Is it possible to do datatype inheritance in Clojure?

Question

For example if I have two different records, but they share a handful of common fields, is there a way to make both records share a common base record? It seems like protocols only allow for declaring method signatures.

Answer 1

I don't think that is possible as of now.

defrecord is just a macro and you can check what it does by using macroexpand, something like:

(macroexpand '(defrecord User [Name Age]))

So if you want such record inheritance you probably need to implement a macro to do so. But I would not suggest that as inheritance is something that I try to avoid because it leads to complexity.

Answer 2

You should be programming against an abstraction, not particular fields, i.e., use a protocol so the records can share a common interface. E.g., if all of your record types need to return a 'string' property, then create a protocol with a 'get-string' method and extend that to each record type.

(defprotocol SQL
  (get-string [t]))

(defrecord Thing [name f1 f2]
  SQL
  (get-string [t] (str name)))

(defrecord AnotherThing [name f1 f2 f3 f4 blah]
  SQL
  (get-string [t] (str name)))

If you find yourself creating the same implementation for a particular protocol method you can always 'defn-' (non-public function) the implementation and use it as the implementation in each record type.