Hi Jens - thanks for the great explanation about how Scheme works in general, as well as the link (I'm reading through right now).  I would assume that using closure as compound value object is one of the perceived uses that have been thought of and that a good compiler would put some work toward making sure it can efficiently support the uses, but I also understand that completely depends on the implementation. While it's unclear for me what model PLT Scheme uses for structs - Robby alluded to that closures is only a constant factor over struct from memory consumption. If there is indeed an upper bound somewhere for closure objects that would be good to know too ;) Thanks, yinso <div>On 5/31/07, Jens Axel Søgaard <<a href="mailto:jensaxel@soegaard.net">jensaxel@soegaard.net</a>> wrote:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> YC wrote: > I've heard that Scheme > doesn't keep variables on the stack, but want to verify. The explanation is a little involved. First the concept of call stack. Consider:    (define (f x) (g (+ x 1))    (define (g y) (h (+ y 2))    (define (h z) (+ x 3))    (f 0) Conceptually f calls g, which calls h. So we have a stack of functions calls here. In most languages the conceptually call stack is implemented as an actual stack. That strategy can't be used naïvely with Scheme. due to call-with-current-continuation. The problem is that call/cc allows a function to return more than once - and thus conceptually you get a call *tree* instead of a call stack. In a simple Scheme implementations the tree is represented as (you guessed it) a tree stored in the normal garbage collected heap. However it is possible to use a stack to store the current branch of the call tree and then do some trickery when control transfers to another branch [actually a little trickery is needed prior to this too]. In short, you can't say "Scheme doesn't keep variables on the stack" because some implementations do and others don't- The canonical [I believe] reference is:    "Three Implementation Models for Scheme"    by R. Kent Dybvig. It is very well-written and explains everything in detail. See especially chapter 3 and 4. -- Jens Axel Søgaard _________________________________________________   For list-related administrative tasks:   <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme"> http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </blockquote></div>