set! semantics

[Stefan Israelsson Tampe]

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