Did some changes:

• Multiline strings are printed correctly now
• Switch away from the δ and λ symbols for derivations and functions respectively
• Went back to the big if else. I don't think it's nicer, but it works out well for another change:
• To print __functor's as functions
• Removed the prefix argument again. This can be achieved with builtins.replaceStrings [ "\n" ] [ "\nprefix" ] as well

I decided to not render single-element lists/attrs without newlines, because I fear this makes it more complicated to understand the output, as the indentation can't be used to see the nesting level anymore.

Here's a sample output for this Nix expression:

{
  value = {
  fun = lib.id;
  funargs = lib.types.submoduleWith;
  multiline = {
    multi = ''
      hello
      there!
    '';
    multi' = ''
      hello
      there!'';
    single = "hello there!";
  };
  pkg = pkgs.hello;
  single = { foo = [ { bar = [ null ]; } ]; };
}

Which renders as

{
  fun = <function>;
  funargs = <function, args: {modules, shorthandOnlyDefinesConfig?, specialArgs?}>;
  multiline = {
    multi = ''
      hello
      there!
    '';
    multi' = ''
      hello
      there!'';
    single = "hello there!";
  };
  pkg = <derivation /nix/store/2nnf3klixf4j2ih8x0zx4albfzlk74l9-hello-2.10.drv>;
  single = {
    foo = [
      {
        bar = [
          null
        ];
      }
    ];
  };
}

I suppose eventually we'd want this to be a builtin function in Nix, since it's needed in the UI anyway, and it's easier on the C++ side to handle circular values etc.