Semantic Gotcha

One of the selling points of protocols is that they are namespaced functions. This mitigates some of the issues of "monkey patching" where someone adds a new function definition to an existing class. In languages where a class is also a namespace, you can run into conflicts if two definitions of a function named get are added to a class at the same time. Since Clojure's protocol functions are namespaced, each person can define their own my.company.project/get and open.source.project/get functions.

However, there is a gotcha. Recall that a protocol also generates a Java interface, and when you extend a protocol in a defrecord, the generated class will implement the protocol's Java interface. Suppose that we defined two protocols:

1 (ns stadig.dog)

2

3 (defprotocol IDog

4   (emote [this]))

1 (ns stadig.cat)

2

3 (defprotocol ICat

4   (emote [this]))

The following code listings do different things:

1 (defrecord CatDog...