Simple example of "import from derivation" in Nix

ifd.nix

{ expr }:

let
  pkgs = import <nixpkgs> { };

  drv =
    pkgs.runCommand "script" { } ''
      echo 'Building during evaluation to generate a Nix expression!'
      ${pkgs.cowsay}/bin/cowsay '${expr}'
      echo '${expr}' > $out
    '';

in
import /* from */ drv

shell.txt

λ nix-instantiate ifd.nix --eval --read-write-mode --argstr expr "2 + 2"
copying path '/nix/store/y6pjym3f1njrcphjyw60ydd1hrfqfrx0-cowsay-3.04' from 'https://cache.nixos.org'...
building '/nix/store/1hf02dd7l1dlq4dmspbzbrkcpq8i4xmq-script.drv'...
Building during evaluation to generate a Nix expression!
 _______
< 2 + 2 >
 -------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||
4