Nix's configuration language is quite powerful, but suffers from a
lack of discoverability, usability and consistency. To name just a few
examples:

* There is no easy way to find out from the command line or from the
 REPL what arguments are supported by functions like
 `stdenv.mkDerivation` or `buildPythonPackage`.

* Mechanisms like the .override attribute provide an almost unlimited
 ability to customize packages, but the only way to figure out what
 you can override is to read the source of the Nix package, and
 *writing* overrides is often black magic.

* NixOS has a nice self-documenting module system, but Nix packages
 are written in a completely different functional style.

* The Nix CLI doesn't know anything about package functions, `.override`
 and `.overrideDerivation`, the NixOS module system, the Nixpkgs
 `config` attribute set, Nixpkgs overlays, or any other customization
 mechanisms that have emerged over the years.

* The syntax and semantics of Nix expressions are…