Interval foreach iteration variable

[bearophile]

This post is just about foreach performed on a range like 0..10, and not about foreach in general (like forerach done on a iota(0,10). Note: the 0..10 syntax may even become syntax sugar for iota(0,10) and the compiler may recognize iota() calls and fully optimize it away).

This D2 program:

import std.stdio: write;
void main() {
    foreach (i; 0 .. 10) {
        write(i, " ");
        i += 1;
    }
}


Outputs:
0 2 4 6 8

But for a person accustomed to Python that looks strange, this is a similar Python2 program:

from sys import stdout
for i in xrange(10):
    stdout.write("%d " % i)
    i += 1

It outputs:
0 1 2 3 4 5 6 7 8 9 


What's good in allowing the D foreach iteration variable when it loops on a interval to be modified like that? I think it's a bit bug prone semantics.

foreach(a; b..c) semantics is not related to the for() semantics, foreach is not present in C language, so D foreach is free to have any sane semantics it desires.

So I think it's better the foreach interval iteration variable to be not mutable.

There are important C style guides that warn against modifying the loop variable inside the loop itself because it's not a very safe&clean thing to do (and if necessary to set a flag that the for loop tests to break). So what I'm suggesting here is not weird.

-------------------

I have recently added a related but different bug report:
http://d.puremagic.com/issues/show_bug.cgi?id=5306

Bye,
bearophile