cps tweaks

[lambda.git] / cps_and_continuation_operators.mdwn

diff --git a/cps_and_continuation_operators.mdwn b/cps_and_continuation_operators.mdwn
index 3b0dda7..2c3f356 100644 (file)
--- 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 -->
 
 
 
@@ -401,40 +403,41 @@ Here are some more examples of using delimited control operators. We present eac
 
 Example 1:
 
-       ; (+ 100 (+ 10 (abort 1))) ~~> 1
+       ; (+ 1000 (+ 100 (abort 11))) ~~> 11
        
-       app2 (op2 plus) (var hundred)
-         (app2 (op2 plus) (var ten) (abort (var one)))
+       app2 (op2 plus) (var thousand)
+         (app2 (op2 plus) (var hundred) (abort (var eleven)))
        
        # Continuation_monad.(run0(
-           abort 1 >>= fun i ->
-           unit (10+i) >>= fun j ->
-           unit (100+j)));;
-       - : int = 1
+           abort 11 >>= fun i ->
+           unit (100+i) >>= fun j ->
+           unit (1000+j)));;
+       - : int = 11
 
 When no `reset` is specified, there's understood to be an implicit one surrounding the entire computation (but unlike in the case of `callcc`, you still can't capture up to *and including* the end of the computation). So it makes no difference if we say instead:
 
        # Continuation_monad.(run0(
            reset (
-             abort 1 >>= fun i ->
-             unit (10+i) >>= fun j ->
-             unit (100+j))));;
-       - : int = 1
+             abort 11 >>= fun i ->
+             unit (100+i) >>= fun j ->
+             unit (1000+j))));;
+       - : int = 11
 
 
 Example 2:
        
-       ; (+ 100 (reset (+ 10 (abort 1)))) ~~> 101
+       ; (+ 1000 (reset (+ 100 (abort 11)))) ~~> 1011
        
-       app2 (op2 plus) (var hundred)
-         (reset (app2 (op2 plus) (var ten) (abort (var one))))
+       app2 (op2 plus) (var thousand)
+         (reset (app2 (op2 plus) (var hundred) (abort (var eleven))))
        
        # Continuation_monad.(run0(
            reset (
-             abort 1 >>= fun i ->
-             unit (10+i)) >>= fun j ->
-              unit (100+j)));;
-       - : int = 101
+             abort 11 >>= fun i ->
+             unit (100+i)
+           ) >>= fun j ->
+           unit (1000+j)));;
+       - : int = 1011
 
 Example 3:
 
@@ -477,10 +480,10 @@ To demonstrate the different adding order between Examples 4 and 5, we use `::`
            (shift (\k. ((op2 plus) (var ten) (app (var k) (var one)))))))
        
        Continuation_monad.(let v = reset (
-         let u = shift (fun k -> k [1] >>= fun x -> unit (10 :: x))
+           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]
 
 
@@ -492,7 +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:
 
@@ -503,7 +522,7 @@ Example 7:
            (shift (\k. app (var k) (app (var k) (var one))))))
        
        Continuation_monad.(let v = reset (
-         let u = shift (fun k -> k 1 >>= fun x -> k x)
+           let u = shift (fun k -> k 1 >>= fun x -> k x)
            in u >>= fun x -> unit (10 + x)
          ) in let w = v >>= fun x -> unit (100 + x)
          in run0 w)