打印并返回Clojure中的另一个值

Question

I have a Clojure beginner question about why the following code doesn't work :

(defn aFun [n]
  (if (< -10 n 10) 
       ((println "Single digit" n) "A return value")
       ((println "More digits" n) "Another return value")))

I get a NullPointerException at runtime. Why ?

This function works :

(defn aFun [n]
  (if (< -10 n 10) 
       "A return value"
       "Another return value"))

And this one too :

(defn aFun [n]
  (if (< -10 n 10) 
       (println "Single digit" n)
       (println "More digits" n)))

I'd like to do both at the same time however : (compute and) return a value, while printing some things first. I'm sure this is pretty basic (not even sure how to title my question), but I can't get what I am doing wrong.

Answer 1

((println "Single digit" n) "A return value")

... attempts to apply (println "Single digit" n) , which returns nil , as a function, to "A return value" .

You probably intend

(do (println "Single digit" n) "A return value")

Try, where you can, to separate side-effects from pure functional computation. Here, for example,

(defn single-digit? [n]
  (< -10 n +10))

... is pure. You can wrap it in stuff that prints what you like.