Don't allow editing accumulation variables outside of accumulation commands

[okamsn]

cl-loop allows this, but iterate does not. Allowing this makes destructured accumulation much slower, which encourages work around rather than using this feature.

It would be better to have the loop be as fast as possible, but that costs flexibility. The question is: Do people actually need to modify accumulation variables outside of loop commands while the loop is running?

User input is needed.

[okamsn]

At this point, I'm definitely leaning towards "no".

As stated above, the current behavior:

• discourages naming accumulation variables due to the speed penalty
• discourages using destructuring accumulations, which require named variables
• encourages using the split flag and knowing more about the macro's internals, just to write (IMO) less readable code

The macro is only useful so long as it is convenient. There are already limitations to how the macro's arguments can be used, and restricting this would change little. Besides, if a user needed to go beyond the accumulation commands, they would probably just use expr in some amount anyway. It is no loss to just use expr fully then.

That being said, it would be nice to add more accumulation commands to lessen the need to go beyond them. E.g., if one can collect at both ends of a list, then one should probably be able to drop at both ends as well.

Disallowing this behavior while adding more accumulation commands should solve this issue, keeping both speed and usefulness.