Clojure Mandatory keyword argument

Question

I have a function like this:

(defn foo [{a :keya b :keyb}]
  (list a b))

And i'm calling it like this:

(foo {:keya "hi"}) ; Returns ("hi" nil)

If I don't give keyb keyword argument, it takes nil for that. Is there a way to ensure that it throws exception for it instead of taking it as nil.

( I know that I can manually check and throw an exception, but is there any special option which enforces the constraints.)

Answer 1

You can use a precondition (http://clojure.org/special_forms#toc9) to assert the key is present:

(defn foo [{a :keya b :keyb}] 
   {:pre [(not (nil? b))]}
   (list a b))

This will throw an AssertionError when the key is nil.

Answer 2

No, but of course because Clojure is a lisp you can define your own macro that handles the boring "manual checking" for you automatically.

Answer 3

aldazosa gave you the right solution, but {:keya nil :keyb nil} is the valid map too. To allow nil values you may use contains? instead of nil? check:

(defn foo [{a :keya b :keyb :as m}]
  {:pre [(every? (partial contains? m)
                 [:keya :keyb])]}
  (list a b))

Answer 4

If you want something more complex look at Validateur or bouncer