Question

I must be misunderstanding something about how to define functions. I’m doing this SICP exercise, “Define a procedure that takes three numbers as arguments and returns the sum of the squares of the two larger numbers.”

I try this in the REPL, and it seems to work:

=> (reduce + (map (fn [x] (* x x)) (rest (sort '(2 1 0)))))
5

But defining a procedure and replacing the numbers with parameters gives me an error:

=> (defn my-procedure [a b c] (reduce + (map (fn [x] (* x x)) (rest (sort '(a b c))))))
ClassCastException clojure.lang.Symbol cannot be cast to java.lang.Number clojure.lang.Numbers.multiply (Numbers.java:146)

What am I not seeing? Thanks!

Answer 1

This should work for you:

(defn my-procedure [a b c]
  (reduce + (map (fn [x] (* x x)) (rest (sort (list a b c))))))

here is why:

When you use a quote form, the form is not evaluated and your vars are not resolved. You are left with a list of symbols in your case.

user=> (def a 1)
#'user/a
user=> '(a)
(a)
user=> (list a)
(1)
user=> `(a) ;; backtick causes vars to be namespace qualified
(user/a)
user=> (class (first '(a)))
clojure.lang.Symbol

Rather than create an un-evaluated form, you should use the list function or another collection

user=> [a] ;; vector
[1]
user=> #{a} ;; set
#{1}

;; (defn sum-of-squares [& args]
;;   (apply + (map #(* % %) args)))