Add 'nix flake' manpages

11 files changed, 1048 insertions, 32 deletions

diff --git a/src/nix/flake-archive.md b/src/nix/flake-archive.md
new file mode 100644
index 000000000..85bbeeb16
--- /dev/null
+++ b/src/nix/flake-archive.md
@@ -0,0 +1,29 @@
+R""(
+
+# Examples
+
+* Copy the `dwarffs` flake and its dependencies to a binary cache:
+
+  ```console
+  # nix flake archive --to file:///tmp/my-cache dwarffs
+  ```
+
+* Fetch the `dwarffs` flake and its dependencies to the local Nix
+  store:
+
+  ```console
+  # nix flake archive dwarffs
+  ```
+
+* Print the store paths of the flake sources of NixOps without
+  fetching them:
+
+  ```console
+  # nix flake archive --json --dry-run nixops
+  ```
+
+# Description
+
+FIXME
+
+)""
diff --git a/src/nix/flake-check.md b/src/nix/flake-check.md
new file mode 100644
index 000000000..dc079ba0c
--- /dev/null
+++ b/src/nix/flake-check.md
@@ -0,0 +1,68 @@
+R""(
+
+# Examples
+
+* Evaluate the flake in the current directory, and build its checks:
+
+  ```console
+  # nix flake check
+  ```
+
+* Verify that the `patchelf` flake evaluates, but don't build its
+  checks:
+
+  ```console
+  # nix flake check --no-build github:NixOS/patchelf
+  ```
+
+# Description
+
+This command verifies that the flake specified by flake reference
+*flake-url* can be evaluated successfully (as detailed below), and
+that the derivations specified by the flake's `checks` output can be
+built successfully.
+
+# Evaluation checks
+
+This following flake output attributes must be derivations:
+
+* `checks.`*system*`.`*name*
+* `defaultPackage.`*system*`
+* `devShell.`*system*`
+* `nixosConfigurations.`*name*`.config.system.build.toplevel
+* `packages.`*system*`.`*name*
+
+The following flake output attributes must be [app
+definitions](./nix3-run.md):
+
+* `apps.`*system*`.`*name*
+* `defaultApp.`*system*`
+
+The following flake output attributes must be [template
+definitions](./nix3-flake-init.md):
+
+* `defaultTemplate`
+* `templates`.`*name*
+
+The following flake output attributes must be *Nixpkgs overlays*:
+
+* `overlay`
+* `overlays`.`*name*
+
+The following flake output attributes must be *NixOS modules*:
+
+* `nixosModule`
+* `nixosModules`.`*name*
+
+The following flake output attributes must be
+[bundlers](./nix3-bundle.md):
+
+* `bundlers`.`*name*
+* `defaultBundler`
+
+In addition, the `hydraJobs` output is evaluated in the same way as
+Hydra's `hydra-eval-jobs` (i.e. as a arbitrarily deeply nested
+attribute set of derivations). Similarly, the
+`legacyPackages`.*system* output is evaluated like `nix-env -qa`.
+
+)""
diff --git a/src/nix/flake-clone.md b/src/nix/flake-clone.md
new file mode 100644
index 000000000..36cb96051
--- /dev/null
+++ b/src/nix/flake-clone.md
@@ -0,0 +1,18 @@
+R""(
+
+# Examples
+
+* Check out the source code of the `dwarffs` flake and build it:
+
+  ```console
+  # nix flake clone dwarffs --dest dwarffs
+  # cd dwarffs
+  # nix build
+  ```
+
+# Description
+
+This command performs a Git or Mercurial clone of the repository
+containing the source code of the flake *flake-url*.
+
+)""
diff --git a/src/nix/flake-info.md b/src/nix/flake-info.md
new file mode 100644
index 000000000..fda3171db
--- /dev/null
+++ b/src/nix/flake-info.md
@@ -0,0 +1,99 @@
+R""(
+
+# Examples
+
+* Show what `nixpkgs` resolves to:
+
+  ```console
+  # nix flake info nixpkgs
+  Resolved URL:  github:NixOS/nixpkgs
+  Locked URL:    github:NixOS/nixpkgs/b67ba0bfcc714453cdeb8d713e35751eb8b4c8f4
+  Description:   A collection of packages for the Nix package manager
+  Path:          /nix/store/23qapccs6cfmwwrlq8kr41vz5vdmns3r-source
+  Revision:      b67ba0bfcc714453cdeb8d713e35751eb8b4c8f4
+  Last modified: 2020-12-23 12:36:12
+  ```
+
+* Show information about `dwarffs` in JSON format:
+
+  ```console
+  # nix flake info dwarffs --json | jq .
+  {
+    "description": "A filesystem that fetches DWARF debug info from the Internet on demand",
+    "lastModified": 1597153508,
+    "locked": {
+      "lastModified": 1597153508,
+      "narHash": "sha256-VHg3MYVgQ12LeRSU2PSoDeKlSPD8PYYEFxxwkVVDRd0=",
+      "owner": "edolstra",
+      "repo": "dwarffs",
+      "rev": "d181d714fd36eb06f4992a1997cd5601e26db8f5",
+      "type": "github"
+    },
+    "original": {
+      "id": "dwarffs",
+      "type": "indirect"
+    },
+    "originalUrl": "flake:dwarffs",
+    "path": "/nix/store/hang3792qwdmm2n0d9nsrs5n6bsws6kv-source",
+    "resolved": {
+      "owner": "edolstra",
+      "repo": "dwarffs",
+      "type": "github"
+    },
+    "resolvedUrl": "github:edolstra/dwarffs",
+    "revision": "d181d714fd36eb06f4992a1997cd5601e26db8f5",
+    "url": "github:edolstra/dwarffs/d181d714fd36eb06f4992a1997cd5601e26db8f5"
+  }
+  ```
+
+# Description
+
+This command shows information about the flake specified by the flake
+reference *flake-url*. It resolves the flake reference using the
+[flake registry](./nix3-registry.md), fetches it, and prints some meta
+data. This includes:
+
+* `Resolved URL`: If *flake-url* is a flake identifier, then this is
+  the flake reference that specifies its actual location, looked up in
+  the flake registry.
+
+* `Locked URL`: A flake reference that contains a commit or content
+  hash and thus uniquely identifies a specific flake version.
+
+* `Description`: A one-line description of the flake, taken from the
+  `description` field in `flake.nix`.
+
+* `Path`: The store path containing the source code of the flake.
+
+* `Revision`: The Git or Mercurial commit hash of the locked flake.
+
+* `Revisions`: The number of ancestors of the Git or Mercurial commit
+  of the locked flake. Note that this is not available for `github`
+  flakes.
+
+* `Last modified`: For Git or Mercurial flakes, this is the commit
+  time of the commit of the locked flake; for tarball flakes, it's the
+  most recent timestamp of any file inside the tarball.
+
+With `--json`, the output is a JSON object with the following fields:
+
+* `original` and `originalUrl`: The flake reference specified by the
+  user (*flake-url*) in attribute set and URL representation.
+
+* `resolved` and `resolvedUrl`: The resolved flake reference (see
+  above) in attribute set and URL representation.
+
+* `locked` and `lockedUrl`: The locked flake reference (see above) in
+  attribute set and URL representation.
+
+* `description`: See `Description` above.
+
+* `path`: See `Path` above.
+
+* `revision`: See `Revision` above.
+
+* `revCount`: See `Revisions` above.
+
+* `lastModified`: See `Last modified` above.
+
+)""
diff --git a/src/nix/flake-init.md b/src/nix/flake-init.md
new file mode 100644
index 000000000..c66154ad5
--- /dev/null
+++ b/src/nix/flake-init.md
@@ -0,0 +1,54 @@
+R""(
+
+# Examples
+
+* Create a flake using the default template:
+
+  ```console
+  # nix flake init
+  ```
+
+* List available templates:
+
+  ```console
+  # nix flake show templates
+  ```
+
+* Create a flake from a specific template:
+
+  ```console
+  # nix flake init -t templates#simpleContainer
+  ```
+
+# Description
+
+This command creates a flake in the current directory by copying the
+files of a template. It will not overwrite existing files. The default
+template is `templates#defaultTemplate`, but this can be overriden
+using `-t`.
+
+# Template definitions
+
+A flake can declare templates through its `templates` and
+`defaultTemplate` output attributes. A template has two attributes:
+
+* `description`: A one-line description of the template, in CommonMark
+  syntax.
+
+* `path`: The path of the directory to be copied.
+
+Here is an example:
+
+```
+outputs = { self }: {
+
+  templates.rust = {
+    path = ./rust;
+    description = "A simple Rust/Cargo project";
+  };
+
+  templates.defaultTemplate = self.templates.rust;
+}
+```
+
+)""
diff --git a/src/nix/flake-list-inputs.md b/src/nix/flake-list-inputs.md
new file mode 100644
index 000000000..250e13be0
--- /dev/null
+++ b/src/nix/flake-list-inputs.md
@@ -0,0 +1,23 @@
+R""(
+
+# Examples
+
+* Show the inputs of the `hydra` flake:
+
+  ```console
+  # nix flake list-inputs github:NixOS/hydra
+  github:NixOS/hydra/bde8d81876dfc02143e5070e42c78d8f0d83d6f7
+  ├───nix: github:NixOS/nix/79aa7d95183cbe6c0d786965f0dbff414fd1aa67
+  │   ├───lowdown-src: github:kristapsdz/lowdown/1705b4a26fbf065d9574dce47a94e8c7c79e052f
+  │   └───nixpkgs: github:NixOS/nixpkgs/ad0d20345219790533ebe06571f82ed6b034db31
+  └───nixpkgs follows input 'nix/nixpkgs'
+  ```
+
+# Description
+
+This command shows the inputs of the flake specified by the flake
+referenced *flake-url*. Since it prints the locked inputs that result
+from generating or updating the lock file, this command essentially
+displays the contents of the flake's lock file in human-readable form.
+
+)""
diff --git a/src/nix/flake-new.md b/src/nix/flake-new.md
new file mode 100644
index 000000000..725695c01
--- /dev/null
+++ b/src/nix/flake-new.md
@@ -0,0 +1,34 @@
+R""(
+
+# Examples
+
+* Create a flake using the default template in the directory `hello`:
+
+  ```console
+  # nix flake new hello
+  ```
+
+* List available templates:
+
+  ```console
+  # nix flake show templates
+  ```
+
+* Create a flake from a specific template in the directory `hello`:
+
+  ```console
+  # nix flake new hello -t templates#trivial
+  ```
+
+# Description
+
+This command creates a flake in the directory `dest-dir`, which must
+not already exist. It's equivalent to:
+
+```console
+# mkdir dest-dir
+# cd dest-dir
+# nix flake init
+```
+
+)""
diff --git a/src/nix/flake-show.md b/src/nix/flake-show.md
new file mode 100644
index 000000000..1a42c44a0
--- /dev/null
+++ b/src/nix/flake-show.md
@@ -0,0 +1,38 @@
+R""(
+
+# Examples
+
+* Show the output attributes provided by the `patchelf` flake:
+
+  ```console
+  github:NixOS/patchelf/f34751b88bd07d7f44f5cd3200fb4122bf916c7e
+  ├───checks
+  │   ├───aarch64-linux
+  │   │   └───build: derivation 'patchelf-0.12.20201207.f34751b'
+  │   ├───i686-linux
+  │   │   └───build: derivation 'patchelf-0.12.20201207.f34751b'
+  │   └───x86_64-linux
+  │       └───build: derivation 'patchelf-0.12.20201207.f34751b'
+  ├───defaultPackage
+  │   ├───aarch64-linux: package 'patchelf-0.12.20201207.f34751b'
+  │   ├───i686-linux: package 'patchelf-0.12.20201207.f34751b'
+  │   └───x86_64-linux: package 'patchelf-0.12.20201207.f34751b'
+  ├───hydraJobs
+  │   ├───build
+  │   │   ├───aarch64-linux: derivation 'patchelf-0.12.20201207.f34751b'
+  │   │   ├───i686-linux: derivation 'patchelf-0.12.20201207.f34751b'
+  │   │   └───x86_64-linux: derivation 'patchelf-0.12.20201207.f34751b'
+  │   ├───coverage: derivation 'patchelf-coverage-0.12.20201207.f34751b'
+  │   ├───release: derivation 'patchelf-0.12.20201207.f34751b'
+  │   └───tarball: derivation 'patchelf-tarball-0.12.20201207.f34751b'
+  └───overlay: Nixpkgs overlay
+  ```
+
+# Description
+
+This command shows the output attributes provided by the flake
+specified by flake reference *flake-url*. These are the top-level
+attributes in the `outputs` of the flake, as well as lower-level
+attributes for some standard outputs (e.g. `packages` or `checks`).
+
+)""
diff --git a/src/nix/flake-update.md b/src/nix/flake-update.md
new file mode 100644
index 000000000..a2ffedd2a
--- /dev/null
+++ b/src/nix/flake-update.md
@@ -0,0 +1,53 @@
+R""(
+
+# Examples
+
+* Update the `nixpkgs` and `nix` inputs of the flake in the current
+  directory:
+
+  ```console
+  # nix flake update --update-input nixpkgs --update-input nix
+  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
+  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
+  ```
+
+* Recreate the lock file (i.e. update all inputs) and commit the new
+  lock file:
+
+  ```console
+  # nix flake update --recreate-lock-file --commit-lock-file
+  …
+  warning: committed new revision '158bcbd9d6cc08ab859c0810186c1beebc982aad'
+  ```
+
+# Description
+
+This command updates the lock file of a flake (`flake.lock`) so that
+it contains a lock for every flake input specified in
+`flake.nix`. Note that every command that operates on a flake will
+also update the lock file if needed, and supports the same
+flags. Therefore,
+
+```console
+# nix flake update --update-input nixpkgs
+# nix build
+```
+
+is equivalent to:
+
+```console
+# nix build --update-input nixpkgs
+```
+
+Thus, this command is only useful if you want to update the lock file
+separately from any other action such as building.
+
+> **Note**
+>
+> This command does *not* update locks that are already present unless
+> you explicitly ask for it using `--update-input` or
+> `--recreate-lock-file`. Thus, if the lock file already has locks for
+> every input, then `nix flake update` (without arguments) does
+> nothing.
+
+)""
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 066430c5d..2b91faa64 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -104,6 +104,13 @@ struct CmdFlakeUpdate : FlakeCommand
         return "update flake lock file";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-update.md"
+          ;
+    }
+
     void run(nix::ref<nix::Store> store) override
     {
         /* Use --refresh by default for 'nix flake update'. */
@@ -134,6 +141,13 @@ struct CmdFlakeInfo : FlakeCommand, MixJSON
         return "list info about a given flake";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-info.md"
+          ;
+    }
+
     void run(nix::ref<nix::Store> store) override
     {
         auto flake = getFlake();
@@ -153,6 +167,13 @@ struct CmdFlakeListInputs : FlakeCommand, MixJSON
         return "list flake inputs";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-list-inputs.md"
+          ;
+    }
+
     void run(nix::ref<nix::Store> store) override
     {
         auto flake = lockFlake();
@@ -211,6 +232,13 @@ struct CmdFlakeCheck : FlakeCommand
         return "check whether the flake evaluates and run its tests";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-check.md"
+          ;
+    }
+
     void run(nix::ref<nix::Store> store) override
     {
         settings.readOnlyMode = !build;
@@ -631,22 +659,11 @@ struct CmdFlakeInit : CmdFlakeInitCommon
         return "create a flake in the current directory from a template";
     }
 
-    Examples examples() override
-    {
-        return {
-            Example{
-                "To create a flake using the default template:",
-                "nix flake init"
-            },
-            Example{
-                "To see available templates:",
-                "nix flake show templates"
-            },
-            Example{
-                "To create a flake from a specific template:",
-                "nix flake init -t templates#nixos-container"
-            },
-        };
+    std::string doc() override
+    {
+        return
+          #include "flake-init.md"
+          ;
     }
 
     CmdFlakeInit()
@@ -662,6 +679,13 @@ struct CmdFlakeNew : CmdFlakeInitCommon
         return "create a flake in the specified directory from a template";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-new.md"
+          ;
+    }
+
     CmdFlakeNew()
     {
         expectArgs({
@@ -681,6 +705,13 @@ struct CmdFlakeClone : FlakeCommand
         return "clone flake repository";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-clone.md"
+          ;
+    }
+
     CmdFlakeClone()
     {
         addFlag({
@@ -720,22 +751,11 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
         return "copy a flake and all its inputs to a store";
     }
 
-    Examples examples() override
-    {
-        return {
-            Example{
-                "To copy the dwarffs flake and its dependencies to a binary cache:",
-                "nix flake archive --to file:///tmp/my-cache dwarffs"
-            },
-            Example{
-                "To fetch the dwarffs flake and its dependencies to the local Nix store:",
-                "nix flake archive dwarffs"
-            },
-            Example{
-                "To print the store paths of the flake sources of NixOps without fetching them:",
-                "nix flake archive --json --dry-run nixops"
-            },
-        };
+    std::string doc() override
+    {
+        return
+          #include "flake-archive.md"
+          ;
     }
 
     void run(nix::ref<nix::Store> store) override
@@ -797,6 +817,13 @@ struct CmdFlakeShow : FlakeCommand
         return "show the outputs provided by a flake";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake-show.md"
+          ;
+    }
+
     void run(nix::ref<nix::Store> store) override
     {
         auto state = getEvalState();
@@ -955,6 +982,13 @@ struct CmdFlake : NixMultiCommand
         return "manage Nix flakes";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "flake.md"
+          ;
+    }
+
     void run() override
     {
         if (!command)
diff --git a/src/nix/flake.md b/src/nix/flake.md
new file mode 100644
index 000000000..440c45dd1
--- /dev/null
+++ b/src/nix/flake.md
@@ -0,0 +1,566 @@
+R""(
+
+# Description
+
+`nix flake` provides subcommands for creating, modifying and querying
+*Nix flakes*. Flakes are the unit for packaging Nix code in a
+reproducible and discoverable way. They can have dependencies on other
+flakes, making it possible to have multi-repository Nix projects.
+
+A flake is a filesystem tree (typically fetched from a Git repository
+or a tarball) that contains a file named `flake.nix` in the root
+directory. `flake.nix` specifies some metadata about the flake such as
+dependencies (called *inputs*), as well as its *outputs* (the Nix
+values such as packages or NixOS modules provided by the flake).
+
+# Flake references
+
+Flake references (*flakerefs*) are a way to specify the location of a
+flake. These have two different forms:
+
+* An attribute set representation, e.g.
+
+  ```nix
+  {
+    type = "github";
+    owner = "NixOS";
+    repo = "nixpkgs";
+  }
+  ```
+
+  The only required attribute is `type`. The supported types are
+  listed below.
+
+* A URL-like syntax, e.g.
+
+  ```
+  github:NixOS/nixpkgs
+  ```
+
+  These are used on the command line as a more convenient alternative
+  to the attribute set representation. For instance, in the command
+
+  ```console
+  # nix build github:NixOS/nixpkgs#hello
+  ```
+
+  `github:NixOS/nixpkgs` is a flake reference (while `hello` is an
+  output attribute). They are also allowed in the `inputs` attribute
+  of a flake, e.g.
+
+  ```nix
+  inputs.nixpkgs.url = github:NixOS/nixpkgs;
+  ```
+
+  is equivalent to
+
+  ```nix
+  inputs.nixpkgs = {
+    type = "github";
+    owner = "NixOS";
+    repo = "nixpkgs";
+  };
+  ```
+
+## Examples
+
+Here are some examples of flake references in their URL-like representation:
+
+* `.`: The flake in the current directory.
+* `/home/alice/src/patchelf`: A flake in some other directory.
+* `nixpkgs`: The `nixpkgs` entry in the flake registry.
+* `nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293`: The `nixpkgs`
+  entry in the flake registry, with its Git revision overriden to a
+  specific value.
+* `github:NixOS/nixpkgs`: The `master` branch of the `NixOS/nixpkgs`
+  repository on GitHub.
+* `github:NixOS/nixpkgs/nixos-20.09`: The `nixos-20.09` branch of the
+  `nixpkgs` repository.
+* `github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293`: A
+  specific revision of the `nixpkgs` repository.
+* `github:edolstra/nix-warez?dir=blender`: A flake in a subdirectory
+  of a GitHub repository.
+* `git+https://github.com/NixOS/patchelf`: A Git repository.
+* `git+https://github.com/NixOS/patchelf?ref=master`: A specific
+  branch of a Git repository.
+* `git+https://github.com/NixOS/patchelf?ref=master&rev=f34751b88bd07d7f44f5cd3200fb4122bf916c7e`:
+  A specific branch *and* revision of a Git repository.
+* `https://github.com/NixOS/patchelf/archive/master.tar.gz`: A tarball
+  flake.
+
+## Flake reference attributes
+
+The following generic flake reference attributes are supported:
+
+* `dir`: The subdirectory of the flake in which `flake.nix` is
+  located. This parameter enables having multiple flakes in a
+  repository or tarball. The default is the root directory of the
+  flake.
+
+* `narHash`: The hash of the NAR serialisation (in SRI format) of the
+  contents of the flake. This is useful for flake types such as
+  tarballs that lack a unique content identifier such as a Git commit
+  hash.
+
+In addition, the following attributes are common to several flake
+reference types:
+
+* `rev`: A Git or Mercurial commit hash.
+
+* `ref`: A Git or Mercurial branch or tag name.
+
+Finally, some attribute are typically not specified by the user, but
+can occur in *locked* flake references and are available to Nix code:
+
+* `revCount`: The number of ancestors of the commit `rev`.
+
+* `lastModified`: The timestamp (in seconds since the Unix epoch) of
+  the last modification of this version of the flake. For
+  Git/Mercurial flakes, this is the commit time of commit *rev*, while
+  for tarball flakes, it's the most recent timestamp of any file
+  inside the tarball.
+
+## Types
+
+Currently the `type` attribute can be one of the following:
+
+* `path`: arbitrary local directories, or local Git trees. The
+  required attribute `path` specifies the path of the flake. The URL
+  form is
+
+  ```
+  [path:]<path>(\?<params)?
+  ```
+
+  where *path* is an absolute path.
+
+  *path* must be a directory in the file system containing a file
+  named `flake.nix`.
+
+  If the directory or any of its parents is a Git repository, then
+  this is essentially equivalent to `git+file://<path>` (see below),
+  except that the `dir` parameter is derived automatically. For
+  example, if `/foo/bar` is a Git repository, then the flake reference
+  `/foo/bar/flake` is equivalent to `/foo/bar?dir=flake`.
+
+  If the directory is not inside a Git repository, then the flake
+  contents is the entire contents of *path*.
+
+  *path* generally must be an absolute path. However, on the command
+  line, it can be a relative path (e.g. `.` or `./foo`) which is
+  interpreted as relative to the current directory. In this case, it
+  must start with `.` to avoid ambiguity with registry lookups
+  (e.g. `nixpkgs` is a registry lookup; `./nixpkgs` is a relative
+  path).
+
+* `git`: Git repositories. The location of the repository is specified
+  by the attribute `url`.
+
+  They have the URL form
+
+  ```
+  git(+http|+https|+ssh|+git|+file|):(//<server>)?<path>(\?<params>)?
+  ```
+
+  The `ref` attribute defaults to `master`.
+
+  The `rev` attribute must denote a commit that exists in the branch
+  or tag specified by the `ref` attribute, since Nix doesn't do a full
+  clone of the remote repository by default (and the Git protocol
+  doesn't allow fetching a `rev` without a known `ref`). The default
+  is the commit currently pointed to by `ref`.
+
+  For example, the following are valid Git flake references:
+
+  * `git+https://example.org/my/repo`
+  * `git+https://example.org/my/repo?dir=flake1`
+  * `git+ssh://git@github.com/NixOS/nix?ref=v1.2.3`
+  * `git://github.com/edolstra/dwarffs?ref=unstable&rev=e486d8d40e626a20e06d792db8cc5ac5aba9a5b4`
+  * `git+file:///home/my-user/some-repo/some-repo`
+
+* `mercurial`: Mercurial repositories. The URL form is similar to the
+  `git` type, except that the URL schema must be one of `hg+http`,
+  `hg+https`, `hg+ssh` or `hg+file`.
+
+* `tarball`: Tarballs. The location of the tarball is specified by the
+  attribute `url`.
+
+  In URL form, the schema must be `http://`, `https://` or `file://`
+  URLs and the extension must be `.zip`, `.tar`, `.tar.gz`, `.tar.xz`
+  or `.tar.bz2`.
+
+* `github`: A more efficient way to fetch repositories from
+  GitHub. The following attributes are required:
+
+  * `owner`: The owner of the repository.
+
+  * `repo`: The name of the repository.
+
+  These are downloaded as tarball archives, rather than
+  through Git. This is often much faster and uses less disk space
+  since it doesn't require fetching the entire history of the
+  repository. On the other hand, it doesn't allow incremental fetching
+  (but full downloads are often faster than incremental fetches!).
+
+  The URL syntax for `github` flakes is:
+
+  ```
+  github:<owner>/<repo>(/<rev-or-ref>)?(\?<params>)?
+  ```
+
+  `<rev-or-ref>` specifies the name of a branch or tag (`ref`), or a
+  commit hash (`rev`). Note that unlike Git, GitHub allows fetching by
+  commit hash without specifying a branch or tag.
+
+  Some examples:
+
+  * `github:edolstra/dwarffs`
+  * `github:edolstra/dwarffs/unstable`
+  * `github:edolstra/dwarffs/d3f2baba8f425779026c6ec04021b2e927f61e31`
+
+* `indirect`: Indirections through the flake registry. These have the
+  form
+
+  ```
+  [flake:]<flake-id>(/<rev-or-ref>(/rev)?)?
+  ```
+
+  These perform a lookup of `<flake-id>` in the flake registry. or
+  example, `nixpkgs` and `nixpkgs/release-20.09` are indirect flake
+  references. The specified `rev` and/or `ref` are merged with the
+  entry in the registry; see [nix registry](./nix3-registry.md) for
+  details.
+
+# Flake format
+
+As an example, here is a simple `flake.nix` that depends on the
+Nixpkgs flake and provides a single package (i.e. an installable
+derivation):
+
+```nix
+{
+  description = "A flake for building Hello World";
+
+  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-20.03;
+
+  outputs = { self, nixpkgs }: {
+
+    defaultPackage.x86_64-linux =
+      # 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.
+
+  * `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 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).
+
+## 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
+this:
+
+```nix
+outputs = { self, nixpkgs, import-cargo }: {
+  ... outputs ...
+};
+```
+
+It is also possible to omit an input entirely and *only* list it as
+expected function argument to `outputs`. Thus,
+
+```nix
+outputs = { self, nixpkgs }: ...;
+```
+
+without an `inputs.nixpkgs` attribute is equivalent to
+
+```nix
+inputs.nixpkgs = {
+  type = "indirect";
+  id = "nixpkgs";
+};
+```
+
+Repositories that don't contain a `flake.nix` can also be used as
+inputs, by setting the input's `flake` attribute to `false`:
+
+```nix
+inputs.grcov = {
+  type = "github";
+  owner = "mozilla";
+  repo = "grcov";
+  flake = false;
+};
+
+outputs = { self, nixpkgs, grcov }: {
+  packages.x86_64-linux.grcov = stdenv.mkDerivation {
+    src = grcov;
+    ...
+  };
+};
+```
+
+Transitive inputs can be overriden from a `flake.nix` file. For
+example, the following overrides the `nixpkgs` input of the `nixops`
+input:
+
+```nix
+inputs.nixops.inputs.nixpkgs = {
+  type = "github";
+  owner = "my-org";
+  repo = "nixpkgs";
+};
+```
+
+It is also possible to "inherit" an input from another input. This is
+useful to minimize flake dependencies. For example, the following sets
+the `nixpkgs` input of the top-level flake to be equal to the
+`nixpkgs` input of the `dwarffs` input of the top-level flake:
+
+```nix
+inputs.nixops.follows = "dwarffs/nixpkgs";
+```
+
+The value of the `follows` attribute is a `/`-separated sequence of
+input names denoting the path of inputs to be followed from the root
+flake.
+
+Overrides and `follows` can be combined, e.g.
+
+```nix
+inputs.nixops.inputs.nixpkgs.follows = "dwarffs/nixpkgs";
+```
+
+sets the `nixpkgs` input of `nixops` to be the same as the `nixpkgs`
+input of `dwarffs`. It is worth noting, however, that it is generally
+not useful to eliminate transitive `nixpkgs` flake inputs in this
+way. Most flakes provide their functionality through Nixpkgs overlays
+or NixOS modules, which are composed into the top-level flake's
+`nixpkgs` input; so their own `nixpkgs` input is usually irrelevant.
+
+# Lock files
+
+Inputs specified in `flake.nix` are typically "unlocked" in the sense
+that they don't specify an exact revision. To ensure reproducibility,
+Nix will automatically generate and use a *lock file* called
+`flake.lock` in the flake's directory. The lock file contains a graph
+structure isomorphic to the graph of dependencies of the root
+flake. Each node in the graph (except the root node) maps the
+(usually) unlocked input specifications in `flake.nix` to locked input
+specifications. Each node also contains some metadata, such as the