[Update: see further contributions to the discussion from Ian Griffiths, Avi Bryant, James Robertson, and Joe Duffy; note also John Cowan's excellent comment below, pointing out that hidden fields work with the back button but not with bookmarks.]
It looks like continuations are back on the discussion board (Gilad Bracha, Tim Bray, and Don Box). I spent some time with Scheme a decade ago and continuations were one of the new features I had to try to understand. Then, as now, I found them more clever than practical.
Gilad sets up a use case for continuations before he goes on to oppose them: in essence, a web application could use continuations to maintain separate stacks, so that as a user hits the back button and then starts down new paths, the web application would not become confused, selling the user a trip to Hawaii instead of Alaska. I can see how continuations would work for that, just as I can see how a bulldozer could turn over the sod in my garden, but I’m far from convinced that either is the right tool for what is really a much simpler problem.
First, a continuation preserves the entire state of a program, including the stack, instruction counter, local variables, etc. How much of that do you really need for a hypothetical travel web app? In reality, you probably need, maybe, 1-5 variable values to restore a previous state in the travel app, so why not just save those explicitly? It would be faster, more secure (less information being saved), and much easier to performance tune and debug (since no magic is happening behind the scenes). Save those variables in a database, in a hash table, in an XML or CSV file, in memcached, or wherever happens to be most convenient. You may be looking at under 100 bytes for each saved state, so if you really want to do this, it’s not going to hurt too badly.
But do you really want to do this? Most of the discussion around REST has focussed on the use of persistent URLs and how to use HTTP verbs like GET, POST, PUT, and DELETE, but there’s another, perhaps more critical idea behind REST — that the resource your retrieve (a web page, XML document, or what-have-you) contains its own transition information.
Let’s say that you load a web page into your browser, load more web pages, then use the back button to return to the original one. Now, select a link. What happens? Did you browser have to go back to the original web server, which was using continuations (or other kinds of saved state) to keep track of the links from every page you visited, so that it won’t send you to the wrong one? Of course not. The web page that you originally downloaded already included a list of all its transitions (links), and intuitive things just happen naturally when you hit the back button.
The web is stateless, but web application toolkits maintain pseudo-sessions (using cookies, URL rewriting, or what-have-you) that makes them look stateful, and that makes programmers lazy. Obviously, you don’t want to stick information like ‘isauthenticated’ on a web page, since it could be forged; likewise, you don’t want to put a credit-card number there. But it is trivially simple to make sure that forms, like links, go to the right place even when you hit the back button — just make the transitions fully independent of any session stored on the server side. For example, consider this:
<form method="post" action="/actions/book-trip"> <button>Book this trip!</button> </form>
Presumably, the trip the person was looking at is stored somewhere in a session variable on the browser. DON’T DO THIS! As Gilad pointed out, someone hitting the back button might end up booking the wrong trip. There are gazillions of ways to push all of the context-sensitive stuff into the web page itself, where it belongs. Here’s one example:
<form method="post" action="/actions/book-trip"> <label>Book your economy trip to Alaska!</label> <input type="hidden" name="destination" value="alaska"/> <input type="hidden" name="package" value="economy"/> <button>Book it.</button> </form>
<form method="post" action="/actions/book-trip/alaska/economy"> <label>Book your economy trip to Alaska!</label> <button>Book it.</button> </form>
This is 100% backbutton-proof and it’s trivially simple to implement. It took me a while after reading Gilad’s (admittedly, strawman) example to realize that there are people who do not develop webapps this way. If they do this much damage just with a Session stack, how much pain will they be able to cause with continuations?
The REST people are right, at least on this point: there’s no need to drive a continuation bulldozer through your webapp, when a little REST garden spade will work quite nicely (and won’t tear up your lawn in the process). Don suggests that there may be other, more legitimate use cases for continuations outside of web applications, and I have no reason to disagree, but I would like to look at them pretty carefully.