[racket] A primitive more fundamental than a continuation?

The subject is interesting, but I'm not sure I understand your question. As
to "what's so special about the third branch?", isn't it special because
that's where you put the let/cc?

i.e. If you wrote (+ (let/cc k 3) (+ 1 2) (+ 2 3)) instead of (+ (+ 1 2)
(let/cc k 3) (+ 2 3)), then you would be able to change the 2nd branch
instead of the 3rd.

If you are asking for an interface for replacing any one of the individual
"branches" with a value in the future, you could always write a macro:

(my/cc k (+ (+ 1 2) (+ 2 3) (+3 4)))

which you could use as (*k* position 12).

my/cc and let/cc can each be written in terms of the other, so the question
of which one is more fundamental is subject to debate. In the
implementation of an interpreter, though, let/cc is very much simpler to
implement than my/cc.

Apologies if I misunderstood the question completely.
  -Patrick

On Sat, Dec 1, 2012 at 4:28 PM, Galler <lzgaller at optonline.net> wrote:

> I've been thinking about a reified continuation:
>
> The documentation (See Guide s10.3) says "A continuation is a value that
> encapsulates a piece of an expression context"
>
> I'm coming to the belief that the continuation is actually  "an ordered
> collection of computation-branches, with facilities provided to manipulate
> one member of the collection"
>
> For example:
>
> (+ (+ 1 2) (let/cc k 3) (+ 2 3))
>
> which yields 11
>
> If we were to capture  k in a module level variable *k*, we can substitute
> 4
> for the result of the third ordinal branch (counting from the left)
>
> (*k* 4)
>
> and get a value of 12, etc. Indeed, we can substitute any value for the
> third ordinal branch.
>
> So my question is:
>
> What's so special about the third branch?
>
> Is there any reason I couldn't change the fourth branch, or second, or even
> first?
>
> Put another way, why doesn't the reified continuation k expose the
> individual branches?
>
>
>
>
>
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