2017-03-24: adjust with to use dynamic-wind. Don’t know how to test this yet.

adjust with to use dynamic-wind. Don’t know how to test this yet.

diff --git a/examples/with.w b/examples/with.w
--- a/examples/with.w
+++ b/examples/with.w
@@ -13,27 +13,44 @@ define-module : examples with
 
 import : oop goops
 
-define : enter thing
+define : enter thing state
        . thing
 define-generic enter
 
-define : exit thing
+define : exit thing state
        . thing
 define-generic exit
 
 define-syntax with
     syntax-rules : as
       : _ thing as name thunk ...
-        let*
-           : name : enter thing
-             res : begin thunk ...
-           exit thing
-           . res
+        let
+          : name #f
+            state #f
+            res #f
+          dynamic-wind
+              λ () : set! name : enter thing state
+              λ () : set! res : begin thunk ...
+                   . res
+              λ () ; : exit thing state
+                   . res
 
-define-method : exit (thing <port>)
+define-method : enter (thing <port>) state
               . "Ensure that a port is always closed at the end of the with-block."
+              when : not : equal? #f state
+                  set-port-column! thing (car state)
+                  set-port-line! thing (car (cdr state))
+              . thing
+
+define-method : exit (thing <port>) state
+              . "Ensure that a port is always closed at the end of the with-block."
+              set! state : list (port-line thing) (port-column thing)
               close-port thing
 
 define : main args
          with (open-file "with.w" "r") as port
               format #t "~a\n" : read port
+              format #t "~a\n" : read port
+              format #t "~a\n" : read port
+
+         ;; TODO: test-continuation