From: Ben Giddings <ben@...>
Date: 2003-08-07T13:42:29+09:00
Subject: Re: Why does Ruby have callcc?

On Wednesday, August 6, 2003, at 08:32  PM, Dan Doel wrote:
> Where cc is the continuation, so you'd often see something like:
>
>    (callcc (lambda (cc) (my-fun cc arg1 arg2 ...))

Ok, and the reason you need to have this lambda is that callcc is a 
function that takes one argument: a function which itself takes one 
argument, the CC.  Because my-fun requries more than one argument, you 
need the lambda, right?

> Which makes an anonymous function that accepts the continuation and 
> passes it, along with some other arguments, on to another function 
> that does stuff.
>
> Now, to do this in ruby (most easily), you'd do:
>
> callcc { |cc|
>    my_fun cc, arg1, arg2 ...
> }

So it looks like Ruby is trying to look like the scheme implementation. 
  Sometimes trying to mimic the way things look in a functional language 
when programming in an imperative language makes for convoluted code.  
There may be some subtlety I'm missing, but it seems like you aught to 
be able to have a "returncc" function in scheme that takes no arguments 
and returns a continuation context thingo, then use it like:

(my_fun (returncc) arg1 arg2 arg3 ...)

It could probably even be implemented as

(lambda (cc) cc)

Though, my scheme is weak to nonexistent so I'm not 100% sure I have 
the syntax right.

I'm just not sure exactly where things end up if you call that 
continuation context... though I suppose you'd just end up right in the 
middle of assigning arguments to a method call, followed by actually 
calling the method.

If that's possible, this should work in Ruby:

my_fun(Continuation.new, arg1, arg2, arg3, ...)

Or, more clear to me:

cc = Continuation.new
my_fun(cc, arg1, arg2, arg3, ...)

Am I out to lunch, or does any of this make sense?

Ben