When and how should custom hierarchies be used in clojure?
- by Rob Lachlan
Clojure's system for creating an ad hoc hierarchy of keywords is familiar to most people who have spent a bit of time with the language. For example, most demos and presentations of the language include examples such as
(derive ::child ::parent)
and they go on to show how this can be used for multi-method dispatch.
In all of the slides and presentations that I've seen, they use the global hierarchy. But it is possible to keyword relationships in custom hierarchies. Some questions, therefore:
Are there any guidelines on when this is useful or necessary?
Are there any functions for manipulating hierarchies?
Merging is particularly useful, so I do this:
(defn merge-h [& hierarchies]
(apply merge-with (cons #(merge-with clojure.set/union %1 %2) hierarchies))
But I was wondering if such functions already exist somewhere.
