mv lab 07 to 06

lectures/09/writer_monad.ml

@@ -0,0 +1,35 @@
+let return x = (x, "")
+
+let ( >>= ) (x, s) f =
+  let (x', s') = f x in
+  (x', s  ^ s')
+
+let ( >> ) mx my =
+  mx >>= fun _ -> my
+
+let square x =
+  let y = x * x in
+  (y, Printf.sprintf "square %d = %d" x y)
+
+let inc x =
+  let y = x + 1 in
+  (y, Printf.sprintf "inc %d = %d" x y)
+
+let dec x =
+  let y = x - 1 in
+  (y, Printf.sprintf "dec %d = %d" x y);;
+
+return 2 >>= square >>= square >>= dec >>= square >>= inc;;
+
+let tell s = ((), s);;
+
+let rec gcd a b =
+  if b = 0 then a else gcd b (a mod b);;
+
+gcd 32 24;;
+
+let rec gcd_logged a b =
+  if b = 0 then tell (Printf.sprintf "gcd = %d" a) >> return a
+  else tell (Printf.sprintf "gcd %d %d: " a b) >> gcd_logged b (a mod b);;
+
+gcd_logged 24 32;;