clojure-koans/src/koans/macros.clj

(defmacro hello [x]
  (str "Hello, " x))

(defmacro infix [form]
  (list (second form) (first form) (nth form 2)))

(defmacro infix-better [form]
  `(~(second form) ; Note the syntax-quote (`) and unquote (~) characters!
    __
    __ ))

(defmacro r-infix [form]
  (cond (not (seq? form))
          __
        (= 1 (count form))
          `(r-infix ~(first form))
        :else
          (let [operator (second form)
                first-arg (first form)
                others __]
            `(~operator
              (r-infix ~first-arg)
              (r-infix ~others)))))


(meditations
 "Macros are like functions created at compile time"
 (= __ (hello "Macros!"))

 "Can I haz some infix?"
 (= __ (infix (9 + 1)))

 "Remember, these are nothing but code transformations"
 (= __ (macroexpand '(infix (9 + 1))))

 "You can do better than that, hand crafting ftw!"
 (= __ (macroexpand '(infix-better (10 * 2))))

 "Things dont always work as you would like them to... "
 (= __ (macroexpand '(infix-better ( 10 + (2 * 3)))))

 "Really, you dont understand recursion until you understand recursion"
 (= 36 (r-infix (10 + (2 * 3) + (4 * 5)))))