adding lisp-interpreter

lisp-interpreter/tests/code/streams.scm

@@ -0,0 +1,32 @@
+(==> (force (delay (+ 1 2))) 3)
+
+(==> (let ((p (delay (+ 1 2))))
+      (list (force p) (force p))) (3 3))
+
+(assert (promise? (delay (+ 1 2))))
+
+
+; promises computed at most once
+(define count 0)
+
+(define p
+  (delay
+    (begin
+        (set! count (+ count 1))
+        (* x 3))))
+
+(define x 5)
+
+(==> count 0)
+(assert (promise? p))
+(==> (force p) 15)
+(assert (promise? p))
+(==> count 1)
+(==> (force p) 15)
+(==> count 1)
+
+
+(define (integers-starting-from n)
+  (cons-stream n (integers-starting-from (+ n 1))))
+
+(assert (equal? (stream-head (integers-starting-from 0) 5) '(0 1 2 3 4)))