WordPress: Filters vs. Actions

So I’ve been messing around with WordPress lately. It’s this massively configurable CMS and blog engine written mostly in PHP. And the way you configure WP (for the most part) is using hooks, which come in two flavors: filters and actions. These two flavors appear to be very similar, but if we read the documentation they are supposed to be used in very different ways.

I was puzzling over the difference between hooks and actions in WordPress when I realized that they make more sense if we imagine that PHP has types.

Using haskell-ish notation, filters have a type like this:

filter :: b -> a -> a

while actions have a type like this:

action :: b -> IO ()

Then after populating a named list of filters with filter1 b1, filter2 b2, et cetera, calling apply_filters does something like this:

apply_filters :: a -> a
apply_filters a = filterN bN $ ... $ filter2 b2 $ filter1 b1 a

and after populating a named list of actions with action1 b1, action2 b2, et cetera, calling do_action does something like this:

do_action :: IO ()
do_action = sequence_ [action1 b1, action2 b2, ..., actionN bN]

Of course PHP doesn’t really have types, meaning that anything can change at any time for any reason (at least that’s how I think about it – I imagine that PHP takes place entirely in a special version of the IO monad that randomly triggers bugs). So in practice the programmer has to be disciplined enough to maintain the semantic difference between the two. But as I understand it these types capture the intended behavior.