news: add module

This new module adds a "news" feature to Home Manager. See #52.

Many thanks to @nonsequitur and @uvNikita for suggestions and
improvements.

modules/default.nix

@@ -11,6 +11,7 @@ let
     ./home-environment.nix
     ./manual.nix
     ./misc/gtk.nix
+    ./misc/news.nix
     ./misc/pam.nix
     ./programs/bash.nix
     ./programs/beets.nix

modules/misc/news.nix

@@ -0,0 +1,146 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.news;
+
+  entryModule = types.submodule ({ config, ... }: {
+    options = {
+      id = mkOption {
+        internal = true;
+        type = types.str;
+        description = ''
+          A unique entry identifier. By default it is a base16
+          formatted hash of the entry message.
+        '';
+      };
+
+      time = mkOption {
+        internal = true;
+        type = types.str;
+        example = "2017-07-10T21:55:04+00:00";
+        description = ''
+          News entry time stamp in ISO-8601 format. Must be in UTC
+          (ending in '+00:00').
+        '';
+      };
+
+      condition = mkOption {
+        internal = true;
+        default = true;
+        description = "Whether the news entry should be active.";
+      };
+
+      message = mkOption {
+        internal = true;
+        type = types.str;
+        description = "The news entry content.";
+      };
+    };
+
+    config = {
+      id = mkDefault (builtins.hashString "sha256" config.message);
+    };
+  });
+
+in
+
+{
+  options = {
+    news = {
+      display = mkOption {
+        type = types.enum [ "silent" "notify" "show" ];
+        default = "notify";
+        description = ''
+          How unread and relevant news should be presented when
+          running <command>home-manager build</command> and
+          <command>home-manager switch</command>.
+
+          </para><para>
+
+          The options are
+
+          <variablelist>
+          <varlistentry>
+            <term><literal>silent</literal></term>
+            <listitem>
+              <para>
+                Do not print anything during build or switch. The
+                <command>home-manager news</command> command still
+                works for viewing the entries.
+              </para>
+            </listitem>
+          </varlistentry>
+          <varlistentry>
+            <term><literal>notify</literal></term>
+            <listitem>
+              <para>
+                The number of unread and relevant news entries will be
+                printed to standard output. The <command>home-manager
+                news</command> command can later be used to view the
+                entries.
+              </para>
+            </listitem>
+          </varlistentry>
+          <varlistentry>
+            <term><literal>show</literal></term>
+            <listitem>
+              <para>
+                A pager showing unread news entries is opened.
+              </para>
+            </listitem>
+          </varlistentry>
+          </variablelist>
+        '';
+      };
+
+      entries = mkOption {
+        internal = true;
+        type = types.listOf entryModule;
+        default = [];
+        description = "News entries.";
+      };
+    };
+  };
+
+  config = {
+    news.entries = [
+      {
+        time = "2017-09-01T10:56:28+00:00";
+        message = ''
+          Hello! This is a news entry and it represents an
+          experimental new feature of Home Manager. The idea is to
+          inform you when something of importance happens in Home
+          Manager or its modules.
+
+          We will try to not disturb you about the same news more than
+          once so the next time you run
+
+              home-manager switch
+
+          or
+
+              home-manager build
+
+          it should not notify you about this text again.
+
+          News items may be conditional and will then only show if the
+          condition holds, for example if they are relevant to your
+          configuration.
+
+          If you want to see all relevant news then please use the
+
+              home-manager news
+
+          command.
+
+          Since this is an experimental feature any positive or
+          negative feedback would be greatly appreciated. For example,
+          by commenting in https://git.io/v5BJL.
+        '';
+      }
+    ];
+  };
+}