Add sitemap.xml to improve indexing by search engines (#551)

* Initial draft of sitemap

* Add newline

* Generate robots.txt

* Fix bug

* Add newline

message-index/robots.txt

@@ -0,0 +1 @@
+Sitemap: https://errors.haskell.org/sitemap.xml

message-index/site.hs

@@ -34,6 +34,11 @@ main = hakyll $ do
     route idRoute
     compile copyFileCompiler
 
+  -- Necessary to point webcrawlers to the correct sitemap
+  match "robots.txt" $ do
+    route idRoute
+    compile copyFileCompiler
+
   match "images/*" $ do
     route idRoute
     compile copyFileCompiler
@@ -131,6 +136,27 @@ main = hakyll $ do
 
   match "templates/*" $ compile templateBodyCompiler
 
+  create ["sitemap.xml"] $ do
+    route idRoute
+    compile $ do
+      let messages = loadAll "messages/*/*"
+      let host = "https://errors.haskell.org"
+      let pageCtx :: Context String
+          pageCtx =
+            mconcat
+              [ modificationTimeField "lastmod" "%Y-%m-%d",
+                constField "host" host,
+                defaultContext
+              ]
+      let sitemapCtx =
+            mconcat
+              [ listField "entries" pageCtx messages,
+                constField "host" host,
+                defaultContext
+              ]
+      makeItem ""
+        >>= loadAndApplyTemplate "templates/sitemap.xml" sitemapCtx
+
   create ["api/errors.json"] $ do
     route idRoute
     compile $ do

message-index/templates/sitemap.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+$for(entries)$
+    <url>
+        <loc>$host$$url$</loc>
+        <changefreq>weekly</changefreq>
+        $if(lastmod)$<lastmod>$lastmod$</lastmod>$endif$
+        <priority>0.8</priority>
+    </url>
+$endfor$
+</urlset>