[bobbyi_settv]

> It uninstall vim and install emacs.

If a package system is well designed, having the package for vim on my system shouldn't be doing any harm even if I'm not currently using it, so the system you are describing isn't really any better than just having a list of packages I want installed in a text file.

[sirn]

That is not the point I want to make. Having both Vim and Emacs doesn't really do any harm. You can even have multiple versions of Vim in the same machine, even with different and incompatible dependency version, and Nix will happily manage that. The point is that with Nix collection, it will try to build the system exactly as described without any kind of side effect.

Using Vim isn't the best example (why would someone want multiple Vim?) but the same apply to languages as well. For example if I want to use Python 3.2, but there are handful of utilities that require 3.3+. I can just install 3.2 to my profile and install those utilities with 3.5 without actually exposing 3.5 to my profile.