Functional Programming: A PragPub Anthology

Macros

To see how, we’ll need to understand Clojure’s macro system. A macro is a special kind of function. It’s intended to take a piece of data that represents code, also known as a form. A macro transforms one form into another before Clojure’s compiler compiles it. Finally, the evaluation rule for a macro is special in that a macro does not evaluate its arguments.

Let’s take a look at a simple macro. This macro takes two arguments, a name and a string to print. It then creates a function that prints the passed-in string.

(defmacro make-printer [name to-print]
  (list 'defn name [] (list 'println to-print)))

Here we’ll use it to create a function named foo.

=> (make-printer foo "this is a foo")
#'matters/foo

=> (foo)
this ...