[plt-scheme] In your words what is continuation passing style and what is its important?

From: Danny Yoo (dyoo at cs.wpi.edu)
Date: Sun Jun 24 19:40:46 EDT 2007

On Sun, 24 Jun 2007, Grant Rettke wrote:

> In your words what is continuation passing style and what is its 
> important?

PLAI talked about CPS in good detail in Section 7, relating it to how 
people write multi-stage web programs:


Also, the paper "Toward Leakage Containment" helped get my head around 
continuation passing style a bit more and why it's useful:


In my own programming, CPS tends to hide but I do conceptually use it. 
Concretely and most recently, I'm working on a next revision of DivaScheme 
--- an extension to DrScheme --- and one part involves taking one's 
location and computing what happens to that location after going across 
some text.  (i.e. I'm on line 5, column 3, and I want to know that after I 
go past "  \n" that I'm on line 6, column 0).

Conceptually, you might think of it as:

     update-location: location text -> location

But in fact, this is not what I directly do.  Rather, I split things up in 
a slightly strange way:

     get-move-after: text -> move
     apply-move: move location -> location

and I get the effect of update-location by doing something like:

     (apply-move (get-move-after a-text) a-location)

Conceptually, my "move" here acts as a continuation that I can later 
choose to apply to a location to shift it around.  The reason I'm doing 
this in such a funny way is because this gives me the freedom to memoize 
the effect of moving across a-text starting from a-loc.  It's just a 
matter of using Dave Herman's memoize PLaneT package, and a very cool 
no-effort optimization.

I can do this only because I'm explicitly holding onto the "what to do 
next" thing as a real object.  And I can reuse this continuation on 
different locations without any problems.  I'll be releasing the code 
shortly on PLaneT so you can take a look.

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