X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=cps_and_continuation_operators.mdwn;h=2c3f35603d9110ed565013c45c68e1c8fb3e6f6c;hp=258f7227f66f093cf5d7e5f1989ea25f943ecc6e;hb=2d0d8e91618b921c03f4764bc7937060f0bec739;hpb=9a23d7fc448e48268a7b60ce9ff3e55222cd833a

diff --git a/cps_and_continuation_operators.mdwn b/cps_and_continuation_operators.mdwn
index 258f7227..2c3f3560 100644
--- a/cps_and_continuation_operators.mdwn
+++ b/cps_and_continuation_operators.mdwn
@@ -173,15 +173,15 @@ So too will examples. We'll give some examples, and show you how to try them out
 
 		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
@@ -268,11 +268,13 @@ That won't work because `k 1` doesn't have type `int`, but we're trying to add i
 
 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 -->
 
 
 
@@ -481,7 +483,7 @@ To demonstrate the different adding order between Examples 4 and 5, we use `::`
 	    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]
 
 
@@ -493,7 +495,23 @@ Example 6:
 	  (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: