Stale code warning
There have been small changes to the protocols code in Clojure. The below post is still useful, but a few details of the example code is different. See part 3 for the updated syntax.
Protocols are a new feature in Clojure, set to be released in the next version. They provide polymorphism in a very Clojure-ish way. I think it's a great lightweight polymorphism implementation that has a lot of potential. In true Clojure style I think it meets the polymorphism objective and yet doesn't need to totally change the way you already write your code in Clojure. I'm breaking this entry into more than one piece to show some different ways that Clojure protocols can be used. Because it's so new, there are not a lot of docs out there on it, but Rich does some good documentation on the macros themselves. If you want to try these examples, make sure you're running off of the 1.2 version of Clojure (from Clojars or a local build from the Clojure git repo). First I'll start by defining a simple protocol:
(defprotocol TextOutput (output-string [x string]))
In Java terms, I'm defining a TextOutput interface (actually a Java interface is being created, but more on that later), that has a single function named output-string that includes no implementation details. The input to this function is a little tricky though. I specified a parameter x and another one called string. The first parameter will be used to pass the implementation of the interface into the function. You don't need to write code to handle the parameter x and when you write your implementation, you'll act like it doesn't exist. A wiki type text output of an italics string would look like:
(deftype ItalicsOutput  TextOutput (output-string [string] (str "_" string "_")))
I have begun thinking about this in Java terms as a class ItalicsOutput that implements the TextOutput interface. Here in the output-string function, I only specify one parameter (not two). Next you can use this implementation with the following code:
(output-string (ItalicsOutput) "stuff") "_stuff_"
I'm telling Clojure I want it to execute the output-string function, on the implementation (ItalicsOutput) (more in this below) with the argument "stuff". I think that below is a little more readable:
(def italics-impl (ItalicsOutput)) (output-string italics-impl "stuff") "_stuff_"
Which just assigns the instantiated implementation to a variable which can then be used. These implementations can also have parameters, like:
(deftype PrefixedOutput [prefix-string] TextOutput (output-string [string] (str prefix-string " " string)))
I think passing a variable in makes the instantiation step make a little more sense:
(def prefix-with-more (PrefixedOutput "more")) (output-string prefix-with-more "stuff") "more stuff"
Both implementations can be used together as well:
(defn print-all  (let [italics-impl (ItalicsOutput) prefix-with-more (PrefixedOutput "more")] (println (output-string italics-impl "stuff")) (println (output-string prefix-with-more "stuff"))))
With output that would look like:
(print-all) _stuff_ more stuff