meta: Merge postgres module with shell

Realistically this might be more related to "system" than shell, however
it may also be advantageous to keep system as minimal as possible since
it could also be argued that interpreted programming languages are a
part of the system.

modules/postgres.nix

@@ -1,25 +0,0 @@
-{ pkgs, ... }:
-
-{
-  services.postgresql = {
-    enable = true;
-
-    ensureUsers = [
-      {
-        name = "user";
-      }
-      {
-        name = "cooldbname";
-        ensureDBOwnership = true;
-      }
-    ];
-
-    ensureDatabases = [
-      "cooldbname"
-    ];
-  };
-
-  environment.systemPackages = with pkgs; [
-    pgcli
-  ];
-}

modules/shell.nix

@@ -2,15 +2,19 @@
 
 let
   inherit (pkgs) fish htop-vim;
-  inherit (lib) mkEnableOption mkIf;
+  inherit (lib) mkEnableOption mkIf mkMerge singleton;
+  inherit (builtins) attrValues;
+  inherit (cfg) postgres;
+  inherit (config.modules.system) username;
 
   cfg = config.modules.shell;
 in
 {
   options.modules.shell = {
+    postgres = mkEnableOption "postgres database and pgcli for containers";
   };
 
-  config = with cfg; {
+  config = {
     users.defaultUserShell = fish;
     environment.shells = [ fish ];
 
@@ -24,8 +28,24 @@ in
       VISUAL = "nvim";
     };
 
-    environment.systemPackages = builtins.attrValues {
-      inherit (pkgs) wget jq eza fd fzf ripgrep;
+    environment.systemPackages = mkMerge [
+      (attrValues {
+        inherit (pkgs) wget jq eza fd fzf ripgrep;
+      })
+
+      (mkIf postgres (attrValues {
+        inherit (pkgs) pgcli;
+      }))
+    ];
+
+    services.postgresql = mkIf postgres {
+      enable = true;
+
+      ensureUsers = singleton {
+        name = username;
+      };
+
+      ensureDatabases = [ username ];
     };
 
     programs = {