A tool for managing your Nix user profile declaratively. Normally
one's Nix profile is managed with tools like nix-env, which is
antithetical to the kind of beliefs which lead one to using tools like
Nix and NixOS in the first place, being a fundamentally imperative
process rather than the declarative one that Nix and NixOS otherwise
champion.
Instead, nix-profile-declarative aims to have you manage your
profile similar to the way you configure NixOS -- by editing the
~/.nix/profile.nix file.
If you're familiar with the Nix ecosystem, you may be wondering why this tool exists in parallel with Home Manager, a tool that does much the same thing.
This is due to one core difference in ethos between
nix-profile-declarative and Home Manager: Modifying the $HOME
folder.
Home Manager configures your applications in a very traditional way, by modifying the many files sitting in your home directory that these applications read for their configuration. This gives Home Manager a lot of freedom, but in my opinion is both messy and a losing game, given that end-user applications are often prone to dropping their own files in their configuration directory then turning around and reading them.
nix-profile-declarative functions much like NixOS, in that when it
wants to configure an application, instead of doing it the messy,
stateful way, it chooses to create a wrapper for the application and
pass command-line flags to it, pointing it to configuration files in
the Nix Store. This means that the only files
nix-profile-declarative will touch are those in the ~/.nix-profile
directory, which is the same place that nix-env already manages.