refactor(web/blog): Move atom feed creation logic to //web/blog

This was previously all inside of my personal homepage configuration,
but that's not really where it belongs.

This moves the blog post -> feed entry logic to //web/blog and moves
some other minor logic (like entry order) into the atom feed
implementation itself.

Change-Id: Idde0241c48e979580de73f2b9afd04e6ca7f4c9a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3770
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/tazjin/homepage/feed.nix

@@ -4,25 +4,11 @@
 with depot.nix.yants;
 
 let
-  inherit (builtins) map readFile sort foldl';
+  inherit (builtins) map readFile;
   inherit (lib) max singleton;
   inherit (pkgs) writeText;
-  inherit (depot.nix) renderMarkdown;
   inherit (depot.web) blog atom-feed;
 
-  postToEntry = defun [ blog.post atom-feed.entry ] (post: rec {
-    id = "https://tazj.in/blog/${post.key}";
-    title = post.title;
-    content = readFile (renderMarkdown post.content);
-    published = post.date;
-    updated = post.updated or post.date;
-
-    links = singleton {
-      rel = "alternate";
-      href = id;
-    };
-  });
-
   pageEntryToEntry = defun [ entry atom-feed.entry ] (e: {
     id = "tazjin:${e.class}:${toString e.date}";
     updated = e.date;
@@ -36,16 +22,13 @@ let
     };
   });
 
-  allEntries = (map postToEntry depot.users.tazjin.blog.posts)
+  allEntries = (map blog.toFeedEntry depot.users.tazjin.blog.posts)
              ++ (map pageEntryToEntry pageEntries);
 
-  mostRecentlyUpdated = foldl' max 0 (map (e: e.updated) allEntries);
-
   feed = {
     id = "https://tazj.in/";
     title = "tazjin's interblag";
     subtitle = "my posts, projects and other interesting things";
-    updated = mostRecentlyUpdated;
     rights = "© 2020 tazjin";
     authors = [ "tazjin" ];
 
@@ -54,6 +37,6 @@ let
       href = "https://tazjin/feed.atom";
     };
 
-    entries = sort (a: b: a.published > b.published) allEntries;
+    entries = allEntries;
   };
 in writeText "feed.atom" (atom-feed.renderFeed feed)

web/atom-feed/default.nix

@@ -5,8 +5,8 @@
 with depot.nix.yants;
 
 let
-  inherit (builtins) map readFile replaceStrings;
-  inherit (lib) concatStrings concatStringsSep removeSuffix;
+  inherit (builtins) foldl' map readFile replaceStrings sort;
+  inherit (lib) concatStrings concatStringsSep max removeSuffix;
   inherit (pkgs) runCommandNoCC;
 
   # 'link' describes a related link to a feed, or feed element.
@@ -67,8 +67,9 @@ let
     title = string;
 
     # Indicates the last time the feed was modified in a significant
-    # way (in seconds since epoch). Recommended element.
-    updated = int;
+    # way (in seconds since epoch). Will be calculated based on most
+    # recently updated entry if unset.
+    updated = option int;
 
     # Entries contained within the feed.
     entries = list entry;
@@ -127,17 +128,23 @@ let
     </entry>
   '');
 
+  mostRecentlyUpdated = defun [ (list entry) int ] (entries:
+    foldl' max 0 (map (e: e.updated) entries)
+  );
+
+  sortEntries = sort (a: b: a.published > b.published);
+
   renderFeed = defun [ feed string ] (f: ''
     <?xml version="1.0" encoding="utf-8"?>
     <feed xmlns="http://www.w3.org/2005/Atom">
       ${elem "id" f.id}
       ${elem "title" f.title}
-      ${elem "updated" (renderEpoch f.updated)}
+      ${elem "updated" (renderEpoch (f.updated or (mostRecentlyUpdated f.entries)))}
       ${concatStringsSep "\n" (map renderAuthor (f.authors or []))}
       ${if f ? subtitle then elem "subtitle" f.subtitle else ""}
       ${if f ? rights then elem "rights" f.rights else ""}
       ${concatStrings (map renderLink (f.links or []))}
-      ${concatStrings (map renderEntry f.entries)}
+      ${concatStrings (map renderEntry (sortEntries f.entries))}
     </feed>
   '');
 in {

web/blog/default.nix

@@ -7,6 +7,11 @@
 with depot.nix.yants;
 
 let
+  inherit (builtins) readFile;
+  inherit (depot.nix) renderMarkdown;
+  inherit (depot.web) atom-feed;
+  inherit (lib) singleton;
+
   # Type definition for a single blog post.
   post = struct "blog-post" {
     key = string;
@@ -31,9 +36,25 @@ let
     oldKey = option string;
   };
 
+  # Rendering fragments for the HTML version of the blog.
   fragments = import ./fragments.nix args;
+
+  # Functions for generating feeds for these blogs using //web/atom-feed.
+  toFeedEntry = defun [ post atom-feed.entry ] (post: rec {
+    id = "https://tazj.in/blog/${post.key}";
+    title = post.title;
+    content = readFile (renderMarkdown post.content);
+    published = post.date;
+    updated = post.updated or post.date;
+
+    links = singleton {
+      rel = "alternate";
+      href = id;
+    };
+  });
+
 in {
-  inherit post;
+  inherit post toFeedEntry;
   inherit (fragments) renderPost;
   includePost = post: !(fragments.isDraft post) && !(fragments.isUnlisted post);
 }