A convenient macro for creating a function with default values for arguments.

default_args.clj

(defmacro defn-defaults [name args body]
  "Create a function that can provide default values for arguments.

  Arguments that are optional should be placed in a hash as the
  last argument with their names mapped to their default values.

  When invoking the function, :<optional-argument-name> <value> 
  specifies the value the argument should take on."

  (if (map? (last args))
    `(defn
       ~name
       ~(let [mandatory-args (drop-last args)
              options (last args)
              option-names (vec (keys options))]
          (vec (concat mandatory-args
                       [(symbol "&") {:keys option-names 
                                      :or options}])))
       ~@body)
    `(defn ~name ~args ~@body)))

; EXAMPLE
(defn-defaults foo [a b {c 5 d 10}]
  (+ a b c d))

(foo 5 10) ;=> 30
(foo 5 10 :c 10 :d 20) ;=> 45
(foo 5 10 :c 0) ;=> 25

Hi. First line should be
(defmacro defn-defaults [name args & body]