endmap.clj

(defn add [& {:keys [:a :b]}]
  (+ a b))

(add :a 1 :b 2)
;;=> 3

(add :a)
;;=  No value supplied for key: :a

(add {:a 1 :b 2})
;;=> 3

;;   WHAT SHOULD HAPPEN?
(add {:a 1 :b 2} {:a 10 :b 20})                                                                 
;;=> WHAT HAPPENS HERE?

(add :a 1 {:b 2})
;;=> 3


;;;; PARTIAL
(def add1 (partial add :a 1))

(add1 :b 2)
;;=> 3

(add1 {:b 2})
;;=> 3

(def add2 (partial add :b 2))

(add2 :a 1)
;;=> 3

(add2 {:a 1})
;;=> 3

;;;; OVERRIDE
(add2 {:a 1 :b 20})
;;=> 21

(add2 {:b 2})
;;=  NPE because (+ nil b)

;;;; with pre-args

(defn frob [a b & {:keys [c d]}]
  [a b c d])

(frob 1 2 :c 3 :d 4)
;;=> [1 2 3 4]

(frob 1 2 {:c 3 :d 4})
;;=> [1 2 3 4]

(frob 1 2 :c 3 {:d 4})
;;=> [1 2 3 4]

((partial frob 1 2 :c 3) :d 4)
;;=> [1 2 3 4]

((partial frob 1 2 :c 3) {:d 4})
;;=> [1 2 3 4]

((partial frob 1 2 :c 3 :d 4) {:d 40})
;;=> [1 2 3 40]