How to properly move newsfeeds in Hakyll

Theoretically, static sites are location-independent, i.e. they don’t care where they are deployed—there are no references, neither in generator nor in the generated HTML and CSS themselves, to the domain and/or path where the site is going to be situated. If you’re using relativizeUrls in all your Hakyll compilers, your site is location-independent already and you can move it easily by just copying the files to a new address.

Unless, of course, you have an Atom or RSS feed, in which case the idyll is broken.

The first reason to this, the universal one, is that feeds contain a link to themselves, or at least to the site from which they can be fetched. Atom feeds carry their own original location in <id> tag inside the <feed>; RSS 2.0 feeds have a <link> to the site inside the <channel>, but can also include <atom:link> (which is just like Atom’s <id>).

The second reason is specific to Hakyll, but to explain it, I have to delve a bit into how newsfeeds work in general.

How computers tell news items apart

A newsfeed is a collection of items (or entries, in Atom’s parlance). An item has a title, an author, a publication date and some contents, plus a number of optional additions like enclosures with pictures or mp3 files.

To us humans, it might appear that these properties are sufficient to figure out if two items are the same, but in reality it isn’t so. Think for a second: if I publish an item and then fix a typo in its title, is that the same item as before, or a new one? Of course it’s the same, you say! But unfortunately, it isn’t that obvious to a machine.

Instead of devising complex comparison strategies, the problem can be solved by addition of yet another property—an ID. The publisher (e.g. Hakyll) generates a unique ID for each item, and readers (the programs, not the humans) can remember IDs they see in order to distinguish updates from new items. Genius, right?

But what does it have to do with Hakyll? How does it make our sites location-dependent?

How Hakyll generates item IDs

Since it is a static generator without any sort of database, it doesn’t have much choice about the way to generate an ID: it has to use some of post’s data, but it can’t rely on title, date or content as these are exactly the things that might be changed during an update.

What’s left is post’s URL, which includes both your site’s domain name and the path to the post itself. This is why there is feedRoot field in Hakyll.Web.Feed.FeedConfiguration—Hakyll already knows post’s path relative to the site’s root, but it has no idea about the domain name and the path where you’ll put the site.

When you move your site, you’ll change feedRoot, which will in turn change all the IDs of your feed items, leading readers (the programs) to believe that these are new ones, yet unseen. This will probably irritate your readers (the humans).

The solution should now be obvious, though: for items that were published before the move, the old feedRoot should be used, while newer posts should use a new value. How can we achieve that?

Let’s write some code!

You must’ve noticed that both renderRss and renderAtom accept Context String parameter. Contexts are important because when a template is being filled out, it’s the context that supplies the values. Cool thing about contexts is that they can associate a key (the thing that appears in your template as a string between dollar signs, e.g. $title$) not only with a constant value which is the same for all items, but also with a function that is run on each item and produces the value to be substituted for the key.

This, and the fact that feedRoot is just another piece of context (bound to key “root”), is enough to implement the solution.

Somewhere in your code, you have a line like this one (it doesn’t really matter if it’s renderRss or renderAtom):

renderRss feedConfig defaultContext

What we need to do is to replace it with the following code:

  (field "root" (\item -> do
                  let id = itemIdentifier item
                  published <- getItemUTC defaultTimeLocale id

                  httpsSwitchDate <-

                  if published > httpsSwitchDate
                    then return newRootUrl
                    else return oldRootUrl)

As you can see, we’re defining a field called “root”, which is mapped to a lambda. The lambda figures out when the given item was published and compares that date to a constant value we have embedded in the code. This constant is the day you’ve moved your site to the new location. Everything posted after it will have new feedRoot, while older posts will keep using the old feedRoot.

Note that we’re mappending defaultContext to our field, not the other way around. This is important, as only the first definition of each field matters. You’ll probably use your own custom context instead of the default one, and it might overwrite “root” key if you mappend in the wrong order. Be careful! (I actually made this mistake when I put this article out.)

To tell the truth, I find the solution a bit messy and hacky, but such is the price of compatibility.

Update 29.06.2016: note that mappending order matters.

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