add lesson 1 a module

lessons/001-a-module/eval.nix

@@ -0,0 +1,20 @@
+let
+  pkgs = import <nixpkgs> {};
+
+  mymodule = {
+    imports = [
+    ];
+    options = {
+    };
+    config = {
+    };
+  };
+in
+  (
+    pkgs.lib.evalModules {
+      modules = [
+        mymodule
+      ];
+    }
+  )
+  .config

lessons/001-a-module/lesson.md

@@ -0,0 +1,39 @@
+# A Module
+
+In this lesson, we will cover what a module is composed of.
+
+In the [eval][eval] file, we have declared an attrset called `mymodule` which is composed of 3
+fields: `imports`, `options` and `config`. This is a module with default values for those fields.
+You can omit any of those fields and they will use those same default values.
+
+[//]: # (./eval.nix)
+
+If you execute the run file (`./run`), you will see printed the empty JSON object `{}`.
+
+[eval]: ./eval.nix
+
+## Module Fields
+
+The `options` field lets you define variables that can be used in the `config` section.
+
+The `config` field assigns values to options defined in other modules. We call this using an option.
+
+The `imports` field lets you import other modules from a module.
+
+We will see what goes in those fields in later lessons.
+
+## Evaluating Modules
+
+In the [eval][eval] file, we use a function called `evalModules`. This function takes an attrset as
+argument which can contain the field named `modules`. In there, we can put as many module as we
+want.
+
+`evalModules` will then merge all `imports`, `options` and `config` fields of all given modules
+following some rules we will see in the next lessons. It then produces an attrset whose only
+interesting field - the result of merging all modules - is found in the `config` field. This is the
+one we print when executing the `./run` file.
+
+## Nixpkgs
+
+The vast majority of nixpkgs is made out of modules, all merged together in the top-level
+`evalModules`.

lessons/001-a-module/run

@@ -0,0 +1 @@
+nix eval -f eval.nix --json | nix run nixpkgs#jq -- .

site/mkdocs.yml

@@ -7,6 +7,8 @@ edit_uri: tree/main/site
 nav:
   - Home: index.md
   - Lessons:
+    - Getting Started:
+      - lessons/001-a-module/lesson.md
     - Types:
       - lessons/010-basic-types/lesson.md