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
This also works and as you can see, delivers the expected answer `101`.
-At the moment, I'm not able to get the third example working with the monadic library. I thought that this should do it, but it doesn't type-check:
+The third example is more difficult to make work with the monadic library, because its types are tricky. I was able to get this to work, which uses OCaml's "polymorphic variants." These are generally more relaxed about typing. There may be a version that works with regular OCaml types, but I haven't yet been able to identify it. Here's what does work:
- # C.(run0 (callcc (fun k -> unit (1,k)) >>= fun (p1,p2) -> p2 (2,unit) >>= fun p2' -> unit (p1,p2')));;
+ # C.(run0 (callcc (fun k -> unit (1,`Box k)) >>= fun (p1,`Box p2) -> p2 (2,`Box unit) >>= fun p2' -> unit (p1,p2')));;
+ - : int * (int * [ `Box of 'b -> ('a, 'b) C.m ] as 'b) as 'a =
+ (2, (2, `Box <fun>))
-If we figure this out later (or anyone else does), we'll come back and report. <!-- FIXME -->
+<!-- FIXME -->