Array foreach in scripting languages

I'm currently building a small scripting language for interactive stuff. Now I
know there's plenty out there, I'm just doing this for fun.
 
A tricky question came up and I'd like to collect opinions and experiences on
this. It's about foreach for dynamic arrays. Consider this C++-like pseudo code:
 
DynamicArray<int> list;
int i;
for(i=0;i<20;i++){list.Add(i);};
foreach(i in list){
    print(i);
    if(i==8){list.Delete(12);};
};
 
The obvious question is how should an active iterator react if the traversed list
changes? To answer this, I simply tried it out in two languages I know that do
provide both dynamic arrays and foreach: STL, Tcl and C#.
 
In STL C++ code, I get an exception because of iterator incompatibility.
In Tcl this problem isn't one because the iterator takes a snapshot of the list
object on start and there's no language construct to actually change this instance.
In C# with the List<int> generic, I get an exception because the iterated object
changes (interestingly, this happens on ANY change to the list, even appending).
 
Now what I'd like to know:
- how do other languages cope with this? (Tcl doesn't need to, C++/C# simply fails)
- what do YOU think would be sensible behaviour?
 
Currently I'm considering having all iterators be elements of a double-linked list
and let the array object be the dl-list head. So if the number of array elements
changes, it can run through all attached iterators and fix their indices or stop them.
 
 
Carsten Orthbandt
Founder + Development Director
SEK SpieleEntwicklungsKombinat GmbH
http://www.sek-ost.de

Wenn ich Visionen habe, gehe ich zum Arzt. - Helmut Schmidt
  
 

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