Am Mittwoch, 23. Januar 2013 schrieb Ludovic Courtès:
> And what if you roll back once you’re at the empty profile?

Then nothing should happen.

> It seems more intuitive for me to error out like this, because there was
> really nothing but nothingness before “hello” was installed.  :-)
> WDYT?

No, I disagree; when I have nothing, install hello and roll back, I should 
be back to nothing. Some other opinions would be useful on this matter.

> This is expected (same behavior as nix-env.)  Profile generations are
> not deleted unless you explicitly do so; this is what guarantees that
> one can roll back anywhere they want.
> 
> But I wonder if this is really worth the trouble.  In my experience, a
> scenario like the one above rarely happens, if ever.

I find the behaviour of roll back currently very confusing, and the 
situation looks reasonable to me:
I install hello, it works, so I keep it.
I install freetype, it does not work, so I drop it again.
I install file, it does not work, so I drop it again.
Now I expect to have only hello, but I have hello and freetype.

Of course, instead of rolling back, I can also uninstall. But the same 
situation occurs when one replaces the package names by different versions, 
I suppose.

I see installing packages and rolling back as steps forward and backward. 
Going into direction B and back, then going into direction C and back 
should not leave me with one step forward in direction B.

Note that I did not use nix before; so I am just arguing from what would 
intuitively be the correct behaviour of --roll-back for me: Being in 
situation A, doing something to bring me into situation B and rolling back 
should put me into situation A again, not something that resembles 
situation A, but with memories of B.

Again, some other opinions would be useful.

Andreas