update lecture 08 notes

lectures/08/infstream.ml

@@ -4,6 +4,26 @@ let hd (Cons (h, _)) = h
 let tl (Cons (_, t)) = t ()
 
 let rec from n = Cons (n, fun () -> from (n + 1))
+
 let rec take n (Cons (h, t)) =
   if n <= 0 then []
   else h :: take (n - 1) (t ())
+
+let rec drop n (Cons (h, t) as s) =
+  if n <= 0 then s
+  else drop (n - 1) (t ())
+
+let rec map f (Cons (h, t)) =
+  Cons (f h, fun () -> map f (t ()))
+
+let rec map2 f (Cons (h1, t1)) (Cons (h2, t2)) =
+  Cons (f h1 h2, fun () -> map2 f (t1 ()) (t2 ()))
+
+let rec fibs =
+  Cons (0, fun () -> Cons (1, fun () -> map2 (+) fibs (tl fibs)))
+
+let rec unfold f x =
+  let (v, x') = f x in
+  Cons (v, fun () -> unfold f x')
+
+let fibs' = unfold (fun (a, b) -> (a, (b, a + b))) (0, 1)