Adding sitemap.xml and robots.txt to a website built with Hakyll is not explicitly documented but it ended up being quite easy with the help of some DuckDuckGo-fu. A quick research returns this post. With a few amendments the solution proposed in the post works like a charm, here’s the revised version I use for my website:

  create ["sitemap.xml"] $ do
    route idRoute
    compile $ do
      posts <- recentFirst =<< loadAll "posts/**"
      pages <- loadAll "pages/*"
        crawlPages =
          sitemapPages pages ++ posts
        sitemapCtx =
          mconcat [ listField "entries" defaultContext (return crawlPages)
                  , defaultContext
      makeItem ""
        >>= loadAndApplyTemplate "templates/sitemap.xml" sitemapCtx
        >>= relativizeUrls

  match (fromList ["robots.txt", "CNAME"]) $ do
    route idRoute
    compile $ getResourceBody >>= relativizeUrls

Notice how the robots.txt and CNAME files - the latter is needed by my domain name registrar - are simply copied since there’s no need to apply any processing to them.