WIP
These are three callables that are often used as arguments in methods that are built to accept callbacks, like array_filter, usort, or call_user_func_array (similiar to javascript's apply array method).
It's a really interesting set of functions for many reasons - for one, like map and reduce, the semantics of the methods provide for a discrete and specific role, which makes thinking about code clearer. In any case, let's review.
usort, uksort and other sorting methods accepts a comparator function, a subset of filters.
vue.js uses a resolver to determine the status of a cached object, and in php, resolvers work similarly.
Discriminators are another subset of filters, and are described in the Slim documentation.