docs(concepts): add nix to concepts

site/src/SUMMARY.md

@@ -11,7 +11,8 @@
   - [Define a Module in a Separate File](./define-module-in-separate-file.md)
   - [Define Custom Flake Attribute](./define-custom-flake-attribute.md)
   - [Dogfood a Reusable Flake Module](./dogfood-a-reusable-module.md)
-- [Explanation]()
+- [Concepts]()
+  - [Nix](./nix.md)
   - [Overlays](./overlays.md)
 - [Reference Documentation]()
   - [Module Arguments](./module-arguments.md)

site/src/nix.md

@@ -0,0 +1,103 @@
+# Nix
+
+This is a brief introduction to nix, in the context of flake-parts.
+
+## What is nix?
+
+When you read `nix` somewhere, it may refer to one or more of the following concepts:
+
+1. An GNU/Linux distribution, called [NixOS](https://nixos.org/)
+2. A language, called the [Nix Language](https://nixos.org/manual/nix/stable/language/index.html)
+3. A package manager, see [package management](https://nixos.org/manual/nix/stable/package-management/package-management.html) or [install nix](https://nix.dev/tutorials/install-nix).
+
+NixOS makes use of the package manager and the nix language.
+But you don't need to run nixos to use the package manager, or the language.
+
+## What are Nixpkgs?
+
+[Nixpkgs](https://nixos.org/manual/nixpkgs/stable/) are one of the biggest collections of packages.
+It's written in the Nix Language, and it's used by the nix package manager. Check the [supported platforms](https://nixos.org/manual/nix/stable/installation/supported-platforms.html).
+
+You can search for packages on [search.nixos.org/packages](https://search.nixos.org/packages)
+
+## How does the nix language look?
+
+The Nix Language is, in a way, **similar** to JSON with support for functions.
+
+```nix
+{
+  string = "hello";
+  integer = 1;
+  float = 3.141;
+  bool = true;
+  null = null;
+  list = [ 1 "two" false ];
+  flat.objects.user = {
+    name = "foo";
+  };
+}
+```
+
+```console
+$ nix eval -f sample.nix
+{ bool = true; flat = { objects = { user = { name = "foo"; }; }; }; float = 3.141; integer = 1; list = [ 1 "two" false ]; null = null; string = "hello"; }
+```
+
+You can even output JSON!
+
+```console
+$ nix eval --json -f sample.nix
+{
+  "bool": true,
+  "flat": { "objects": { "user": { "name": "foo" } } },
+  "float": 3.141,
+  "integer": 1,
+  "list": [1, "two", false],
+  "null": null,
+  "string": "hello"
+}
+```
+
+And a function:
+
+```nix
+{ name }: {
+  hello = "My name is ${name}, running on ${builtins.currentSystem}";
+}
+```
+
+```console
+$ nix-instantiate --eval hello.nix --argstr name jon -A hello
+"My name is jon, running on x86_64-darwin"
+```
+
+To learn more, head to
+
+- [nix.dev's nix language tutorial](https://nix.dev/tutorials/first-steps/nix-language) or;
+- [A tour of Nix](https://nixcloud.io/tour/?id=1).
+
+## What are people using nix for?
+
+- To package applications (with flakes)
+- A declarative CI ([hercules-ci](https://hercules-ci.com/))
+- Control a fleet of machines (using [nixops](https://nixos.wiki/wiki/NixOps), [deploy-rs](https://github.com/serokell/deploy-rs))
+- Manage dotfiles
+- As a configuration or template language ([terranix](https://terranix.org/))
+
+See more in [awesome-nix](https://github.com/nix-community/awesome-nix).
+
+## What are nix flakes?
+
+Nix flakes are one the newest features in the nix ecosystem, built on top of the experience with nix and its ecosystem.
+
+The main idea is to have a file called `flake.nix` with a set of [standardized outputs](https://nixos.wiki/wiki/Flakes#Output_schema). You can also reference to other flakes, and a `flake.lock` file is used to guarantee reproducibility.
+
+Still **considered experimental**, flakes enable packaging applications in a distributed manner.
+
+- [nix flake manual](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html)
+- [nix.dev flakes](https://nix.dev/concepts/flakes)
+
+## What is flake-parts for?
+
+flake-parts makes writing flakes easier, by establishing a way of working with [system](./system.md).
+On top of that, it makes easier to write and reuse modules.