feat(flake): modular structure with flake-file and flake-parts

flake/default.nix

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+# Flake
+
+/**
+  Entry point of NixOS configuration
+
+  - Defines external sources and locking their commit, which gives up __purity__.
+  - Declares ready-to-use configurations (e.g. development shells, packages, etc.)
+
+  Essentially, `flake.nix` is like `Cargo.toml` in Rust or `package.json` in Node.js, but for an entire operating system.
+*/
+{ inputs, ... }:
+{
+  imports = [ inputs.flake-file.flakeModules.default ];
+
+  systems = inputs.nixpkgs.lib.systems.flakeExposed;
+
+  flake-file.inputs = import ./inputs;
+  flake-file.outputs = "inputs: inputs.flake-parts.lib.mkFlake { inherit inputs; } ./flake";
+  flake-file.description = "A very basic flake";

flake/inputs/core.nix

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+# Core Inputs
+
+/**
+  Essential inputs that form
+  the foundation of the NixOS configuration
+*/
+{
+  /**
+    The Core of NixOS, providing packages and modules.
+    Points to `nixos-unstable` branch for latest features an updates
+  */
+  nixpkgs = {
+    url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  /**
+    A modular flake framework that simplifies flake organizing
+    and reduces boilerplate code.
+  */
+  flake-parts = {
+    url = "github:hercules-ci/flake-parts";
+  };
+
+  /**
+    Like flake-parts, but for flake inputs
+  */
+  flake-file = {
+    url = "github:vic/flake-file";
+  };
+}

flake/inputs/default.nix

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+# Flake inputs
+
+/**
+  Declares configuration’s dependencies. Inputs
+  are sorted by their purpose and functionality.
+  See sub chapters to learn which inputs are intended for.
+
+  > [!WARNING]
+  > Because nix doesn’t support splitting flake inputs into several files,
+  > `flake.nix` is generated via [flake-file](https://github.com/vic/flake-file). Then adding new input,
+  > remember running `nix run .#write-flake` to apply changes!
+  >
+  > In next chapters, you'll see attributes of type `flake.inputs.<class>.<input>`.
+  > Then accessing inputs in repl, remove <class>, so instead of calling e.g. `flake.inputs.core.nixpkgs`,
+  > you call `flake.inputs.nixpkgs`.
+*/
+import ./core.nix