Pure functions

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

Note::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

;; function takes 2 arguments
;; function uses both arguments for result
(defn add-numbers [number1 number2]
  (+ number1 number2))

;; specific values are passed as arguments
(add-numbers 1 2)

An example with map

Note Define a collection called numbers and write a named function that increments each number of the numbers collection. Is your function pure or impure ?

(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 function 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.