Let's start with a minimum function definition. Here is a minimal syntax of defn:

(defn funtion-name [arg1 arg2 ...] 
  expr-1  
  expr-2 
  .. 
  expr-n 
  ) 

defn is a special form. The first argument is a function name and is followed by a vector of one or more arguments, then one or more expressions. The last expression is returned to the caller.

Here, we define a very simple function. The hello function returns a Hello world string:

(defn hello [s] 
  (str "Hello world " s " !")) 
;;=> #'living-clojure.core/hello 
(hello "Nico") 
;;=> "Hello world Nico !" 
(hello "Makoto") 
;;=> "Hello world Makoto !" 

The next sample defines a simple adder function:

(defn simple-adder [x y] 
  (+ x y) 
  ) 
;;=> #'living-clojure.core/simple-adder 
(simple-adder  2 3) 
;;=> 5 

A variadic function allows a variable number of arguments. The next example defines another adder. It may have an arbitrary number of arguments:

(defn advanced-adder [x & rest] 
  (apply + (conj rest x)) 
  ) 
;;=> #'living-clojure.core/advanced-adder 
(advanced-adder 1 2 3 4 5) 
;;=> 15 

Here, we will introduce the multiple arity function. The following function defines a single argument function and a couple of argument functions with the same defn:

(defn multi-arity-hello 
  ([] (hello "you")) 
  ([name] (str "Hello World " name " !"))) 
;;=> #'living-clojure.core/multi-arty-hello 
(multi-arity-hello) 
;;=> Hello World you ! 
(multi-arity-hello "Nico") 
  ;;=> Hello World Nico ! 

Sometimes, specifying a keyword is useful, since it is not necessary to remember the order of arguments.

The next example shows how to define such a function. The options are :product-name, :price, and :description. The :or expression supplies default values if any values in keys are omitted:

(defn make-product-1 
  [serial & 
   {:keys [product-name price description] 
    :or {product-name "" price nil description "no description !"} 
    }  
   ] 
   {:serial-no serial :product-name product-name 
    :price price :description description} 
  ) 
;;=> #'living-clojure.core/make-product-1 
 
(defn make-product-2 
  [serial & 
   {:keys [product-name price description] 
    :or {:product-name "" :description "no description !"} 
    }    
   ] 
   {:serial-no serial :product-name product-name 
    :price price :description description} 
  ) 
;;=> #'living-clojure.core/make-product-2 
 
(make-product-1 "0000-0011") 
;;=> {:serial-no "0000-0011", :product-name "", :price nil, :description "no description !"} 
(make-product-2 "0000-0011") 
;;=> {:serial-no "0000-0011", :product-name nil, :price nil, :description nil} 

Clojure can define functions with pre-condition and post-condition. In the following defn, :pre checks whether an argument is positive. :post checks whether the result is smaller than 10:

(require '[clojure.math.numeric-tower :as math]) 
;;=> nil 
(math/sqrt -10) 
;;=> NaN 
(defn pre-and-post-sqrt [x] 
  {:pre  [(pos? x)] 
   :post [(< % 10)]} 
   (math/sqrt x)) 
;;=> #'living-clojure.core/pre-and-post-sqrt 
(pre-and-post-sqrt 10) 
;;=> 3.1622776601683795 
(pre-and-post-sqrt -10) 
;;=> AssertionError Assert failed: (pos? x)  user/pre-and-post-sqrt (form-init2377591389478394456.clj:1) 
(pre-and-post-sqrt 120) 
AssertionError Assert failed: (< % 10)  user/pre-and-post-sqrt (form-init2377591389478394456.clj:1) 

Moreover, in this recipe, we will show a more complicated function. The make-triangle function prints a triangle with a character. If this function is called without a :char argument, it prints a triangle made of asterisks. If it is called with a :char argument, it prints a triangle comprising characters specified by :char:

(defn make-triangle 
  [no & {:keys [char] :or {char "*"}}] 
  (loop [x 1] 
    (when (<= x no)  
      (dotimes  
          [n (- no x)] (print " ")) 
      (dotimes  
          [n  
           (if (= x 1)  
             1     
             (dec (* x 2)))] 
        (print char)) 
      (print "\n") 
      (recur (inc x)) 
      ) 
    ) 
  ) 
(make-triangle 5)     
;;=>     * 
;;=>    *** 
;;=>   ***** 
;;=>  ******* 
;;=> ********* 
;;=> nil 
(make-triangle 6 :char "x"))     
;;=>      x 
;;=>     xxx 
;;=>    xxxxx 
;;=>   xxxxxxx 
;;=>  xxxxxxxxx 
;;=> xxxxxxxxxxx