lesson/add-submodule-types (#32)

Added content for the submodule lesson.

lessons/submodule-types/character.nix

@@ -0,0 +1,40 @@
+{
+  lib,
+  options,
+  config,
+  ...
+}: let
+  cfg = config;
+  opts = options;
+in {
+  options = {
+    name = lib.mkOption {
+      type = lib.types.str;
+    };
+    title = lib.mkOption {
+      type = lib.types.str;
+    };
+    origin = lib.mkOption {
+      type = lib.types.str;
+    };
+
+    greeting = lib.mkOption {
+      type = lib.types.str;
+    };
+  };
+
+  config = {
+    greeting = (
+      lib.concatStringsSep
+      "\n"
+      (
+        [
+          "Hello"
+        ]
+        ++ (lib.optional opts.name.isDefined "My name is ${cfg.name}.")
+        ++ (lib.optional opts.title.isDefined "I am a ${cfg.title}.")
+        ++ (lib.optional opts.origin.isDefined "I am from ${cfg.origin}.")
+      )
+    );
+  };
+}

lessons/submodule-types/config.nix

@@ -0,0 +1,15 @@
+{...}: {
+  config = {
+    characters = [
+      {
+        name = "Boaty McBoatface";
+        origin = "England";
+      }
+      {
+        name = "djacu";
+        title = "Nix Enthusiast";
+        origin = "USA";
+      }
+    ];
+  };
+}

lessons/submodule-types/eval.nix

@@ -0,0 +1,11 @@
+{pkgs}:
+(
+  pkgs.lib.evalModules {
+    modules = [
+      ./options.nix
+      ./config.nix
+    ];
+  }
+)
+.config
+.greeting

lessons/submodule-types/lesson.md

@@ -1 +1,41 @@
 # Submodule Types
+
+After learning about composed types, you can imagine that nesting can get deep quickly.
+The logic for tracking each layer can become overwhelming.
+This is where one of the most useful types included in type system comes into play: `submodule`.
+It allows you to define nested modules with their own option imports, declarations, and definitions.
+
+First we will define our submodule.
+In `character.nix` we have something that looks very similar to what we built before.
+We have `name`, `title`, and `origin` as inputs and a greeting that combines all these as an output.
+
+[//]: # (./character.nix)
+
+In our `options.nix`, we have a `characters` option that is a list of submodule that points to our `character.nix` file.
+We have also defined a `greeting` option that will take all `greeting` definitions from our characters and join them into a single string.
+Note: these two `greeting` options are not the same; `character.nix` and `options.nix` each have their own `greeting` option.
+
+!!! note
+    We could have written the `character.nix` submodule in place.
+    For the sake of readability, we gave it its own file.
+    Also, writing it in its own file allows us to use it elsewhere if desired.
+
+[//]: # (./options.nix)
+
+In our `config.nix`, we define all of our characters.
+The behavior of the submodule type is attr-*like*.
+So inside our list we have attribute sets; each one can have a value for attributes `name`, `title` and `origin`.
+
+[//]: # (./config.nix)
+
+Setup an `eval.nix` to evaluate our modules and return the `config.greeting` attribute.
+
+[//]: # (./eval.nix)
+
+Create a `run.sh` run script to evaluate the `eval.nix` file.
+
+[//]: # (./run.sh)
+
+And if we run the script (`./run.sh`), we have our configuration.
+
+[//]: # (self.eval)

lessons/submodule-types/options.nix

@@ -0,0 +1,27 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  cfg = config;
+in {
+  options = {
+    characters = lib.mkOption {
+      type = lib.types.listOf (lib.types.submodule ./character.nix);
+    };
+    greeting = lib.mkOption {
+      type = lib.types.str;
+    };
+  };
+
+  config = {
+    greeting =
+      lib.concatStringsSep
+      "\n\n"
+      (
+        builtins.map
+        (character: character.greeting)
+        cfg.characters
+      );
+  };
+}

lessons/submodule-types/run.sh

@@ -0,0 +1,3 @@
+nix eval -f eval.nix \
+    --apply 'x: x {pkgs = import <nixpkgs> {};}' \
+    --json | nix run nixpkgs#jq -- -r