outputs = { self, nixpkgs }: {

    packages.x86_64-linux.default =
      # Notice the reference to nixpkgs here.
      with import nixpkgs { system = "x86_64-linux"; };
      stdenv.mkDerivation {
        name = "hello";
        src = self;
        buildPhase = "gcc -o hello ./hello.c";
        installPhase = "mkdir -p $out/bin; install -t $out/bin hello";
      };

  };
}
```

The following attributes are supported in `flake.nix`:

* `description`: A short, one-line description of the flake.

* `inputs`: An attrset specifying the dependencies of the flake
  (described below).

* `outputs`: A function that, given an attribute set containing the
  outputs of each of the input flakes keyed by their identifier,
  yields the Nix values provided by this flake. Thus, in the example
  above, `inputs.nixpkgs` contains the result of the call to the
  `outputs` function of the `nixpkgs` flake.

  In addition to the outputs of each input, each input in `inputs`
  also contains some metadata about the inputs. These are:

  * `outPath`: The path in the Nix store of the flake's source tree.
     This way, the attribute set can be passed to `import` as if it was a path,
     as in the example above (`import nixpkgs`).

  * `rev`: The commit hash of the flake's repository, if applicable.

  * `revCount`: The number of ancestors of the revision `rev`. This is
    not available for `github` repositories, since they're fetched as
    tarballs rather than as Git repositories.

  * `lastModifiedDate`: The commit time of the revision `rev`, in the
    format `%Y%m%d%H%M%S` (e.g. `20181231100934`). Unlike `revCount`,
    this is available for both Git and GitHub repositories, so it's
    useful for generating (hopefully) monotonically increasing version
    strings.

  * `lastModified`: The commit time of the revision `rev` as an integer
    denoting the number of seconds since 1970.

  * `narHash`: The SHA-256 (in SRI format) of the
    [Nix Archive (NAR) serialisation][Nix Archive]
    NAR serialization of the flake's source tree.

  The value returned by the `outputs` function must be an attribute
  set. The attributes can have arbitrary values; however, various
  `nix` subcommands require specific attributes to have a specific
  value (e.g. `packages.x86_64-linux` must be an attribute set of
  derivations built for the `x86_64-linux` platform).

* `nixConfig`: a set of `nix.conf` options to be set when evaluating any
  part of a flake. In the interests of security, only a small set of
  set of options is allowed to be set without confirmation so long as [`accept-flake-config`](@docroot@/command-ref/conf-file.md#conf-accept-flake-config) is not enabled in the global configuration:
   - [`bash-prompt`](@docroot@/command-ref/conf-file.md#conf-bash-prompt)
   - [`bash-prompt-prefix`](@docroot@/command-ref/conf-file.md#conf-bash-prompt-prefix)
   - [`bash-prompt-suffix`](@docroot@/command-ref/conf-file.md#conf-bash-prompt-suffix)
   - [`flake-registry`](@docroot@/command-ref/conf-file.md#conf-flake-registry)
   - [`commit-lock-file-summary`](@docroot@/command-ref/conf-file.md#conf-commit-lock-file-summary)

## Flake inputs

The attribute `inputs` specifies the dependencies of a flake, as an
attrset mapping input names to flake references. For example, the
following specifies a dependency on the `nixpkgs` and `import-cargo`
repositories:

```nix
# A GitHub repository.
inputs.import-cargo = {
  type = "github";
  owner = "edolstra";
  repo = "import-cargo";
};

# An indirection through the flake registry.
inputs.nixpkgs = {
  type = "indirect";
  id = "nixpkgs";
};
```

Alternatively, you can use the URL-like syntax:

```nix
inputs.import-cargo.url = "github:edolstra/import-cargo";
inputs.nixpkgs.url = "nixpkgs";
```

Each input is fetched, evaluated and passed to the `outputs` function
as a set of attributes with the same name as the corresponding
input. The special input named `self` refers to the outputs and source
tree of *this* flake. Thus, a typical `outputs` function looks like