As with all Lisps, Clojure’s inherent homoiconicity gives you access to the full extent of the language to write code-generation routines called “macros”. Macros provide a powerful way to tailor the language to your needs.
Be careful though. It’s considered bad form to write a macro when a function will do. Use a macro only when you need control over when or if the arguments to a form will be evaluated.
You’ll want to be familiar with Clojure. Make sure you understand everything in Clojure in Y Minutes.
;; Define a macro using defmacro. Your macro should output a list that can ;; be evaluated as clojure code. ;; ;; This macro is the same as if you wrote (reverse "Hello World") (defmacro my-first-macro  (list reverse "Hello World")) ;; Inspect the result of a macro using macroexpand or macroexpand-1. ;; ;; Note that the call must be quoted. (macroexpand '(my-first-macro)) ;; -> (#<core$reverse clojure.core$reverse@xxxxxxxx> "Hello World") ;; You can eval the result of macroexpand directly: (eval (macroexpand '(my-first-macro))) ; -> (\d \l \o \r \W \space \o \l \l \e \H) ;; But you should use this more succinct, function-like syntax: (my-first-macro) ; -> (\d \l \o \r \W \space \o \l \l \e \H) ;; You can make things easier on yourself by using the more succinct quote syntax ;; to create lists in your macros: (defmacro my-first-quoted-macro  '(reverse "Hello World")) (macroexpand '(my-first-quoted-macro)) ;; -> (reverse "Hello World") ;; Notice that reverse is no longer function object, but a symbol. ;; Macros can take arguments. (defmacro inc2 [arg] (list + 2 arg)) (inc2 2) ; -> 4 ;; But, if you try to do this with a quoted list, you'll get an error, because ;; the argument will be quoted too. To get around this, clojure provides a ;; way of quoting macros: `. Inside `, you can use ~ to get at the outer scope (defmacro inc2-quoted [arg] `(+ 2 ~arg)) (inc2-quoted 2) ;; You can use the usual destructuring args. Expand list variables using ~@ (defmacro unless [arg & body] `(if (not ~arg) (do ~@body))) ; Remember the do! (macroexpand '(unless true (reverse "Hello World"))) ;; -> ;; (if (clojure.core/not true) (do (reverse "Hello World"))) ;; (unless) evaluates and returns its body if the first argument is false. ;; Otherwise, it returns nil (unless true "Hello") ; -> nil (unless false "Hello") ; -> "Hello" ;; Used without care, macros can do great evil by clobbering your vars (defmacro define-x  '(do (def x 2) (list x))) (def x 4) (define-x) ; -> (2) (list x) ; -> (2) ;; To avoid this, use gensym to get a unique identifier (gensym 'x) ; -> x1281 (or some such thing) (defmacro define-x-safely  (let [sym (gensym 'x)] `(do (def ~sym 2) (list ~sym)))) (def x 4) (define-x-safely) ; -> (2) (list x) ; -> (4) ;; You can use # within ` to produce a gensym for each symbol automatically (defmacro define-x-hygienically  `(do (def x# 2) (list x#))) (def x 4) (define-x-hygienically) ; -> (2) (list x) ; -> (4) ;; It's typical to use helper functions with macros. Let's create a few to ;; help us support a (dumb) inline arithmetic syntax (declare inline-2-helper) (defn clean-arg [arg] (if (seq? arg) (inline-2-helper arg) arg)) (defn apply-arg "Given args [x (+ y)], return (+ x y)" [val [op arg]] (list op val (clean-arg arg))) (defn inline-2-helper [[arg1 & ops-and-args]] (let [ops (partition 2 ops-and-args)] (reduce apply-arg (clean-arg arg1) ops))) ;; We can test it immediately, without creating a macro (inline-2-helper '(a + (b - 2) - (c * 5))) ; -> (- (+ a (- b 2)) (* c 5)) ; However, we'll need to make it a macro if we want it to be run at compile time (defmacro inline-2 [form] (inline-2-helper form)) (macroexpand '(inline-2 (1 + (3 / 2) - (1 / 2) + 1))) ; -> (+ (- (+ 1 (/ 3 2)) (/ 1 2)) 1) (inline-2 (1 + (3 / 2) - (1 / 2) + 1)) ; -> 3 (actually, 3N, since the number got cast to a rational fraction with /)
Originally contributed by Adam Bard, and updated by 4 contributor(s).