In Clojure how is a macro different from a function?

Question

I just started with macros and made a simple one to return maximum number out of a list.

(defmacro macro-max [list] (apply max list))

and If I make a function to do the same, it will be.

(defn  macro-max [list] (apply max list))

I am just exploring Clojure, so I don't know much.

I may sound completely silly to experts, But it looks like I can pretty much define a function instead of a macro.

Answer 1

The difference comes to life when you don't want to evaluate your arguments unless needed.

Consider this example:

(defn unless [pred body]
  (when (not pred)
    body))

This doesn't work since arguments to a function are eagerly evaluated. So body always runs, as shown below:

(unless (zero? 2)
        (prn "Not zero!"))
;; "Not zero!"

(unless (zero? 0)
        (prn "Not zero!"))
;; "Not zero!"

Both executions above print "Not zero!", which is clearly wrong.

The only way to write our unless utility is by using a macro:

(defmacro unless [pred body]
  `(when (not ~pred)
     ~@body))

Now if we give it a try, we'll see it works as expected:

(unless (zero? 2)
        (prn "Not zero!"))
;; "Not zero!"

(unless (zero? 0)
        (prn "Not zero!"))
;; this prints nothing

Macros only evaluate its arguments when the developer decides to.

You'll notice some special characters in the macro such as `, ~ and ~@. They stand for syntax-quote, unquote and unquote-splicing, respectively and are explained here.

I suggest you study these symbols.

Hope this helps.