update lecture 08 notes

lectures/08/lazystream.ml

@@ -1,13 +1,17 @@
 type 'a lazystream = Cons of 'a * 'a lazystream Lazy.t
 
-let hd (Cons (h, _)) = h;;
-let tl (Cons (_, t)) = Lazy.force 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 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);;
+  else h :: take (n - 1) (Lazy.force t)
+
+let rec drop n (Cons (h, t) as s) =
+  if n <= 0 then s
+  else drop (n - 1) (Lazy.force t)
 
 let rec map f (Cons (h, t)) =
   Cons (f h, lazy (map f (Lazy.force t)))
@@ -17,3 +21,9 @@ let rec map2 f (Cons (h1, t1)) (Cons (h2, t2)) =
 
 let rec fibs =
   Cons (0, lazy (Cons (1, lazy (map2 (+) fibs (tl fibs)))))
+
+let rec unfold f x =
+  let (v, x') = f x in
+  Cons (v, lazy (unfold f x'))
+
+let fibs' = unfold (fun (a, b) -> (a, (b, a + b))) (0, 1)