, 40 insertions, 0 deletions
diff --git a/js/games/nluqo.github.io/~bh/61a-pages/Lectures/4.2/pairs.scm b/js/games/nluqo.github.io/~bh/61a-pages/Lectures/4.2/pairs.scm
new file mode 100644
index 0000000..37e3a98
--- /dev/null
+++ b/js/games/nluqo.github.io/~bh/61a-pages/Lectures/4.2/pairs.scm
@@ -0,0 +1,40 @@
+;;; The integers.
+
+(define (ints-from n)
+  (cons-stream n (ints-from (+ n 1))))
+
+(define integers (ints-from 1))
+
+;;; pairs of integers, A&S version
+
+(define (pairs s t)
+  (cons-stream
+   (list (stream-car s) (stream-car t))
+   (interleave
+    (stream-map (lambda (x) (list (stream-car s) x))
+		(stream-cdr t))
+    (pairs (stream-cdr s) (stream-cdr t)))))
+
+;;; Louis Reasoner's version, ex. 3.68
+
+(define (louis-pairs s t)
+  (interleave
+   (stream-map (lambda (x) (list (stream-car s) x))
+	       t)
+   (louis-pairs (stream-cdr s) (stream-cdr t))))
+
+;;; Fixing Louis's version with explicit DELAY and FORCE
+
+(define (fixed-pairs s t)
+  (interleave-delayed
+   (stream-map (lambda (x) (list (stream-car s) x))
+	       t)
+   (delay (fixed-pairs (stream-cdr s) (stream-cdr t)))))
+
+(define (interleave-delayed s1 delayed-s2)
+  (if (stream-null? s1)
+      (force delayed-s2)
+      (cons-stream (stream-car s1)
+                   (interleave-delayed (force delayed-s2)
+                                       (delay (stream-cdr s1))))))
+