Theory: defroutes is a Clojure macro

The Compojure function defroutes is actually a Clojure macro. The defroutes macro provides a simple syntax for defining routes and associating handler functions.

What is a macro?

Clojure has a programmatic macro system which allows the Clojure community to extend the language, rather than wait for the language designers. This macro approach also helps keep the language very compact, with a minimum of primitives.

We have already used several macros in our code. In our project.clj configuration we use the defproject macro to make it easy to define our Clojure project. In our code we have used the defn macro to define names (symbols) for functions.

Peeking under the covers

You can always look at what a macro is doing by using the macroexpand or macroexpand-all functions. These functions show you what the code looks like after the macro-reader has processed the macro.

To expand a macro, require the clojure.walk library in your namespace

[clojure.walk :as walk]

Then wrap the macro you wish to explore with the macroexpand-all function.

(walk/macroexpand-all
 '(defroutes myapp
    (GET "/" [] "Show something")))


(def myapp
  (compojure.core/routes
   (compojure.core/make-route
    :get #clout.core.CompiledRoute{:source "/", :re #"/", :keys [], :absolute? false}
    (fn* ([request__13075__auto__] (let* [] "Show something"))))))

You can see that the defroutes function expands to a make-route function that creates the details of the route and associates it with a handler or response map. The routes function join multiple routes together.

For further examples, see http://learnxinyminutes.com/docs/clojure-macros/