Impure functions

Using external values rather than the arguments that were passed

(def random-value '(5 4 3 2 1))

(defn impure-increment-numbers [number-collection]
  (map inc random-value))

(impure-increment-numbers '(1 2 3 4 5))

Using a global value rather than the argument passed makes this function non-deterministic

Side effect example

(defn print-to-console [value-to-print]
  (println "The value is" value-to-print "- the returned value is below:"))

(print-to-console "a side effect")

(rand)                         ; reading (from a random number generator) is too

Impurity via host language interoperability

Using code from the host environment is often a source of impurity, especially when that language is very stateful.

This example creates a new date object using the JavaScript Date class from the host environment.

(defn task-complete [task-name]
  (str "Setting task " task-name " completed on " (js/Date.)))

(task-complete "hack clojure")

When we call task complete we have no control over the date that the function uses.

(:import java.util.Date)

(defn task-complete [task-name]
  (str "Setting task " task-name " completed on " (java.util.Date.)))

(task-complete "hack clojure")

A pure approach to host language Interoperability

It keeps functions simpler if you use generated numbers outside of a function and passed them as an argument.

In this example we use (.getTime (js/Date.)) to get the time from JavaScript

(defn task-complete [task-name completed-date]
  (str "Setting task " task-name " completed on " completed-date))

(task-complete "hack clojure" (js/Date.))

(:import java.util.Date)

(defn task-complete [task-name completed-date]
  (str "Setting task " task-name " completed on " completed-date))

(task-complete "hack clojure" (java.util.Date.))