diff --git a/16-formatter-and-checks/README.md b/16-formatter-and-checks/README.md
new file mode 100644
index 0000000..9f4e21d
--- /dev/null
+++ b/16-formatter-and-checks/README.md
@@ -0,0 +1,16 @@
+# 16-formatter-and-checks
+
+This example shows `formatter.<system>` and `checks.<system>.*`.
+
+It includes:
+
+- a `formatter` output pointing at `nixfmt`, and
+- a check that uses that formatter on a file in the example.
+
+Useful commands:
+
+```bash
+nix fmt sample.nix
+nix flake show
+nix flake check
+```
diff --git a/16-formatter-and-checks/flake.lock b/16-formatter-and-checks/flake.lock
new file mode 100644
index 0000000..dfdfdf9
--- /dev/null
+++ b/16-formatter-and-checks/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1776548001,
+        "narHash": "sha256-ZSK0NL4a1BwVbbTBoSnWgbJy9HeZFXLYQizjb2DPF24=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "b12141ef619e0a9c1c84dc8c684040326f27cdcc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/16-formatter-and-checks/flake.nix b/16-formatter-and-checks/flake.nix
new file mode 100644
index 0000000..7fee7b7
--- /dev/null
+++ b/16-formatter-and-checks/flake.nix
@@ -0,0 +1,26 @@
+{
+  # Exposes a formatter output and a check that uses the same formatter on
+  # a file in this example directory.
+  description = "A minimal formatter and checks example";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { nixpkgs, ... }:
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+      formatter.${system} = pkgs.nixfmt;
+
+      checks.${system}.formats = pkgs.runCommand "formatter-check" { } ''
+        cp ${./sample.nix} ./sample.nix
+        ${pkgs.nixfmt}/bin/nixfmt ./sample.nix >/dev/null
+        grep -q 'greeting = "hello";' ./sample.nix
+        echo ok > "$out"
+      '';
+    };
+}
diff --git a/16-formatter-and-checks/sample.nix b/16-formatter-and-checks/sample.nix
new file mode 100644
index 0000000..2d5e513
--- /dev/null
+++ b/16-formatter-and-checks/sample.nix
@@ -0,0 +1,3 @@
+{
+  greeting = "hello";
+}
diff --git a/17-package-from-source/README.md b/17-package-from-source/README.md
new file mode 100644
index 0000000..f43b03c
--- /dev/null
+++ b/17-package-from-source/README.md
@@ -0,0 +1,18 @@
+# 17-package-from-source
+
+This example shows packaging a selected local source tree.
+
+It includes:
+
+- a derivation built from a local source tree,
+- `lib.fileset.toSource` to include only selected files, and
+- a check that proves excluded files are not in the packaged source.
+
+Useful commands:
+
+```bash
+nix build
+./result/bin/source-greet
+nix run
+nix flake check
+```
diff --git a/17-package-from-source/flake.lock b/17-package-from-source/flake.lock
new file mode 100644
index 0000000..dfdfdf9
--- /dev/null
+++ b/17-package-from-source/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1776548001,
+        "narHash": "sha256-ZSK0NL4a1BwVbbTBoSnWgbJy9HeZFXLYQizjb2DPF24=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "b12141ef619e0a9c1c84dc8c684040326f27cdcc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/17-package-from-source/flake.nix b/17-package-from-source/flake.nix
new file mode 100644
index 0000000..833e1b8
--- /dev/null
+++ b/17-package-from-source/flake.nix
@@ -0,0 +1,62 @@
+{
+  # Packages a selected local source tree so the derivation only sees the
+  # files that the example deliberately includes.
+  description = "Package a local source tree with fileset";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs, ... }:
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+      sourceFiles = pkgs.lib.fileset.unions [
+        ./src/message.txt
+        ./src/run.sh
+      ];
+      selectedSource = pkgs.lib.fileset.toSource {
+        root = ./.;
+        fileset = sourceFiles;
+      };
+    in
+    {
+      packages.${system}.default = pkgs.stdenv.mkDerivation {
+        pname = "source-greet";
+        version = "0.1.0";
+        src = selectedSource;
+        dontBuild = true;
+        dontUnpack = true;
+
+        installPhase = ''
+          mkdir -p $out/bin $out/share/source-greet
+          cp $src/src/message.txt $out/share/source-greet/message.txt
+          cp $src/src/run.sh $out/bin/source-greet
+          chmod +x $out/bin/source-greet
+
+          substituteInPlace $out/bin/source-greet \
+            --replace-fail "@bash@" "${pkgs.bash}/bin/bash" \
+            --replace-fail "@message@" "$out/share/source-greet/message.txt"
+        '';
+      };
+
+      apps.${system}.default = {
+        type = "app";
+        program = "${self.packages.${system}.default}/bin/source-greet";
+        meta.description = "Run the package built from the selected local source tree.";
+      };
+
+      checks.${system}.selected-source = pkgs.runCommand "selected-source-check" { } ''
+        [ ! -e ${selectedSource}/ignored.txt ]
+        output="$(${self.packages.${system}.default}/bin/source-greet)"
+
+        if [ "$output" = "hello from the selected source tree" ]; then
+          echo ok > "$out"
+        else
+          echo "unexpected output: $output" >&2
+          exit 1
+        fi
+      '';
+    };
+}
diff --git a/17-package-from-source/ignored.txt b/17-package-from-source/ignored.txt
new file mode 100644
index 0000000..38ab5fb
--- /dev/null
+++ b/17-package-from-source/ignored.txt
@@ -0,0 +1 @@
+this file is intentionally excluded from the package source
diff --git a/17-package-from-source/src/message.txt b/17-package-from-source/src/message.txt
new file mode 100644
index 0000000..2ea19ee
--- /dev/null
+++ b/17-package-from-source/src/message.txt
@@ -0,0 +1 @@
+hello from the selected source tree
diff --git a/17-package-from-source/src/run.sh b/17-package-from-source/src/run.sh
new file mode 100644
index 0000000..a2d0cd8
--- /dev/null
+++ b/17-package-from-source/src/run.sh
@@ -0,0 +1,2 @@
+#!@bash@
+cat @message@
diff --git a/18-home-manager-module/README.md b/18-home-manager-module/README.md
new file mode 100644
index 0000000..8e57823
--- /dev/null
+++ b/18-home-manager-module/README.md
@@ -0,0 +1,17 @@
+# 18-home-manager-module
+
+This example shows a small reusable Home Manager module.
+
+It includes:
+
+- a module stored in `module.nix`,
+- one option namespace,
+- one config effect, and
+- a check that evaluates a throwaway Home Manager configuration.
+
+Useful commands:
+
+```bash
+nix flake show
+nix flake check
+```
diff --git a/18-home-manager-module/flake.lock b/18-home-manager-module/flake.lock
new file mode 100644
index 0000000..9570385
--- /dev/null
+++ b/18-home-manager-module/flake.lock
@@ -0,0 +1,48 @@
+{
+  "nodes": {
+    "home-manager": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1776950293,
+        "narHash": "sha256-t6KMARLILjPuTBSRoYanUxV+FU50IFZ7L5XVdOcdtaY=",
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "rev": "6837e0d6c5eda81fd26308489799fbf83a160465",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1776548001,
+        "narHash": "sha256-ZSK0NL4a1BwVbbTBoSnWgbJy9HeZFXLYQizjb2DPF24=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "b12141ef619e0a9c1c84dc8c684040326f27cdcc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "home-manager": "home-manager",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/18-home-manager-module/flake.nix b/18-home-manager-module/flake.nix
new file mode 100644
index 0000000..a586766
--- /dev/null
+++ b/18-home-manager-module/flake.nix
@@ -0,0 +1,59 @@
+{
+  # Evaluates a minimal Home Manager module from `module.nix` and verifies
+  # its effect through a throwaway Home Manager configuration.
+  description = "A minimal Home Manager module";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+    home-manager = {
+      url = "github:nix-community/home-manager";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      home-manager,
+      ...
+    }:
+    let
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+
+      testConfig = home-manager.lib.homeManagerConfiguration {
+        inherit pkgs;
+        modules = [
+          ./module.nix
+          {
+            home = {
+              username = "learner";
+              homeDirectory = "/home/learner";
+              stateVersion = "24.11";
+            };
+            playground.welcome = {
+              enable = true;
+              name = "flakes";
+            };
+          }
+        ];
+      };
+    in
+    {
+      checks.${system}.welcome =
+        pkgs.runCommand "home-manager-welcome-check"
+          {
+            got = testConfig.config.home.file."welcome.txt".text;
+            expected = "hello, flakes";
+          }
+          ''
+            if [ "$got" = "$expected" ]; then
+              echo ok > "$out"
+            else
+              echo "unexpected welcome text: $got" >&2
+              exit 1
+            fi
+          '';
+    };
+}
diff --git a/18-home-manager-module/module.nix b/18-home-manager-module/module.nix
new file mode 100644
index 0000000..9910785
--- /dev/null
+++ b/18-home-manager-module/module.nix
@@ -0,0 +1,20 @@
+{ lib, config, ... }:
+
+let
+  cfg = config.playground.welcome;
+in
+{
+  options.playground.welcome = {
+    enable = lib.mkEnableOption "the playground Home Manager welcome file";
+
+    name = lib.mkOption {
+      type = lib.types.str;
+      default = "world";
+      description = "Name placed in the generated welcome file.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    home.file."welcome.txt".text = "hello, ${cfg.name}";
+  };
+}
diff --git a/19-devshell-vs-shellfor/README.md b/19-devshell-vs-shellfor/README.md
new file mode 100644
index 0000000..dcf7322
--- /dev/null
+++ b/19-devshell-vs-shellfor/README.md
@@ -0,0 +1,20 @@
+# 19-devshell-vs-shellfor
+
+This example compares two dev shell styles for the same local package.
+
+It includes:
+
+- a generic `mkShell` shell,
+- a Haskell-specific `shellFor` shell,
+- one local package that both shells support, and
+- a test suite run by `nix flake check`.
+
+Useful commands:
+
+```bash
+nix develop .#generic
+nix develop .#shellFor
+nix build
+./result/bin/mini-shell-choice
+nix flake check
+```
diff --git a/19-devshell-vs-shellfor/app/Main.hs b/19-devshell-vs-shellfor/app/Main.hs
new file mode 100644
index 0000000..5fef45c
--- /dev/null
+++ b/19-devshell-vs-shellfor/app/Main.hs
@@ -0,0 +1,6 @@
+module Main where
+
+import MiniShellChoice.Message (message)
+
+main :: IO ()
+main = putStrLn message
diff --git a/19-devshell-vs-shellfor/flake.lock b/19-devshell-vs-shellfor/flake.lock
new file mode 100644
index 0000000..dfdfdf9
--- /dev/null
+++ b/19-devshell-vs-shellfor/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1776548001,
+        "narHash": "sha256-ZSK0NL4a1BwVbbTBoSnWgbJy9HeZFXLYQizjb2DPF24=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "b12141ef619e0a9c1c84dc8c684040326f27cdcc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/19-devshell-vs-shellfor/flake.nix b/19-devshell-vs-shellfor/flake.nix
new file mode 100644
index 0000000..30bdffa
--- /dev/null
+++ b/19-devshell-vs-shellfor/flake.nix
@@ -0,0 +1,54 @@
+{
+  # Compares a generic dev shell with a Haskell-specific `shellFor` shell
+  # while building the same local package in both cases.
+  description = "Compare mkShell and shellFor";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs, ... }:
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+      haskellPackages = pkgs.haskellPackages.override {
+        overrides = final: _: {
+          mini-shell-choice = final.callCabal2nix "mini-shell-choice" ./. { };
+        };
+      };
+      project = haskellPackages.mini-shell-choice;
+      checkedProject = pkgs.haskell.lib.doCheck project;
+      genericShell = pkgs.mkShell {
+        packages = [
+          haskellPackages.ghc
+          pkgs.cabal-install
+          pkgs.haskell-language-server
+        ];
+      };
+      shellForShell = haskellPackages.shellFor {
+        packages = hp: [ hp.mini-shell-choice ];
+        nativeBuildInputs = [
+          pkgs.cabal-install
+          pkgs.haskell-language-server
+        ];
+      };
+    in
+    {
+      packages.${system}.default = project;
+
+      apps.${system}.default = {
+        type = "app";
+        program = "${self.packages.${system}.default}/bin/mini-shell-choice";
+        meta.description = "Run the package used by both shell styles.";
+      };
+
+      devShells.${system} = {
+        default = genericShell;
+        generic = genericShell;
+        shellFor = shellForShell;
+      };
+
+      checks.${system}.test-suite = checkedProject;
+    };
+}
diff --git a/19-devshell-vs-shellfor/mini-shell-choice.cabal b/19-devshell-vs-shellfor/mini-shell-choice.cabal
new file mode 100644
index 0000000..13ed1a4
--- /dev/null
+++ b/19-devshell-vs-shellfor/mini-shell-choice.cabal
@@ -0,0 +1,27 @@
+cabal-version:      2.4
+name:               mini-shell-choice
+version:            0.1.0.0
+build-type:         Simple
+
+library
+  exposed-modules:  MiniShellChoice.Message
+  hs-source-dirs:   src
+  build-depends:    base >=4.14 && <5
+  default-language: Haskell2010
+
+executable mini-shell-choice
+  main-is:          Main.hs
+  hs-source-dirs:   app
+  build-depends:
+      base >=4.14 && <5,
+      mini-shell-choice
+  default-language: Haskell2010
+
+test-suite mini-shell-choice-test
+  type:             exitcode-stdio-1.0
+  main-is:          Main.hs
+  hs-source-dirs:   test
+  build-depends:
+      base >=4.14 && <5,
+      mini-shell-choice
+  default-language: Haskell2010
diff --git a/19-devshell-vs-shellfor/src/MiniShellChoice/Message.hs b/19-devshell-vs-shellfor/src/MiniShellChoice/Message.hs
new file mode 100644
index 0000000..2c04e4b
--- /dev/null
+++ b/19-devshell-vs-shellfor/src/MiniShellChoice/Message.hs
@@ -0,0 +1,4 @@
+module MiniShellChoice.Message where
+
+message :: String
+message = "compare the generic shell and the shellFor shell"
diff --git a/19-devshell-vs-shellfor/test/Main.hs b/19-devshell-vs-shellfor/test/Main.hs
new file mode 100644
index 0000000..0348839
--- /dev/null
+++ b/19-devshell-vs-shellfor/test/Main.hs
@@ -0,0 +1,11 @@
+module Main where
+
+import MiniShellChoice.Message (message)
+import System.Exit (die)
+
+main :: IO ()
+main =
+  if message == "compare the generic shell and the shellFor shell" then
+    putStrLn "test passed"
+  else
+    die "unexpected message"
diff --git a/notes/019-formatter-and-checks.md b/notes/019-formatter-and-checks.md
new file mode 100644
index 0000000..01508e4
--- /dev/null
+++ b/notes/019-formatter-and-checks.md
@@ -0,0 +1,38 @@
+# Formatter and Checks
+
+This note covers `16-formatter-and-checks/`, which exposes `formatter.<system>` and a small `checks.<system>.*` derivation that uses the same formatter.
+
+---
+
+## 1. Why These Outputs Matter
+
+Two flake outputs are especially practical in day-to-day use:
+
+- `formatter.<system>`, which powers `nix fmt`, and
+- `checks.<system>.*`, which powers `nix flake check`.
+
+This example keeps both outputs small so the wiring is obvious.
+
+---
+
+## 2. The Important Connection
+
+The flake points `formatter.<system>` at `pkgs.nixfmt`, then reuses that same formatter in a check:
+
+```nix
+formatter.${system} = pkgs.nixfmt;
+```
+
+That shows the main idea: a formatter output is just another derivation, so checks can use it too.
+
+---
+
+## 3. Commands to Try
+
+```bash
+cd 16-formatter-and-checks
+
+nix fmt sample.nix
+nix flake show
+nix flake check
+```
diff --git a/notes/020-package-from-source.md b/notes/020-package-from-source.md
new file mode 100644
index 0000000..c5251bc
--- /dev/null
+++ b/notes/020-package-from-source.md
@@ -0,0 +1,41 @@
+# Package from Source
+
+This note covers `17-package-from-source/`, which packages a selected local source tree and excludes unrelated files from the build input.
+
+---
+
+## 1. Why Source Selection Matters
+
+`src = ./.;` copies the whole example directory into the store. That is fine for small demos, but real packages often need a narrower source tree.
+
+This example uses `lib.fileset.toSource` so the derivation only sees:
+
+- `src/run.sh`, and
+- `src/message.txt`.
+
+That leaves `ignored.txt` out of the packaged source on purpose.
+
+---
+
+## 2. Why the Check Looks at the Source Path
+
+The check does two things:
+
+- it runs the packaged binary, and
+- it asserts that the generated source path does not contain `ignored.txt`.
+
+That keeps the example focused on the real behavior being taught: selecting build inputs, not just producing a runnable script.
+
+---
+
+## 3. Commands to Try
+
+```bash
+cd 17-package-from-source
+
+nix build
+./result/bin/source-greet
+
+nix run
+nix flake check
+```
diff --git a/notes/021-home-manager-modules.md b/notes/021-home-manager-modules.md
new file mode 100644
index 0000000..ce6dc97
--- /dev/null
+++ b/notes/021-home-manager-modules.md
@@ -0,0 +1,40 @@
+# Home Manager Modules
+
+This note covers `18-home-manager-module/`, which defines a small reusable Home Manager module in `module.nix` and verifies it by evaluating a throwaway Home Manager configuration.
+
+---
+
+## 1. What This Example Mirrors
+
+This example is the Home Manager counterpart to `04-nixos-module/`.
+
+It has the same shape:
+
+- one option namespace,
+- one configuration effect, and
+- one check that evaluates a temporary configuration.
+
+The important difference is the target system. Home Manager modules work on `home.*` options rather than `services.*`, `environment.*`, or other NixOS module options.
+
+---
+
+## 2. Why the Check Reads a Config Value
+
+The flake does not need to switch a real Home Manager configuration. It only needs to prove that the module evaluates and contributes the expected value.
+
+That is why the check reads:
+
+- `testConfig.config.home.file."welcome.txt".text`
+
+instead of trying to activate a real profile.
+
+---
+
+## 3. Commands to Try
+
+```bash
+cd 18-home-manager-module
+
+nix flake show
+nix flake check
+```
diff --git a/notes/022-devshell-vs-shellfor.md b/notes/022-devshell-vs-shellfor.md
new file mode 100644
index 0000000..07c612c
--- /dev/null
+++ b/notes/022-devshell-vs-shellfor.md
@@ -0,0 +1,40 @@
+# Dev Shell versus shellFor
+
+This note covers `19-devshell-vs-shellfor/`, which builds one local package and exposes two dev shells around it.
+
+---
+
+## 1. What Is Being Compared
+
+The example defines:
+
+- `devShells.<system>.generic`, built with `pkgs.mkShell`, and
+- `devShells.<system>.shellFor`, built with `haskellPackages.shellFor`.
+
+Both shells support the same local package, but they are assembled differently.
+
+---
+
+## 2. Why the Difference Matters
+
+`mkShell` is generic. You list tools directly.
+
+`shellFor` is package-set-aware. It starts from Haskell packages and builds a shell around their dependencies.
+
+That makes `shellFor` more tightly coupled to the package graph, while `mkShell` stays more explicit and general-purpose.
+
+---
+
+## 3. Commands to Try
+
+```bash
+cd 19-devshell-vs-shellfor
+
+nix develop .#generic
+nix develop .#shellFor
+
+nix build
+./result/bin/mini-shell-choice
+
+nix flake check
+```