Re: CPS in Parrot (was: A plea for a new old language)

[Michael Vanier <address@hidden>]

I think the difficulty with continuations is even more basic than you
describe.  Continuations used in the call/cc sense violate our most basic
assumption about functions, which is that a function called from HERE is
going to return back HERE (the same place).  Continuations let you say: a
function called from HERE is going to return back WHEREVER, where WHEREVER
is some location in the code that was previously captured in the
continuation, lexicals and all.  This idea that you can control where a
function call is going to return is tremendously powerful, but is also very
tough to wrap your mind around (I still don't feel I fully get it).

Mike

> Date: Tue, 5 Aug 2003 18:10:14 -0400
> From: Dan Sugalski <dan@sidhe.org>
> 
> At 3:22 PM -0400 8/5/03, Geoffrey Knauth wrote:
> >On Tuesday, Aug 5, 2003, at 13:00 US/Eastern, Dan Sugalski wrote:
> >>I'm convinced at this point that all of the fear people have about 
> >>continuations is a direct result of how they're taught and what's 
> >>associated with them
> >
> >I haven't feared what I don't know.  I'm afraid to ask, "What's 
> >associated with them?"
> 
> Lisp and/or Scheme, though the way the design of computer languages 
> is taught (or, rather, not taught, until far too late) doesn't help. 
> The only place continuations are brought up, assuming they're dealt 
> with at all, is in a Lisp class, but not until the students have 
> already had a number of years (upwards of 5-10 for some) of 
> procedural and/or OO programming (C, C++, VB, even Java to some 
> extent), which hammers some assumptions about the intrinsic 
> stack-based nature of control-flow into students heads.
> 
> So, they walk into a lisp class with either the C model or the BASIC 
> "They're all globals!" model deeply internalized, then get hit in 
> rapid succession by Lisp's very different paradigm then a control 
> flow and variable model completely at odds with what they know to be 
> true. You then end up with a class full of people that either dislike 
> continuations because they clash with what they know to be true about 
> control and/or variables, or dislike continuations because lisp 
> clashes with what they know and any concepts introduced with it get 
> disliked by association. (Or they never understood it in the first 
> place because they couldn't get past the confusion of dealing with a 
> functional language, so everything explained in terms of the language 
> didn't stick)
> 
> It doesn't help that many of the folks publicly advocating 
> continuations tend to be a little... odd, so when you do run across a 
> paper or proposal, your first impression is that the author's either 
> mad or doing the CS equivalent of particle physics.
> 
> Not true for everyone, certainly -- a lot of people never get taught 
> anything about continuations, and all they know about them is that 
> most folks think they're Deep Black Magic.
> -- 
>                                          Dan
> 
> --------------------------------------"it's like this"-------------------
> Dan Sugalski                          even samurai
> dan@sidhe.org                         have teddy bears and even
>                                        teddy bears get drunk
>