Useful alternative control structures?

Question

Sometimes when I am programming, I find that some particular control structure would be very useful to me, but is not directly available in my programming language. I think my

Answer 1

Most languages have built-in functions to cover the common cases, but "fencepost" loops are always a chore: loops where you want to do something on each iteration and also do something else between iterations. For example, joining strings with a separator:

string result = "";
for (int i = 0; i < items.Count; i++) {
    result += items[i];
    if (i < items.Count - 1) result += ", "; // This is gross.
    // What if I can't access items by index?
    // I have off-by-one errors *every* time I do this.
}

I know folds can cover this case, but sometimes you want something imperative. It would be cool if you could do:

string result = "";
foreach (var item in items) {
    result += item;
} between {
    result += ", ";
}