Named parameters functions in Clojure

Question

This code discussing named arguments in Clojure from "The Joy of Clojure":

(defn slope [& {:keys [p1 p2] :or {p1 [0 0] p2 [1 1]}}] 
   (float (/ (- (p2 1) (p1 1))
             (- (p2 0) (p1 0)))))

(slope :p1 [4 15] :p2 [3 21])

The function itself, I understand it -no problem with destructuring- but I don't understand the calling.
Are we passing four arguments to slope? how vectors are getting assigned to :p1 and :p2?

Answer 1

You are passing four arguments to slope, yes. The [] part of slope specifies the parameters, in which & means "slurp all additional parameters into this form", which then specifies that it is looking for arguments that form a map with keys p1 and p2 (and gives default values if either doesn't exist).