add template showing nixpkgs option usage

Author: mibmo

flake.nix

@@ -21,6 +21,10 @@
           description = "Default template suitable as starting point";
           path = ./templates/default;
         };
+        nixpkgs = {
+          description = "Template showing nixpkgs overlay being applied";
+          path = ./templates/nixpkgs;
+        };
       };
     };
 }

templates/nixpkgs/default.nix

@@ -0,0 +1,53 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+
+    conch = {
+      url = "github:mibmo/conch";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    { conch, ... }:
+    conch.configure {
+      nixpkgs.overlays = [
+        (self: super: {
+          hello = super.hello.overrideAttrs (
+            final: prev: {
+              patches = prev.patches or [ ] ++ [
+                (builtins.toFile "hello-message.patch" ''
+                  diff '--color=auto' -urpN old/src/hello.c new/src/hello.c
+                  --- old/src/hello.c	2026-01-17 20:24:39.000000000 +0100
+                  +++ new/src/hello.c	2026-06-09 19:10:47.953160544 +0200
+                  @@ -145,7 +145,7 @@ main (int argc, char *argv[])
+                   #endif
+                   
+                     /* Having initialized gettext, get the default message. */
+                  -  greeting_msg = _("Hello, world!");
+                  +  greeting_msg = _("Hello, conch!");
+                   
+                     /* Even exiting has subtleties.  On exit, if any writes failed, change
+                        the exit status.  The /dev/full device on GNU/Linux can be used for
+                '')
+              ];
+              doCheck = false;
+            }
+          );
+        })
+      ];
+      packages =
+        { pkgs, ... }:
+        {
+          # this references `pkgs`, so uses patched version
+          inherit (pkgs) hello;
+        };
+      devShells.default =
+        { pkgs, ... }:
+        {
+          # same here! try entering the dev shell!
+          packages = with pkgs; [ hello ];
+          hook = "hello";
+        };
+    };
+}