A function is considered pure if does not side effects or is affected by side causes. A pure function does not change any other part of the system and is not affected by any other part of the system.
When you pass arguments to a function and that function returns a value without interacting with any other part of the system, then that function is considered pure.
Should something from outside a function be allowed to affect the result of evaluating a function, or if that function be allowed to affect the outside world, then its an impure function.
So lets look at a simple code example
Write a pure function that adds two numbers together ?
(defn add-numbers [number1 number2] (+ number1 number2)) (add-numbers 1 2)
Lets look at each line of this suggested answer
(defn add-numbers [number1 number2] ;; function takes 2 arguments (+ number1 number2)) ;; function uses both arguments for result (add-numbers 1 2) ;; specific values are passed as arguments
An example with map
Define a collection called numbers and write a named function that increments each number of the numbers collection.
Is your function pure or impure ?
Impure function example.
(def numbers '(5 4 3 2 1)) (defn increment-numbers  (map inc numbers)) (increment-numbers)
The function takes no arguments and is pulling in a value from outside the function. This is a trivial example, but if all your code is like this it would be more complex. If the value pointed to by
numbers is mutable and changes before the
increment-numbers functiion is called then you will get different results.
Here is a Pure function example
(def numbers '(5 4 3 2 1)) (defn increment-numbers [number-collection] (map inc number-collection)) (increment-numbers numbers)
In this example we are explicitly passing the
numbers collection to the function. The function works on passed value and returns a predictable result.