Added 5th part of the tutorial.

examples/hakyll/tutorials/part05.markdown

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+---
+title: Creating feeds
+what: adds an rss feed to the simple blog
+---
+
+## Adding Feeds
+
+In this tutorial, we're going to add an RSS feed to the blog we wrote in the
+previous part. Here is a [zip file] containing the source.
+
+[zip file]: examples/feedblog.zip
+
+You will be glad to hear that Hakyll has native support for RSS as well as Atom
+feeds[^1]. This simplifies our object a lot.
+
+[^1]: Since Hakyll-2.0
+
+This is the first time that the absolute URL of your site you have to give to
+the `hakyll` function actually matters. That's because the specifications of
+those feed formats dictate you need the full URL of them.
+
+## Creating a configuration
+
+The first thing to do is creating a configuration for your feed. You could
+place this code outside of your main function, as is done in the example.
+
+~~~~~{.haskell}
+myFeedConfiguration = FeedConfiguration
+    { feedUrl         = "rss.xml"
+    , feedTitle       = "SimpleBlog RSS feed."
+    , feedDescription = "Simple demo of a feed created with Hakyll."
+    , feedAuthorName  = "Jasper Van der Jeugt"
+    }
+~~~~~
+
+Note that we enter the url of the feed in our configuration. So the function
+to render our feed only takes two arguments, the configuration and a list of
+items to put in the feed. Let's put the three most recent posts in our feed.
+
+~~~~~{.haskell}
+renderRss myFeedConfiguration (take 3 postPages)
+~~~~~
+
+## But it's not that easy
+
+If you look at our generated RSS feed (build the site), you will see
+`$description` tags appearing in our final render. We don't want that! How
+did they get there in the first place?
+
+To render feeds, Hakyll expects a number of fields in the renderables you put
+in the feed. They are:
+
+- `$title`: Title of the item. This is set in our posts, since we use a `title`
+  metadata field.
+- `$url`: Url of the item. This is automatically set by Hakyll, so you shouldn't
+  worry about it.
+- `$description`: A description of our item to appear in the feed reader.
+
+The latter is obviously the problem: we don't have a description in our posts.
+In fact, we would like to copy the `$body` key to the `$description` key, so
+people can read the full post in their feed readers.
+
+## Where arrows come in
+
+The `Text.Hakyll.ContextManipulations` module contains a number of simple
+functions that create Arrows for us. One of these functions is `copyValue`,
+which takes a source and a destination key. So, we need to pass our
+items through this Arrow first.
+
+~~~~~{.haskell}
+renderRss myFeedConfiguration $
+    map (>>> copyValue "body" "description") (take 3 postPages)
+~~~~~
+
+And that's that, now our feed gets rendered properly. Exercise for the reader
+is to add a Atom feed[^2].
+
+[^2]: Hint: look around in the [reference]($root/reference.html).
+
+## The gist of it
+
+- Hakyll has native support for RSS and Atom feeds.
+- The items must contain `$title` and `$description` fields.
+- Arrows can be used to copy values in a `Context`.