Gilad Bracha's provocative Will Continuations Continue? has stirred up some controversy this week. His objection to continuations stems from the observation that in Ajax applications, the back button is less important than in traditional web programs, sometimes even effectively disabled. For an architect deciding whether to implement continuations for a web framework or its underlying VM, this has to alter the equation. Are Ajax applications rendering continuations obsolete? [1]
I'm a little late to respond, thanks to several days of netlessness. Gilad's post has generated some thoughtful discussion: Tim Bray protests that the web's simple UI model is well understood by users, Don Box points out that the web is not necessarily the only killer app for continuations, and Avi Bryant explains a few of the different kinds of state in web applications and where continuations fit into the picture. Later, Ian Griffiths raises deeper technical complaints about server-side continuations persistence about resource management (what happens when the user walks away?) and thread migration; Ezra Cooper responds that these problems don't exist for Links, where continuations live on the client side.
What makes Gilad's argument unrealistic is that Ajax applications currently account for some miniscule percentage of the web. Even if interactive applications are the future, the web never forgets its past. It's as naive to think you can stop supporting the back button as it would be for browsers to stop supporting HTML 4. You could protest that only the browsers have to support these "legacy" web pages, whereas on the server side, web programmers should start writing Ajax apps right now. But I don't see amazon.com or hotwire.com throwing away their shopping carts in favor of a Ruby on Rails codebase. There are way more existing applications with huge codebases that could benefit from a backwards-compatible solution to the Orbitz problem than there are companies itching to throw their entire codebase away to make way for an all-Ajax site.
None of this changes the fact that adding continuations to the JVM isn't easy. It's still a design decision with the attendant trade-offs. But speculation that everyone will be writing programs that make no use of the back button, say, 10 years from now, is not a compelling excuse not to support those programs now.
[1] As a pedantic aside, continuations could no more become obsolete than variables, or functions, or algorithms could. Continuations are an abstract concept, not a programming construct. More precisely, the real question is whether Ajax applications are rendering obsolete the use of first-class continuations as an implementation strategy for web application frameworks. In particular, I think a number of people have the misconception that programmers have to understand and use call/cc in order to program with these frameworks. In fact, the frameworks only use continuations under the hood to allow the programmer to write their web programs in a natural style.
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Speaking of continuations, I just posted an article on IBM's DeveloperWorks which attempts to give a tutorial on continuations to those who have not heard of them.
I wouldn't call your footnote [1] just a pedantic aside -- a pedagogic aside, perhaps. It's important to learn about continuations for the Web regardless of whether first-class continuations stored on the server side make sense.
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