Updated documentation

1 files changed, 238 insertions, 323 deletions

diff --git a/doc/flakes/design.md b/doc/flakes/design.md
index 63198e577..c9520bcbf 100644
--- a/doc/flakes/design.md
+++ b/doc/flakes/design.md
@@ -2,92 +2,83 @@
 
 ## Goals
 
-* To provide Nix repositories with an easy and standard way to
-  reference other Nix repositories.
+* Standard and easy way for Nix repos to reference other Nix repos as
+  dependencies
 
-* To allow such references to be queried and updated automatically.
+* Discoverability: Be able to query and update these references to Nix repos
+  automatically
 
-* To provide a replacement for `nix-channel`, `NIX_PATH` and Hydra
-  jobset definitions.
+* To provide a replacement for `nix-channel`, `NIX_PATH` and Hydra jobset
+  definitions
 
-* To enable reproducible, hermetic evaluation of packages and NixOS
-  configurations.
+* Reproducibility: Evaluate packages and NixOS configurations hermetic by
+  default
 
-Things that we probably won't do in the initial iteration:
+Upcoming but not yet implemented:
 
-* Sophisticated flake versioning, such as the ability to specify
-  version ranges on dependencies.
+* Sophisticated flake versioning, such as the ability to specify version ranges
+  on dependencies.
 
-* A way to specify the types of values provided by a flake. For the
-  most part, flakes can provide arbitrary Nix values, but there will
-  be some standard attribute names (e.g. `packages` must be a set of
-  installable derivations).
+* A way to specify the types of values provided by a flake. For the most part,
+  flakes can provide arbitrary Nix values, but there will be some standard
+  attribute names (e.g. `packages` must be a set of installable derivations).
 
 
 ## Overview
 
-* A flake is (usually) a Git repository that contains a file named
-  `flake.nix` at top-level.
+* A flake is (usually) a Git repository that contains a file named `flake.nix`
+  at top-level
 
-* Flakes *provide* an attribute set of values, such as packages,
-  Nixpkgs overlays, NixOS modules, library functions, Hydra jobs,
-  `nix-shell` definitions, etc.
+* A flake *provides* an attribute set of values, such as packages, Nixpkgs
+  overlays, NixOS modules, library functions, Hydra jobs, `nix-shell`
+  definitions, etc.
 
-* Flakes can *depend* on other flakes.
+* Flakes can *depend* on other flakes or other repositories which aren't flakes
 
-* Flakes are referred to using a *flake reference*, which is either a
-  URL specifying its repository's location
-  (e.g. `github:NixOS/nixpkgs/release-18.09`) or an identifier
-  (e.g. `nixpkgs`) looked up in a *lock file* or *flake
-  registry*. They can also specify revisions,
-  e.g. `github:NixOS/nixpkgs/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f`.
+* Flakes are referred to using a *flake reference*, which is either a URL
+  specifying its repository's location or an identifier looked up in a *lock
+  file* or *flake registry*.
 
-* The *flake registry* is a centrally maintained mapping (on
-  `nixos.org`) from flake identifiers to flake locations
-  (e.g. `nixpkgs -> github:NixOS/nixpkgs/release-18.09`).
+* A *flake registry* is a mapping from flake identifiers to flake locations
+  (e.g. `nixpkgs -> github:NixOS/nixpkgs/release-18.09`). There is a centrally
+  maintained flake registry on `nixos.org`.
 
-* A flake can contain a *lock file* (`flake.lock`) used when resolving
-  the dependencies in `flake.nix`. It maps flake references to
-  references containing revisions (e.g. `nixpkgs ->
+* A flake can contain a *lock file* (`flake.lock`) used when resolving the
+  dependencies in `flake.nix`. It maps mutable flake references
+  (e.g. `github:NixOS/nixpkgs/release-18.09`) to references containing revisions
+  (e.g. `nixpkgs ->
   github:NixOS/nixpkgs/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f`).
 
-* The `nix` command uses the flake registry as its default
-  installation source. For example, `nix build nixpkgs.hello` builds the
-  `hello` package provided by the `nixpkgs` flake listed in the
-  registry. `nix` will automatically download/upload the registry and
-  flakes as needed.
+* The `nix` command uses the flake registry as its default installation source.
+  For example, `nix build nixpkgs.hello` builds the `hello` package provided by
+  the `nixpkgs` flake listed in the registry. `nix` will automatically
+  download/upload the registry and flakes as needed.
 
 * `nix build` without arguments will build the flake in the current
   directory (or some parent).
 
-* The command `nix flake update` generates/updates `flake.lock` from
-  `flake.nix`. This should probably also be done automatically when
-  building from a local flake.
+* `nix flake update` generates `flake.lock` from `flake.nix`, ignoring the old
+  lockfile.
 
-* `nixos-rebuild` will build a configuration from a (locked)
-  flake. Evaluation will be done in pure mode to ensure there are no
-  unaccounted inputs. Thus the NixOS configuration can be reproduced
-  unambiguously from the top-level flake.
+* `nixos-rebuild` will build a configuration from a (locked) flake. Evaluation
+  is done in pure mode to ensure there are no unaccounted inputs. Thus the
+  NixOS configuration can be reproduced unambiguously from the top-level flake.
 
-* Nix code can query flake metadata such as `commitHash` (the Git
-  revision) or `date` (the date of the last commit). This is useful
-  for NixOS to compute the NixOS version string (which will be the
-  revision of the top-level configuration flake, uniquely identifying
-  the configuration).
+* Nix code can query flake metadata such as `commitHash` (the Git revision) or
+  `epoch` (the date of the last commit). This is useful for NixOS to compute
+  the NixOS version string (which will be the revision of the top-level
+  configuration flake, uniquely identifying the configuration).
 
-* Hydra jobset configurations will consist of a single flake
-  reference. Thus we can get rid of jobset inputs; any other needed
-  repositories can be fetched by the top-level flake. The top-level
-  flake can be locked or unlocked; if some dependencies are unlocked,
-  then Nix will fetch the latest revision for each.
+* Hydra jobset configurations will consist of a single flake reference. Thus we
+  can get rid of jobset inputs; any other needed repositories can be fetched by
+  the top-level flake. The top-level flake can be locked or unlocked; if some
+  dependencies are unlocked, then Nix will fetch the latest revision for each.
 
 
 ## Example flake
 
-A flake is a Git repository that contains a file named
-`flake.nix`. For example, here is the `flake.nix` for `dwarffs`, a
-small repository that provides a single package and a single NixOS
-module.
+Let us look at an example of a `flake.nix` file, here for `dwarffs`, a small
+repository that provides a single package and a single NixOS module.
 
 ```nix
 {
@@ -101,23 +92,26 @@ module.
   # Some other metadata.
   description = "A filesystem that fetches DWARF debug info from the Internet on demand";
 
-  # A list of flake references denoting the flakes that this flake
-  # depends on. Nix will resolve and fetch these flakes and pass them
-  # as a function argument to `outputs` below.
+  # The flake dependencies. Nix will resolve and fetch these flakes and pass
+  # them as a function argument to `outputs` below.
   #
-  # `flake:nixpkgs` denotes a flake named `nixpkgs` which is looked up
+  # "nixpkgs" denotes a flake named `nixpkgs` which is looked up
   # in the flake registry, or in `flake.lock` inside this flake, if it
   # exists.
   inputs = [ flake:nixpkgs ];
 
+  # An attribute set listing dependencies which aren't flakes, also to be passed as
+  # a function argument to `provides`.
+  nonFlakeRequires = {};
+
   # The stuff provided by this flake. Flakes can provide whatever they
   # want (convention over configuration), but some attributes have
-  # special meaning to tools / other flakes: for example, `packages`
+  # special meaning to tools / other flakes. For example, `packages`
   # is used by the `nix` CLI to search for packages, and
   # `nixosModules` is used by NixOS to automatically pull in the
   # modules provided by a flake.
   #
-  # `outputs` takes a single argument named `deps` that contains
+  # `outputs` takes a single argument (`deps`) that contains
   # the resolved set of flakes. (See below.)
   outputs = deps: {
 
@@ -153,7 +147,11 @@ module.
     nixosModules.dwarffs = import ./module.nix deps;
 
     # Provide a single Hydra job (`hydraJobs.dwarffs`).
-    hydraJobs = deps.this.packages;
+    hydraJobs.build.x86_64-linux = packages.dwarffs;
+
+    # A bunch of things which can be checked (through `nix flake check`) to
+    # make sure the flake is well-defined.
+    checks.build = packages.dwarffs;
   };
 }
 ```
@@ -170,8 +168,11 @@ Similarly, a minimal `flake.nix` for Nixpkgs:
 
   outputs = deps:
     let pkgs = import ./. {}; in
+    let pkgs = import ./. { system = "x86_64-linux"; }; in
     {
-      lib = import ./lib;
+      lib = (import ./lib) // {
+        nixosSystem = import ./nixos/lib/eval-config.nix;
+      };
 
       builders = {
         inherit (pkgs) stdenv fetchurl;
@@ -180,145 +181,124 @@ Similarly, a minimal `flake.nix` for Nixpkgs:
       packages = {
         inherit (pkgs) hello nix fuse nlohmann_json boost;
       };
+
+      legacyPkgs = pkgs;
     };
 }
 ```
-Note that `packages` is an unpolluted set of packages: non-package
-values like `lib` or `fetchurl` are not part of it.
-
+Note that `packages` is an unpolluted set of packages: non-package values like
+`lib` or `fetchurl` are not part of it.
 
-## Flake identifiers
+## Flake registries
 
-A flake has an identifier (e.g. `nixpkgs` or `dwarffs`).
+Note: If a flake registry contains an entry `nixpkgs -> github:NixOS/nixpkgs`,
+then `nixpkgs/release-18.09` will match to become
+`github:NixOS/nixpkgs/release-18.09`. This is referred to as "fuzzymatching".
 
 
 ## Flake references
 
-Flake references are a URI-like syntax to specify the physical
-location of a flake (e.g. a Git repository) or to denote a lookup in
-the flake registry or lock file.
-
-* `(flake:)?<flake-id>(/rev-or-ref(/rev)?)?`
-
-  Look up a flake by ID in the flake lock file or in the flake
-  registry. These must specify an actual location for the flake using
-  the formats listed below. Note that in pure evaluation mode, the
-  flake registry is empty.
-
-  Optionally, the `rev` or `ref` from the dereferenced flake can be
-  overriden. For example,
-
-  > nixpkgs/19.09
-
-  uses the `19.09` branch of the `nixpkgs` flake's GitHub repository,
-  while
-
-  > nixpkgs/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f
-
-  uses the specified revision. For Git (rather than GitHub)
-  repositories, both the rev and ref must be given, e.g.
-
-  > nixpkgs/19.09/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f
-
-* `github:<owner>/<repo>(/<rev-or-ref>)?`
-
-   A repository on GitHub. These differ from Git references in that
-   they're downloaded in a efficient way (via the tarball mechanism)
-   and that they support downloading a specific revision without
-   specifying a branch. `rev-or-ref` is either a commit hash (`rev`)
-   or a branch or tag name (`ref`). The default is `master` if none is
-   specified. Note that in pure evaluation mode, a commit hash must be
-   used.
-
-   Flakes fetched in this manner expose `rev` and `date` attributes,
-   but not `revCount`.
-
-   Examples:
-
-   > github:edolstra/dwarffs
-
-   > github:edolstra/dwarffs/unstable
-
-   > github:edolstra/dwarffs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553
-
-* > https://<server>/<path>.git(\?attr(&attr)*)?
+Flake references are a URI-like syntax to specify the physical location of a
+flake (e.g. a Git repository) or to denote a lookup in the flake registry or
+lock file. There are four options for the syntax:
 
-  > ssh://<server>/<path>.git(\?attr(&attr)*)?
+* Flake aliases
+  A flake alias is a name which requires a lookup in a flake
+  registry or lock file.
 
-  > git://<server>/<path>.git(\?attr(&attr)*)?
+  Example: "nixpkgs"
 
-  > file:///<path>(\?attr(&attr)*)?
+* GitHub repositories
+  A repository which is stored on GitHub can easily be fetched using this type.
+  Note:
+  * Only the code in this particular commit is downloaded, not the entire repo
+  * By default, the commit to download is the last commit on the `master` branch.
+    See later for how to change this.
 
-   where `attr` is one of `rev=<rev>` or `ref=<ref>`.
+  Example: `github:NixOS/nixpkgs`
 
-   A Git repository fetched through https. Note that the path must end
-   in `.git`. The default for `ref` is `master`.
-
-   Examples:
-
-   > https://example.org/my/repo.git
-   > https://example.org/my/repo.git?ref=release-1.2.3
-   > https://example.org/my/repo.git?rev=e72daba8250068216d79d2aeef40d4d95aff6666
-
-* > /path.git(\?attr(&attr)*)?
-
-  Like `file://path.git`, but if no `ref` or `rev` is specified, the
-  (possibly dirty) working tree will be used. Using a working tree is
-  not allowed in pure evaluation mode.
+* `ssh/https/git/file`
+  These are generic `FlakeRef`s for downloadding git repositories or tarballs.
 
   Examples:
-
-  > /path/to/my/repo
-
-  > /path/to/my/repo?ref=develop
-
-  > /path/to/my/repo?rev=e72daba8250068216d79d2aeef40d4d95aff6666
-
-* > https://<server>/<path>.tar.xz(?hash=<sri-hash>)
-
-  > file:///<path>.tar.xz(?hash=<sri-hash>)
-
-   A flake distributed as a tarball. In pure evaluation mode, an SRI
-   hash is mandatory. It exposes a `date` attribute, being the newest
-   file inside the tarball.
-
-   Example:
-
-   > https://releases.nixos.org/nixos/unstable/nixos-19.03pre167858.f2a1a4e93be/nixexprs.tar.xz
-
-   > https://releases.nixos.org/nixos/unstable/nixos-19.03pre167858.f2a1a4e93be/nixexprs.tar.xz?hash=sha256-56bbc099995ea8581ead78f22832fee7dbcb0a0b6319293d8c2d0aef5379397c
-
-Note: currently, there can be only one flake per Git repository, and
-it must be at top-level. In the future, we may want to add a field
-(e.g. `dir=<dir>`) to specify a subdirectory inside the repository.
+  - https://example.org/my/repo.git
+  - ssh://git@github.com:NixOS/nix.git
+  - git://github.com/edolstra/dwarffs.git
+  - file:///home/my-user/some-repo/some-repo.git
+  - https://releases.nixos.org/nixos/unstable/nixos-19.03pre167858.f2a1a4e93be/nixexprs.tar.xz
+  - file:///<path>.tar.xz
+
+* Local, dirty paths
+  This `FlakeRef` is the equivalent of `file://<path>` used for dirty paths.
+
+  Example: /path/to/my/repo
+
+Notes:
+- Each FlakeRef (except for the Path option) allows for a Git revision (i.e.
+  commit hash) and/or referenceo(i.e. git branch name) to be added. For
+  tarbals, an SRI hash needs to be added.
+  Examples:
+  * `"nixpkgs/release-18.09"`
+  * `github:NixOS/nixpkgs/1e9e709953e315ab004951248b186ac8e2306451`
+  * `git://github.com/edolstra/dwarffs.git?ref=flake&rev=2efca4bc9da70fb001b26c3dc858c6397d3c4817`
+  * file:///<path>.tar.xz(?hash=<sri-hash>)
+- In full pure mode, no mutable `FlakeRef`s can be used
+  * No aliases, because they need to be looked up
+  * `github` requires a specified `rev`
+  * `ssh/https/git/file` require a specified `ref` _and_ `rev`
+  * `path` is always mutable
+- Flakes don't need to be top-level, but can also reside in a subdirectory. This is shown by adding `dir=<subdir>` to the `FlakeRef`.
+  Example: `./foo?dir=bar`
 
 
 ## Flake lock files
 
-This is a JSON file named `flake.lock` that maps flake identifiers
-used in the corresponding `flake.nix` to "immutable" flake references;
-that is, flake references that contain a revision (for Git
-repositories) or a content hash (for tarballs).
+A lockfile is a JSON file named `flake.lock` which contains a forrest of
+entries mapping `FlakeRef`s to the immutable `FlakeRef` they were resolved to.
 
 Example:
 
 ```json
 {
-  "nixpkgs": "github:NixOS/nixpkgs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553",
-  "foo": "https://example.org/foo.tar.xz?hash=sha256-56bbc099995ea8581ead78f22832fee7dbcb0a0b6319293d8c2d0aef5379397c"
+  "nixpkgs": {
+    "uri": "github:NixOS/nixpkgs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553",
+    "content-hash": "sha256-vy2UmXQM66aS/Kn2tCtjt9RwxfBvV+nQVb5tJQFwi8E="
+  },
+  "foo": {
+    "uri": "https://example.org/foo.tar.xz?hash=sha256-56bbc099995ea8581ead78f22832fee7dbcb0a0b6319293d8c2d0aef5379397c",
+    "content-hash": "sha256-vy2UmXQM66aS/Kn2tCtjt9RwxfBvV+nQVb5tJQFwi8E="
+  }
 }
 ```
 
+Lockfiles are used to help resolve the dependencies of a flake.
+- `nix build github:<..>` uses the remote lockfile and update it
+- `nix build /home/user/dwarffs` uses the local lockfile, updates it and writes the result to file
+- `nix flake update <flakeref>` recreates the lockfile from scratch and writes it to file
+- `--no-registries` makes the command pure, also when fetching dependencies
+- `--no-save-lock-file`: Several commands will update the lockfile (e.g. `nix
+  build`). This flag prevents the updated lockfile to be written to file.
+- `--recreate-lock-file` makes prevents the current lockfile from being used
 
 ## `outputs`
 
-The flake attribute `outputs` is a function that takes an argument
-named `deps` and returns a (mostly) arbitrary attrset of values. Some
-of the standard result attributes:
+The function argument `deps` is an attrset containing all dependencies listed
+in `requires` and `nonFlakeRequires` as well as `path` (for the flake's source
+code) and an attribute `meta` with:
+- `description`
+- `commitHash` (not for tarball flakes): The Git commit hash.
+- `date`: The timestamp of the most recent commit (for Git repos), or of the
+  most recently modified file (for tarballs)
+- `revCount` (for Git flakes, but not GitHub flakes): The number of ancestors
+  of the revision. Useful for generating version strings.
+
+The flake attribute `outputs` is a function that takes an argument named `deps`
+and returns an attribute set. Some of the members of this set have protected
+names:
 
-* `packages`: A set of installable derivations used by the `nix`
-  command. That is, commands such as `nix install` ignore all other
-  flake attributes.
+* `packages`: A set of installable derivations used by the `nix` command. That
+  is, commands such as `nix install` ignore all other flake attributes. It
+  cannot be a nested set.
 
 * `hydraJobs`: Used by Hydra.
 
@@ -329,213 +309,155 @@ of the standard result attributes:
   we need to avoid a situation where `nixos-rebuild` needs to fetch
   its own `nixpkgs` just to do `evalModules`.)
 
-* `devShell`: A specification of a development environment in some TBD
-  format.
-
-The function argument `flakes` is an attrset that contains an
-attribute for each dependency specified in `inputs`. (Should it
-contain transitive dependencies? Probably not.) Each attribute is an
-attrset containing the `outputs` of the dependency, in addition to
-the following attributes:
-
-* `path`: The path to the flake's source code. Useful when you want to
-  use non-Nix artifacts from the flake, or if you want to *store* the
-  source code of the dependency in a derivation. (For example, we
-  could store the sources of all flake dependencies in a NixOS system
-  configuration, as a generalization of
-  `system.copySystemConfiguration`.)
-
-* `meta`: An attrset containing the following:
-
-  * `description`
-
-  * `commitHash` (or `rev`?) (not for tarball flakes): The Git commit
-    hash.
+* `devShell`: A derivation to create a development environment
 
-  * `date`: The timestamp of the most recent commit (for Git
-    repositories), or the timestamp of the most recently modified file
-    (for tarballs).
-
-  * `revCount` (for Git flakes, but not GitHub flakes): The number of
-    ancestors of the revision. Useful for generating version strings.
-
-
-## Non-flake dependencies
-
-It may be useful to pull in repositories that are not flakes
-(i.e. don't contain a `flake.nix`). This could be done in two ways:
-
-* Allow flakes not to have a `flake.nix` file, in which case it's a
-  flake with no inputs and no outputs. The downside of this
-  approach is that we can't detect accidental use of a non-flake
-  repository. (Also, we need to conjure up an identifier somehow.)
-
-* Add a flake attribute to specifiy non-flake dependencies, e.g.
-
-  > nonFlakeInputs.foobar = github:foo/bar;
+* `self`: The result of the flake's output which is passed to itself
+  Example: `self.outputs.foo` works.
 
 
 ## Flake registry
 
-The flake registry maps flake IDs to flake references (where the
-latter cannot be another indirection, i.e. it must not be a
-`flake:<flake-id>` reference).
-
-The default registry is kept at
-`https://nixos.org/flake-registry.json`. It looks like this:
+A flake registry is a JSON file mapping flake references to flake references.
+The default/global registry is kept at
+`https://github.com/NixOS/flake-registry/blob/master/flake-registry.json` and
+looks like this:
 
 ```json
 {
-    "version": 1,
     "flakes": {
         "dwarffs": {
             "uri": "github:edolstra/dwarffs/flake"
         },
+        "nix": {
+            "uri": "github:NixOS/nix/flakes"
+        },
         "nixpkgs": {
-            "uri": "github:NixOS/nixpkgs/release-18.09"
+            "uri": "github:edolstra/nixpkgs/release-19.03"
+        },
+        "hydra": {
+            "uri": "github:NixOS/hydra/flake"
+        },
+        "patchelf": {
+            "uri": "github:NixOS/patchelf"
         }
-    }
+    },
+    "version": 1
 }
 ```
 
-Nix automatically (re)downloads the registry. The downloaded file is a
-GC root so the registry remains available if nixos.org is unreachable.
-TBD: when to redownload?
+Nix automatically (re)downloads this file whenever you have network access. The
+downloaded file is a GC root so the registry remains available if nixos.org is
+unreachable.
 
+In addition to a global registry, there is also a user registry stored in
+`~/.config/nix/registry.json`.
 
-## Nix UI
 
-Commands for registry / user flake configuration:
+## Nix UI
 
-* `nix flake list`: Show all flakes in the registry.
+There is a list of new commands added to the `nix` CLI:
 
-* `nix flake add <flake-ref>`: Add or override a flake to/in the
-  user's flake configuration (`~/.config/nix/flakes.nix`). For
-  example, `nix flake add nixpkgs/nixos-18.03` overrides the `nixpkgs`
-  flake to use the `nixos-18.03` branch. There should also be a way to
-  add multiple branches/revisions of the same flake by giving them a
-  different ID, e.g. `nix flake add --id nixpkgs-ancient
-  nixpkgs/nixos-16.03`).
+* `nix flake list`: Show all flakes in the registry
 
-* `nix flake remove <flake-id>`: Remove a flake from the user's flake
-  configuration. Any flake with the same ID in the registry remains
-  available.
+* `nix flake add <alias FlakeRef> <resolved FlakeRef>`: Add or override a flake
+  to/in the user flake registry.
 
-* `nix flake lock <flake-id>`: Lock a flake. For example, `nix flake
-  lock nixpkgs` pins `nixpkgs` to the current revision.
+* `nix flake remove <alias FlakeRef>`: Remove a FlakeRef from the user flake
+  registry.
 
-Commands for creating/modifying a flake:
+* `nix flake pin <alias FlakeRef>`: Look up to which immutable FlakeRef the
+  alias FlakeRef maps to currently, and store that map in the user registry.
+  Example: `nix flake pin github:NixOS/nixpkgs` will create an entry
+  `github:NixOS/nixpkgs ->
+  github:NixOS/nixpkgs/444f22ca892a873f76acd88d5d55bdc24ed08757`.
 
-* `nix flake init`: Create a `flake.nix` in the current directory.
+* `nix flake init`: Create a `flake.nix` in the current directory
 
-* `nix flake update`: Update the lock file for the `flake.nix` in the
-  current directory. In most cases, this should be done
-  automatically. (E.g. `nix build` should automatically update the
-  lock file is a new dependency is added to `flake.nix`.)
+* `nix flake update`: Recreate the lock file from scratch, from the `flake.nix`.
 
 * `nix flake check`: Do some checks on the flake, e.g. check that all
   `packages` are really packages.
 
-* `nix flake clone`: Do a Git clone of the flake repository. This is a
-  convenience to easily start hacking on a flake. E.g. `nix flake
-  clone dwarffs` clones the `dwarffs` GitHub repository to `./dwarffs`.
-
-TODO: maybe the first set of commands should have a different name
-from the second set.
+* `nix flake clone`: `git clone` the flake repo
 
 Flags / configuration options:
 
-* `--flakes (<flake-id>=<flake-ref>)*`: add/override some flakes.
+* `--flakes (<alias FlakeRef>=<resolved FlakeRef>)*`: add/override some
+  FlakeRef
 
-* (In `nix`) `--flake <flake-ref>`: set the specified flake as the
-  installation source. E.g. `nix build --flake ./my-nixpkgs hello`.
+* `--flake <flake-ref>`: set the specified flake as the installation source
+  E.g. `nix build --flake ./my-nixpkgs hello`.
 
-The default installation source in `nix` is the `packages` from all
-flakes in the registry, that is:
+The default installation source in `nix` is the `packages` from all flakes in
+the registry, that is:
 ```
 builtins.mapAttrs (flakeName: flakeInfo:
   (getFlake flakeInfo.uri).${flakeName}.outputs.packages or {})
   builtins.flakeRegistry
 ```
-(where `builtins.flakeRegistry` is the global registry with user
-overrides applied, and `builtins.getFlake` downloads a flake and
-resolves its dependencies.)
-
-It may be nice to extend the default installation source with the
-`packages` from the flake in the current directory, so that
-
-> nix build hello
-
-does something similar to the old
-
-> nix-build -A hello
-
-Specifically, it builds `packages.hello` from the flake in the current
-directory. Of course, this creates some ambiguity if there is a flake
-in the registry named `hello`.
-
-Maybe the command
-
-> nix dev-shell
-
-should do something like use `outputs.devShell` to initialize the
-shell, but probably we should ditch `nix shell` / `nix-shell` for
-direnv.
+where `builtins.flakeRegistry` is the global registry with user overrides
+applied, and `builtins.getFlake` downloads a flake and resolves its
+dependencies.
 
 
 ## Pure evaluation and caching
 
-Flake evaluation should be done in pure mode. Thus:
+Flake evaluation is done in pure mode. Thus:
 
-* Flakes cannot do `NIX_PATH` lookups via the `<...>` syntax.
+* Flakes cannot use `NIX_PATH` via the `<...>` syntax.
 
-* They can't read random stuff from non-flake directories, such as
+* Flakes cannot read random stuff from non-flake directories, such as
   `~/.nix/config.nix` or overlays.
 
-This enables aggressive caching or precomputation of Nixpkgs package
-sets. For example, for a particular Nixpkgs flake closure (as
-identified by, say, a hash of the fully-qualified flake references
-after dependency resolution) and system type, an attribute like
-`packages.hello` should always evaluate to the same derivation. So we
-can:
+This enables aggressive caching or precomputation of Nixpkgs package sets. For
+example, for a particular Nixpkgs flake closure (as identified by, say, a hash
+of the fully-qualified flake references after dependency resolution) and system
+type, an attribute like `packages.hello` should always evaluate to the same
+derivation. So we can:
 
-* Keep a local evaluation cache (say `~/.cache/nix/eval.sqlite`)
+* Keep a local evaluation cache (say `~/.cache/nix/eval-cache-v1.sqlite`)
   mapping `(<flake-closure-hash, <attribute>) -> (<drv-name>,
   <drv-output-paths>, <whatever other info we want to cache>)`.
 
-* Download a precomputed cache
-  (e.g. `https://releases.nixos.org/eval/<flake-closure-hash>.sqlite`). So
-  a command like `nix search` could avoid evaluating Nixpkgs entirely.
-
-Of course, this doesn't allow overlays. With pure evaluation, the only
-way to have these is to define a top-level flake that depends on the
-Nixpkgs flake and somehow passes in a set of overlays.
+* Download a precomputed cache, e.g.
+  `https://releases.nixos.org/eval/<flake-closure-hash>.sqlite`. So a command
+  like `nix search` could avoid evaluating Nixpkgs entirely.
 
-TODO: in pure mode we have to pass the system type explicitly!
+Of course, this doesn't allow overlays. With pure evaluation, the only way to
+have these is to define a top-level flake that depends on the Nixpkgs flake and
+somehow passes in a set of overlays.
 
 
 ## Hydra jobset dependencies
 
-Hydra can use the flake dependency resolution mechanism to fetch
-dependencies. This allows us to get rid of jobset configuration in the
-web interface: a jobset only requires a flake reference. That is, *a
-jobset is a flake*. Hydra then just builds the `hydraJobs` attrset
-`provide`d by the flake. (It omitted, maybe it can build `packages`.)
+Hydra can use the flake dependency resolution mechanism to fetch dependencies.
+This allows us to get rid of jobset configuration in the web interface: a
+jobset only requires a flake reference. That is, a jobset *is* a flake. Hydra
+then just builds the `hydraJobs` attrset
 
 
 ## NixOS system configuration
 
-NixOS currently contains a lot of modules that really should be moved
-into their own repositories. For example, it contains a Hydra module
-that duplicates the one in the Hydra repository. Also, we want
-reproducible evaluation for NixOS system configurations. So NixOS
-system configurations should be stored as flakes in (local) Git
-repositories.
+NixOS currently contains a lot of modules that really should be moved into
+their own repositories. For example, it contains a Hydra module that duplicates
+the one in the Hydra repository. Also, we want reproducible evaluation for
+NixOS system configurations. So NixOS system configurations should be stored as
+flakes in (local) Git repositories.
 
 `my-system/flake.nix`:
-
 ```nix
 {
+  name = "my-system";
+
+  epoch = 201906;
+
+  inputs =
+    [ "nixpkgs/nixos-18.09"
+      "dwarffs"
+      "hydra"
+      ... lots of other module flakes ...
+    ];
+
   outputs = flakes: {
     nixosSystems.default =
       flakes.nixpkgs.lib.evalModules {
@@ -549,13 +471,6 @@ repositories.
           ];
       };
   };
-
-  inputs =
-    [ "nixpkgs/nixos-18.09"
-      "dwarffs"
-      "hydra"
-      ... lots of other module flakes ...
-    ];
 }
 ```
 
@@ -563,5 +478,5 @@ We can then build the system:
 ```
 nixos-rebuild switch --flake ~/my-system
 ```
-This performs dependency resolution starting at `~/my-system/flake.nix`
-and builds the `system` attribute in `nixosSystems.default`.
+This performs dependency resolution starting at `~/my-system/flake.nix` and
+builds the `system` attribute in `nixosSystems.default`.