add lecture 08

lectures/08/lazystream.ml

@@ -0,0 +1,19 @@
+type 'a lazystream = Cons of 'a * 'a lazystream Lazy.t
+
+let hd (Cons (h, _)) = h;;
+let tl (Cons (_, t)) = Lazy.force t;;
+
+let rec from n = Cons (n, lazy (from (n + 1)));;
+
+let rec take n (Cons (h, t)) =
+  if n <= 0 then []
+  else h :: take (n - 1) (Lazy.force t);;
+
+let rec map f (Cons (h, t)) =
+  Cons (f h, lazy (map f (Lazy.force t)))
+
+let rec map2 f (Cons (h1, t1)) (Cons (h2, t2)) =
+  Cons (f h1 h2, lazy (map2 f (Lazy.force t1) (Lazy.force t2)))
+
+let rec fibs =
+  Cons (0, lazy (Cons (1, lazy (map2 (+) fibs (tl fibs)))))