the evaluation cache. This is made possible by the hermetic
  evaluation model of flakes.

  Intermediate results are not cached.

* The new `--offline` flag disables substituters and causes all
  locally cached tarballs and repositories to be considered
  up-to-date.

* The new `--refresh` flag causes all locally cached tarballs and
  repositories to be considered out-of-date.

* Many `nix` subcommands now have a `--json` option to produce
  machine-readable output.

* `nix repl` has a new `:doc` command to show documentation about
  builtin functions (e.g. `:doc builtins.map`).

* Binary cache stores now have an option `index-debug-info` to create
  an index of DWARF debuginfo files for use by
  [`dwarffs`](https://github.com/edolstra/dwarffs).

* To support flakes, Nix now has an extensible mechanism for fetching
  source trees. Currently it has the following backends:

  * Git repositories

  * Mercurial repositories

  * GitHub and GitLab repositories (an optimisation for faster
    fetching than Git)

  * Tarballs

  * Arbitrary directories

  The fetcher infrastructure is exposed via flake input specifications
  and via the `fetchTree` built-in.

* **Languages changes**: the only new language feature is that you can
  now have antiquotations in paths, e.g. `./${foo}` instead of `./. +
  foo`.

* **New built-in functions**:

  - `builtins.fetchTree` allows fetching a source tree using any
    backends supported by the fetcher infrastructure. It subsumes the
    functionality of existing built-ins like `fetchGit`,
    `fetchMercurial` and `fetchTarball`.

  - `builtins.getFlake` fetches a flake and returns its output
    attributes. This function should not be used inside flakes! Use
    flake inputs instead.

  - `builtins.floor` and `builtins.ceil` round a floating-point number
    down and up, respectively.

* Experimental support for recursive Nix. This means that Nix
  derivations can now call Nix to build other derivations. This is not
  in a stable state yet and not well
  [documented](https://github.com/NixOS/nix/commit/c4d7c76b641d82b2696fef73ce0ac160043c18da).

* The new experimental feature `no-url-literals` disables URL
  literals. This helps to implement [RFC
  45](https://github.com/NixOS/rfcs/pull/45).

* Nix now uses `libarchive` to decompress and unpack tarballs and zip
  files, so `tar` is no longer required.

* The priority of substituters can now be overridden using the
  `priority` substituter setting (e.g. `--substituters
  'http://cache.nixos.org?priority=100 daemon?priority=10'`).

* `nix edit` now supports non-derivation attributes, e.g. `nix edit
  .#nixosConfigurations.bla`.

* The `nix` command now provides command line completion for `bash`,
  `zsh` and `fish`. Since the support for getting completions is built
  into `nix`, it's easy to add support for other shells.

* The new `--log-format` flag selects what Nix's output looks like. It
  defaults to a terse progress indicator. There is a new
  `internal-json` output format for use by other programs.

* `nix eval` has a new `--apply` flag that applies a function to the
  evaluation result.

* `nix eval` has a new `--write-to` flag that allows it to write a
  nested attribute set of string leaves to a corresponding directory
  tree.

* Memory improvements: many operations that add paths to the store or
  copy paths between stores now run in constant memory.

* Many `nix` commands now support the flag `--derivation` to operate
  on a `.drv` file itself instead of its outputs.

* There is a new store called `dummy://` that does not support
  building or adding paths. This is useful if you want to use the Nix
  evaluator but don't have a Nix store.

* The `ssh-ng://` store now allows substituting paths on the remote,
  as `ssh://` already did.

* When auto-calling a function with an ellipsis, all arguments are now
  passed.

* New `nix-shell` features:

  - It preserves the `PS1` environment variable if
    `NIX_SHELL_PRESERVE_PROMPT` is set.