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Re: The fundamental concept of continuations


From: Bakul Shah
Subject: Re: The fundamental concept of continuations
Date: Mon, 08 Oct 2007 23:07:34 -0700
User-agent: Thunderbird 2.0.0.6 (Macintosh/20070728)

address@hidden wrote:
> Again I am depressed to encounter a fundamentally new concept that I
> was all along unheard of.

The concept is 37 years old.  Wadsworth in his "Continuation
Revisited" paper says he & Strachey were struggling with
extending the technique of denotational semantics to describe
jumps and not finding a satisfactory answer.  Then, in his
words:

  in October 1970 Strachey showed me a paper "Proving
  algorithms by tail functions" by Mazurkiewicz [2] which he
  had obtained from an IFIP WG2.2 meeting. Just the phrase
  "tail functions" in the title was enough -- given the
  experience of our earlier struggles -- for the ideas to
  click into place! The (meaning of the) "rest of the program"
  was needed as an argument to the semantic functions -- just
  so those constructs that did not use it, like jumps, could
  throw it anyway. The term "continuation" was coined as
  capturing the essence of this extra argument (though I
  often wished to have a shorter word!) and the rest, as they
  say, is history.

> Its not even in paul graham's book where i
> learnt part of Lisp. Its in Marc Feeley's video.
>
> Can anyone explain:
>
> (1) its origin
> (2) its syntax and semantics in emacs lisp, common lisp, scheme
> (3) Is it present in python and java ?
> (4) Its implementation in assembly. for example in the manner that
> pointer fundamentally arises from indirect addressing and nothing new.
> So how do you juggle PC to do it.
> (5) how does it compare to and superior to a function or subroutine
> call. how does it differ.
>
> Thanks a lot.
>
> (6) any good readable references that explain it lucidly ?

You might like this one:

http://www.intertwingly.net/blog/2005/04/13/Continuations-for-Curmudgeons


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