let callcc body = fun outk -> body (fun v localk -> outk v) outk
-<!-- GOTCHAS?? -->
+ <!-- GOTCHAS?? -->
3. `callcc` was originally introduced in Scheme. There it's written `call/cc` and is an abbreviation of `call-with-current-continuation`. Instead of the somewhat bulky form:
- (call/cc (lambda (k) ...))
+ (call/cc (lambda (k) ...))
-I prefer instead to use the lighter, and equivalent, shorthand:
+ I prefer instead to use the lighter, and equivalent, shorthand:
- (let/cc k ...)
+ (let/cc k ...)
Callcc examples
let u = shift (fun k -> k [1] >>= fun x -> unit (10 :: x))
in u >>= fun x -> unit (100 :: x)
) in let w = v >>= fun x -> unit (1000 :: x)
- in run0 w)
+ in run0 w);;
- : int list = [1000; 10; 100; 1]
(app (reset (app2 (op2 plus) (var ten)
(shift (\k. (var k))))) (var one))
- (* not sure if this example can be typed as-is in OCaml. We may need a sum-type *)
+ (* not sure if this example can be typed as-is in OCaml... this is the best I an do at the moment... *)
+
+ # type 'x either = Left of (int -> ('x,'x either) Continuation_monad.m) | Right of int;;
+ # Continuation_monad.(let v = reset (
+ shift (fun k -> unit (Left k)) >>= fun i -> unit (Right (10+i))
+ ) in let w = v >>= fun (Left k) ->
+ k 1 >>= fun (Right i) ->
+ unit (100+i)
+ in run0 w);;
+ - : int = 111
+
+<!--
+# type either = Left of (int -> either) | Right of int;;
+# let getleft e = match e with Left lft -> lft | Right _ -> failwith "not a Left";;
+# let getright e = match e with Right rt -> rt | Left _ -> failwith "not a Right";;
+# 100 + getright (let v = reset (fun p () -> Right (10 + shift p (fun k -> Left k))) in getleft v 1);;
+-->
Example 7: