Set defaults for inputs from flake.lock

If the `src` passed to flakelight contains a flake.lock, it will now be
read and used to set defaults for `inputs`. Explicitly passed inputs
will override this.

API_GUIDE.md

@@ -87,8 +87,31 @@ This section covers the options available to modules.
 Type: AttrsOf FlakeInput
 ```
 
-The `inputs` option allows setting the flake inputs used by modules. To set the
-nixpkgs used for building outputs, you can pass your flake inputs in as follows:
+The `inputs` option is an attrset of the flake inputs used by flakelight
+modules. These inputs get passed as the `inputs` module argument, and are used
+for `inputs` and `inputs'` in the package set.
+
+Default values are automatically initialized from your flake inputs by reading
+your `flake.lock`. Note that this does not include the `self` argument, which
+must be passed explicitly if you want to use it.
+
+Flakelight will add a recent `nixpkgs` input if your flake does not have one.
+Other flakelight modules may provide default inputs for their dependencies.
+
+To use a different nixpkgs from the built-in default:
+
+```nix
+{
+  inputs = {
+    flakelight.url = "github:nix-community/flakelight";
+    nixpkgs.url = "nixpkgs/nixpkgs-unstable";
+  };
+  outputs = { flakelight, ... }:
+    flakelight ./. { };
+}
+```
+
+You can also explicitly pass in inputs, overriding defaults, as follows:
 
 ```nix
 {

README.md

@@ -73,9 +73,8 @@ To use a different nixpkgs, you can instead use:
     nixpkgs.url = "nixpkgs/nixpkgs-unstable";
     flakelight.url = "github:nix-community/flakelight";
   };
-  outputs = { flakelight, ... }@inputs:
+  outputs = { flakelight, ... }:
     flakelight ./. {
-      inherit inputs;
       devShell.packages = pkgs: [ pkgs.hello pkgs.coreutils ];
     };
 }

builtinModules/autoInputs.nix

@@ -0,0 +1,13 @@
+# flakelight -- Framework for simplifying flake setup
+# Copyright (C) 2023 Archit Gupta <archit@accelbread.com>
+# SPDX-License-Identifier: MIT
+
+{ lib, src, ... }:
+let
+  inherit (builtins) mapAttrs pathExists;
+  inherit (lib) mkOverride;
+  lock2inputs = import ../misc/lock2inputs.nix { inherit lib; };
+  lockFound = pathExists (src + "/flake.lock");
+  autoInputs = if lockFound then lock2inputs src else { };
+in
+{ config.inputs = mapAttrs (_: mkOverride 950) autoInputs; }

misc/lock2inputs.nix

@@ -0,0 +1,46 @@
+# flakelight -- Framework for simplifying flake setup
+# Copyright (C) 2023 Archit Gupta <archit@accelbread.com>
+# SPDX-License-Identifier: MIT
+
+# Get a flakes inputs
+{ lib, ... }:
+src:
+let
+  inherit (builtins) fromJSON head isString mapAttrs readFile tail;
+  inherit (lib) fix;
+
+  json = fromJSON (readFile (src + "/flake.lock"));
+  inherit (json) nodes;
+  rootNode = nodes.${json.root};
+
+  getInputName = base: ref:
+    let next = getInputName json.root nodes.${base}.inputs.${head ref}; in
+    if isString ref then ref
+    else if ref == [ ] then base
+    else getInputName next (tail ref);
+
+  getInput = ref: resolved.${getInputName json.root ref};
+
+  fetchNode = node: fetchTree (node.info or { } //
+    removeAttrs node.locked [ "dir" ]);
+
+  resolveFlakeNode = node: fix (self:
+    let
+      sourceInfo = fetchNode node;
+      outPath = sourceInfo +
+        (if node.locked ? dir then "/${node.locked.dir}" else "");
+      inputs = (mapAttrs (_: getInput) (node.inputs or { })) //
+        { inherit self; };
+      outputs = (import (outPath + "/flake.nix")).outputs inputs;
+    in
+    outputs // sourceInfo // {
+      _type = "flake";
+      inherit outPath inputs outputs sourceInfo;
+    });
+
+  resolveNode = node:
+    if node.flake or true then resolveFlakeNode node else fetchNode node;
+
+  resolved = mapAttrs (_: resolveNode) nodes;
+in
+mapAttrs (_: v: resolved.${v}) rootNode.inputs