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?

dfan · Accepted Answer · 2011-04-12 14:33:24Z

8

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).

answered Apr 12, 2011 at 14:33

dfan

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2 Comments

Chiron Over a year ago

I got that but when calling: (slope :p1 [4 15] :p2 [3 21]) how p1 gets the value [4 15] and p2 [3 21] ?

dfan Over a year ago

p1 got the value [4 15] because the arguments :p1 [4 15] :p2 [3 21] are treated as a map, and the parameter block in slope says to bind the variables p1 and p2 to their corresponding values in that map.

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