Re: Demystifying Continuations

[address@hidden (Gregory T. Sullivan)]

>>>>> "DS" == David Simmons <David.Simmons@smallscript.net> writes:

 DS> Continuations are usually talked about in Procedural language settings, 
 DS> not Object Oriented settings. Specifically because a continuation is 
 DS> easily implemented as a goto.

 DS> One of the reasons the picture becomes complex in the OO world is 
 DS> because of all the throw/unwind logic. For historical purposes, I would 
 DS> say that a 'normal' continuation would NOT unwind, but would indeed
 DS> replace.

Have you studied the interaction between 
  (dynamic-wind before thunk after) 
and call-with-current-continuation in Scheme?  Scheme is not
object-oriented, but of course has continuations and, more recently,
the notion of unwind protected blocks.  The R5RS definition of Scheme
can be found at these fine websites:

  http://www.schemers.org/Documents/Standards/R5RS/
  http://www.cs.indiana.edu/scheme-repository/doc.standards.html
  http://www.swiss.ai.mit.edu/~jaffer/r5rs_toc.html

Some of the relevant text is:

     BEFORE is called whenever execution enters the dynamic extent of
     the call to THUNK and AFTER is called whenever it exits that
     dynamic extent.  The dynamic extent of a procedure call is the
     period between when the call is initiated and when it returns.
     In Scheme, because of `call-with-current-continuation', the
     dynamic extent of a call may not be a single, connected time
     period.  It is defined as follows:

        * The dynamic extent is entered when execution of the body of
          the called procedure begins.

        * The dynamic extent is also entered when execution is not
          within the dynamic extent and a continuation is invoked that
          was captured (using `call-with-current-continuation') during
          the dynamic extent.

        * It is exited when the called procedure returns.

        * It is also exited when execution is within the dynamic extent
          and a continuation is invoked that was captured while not
          within the dynamic extent.


-- 
Greg      gregs@ai.mit.edu (617)253-5807
Sullivan  http://www.ai.mit.edu/~gregs/