Lazy version of evaluator and tests.

[sicp.git] / src / sicp / streams.rkt

diff --git a/src/sicp/streams.rkt b/src/sicp/streams.rkt
index 42bde472c9a91079104933978dbba739a8f67d0f..b0540aedb1246cec7e517dd7bc2401a477704ccb 100644 (file)
--- a/src/sicp/streams.rkt
+++ b/src/sicp/streams.rkt
@@ -1,5 +1,7 @@
 #lang planet neil/sicp
 
+(#%require (only racket random))
+
 (define (stream-car s) (car s))
 (define (stream-cdr s) (force (cdr s)))
 
@@ -105,3 +107,24 @@
                          (stream-cdr stream)))))
                
 (define primes (sieve (integers-starting-from 2)))
+
+(define (interleave s1 s2)
+  (if (stream-null? s1)
+      s2
+      (cons-stream (stream-car s1)
+                   (interleave s2 (stream-cdr s1)))))
+
+(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)))))
+
+(define (integral integrand initial-value dt)
+  (define int
+    (cons-stream initial-value
+                 (add-streams (scale-stream integrand dt)
+                              int)))
+  int)