Question

I have been working recently with Clojure’s types and records recently, and I was wondering if these are very new concepts, or are they inspired by the Common Lisp Object System?

Answer 1

I believe they are pretty much new innovations within Clojure.

CLOS is a fairly complex, fully featured object oriented system. It features various OOP techniques you often hear mentioned and which exist in other object oriented languages – multiple inheritance, dynamic dispatch, generic functions etc.

Clojure takes a different approach – types and records are much simpler than full OOP and are not intended to constitute a full OOP system. Rather, I understand that the design is motivated by:

• The belief that many OOP techniques are actually harmful when building large systems – implementation inheritance for example
• The opportunity to get maximum performance (i.e. the same as Java) for the most common case of polymorphism (i.e. single dispatch on type)
• The desire to solve the “expression problem“, which you can do in Clojure using deftype / defrecord together with protocols
• The intent to make all records/types immutable in order to fit with the rest of Clojure’s philosophy

If you want a traditional object oriented system like CLOS, it would be possible to build this in Clojure on on top of types and records. You can also use Java-style object orientation directly within Clojure. However I believe it’s not usually recommended by Clojure experts – Clojure usually offers you different or better ways to solve the same problems.

In general – Clojure tends to provide you with “simple” tools that you can compose to solve the problem at hand, rather than prescribing a complex framework at any point. It’s an interesting philosophy discussed at some length in this video by Stuart Halloway.