Thursday, August 05, 2004

Continuations - part 1.

Ok, so why did I write a post on last-call-optimisation? Well I find it very difficult to explain continuations to someone who dosn't understand LCO, so I thought I should post a quick example of LCO before this. Of course the reason behind this sudden urge to actually fulfil my promise to explain continuations here is Lambda again. This time an extended discussion on continuations sparked by Keith Devens' request for help understanding the things. Itself sparked by a particularly interesting analogy by Luke Palmer in news:perl.perl6.language, now forever dubbed the "Continuation Sandwich" --- the relevant part of his post is:

Say you're in the kitchen in front of the refrigerator, thinking about a sandwitch. You take a continuation right there and stick it in your pocket. Then you get some turkey and bread out of the refrigerator and make yourself a sandwitch, which is now sitting on the counter. You invoke the continuation in your pocket, and you find yourself standing in front of the refrigerator again, thinking about a sandwitch. But fortunately, there's a sandwitch on the counter, and all the materials used to make it are gone. So you eat it. :-)

Now before we begin I'm going to deal with continuation-passing-style and call-with-current-continuation seperately, but as a taste of what we're in for interested parties can check my CPS-in-Java example on the wiki, or the most lucid description of call/cc in Chapter 13 of Teach Yourself Scheme in Fixnum Days.

In the meantime, it's 0021 hours here, so I'm going to sleep first, and finish this later.

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