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From Stephan Michels <step...@apache.org>
Subject RE: [Vote] Controller/Sitemap integration
Date Thu, 17 Jul 2003 14:48:02 GMT


> > So, here we have a DFA.
> >
> > The 'continuation' means to freeze the current state of the
> > execution. The benefit of the continuations are that you have
> > a history of the states, which you had traversed. And like a
> > backtracking algorithm, you can go back to a previous state
> > and follow another route.
> >
> > So, the answer is, it is sufficient to have the 'history of
> > states' to take a new name like 'continuation' instead of
> > simple 'state'? I think yes.
>
> Sorry, I don't think so. I'm not a specialist in Scheme but I think
> there is a concept behind continuations and the general use for all
> controllers in Cocoon would be misleading.

Okay, to be sure I googled around, and found:
http://www.cs.utexas.edu/users/wilson/schintro/schintro_141.html

> call-with-current-continuation
>
> call-with-current-continuation is a very powerful control construct,
> which can be used to create more conventional control constructs, like catch and
> throw in Lisp (or setjmp and longjmp in C), or coroutines, and many
> more. It is extremely powerful because it allows a program to manipulate its own
> control "stack" so that procedure calls and returns needn't follow the
> normal depth-first textual call ordering.
>
> Recall that we said [ WHERE? ] that Scheme's equivalent of an activation
> stack is really a chain of partial continuations (suspension records),
> and this chain is known as a full continuation. And since continuations are
> immutable, they usually form a tree reflecting the call graph (actually,
> only the non-tail calls). Normally, the parts of this tree that are not
> in the current continuation chain are garbage, and can be garbage
> collected.
>
> If you take a pointer to the current continuation, and put it in a live
> variable or data structure, however, then that continuation chain will
> remain live and not be garbage collected. That is, you can "capture" the
> current state of the stack.

It is not exactly what I want, but the core statement is

'A continuation is chain of partial continuations, or captured
states of the stack'

I think :) But maybe I'm wrong?!

It's funny, but the whole continuation concept reminds me of the Tomita
algorithm for simulating a GLR parser, which I currently working on ;-)

http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps

Stephan.


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