Sometimes a package installation order exists such that it is possible to avoid having two conflicting packages installed simultaneously. If a currently installed package conflicts with a new package that is planned to be installed, it may be possible to solve the conflict by replacing the installed package with a different package that occupies the same slot.
In order to avoid a conflict, a package may need to be uninstalled rather than replaced. The following constraints protect inappropriate packages from being chosen for automatic uninstallation:
Installed packages that have been pulled into the current dependency graph will not be uninstalled. Due to dependency neglection and special properties of packages in the "system" set, other checks may be necessary in order to protect inappropriate packages from being uninstalled.
An installed package that is matched by a dependency atom from the "system" set will not be uninstalled in advance since it might not be safe. Such a package will only be uninstalled through replacement.
An installed package that is matched by a dependency atom from the "world" set will not be uninstalled if the dependency graph does not contain a replacement package that is matched by the same dependency atom.
In order to ensure that package files remain installed in a usable state whenever possible, uninstallation operations are not executed until after all associated conflicting packages have been installed. When file collisions occur between conflicting packages, the contents entries for those files are removed from the packages that are scheduled for uninstallation. This prevents uninstallation operations from removing overlapping files that have been claimed by conflicting packages.