Hello, I believe that keeping the box identity but not the box value is the correct behavior of continuations under the Scheme standard. I agree it's not intuitive, though.
I have been thinking that we could eliminate variable boxes for mutable variables when we can prove that the continuation is never captured. I need to understand more about how continuations work, but I don't think we need a new variable type for efficiency reasons. If you want a new type of variable because you want different semantics, that's a different issue. Could you give more of an example? I assume that you're talking about something like this: (loop ;; the macro defines a loop counter (call/cc some-other-function)) So when the continuation of the loop is called more than once, the loop counter is incremented too much. Is that right? Best, Noah On Sun, Mar 17, 2013 at 9:07 AM, Stefan Israelsson Tampe < stefan.ita...@gmail.com> wrote: > Dear all, > > I just noted an issue with a programming style, that I suspect > schemers try to avoid but still is quite common in the lisp community > and actually makes for simpler macros when e.g. writing loop macros. > > The issue here is that one setup local variables with a let and then > modify them using set!. > > The problem is that this code will always be boxed. Now the overhead > of this is almost invisible due to other overhead. What I ask is what > happens when we store the stack at a call/cc setup? What will happen > is that the box identity is stored but not the box value and hence > there will be problems with recurrent calls of a continuation. > > Now I could be pretty fine with this because it's a clean statement to > say that this behavior is included in the set! semantic. And we > actually need keep this semantic to not break backward compability. > > Anyway. It's still pretty useful to have a local-set that will only > set a variable if it safely can set the local variable on the stack > directly I therefore propse for guile to include a new variable > binding construct called local-set! personally I would in most cases > avoid it's use but I would love to have it in my loop macros. > > WDYT > /Stefan > >
