Improve generators.toPretty

[infinisil]

Motivation for this change

For big Nix expressions, toPretty can be very hard to read, as it doesn't output any newlines. This PR implements the multiline option for toPretty, enabled by default, which makes it output newlines with proper indentation. Example output:

{
  foo = {
    bar = 10;
    baz = [
      "baz"
    ];
  };
}

In addition, I made some other improvements:

• Don't quote attribute names if they don't need to be quoted by using escapeNixIdentifier, introduced in f9eb3d1
• Switch away from the δ and λ symbols for derivations and functions respectively
• Multiline strings are printed correctly now

Ping @Profpatsch @rycee

Best reviewed commit-by-commit. Specifically there's one commit in there that just refactors the function without changing its behavior.

Things done

• Added a new test case for all functionality

[Profpatsch]

I love this, I’ve been wanting to have multiline toPretty in forever.

Two additional things:

1. I’d leave the if/then/else form instead of going vial typeOf, since it’s more straightforward and easier to adjust.
2. I’d render lists and attrsets that only have one element on one line to be slightly more compact. From 2 elements, split onto newlines. Though that might be easier said than done …

[infinisil]

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
        ];
      }
    ];
  };
}

[Profpatsch]

Looks very good to me 👍

Haven’t reviewed the logic in-depth, but we have tests so it’s fine. LGTM

[edolstra]

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.