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")
Other impure examples
(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 funciton uses.

Java version of the example

This example is the same as above, except it uses the Java Date class.

(java.util.Date.) creates a new date object with the current date

(: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.))
Java version of the example
(: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.))

Hint

Where impure functions cannot be avoided, it is common to wrap impure code inside a function in order to keep all your other functions pure.

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